from city’s station to station city v journal of facade design & engineering volume 6 / number 3 / 2018 editorial this special issue is linked to the conference façade 2018 – adaptive!, the fifth conference that has been organised by lucerne university of applied science and arts within the framework of the european façade network, efn. façade 2018 is also the final conference of the cost action tu1403 ‘adaptive façades network’ (www.tu1403.eu) and dedicated to multifunctional, adaptive, and dynamic building envelopes. approximately one third of all energy consumed by end-users in europe today is for space heating& cooling, ventilation, and lighting of buildings. therefore, the energy performance of future building envelopes will play a key role in meeting the eu’s climate and energy sustainability targets. while most of our façades today are passive systems and are largely exhausted from an energetic point of view, multifunctional, adaptive, and dynamic façades can be considered as the next big milestone in façade technology. adaptive building envelopes are able to interact with the environment and the user by reacting to external influences and adapting their behaviour and functionality accordingly: the building envelope insulates only when necessary, it produces energy when possible, and it shades or ventilates when the indoor comfort so demands. nevertheless, the development and realisation of adaptive building envelopes is still in the initial stages. in order to advance the development and application of adaptive façades, cost action tu1403 ‘adaptive façades network’ was initiated in 2014. the cost action, which started in 2015 and will end in late 2018, is a european research project with the objective to support trans-national cooperation between researchers and industry through science and technology networks. over four years, more than 210 participants from 27 countries were involved in numerous cost networking activities: 15 meetings, two training schools, two industry workshops, 31 short term scientific missions, and two conferences. the main objectives of the cost action were to: – increase knowledge sharing between the various european research centres and between these centres and the industry; – foster the development of novel concepts and technologies and/or new combinations of existing technologies for adaptive façades; – foster the development of effective evaluation tools, methods, and metrics for adaptive façades. the work was managed in four working groups: – wg1. adaptive technologies and products – wg2. component performance and characterisation methods – wg3. whole building integration and whole-life evaluation methods of adaptive façades – wg4. dissemination and future research vi journal of facade design & engineering volume 6 / number 3 / 2018 the more than 60 presentations and proceeding contributions at the conference façade 2018 are the result of cost tu 1403 and give an overview of the state of the art in adaptive façade technology. the thirteen articles in this special issue façade 2018 – adaptive! were proposed by the guest editors, who are working group (wg) leaders, and the scientific committee to undergo the double-blind peer review process of jfde. the criteria for the selection of papers from proceeding contributions were the completeness of work and the scientific relevance, so that the special issue constitutes a representation of the work executed by the action’s working groups. as guest editor, i would like to thank the authors and reviewers for their significant contributions to this special issue. moreover, i sincerely thank susanne gosztonyi and stephanie ly-ky for their great assistance during the whole process. i also want to thank cost (european cooperation in science and technology). this special issue is based upon work from cost action tu1403, supported by cost. last but not least, constructive comments and great help of the jfde editor thaleia konstantinou, and editors in chief, ulrich knaack and tillmann klein, are gratefully acknowledged. andreas luible chair of cost action tu1403 this special issue is based upon work from cost action tu1403 ‘adaptive façades network’, supported by cost (european cooperation in science and technology). cost (european cooperation in science and technology) is a funding agency for research and innovation networks. our actions help connect research initiatives across europe and enable scientists to grow their ideas by sharing them with their peers. this boosts their research, careers, and innovation. funded by the horizon 2020 framework programme of the european union from city’s station to station city v journal of facade design & engineering volume 7 / number 1 / 2019 editorial welcome to our final jfde issue of 2020, a year defined by the global covid-19 outbreak, which has had a drastic impact on all our lives, ranging from global politics to our daily routines. conversely, most academic activities had to switch from a physical setting to a virtual environment (a situation that is still very much the case in europe). we have had to learn how to interact via digital tools and especially seize and promote the value of not just written information, but also the potential behind virtual conferences, workshops, and events to keep in touch with our colleagues, friends, and fellow researchers. well, what else can we say… it has worked surprisingly well. although most of us are certainly looking forward to regaining the closeness that regular, daily interaction brings to our academic lives -may this be in education or research-, we are also truly aware of, and amazed by, the benefits that have come from this widespread digital conversion to virtual settings. we currently have the possibility to easily engage with researchers and students from all over the world, increasing the dissemination of relevant knowledge, and sparking the formation of new exchange networks, regardless of our physical distance. a special mention in this regard goes to the potential co 2 emissions that may be saved by decreasing our work-related travels. does it really make sense to fly around the globe for just one talk at a conference? maybe not. and yes, we can even have social interaction via digital tools, which of course works, but this is one aspect in which it can be shown that things are not the same. personal contact cannot be replaced. organised matches at scientific events, spontaneous meetings over coffee, and, of course, the direct contact among students are all essential activities that we will be happy to resume once this crisis is over, while hopefully we will continue to reap the unintended benefits and lessons that came with it. so, expect hybrid to be the new normal! now, about the articles showcased in this issue: prefab, bricks, and water seem to be the core topics addressed by the researchers. it is interesting to see so many different approaches when it comes to prefabrication as a general strategy for the development of a design concept. the same goes for the articles that explore the use of water, either as a thermal energy carrier, or its impact on the hygrothermal performance of buildings. finally, it is always fascinating to see clear links between theoretical scientific developments and the design and production of components for application in real buildings. so, once again we have quite a wide range of themes throughout this issue, but as it often is with the built environment, all of these different aspects add to the general knowledge about façades, finding their way into real building envelopes that perform and function under a holistic approach. the editors in chief, ulrich knaack tillmann klein blank page blank page journal of facade design and engineering 2 (2014) 1–2 doi 10.3233/fde-150017 ios press 1 editorial jfde special issue glass facade design and engineering is a multidisciplinary field that touches many other scientific disciplines. glass is one of the key materials for building envelopes, and a strong scientific community has developed over the last decade. designers love glass for its transparency. it is strong but brittle and very demanding in terms of engineering. we continuously see new innovative developments in terms of its climatic performance, structural possibilities, construction design and new applications. reason enough to dedicate this special issue to the topic. the issue would not have been possible without the contribution of our special editors jan belis and christian louter, who contributed through their outstanding editorial work and network. most of the papers in this issue were carefully selected from of a number of invited submissions and conference papers of the cost action tu0905 mid-term conference, april 17+18 2013, porec, (crc press/balkema, leiden) and subsequently subjected to the regular blind review process of the journal. glass as a building material demonstrates the nature of the architectural discipline, where science and building practice are closely linked. buildings are the live testing bed for scientific research and, at the same time, building practice formulates new research questions. we found that many articles sent to us deal with this relation. therefore we decided to introduce the new category ’applied practice’ for certain journal paper contributions, which from now on can be found at the end of each issue. although they do not need to be purely scientific, ’applied practice’ papers will always discuss new developments, will have a clear structure and are subjected to the strict jfde review process. façade design and engineering is a peer reviewed, open access journal, funded by the netherlands organisation for scientific research nwo (www.nwo.nl). we see ’open access’ as the future publishing model. but it certainly requires new financial models which we will have to explore in the coming years. the introduction of ’applied practice’ papers also serves a role here by linking the journal to a scientific as well as to a large professional audience. in this manner, it can fulfil its role to disseminate scientific knowledge. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. 2 u. knaack and t. klein / editorial we are pleased to announce that we have been able to broaden the team around jfde with thaleia konstantinou and marjan vrolijk as managing editors who will, along with the team members from ios press amsterdam, certainly contribute to the quality of this publication. the editors in chief, ulrich knaack and tillmann klein from city’s station to station city v journal of facade design & engineering volume 7 / number 1 / 2019 editorial powerskin 2019 this issue of the journal of façade design and engineering is a result of the second façade conference, powerskin, held on january 17th 2019, in the context of the building trade fair ‘bau’ in munich. the conference was organized collaboratively by tu munich, tu darmstadt, and tu delft. all three universities conduct high-impact research and education in the field of building envelopes. the conference featured a mix of practice and education experiences, as well as scientific contributions, and aimed to answer the key question of the 2019 conference: how can digital tools and methods promote changes that aim towards the decarbonisation of the built environment and the improvement of well-being? the 2019 conference focused on the envelope as an integral part of the building and its energy system, as well as the main driver to create comfort. thus, the envelope is understood in its condition of being a complex interface to the social, economic, and climatic environment in which we build our cities. the interaction between these topics, the influences they create, and the digital tools that have been developed and used to design and engineer building envelopes are the main focus points of each of the papers published in this issue. we thank our guest editors thomas auer (tu munich) and jens schneider (tu darmstadt), and their respective teams, who have been key partners in creating this special issue. the editors in chief, tillmann klein and ulrich knaack. doi 10.7480/jfde.2019.1.3025 from city’s station to station city v journal of facade design & engineering volume 6 / number 2 / 2018 jfde special icae2018 – envelope 4.0 dear scientists, engineers, and designers, with this new edition we continue the collaboration with our funding member tecnalia and the international congress on architectural envelopes (www.icae2018.eu), which they organise every 3 years in san sebastian (basque country, spain). the eleven articles found in this new issue were carefully selected from 50 abstracts that will be presented during the scientific section of the congress. the final selected papers were subjected to the regular double-blind review process of the journal. with this selection of papers, we want to give an overview of the traditional technologies that are normally discussed in the context of façades, while also making some mention of new technologies, which, some time ago, existed only in the realm of the laboratory but which have begun to appear in real buildings and construction in recent years. in fact, the main topic of the conference this year is the envelope 4.0, and consequently the aim of the magazine. under this title, the aim is to unify two elements: the architectural envelope, due to it being the main reason for holding the conference, and the term “industry 4.0” as a representation of the need to improve the manufacturing, assembly, distribution, and installation processes of many of the products included in today’s envelopes in order to be competitive in the market. we hope that with this selection we can provide you with new ideas and possibilities. an interesting set of approaches to new technologies is demonstrated (using robotics systems to install façades), new materials for the envelope (biocomposites, smart and multifunctional materials, and new types of membranes), use of bim from design to manufacturing, 3d printed façades for thermal regulation, solar heating and cooling technologies in the envelope, characterisation of the thermal performance in roof solutions, and numerical and experimental performance assessment of structural thermal bridges. in conclusion, we want to thank our special editors, julen astudillo and jose antonio chica from tecnalia, for their effort in making this partnership happen. the editors of this special edition, julen astudillo jose antonio chica ulrich knaack tillmann klein http://www.icae2018.eu v journal of facade design & engineering volume 8 / number 1 / 2020 editorial welcome to this new issue of our journal of façade design and engineering. we are very pleased to be able to release a new issue under the current global circumstances, to keep supporting research and the engineering of new technologies, materials, and methods for the design of our envelopes. this special issue features a wide range of topics, stemming from research activities of members from the european façade network (efn). the efn seeks to advance and promote façade design and engineering at a european level and beyond, through inclusive collaboration between european research centres, universities, and alumni, resulting in skills and knowledge transfer in education, research, and development. consequently, this special issue showcases a selection of research experiences presented at two scientific events sponsored by the efn. the first scientific event was the conference “facades19” held in lisbon on november 22nd, 2019, which was organised by dr. daniel aelenei from the department of civil engineering at nova school of science and technology. the second scientific event refers to a special “efn session“ hosted at the façade tectonics 2020 world congress, held online in august 2020. this session within the larger congress was coordinated by daniel artzmann, mikkel kragh, annalisa andaloro, and ulrich knaack. the selection of the papers from both events was based on their relevance to the scope of our journal and went through the double-blind peer review process of the jfde. we thank all supporters and contributors who made these events a success and especially the authors of the articles compiled in this issue. the editors in chief, ulrich knaack tillmann klein https://urldefense.proofpoint.com/v2/url?u=https-3a__www.fct.unl.pt_en&d=dwmfaq&c=xyzuhxbd2cd-cornpt4qe19xojbbry-tbplk0x9u2o8&r=jlr9ztrqy815mcc1a8kmmjjfu3alv9skdpkdfjgzna0&m=yprduuia8x8wuvl3_6knt3ahq6ja7rjrt_ix-fz9_i8&s=_dgwfthoh69iyc1mjzfkzrbta5vfh-rijh8ywj5dlfq&e= https://urldefense.proofpoint.com/v2/url?u=https-3a__www.fct.unl.pt_en&d=dwmfaq&c=xyzuhxbd2cd-cornpt4qe19xojbbry-tbplk0x9u2o8&r=jlr9ztrqy815mcc1a8kmmjjfu3alv9skdpkdfjgzna0&m=yprduuia8x8wuvl3_6knt3ahq6ja7rjrt_ix-fz9_i8&s=_dgwfthoh69iyc1mjzfkzrbta5vfh-rijh8ywj5dlfq&e= jfde-vo_08-is_01-web from city’s station to station city 053 journal of facade design & engineering volume 6 / number 2 / 2018 bim from concept design to fabrication: a customised methodology for façade consultancies ana gallego fernández, miguel a. núñez díaz, a. mateo marcos núñez enar. architectural envelopes, madrid, spain abstract when an architect ideates a complex building envelope, they often rely on a façade consultancy to develop the final detailed solution of their design. the purpose of this paper is to describe a customised bim methodology to develop complex building envelopes, evaluating the process followed to convert an architectural concept design to a fabrication reality. over recent years, building information modelling has developed greatly in terms of architectural, structural and mep disciplines. it conveniently advances and analyse the variables of a concept design, and furthermore, coordinates disciplines during the detailed design phases. however, when a technical approach to the envelope’s design must be implemented, we need detailed engineering tools to simulate the environmental data, and to analyse and develop the system’s fabrication features and assemblies, which are tedious to incorporate on bim basis. this paper describes the process followed to develop and execute a building envelope project, starting with a concept design and incorporating virtual simulation processes for the solution to meet its structural and thermal requirements. the final aim is to have detailed drawings and documents of the envelope’s elements, with coordinated information for construction and fabrication purposes. keywords building information modelling (bim), prototyping, digital manufacturing doi 10.7480/jfde.2018.2.2088 054 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction the architecture, engineering, and construction industry (aec) is facing an era in which technology is improving the way we develop projects, as well as the techniques and the materials used for those projects. blanco, mullin, pandya, and sridhar (2017), from the mckinsey institute, explain how new applications and tools are changing the way companies design, plan, and execute construction projects. since such designs are becoming increasingly complex and expensive, managers must take into consideration solutions for cost improvements, the timelines of projects, and the overall efficiency of the construction process. the construction industry is one of the largest in the world economy, despite having the dubious honour of being at the tail-end of labour productivity in most countries. fig.1 shows how the construction sector labour-productivity growth averaged 1% per year over the past two decades, compared with 2.8% for the total world economy, and 3.6% for manufacturing. the complete study is found in the mckinsey report (barbosa et al., 2017). fig. 1 global productivity growth trends. source: mckinsey global institute the mckinsey report outlines how productivity can be improved in the sector and suggests seven actions that the construction industry should adopt: (1) reshape regulation and increase transparency; (2) rewire the contractual framework; (3) rethink design and engineering processes; (4) improve procurements and supply-chain management; (5) improve on-site execution; (6) infuse digital technologies, new materials and advanced automation; and (7) reskill the workforce. this report also profiles how productivity in the construction sector will increase if some parts of the industry move to a manufacturing-style production system, such as the projects developed by barcelona housing systems or the osirys research project: forest based composites for façades and interior partitions to improve indoor air quality in new builds and restorations. the aec industry needs to adopt an integrated advanced platform that spans project-planning, design, construction, operations, and maintenance. companies can start by making 3d building information modelling (bim) universal within the company’s workflows along with digital collaboration platforms to establish transparency in the design, costing, and progress visualisation of a project. frontrunners in the construction industry are embracing the bim methodology, as they https://www.mckinsey.com/our-people/andrew-mullin https://www.mckinsey.com/our-people/mukund-sridhar 055 journal of facade design & engineering volume 6 / number 2 / 2018 have with other new technologies such as 3d printing, cloud computing and big data. the need for synchronised information throughout the project life cycle is beneficial not only for project owners and for contractors, but for the efficiency of every design team involved in the construction sector. in addition, the advanced analytics enabled by the internet of things improves on-site monitoring of materials, labour, and equipment productivity. when the bim methodology is adopted correctly, it facilitates an integrated design and construction process, and enables an improved on-site execution. this is verified in the mckinsey report by means of case studies that have been developed in different countries around the world, where the use of new technologies, along with building information modelling, allows them to achieve greater productivity. 2 building information modelling of architectural envelopes building information modelling (bim) is a very broad term that describes the process of creating and managing information about a built asset through one or more virtual models that are digitally constructed. bim technology supports design through its phases, allowing better analysis and control than with manual processes. when completed, these computer-generated models contain precise geometry and data needed to support the construction, fabrication, and procurement activities through which the building is realised. in 2014, the european union urged member countries to adopt bim on public projects to improve the productivity of the aec industry. the efficiency outcomes are realised in viewing the design, construction, and operation as a whole: the life cycle of the asset. traditional construction relies on 2d drawings for information sharing, while bim provides an added dimension to design, communication, and strategic planning. 3d digital models allow clients and project teams to view a design with more accuracy than any flat image can provide. in 2017, the british standards institution published the new iso 19650, describing the international standards that must be followed for information management using bim. this standard gives recommendations for a structured framework to manage, exchange, and organise information through the whole life cycle of a built asset, and guidance for organisations to develop the right commercial and collaborative environment so that information is produced in an effective and efficient manner during the project’s delivery phases, reducing wasteful activities. all of these standards are also applicable for the envelope models, where information exchange amongst the architectural and structural teams is crucial. the correct organisation of the information required to develop the project within the team is also a key factor. the following points outline the basis of the bim methodology and provide specific guidance for the development of the envelope information models. 2.1 dimensions within the digital models everybody familiar with the bim methodology knows about the dimensions of the models, which comprise the basic difference as well as the benefit of using this technology. building elements have geometrical and classification standards as basic information, which lead to automated 056 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 2 the new dimensions of bim models. (image by gammon construction ltd, 2015) measurements and billing of the components in the model. we can also associate timeline information to the elements that compose the envelope for planning construction, procurement, and logistics. the sixth dimension of the virtual models is particularly relevant for building envelopes because the solar and thermal simulations will determine a sustainable solution for the performance of the building. finally, strategic decisions regarding the maintenance and operation of the asset can be made prior to the construction, making the project 7d. this is important in technological envelopes, where the building maintenance unit (bmu) is often needed in the project, and therefore the consideration of the maintenance of the glass elements of the façade is essential in the design phase. by using bim to develop design options during a project, we can see in a matter of hours the consequences that will result from decisions made in all dimensions of the project. we can see in fig.2 how new dimensions are being introduced in the construction scheme to integrate new fabrication methods, such as the use of robots in the process of assembling façade components, or the introduction of the iot (internet of things) in the entire construction process. what is referred to here as bim 8d is especially interesting in terms of the optimisation of the design process of the envelope, having an automated analysis and a calculation of the façade’s external condition, and integrating the standards with which the envelope needs to comply in the same process will provide us with a satisfying design basis. 2.2 coordination with other design disciplines. information exchange. the building envelope is currently considered as an independent design discipline in the construction schema, and this is a new departure, since it was always part of the architectural model. this is undeniable when we consider the definition of the discipline provided by the american nation institute of building sciences: “each design discipline has a different set of skills, professional standards and issues that drive how they operate in the building process”. the building envelope must be fully coordinated with the structural engineers’ information, but also with architectural finishes, hvac, plumbing, and electrical systems. an integrated design of a building requires the various stakeholders and disciplines to interact as early as possible in the process, and making available the clear information that is required at each stage of the project for the resolution of design objectives. 057 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 3 diagram of a bim design collaboration team. “a building envelope must reconcile many requirements –ventilation, solar heat gain, glare control, daylight levels, thermal insulation, water management, materials, assembly, sound and pollution control– making the design a complicated balancing act.” (lovel, 2013). the integration of environmental systems in the solution of the envelope must be a synthesised and integral part of the design process. for this to be accomplished there must be a comprehensive and clear information exchange between all the disciplines involved in the design of the asset, which is achieved more easily using bim models. design discipline required information information delivered architecture geometry, materials, aesthetics surface perimeter lines space characteristics, occupancy accesses, comfort quality structure supporting elements façade system loads characteristics, resistance anchorages & stiffeners mep systems ventilation, heating and cooling thermal gains drainage strategy, electric support façade transmittance table 1 basic information exchange for envelope’s development. 2.3 manufactured objects for bim many platforms facilitate façade components for virtual models, where thousands of objects and construction systems can be downloaded with all the accompanying information to cover all the levels of development (lod) needed in the project. this data is useful when the aim is to create an as-built model for operations and we need to incorporate all the characteristics of the building components for their future replacement or maintenance (lod 400). however, all this information is unnecessary in the project phase, when we need to define the solution clearly in order to develop shop drawings. 058 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 4 level of development of a curtain wall system. (image by bim forum) the problem that many designers face when downloading these elements is the size of the files, due to all the associated information and the custom variables that the elements present. as façade consultants, we often work with customised solutions or non-commercial building components made for a specific project, and therefore, we must create new specific components for each project. following the project progress timeline, which can be associated with the different lod of the building components, we start by defining the 3d geometry and introduce the information and details needed to define the element. the detailed development of these bespoke assemblies is one of the frameworks of this optimisation. part of the engineering proposals given by the façade company is the provision of an extremely detailed solution, which is in contrast with a typical architecture detail. experience has taught us that we cannot associate every detail of the building elements in order for them to appear when making a section or a plan view of the model. maintaining the same level of detail of components on a bim model as if it were a cad drawing will simply cause the model to crash. the aim of the 3d model is to coordinate the façade solution with other disciplines and share information with them. most of these 2d details are unnecessary for this purpose, but are required for drawings produced for fabrication purposes and for detailed specifications. being able to discern what information is necessary for the specific purpose in each phase of the project is the first step of the optimisation process. fig. 5 a detailed envelope drawing versus the bim model. 059 journal of facade design & engineering volume 6 / number 2 / 2018 2.4 fragmented delivery process versus integrated strategies one reason for the industry’s poor productivity record is that it still relies mainly on paper to manage its processes and deliverables such as blueprints, design drawings, procurement and supply-chain orders, equipment logs, daily progress reports, and punch lists. due to the lack of digitisation, information sharing is delayed and may not be universal. fig. 6 represents the different design approaches taken by the consultancy when delivering a project. the first diagram represents the traditional workflow and technical roles, while the second shows an integrated bim methodology with the software simulations tools within the project. most of the bim protocols focus on collaboration techniques and the various disciplines involved in a project, as well as the information exchange between them, which is key to developing a comprehensive and well-coordinated project. these fundamentals are also applicable to the work developed by the team of consultants when creating and executing a building project. it is necessary to move over to integrated strategies and collaboration platforms for sharing information within the companies to increase productivity, and avoid the loss of information and lack of coordination. incorporating bim into the design workflow of the envelope project allows us to evaluate it as a whole and comprises a unique database for all the information associated with the components of the building. the benefits of bim for subcontractors and fabricators are widely analysed in the bim handbook (eastman, teicholz, & sacks, 2011). fig. 6 traditional design workflow versus a bim integrated workflow. 060 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 7 detailed fabrication model. (image by f+a+t facades, u.k) 3 optimisation of the detailed design methodology as a consultancy, we must adapt our workflow to the criteria established by our clients, and each one has a different way of handling the information they give to the design disciplines involved in the construction project. we work with the bim software revit, from autodesk®, but our clients may not work with the same platform or even with bim at all. therefore, the first step for our design team is to translate the basic architectural and structural information received -when they are not revit models in order to define a common starting point from which to develop the project and follow the same bim methodology regardless of what the incoming source is. we also have to define an exchange procedure to coordinate the design process, and this depends on the software used by our clients. the type of building determines the objectives and the main characteristics that need to be obeyed by the envelope. we develop a wide variety of projects, such as offices, hotels, greater complexes such as shopping malls or refurbishment of existing buildings. whatever the type of building, it is important to establish how our team will use the model, as well as determining the uses our client needs for the efficiency of the design process. in multiple phases of a building project, the envelope designers need to interact with various simulation tools to predict the performance of the model, which makes interoperability among different programs a necessity (chi, wang, & jiao, 2015). we use mainly autodesk® tools, which implies that they have a priori good interoperability between each other, although this is not always directly achieved. for example, within the autodesk® tools, there is a gap between the software used in architecture and software intended for fabrication: architecture, engineering, and construction (aec) is a different collection than the one intended for product design & manufacturing (pdm). every façade solution is preceded by the geometry and the materials ideated by the architect. the design of the building envelope takes into consideration the form, size, and type of the glazing elements in order to create an energy efficient model with a maximum level of acceptable daylight. the bim-based performance optimisation described in the bpopt article (asl, zarrinmehr, bergin, & yan, 2015) can be used to evaluate the variables and search for an optimal solution at an early stage of the design, which is frequently done prior to the detailed development of the project that supports the architect in the design of the façade. the consultancy has to find the best technical solution to satisfy the design intent of the architect, while also considering the structural stability of the façade, the thermal conditions required in the building, and the compliance of international construction standards without disregarding the aesthetic of the envelope. 061 journal of facade design & engineering volume 6 / number 2 / 2018 task developed input process software output architectural and structural base for the façade design 2d .dwg files 3d .sat models autocad dynamo coordination model envelope geometry coordination model revit base model basic solar analysis façade base model revit sun path / insight design criteria structural constructability façade base model dynamo / excell structural criteria detailed development detailed drawings inventor / revit fabrication module detailed sun analysis geometry model dynamo / ecotect performance detailed structural solution façade model revit / dynamo / robot structure model execution project envelope models revit / dynamo / inventor / autocad drawing production table 2 software integration and automation processes considered. we usually start the detailed project from 2d cad drawings or 3d rhino files received from our clients, where the geometry of the building is defined and the structure outlined. using the bim platform along with its open source visual programming tool dynamo allows this process to be automated and have a reliable construction model with which to start the process of detailed design of the envelope. bim represents the building as an integrated database of coordinated information, and enables sharing this data for interoperability between prevalent software tools for specific simulations. once we start to develop the solution, we have to analyse the characteristics of the structure, and the geometry of the project as a whole, to consider an optimal technical solution. the size and form of the building elements have already been defined on a 3d coordination basic model. performing a basic analysis of the solar path and the radiation reaching the façade within the bim model will establish the design criteria we need to consider. having an initial estimation of the wind loads and the stability of the structural components is also necessary at this stage to have reliable dimensions of the components that will be assembled in the solution. this process can be optimised using dynamo to extract the geometrical information needed from the model and automatically introduce it into calculation sheets. once these analyses have been completed we have the technical criteria to do a first sketch of the typical detailed solution of the façade that needs to be validated by the architect in order to establish the coordination dimensions with the structure and the rest of the disciplines. we can then start to develop the detailed envelope model with the typical solutions considered. using bim coordinated models with other disciplines allows us to detect and analyse different encounters that need to be solved on the project being executed. performing clash detection with structure, interior finishes, and mep systems is a key process for automating this task. when the structural elements of the façade are complex, we introduce specific analysis tools like autodesk robot® to our workflow. finally, when the assemblies of the components are especially challenging we also use fabrication software to develop a detailed 3d model of a façade module. autodesk inventor® allows us to assemble the components of a module and have a detailed analysis of the element’s structural performance. this software will also allow us to generate drawings showing the assembly instructions of the components on a timeline basis. whichever software is used throughout the envelope’s development, the final goal of the detailing process is to have all the documents needed for the construction of the building. the implementation plans and technical documentation of the systems must be clear, coordinated, and comprehensive. 062 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 8 digital construction organization. source: mckinsey global institute 4 evaluation of the process the knowledge generated with each project that we develop in the consultancy with the bim methodology differs, in part because we never work on the same building types or with the same materials, and partially because our workflow must be adaptable to the one our clients follow. fig. 8 illustrates the five trends that will shape the digital construction organisation of the near future, developing the next generation of digital-native leaders to deliver projects of the future: (1) higher definition surveying and geolocation, with rapid digital mapping and estimating; (2) nextgeneration of 5d building information modelling; (3) digital collaboration and mobility, moving to paperless projects; (4) the internet of things and advanced analytics, that will enable intelligent asset management and decision-making; and (5) future-proof design and construction, by integrating materials and methods of the future into our designs. every project becomes an opportunity to test and refine our workflow, optimise the interoperability and coordination processes within our team and discover new digital solutions that can be tested on future projects. for this to be accomplished, there needs to be not only a commitment to invest on innovation, but also continuous training of team members -as well as the leaderswho need to use the latest equipment and digital tools. 5 conclusions in the spanish construction system, there are several phases of the design project to undertake before we can obtain the construction permits given by the appropriate authorities, and once these permissions are obtained and the design is finalised, the builders are chosen to materialise the project. using building information models allows fewer errors during the on-site construction process. developing and executing a project solely of the façade does not save time in the construction process. quite the contrary, we must develop a virtual coordinated construction model of the entire envelope, instead of developing the typical details of the façade components needed for the subsequent fabrication process. not only should the time spent in developing a façade engineering project increase, but the owner’s budget for the design phase should also increase accordingly, since, with the use of bim, savings are realised in the construction phase and the management of the asset. 063 journal of facade design & engineering volume 6 / number 2 / 2018 the process of digital collaboration and mobility means moving away from paper toward online, real-time sharing of information to ensure transparency and collaboration within a project. for this to be accomplished, there needs to be a change of mentality in many fields within the construction industry, and this goal is, unfortunately, a remote one today. references asl, m. r., zarrinmehr, s., bergin, m., & yan, w. (2015). bpopt: a framework for bim-based performance optimization. energy and buildings, 108, 401-412. barbosa, f, woetzel, j., mischke, j., ribeirinho, m. j., sridhar, m., parsons, m., & bertram, n. (2017). reinventing construction: a route to higher productivity. mckinsey & company. blanco j. l., mullin a., pandya k., & sridhar m. (july 2017). the new age of engineering and construction technology. mckinsey & company capital projects & infrastructure. chi, h. l., wang, x., & jiao, y. (2015). bim-enabled structural design: impacts and future developments in structural modelling, analysis and optimisation processes. archives of computational methods in engineering, 22(1), 135-151. eastman, c. m., teicholz, p., & sacks, r. (2011). bim handbook: a guide to building information modeling for owners, managers, designers, engineers and contractors. hoboken, new jersey: john wiley & sons. lovel j. (2013). building envelopes: an integrated approach. new york: princeton architectural press. international organization for standardization, 2017. iso 19650-1 & iso 19650-2 organization of information about construction works information management using building information modelling part 1: concepts and principles part 2: delivery phase of assets bimforum (2017). level of development specification. retrieved from http://bimforum.org/lod/ https://www.mckinsey.com/our-people/mukund-sridhar from city’s station to station city 085 journal of facade design & engineering volume 6 / number 2 / 2018 an analysis of the potential of envelope-integrated solar heating and cooling technologies for reducing energy consumption in european climates peru elguezabal, beñat arregi tecnalia sustainable construction division, parque tecnológico de bizkaia, derio spain abstract there is a clear trend towards the increased contribution of renewable energy at european level, and eu policies are oriented in this direction. the building sector is no exception and presents an urgent need for increasing the share of renewable energy sources (res) to reduce the impact on the environment. the aim of this paper is to examine the potential of solar heating and cooling technologies in reducing energy consumption by incorporating solar thermal and pv collectors within the building’s envelope. although generally envisaged as being integrated into the roof, preferably oriented to the south, this study explores their potential for integration into building façades. external climate influences both the demand for space heating and cooling (influenced by temperature) and the potential of solar renewable energy (incident global irradiation). however, a time lag exists since supply and demand peak at different times within the day as well as during the year. this study assesses the interplay of solar energy supply with heating and cooling energy demand. an analysis is performed over climate data files for five european locations, based on daily weather data. besides the extent of incident solar irradiation, its seasonal usability is assessed with regard to the thermal demand. the impact of the inclination of solar collector devices is assessed by comparing their placement on a horizontal plane, on the inclination of maximum exposure for each climate, and on vertical planes for the four cardinal directions. as a conclusion, the utilisation of solar energy in different scenarios is assessed and a discussion on the integration of solar thermal and pv collectors on façades is presented, building on the potential of these technologies for developing innovative solutions that could significantly upgrade the buildings’ energy performance in the near future. keywords façade integrated solar technologies, solar heating, solar cooling, solar collectors doi 10.7480/jfde.2018.2.2102 086 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction the poor state of the building sector in relation to energy consumption, efficiency, and associated impacts such as co 2 emissions is a well-known problem (unep sbci, 2009). the challenge posed by the eu’s ambitious goals for 2020, 2030, and 2050 (european commission, 2017) has stated a clear position looking to correct this course and achieve a significant improvement over the current situation. in such a scenario the necessity for a higher contribution to come from renewables is undisputable. this is clearly stated by the eu in the nearly zero energy building (nzeb) definition (european commission, 2010), in which, in addition to the maximum admissible primary energy consumption in buildings, a minimum contribution from renewable energy sources (res) is demanded. the directive also specifies that this energy must be produced “on-site or nearby”. due to the complex development of the nzeb concept within different member states with varied interpretations of that definition, the commission has also published some recommendations (european commission, 2016), to provide quantifiable references that could help to better explain what is expected. specifically, it distinguishes four major climatic zones in europe (mediterranean, oceanic, continental, and nordic) and sets the minimum renewable contribution in relation to each of these zones, within a range of 28-77% for new single-family houses and 30-70% for the case of office buildings. this represents an effective yearly coverage over the primary energy of 25-50 kwh/ m2 and 30-60 kwh/m2 respectively. the path described for these cases should be the main goal to be achieved by all buildings in any future scenario, and places significant focus on renovation works. this is a critical point as, according to the international energy agency, approximately 60% of the current building stocks will still be in use in 2050 in the european union, united states, and russia (iea, 2013). once the requirement to incorporate res in buildings is accepted, there are various ways to execute these technologies in buildings, from independent devices attached to buildings to solutions that look for a higher integration, taking into consideration that this concept accepts different interpretations. given the complexity of the processes of energy harvesting, storage, and distribution, the method for combining renewable systems and bringing them into buildings is a major challenge. at present, there is a set of systems and technologies that, taking benefit from the energy provided by the sun, can significantly contribute to the reduction of energy consumption in buildings. these systems, which are labelled as renewable, are already commercially available in some cases, while others are still in development. solar thermal collectors, pv collectors, heat pumps over a certain performance, thermal storages, batteries, and management systems are the key components that have been identified as the means to achieve solar heating and solar cooling transformation. on the other hand, there seems to be no single measure that can solve the whole problem, but combinations and systems brought together can provide various solutions under holistic approaches. although existing technologies and solutions do have great potential under optimum conditions, when these are brought into buildings, the impact and effect of different configurations is not fully obvious. solar collectors are very rarely, if ever, displayed in a fully horizontal position. discarding sun-following panels and assuming a fixed position, collectors are commonly oriented to the south (as far as possible) and with a certain slope that seeks to maximise solar gains over the complete year, this angle being a function of the latitude and the sun declination (stanciu & stanciu, 2014). in the search for better architectural integration, specific and unique design solutions for solar collector devices are devised to suit individual buildings, considering the aesthetic benefits of integrating flush mounted panels in the roof, in the façade, or in any other angled plane limited by the constraints of the construction and type of the building. the integration of collectors on building 087 journal of facade design & engineering volume 6 / number 2 / 2018 façades can be especially appealing when installing them as seamless solutions. in addition to this, their placement on vertical planes offers additional benefits in terms of a more stable energy production when oriented to south (munari probst & roecker, 2012). this approach foregoes the orientation and/or tilt that receives the highest overall annual irradiation, getting less energy in absolute terms. nevertheless, a more regular production is obtained throughout the whole year, allowing a better management and distribution of energy and, additionally, avoiding or minimising problems in summer conditions, such as overheating and stagnation for thermal collectors, and efficiency losses in pv panels. in summary, many different possibilities are feasible for the integration of solar panels within the building envelope, but at present there is no clear strategy or guidelines in the bibliography to help in the definition of such integrated solutions and their suitability to specific european climates. the main aim of this study is to address the potential of solar systems incorporated into the building envelope (o’hegarty, kinnane, & mccormack, 2016) for space heating and cooling, taking into consideration different climates and orientations, under a general scope that seeks to balance solar production and demand. as a result of the assessment, some recommendations and criteria for designing solar façades are provided, and a number of currently available technological solutions for the integration of solar collection devices are presented. 2 methodology an analysis of climatic data for five european locations is performed to determine the theoretical potential of solar collection technologies. the selected approach is based on the correlation between solar potential, expressed as global irradiance, and thermal demand, expressed in heating degree days (hdd) and cooling degree days (cdd). given the variety in performance for different solar collection technologies, and the greatly varying insulation levels for different eu regions and construction periods (elguezabal et al., 2018), this method has been selected to represent the generic solar potential of each climate, without restricting it to specific technologies or building types. the adopted methodology is explained through the following steps: 1 the starting point is the data obtained from a statistical weather database (meteonorm 6.0) (meteotest, n.d.), providing the external ambient temperature as well as the incident solar irradiation over a horizontal plane with hourly resolution during a representative year. 2 taking external temperatures as a reference, the necessity for heating is assumed to be activated once the external temperature goes below 15 ºc, and correspondingly, there is cooling requirement once ambient air exceeds 20 ºc. these base temperatures have been selected to allow comparison among different cases; actual base temperatures are dependent upon the specific case considered, influenced by the type of construction, insulation properties, etc. (schoenau & kehrig, 1990). the temperature difference from these base values, integrated over a whole year, will give an indication of the demand for heating and cooling, expressed as (hdd) and (cdd). (1) (2) 088 journal of facade design & engineering volume 6 / number 2 / 2018 3 the next step is to determine the available irradiation to cover these needs. as the database only provides hourly values for global irradiation over horizontal surfaces, the anisotropic sky model (perez, stewart, seals, & guertin, 1998) has been used to estimate the incident radiation over surfaces at different orientations and tilts. 4 a daily resolution has been adopted to uncouple the calculation from the specific storage system chosen. this assumes a thermal storage than can make use of the solar energy received at any moment over a given day, but cannot store the excess energy for longer than that 24-hour period. seasonal energy storage is therefore not considered. 5 the usable solar irradiation has been defined as the irradiation received when a heating or cooling demand exists. taking into consideration the average daily external temperature and comparing it with the thresholds (base temperatures) stated above, the irradiation received for a given day is assigned for heating or cooling. in the same line, the irradiation is considered unusable if a daily demand does not exist. 6 once the heating/cooling demand and the solar production are combined, the possibilities of the solar energy to respond to these demands can be appreciated for each location and orientation. 7 following the above method, the theoretical potential of the solar energy has been determined for different orientations and inclinations. in practice, due to different constraints, the optimal surface may not be always available or provide the sufficient area, and the solar potential might be affected by local conditions such as exposure to wind or shadowing from neighbouring buildings or objects. 8 finally, practical recommendations are provided in terms of possibilities and interest for integrating different systems in building envelopes. 3 assessment of potential for different european climates five locations have been selected as representative situations for a variety of conditions within europe. large cities with some of the highest heating and cooling demands are studied, as well as some more balanced scenarios. the selected cities were stockholm, dublin, budapest, madrid, and athens. for these cities, fig. 1 provides information represented at three different levels: – in the upper row, total global irradiation, expressed monthly, for a set of different planes: horizontal plane (commonly used in irradiation atlases), tilt of maximum incidence depending on the location, and vertical south, west, north, and east planes. – in the middle row, monthly accumulated heating and cooling demand, expressed in hdd and cdd. – in the bottom row, the share of usable overall irradiation that is exploitable for heating and cooling purposes, depending on the adopted plane (horizontal, maximum irradiation, south or west, assuming that west and east are practically equivalent). 089 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 11 monthly global irradiation over different planes: horizontal, maximum incidence depending on the location, and vertical south, west, north, and east (top). monthly accumulated heating and cooling demand (middle). exploitable irradiation for heating and cooling purposes depending on plane (bottom). 4 results and discussion besides the total available irradiation, it is crucial to consider what that energy is destined for, as well as the period when production and consumption are able to be combined. as expected, the predominance of the heating demand in most of the cases is clear, with more intensive demands at northern latitudes such as stockholm, but also in severe continental climates, as for budapest. cooling demand is insignificant for stockholm and dublin, starts to appear at more southern latitudes with a very small requirement for budapest, becomes higher in madrid, and is the predominant demand in athens. the non-usable energy (grey in fig. 1 bottom) relates to the solar irradiation received in those days where the space heating or cooling demand is null. for both stockholm and dublin, this portion is small, as some heating demand exists even during the months with maximum irradiance in summer (middle row in fig. 1). in contrast, for the case of budapest, the demand is quite low for those months, thus the solar energy is not conveniently exploited, making the non-usable proportion more significant. on the other hand, in madrid and athens, the energy harvested during the summer has a strong potential to be used for solar cooling. when considering the usable solar energy in its maximum amplitude, dublin presents an interesting result: while being the city with lowest irradiation (even below stockholm which is at a higher latitude), most of this irradiation is potentially usable for heating purposes as described in fig. 1 bottom. 090 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 12 relationship between usable annual solar irradiation for heating (left) and cooling (right) in relation to the total demand expressed as annual hdd and cdd, respectively. location orientation usable for heating usable for cooling madrid horizontal 77% 99% vertical south 80% 48% vertical east 48% 56% athens horizontal 78% 97% vertical south 83% 49% vertical east 47% 53% budapest horizontal 78% 99% vertical south 79% 55% vertical east 49% 57% stockholm horizontal 76% 95% vertical south 79% 65% vertical east 53% 59% dublin horizontal 83% – vertical south 75% – vertical east 52% – table 1 percentage of solar energy usable for space heating and cooling, for each city and plane orientation, in comparison with the optimal angle for solar collection. fig. 2 and table 1 assess the usability of solar gains for space heating and cooling, depending on the specific climate and orientation. taking as a reference the maximum angle where the total solar collection is maximised, there is always a reduction of the collected energy when a horizontal, south, or west/east plane is adopted, aiming to integrate these solutions within the envelope. looking at the share of expected gains that is exploitable for heating and cooling purposes, some general recommendations can be appreciated and considerations given. for solar heating production, the southern façade is the most interesting orientation as it provides a regular profile throughout the whole year, smoothing the summer profile and reducing peaks in that season when heating is not requested (top row in fig. 1). it represents an interesting option for heating, providing 75% 83% of the energy that would have been collected at the maximum orientation among the cases studied (table 1). it is worth noting that, in all cities assessed except dublin, the irradiation usable for heating in a south-facing plane is higher than for a horizontal plane, which is the commonly used source of information in weather databases. 091 journal of facade design & engineering volume 6 / number 2 / 2018 for solar cooling production, the more interesting angles are those that maximise absolute gains, because the most intensive cooling needs are coupled with the period of highest solar production. even horizontal surfaces can harvest 95% 99% compared to the orientation of maximum gains (table 1). however, if roof surfaces are limited or unusable, especially when retrofitting, west and east façades replicate the distribution of solar gains over the duration of the year (top row in fig. 1), although this results in a significant reduction and meets around 53% 57% of energy demand for those cases with relevant cooling requirements. 5 examples of integrated solar technologies the results presented above have demonstrated that a significant quantity of solar energy reaches the different planes of buildings. this energy, which can be employed to produce heating and cooling, represents an interesting opportunity to integrate solar technologies within buildings. vacuum pipe flat plate unglazed temperature (°c) 100 – 140 50 – 100 25 – 50 power (kw/m²) 1.6 – 2.2 1.5 – 2 1 – 1.2 average production (kwh/ m²a) 480 – 650 400 – 600 300 – 350 average cost (€/m²) 800 370 220 table 2 characteristics of the three main technologies for solar thermal panels. an overview of currently available solar technologies for thermal collectors is presented in table 2, based on research for panels installed in switzerland (munari probst & roecker, 2012). with regard to photovoltaic technologies, the wide range of currently available systems and recent developments have boosted the efficiency of such systems, while the cost is continuously decreasing. in any case, values ranging between 0.15 – 0.3 kw/m² for an annual production of 100 300 kwh/m2 and a cost of 1500 4000 €/m2 are representative of such solutions (irena, 2012; irena, 2017). when combining the available solar energy with the characteristics of each technology, the potential for integrating these into the envelope can be appreciated. considering mainly thermal solutions due to their higher efficiency, the possibilities for solar heating are wide and all the technologies can contribute to reducing the demand for non-renewable energy sources. on the other hand, solar cooling requires high temperature levels achievable only by vacuum tube collectors and some flat plate systems. the exploitation of electricity from pv panels is more straightforward for both heating and cooling, reducing the electricity consumption of energy provision devices such as heat pumps. 092 journal of facade design & engineering volume 6 / number 2 / 2018 location priority production highest potential for solar integration orientations technologies stockholm heating (100%) south vertical unglazed, flat plate, vacuum, pv dublin heating (100%) south vertical unglazed, flat plate, vacuum, pv budapest heating (94%) south vertical unglazed, flat plate, vacuum, pv madrid heating (80%) south vertical unglazed, flat plate, vacuum, pv athens cooling (52%) max. exposure (30º), west, east flat plate, vacuum, pv table 3 assessment of the interest for integrating solar technologies as a function of location and of most beneficial orientation for each case. discarding the horizontal plane and taking into consideration the predominant demand in each climate, table 3 assesses the interest for integrating a number of solar technologies in the specific locations and orientations assessed in this study, considered to be representative of many locations within europe. the choice of systems to integrate is quite varied, except for unglazed panels in locations where cooling is pursued. this limitation is not so important in climates with a predominant demand for heating, where such solutions are very interesting due to their lowest investment required. on the other hand, the consideration of which orientation has the highest interest is critical for an effective integration process and ultimately for achieving a cost-effective payback of the intervention. as exemplar applications of such solutions, flat plate, unglazed, and pv panels provide a high level of integration for opaque areas, while vacuum pipe panels offer interesting possibilities for glazed areas as well as for opaque zones. the external glass of pv and flat plate technologies also allows certain combinations and interactions with glazed areas, although the collectors will not allow natural light to get inside the building. fig. 3 shows flat plate and unglazed panels covering opaque areas, providing good efficiencies for heating in south vertical façades. an application for glazed areas where vacuum pipe panels are integrated is portrayed in fig. 4. for a case such as the one in athens, where cooling and heating loads are quite similar, such panels can be employed in a south orientation for heating and in east or west orientation for cooling production. in addition, these solutions are also applicable for heating generation in other locations and in the south façade, although the effect of solar gains will require a detailed assessment to avoid overheating during summer, probably requiring filtered glasses within the solution. fig. 1 left. glazed flat plate collector integrated into a modular façade system (winkler solar, n.d.), right. prototype of unglazed collector integrated into a metal cladding panel (elguezabal, garay & martin, 2017). 093 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 2 left. vacuum pipe collectors integrated into a modular curtain wall providing a glazed façade. right. detail of the prototype (university of stuttgart, n.d.) 6 conclusions the potential for the exploitation of solar energy is very significant, since it is a renewable resource available in the environment in high quantities. this challenge supposes that, in addition to conventional measures such as increasing insulation levels and equipment efficiency, it is necessary to seek to integrate renewables in buildings through their envelope, turning them into a key element in the transformation of buildings in relation to energy exploitation. it is very important to consider the coupling of production and consumption to improve system performance and avoid inefficiencies in management and distribution. endeavouring to meet the nzeb requirements, buildings will increasingly be required to be more and more self-sufficient, while their connection to the main supply networks should decrease until they eventually become negligible. this will imply the introduction of solar collecting devices comprising a significant surface area. the available surface in the optimal orientations varies depending on the demand that is expected to be covered, but limitations are common especially when retrofitting is pursued. in such situations, alternatives to the optimal orientations need to be considered. the present study has investigated different scenarios regarding the integration of solar collecting devices into the building envelope and their potential to be exploited for space heating and cooling, resulting in some interesting key findings. it is important to consider the dominant demand for each climate since this has a significant impact on the maximum usable energy. in many cases, the placement of solar collectors in vertical façades instead of on the roof could provide benefits, especially for solar heating, as collection and demand achieve a better coupling on south-facing orientations. to cover space heating demand with solar energy, the south vertical orientation is very promising in all five cities studied. although some energy is not used, the benefits of getting a regular production profile could represent a general improvement compared to the maximum angle orientation. as an additional reason to consider vertical façades for the incorporation of energy harvesting systems, the available surface of the façades is usually greater than that of the roof, and increases more for slender and block buildings. if the cooling demand is greater than the heating demand, or in a similar range of magnitude, the advantage of the south orientation is not so obvious, since the east and west cases can maximise cooling production in summer, but their production goes below the south case in winter. for such applications, more detailed studies would be needed, making comparative designs between different orientations and combinations and taking into account the cost and return on investment associated with energy savings. 094 journal of facade design & engineering volume 6 / number 2 / 2018 finally, some currently available and recently developed thermal and pv solutions have been shown as examples of the applicability for integrating solar technologies. taking into consideration their performance, efficiencies, and costs, as well as the main interest for placing them in specific envelope orientations, the potential of these solutions has been highlighted, providing general recommendations about the convenience of their integration, aiming to contribute to a higher penetration of res within buildings. future research could build on the findings from these studies, by considering the impact of the efficiency curves for different solar technologies, on the one hand, and the influence of building shape and insulation levels on the energy demand, on the other hand. this would allow a direct comparison of on-site energy production and consumption, as well as their distribution over time and the usage of common metrics (cao, hasan, & sirén, 2013) for assessing the correlation between supply and demand. references cao, s. hasan, & a., sirén, k. (2013). on-site energy matching indices for buildings with energy conversion, storage and hybrid grid connections. energy & buildings, vol. 64., pp. 423-438. european commission (2016). commission recommendation (eu) 2016/1318 of 29 july 2016 on guidelines for the promotion of nearly zero-energy buildings and best practices to ensure that, by 2020, all new buildings are nearly zero-energy buildings european commission (2010). directive 2010/31/eu of the european parliament and of the council of 19 may 2010 on the energy performance of buildings (recast) elguezabal, p. arregi, b., schuetz, p., gwerder, d., scoccia, r., tsatsakis, k., biosca, j., bortkiewicz, a., waser, r. & sturzenegger, d. (2018). review of the european dwelling stock and its potential for retrofit interventions using solar-assisted heating and cooling. conference proceedings. in rehabend 2018, cáceres, spain. 15-17 may 2018. elguezabal, p., garay, r. & martin, k. (2017). experimentation under real performing conditions of a highly integrable unglazed solar collector into a building façade. energy procedia. vol. 122 (2017), pp. 775-780. european commission (2017). energy strategy and energy union. secure, competitive, and sustainable energy. retrieved from https://ec.europa.eu/energy/en/topics/energy-strategy-and-energy-union. iea international energy agency. (2013).transition to sustainable buildings. strategies and opportunities to 2050. irena international renewable energy agency. (2012).. renewable energy technologies. cost analysis series. volume 1: power sector issue 4/5. solar photovoltaics irena international renewable energy agency. (2017). cost and competitiveness indicators. rooftop solar pv meteonorm weather database. meteotest. retrieved from http://www.meteonorm.com [accessed 15/02/2018] munari probst, m.c. & roecker, c. (eds.). (2012). solar energy systems in architecture – integration criteria and guidelines. iea shc task 41, subtask a o’hegarty, r., kinnane, o. & mccormack, s. j. (2016). review and analysis of solar thermal facades. solar energy. vol. 135 (2016), pp. 408-422. perez, r., stewart, r., seals, r., & guertin t. (1998). the development and verification of the perez diffuse radiation model. sandia report sand88-7030 schoenau, g. & kehrig, r. (1990). method for calculating degree-days to any base temperature. energy & buildings. vol. 14, issue 4 (1990), pp. 299–302. stanciu, c. & stanciu, d. (2014). optimum tilt angle for flat plate collectors all over the world – a declination dependence formula and comparisons of three solar radiation models. energy conversion and management. vol. 81 (2014), pp. 133-143. unep sbci, united nation’s environment programme’s sustainable building and climate initiative (2009). buildings and climate change. summary for decision – makers. retrieved from http://staging.unep.org/sbci/pdfs/sbci-bccsummary.pdf university of stuttgart (2018). ibk2. retrieved from http://www.uni-stuttgart.de/ibk2/index.php winkler solar (2018). winkler solar. retrieved from http://www.winklersolar.com http://www.meteonorm.com from city’s station to station city 001 journal of facade design & engineering volume 6 / number 2 / 2018 development of a modular end effector for the installation of curtain walls with cable-robots meysam taghavi1, homero heredia2, kepa iturralde1, håvard halvorsen2, thomas bock1 1 chair of building realization and robotics, technical university of munich, germany 2 nlink as, norway abstract the installation of façade enclosures is a manual, dangerous, and time-consuming construction task. however, thanks to the capability of automated systems, the application of automation in construction is increasing, and therefore, manual work and risky situations can be avoided. despite this, only a few robotic systems are capable of spanning such a vast work space, i.e. the façade of a building. among these systems is the cable driven parallel robot (cdpr). furthermore, the cdpr could carry heavy loads such as unitised curtain wall modules (cwm). nevertheless, the tools and devices required for installing the cwm need to be innovated. firstly, in order to cover that research gap, the current manual procedure was analysed in detail. after that, the development team evaluated several options for performing the tasks. finally, an optimal solution was chosen: the so-called modular end-effector (mee). the mee comprises several tools in order to achieve various tasks. mainly, these tasks are: drilling the concrete slab, bracket installation, and cwm handling and positioning. in addition to the aforementioned tasks, the mee should accurately fix all elements with a desired tolerance less than 1 mm. meanwhile, the mee should compensate for the perturbation movement due to external forces such as wind that affect the system. as part of the study, a detailed workflow for the automated installation of cwms was elaborated. the drilling step of the workflow was tested and the result is presented in this paper. keywords modular end effector, cable robot, construction robotics doi 10.7480/jfde.2018.2.2067 002 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 1 left: bracket and cwm level adjustment. two possible directions of bracket adjustment (x & y) and one direction of cwm adjustment (z). right: conventional installation of a cwm, possible dangers for labourers (e.g. falling) and the number of workers involved in the installation process. (images by focchi spa-2017) 1 introduction the existing conventional cwm installation is a manual procedure that is based on two major steps: bracket fixation and installation of the cwm. installing a cwm on a building requires the prior installation of an interface, or bracket. as of now, these brackets get anchored to the edge of the concrete slabs, thanks to cast-in channels. apart from hosting and holding the cwm load, these elements are designed to cope with deviations between theoretical and actual construction geometries. the groove in these channels, in combination with the slotted cuts in the bracket plate, allow the manual adjustment of the location and orientation of the bracket itself. see fig. 1 (left). however, when considering their use by automated systems, it not only becomes cumbersome, but redundant, as automated systems can eliminate or reduce the variation between actual and theoretical states in previous cwm installation stages. this can be achieved by using another correct, but less popular, method for cwm interface installation: slab drilling and setting of expansion anchors. this method allows for the preparation of the structure and work in the correct location right before the installation of a cwm without the need for further adjustments. aside from the ease of automation, a relevant advantage of drilling (over cast-in channels) is the reduction of complexity during the design, planning, and concrete casting of slabs. furthermore, using expansion anchors allows the installation of cwm on new construction sites as well as on existing ones that require renovation work. once all brackets are installed, and adjusted in their locations with a tolerance of 1 mm, the cwms are transported to the desired building level with aid of a crane or elevator and, finally, are manually located, mounted, and fixed in the planned position. fig. 1 (right) shows the cwm being lifted up using a crane, and several workers, who are working in a dangerous situation. the amount of labour work, danger, and required time is considerable. workers could use the adjustment screw to level the cwm in the z-direction (fig. 1 (left)). automating the cwm lift and installation will increase the safety of labour. in foresight, the higher installation speed could be reached, while maintaining the required accuracy. 003 journal of facade design & engineering volume 6 / number 2 / 2018 1.1 previous robotic experiences for installing elements and modules in construction the automation level of different industries has been increased notably in the last decades. although the construction sector has been demanding the use of automation and robotics in recent times (bock t. , 2015), the presence of proper automatic systems is rare (hastak, 1998) (vähä, heikkil, kilpelinen, jrviluoma, & gambao, 2013). as an example of robotics working in the construction industry, a prototype of a moving platform hosting a modular end effector for an overhead (ceiling) nailing system was developed by the company lindner and the technical university of munich (bock & linner, 2016). another commercial robot for drilling the ceiling was produced by the company nlink in norway (nlink, 2018). the corresponding required features of the robot, such as flexibility, heavy duty, reliability, and big workspace should be deeply considered when developing automation systems in the construction sector (moreira, et al., 2015). for instance, the installation of the automatic curtain wall module (cwm) requires a large work space (the façade of the building), heavy payload (the cwm weight is about 300 kg), and it happens in an outdoor environment. several automatic systems for cwm installation have already been proposed. for instance, the patented technology from brunkeberg systems ab (usa patent no. us8695308, 2014) is in use. it brings the railing system into action just for their dedicated cwm. alternatively, a mobile robot that installs the cwm from inside the building is used by yu et al. (yu, lee, han, lee, & lee, 2007). this method automates only the cwm installation and not the bracket installation. a telescopic, tele-operated hydraulic system was designed and tested to nail the sandwich panels from outside the building by (cinkelj, kamnik, cepon, mihelj, & munih, 2010). the system is also designed to work only with specialised panels. as previously mentioned, these systems have their own limitations. the cdpr provides the big workspace and high payloads. this makes the cdpr a proper solution for construction automation and especially for cwm installation. therefore, within the call ict-252016-2017 (eu, 2017) from the european union, the hephaestus project (hephaestus, 2018) was founded. the project, as well as the subject of this paper, focuses on cwm installation using cable driven parallel robot (cdpr). 1.2 sub-systems and tasks for cwm installation to automate the cwm installation, the system in the hephaestus project is divided into three subsystems: 1) cdpr, 2) mee, and 3) controlling system. this paper addresses the challenges of the mee design in detail. the requirements for the mee were carefully analysed and considered as: – drilling the concrete – installing the bracket (the interface between the cwm and building) – positioning and mounting the cwm on the installed brackets moreover, the mee should be able to work outdoors and provide required accuracy for the system. it means that the system should be stable in case of environmental loads and vibration caused by, for example, wind, and that the bracket should be installed with a tolerance of 1 mm. 004 journal of facade design & engineering volume 6 / number 2 / 2018 2 development of the modular end effector the end-effectors are tools that are attached to the robotic manipulator, and interact with the environment. in the case of the hephaestus project, this end-effector operates within the platform of the cable robot and it is modular, meaning it contains several devices including the end-effectors. in this project the main task is to install a curtain wall module (cwm). within the task, there are four main requirements. first, the mee should perform tasks such as drilling, fixing a bracket and handling the cwm. second, the mee should fulfil the tasks accurately. the issue is that the cable robot does not provide the required accuracy. therefore, the mee must re-adjust to the location. a secondary robotic device will be necessary for the completion of this subtask. the third requirement is that the mee needs to be stable while performing these tasks and the outdoor hazards, including their forces, need to be neutralised. a fourth and final requirement is that the cwm module needs to be lifted and fixed into its correct position. for each of the requirements of the mee, several possible options were considered. these options were analysed carefully and evaluated. in the selection method, careful considerations occurred for the following indicators: – the preferred option should fulfil the requirements of the system completely with smallest risk of failure. – it should be safe. – its design and manufacturing should be possible within the available resources of the hephaestus project. once the analysis was carried out, an optimal solution for each of the requirements was selected. the evaluation process and the selected solutions are explained in the next sub-chapters. 2.1 secondary robotic system the cable robot is expected to have an accuracy of about 40 mm, thus a secondary robotic system is required to achieve the 1 mm positioning need for the different tasks, from the structural preparation of the building to the installation of necessary mounting hardware. that is, the cable robot will achieve a ‘macro’ positioning throughout the building façade plane, and the secondary robotic system will do the ‘micro’ positioning of tools and components at the required locations. once the coordinates for the cwm and the anchors’ design locations are known, it is possible to locate the cable robot frame in a nearby position, where the anchor points are accessible to the tooltip of the robotic arm. a thorough consideration of options for this secondary system included: cartesian system, pyramid based movement, hexapod, and finally, a serial arm robot. to evaluate these options, sub-tasks and useful end-effector tools were analysed for completing the tasks. for example, a drill with a rotary hammer was considered. given the tasks, tool availability, and project resources, it was defined that the most appropriate option for a secondary system is a 10 kg payload serial robotic arm. this system will then manipulate off-the-shelf tools that are adapted and mounted at the robot head via pneumatic or electric tool changers, which allow for fast and automated swapping throughout the cwm installation process. universal robots ur-10 is tentatively the system to be integrated. 005 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 2 robotised bracket installation process. left: rebar scanning, rebar scanner in red colour is handled by serial arm robot and scan a specific area to check if there exist a rebar or not. centre: drilling, the driller in blue colour is handled by serial arm robot to do the drilling. right: bracket adjustment and installation, a gripping tool is handled by serial arm robot. 2.2 stabilising the mee it is considered that the hephaestus robot works under wind conditions with a maximum speed of 15 m/s; this point initially brings the stiffness characteristic of the cable robot into consideration. the stiffness of the cable robot could be translated as the relation of the applied external force and the resulting movement. it is a parameter that can be considered during design of the cable robot. however, there are some limitations, since higher stiffness needs higher cable tension, while the other factors remain constant. for higher tension in the cables, there are cost and technical constraints. the stiffness of the cable robot influences the accuracy of the performance of the mee, since external loads imposed on the cable robot and its platform (e.g. wind) will cause the cable robot to move, which consequently means the movement of the mee. the mee is the final performer of the task on the building and it should be stable during the performance. a driller is an example of a final tool performing the task on the building, which should not move due to external loads while drilling. for filling the gap between the stability of mee and the stiffness of the cable robot, the stabilising system was designed in this project. in the system, the mee will grip the building in order to be stabilised during the performance of the task. 2.3 performing tasks on the building in order to mount cwms, the first requirement is to install the cwm mounting bracket on the edge of the slabs at each level of the building. the sequence for this interface installation can be defined by three main phases: structural preparation (steps 1 and 2), bracket installation (steps 3-5), and quality assurance (step 6), which consists of 6 steps in total: 1. rebar scanning: to avoid structural damage or potential drill bit jamming, it is imperative to scan the slab surface to detect embedded reinforcement bars. this task will be performed by adapting and manipulating an off-the-shelf manual rebar scanner. fig. 2 (left) shows the concept of the automated method of rebar scanning in this project. 2. drilling: following the identification of optimal drilling locations, the actual drilling takes place using a rotary hammer tool. this operation needs to be followed by, or include, an integrated dust management system to leave the hole ready for further steps. fig. 2 (centre) shows the concept of automated drilling in this project. 006 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 3 cwm handling. left: cwm crates [picture rights: courtesy of focchi spa]. centre: picking up the cwm vertically with no rotation, green is the reference bed and blue is the curtain wall module. yellow is the cable robot frame. right: picking up the cwm horizontally, two degree of rotation in roll and pitch direction is implemented. 3. expansion anchor setting: setting an anchor requires a gripping mechanism to manipulate the anchor from a magazine onto the drilled hole. once the location is locked, a hammering device (e.g. rotary hammer with hammering-only function) inserts the element down into the hole. this operation is done for two anchors: the first anchor with the bracket plate (see next step), and the second anchor alone to secure bracket plate rotation about the z-axis. 4. bracket plate setting: the bracket plate, which will create the interface between the building and the cwm, will be gripped, manipulated, and positioned on the slab at the correct location. here, it will be locked by the 1st installed expansion anchor (from previous step). fig. 2 (right) shows the robotic procedure of the bracket adjustment and installation 5. nut tightening: to fully secure the interface hardware, the nuts from both previous anchors must be tightened to the right torque. for this operation an impact driver is used. 6. installation quality: prior to installing the cwm using the installed bracket, the correct installation of components needs to be verified. for this a 3d camera is utilised to remotely verify all components are in place and ready to take the panel load. the six steps above describe the installation of bracket plates and required hardware. the combination of a robotic arm manipulator, tool changer, and off-the-shelf components conveniently allows the change or addition of tools. this feature benefits the utilisation of the same configuration for other applications such as façade washing or painting. e.g. mounting a spray paint nozzle onto the robot arm and moving it across the building exterior walls for painting complex surfaces. 2.4 handling the cwm the transportation and arrival of the cwm is achieved using racks, with which the cwms are transported horizontally to the construction site, as shown in fig. 3 (left). for the cdpr, the necessary manoeuvres could be minimised if the cwms were transported vertically (fig. 3 (centre)), but that would decrease the optimisation of the transportation of the modules. at this point, it was decided that the cdpr needs to pick up the cwms from the horizontally stacked position, and must then mount them in a vertical position parallel to the building façade. for solving this issue, a twodirectional rotary handling system with suction cups has been preconceived as a preliminary solution (fig. 3 (right)). 007 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 4 drilling test. left: serial arm robot ur10 mounted hosting a driller with integrated dust suctioning system. centre: drilling with a 16 mm drill bit into sample concrete. right: showing the quality of a drilled hole. 2.5 initial proof of concept in order to initiate the validation of the developed mee, the secondary robotic system, the process, and the tools to be used, an initial test experiment was set up. a rotary hammer with a 16 mm diameter drill-bit installed was mounted on the serial arm ur-10 and driven into a concrete box that simulated the structure found in the slabs of a building. in parallel, the tool was equipped with a vacuum cleaner hose to remove dust during the drilling process. the test was successful and the resulting hole proved to be ready and useful for the installation of an expansion anchor, by following the procedure in the workflow defined in chapter 2.3 of this paper. fig. 4 shows the process and result. 3 conclusion current practices for the installation of curtain wall modules imply several adversities, including the safety of workers. in order to achieve a more automated process, ongoing research based on an adaptation of a cable driven parallel robot will focus on this topic. this paper describes the elaboration process of a novel robotic modular end-effector or tool system that, in the near future, should perform the tasks for installing a curtain wall. first, several options were gathered and evaluated. then a final solution was selected based on achievability criteria. the chosen modular end-effector integrates and makes use of market-ready solutions that can sufficiently perform the set of tasks involved in the installation of cwm. this development approach enabled the accessibility to a high feasibility-rate scenario, where it is possible to validate the concept in small and progressive steps without the need to develop completely new concepts and technology. validating the system solution as a whole will enable further refinement of the solution itself. it will also enable continual abundance of automation solutions to the construction industry, which can address its most recurrent issues, such as inefficiency and safety. for progressive movement towards the full construction and use of the cwm installation system, the different sub-tasks and tools will be tested individually in a similar way to the experiment described in chapter 2.5. some of these experiments will comprise mounting tools on the serial arm ur-10 to prove the effectiveness of the workflow defined in chapter 2.3. in order to refine the design, and validate the function of the bigger mee elements (e.g. building gripping damping system), digital simulations and physical experiments will follow. finally, after designing and validating the mee, it will be manufactured and the system will be tested on the real building together with cable robot. 008 journal of facade design & engineering volume 6 / number 2 / 2018 acknowledgements the research leading to these results has received funding from the european union's h2020 programme (h2020/2014-2020) under grant agreement n° 732513. references bock, t. (2015). construction robotics enabling innovative disruption and social supportability. international symposium on automation and robotics in construction (isarc), 32, pp. 1-11. finland. bock, t., & linner, t. (2016). construction robots, elementary technologies and single-task construction robots, cambridge handbooks on construction robotics. new york: cambridge university press. cinkelj, j., kamnik, r., cepon, p., mihelj, m., & munih, m. (2010). closed-loop control of hydraulic telescopic handler. journal of automation in construction, 19(07), 954-963. eu. (2017, 01 12). eu call, ict-25-2016-2017. retrieved from advanced robot capabilities research and take-up: http://cordis. europa.eu/programme/rcn/700616_en.html falk, j. h., augustinson, d. f., & brunkeberg systems ab. (2014). usa patent no. us8695308. gray, c., & hughes, w. (2001). building design management. butterworth: oxford. h., y. s. (2007). development of the curtain wall installation robot: performance and efficiency tests at a construction site. autonomous robots, 281--291. hastak, m. (1998). advanced automation or conventional construction process. journal of automation in construction, 7(4), 299-314. hephaestus. (2018, 01 13). retrieved from about the project: http://www.hephaestus-project.eu/ moreira, e., pinto, a. m., costa, p. j., moreira, a. p., veiga, g., lima, j., . . . costa, p. g. (2015). cable robot for non-standard architecture and construction: a dynamic positioning system. international conference on industrial technology (icit) (pp. 3184-3189). seville: ieee. nlink. (2018, 01 19). retrieved from the ceiling drilling robot: www.nlink.no vähä, p., heikkil, t., kilpelinen, p., jrviluoma, m., & gambao, e. (2013). extending automation of building construction survey on potential sensor technologies and robotic applications. journal of automation in construction, 36(0), 168-178. yu, s. n., lee, s. y., han, c. s., lee, k. y., & lee, s. h. (2007). development of the curtain wall installation robot: performance and efficiency tests at a construction site. journal of autonomous robots, 22(3), 281-291. from city’s station to station city 107 journal of facade design & engineering volume 6 / number 2 / 2018 exploring the potential of smart and multifunctional materials in adaptive opaque façade systems miren juaristi1, aurora monge-barrio1, ana sánchez-ostiz1, tomás gómez-acebo2 1 universidad de navarra, school of architecture 2 universidad de navarrra, tecnun, school of engineers abstract climate adaptive façades are considered promising breakthroughs for the reduction of energy consumption, as energy exchange is enabled when the weather conditions offer benefits instead of threats. so far, conventional building envelopes enhance thermal performance through opaque façade components and static insulations. therefore, natural resources from the building environment remain untapped. little research has been done in adaptive opaque façades, even if their dynamic behaviour shows a strong potential to exploit environmental resources. for the successful development of these innovative façade systems, a balance between sophistication and benefit is necessary. to manage this objective, the implementation of smart and multifunctional materials in the envelopes seems promising, as they are able to repeatedly and reversibly change some of its functions, features, or behaviour over time in response to environmental condition. consequently, to trigger the response of the envelope, no external actuator or complex software management would be necessary. nevertheless, these materials do not fulfil all of the façade requirements by themselves. thus, they need to be combined with other adaptive technologies and building elements. this paper shows an initial definition of different façade configurations that include reactive materials, which enable the adaptiveness of opaque façade systems. the desired results are new façade roles suitable for a temperate climate, according to the potential of these multi-performance materials in the external layer of the envelope: the dynamic temperature change of the external cladding through the solar reflectance change and the enhancement or prevention of thermal losses through shape-changing ventilated façades. to achieve these new high performances, an ideal approach to the thermal behaviour of each façade layer was taken, and the required physical properties of each element was highlighted. as a result, we propose a mapping of a potentially suitable combination of reactive materials with other building elements that might enable holistic adaptive thermal performance. keywords climate response, environmental resources, temperate climate, thermal performance, adaptive technologies, innovative systems doi 10.7480/jfde.2018.2.2216 108 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction traditionally, the main objective when designing building envelopes was to balance the optimal performance for average climatic situations with their reasonable behaviour under adverse conditions. most of the time, this meant that the envelope was not optimised from the point of view of construction design and performance. however, nowadays, adaptive or smart technologies can provide suitable and more efficient outputs under diverse climatic conditions. smart and multifunctional materials belong to these kinds of technologies, as they are engineered materials that respond automatically and reversibly to environmental stimuli. some smart materials (sms) can change one or more of their properties as direct responses, whilst other sms transform one energy form into another (addington & schodek, 2005). multifunctional materials (mm), also known as information materials, are less sophisticated raw materials whose “multi-properties” were designed by manipulating their structure by computational techniques (kretzer, 2017). these advanced materials are applied in different fields, such as robotics or biomedicine, and some researchers have already pointed out their potential to develop successfully climate adaptive building envelopes (loonen, trčka, cóstola, & hensen, 2013). even if there are some conceptual proposals for their application in the façade industry (badarnah & knaack, 2005; lelieveld, 2013), they are still at an early research stage. this could be due to their technical limitations, as will be discussed in section 3.2, but may also be due to the fact that their application and potential performances haven’t yet been proposed in a defined, holistic way. to address the above-mentioned challenges, we propose two new roles of opaque adaptive façades and we follow a holistic first-stage design strategy, based on a literature review about reactive materials and responsive building elements. the objective of the proposed adaptive opaque façade systems is to enhance their thermal performance due to the dynamic response of smart and multifunctional materials, even when they are combined with other façade components. these roles are specially proposed for temperate climates with small day-night oscillations, characterised by their diverse climatic conditions changing in short time periods, as well as by their rich environmental resources that could contribute in the reduction of energy demand. 2 new roles of the opaque façades: outlining the ideal thermal performance 2.1 the potential of exterior claddings which adapt their solar reflectance vernacular architecture makes use of different colour coatings in façades depending on the thermal behaviour that is required for each location. however, when we need to define the finishing for a temperate climate, a conflict exists and we must make a choice that won’t always be ideal, especially if the temperate climate doesn’t have any dominant climatic condition. in these climates, when ambient temperatures are below the comfort temperature range, the transmission of solar heat gains through façades are effective in the reduction of energy demand. high absorptance and emittance materials, which are usually dark in colour, are the perfect choice to enhance these gains (table 1). meanwhile, if the exterior temperatures are above the comfort temperatures, the use of cladding materials with low absorptance and emittance (clear or light coloured materials) is advisable to prevent overheating (ibañez-puy, vidaurre-arbizu, sacristán-fernández, & martín-gómez, 2017; sánchez-ostiz gutiérrez, 2011). therefore, the ideal material is an adaptive one that could change its reflection coefficient. smart materials called thermochromics have the ability to reversibly change 109 journal of facade design & engineering volume 6 / number 2 / 2018 these properties upon reaching a specific temperature. in fact, the experiments undertaken by ma and zhu (2009) and karlessi, santamouris, apostolakis, synnefa, and livada (2008) reveal that the reflectance of these materials increases towards long wavelengths, which provokes a reduction in the temperature of the coating. furthermore, exterior cladding material also influences the intensity of thermal flux depending on their thermal diffusivity and conductivity. in this sense, we couldn´t find any multifunctional materials that could dynamically adapt their diffusivity and conductivity, so the heat transferring to the inner façade layer will depend on the “static” material that we choose, as shown in table 1. thermal performance physical properties a) enhance solar gain b) thermal dissipation technology heat gain absorptance high low thermochromic finish reflectance low high heat transfer to the inner façade layers emittance high low thermochromic finish thermal diffusivity high low a) metallic cladding b) ceramic, stony cladding thermal conductivity high low a) metallic cladding b) ceramic, stony cladding table 1 relevant physical properties of exterior cladding materials to obtain adaptive thermal control. thermal performance physical properties a) enhance solar gain b) thermal dissipation technology thermal conservation thermal conductivity high low adaptive air cavities dynamic insulations adaptive insulationsthermal diffusivity high low thermal storage time-lag low high a) lightweight a) heavyweight thermal diffusivity high low a) air, metal b) ceramic, stony material sensible heat content high low a) stone, concrete, ceramic, clay, water b) air, light wood, thermal insulation material latent heat content high low pcm heat transfer to the interior space thermal diffusivity high low a) metallic cladding b) ceramic, stony cladding thermal conductivity high low a) metallic cladding b) light woods, synthetic cladding table 2 relevant physical properties of interior façade materials to obtain adaptive thermal behaviour. possible adaptive control of solar thermal radiation through the cladding needs to be properly understood in terms of the overall thermal flux of the system. for instance, it would be useless to foster solar heat gains under certain climatic conditions if the internal layers of the façade component were blocking that thermal flux. therefore, to meet all the environmental boundary conditions, they must also include a dynamic behaviour to afford suitable heat transfer, thermal conservation and storage. table 2 summarises the determining physical properties in each stage of the thermal flux, and promising technologies and building materials that could provide the target performance, are detected. 110 journal of facade design & engineering volume 6 / number 2 / 2018 2.2 shape-changing claddings to seek high performing ventilated façades opaque ventilated façades enhance thermal performance under mild and warm climatic conditions. these façade systems consist of two opaque layers with an air cavity between. the convective movements in the cavity cause heat dissipation and decrease the surface temperature of the second opaque layer (ibañez-puy et al., 2017; sánchez-ostiz gutiérrez, 2011). nonetheless, in cold periods this behaviour is detrimental as it increases thermal losses, especially if the convective movements reach high velocities. ibañez-puy et al. point out that the appropriated outer skin, as well as joints configuration, change depending whether conditions are hot or cold and windy (ibañez-puy et al., 2017). transferring that to a temperate climate, where sequences of threatening climatic scenarios coexist with mild scenarios (even in the same season), it is impossible to choose an optimal, static solution. adaptive configuration of the outer skin seems like a promising solution to meeting all climatic scenarios, and smart and multifunctional materials may play a role. there are two material families that look favourable: shape memory alloys (sm) and thermobimetals (mm). the former, such as ni-ti alloys, are capable of returning to their original shape from a deformed state, when a certain temperature is reached. thermobimetals, like ni-fe alloys, are two different sheets of differing metal alloys that, laminated together, expand at different rates within a few seconds, causing the bending of the piece (fig. 1). when the heat source is gone, they can return to their original shape (lópez, rubio, martín, croxford, & jackson, 2015). fig. 1 thermobimetals bend when they are exposed to the operational temperature, as they are composed of two different metal alloys that expand at different rates. however, shape changes in the exterior cladding, which aim to control convective heat transfer, would only be effective if physical events are analysed in a holistic way. firstly, we need to understand that convective movements inside the cavity can happen because of two phenomena: a temperature gradient due to solar gains or wind pressure. both phenomena can enhance thermal dissipation if physical parameters are considered in the design. if the exterior cladding controls thermal dissipation triggered by solar radiation, then cladding material is a critical factor. for instance, materials with high absorptance and emittance would be suitable to boost convective insulation. otherwise, when playing with wind action to promote thermal dissipation, material choice has little relevance (table 3). 111 journal of facade design & engineering volume 6 / number 2 / 2018 thermal performance physical properties thermal dissipation convective insulation promising responsive materialsolar gain wind action heat gain absorptance low little relevance high thermochromic finishreflectance high little relevance low heat transfer emittance low little relevance high thermochromic finish thermal diffusivity * little relevance low * thermal conductivity * little relevance low * *further numerical and experimental assessments are needed to discover required physical property table 3 physical property requirements of exterior cladding materials to shift from thermal dissipation to convective insulation in addition, the morphology and dimensions of the air cavity are important for thermal performance (table 4). ventilated façades with significant height and low roughness, will more easily prevent overheating. when this dissipation is due to solar heat gains, it is more appropriate to have a ventilated cavity and a cladding element with closed joints, as the stack effect is boosted, whereas to prevent overheating by making use of the wind, it is better that joints are open (ibañez-puy et al., 2017). on the other hand, if we want to avoid the thermal losses that wind would create, the air cavity should be 1-5cm thick, unventilated, and with the lowest height possible (sánchezostiz gutiérrez, 2011). thermal performance ideal morphology thermal dissipation convective insulation promising responsive material solar gain wind action heat transfer opening degree (ibañez-puy et al., 2017) ventilated, closed-joint ventilated, open-joint no ventilated sma thermobimetals thickness 7-35cm(balocco, 2002) * 1-5cm(sánchez-ostiz gutiérrez, 2011) roughness low low high height higher, better higher, better higher, better * further numerical and experimental assessments are needed to discover required physical property table 4 morphological requirements of the air cavity to shift from thermal dissipation to convective insulation thermal dissipation by both wind and solar action broadens the scenarios in which the façade would perform in an optimal way. it must be considered that dissipation by solar heat gains becomes less effective as wind velocity increases, and is negligible over 2.5m/s. besides, convective thermal dissipation overnight can occur in windy conditions, or in situations in which there is not enough solar radiation (ibañez-puy et al., 2017). 112 journal of facade design & engineering volume 6 / number 2 / 2018 3 possible implementations of sm & mm in adaptive opaque façades: a proposition of promising façade configurations and their challenges 3.1 suitable combination of sm or mm with other façade technologies adaptive solar control façades result from the adequate combination of thermochromic-coated claddings with other façade components. it can be concluded from the information shown in table 1 that when the façade aims to gain thermal energy from solar radiation, heat transfer needs to be as fast and effective as possible in the cladding, in such a way that it can be stored in the internal layer or transferred to the interior environment (fig. 2). heat gained in the outer skin is transferred to the internal layers, where it should be stored by thermal mass, sensible heat, or latent heat. finally, it is transferred to the interior environment through convection and radiation. when solar gains are detrimental to thermal comfort, the radiant heat that can’t be reflected by the exterior cladding is dissipated by convective movements (if there is an air cavity placed just behind the outer skin). this, coupled with thermal conservation layers, minimises thermal gains by conduction in the indoor environment. fig. 2 possible combination of thermochromics with other technologies. 113 journal of facade design & engineering volume 6 / number 2 / 2018 the thermal behaviour shown in fig. 2 might be achieved through a different configuration of thermochromics with other building elements. this can be illustrated briefly by the cladding material, which modifies the intensity of the thermal flux depending on the material type that is chosen. when applying a thermochromic finish in a metallic cladding, thermal flux is more intense than if ceramic, stony, or synthetic materials were chosen. regarding thermal conservation, as energy exchange is profitable at certain climatic conditions, it would be inadequate to use regular insulation materials. indeed, an adaptive conservation layer should be placed in the internal layers. a possible solution is to place an adaptive air cavity behind the external cladding, which could include an autoreactive damper (made by sm/mm or an electro-mechanical actuator). to open this damper, heat would be transferred to the intermediate layer by introducing pre-heated air, and a second air cavity would, by necessity, be in contact with the storage façade element (fig. 3). fig. 3 possible configuration combining thermochromic finish applied in exterior cladding and adaptive air cavities to achieve adaptive thermal performance in an opaque façade system. another option is to replace the traditional insulation material with a dynamic insulation element, which changes its behaviour to enhance or block thermal conduction (fig. 4), or adaptive insulations (favoino, jin, & overend, 2017), which adapt their features to change their thermal performance (fig. 5). finally, thermal storage could be achieved by combining the aforementioned elements with an internal layer of high sensible heat content, such as concrete, ceramic, or water, or high latent heat, such as phase change materials (fig. 2). 114 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 4 possible configuration combining a thermochromic finish applied in exterior cladding and a dynamic insulation element to achieve adaptive thermal performance in an opaque façade system. fig. 5 possible configuration combining thermochromic finish applied in exterior cladding and adaptive insulation element to achieve adaptive thermal performance in an opaque façade system. with respect to shape-changing ventilated façades, morphology variations of the outer cladding must be set properly within the system to enhance or reversibly block thermal flux. in order to meet all the possible requirements in each climatic condition, three possible morphological configurations are proposed for a ventilated façade system. the first shape configuration provides a ventilated façade with a closed air cavity, as the external cladding geometry has a closed-joint arrangement and furthermore, the upper and lower dampers of the cavities are closed. the second configuration enables heat dissipation by solar radiation, as the outer skin has a closed-joint geometry but the dampers of the cavities are opened, which enhances the stack effect due to the temperature gradient. the third and final configuration makes thermal dissipation possible by wind action, as both cavity and cladding joints are opened. 115 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 6 possible configurations of a shape-changing ventilated façade using sm or mm as actuators embedded in the cladding and in the dampers. different morphologies would allow the control of thermal losses due to convective movements. configuration (a) would allow a convective insulation, (b) would enable thermal dissipation triggered by solar thermal radiation, and (c) would prevent overheating in the interior space, enhancing thermal losses by wind action. sm and mm would be the actuators of the kinetic behaviour and they would react automatically and reversibly to a set operational temperature. this would prevent or allow ventilation of the air cavity. besides, the opening degree of the outer skin would be controlled by opening or closing the joints of the cladding and/or by partitioning the air cavity at specific temperatures. when thermal dissipation is intended, the direction of energy exchange would be from indoor to outdoor (fig. 7). in order to reduce the temperature of the interior environment, different strategies could be provided by a single façade system. on one side, internal heat loads could be minimised by the storage layers, to redistribute that energy by conduction when it is needed. furthermore, disabling conservation layers could improve thermal comfort when the exterior conditions are more suitable than the interior ones, as heat could be transferred from the interior to the cavity by conductive and convective fluxes. finally, as previously discussed, convective movements in the air cavity could cause significant thermal losses. on the contrary, to use the exterior cavity as a convective insulation element, it should be fully closed, and the conservation layer should be activated, impeding convective flows and blocking thermal flux by conduction. in this case, material combinations fit with the ones that were exposed in fig. 2. once more, adaptive flow from the internal layers to the exterior cavity would only be possible if the conservation layers performed in a dynamic way according to different boundary conditions. fig. 7 possible combination of shape-changing materials with other technologies 116 journal of facade design & engineering volume 6 / number 2 / 2018 3.2 technical limitations technical limitations delay the success of some smart and multifunctional materials in the building industry. their drawbacks need to be properly understood to overcome this challenge, in such a way that they could be addressed by finding suitable combinations with other building technologies. for instance, according to the literature reviewed, thermochromic materials have a serious problem with degradation, especially when they are exposed to ultraviolet radiation (addington & schodek, 2005) and their mechanical properties decay (ma & zhu, 2009). this is the reason why the number of reversible adaptation cycles are considered too short for façade application. however, we can find commercialised products containing thermochromics in the glass industry, which ensure optimal fatigue life. in fact, they are usually applied as thin films between glass panels; the glass prevents uv radiation. when applying smas or thermobimetals, we must combine these materials in a multilayered façade system to face thermal, hygrothermal, and acoustic requirements. in addition, these alloys are oxidised when they are in contact with aggressive environments (marine or industrial). but while the oxide layer of some smas, such as ni-ti alloys, is compact, thin, and passive, and it acts as a protection layer, the oxide layer of thermobimetals (ni-fe alloys) is porous and, therefore, destructive. as these materials act kinetically, a hypothetical protection layer would be useless, as it could be broken by the repetitive deformation of the piece. consequently, thermobimetals shouldn’t be used in marine or industrial atmospheres. 4 conclusions and future developments this paper has shown different possible façade configurations in which to apply smart and multifunctional materials in adaptive opaque façades. to reach this stage, it was necessary to consider the physical events in a conceptual way, so that the ideal thermal behaviours in the overall systems were designed. this analysis allowed for the anticipation of the most appropriate physical properties for each layer of the systems, according to the pursued dynamic roles. based on the literature review, we highlighted promising materials and technologies that could meet these requirements and we explained briefly how they could work in a holistic way. to more accurately scope the potential of these new systems and to find out which configurations are the most suitable ones, further research is still needed, starting with numerical assessments. they would allow for the optimisation of the adaptability range, the velocity of adaptation, and the operational scenario (operational temperature setting) of these materials. moreover, they would allow for the determination of whether the proposed combinations with other building materials would enable a holistic responsive performance. lastly, these new façades must be validated through experimental assessments to prove that current technologies can offer suitable adaptive responses over an adequate lifespan. acknowledgements the authors would like to thank the support to the asociación de amigos of the universidad de navarra. in addition, we would like to gratefully acknowledge cost action tu1403 “adaptive façade network” for providing excellent research networking. 117 journal of facade design & engineering volume 6 / number 2 / 2018 references addington, d. m., & schodek, d. l. (2005). smart materials and new technologies : for the architecture and design professions. amsterdam: elsevier, architectural press. badarnah, l., & knaack, u. (2005). bionic breathing skin for buildings. ebooks.iospress.nl, (nerdinger), 612–619. balocco, c. (2002). a simple model to study ventilated façades energy performance. energy and buildings, 34(5), 469–475. http://doi. org/10.1016/s0378-7788(01)00130-x favoino, f., jin, q., & overend, m. (2017). design and control optimisation of adaptive insulation systems for office buildings. part 1: adaptive technologies and simulation framework. energy, 127, 301–309. http://doi.org/10.1016/j.energy.2017.03.083 ibañez-puy, m., vidaurre-arbizu, m., sacristán-fernández, j. a., & martín-gómez, c. (2017). opaque ventilated façades : thermal and energy performance review. renewable and sustainable energy reviews, 79(may), 180–191. http://doi.org/10.1016/j. rser.2017.05.059 karlessi, t., santamouris, m., apostolakis, k., synnefa, a., & livada, i. (2008). development and testing of thermochromic coatings for buildings and urban structures. solar energy, 83(4), 538–551. http://doi.org/10.1016/j.solener.2008.10.005 kretzer, m. (2017). information materials. springer international publishing ag switzerland. http://doi.org/10.1007/978-3-31935150-6 lelieveld, c. m. j. l. (2013). smart materials for the realization of an adaptive building component. delft university of technology. http://doi.org/10.1017/cbo9781107415324.004 loonen, r. c. g. m., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483–493. http://doi.org/10.1016/j.rser.2013.04.016 lópez, m., rubio, r., martín, s., croxford, b., & jackson, r. (2015). active materials for adaptive architectural envelopes based on plant adaptation principles. journal of façade design and engineering, 3(1), 27–38. http://doi.org/10.3233/fde-150026 ma, y., & zhu, b. (2009). research on the preparation of reversibly thermochromic cement based materials at normal temperature. cement and concrete research, 39(2), 90–94. http://doi.org/10.1016/j.cemconres.2008.10.006 sánchez-ostiz gutiérrez, a. (2011). fachadas: cerramientos de edificios. [facades: enclosures of buildings]. madrid : cie inversiones editoriales dossat-2000, 2011. from city’s station to station city 121 journal of facade design & engineering volume 6 / number 2 / 2018 unglazed solar thermal systems for building integration, coupled with district heating systems. conceptual definition, cost and performance assessment mikel lumbreras1, roberto garay2, koldobika martin1 1 department of thermal engineering, university of the basque country (upv/ehu) 2 sustainable construction division, tecnalia. abstract in this paper, the energy performance of a solar thermal (st) façade system is studied in relation to its connection to a district heating system. this concept allows for the direct use of st heat in the building, while taking profit from the network for delivery/selling of excess heat and purchase of heat during periods of underproduction. the use of unglazed collectors for low-intrusive architectural interaction in façades is discussed. studies are carried out on the heat production of the system and its capacity to cope with local demands. economic studies are carried out in order to balance the investment and operational costs/profits of the system. keywords solar systems, thermal energy, building integration, energy systems 122 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction in developed countries, there is a clear need, and political impulse, to achieve an energetic transition from fossil fuels to renewable sources. within renewable energy sources, the potential of solar power and associated technologies is well known. in particular, solar thermal systems are a proven renewable heating technology. although the exergy of this energy source is quite low, the potential of solar energy is still one of the greatest on the planet (ehsanul, kumar, kumar, adelodun, & kim, 2018). in this context, solar energy must be one of the main pillars of a renewable energy strategy. a clear example of the drive towards this transition is the minimum solar contribution required by national building codes in developed countries, such as the spanish cte (2013). in most traditional heating system designs, solar thermal (st) systems are sized to meet only a fraction of the entire demand for thermal energy. solar production and heat loads in buildings have daily and seasonal variations due to transient and variable weather conditions. in most climates, space heating (sh) load is interrupted in summer periods, but domestic hot water (dhw) loads are stable all year round. in winter, the available st heat is not sufficient to cover heat loads in buildings, while in summer, solar heat production clearly exceeds the demand of the building. st systems are commonly sized not to exceed heat loads over the spring-autumn period. most st systems are incorporated in roofs. in order to meet the increased requirements for st installation, larger surfaces will need to be activated for st installations. for this, building façades need to be considered as candidate areas due to their large available surface, although challenges of overshadowing in high urban building densities must still be resolved. with the steady incorporation of nzeb (nearly zero energy buildings) in cities, relevant reductions in heat loads can be foreseen in the near future. the utilisation of renewable energies in these same buildings will reach a point where the directionality in the production-consumption role will be altered. the increase in st installation with the reduction of heat loads in buildings modifies traditional st sizing criteria. as a result, excess heat may be available from these st systems. in this paper, the connection to district heating (dh) is explored in order to allow this excess production to be used in adjacent buildings. dh systems are one of the most efficient ways to cover heat loads in urban areas. traditionally, dh systems have been based on large boilers or chp (combined heat & power) systems. nowadays, it is increasingly common to find dh networks that incorporate distributed energy sources (monsalvete álvarez de uribarri, eicker, & robinson, 2017), commonly with lower exergy in comparison with traditional high temperature power plants. this includes the exploitation of industrial waste heat and solar thermal systems, among others. all this results in a reduction of fossil fuel dependence and contributes to a decarbonised environment. heat losses in the dh system are proportional to the temperature gradient between supply temperature and environment temperature. with lower operational temperatures and distributed energy sources, there is a substantial improvement in system performance. in this paper, the potentialities, constraints, and performance levels of façade-integrated solar thermal systems coupled with low temperature district heating (ltdh) are studied, comprising their 123 journal of facade design & engineering volume 6 / number 2 / 2018 thermal and economic performance. the techno-economic viability of unglazed façade-integrated solar thermal systems when combined with low temperature district heating systems. 1.1 unglazed solar thermal collectors in general, st systems are composed of solar thermal collectors, where a heat transfer fluid is circulated in a pressurised circuit. solar heat is absorbed and transferred to the fluid, resulting in an increment of the temperature. depending on external conditions and collectors` characteristics, the performance of each collector is different. their performance definition is described in detail in (duffie and beckman, 1980). fig. 1 collector efficiency vs (t m -t a ). g t = 800w/m2. source: stickney, b. & soifer, b, (2009). fig. 1 shows that with a low temperature difference, unglazed collectors present better efficiency levels than other systems. the possibility to incorporate unglazed systems in buildings has been studied in diverse investigations such as (a. giovanardi, 2016) but only a few companies propose integration into the façade, and the technology is still under-exploited. 1.2 architectural integration of st systems building integration of st systems has been historically limited due to the need to accommodate glazed areas and tubular assembles in the architectural composition of buildings. despite this, smart but marginal integration solutions for vacuum tubes have been achieved in balconies or transparent areas (o’hegarty, kinnane, and mccormack, 2016) as for the unglazed collectors, the method for their integration in façades is explained in (garay martinez, r., arregi goikolea, b., bonnamy, p. & lopez, j., 2017). having no glass or tubular covers, the unglazed collectors are the only ones that can be integrated without modifying the aesthetics of the building. specifically, the unglazed solar collector enables varied forms (shape, size, and typology) and materials (colour, texture, transparency etc.). 124 journal of facade design & engineering volume 6 / number 2 / 2018 in broad terms, it must be considered that façades are the prominent image of the building. in the selection of st technologies and their integration with solar thermal façades (stf), this aspect needs to be taken into account. the existence of a wide range of architectural façades requires delivery of a wide range of stf products, to ensure freedom of design intent. in (garay martinez, arregi goikolea, bonnamy, and lopez, 2017), an experimental study is performed on unglazed st collectors and their potential to deliver heat to hvac systems in buildings. in this work, it is identified that façades are the biggest area on to which collectors can be installed, and that unglazed collectors are one of the most sensible alternatives to achieve st production and architectural integration. 1.3 st connection to dh networks with the trend to incorporate decentralised and decarbonised heat, st is an increasingly common alternative heat source being incorporated in dh. there are two integration alternatives for st in dh: centralised and distributed st systems. to date, most st installations have consisted of centralised st plants outside cities. this paper explores the possibility to integrate distributed st systems in buildings, by means of their integration in building façades. this allows for the moving of energy sources closer to consumption points to reduce the transmission losses associated with the aforementioned centralised plant. dh connection of st systems can be performed in different ways, with different functionalities. (sanchez zabala and garay martinez, 2017) describe several types of connections. fig. 2. shows different types of st integration schemes into a dh network. fig. 2 st integration into dh networks. a. st & dh in parallel. b. delivery of excess heat to dh. c. hybrid system without storage (sanchez zabala & garay martinez, 2017) this paper explores a direct st and dh connection to central hvac manifolds in building, allowing bi-directional heat transfer to the dh. in this concept, local storage is avoided. 125 journal of facade design & engineering volume 6 / number 2 / 2018 2 methodology in this paper, simulations are performed in order to assess the technical and economic viability of unglazed st systems connected to dh networks in order to deliver decarbonised heat within reasonable economic metrics. for this purpose, simulation studies are performed for a multi-storey building in the region of bordeaux (france). according to the koppen-geiger climate definition, described by (kottek, m., grieser, j., beck, c., rudolph, b., & rubel, f., 2006), bordeaux is classified as having a c fb climate, which covers most climates in western europe, from the north of spain to central eu latitudes such as uk, the netherlands, etc... for this reason, bordeaux is considered to be a representative location for west-eu climates. the heat load, for dhw and space heating is calculated by means of dynamic simulation methods with an hourly resolution. heat production of a south-oriented st façade is simulated for the same climate with the same resolution. the economic viability of dh-connected st systems is evaluated by means of a comparative study against fossil-fuel alternatives. various st connection & heat pricing schemes are studied. fig. 3 methodology description 2.1 heat load modelling the first step in this study is developing the building model and defining its main characteristics, depending on which the heat loads vary considerably. the selected case study comprises a 5-storey building, with a façade surface area of 1250 m2. in fig. 4, a general view of the building and its basic connection scheme to dh is shown. the u-value of the walls is 0.8 w/m2. the window-to-wall ratio is 40% with a u-value of 1.4 w/m2. fig. 4 shows general prototypes for st integration in the building and its connection to dh substation. 126 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 4 3d model of the building geometry & st system on the south façade (left); connection scheme within the collector field and connection to dh (right) dhw heat load has been calculated according to (código técnico de la edificación [technical building code], 2013), considering a total floor area of 5000m2 divided in 80m2 apartments inhabited by families comprising 3 people. according to the calculation procedure, 22 litres of dhw (60ºc) are consumed per person per day. for sh, a pseudo dynamic calculation has been used, with a self-developed procedure that is compliant with une-en iso 13790:2011. 2.2 modeling of the unglazed st system for the modelling of the st façade, a self-developed model in the software r has been used for thermal calculations. this software tool allows big databases to be worked with as vectors, thereby reducing calculation times. within this model, specific collector energie solaire kollektor as (2012) data has been used in order to calculate efficiency and other parameters used in the model. as for the working temperatures, the inlet temperature has been fixed in order to be the same as the dh return line temperature (± 30 ºc) and the outlet temperature from the collector field has been set according to each of the simulation cases, which are further defined later in the text. moreover, load losses have been estimated to be 10%. 2.3 considerations for economic metrics the economic assessment is based on general economic metrics which can be found in general purpose economic literature such as (harris, 2018). for their calculation, the investment necessary for the installation and exploitation of each technology has been calculated, leading to the calculation of the yearly revenues and operational costs. this will include the consumption of primary energy sources and the heat purchase and delivery. as it is a theoretical study, the maintenance costs have been avoided for being much lower in comparison with other cash flows. specifically, the metrics used for this economic assessment are as follows: return on investment (roi), which is the time taken to recover the investment; cash flow, the net amount of cash moving in and out of each technology; and net present value (npv), which is the difference between the present value of cash inflows and the present value of cash outflows over a period of time. 127 journal of facade design & engineering volume 6 / number 2 / 2018 3 thermal performance of unglazed st system a st collector system is studied in a high-rise building. this collector field comprises 240m2 of south-oriented unglazed st. the field is arranged in 20 parallel circuits, comprising 6 collectors in a serial arrangement, with each collector covering and area of 2m2. data relating to the specific collector used for this installation can be found in energie solaire kollektor as (2012). in general, collectors achieve better efficiency when the temperature difference between the collector (average) and environment is low. in order to limit the average is the solar field, a sensitivity analysis is carried out to ensure that the inlet-outlet temperature difference is limited to 10 ºc. that temperature difference is defined by difference between the average temperature in the collector battery and the ambient temperature. this results in the aforementioned configuration. in an isolated system, the service temperature needs to be met by solar thermal collectors. in these systems, it is common to use the lowest possible service temperature to increase the overall performance of st. in the case of considering st systems coupled with dh, there is no need to configure st systems to raise fluid temperature to the overall flow temperature in the dh network. there is a minimum temperature difference (normally 3-5ºc) that the system must achieve before it is worth activating the pumps to circulate the fluid. if the heat output is delivered in the return pipe of the dh, greater performance is achieved due to the lower service temperature compared to insulated st systems. in fig. 5, the results from the simulation of unglazed st when reacting to different situations are shown. fig. 5 collector in serial arrangement performance for different situations depending on the service temperature required, the number of collectors arranged in series increases, so that the surface needed increases proportionally. taking into account the fact that the inlet-outlet temperature difference is limited, the number of collectors is also limited. nowadays, ltdh (low temperature district heating) is considered as an alternative to conventional dh, and the working temperatures of this system are much lower, reducing heat losses in supply temperatures. in this case, the best solution for installing integrated st is to install the st system to the return pipe of ltdh system. 128 journal of facade design & engineering volume 6 / number 2 / 2018 4 results 4.1 heat production solar production is simulated for the climate of bordeaux for various operational conditions. these conditions consider various inlet temperatures, among which low temperatures are incorporated and used in line with ltdh. temperatures in the range of 50-60ºc are representative of flow temperatures, while temperatures of 20-30ºc are representative of dh return lines. in fig. 6, the total heat production of a solar unglazed collector (energie solaire kollektor as, 2012) and the total efficiency defined by the production, divided by the total solar radiation, are shown. the bar plot refers to the total solar production by the unglazed collector; the lines refer to the total efficiency. the results indicate that better results are achieved for the cases in which the inlet-outlet temperature difference is limited to 10ºc, no matter what the inlet temperature is. fig. 6 solar production and total solar efficiency for different inlet temperatures temperature differences in the range of 10ºc can be used to inject heat to the return line of the dh system. 129 journal of facade design & engineering volume 6 / number 2 / 2018 4.2 economic viability the economical assessment of dh connected st system is studied against several benchmark cases (natural gas boilers, electric heaters etc.). investment and operational costs are calculated and the economic performance of the system is calculated over the service life of the system. investment and operational costs have been calculated for the cases under review. investment costs cover the equipment and installation costs of each system. representative hvac systems for multistorey buildings have been used and their cost normalised per kw. data has been taken from (precio centro guadalajara, 2018) and from (tarifa de precios solar térmica salvador escoda, 2018). technology material installation costs (€/kw) natural gas boiler 85 joule heater 5 ground source heat pump 692 unglazed collector without dh 915 unglazed collector with dh 608 table 1 installation cost per unit of power for different technologies. in table 2, the breakdown of the investment costs is presented. concept unitary cost (€/unit) quantity total € solar collector rk // alpin rkm 2001 2m2 305 120 36630 € support assembly for façades // sfv-ar 120 120 14400 € valves and other installation materials ----4041 € workforce costs ----7251€ total ----62023 € table 2 cost estimation for st installation. source: tarifa de precios solar térmica salvador escoda [salvador escoda solar thermal price list] (2018). operational costs incorporate the primary energy consumed by heating systems. the cost of primary energy sources are shown in table 3. primary energy price (€/kwh) natural gas 0.05 electricity 0.14 st 0 dh heat (une-en iso 13790:2011) 0.0685 heat purchase (estimated 70% of dh heat cost) (une-en iso 13790:2011) 0.04795 table 3 prices for primary sources (2016). 130 journal of facade design & engineering volume 6 / number 2 / 2018 the cost of dh has been obtained from the commercial price of heat in the dh network in paris, specifically; data has been obtained from (tarifs de vente cpcu, 2016), which is one of the largest networks in eu. as for the dh cases, it has been assumed that the heat produced by st system could be sold to dh network at two price points: 100% of heat price produced in dh and 70% of the heat price produced in dh. this is simply a consideration in order to simulate an ideal case, as well as a more realistic one. investment and operational costs of all alternative systems studied are recorded in fig. 7. fig. 7 initial investment and operational costs for each technology the case of the st system coupled with dh has negative operational costs. for the calculation of such operational costs, it has been considered that all of the heat demand from the building is obtained from the dh supply line and the heat produced by the st is, in its entirely, sold to dh. in this way, general data used for these calculations is recorded in table 4. heat load (sh+dhw) (kwh/year) 424040 solar production (kwh/year) 138196 income from st 70% (€/year) 9466 income from st 100% (€/year) 6626 table 4 operational cost overview it is clearly seen that both st systems, with and without the dh network, require a larger initial investment than typical natural gas boilers or electrical heaters. if a dh network is available, the possibility to deliver excess heat to the dh offsets the cost of heat purchased in winter periods. this results in negative operational costs. the income from heat sold to dh is higher than the cost of heat purchases from the dh network. 131 journal of facade design & engineering volume 6 / number 2 / 2018 the evolution of the cumulative cost of each of the technologies is shown in fig. 8. natural gas boilers have been taken as a reference, thus, the accumulated differential cost against this technology is provided. the case of joule heating has been removed due to its clearly anti-economic performance and in order to see the cost comparison in more detail. for the purposes of calculation in fig. 8, the interest rate has not been considered. fig. 8 cost evolution for different technologies, with gas boiler as reference from fig. 8, it is resolved that st connected to dh return line shows better economic performance than conventional gas boilers when the second year has passed. table 5 presents the return of investment (roi) for dh-connected st façades for various interest rates. interest rates of 5% and 10% are considered. st to dh return 100% st to dh 70% roi (i = 5%) 5.554 6.858 roi (i = 10%) 5.097 5.853 table 5 return of investment for cases where dh is installed (years) 5 discussion in this work the possibility to incorporate unglazed solar thermal collectors in the context of dh is studied. the economic metrics of the presented case study show good feasibility, with rois in the range of 5-6 years. in this context, it is crucial to understand that heat purchase agreements need to be defined at a dh scale. these agreements substantially affect the economic metrics. in the presented study, the payback period is reduced by 1-2 years when heat purchase price is reduced to 70%. 132 journal of facade design & engineering volume 6 / number 2 / 2018 considering the data presented in fig. 8, the connection to dh substantially improves the economic metrics of the st systems, with payback periods reduced from 6-7 years to ~2 years. although the st collector field incorporates relevant capital costs for the installation of the st system, the connection to the dh avoids the need for large heat production systems to be installed for back-up, through reducing investments in auxiliaries, and reducing operational costs. the main problem that may be faced by these installations is the capacity of the dh return line to absorb heat from st when there are lots of distributed systems connected to them. although, in actuality, this problem does not exist due to the development situation, in the future it will need to be taken into account to avoid the collapse of the dh network. 6 conclusion & further work this paper has studied the technical and economic feasibility for the integration of unglazed st collectors in buildings, and its connection to dh infrastructure. the presented solution relies on the dh in order to balance excess heat production and to supply energy in periods without local production. overall, dh-connected st seems to be a promising solution, as it pays back in the most favourable case within ~2 years when compared to traditional heating solutions. with proven performance levels at collector level, and several standalone installations, the adaptation of st into dh applications needs to be undertaken. this activity will be carried out within the eu h2020 project related (2017). within this project, among other activities, an unglazed st system will be adapted for dh operation, and tested under a controlled test environment in the north of spain. this same system will be integrated in up to 4 dh networks across europe. acknowledgements this project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no. 768567. references ehsanul, k., kumar, p., kumar, s., adelodun, a. a., & kim, k. (2018). solar energy: potential and future prospects. renewable and sustainable energy reviews, volume 82, part 1, pp. 894-900, issn 1364-0321, https://doi.org/10.1016/j.rser.2017.09.094. código técnico de la edificación [technical building code] (cte) (2013). documento básico de ahorro de energía [basic document of energy saving] (db-he). retrieved from https://www.codigotecnico.org/images/stories/pdf/ahorroenergia/dbhe.pdf monsalvete álvarez de uribarri, p., eicker, u., & robinson, d. (2017). energy performance of decentralized solar thermal feed-in to district heating networks. energy procedia, volume 116, pages 285-296, issn 1876-6102, https://doi.org/10.1016/j.egypro.2017.05.075.] stickney, b. & soifer, b, (2009). solar thermal hydronic collector comparison and selection. retrieved from http://solarprofessional.com/articles/products-equipment/solar-heating/solar-thermal-hydronics#.wof68emwzxm duffie, j.a. & beckman, w.a. (1980). solar engineering of thermal processes (second edition). new york: john wiley & sons, inc. 133 journal of facade design & engineering volume 6 / number 2 / 2018 o’hegarty, r, kinnane, o., & mccormack, s.j. (2016). review and analysis of solar thermal façades. solar energy,volume 135, pp 408-422, issn 0038-092x giovanardi, a. (2012), integrated solar thermal façade component for building energy retrofit. eurac research. garay martinez, r., arregi goikolea, b., bonnamy, p., & lopez, j. (2017). concept, development and thermal characterization of an unglazed solar thermal collector for façade integration. dyna ingenieria e industria. 92. pp466-472. 10.6036/8108. sanchez zabala, v., & garay martinez, r. (2017). design of consumer thermal substations for the integration of distributed solar technologies in district heating systems. kottek, m., grieser, j., beck, c., rudolph, b., & rubel, f. (2006), world map of the köppen-geiger climate classification updated, meteorologische zeitschrift, vol.15, no. 3, 259-263 une-en iso 13790:2011, energy performance of buildings calculation of energy use for space heating and cooling (iso 13790:2008) solar collector factsheet (2012), energie solaire kollektor as. retrieved from https://www.energie-solaire.com/jt_files/pdf/scf1209de.pdf harris, r. (2018). value creation, net present value and economic profit. darden business publishing. university of california precio centro guadalajara [price center guadalajara]. visited in 2018. retrieved from http://preciocentro.com/tienda/productos-edicion-2017/55-base-edificacion-urbanizacion-2017.html [cd-rom]. tarifa de precios solar térmica salvador escoda [salvador escoda solar thermal price list] (2018). retrieved from http://www. salvadorescoda.com/tarifas/energias_renovables_tarifa_pvp_salvadorescoda.pdf tarifs de vente cpcu [sales rates cpcu], (2016). retrieved from http://www.cpcu.fr/qui-sommes-nous/documentations-cpcu related, renewable low temperature district, eu h2020 ga nº 768567 (2017-2021), retrieved from www.relatedproject.eu from city’s station to station city 009 journal of facade design & engineering volume 6 / number 2 / 2018 on the development of a façade‑integrated solar water storage matteo d’antoni, paolo bonato, roberto fedrizzi eurac research, institute for renewable energy, bolzano, italy abstract the integration of active solar thermal technologies into building envelopes has recently been receiving greater attention, and has been promoted within international projects such as iea task 56 and cost action 1403. although the façade integration of solar thermal collectors is a topic that has been debated at length, little attention has been paid to the building integration of solar water storage. the scope of this paper is to highlight the main barriers that are experienced in the development of façade-integrated solar water storage. this activity is a part of the sunrise project that aims to develop a new unitised curtain wall element for tertiary office buildings. the façade element integrates a complete solar thermal system consisting of a solar collector, hot water storage, a radiant panel, and all the required operation components. a mock-up of the solar façade is manufactured to identify practical constructional issues. the thermal behaviour of the tank is analysed through fem simulations and laboratory tests. keywords solar active envelope systems, building integration, product development, heat storage doi 10.7480/jfde.2018.2.2048 010 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction in recent years, the attention given to the building‑integration of active solar thermal systems (bists) has risen. the market penetration of bists, however, is still low, and a greater number of market‑ available solutions is desired to improve the acceptability of solar envelopes and increase the amount of building loads covered by solar energy. international efforts like iea shc task 56 (iea, 2016) and cost action 1403 (cost action tu, 2013) are investigating this topic from market, technological, and energy perspectives with the aim of studying and promoting solar envelope systems. scientific literature highlights that bists suffer from technological and economical barriers (iea, 2009) such as high investment costs and mistakes during installation that can be effectively tackled with the development of turnkey solutions and the application of prefabrication techniques (soppelsa et al., 2016). from this perspective, r&d projects together with industry become fundamental. as a concrete example for overcoming such limitations, the integration of a complete solar thermal system in a curtain wall element is studied within the framework of the sunrise project. this r&d collaboration with a façade manufacturer aims to design a new turnkey plug‑and‑play façade element that is suitable for new and retrofitted tertiary buildings. whenever solar radiation is considered as a means to cover thermal building loads, a a thermal energy storage (tes) is required for making collected heat available at times when the solar resource is not present. the heat harvested by solar façades is typically stored in the building’s thermal mass or in water‑based tess. low‑tech passive design strategies recommend exposing the building’s thermal mass in order to collect solar radiation during the day and slowly release the collected heat overnight. as in the case of trombe walls (givoni, 1998), air‑based solar concepts can be assisted by fans and dampers or can be operated on natural convection principles. the thermal capacity of building materials can be further enhanced with phase change materials (pcm) incorporated in the wall structure or mixed with the construction materials (de gracia & cabeza, 2015; kolaitis et al., 2015). the thermal mass of the envelope is also exploited by massive solar thermal collectors (mstcs) (d’antoni & saro, 2012), which are capacitive building envelope structures capable of removing the absorbed heat through an embedded pipe loop. mstcs can serve water‑based active solar systems for a direct or indirect use of collected heat in covering building loads. in existing façade‑integrated solar thermal systems, the collected heat can be stored in conventional water tess that are detached from the building façade (risholt et al., 2015; waf, 2018). alternatively, solar collectors and water thermal storage can be combined into a single unit, so that neither external storage nor connection pipes are needed. this energy concept has a long tradition in hot climates but continuous research (singh, lazarus, & souliotis, 2016) has resulted in improved designs, such as the installation of water storage as a separate layer behind the solar collector (sopian, syahri, abdullah, othman, & yatim, 2004), the combination of water and pcm storage (reddy, 2007) or the merging of water storage and absorber plate in a unique tubular structure (khalifa & jabbar, 2010). from this literature review, it emerges that only a limited number of concepts and solutions of building‑integrated solar water storage have been developed. as a contribution for future bist concepts, this paper aims to share the experience acquired in the development of a building‑ integrated water tes, considering both technological and thermal aspects. in the framework of the sunrise project, a mock‑up of the solar façade that integrates such compact water tes is manufactured to identify practical constructional issues. in addition, the thermal behaviour of the tes is analysed through fem simulations and performance tests carried out in the laboratory facilities at the institute for renewable energy of eurac research. 011 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 1 building‑integration of the solar thermal system into the façade. 2 façade concept 2.1 concept overview the energy concept consists of a solar thermal system integrated in the opaque part of a unitised curtain wall element (fig. 1). such a system is composed of a flat‑plate solar thermal collector installed on the external side of the façade element, a compact water tes incorporated in the parapet, and a radiant panel fixed to the inside as the finishing layer (fig. 2). the solar system includes a hydraulic module containing the required hydronic components (i.e. circulating pump, tempering valves, expansion vessel, safety valves, fill and drain ports, deaerator, manometer, and thermometer) and an electronic board. from a construction perspective, these components are embedded within two metal semi‑shells, which, once joined, guarantee air and water tightness of the spandrel panel. the solar thermal collector is hung on the external metal shell and a 3 cm air gap is left in between to facilitate installation operations. the façade‑integrated system can harvest solar radiation through the solar collector, store the collected heat in the water tes and either deliver it to the thermal zone or to a centralised dhw tank. each façade element is equipped with two water circuits that are independent and hydraulically separated: the solar loop (sc) transfers the heat collected by the solar collector to the façade‑integrated tank or to a centralised dhw tank, whereas the heating and cooling loop (hc) delivers heating/cooling power to the occupied space through the radiant panel. the heating water can be either drawn from the façade‑integrated water storage or supplied by an external back‑ up system (i.e. gas boiler), depending on the availability of stored solar heat. the cooling water is instead produced at central system level with an auxiliary chiller and directed to the radiant panel, bypassing the tes. 012 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 2 view from inside (left) and outside (right) of façade prototype taken during the tests in the climatic chamber. obviously, the development of a robust control strategy is of key importance in the effective management of heat fluxes from/to the façade, the achievement of high final energy savings, and to avoid stagnation in the solar loop. the control developed for the sunrise façade is based on an irradiance detector and a number of temperature sensors placed in the water loops and in the tes. 2.2 drivers for the façade concept the development of new energy concepts should consider not only energy performance targets but all issues connected to the design, manufacture, installation, and operation of the system. from the energy perspective, the sunrise façade concept aims: – to cover part of heating and dhw loads through solar energy available on the external façade; – to maximise the local exploitation of solar energy in order to reduce parasitic heat losses in distribution pipework (quantified in 20% of the collected heat (dgs, 2005)); – to share the excess collected heat at building level. the key technological driver is the prefabrication of the hydronic circuit in order to (1) reduce construction and installation costs and (2) achieve higher quality workmanship. prefabrication principles such as modularity and standardisation of components are prerequisites in this activity. the façade‑integration of a solar thermal system influences the layout of generation, transmission, and emission systems. the manufacturing of a façade element integrating a heating/cooling terminal allows for to savings in material costs and costs incurred during installation phases. at building level, the installation of a decentralised tes in the building façade allows for resizing or elimination of the volume of the centralised tank and, therefore, the reduction of technical spaces. furthermore, the thermal insulation required in the façade element could be replaced in part by the rear‑side insulation of the solar collector, which could be also exploited by the tank. 013 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 3 design of the tes integrated in the building façade. 2.3 sunrise solar water storage the design of the tes installed in the sunrise façade is developed with consideration given to the following technological requirements: – it has to include two independent and hydraulically separated loops. – it must handle the thermal and mechanical stress induced by the solar system operation. – it cannot cause detrimental conditions for the façade element. – it has to have a minimum weight and thickness. the sunrise storage is custom‑made and consists of a series of steel cylinders connected by two headers at both ends, as shown in fig. 3. the overall dimensions of the tank are about 1.2 m high, 1.3 m wide and 0.06 m thick for a total water capacity of about 46 litres. an immersed heat exchanger is installed in the lower header to allow the heat transfer between sc loop and water storage. the connection with the hc loop is realised via two openings in the upper header (water outlet) and lower header (water inlet). the thermal insulation of the tank consists of a 10cm thick mineral wool layer on the inside and a 4cm thick layer on the outside. 3 characterisation of the façadeintegrated water storage the experiences gained during the product development are reported here. with respect to the façade‑integration of a water tes, the analysis has a two‑fold perspective: a focus on the individual component (tes) and a focus on the façade. 014 journal of facade design & engineering volume 6 / number 2 / 2018 3.1 component level 3.1.1 geometry the integration of any new component into a façade element must deal with dimensional restrictions. the overall dimensions of the tank are limited to the height of the spandrel panel, the width of the façade element, and the maximum acceptable thickness. moreover, the building‑ integration of a water storage element suggests a parallelepipedal tank shape conceived as a slim flat layer inserted in the spandrel panel’s multi‑layer structure. the common cylindrical shape of water storage elements is hardly an option in this case because of its excessive thickness. another limit to the tank geometry is posed by the mechanical stress induced by the water operating pressure (in the range of 4‑6 bars), which can cause structural deformations in the tank’s walls and decrease its lifespan. in the sunrise façade, the tank shape is designed as the junction of more vertical cylinders so that the pressure distribution on the tank walls is homogeneous. the resistance to mechanical stress is enhanced at the price of a lower water capacity and compactness of the tes. the energy efficiency of water storages is affected by the compactness of the tank expressed with the surface‑to‑volume ratio or s/v. given a certain tank capacity, larger external surfaces mean indeed larger dispersing areas and higher heat losses. table 1 compares the s/v ration for different shapes of tes with equal water volume. the external surface area of the sunrise tes design is about 2 and 6 times larger than for full parallelepipedal and cylindrical water storages, respectively. the heat capacity of the tes is the result of a trade‑off between tank size and the target amount of solar coverage of building loads. insufficient heat capacity can easily worsen the performance of the whole energy system as higher heat losses through the tank walls and lower solar collector efficiencies can be expected. in the case of residential solar combi‑systems, the typical design ration between tes volume and solar field area is between 40‑100 l/m2, whereas here it is limited to 18 l/m2. in fact, geometrical constraints limit the overall dimensions of the tes and its shape further reduces the water volume to only 52% of the occupied gross volume. although these calculations testify a sub‑optimal behaviour of the sunrise tes in terms of both heat retention and thermal capacity, thermal performance requirements were revealed to be of secondary importance with respect to geometry limitations. tes geometry volume [l] external surface [m2] compactness s/v [1/m] sunrise tes 46 4.15 90 parallelepiped 2.22 48 cylinder 0.76 16 table 1 comparison of compactness ratio between different tes geometries. 015 journal of facade design & engineering volume 6 / number 2 / 2018 3.1.2 thermal insulation proper thermal insulation is essential for any water storage in order to reduce the heat losses through the mantle and improve the storage efficiency. when the tes is integrated in the façade, the insulating material must not only guarantee satisfying thermal performances, but also respect technological specifications such as fire resistance class and permeability to water vapour. as already pointed out, the insulation layer installed in the spandrel panel of traditional façade elements or on the back of rear‑insulated solar thermal collectors can synergistically reduce the thickness of the tank insulation layer. different insulation thicknesses and materials (mineral wool and vip vacuum insulation panel) are considered during the design process of the sunrise tes. in spite of the excellent thermal behaviour of vip, its use in the sunrise mock‑up is discarded because of its susceptibility to mechanical damages, the complexity of application, and the high cost of the materials used in the panel’s core. instead, the chosen insulation material is mineral wool because of its good thermal insulation proprieties, its excellent behaviour in fire, and the ease of its application. in light of these considerations, two insulation layers of 10cm and 4cm are installed on the internal and external sides of the sunrise tes, respectively. a thicker insulation layer (10cm) is applied on the internal side in order to embed the water pipes, whereas the external layer can benefit from the insulation in the rear side of the solar thermal collector. the heat transfer coefficients (u2d) calculated with comsol simulations under the hypothesis of stationary conditions and bi‑dimensional heat transfer are shown below (fig. 4 and fig. 5). the total heat losses correspond to 1.9 w/k (or 85 w for a δt = 45k) for the chosen configuration, that is equivalent to a class e efficiency hot‑water storage according to the commission delegated regulation (eu) no 812/2013. the main cause of such poor energy performance is undoubtedly the shape of the tank, which is characterised by a large heat dispersing surface per unit of volume. fig. 4 fem simulation of the tes assuming t water = 65°c, t indoor = 20°c, t outdoor = 4°c. fig. 5 heat transfer coefficient calculated from fem analysis for different insulation thicknesses. 016 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 6 adimensional temperature in the tes during charging phase on the left (t min =23.5 °c, t max =76.1 °c, δτ=5 h, h=1.2 m) and discharging phase on the right (t min =21.1 °c, t max =76.1 °c, δτ=72 h, h=1.2 m). 3.1.3 thermal stratification the thermal stratification in tess is beneficial to the performance of solar systems as higher solar collector efficiencies are achieved, the heat losses from equipment are reduced, and the set point temperatures for heat delivery can be more easily reached. in this perspective, a key factor in tes design is the definition of the height‑to‑diameter ratio of the tank. the typical recommended ratio for tes with optimum stratification is at least 2.5:1, whereas the ratio in the developed tes concept is about 20:1 and thus a good thermal stratification is expected. fig. 6 shows the temperature profiles of the charging (left) and discharging (right) phases measured during the experimental activities carried out in a climatic chamber. the hot and the cold chambers are kept at a constant temperature of 20°c and a sun simulator is used to provide a constant irradiance of 1000 w/m2 on a solar collector surface during the charging test. the temperature stratification is expressed with the notion of adimensional temperature t* (=δt/δt max ), calculated using minimum and maximum temperature measured during the test. temperature sensors are installed at different heights of the tes and, in particular, at a relative height h* of 0.15 and 0.85. the transient variation of temperature is expressed as function of adimensional time τ*. the charging phase starts when the tes is at fully‑mixed conditions and in thermal equilibrium with hot and cold chambers. it can be observed in fig. 6 that fully‑mixed conditions are maintained during the whole charging process as the solar heat exchanger is installed in the lower header of the sunrise tes. the test is interrupted after 5 hours when the maximum temperature of about 76°c is reached. at this point, the charging phase test is interrupted and the artificial sun is switched off. the discharging phase test consists of the monitoring of the tes temperature variation due to the sole thermal losses across storage mantle. this process is much slower than the previous one and it takes 72 hours before initial temperature conditions are restored. 017 journal of facade design & engineering volume 6 / number 2 / 2018 3.1.4 product availability and cost the façade‑integration of the water tank must deal with shape and technological constraints that go beyond the range of products that are commercially available, so tailored solutions have to be designed. the price per unit of volume of the tank is expected to be significantly higher with respect to the solar tes available on the market. this is due to three factors: the small size, the high rate of consumption of raw material per unit of volume, and the customisation of the product. assuming a specific list price of €2/l that can be achieved for standard solar tanks, the target price for the sunrise storage should be less than €100, which is unrealistic at the current stage. 3.2 façade‑integration level 3.2.1 weight and thickness of the façade the integration of water storage into the spandrel panel undoubtedly increases the complexity of the façade in several aspects, but the most visible and straightforward effect is an increase in the weight and thickness of the façade. in the manufactured mock‑up, the integration of the water storage causes an increase of the façade weight of about one quintal, of which around half is due to the weight of the steel and half is due to the weight of the water. the use of plastic materials in place of steel could reduce the overall weight, but it is not recommended because of the lower resistance to thermal and mechanical stresses. the primary and secondary load bearing structures of the façade shall be appropriately sized to account for the additional weight and prevent deformations overtime. the thickness of the façade element is increased by the integration of the tes in the layered structure of the spandrel panel. such effect, however, is relatively limited when a slim parallelepipedal tank is installed, as in the case of the sunrise tes that is only 6 cm thick. the need for thermal insulation in addition to that already installed also increases the weight and the overall thickness of the façade. 3.2.2 heat fluxes when the façade‑integrated tank is charged, a fraction of the stored heat is progressively lost over time through the tank walls. the heat losses can be reduced improving the insulation quality, increasing the insulation thickness, decreasing the external surfaces and minimising thermal bridges. a fraction of heat losses is directed to the inside and can affect the indoor microclimate. in particular, parasitic losses from tes can be considered a beneficial side effect during the heating season: the temperature of the zone slightly rises, the user comfort is improved and the active heat transfer from space heating terminals can be reduced. if this phenomenon is beneficial during the winter season, in summer it represents an undesirable event that shall be avoided as much as possible since it might cause an increase of space cooling energy demand. the magnitude of heat losses is affected by the water temperature in the storage element and thus also by the heat capacity of the water storage and the typology of solar thermal collector (unglazed, glazed, or evacuated tube). 018 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 7 comparison of temperature gradient and heat fluxes under between the façade‑integrated tes (left) and an insulated assembly (right) during the winter season (t zone =20°c, t amb =5°c, t cavity =25°c). the heat losses from the front and rear sides of the charged tank are compared in fig. 7 against a traditional façade construction with the same insulation thickness (14cm of mineral wool) and identical boundary temperatures (indoor: t zone =20°c, outdoor: t amb =5°c). the calculation is performed considering a mono‑dimensional heat transfer model and stationary conditions. the façade‑ integrated solution is assumed to be adjacent to an air cavity at 25°c (average temperature measured during solar collector operation). as can be observed, the heat flux is directed outwards in case of the traditional façade assembly, whereas the thermal losses of the tes represent a heat gain for the thermal zone for the solar active façade. 3.2.3 hydraulic layout of the façade element the installation of the tes in the façade element increases the complexity of the façade and, therefore, the requirement for inspections and maintenance as well as the risk of failure. hydronic components such as an expansion vessel, valves, and charging and discharging points might be required depending on the design of the hydraulic system. the sunrise solar façade is designed so that the solar field can be enlarged by connecting multiple façade elements to each other with limited design efforts since a single hydraulic module can serve up to 10 solar façade elements. this is possible through a number of water pipes embedded in the façade thickness that pass through the mullion and a set of throttles that ease the connection between adjacent façade elements. 3.2.4 accessibility to the stored solar heat the exploitation of the stored solar heat can be limited in case of façade‑integrated tanks, as some services such as dhw preparation are centralised. viewed from a broader perspective, the interaction between façade‑integrated solar thermal collectors and heating distribution networks can be easily realised if a central water storage is installed. in this case, the solar thermal collectors might cover not only the space heating demand of the zone nearby the installation site, but also other thermal loads of the building (i.e. dhw). 019 journal of facade design & engineering volume 6 / number 2 / 2018 4 conclusions the paper reviews the development process of a façade‑integrated solar water storage. this activity is carried out within the framework of an r&d project where the scope is the development of a new solar envelope system for the tertiary building sector. during the project, a mock‑up of the façade concept is manufactured, simulated, and tested in the climatic chamber of the institute for renewable energy of eurac research (bolzano, italy). from the project activites, it emerges that the integration of a solar water storages is not an easy task. looking at the sunrise façade development, the respect of the technological requirements and the achievement of high thermal performances are difficult goals to attain at the same time. moreover, as the solution is building‑integrated, geometrical requirements (i.e. thickness, compactness) drive the development of the façade concept and play a much more relevant role than energy efficiency criteria. more specifically, the façade‑integration of the tes binds its design to geometrical limits that are unfavourable to the purpose of heat storing. the overall dimensions are constrained and the high surface‑to‑volume ratio is responsible for higher thermal losses compared to traditional shapes. the design of the thermal insulation of the tank is also restricted in terms of thickness and material properties by the façade structure. in addition, such tanks are not available on the market and therefore must be customised at higher costs. focusing instead on the façade‑integration, the overall thickness and weight of the façade elements are increased by the addition of the water tank. parasitic heat fluxes toward the interior are expected to occur, with a positive fallout during the heating season and a negative one during the cooling season. finally, the accessibility of the stored solar heat is limited by the delocalisation of the stored heat and integration of central services or thermal loads could be difficult without an additional central tank. although the results are not so encouraging, the experience gained is considered highly valuable and can set new design drivers and incentives for future developments on solar envelope systems. reference international energy agency (iea), solar heating and cooling (shc) programme (2016). task 56 – building integrated solar envelope systems for hvac and lighting. retrieved from http://task56.iea‑shc.org/. cost action tu 1403 (2014). adaptive façade network. international energy agency (iea), solar heating and cooling (shc) programme (2009). task 41 – solar energy and architecture. retrieved from http://task41.iea‑shc.org/. soppelsa, a., fedrizzi, r., buffa s., bertesina, d., nouvel r., & cotrado, m. (2016). d4.3 report on assessed industrialized energy generation kit, inspire eu fp7 project. retrieved from www.inspirefp7.eu. givoni, b. (1998). climate considerations in building and urban design. hoboken, new jersey: john wiley & son. de gracia, a., & cabeza, l. f. (2015). phase change materials and thermal energy storage for buildings. energy and buildings, 103, 414‑419. doi.org/10.1016/j.enbuild.2015.06.007 kolaitis, d., garay martinez, r., & founti, m. a. (2015). an experimental and numerical simulation study of an active solar wall enhanced with phase change materials. journal of façade design and engineering, 3, 71‑80. doi.org/10.3233/fde‑150027 d’antoni, m., & saro, o. (2012). massive solar thermal collectors: a critical literature review. renewable and sustainable energy reviews, 16, 3666‑3679. doi.org/10.1016/j.rser.2012.02.076 risholt, b., asphaug, s., busklein, j.o., uvsløkk, s., rognvik, e., & grynning, s. (2015). window with integrated solar collector. climate resistance evaluation report. sintef academy press. waf solar façade. retrieved march 27, 2018, from http://www.waf.at/solar‑systems/ singh, r., lazarus, i.j., and souliotis, m. (2016). recent developments in integrated collector storage (ics) solar water heaters: a review. renewable and sustainable energy reviews, 54, 270‑298. doi.org/10.1016/j.rser.2015.10.006 sopian, k., syahri, m., abdullah, s., othman, m.y., & yatim, b. (2004). performance of a non‑metallic unglazed solar water heater with integrated storage system. renewable energy, 29, 1421‑1430. doi.org/10.1016/j.renene.2004.01.002 020 journal of facade design & engineering volume 6 / number 2 / 2018 reddy, k.s. (2007). thermal modeling of pcm‑based solar integrated collector storage water heating system. transactions of the asme, 129, 458‑464. doi:10.1115/1.2770753 khalifa, a.j., & jabbar, r.a.a. (2010). conventional versus storage domestic solar hot water systems: a comparative performance study. energy conversion and management, 51, 265‑270. doi.org/10.1016/j.enconman.2009.09.021 dgs (2005). planning and installing solar thermal systems. a guide for installers, architects and engineers. james & james / earth‑ scan. from city’s station to station city v journal of facade design & engineering volume 9 / number 1 / 2021 editorial the powerskin conference series is a biennial event organised cooperatively between tu münchen, tu darmstadt, and tu delft, which is already in its third edition, having started in 2017. the conference aims to address the role of building skins in accomplishing a carbon neutral building stock. the presented papers showcase recent scientific research and developments as well as projects related to building skins from the perspectives of material, technology, and design. topics such as building operation, embodied energy, energy generation and storage in context of the envelope, energy, and environment are considered. the building envelope largely determines the energetic performance of the building, plays a significant part in the embodied energy of construction activities, defines the indoor qualities for the user and – quite importantly – defines the appearance of the building in an urban content. so, being central to all these aspects, the building envelope is the focus of research & development, engineering, and design. this is the scope of the powerskin conferences, bringing research, industry, and users together to share and discuss new knowledge in an interdisciplinary environment (albeit an online environment this time around). the focus of the powerskin conference 2021 deals with the question of whether simplicity and robustness are in contradiction to good performance or whether they even complement each other. hence the question simplicity vs. performance? is tackled throughout the conference from three points of view which define the thematic sessions of the conference: energy, envelope, and environment. this special issue of the journal of façade design and engineering dedicated to powerskin 2021 showcases the most prominent and relevant papers of the conference, with the aim of enhancing their visibility for a larger audience. ulrich knaack thomas auer jens schneider editorial mono-material wood wall effects of phase change materials on heat flows through double skin façades holistic design explorations of building envelopes supported by machine learning potential of façade-integrated pvt with radiant heating and cooling panel supported by a thermal storage for temperature stability and energy efficiency additive manufacturing of thermally enhanced lightweight concrete wall elements with closed cellular structures exploring the possibility of using bioreceptive concrete in building façades photovoltaic warm façades with phase change materials in european climates smart textile sun shading a full performance paper house skin metrics paoss from city’s station to station city 001 journal of facade design & engineering volume 7 / number 1 / 2019 trombe curtain wall façade thomas wüest1, andreas luible2 1 competence centre building envelope, institute of civil engineering, lucerne university of applied sciences and arts, horw, switzerland, email: thomas.wueest@hslu.ch 2 competence centre building envelope, institute of civil engineering, lucerne university of applied sciences and arts, horw, switzerland abstract in times of energy use awareness, decarbonisation, and resource efficiency, the performance of wellknown façade components must be pushed beyond current limits through innovative designs and new combinations in construction. this paper presents an unconventional redesign of a double skin façade (dsf), based on trombe wall principles, to enlarge solar gains in heating seasons and avoid overheating issues in summertime. the dsf variant is equipped with a thermal storage mass in the dsf cavity and interior insulation. the thermal mass, in this case concrete, is of a dark colour for high solar absorption, whereas the shading device is highly reflective. in contrast to traditional trombe wall systems, this tcw is not supposed to actively heat interior space or transfer thermal energy. instead, the tcw aims to regulate heat flux within the façade level by the management of solar thermal energy fluxes. the potential to reduce buildings’ heat losses through solar energy use is shown and compared to a traditional external thermal insulation composite system (etics) with an appropriate insulation thickness for renovation purposes in switzerland. the u-value is therefore considerably lower, 0.25 instead of 0.41 for the tcw. due to the innovative design and fully transient operation, a highly detailed and flexible simulation tool is needed to analyse and assess the façade performance. the decision to simulate the novel system was made for modelica-dymola, with its object-oriented, equation-based simulation language. the simulations of both tcw and etics show potential for heat loss reduction due to solar energy storage on every orientation. however, the tcw shows a high solar energy usage due to its ‘natural’ overheating tendency. furthermore, heat losses are significantly lower than the u-value predicts and, in some cases, even lower than the etics heat losses. in addition, due to its lower use of material and lower weight, the system can be used as a curtain wall system instead of traditional dsfs, which have higher heat losses in winter and higher solar gains in summer. keywords passive façade, low-tec, energy efficiency, adaptive façade, performance gap, adaptive g-value doi 10.7480/jfde.2019.1.2619 002 journal of facade design & engineering volume 7 / number 1 / 2019 1 introduction in recent decades, there has been a constant rise in awareness of energy. according to the international energy agency (2013), the largest energy consumer is the building sector, which is responsible for one-third of global energy consumption and carbon dioxide emissions. if energy efficiency is not improved, the energy demand is expected to rise by 50% by 2050. however, the eu energy roadmap 2050 (european commission, 2012) states that the buildings that will be in use in 2050 are being designed and built now. to achieve those energy goals, higher energy efficiency in new and existing buildings plays a key role. in addition, zero energy buildings (zeb) should become the standard for new buildings. achieving high energy efficiency in buildings is a complex process with various influences. the role of its envelope, as a physical separator between controlled and uncontrolled environments, is an important factor. a façade controls the energy and mass transfer between indoors and outdoors, making it one of the most promising solutions for the improvement of energy consumption in buildings. therefore, improved façades will lead to lower energy demand for cooling and heating. furthermore, the façade can support hvac systems and reduce their size. undoubtedly, more efficient hvac installations also contribute to a building’s energy savings. however, the façade still makes the highest demands on hvac energy. as perino & serra (2015) emphasise, new and more revolutionary concepts and technologies to achieve a zeb or nearly zeb standard need to be developed. various research efforts in the area of energy efficient and / or adaptive façades are in progress. while some focus on the limits of available simulation tools (loonen, 2010), others explore the potential of smart materials, such as a phase change material (pcm) filled double glazing unit (goia, zinzi, carnielo, & serra, 2015) or other smart windows (casini, 2015) for improving the energy efficiency of buildings. furthermore, kinematic (schleicher et al., 2011; suralkar, 2011) solutions are being researched. however, the possibilities of active or passive solar energy use within façades are also under exploration. for this topic, a review of quesada rousse, dutil, badache, & hallé (2012) gives a good overview of opaque solar façades, which have significant potential to reduce heating loads. thermally active solar façades usually need fans or pumps to distribute the solar thermal gains for heating, cooling, or storage purposes. passive systems instead utilise natural processes of heat transfer (conduction, convection, radiation). the most popular solar walls are trombe walls and its variants. trombe walls are rather simple constructions elements; they consist of an external glass layer and an internal, high absorptive thermal mass, divided by an air gap, which uses the greenhouse effect. active or passive controllable flaps enable the natural airflow to be controlled and, therefore, the internal heat gain. hu, he, ji, & zhang, (2017), quesada et al. (2012), and omrany, ghaffarianhoseini, ghaffarianhoseini, rhaahemifar, & tookey (2016) agree that trombe walls (tw) have the potential to significantly reduce a building’s energy consumption. however, most tws are often considered as massive, thick walls that allow significant thermal transfer to the room. this study introduces a new tw façade with solar thermal storage, which can be used as “lightweight” curtain wall façade element. in contrast to earlier attempts, this new tw façade renounces to “heat” interior spaces; instead, its purpose is to passively balance out thermal heat gains on the façade level. in addition, this study aims to use low-tech solutions as far as possible. due to the low-tech principle, this feasibility study of a trombe curtain wall element is strongly related to currently available materials and elements. 003 journal of facade design & engineering volume 7 / number 1 / 2019 the façade is based on non-ventilated double skin façade (dsf) elements with a high reflective shading device. the inner insulated glazing unit (igu) layer of traditional dsfs is replaced by a 50mm thermal mass and 75mm internal insulation. this configuration allows it to control heat fluxes trough the façade on seasonal, daily, or hourly levels. while the “natural” overheating tendency of dsfs is used to load the thermal storage in heating seasons, the high reflective shading device, in cooperation with the thermal mass, reduces overheating in cooling seasons. this enables the trombe curtain wall (tcw) system to actively employ an adaptive g-value. in this paper, the performance of the new tcw prototype is compared to a traditional external thermal insulation composite system and shows the potential of solar energy use to reduce heating demands of buildings. this is an extract of the original project “solar energy balanced façade” (sebf), in which the façade element is partly transparent and the tcw element is used as a parapet. 2 methodology to investigate the thermal performance of this new tcw prototype, a detailed dynamic calculation model must be applied. unfortunately, most building performance simulation (bps) tools are not supposed to investigate such a dynamic system in a detailed way. therefore, the decision was made to use modelica, an object-orientated, equation-based language that can describe physical systems in various domains. detailed descriptions of the various advantages of equation-based modelling with modelica can be found in wetter (2011). one of the features is the possibility to encapsulate physical functions into classes, which can be tested separately and reused to build up systems that are more complex. in addition, the user can concentrate on describing the physical constraints instead of describing the intended performance. because of its high level of detail, the study is undertaken only at a façade level. 2.1 modelling approach the trombe wall system is divided into its four main elements described in the simulation model (see fig. 1): b) external glass layer; c) air cavity with shading device; d) thermal mass; e) insulation layer. the external glazing (fig 1.b) is modelled as a pane with two surface nodes and a centre node, connected by two conduction elements. the mass of the glazing is concentrated at the centre node, where the solar heat gain is applied. so far, these are standard modelica library elements with extended parameters to simplify model changes and encapsulation. the implemented optical model uses direct radiation, diffuse radiation, and the angle of incidence to calculate the fresnel corrected reflection and transmission of the glazing. 004 journal of facade design & engineering volume 7 / number 1 / 2019 fig. 1 modelica model scheme for the four main components of the trombe wall system. bottom row, from left to right: a) external surface heat transfer, b) single glazing element, c) air cavity, d) storage mass, e) insulation, and f) interior surface transfer. upper row: detailed scheme for the air gap model, two models are combined: a) air gap without shading and b) air gap with shading. both possible heat fl uxes are completely modelled by one (or two) convective and radiative (standard) conduction elements based on the iso 15099 formulation (section 8.3.2.2 and 8.4.3.1). the shading device is modelled in the same manner as the single glazing, two conductors, one mass, and heat gain elements. due to the small thickness (1mm) and high conductivity (λ=160w/mk) of aluminium, this is regarded as a valid simplifi cation. all conduction elements in the air gap were connected to the shading control by a shading area factor (0…1). in case of activation or deactivation of the shading device, a critical damping element smoothens the transition from open to closed, or vice versa. the optical model is also connected to the shading area factor to calculate transmission, refl ection, and solar heat gain on the shading mass. the wall is divided into 10 equidistant conductors and 11 masses, nine of which represent 1/10th of the total thickness with the fi nal two representing 1/20th of the total thickness as surface elements. the solar heat gain is applied to the outer surface mass, depending on the surface’s characteristics. the insulation layer is a simplifi ed wall model in which only fi ve conductors are used instead of 10. this also applies to the masses, four of which represent 1/5th and two represent 1/10th of the surface. internal and external surface heat exchange is modelled according to en 15099 with a radiation temperature equal to air temperature. the defi nition of those basic elements allows the main components to be reconnected to a comparison model of a standard insulated wall system (see section 2.3). 2.2 element description for the investigations shown in this paper, the results are based on elements of 2.5m², with 1m width and 2.5m height. to compare the results, two elements are considered: the trombe curtain wall element and a standard insulated wall for renovation purposes (sia 380/1, 2009). the trombe curtain wall (tcw) element consists of: – 6mm external glazing – 150mm air gap (shading mounted centrally) – shading device (1mm aluminium) – 50mm of concrete – 75mm insulation – u-value: 0.44 / 0.41 w/(m2k) (without and with shading) 005 journal of facade design & engineering volume 7 / number 1 / 2019 the second wall, a usual external thermal insulation composite system (etics); – 15mm plaster – 150mm insulation – 150mm concrete – u-value 0.25 w/(m²k) 2.3 material properties each material used is considered in terms of “common” values from practice, according to related literature (see table 1). this is not to distort the results of the comparison with standard façades. the adjustment of material properties and therefore the improvement of the façade performance are topics for further investigations. material ρ [-] τ [-] λ [w/(mk)]] ρ [kg/m3] c p [j/(kgk)] ε [-] d [mm] glass (orthogonal) 0.08 0.82 1.0 2700 750 0.84 6 insulation 0.5 0 0.04 80 600 0.9 75 concrete 0.8 0 2.1 2400 1000 0.9 150 / 50 plaster 0.5 0 0.87 1600 1000 0.9 15 shading device 0.85 0 160 2800 880 0.9 1 table 1 material properties 2.4 boundary conditions internal air temperature is set to 22 °c, and influences of occupancy, which lead to temperature variations, are not considered here. the climatic outdoor conditions are given by a standard design year for zürich (ch) on an hourly basis from meteonorm. all parameters, such as temperature, wind speed, façade irradiation, and sun position, are interpolated linearly from hourly values. 2.5 shading control the tcw makes use of solar gains in cold periods and prevents overheating during warmer periods, for which a customised shading control is responsible. first, the shading control must decide whether it is a heating or cooling period. therefore, the mean value of external air temperature over 24 hours is used as deciding factor. if the mean outside temperature is below 12 °c, which is more or less the heating limit temperature in switzerland, the ‘heat gain’ mode is activated. consequently, the ‘avoid heat gain’ mode is enabled for mean temperatures above 12°c. for mean outside temperatures higher than 15°c, the system enlarges night time heat losses. to prevent heat losses, the shading is activated at night time (global radiation < 25 w/m²) with a mean outside temperature below 15°c as an additional thermal resistance. during daytime with 006 journal of facade design & engineering volume 7 / number 1 / 2019 solar irradiation >25 w/m² and mean outside temperatures below 12 °c, the high absorptive thermal storage accumulates solar energy. to avoid solar gains, the high reflective shading device is activated on days with mean outside temperatures above 12°c. in addition, during nights with mean outside temperatures above 15°c, the shading device is disabled to enlarge heat losses and ‘pre-cool’ the thermal storage. this tcw control strategy is determined to react to seasonal needs. at the moment, this is undoubtedly a simplified approach to prove the feasibility of the system for the central european climate. further investigation to define the right set points and, perhaps, additional input parameters is in progress. 3 results in this chapter, the simulation results are presented. to begin with, the systems are simulated in a detailed transient manner without solar gains in order to compare it to the u-value calculation. based on those “no-sun” results, the solar energy potential for each façade system in the four main orientations is presented. finally, the orientation-dependent transient results are compared directly. 3.1 non-solar results based on chapters 2.2 and 2.3, the u-value of the tcw system is 64% higher. the transient simulation, without solar gains, shows similar results to the simplified calculation (see table 2). however, the shading control system within the tcw façade receives the same data and therefore provides the same control output as when solar gains occur. the relatively higher heat losses in summer months (june to september) is due to the night-cooling mode and therefore the discharged thermal storage. furthermore, the shading device is closed during “sunny” hours and reduces daily heat gains due to higher outside temperatures. jan feb mar apr jun jul aug sep oct nov dec massive -9.2 -8.0 -7.6 -5.6 -3.9 -2.1 -1.6 -1.7 -3.2 -5.0 -7.3 trombe -15.0 -13.0 -12.4 -9.4 -6.6 -3.9 -3.1 -3.2 -5.7 -8.5 -11.9 % 163% 163% 163% 167% 171% 185% 194% 192% 179% 170% 164% table 2 heat flux through façade elements per month, without solar influences in [kwh] per element the u-value prediction is confirmed and an overheating of the dsf is, due to the lack of solar irradiation, not observed. 3.2 solar energy potential for the etics the solar potential is strongly dependent on orientation (available solar irradiation) and solar absorption. in this study, the solar absorption coefficient of the massive wall system is assumed to be 007 journal of facade design & engineering volume 7 / number 1 / 2019 0.5, which is relatively high for painted surfaces and low for bricks. however, it is an average value to demonstrate the solar potential and to provide a comparison with the trombe wall system. as shown in fig. 2, the solar irradiance has an infl uence on all façade orientations. for the four main directions (north = 0°, east = 90°, south = 180°, and west = 270°), a reduction of at least 2% and increases up to 135% in the monthly energy losses are observed, where a reduction of 100% means that no energy losses occur and values over 100% mean that gains occur. for the heating demand, the winter months (december to february) are considered. within this time range, north, east, and west façades show a reduction of less than 10%. the south facing wall, shows a signifi cant reduction of 9% to 17%. for the summer period, east, south, and west can turn into zero or even plus energy façades. fig. 2 monthly energy losses of the etics wall element 3.3 solar energy potential tcw the tcw system is proposed to increase solar gains during heating season and to prevent overheating in cooling periods. the material properties in section 2.2 and shading control strategy in section 2.3 support this performance. fig. 3 presents the monthly energy losses for tcw under identical boundary conditions as the etics. the reduction in energy losses of a north facing tcw in the winter months are at least 14%, with maximum of 29%. south and west facing walls perform between 21% and 60% compared to the system without solar gains. the south facing tcw reduces heat losses in a range between 59% and 111%. this means that a south facing tcw produced solar gains in february instead of losses. over all, the tcw presents a highly eff ective use of available solar energy in wintertime. this is mostly due to the exploitation of the dsfs overheating tendency. additionally, due to the highly refl ective shading device, overheating and unwanted solar gains during the cooling period are mainly avoided. the maximum reduction in heat losses reaches 160% for the east facing tcw in july. the results in fig. 3 support the claim that the tcw energy balance is seasonally adaptive. 008 journal of facade design & engineering volume 7 / number 1 / 2019 fig. 3 monthly energy losses of the tcw façade element 3.4 tcw and etics comparison for a comprehensive performance evaluation, the transient results of tcw and etics are compared directly. as mentioned in chapters 2.3 and 3.1, the tcw is supposed to lose 64% more heat than the etics. whereas the etics is still a static construction, the tcw is seasonally adaptive. figs. 4 to 7 present the monthly energy balance of tcw and etics for the four main directions. north, east, and west facing tcws still perform worse than etics in wintertime. the northern tcw’s energy loss is only about 21% to 43% higher than the etics. for east and west directions, the tcw loses a maximum of 25% more energy than the etics. the southern tcw performs consistently better than the etics. in spring and autumn, the tcw system on all orientations is more effi cient than the etics. even north facing façades (fig. 4) show lower heat losses in march and april, while the east and west facing ones (figs. 5 and 7) show lower heat losses than in the february to october period. the south facing elements, depicted in fig. 6, demonstrate even positive energy balances in february to april. the summer performance of the tcw shows no signifi cant overheating problem when compared to the etics façade. in many cases, the energy balance is on a similar level. only east and west façades show signifi cantly higher heat gains up to 1.9 kwh instead of 0.6 kwh for an etics element (july, east, see fig. 5). 009 journal of facade design & engineering volume 7 / number 1 / 2019 fig. 4 monthly energy losses of tcw and etics with solar gains for north orientation fig. 5 monthly energy losses of tcw and etics with solar gains for east orientation fig. 6 monthly energy losses of tcw and etics with solar gains for south orientation fig. 7 monthly energy losses of tcw and etics with solar gains for west orientation 010 journal of facade design & engineering volume 7 / number 1 / 2019 4 discussion & conclusions undoubtedly, the comparison between those two façade elements seems to be “unfair”. on the one hand, we have twice the amount of insulation and, on the other hand, a dsf system that is predicted to overheat in summertime. following these arguments, the tcw system’s energy performance has no chance. goia, romeo, & perino (2017) showed that simplified metrics (uand g-values) are not always suitable for determining the performance of advanced façades. the presented results show that both systems, tcw and etics, can benefit from solar irradiation to reduce heat losses. in case of the etics, the reductions during heating seasons are constantly low, with the exception of south facing walls, whereas the tcw system presents a continuously high use of solar energy. the tcw presents higher solar gains than the etics, due to the utilisation of dsf’s tendency to overheat to charge the thermal storage within the cavity. in summertime, both systems, tcw and etics, show considerably lower heat losses or even solar gains. however, the g-value of the tcw with closed shading is below 0.02, which is hardly reachable in a dynamic environment. furthermore, this is still one quarter of a traditional fully glazed closed cavity façade (ccf) (rudolf, 2015). this paper has shown that even if the tcw u-value is significantly higher, the transient energy simulation shows a high potential for solar thermal energy use to reduce energy losses of buildings at the façade level. the tcw construction tries to exploit the full potential of the used materials by using an unconventional redesign of a dsf system. in addition, the lower material usage leads to lower weight, which makes it feasible to be used as curtain wall system. however, the material properties have not yet been tuned to their limits. to sum up, even if the system’s potential is not yet fully revealed, the tcw’s potential to contribute to the future energy challenges in the building sector is evident. further investigations on shading control, material selection, optical, and radiative properties shall study additional enhancements of the system during each season. the main sebf project is still in progress. the first simplified laboratory tests on small-scale function models confirmed that the main simulation components are operating correctly. a 1:1 mock-up with a tcw parapet is currently being built to perform measurements and validate the simulation model. more detailed experiments to improve the sebf / tcw system and the simulation model are planned in the future. acknowledgements the authors gratefully acknowledge the funding provided by the swiss national science foundation (snf grant no. izcnz0-174562) and support of the lucerne university of applied sciences and arts. references casini, m. (2015, april 30). smart windows for energy efficiency of buildings. international journal of civil and structural engineering ijcse, 2(1), pp. 230-238. european commission. (2012). energy roadmap 2050. luxembourg: publications office of the european union. doi:10.2833/10759 goia, f., romeo, m., & perino, m. (2017). simplified metrics for advanced window systems. effects on the estimation of energy use for space heating and cooling. energy procedia 122, pp. 613-618. goia, f., zinzi, m., carnielo, e., & serra, v. (2015). spectral and angular solar properties of a pcm-filled double glazing unit. energy and buildings (87), pp. 302-312. hu, z., he, w., ji, j., & zhang, s. (2017). a review on the application of trombe wall system in buildings. renewable and sustainable energy reviews 70, pp. 976-987. 011 journal of facade design & engineering volume 7 / number 1 / 2019 international energy agency (2013). transition to sustainable buildings strategies and opportunities to 2050. paris: international energy agency. loonen, r. (2010). climate adaptive building shells. eindhoven: technische universiteit eindhoven. omrany, h., ghaffarianhoseini, a., ghaffarianhoseini, a., rhaahemifar, k., & tookey, j. (2016). application of passive wall systems for improving the energy efficiency in buildings: a comprehensive review. renewable and sustainable energy reviews 62, pp. 1252-1259. perino, m., & serra, v. (2015). switching from static to adaptable and dynamic building envelopes: a paradigm shift for the energy efficiency in buildings. journal of façade design and engineering (3), pp. 143-163. quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012). a comprehensive review of solar façades. opaque solar façades. renewable and sustainable energy reviews 16, pp. 2820-2832. rudolf, b. (2015). closed cavity fassaden und d3-fassaden: geschlossene zweiund dreischalige fassaden [closed cavity and d3 facades: closed double and triple layered facades]. bauphysik [building physics] (37), pp. 244-249. schleicher, s., lienhard, j., poppinga, s., masselter, t., speck, t., & knippers, j. (2011). adaptive façade shading systems inspired by natural elastic kinematics. london: international adaptive architecture conference. sia 380/1. (2009). sia 380/1 thermische energie im hochbau [thermal energy in building construction]. zürich: schweizer ingenieurund architektenverein [swiss society of engineers and architects]. suralkar, r. (2011). solar responsive kinetic facade shading systems inspired by plant movements in nature. in proceedings of conference: people and buildings held at the offices of arup uk, 23rd. wetter, m. (2011). a view on future building system modeling and simulation. in j. l. hensen, & r. lamberts, building performance simulation for design and operation. london uk: routledge. 012 journal of facade design & engineering volume 7 / number 1 / 2019 journal of facade design and engineering 1 (2013) 85–95 doi 10.3233/fde-130003 ios press 85 tectonic thinking in contemporary industrialized architecture anne beim∗ cinark – centre for industrialized architecture, institute of architectural technology, the royal danish academy of fine arts – school of architecture, copenhagen, denmark received: 6 june 2013 accepted: 16 october 2013 abstract. this paper argues for a new critical approach to the ways architectural design strategies are developing. contemporary construction industry appears to evolve into highly specialized and optimized processes driven by industrialized manufacturing, therefore the role of the architect and the understanding of the architectural design process ought to be revised. the paper is based on the following underlying hypothesis: ‘tectonic thinking – defined as a central attention towards the nature, the properties, and the application of building materials (construction) and how this attention forms a creative force in building constructions, structural features and architectural design (construing) – helps to identify and refine technology transfer in contemporary industrialized building construction’.1 through various references from the construction industry, business theory and architectural practice the paper offers various analyses, comparisons and concrete design approaches. how architectural design processes and the tectonic design can benefit from integrated product deliveries, mass-customization and design for disassembly is examined and discussed. the paper concludes by presenting a series of arguments that call for adaptable systems based on sufficient numbers of industrialized building products of high quality and a great variety of suppliers, and point at the need for optimizing our use of resources in order to reach sustainable solutions in architecture. keywords: architectural design, facade design, design, construction, construction industry, building envelopes, facades, residential building, standards, sustainable development, project management, environmental impact, life-cycle 1. introduction when speaking about the future demands for industrialized architecture – or how to translate industrialized processes into tectonic sustainable design strategies in architecture – several questions come to mind. first of all, why is the building industry in comparison to the production industry of consumer goods, apparently developing slowly in terms of incorporating new technologies, products and practices? secondly, which general obstacles can be identified – and finally what are the potentials, which seem to be embedded in knowledge production of related industries and professions? ∗corresponding author: professor anne beim, ph.d., cinark – centre for industrialized architecture, institute of architectural technology, the royal danish academy of fine arts – school of architecture, phillip langes allé 10, dk-1435 copenhagen, denmark. tel.: +45 4170 1623; e-mail: anne.beim@kadk.dk. 1this definition of tectonic thinking forms part of a large, central research project: towards a tectonic sustainable building practise, that is presently (2010–2014) executed in collaboration between; the royal danish academy of fine arts – school of architecture, aarhus school of architecture, and the danish building research institute. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:anne.beim@kadk.dk 86 a. beim / tectonic thinking in contemporary industrialized architecture although these questions raise a widespread discussion, one could argue that the building industry can benefit from different ways of architectural synthesis thinking as a basis for improving. this understood in such a way that industrialized manufacturing technologies and products should be driven by ideas and innovation, which enrich and improve the building designs – the architecture – both in terms of customization when manufactured, adaptability, sustainable solutions and aesthetics. in the following, this series of questions and arguments will be further analyzed and discussed primarily in order to address the heading of this article, but also quite importantly in order to test the underlying hypothesis that: tectonic thinking can be used as an explicit design strategy. 1.1. the meaning of industrialization in contemporary building industry if we look at contemporary building industry, it is generally characterized by a great number of parallel domains of knowledge, technologies and practices. due to this fundamental condition, it holds a series of insoluble, but also interesting paradoxes. today’s building industry, embraces highly advanced levels of technologies yet one can find construction practices and technical solutions that have not changed noticeable for many centuries. to exemplify, nano technology used for improving the properties of glazing surfaces can be built into a structure next to traditional stone masonry. similar to other manufacturing industries that provide for fundamental human needs, the building industry is resting on the shoulders of previous technologies and traditions of manufacture. as such, the building industry is rooted in the crafts, practices and codes of conduct that arise from particular economies, societal systems and cultural settings. layers of improved technologies are continuously added and the use of new materials and manufacturing procedures are expanding the existing fields of knowledge and practices. it is therefore predominantly characterized by forming an evolutionary track. only rarely we see radical changes – or significantly new methods and technologies implemented ‘over night’ in the building industry. as strike notes some of the most inventive ideas in terms of improving building technology that have informed and changed the building industry have been envisioned by architects. this influence was in particular prevailing in the late 19th and 20th centuries. due to its heterogeneous and ‘slow-moving’ nature, the building industry is often mentioned for its lack of true technological sophistication and for its shortcomings in terms of productivity, when compared to other industries of consumer products – the car industry or ship building industry in particular. for the past century, the car industry and ship building industries have been pointed out as comparable industries, when speaking about productivity and future manufacturing for buildings. however, this idea is usually argued for from an economically point of view – not taking into account that buildings are fundamentally different being prototypes, grounded on a specific site, rooted in a cultural and societal context, as well as they are planned to last for decades or maybe centuries. due to these critical differences, the car industry and ship building industries do not seem sufficient models for the building industry. however, these conclusions are not truly valid. the poor reputation partly has to do with statistical logics of categorization. when building trades grow to be industrialized, e.g. the window industry, they usually move from the building industry category to the product industry category (at least in the danish statistical systems). in that sense, the statistical records of the building industry will always bear the mark of ‘the poor performative parts’ that seems to remain: the complex inter-disciplinary fields of knowledge that have to collaborate on the building designs/execution and issues that concerns procedures on the site, will always seem insufficient in regard of conventional economical productivity, and will always figure in the statistics. a. beim / tectonic thinking in contemporary industrialized architecture 87 fig. 1. wall assembly at the berchtold holzbau factory in voralberg. there is no doubt that the building industry has developed significantly in terms of industrialized manufacturing. in particular, the introduction of digital technologies has provided new and different ways of fabrication through the past couple of decades. these new digital technologies make long series of identical objects unnecessary, industrially manufactured components can be unique, and optimized to fit a particular construction design (fig. 1). this counts both for smaller objects and large scale; including advanced production of integrated building components; complex volumetric building elements as well as different sorts of all-encompassing construction solutions. in addition, a great number of building practices including various trades at the construction site have become more ‘automized’ and have been developed into more efficient fabrication procedures by use of mechanized tools and gear that improve the execution of the physical construction as demonstrated by kieran & timberlake (2004). 2. mass production – mass customization custom made inspired by a tendency in the product industry contemporary building industry is slowly moving from an ‘old-fashioned’ manufacturing concept of mass production – to a more flexible manufacturing concept of mass customization. where mass production primarily is directed towards an economical logic of efficiency of organization and mechanical manufacturing procedures, which depends on standardization, repetition, reduction of variables, and a market of ‘anonymous masses’ (taylorism).2 mass customization is rather based on a consumer oriented economical rationale, making use of 2“taylorism was a theory of management that analyzed and synthesized workflows. its main objective was to improve economic efficiency, especially labor productivity. it was one of the earliest attempts to apply science to the engineering of processes and to management. its development began with frederick winslow taylor in the 1880s and 1890s within the manufacturing industries. although scientific management as a distinct theory or school of thought was obsolete by the 1930s, most of its themes are still important parts of industrial engineering and management today. these include analysis; synthesis; logic; rationality; empiricism; work ethic; efficiency and elimination of waste; standardization of best practices; disdain for tradition preserved merely for its own sake or merely to protect the social status of particular workers with particular skill sets; the transformation of craft production into mass production; and knowledge transfer between workers and from workers into tools, processes, and documentation”; http://en.wikipedia.org/wiki/scientific management. management http://en.wikipedia.org/wiki/scientific_management 88 a. beim / tectonic thinking in contemporary industrialized architecture computer-based configuration for standardization. here the object is to meet the market forces of individual needs.3 mass customization forms a compromise between highly industrialized manufacturing processes and a customized end-user perspective. yet, one has to keep in mind that the manufacturing processes and products are still very standardized in a number of ways. to some extent mass customization – can be regarded as based on standardization of processes instead of products. in this interpretation, the products are no longer off-the-shelf items, but now liberated from a purely mechanistic logic to also include variation in product properties, flexibility in design solutions and development of added services and branding strategies linked to the product. this way of thinking in contemporary production industry leads to new sorts of products and supplies that challenge the traditional or existing premises of the construction industry, which is permeated by the contracting culture of procurement and tendering. however, in the building industry a quite flexible – but as mentioned earlier by no means efficient – production system already exists and the products and construction solutions (designs) are for the most part individually customized. as such, the building industry is still very dominated by the custom made. this is not in order to fulfill the needs of the end-user defined by an individualized market, but rather to satisfy a particular client. as such, this notion is much in line with an architectural rationale – which is oriented towards the specific project and a market rooted in specified and cultural values. the idea of mass customization in regard to the building industry is however still interesting, where the manufacturers of consumer goods seek to individualize the mass produced, the objective of the building industry will be to mass-produce the individualized as stated by beim, nielsen and vibæk (2010). by adopting the concept of mass customization, although from the side of the unique – the building industry becomes more aligned with the production industry, both in terms of manufacturing, quality control and business strategies. from an architectural point of view, it is particularly important to focus on the flexibility of digitalized industrial manufacturing, the quality improvement, and the end-user perspective, when taking part in this inevitable development of the building industry. kieran and timberlake (2004) have described how the architectural project design can benefit if architects take on the responsibility of developing the building industry. 2.1. system products and integrated product deliveries – a new sort of building industry? mass customization is also conceptually applied when developing integrated product deliveries (ipd), which refer to a new industrialized article of trade that can be identified as, ‘a multi-technological complex part of a building’ that can ‘be configured and customized’ to a specific construction project. it is furthermore ‘developed in a separate product development process based on the principles of integrated product development’ (fig. 2). in it’s actually produced and specifically customized state and when delivered to a customer this building assembly becomes an integrated product delivery that – as a kind of super level – also can include ‘marketing, shipment and servicing’ as described by mikkelsen, beim, hvam & tølle (2005). a building element or construction system based on integrated product delivery (ipd), holds a well integrated, structured organization of knowledge disciplines and systemized 3“consumerism is a social and economic order that is based on the systematic creation and fostering of a desire to purchase goods and services in ever greater amounts. in economics, consumerism refers to economic policies placing emphasis on consumption. in an abstract sense, it is the belief that the free choice of consumers should dictate the economic structure of a society. the term “consumerism” was first used in 1915 to refer to ‘advocacy of the rights and interests of consumers”’. http://en.wikipedia.org/wiki/consumerism. http://en.wikipedia.org/wiki/consumerism a. beim / tectonic thinking in contemporary industrialized architecture 89 fig. 2. the development towards an integrated product delivery. illustrated by an i-phone and an intelligent façade system. processes, it is supported by use-driven innovation, and have service elements and business strategies incorporated. to sum up: • a system product is a complex (sub-)system in buildings developed as finished product • a system product is made for mass customization and thus to a certain extend it can be fitted to individual needs and demands. customization is prepared by computer-based configuration! • a system product is an industrialized product – developed product-wise and process-wise via an industrial process from configuration through use to facility management • an integrated product delivery is a customized delivery of a configured system product for a building • a system product is a thoroughly designed and quality proofed product! however, where is the challenge in order to incorporate this advanced technological aim into the building industry? it is curious to see that building systems of today for the most part are conceptualized as whole units or modules (physical) – seen as large parts of the buildings physical structure; e.g. facade elements, ceiling panels, load bearing constructions. the aim when developing a system product for an integrated product delivery is to raise the system product to a higher level of functionality, which is integrating several features or performances. these could be developed for indoor climate systems, energy supply systems, laboratory spaces or bathroom pods. it is important to understand that this is a new way to develop products and systems for buildings and it relies on functional thinking and connection to complex systems. it is based on new product ideas that are created in the dialogue between different professions/disciplines, where the challenge is to create coherence between various fields of knowledge, technologies and building systems. part 90 a. beim / tectonic thinking in contemporary industrialized architecture of the objective is to develop new knowledge based products that can be sold at an international market. mikkelsen, beim, hvam & tølle (2005). 2.2. product development – beyond the development of the product? the american economist joseph pine (1992/2007) and the german/american professor in business theory, frank piller have both in various ways offered a number of theoretical models within this field of business management theory. the latter is a central figure in the “international institute on mass customization and personalization” (iimcp or short: mcp institute). it is a society to provide a platform for interaction between researchers and practitioners on mass customization, personalization and related issues. the proposed models sustain a development towards business strategies that draw upon the (end) users as creators for innovation. the theoretical framework is primarily meant for the progression of economic value in the industry of consumer goods, but similar ideas are now also being picked up and adjusted to the specific features of the building industry in research environments of construction management and architectural management. in order to explain pines ideas further they are tested on a velux skylight window system (fig. 3). where the traditional building industry, producing building components and construction elements, usually focus at the first two levels of commodities and goods in terms of what to offer to the client, the manufacturer of an integrated product delivery offers a customized delivery of additional levels of product development. these can be purchased supplementary and will be delivered as unique addons, provided by a specific sales organization and delivered as a ‘special’ delivery. in the case of the velux skylight window, it can be particular services (e.g. extended warranties, maintenance, facility management etc.), developed and customized according to the clients needs. similarly, a specific experience of daylight can be offered by a customized combination of solutions and services. the fig. 3. model for customization and commoditization, by joseph pine, 2000 tested on velux skylight window systems. a. beim / tectonic thinking in contemporary industrialized architecture 91 final level of customization is directed towards transformation. this can be new ways of using the architectural space, provided by a customized series of former deliveries. another part of pines theory described in fig. 3 shows how these various levels of output can be transformed into commoditization. these can developed closely related to the integrated product delivery or can be developed as new separate supplies. from the experience level, one can develop lifestyle packages; from the service level one could develop special packages combining e.g. warranties and maintenance. at the level of goods, the commoditization could be the branding of the product as maybe highly sustainable, long lasting materials, which offers the right sentiment about the product. in conclusion, it requires creative imagination and synthesis thinking to develop the product beyond the development of the product. 2.3. translation of industrialized processes into tectonic design strategies in architecture fredrik nilsson a professor in architecture at chalmers, and head of r&d at white architects, which is sweden’s largest architectural office, has researched into building construction and design procedures in architecture in great depth and how these are being influenced by contemporary industrialized processes and manufacturing. nilsson (2007) has put forward the argument that industrialized architecture can be seen as a tectonic strategy: “prefabricated elements can today be made optimal and unique, following the lines of forces in construction and having other geometries, opening up possibilities for new architectural expressions as well as more economical, resource efficient and sustainable building. here are new possibilities for interesting development of different tectonic expressions in architecture.” (2007, pp. 2-3). in addition to this, nilsson points out the tectonic discipline as a useful way to approach the new challenges of the building industry. in this article described as, resource management, industrialized processes and manufacturing, integrated product deliveries, commercialization of building from commodity to architectural experience. as a reply to these serious challenges, nilsson highlights an argument by the american architectural theoretician, kenneth frampton, which is presented in his book: studies in tectonic culture: the poetics of construction in nineteenth and twentieth century architecture: “[. . . ] our built environment is produced in interplay of three aspects – topos, place; typos, building type; and the tectonic. the tectonic is according to frampton the aspect best suited to counter present tendencies to legitimize architecture in discourses outside its own discipline” (1995). as stated in the introduction of this article – tectonic thinking concerns the materials/resources and their use in an architectural creative context. by giving emphasis to how buildings are made and how this making is detectable in the building, tectonic thinking can be pushed forward as a way to challenge the standardization and commodification of building production. this understanding has also been pushed forward by torben dam (2007) a landscape researcher, who has looked into how tectonics make meaning in landscape architecture. here ‘the making’ is specifically related to site and context. in many ways, contemporary industrialized manufacturing can be compared to some of the characteristics in tectonic thinking such as: resource thinking, process thinking, and system thinking. each dimension considered from a synthesis perspective. as such it also forms a mode of practice or conduct that is embedded in architectural making. tectonic thinking can be defined as a key that both hold on to the essential nature of architectural making, as well as it provides a ‘systemized language’ of principles by which architects can specify 92 a. beim / tectonic thinking in contemporary industrialized architecture technical knowledge used for technology transfer and hereby for developing new products for the building industry. a new attempt to translate and conceptualize industrialized building components and processes into tectonic design strategies in architecture and for the building industry, has been developed in the industrial phd-study of søren nielsen, who is architects and partner in the danish architectural firm, vandkunsten. in this study he focuses on: adaptive architecture and sustainability: adaptability as tectonic strategy. as part of his case study for a new façade cladding system that can be re-used, he has tried to develop a design/construction strategy for design for disassembly (dfd). this study has developed into a sort of language or rather syntax for a construction of knowledge relating to three stages of production: 1. production of systemized technical protocols for programmatic purposes, consisting of diagrams 2. production of experimental knowledge through implementation of technical protocols, resulting in drawings, models, prototypes and buildings 3. production of analytical diagrams from case-studies resulting in documentation diagrams and pictograms the following three diagrams are results of the first and third stage of production. they show how søren nielsen (2011)systematically maps the principles of tectonic articulation and the technical fig. 4. diagram showing the tectonic and technical principles of a particular case study; drawing and prototype of a assembling detail for a reversible façade cladding panel, by søren nielsen, vandkunsten architects. a. beim / tectonic thinking in contemporary industrialized architecture 93 fig. 5. diagram showing the correlation between technical solutions, by søren nielsen, vandkunsten architects. fig. 6. pictogram showing tectonic articulation, by søren nielsen, vandkunsten architects. principles in protocols for empiric analysis (fig. 4). by scanning a number of cases in a matrix for tectonic and technical principles, he provides a method to identify the relations between the different principles. he describes the scanning process as generative or self-supplying because the taxonomy of building principles is generated as a gradually more precise tool when utilized. a central part of søren nielsen’s work is to extract the tectonic and technical principles from highly situated and specified solutions into more generic data. nielsen (2011) describes the extracting processes as following: “diagrammatic representation of architectural solutions: in technical diagrams a building, its part and details, can be dissolved into a number of singular variables with a general relevance for all similar operations. the diagram accounts for and documents programmatic requests, but reveals nothing about the tectonic articulation. however, the tectonic articulations can be identified as form-generating types of phenomena, which can be illustrated in pictograms and perhaps other abstract representation. the very relationship between tectonic articulation and technical principles remains situated, embedded in the material evidence of the practical cases”. by a systematic and visual approach as illustrated in figs. 5, 6, søren nielsen offers a transparent methodology, which shows how to develop various tectonic features of a specific construction solutions or architectural designs. the diagrams make it possible to follow the different principle levels or tracks of design decisions; also, they provide a clear (tectonic) terminology for communication 94 a. beim / tectonic thinking in contemporary industrialized architecture (technology transfer) and dissemination of a product or construction detail to other disciplines within the building industry. the future demands of industrialized architecture – concluding arguments. a fully developed industrialized architecture may help to transfer knowledge though systematic product development, as proposed in the models of integrated product deliveries (ipd). a fully developed industrialized architecture ought to be based on tectonic principles, focusing on the interplay of ‘construction and construing’. at product level of building components focusing on assembly of various elements at system level focusing on integration of various systems at the level of all-encompassing systems focusing on conceptualizing of various building constructions/designs (eg. light weight wooden elements) a fully developed industrialized architecture will have to be based on sufficient numbers of products of high quality and it requires a great variety of suppliers offering a higher level of system solutions. if so, it may create new international markets for integrated products and systems. a fully developed industrialized architecture may help to direct and optimize our use of resources – in terms of design for disassembly (dfd) and conceptualized as cradle to cradle in order to reach sustainable solutions. acknowledgments i want to thank søren nielsen, architect, and partner of vandkunsten architects, who in his present research into design strategies for sustainable construction – in this article also called tectonic design strategies – has inspired to this article and enriched the scientific field of architecture with vital knowledge. also, i want to thank kasper sanchez vibæk, associate professor in cinark and former colleague, for having developed the topic of integrated product deliveries from an architectural point of view by doing the hard work of challenging its rigid nature by scientific means instead of ignoring it due to academic fear or prejudice. references beim a., nielsen j., & vibæk k. s. (2010). three ways of assembling a house, cinark research, the royal danish academy of fine arts school of architecture publishers, 2010, 52 dam t. (2007). conference paper: does tectonics make meaning in landscape architecture, international conference: tectonics making meaning, university of eindhoven, dec. gilmore, j. h., & pine j. b. (2007). authenticity: what consumers really want, harvard business review press. kieran, s., & timberlake j. (2004). refabricating architecture: how manufacturing methodologies are poised to transform building construction, mcgraw hill co., ny. mikkelsen, h., beim a., hvam l., & tølle m. (2005). selia – systemleveranceri byggeriet – en udredning til arbejdsbrug (selia – integrated product deliveries in construction – a survey for project development); dtu, denmark’s technical university, institute of production & management. nielsen, s. (2011). ‘adaptability as a tectonic strategy’, in; the role of material evidence in architectural research: drawings, models, experiments. thomsen, r. m. (ed.) & beim a. (ed.), the royal danish academy of fine arts, schools of architecture, design and conservation. nilsson f. (2007). new technology, new tectonics? on architectural and structural expressions with digital tools, international conference: tectonics making meaning, university of eindhoven. a. beim / tectonic thinking in contemporary industrialized architecture 95 pine j. b. (1992). mass customization: the new frontier in business competition, harvard business review. strike, j. (1991). construction into design: the influence of new methods of construction on architectural design 1690 1990, butterworthheinemann ltd., oxford. journal of facade design and engineering 2 (2014) 19–32 doi 10.3233/fde-130010 ios press 19 tec – thin environmental cladding glass as functional facade element alan tomasia,∗, danijel mociboba, bert van de lindea, frank wellershoffb and kristian koldtoftc apermasteelisa group, vittorio veneto, italy bhafencity universität hamburg, germany cfiberline composites, middelfart, denmark submitted 7 may 2014 revised 16 october 2014 accepted 23 october 2014 abstract. permasteelisa group developed with fiberline composites a new curtain wall system (thin environmental cladding or tec), making use of pultruded gfrp (glass fiber reinforced polymer) material instead of traditional aluminum. main advantages using gfrp instead of aluminum are the increased thermal performance and the limited environmental impact. selling point of the selected gfrp resin is the light transmission, which results in pultruded profiles that allow the visible light to pass through them, creating great aesthetical effects. however, gfrp components present also weaknesses, such as high acoustic transmittance (due to the reduced weight and anisotropy of the material), low stiffness if compared with aluminum (resulting in higher facade deflection) and sensible fire behavior (as combustible material). this paper will describe the design of the tec-facade, highlighting the functional role of glass within the facade concept with regards to its acoustic, structural, aesthetics and fire behavior. keywords: facade design, structural design, structural analysis, building envelopes, curtain walls, glass facades, structural panels, building materials, glass, day-lighting, sustainable development, performance assessment 1. introduction tec, thin environmental cladding, is the latest technological development completed by the permasteelisa group r&d department. this is part of permasteelisa’s alter technology, which looks at innovative design systems and materials. more specifically, tec represents a curtain wall system entirely composed of pultruded composite material. the expert knowledge of pultrusion techniques and gfrp (glass fibre reinforced polymer) properties was provided by fiberline composites with a common and challenging objective: the development of a high performing, top quality product characterized by a unique architectural appearance. figure 1 shows the final product applied in the full-scale test area. as can be seen, gfrp material (and chosen insulating material for cavity filling) allows for high-diffused light transmission levels. this results in great aesthetic effects during night-time and in natural lighting during day-time. ∗corresponding author: alan tomasi, group r&d project manager, permasteelisa s.p.a., viale e. mattei 21/23 | 31029 vittorio veneto, treviso, italy. tel.: +39 0438 505207; e-mail: a.tomasi@permasteelisagroup.com; www.permasteelisagroup.com. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:a.tomasi@permasteelisagroup.com www.permasteelisagroup.com 20 a. tomasi et al. / tec – thin environmental cladding fig. 1. tec views: mock-up panels during day-time (a) and during night-time (b); demonstrator panel exposed at palazzo giustinian lolin in venice (c). as can be seen, in order to give to tec this unique and innovative light skin appearance, permasteelisa decided to openly show the gfrp on both facade faces, designing large, translucent framing and infill elements. this peculiarity – important for the characterization of the product – implied a challenging design work to keep the facade performance to the reference high levels. within the tec, glass has been applied not only as simple frame infill, but also as functional component to pass over the gfrp disadvantages and, at the same time, to improve the facade performances assuring the desired aesthetics. thin translucent gfrp surfaces need to be made heavy to achieve the desired sound insulation values. the only transparent material with adequate weight is glass, which has been included as an aesthetic shield on the external surface. to achieve the desired aesthetics, glass on framing has been laminated to gfrp plates using a cold lamination process (due to the maximal temperature that the gfrp can sustain, standard autoclave processes are not feasible). due to acoustic insulation, the decoupling of internal and external skins was mandatory, although coupling them would result in higher structural stiffness, increasing the effective inertia due to the composite effect. an intensive investigation on structural adhesives has been conducted with the purpose to find the best compromise (not too rigid, not too weak – acoustic decoupling vs. sandwich effect). the best solution has been found in a high modulus silicone, which assures sufficient stiffness, stable and durable, which allows reducing the thicknesses of the framing, proportionally reducing the cost and environmental impact of the product. a. tomasi et al. / tec – thin environmental cladding 21 another aspect where glass was exploited is to improve the fire reaction property of the facade skins. being a combustible material, the burning behaviour of gfrp is sensitive to its thickness. the glass pane laminated with the gfrp increases the capability of conducting the heat away from the burning area (glass is more conductive than gfrp), which decreases the burning rate, which allows reducing the gfrp plate thicknesses, maximizing the light transmission of framing and spandrel areas. 2. tec-facade design the tec-facade research project began with the idea of developing a simple and unique unitized facade system entirely composed by gfrp pultruded profiles. the pultrusion process allows very large shapes to be produced with good tolerances. this possibility pushed the design toward a solution a b fig. 2. tec design: preliminary design (a) and final design (b). 22 a. tomasi et al. / tec – thin environmental cladding composed by large, hollow mullions and a spandrel area composed by one entire hollow profile (fig. 2a). this solution, despite the intrinsic lack of flexibility in panel width (given by the width of each profile), would have given to the facade concept a unique and personal design, completely exploiting the possibilities given by the innovative material and production process. however, the design solution presented a few lacks, which caused a challenging design work to be improved. pultruded composite profiles are characterized by an important anisotropy, which become even more important when large sections are considered. material stiffness in orthogonal direction to the pultrusion is about 1/3 of the stiffness in parallel with the reinforcements (mono-directional glass fibres). this property led to inadequacies of the design system considered as a whole. even if all the profiles considered alone appeared to be correctly designed, the results of simulations performed on the panel as a whole were poor. due to material anisotropy, in addition to the vertical component of deflection (usual governing factor in unitized cw design), a horizontal one has to be superimposed, which exceeds the total deflection above the limits. beyond structural inadequacy, also the acoustic behaviour of the preliminary tec resulted poor. gfrp material is in fact characterized by very low specific weight which is, regarding the acoustic, a disadvantage: at low frequencies the sound insulation values of a homogeneous partition is proportional to its mass. moreover, in large hollow profiles, profile ribs act as sound transfer mean, rigidly connecting internal and external facade skins. for acoustic, decoupling of internal and external skins is as much important as the high surface mass. driven by the need to solve acoustic and structural performances, the preliminary concept was redesigned to a solution which implements the three exposed principles: increased stiffness in panel width, increased surface weight and decoupling of the external and internal skins, maintaining at the same time the characteristic appearance. the final design solution consists of an external glass/gfrp laminate skin structurally bonded to a main gfrp frame. the float glass pane, 12mm thick, would assure the right surface weight, contributing also to the panel stiffening in the width, while decoupling of facade skins would have been assured by the structural adhesive used to fix the external laminates (fig. 2b). 3. structural analyses first step was the validation of structural adequacy of the facade concept. in this particular case, because of the slenderness of the facade frame, the contribution of glass stiffness and glass/gfrp sandwich effect is crucial. structural calculations have been performed on the entire panel, considering strength and stiffness of the adhesive joint, connecting the external skin as a parameter. facade deflection and tensile stresses on external glass skin, detected as the most critical points, have been monitored with high accuracy and is presented further on. numerical simulation is done, using fe (finit element) software straus7 (straus7, 2004). isotope plate elements are used for modelling the glass panel and aluminium bracket, while orthotropic plate element is used for simulating the gfrp mullion. beam elements are used to model the gfrp transoms, while the adhesive was modelled using brick elements (fig. 3a). being an anisotropic material, pultruded gfrp profiles are characterized by different mechanical characteristics in different directions, in accordance with manufacturer guidelines (fiberline, 2002). a panel is restrained at bracket position, while connections between gfrp mullion and transom are modelled using master-slave links, joining a pair of nods in all directions (fig. 3a). due to panel and a. tomasi et al. / tec – thin environmental cladding 23 fig. 3. fe modelling: (a) connection details (b) symmetrical model (c) load path (d) gerber effect. load condition symmetry, only one half of the facade is modelled (fig. 3b). the governing load for serviceability limit state of facade is the wind load. a reference value of 2.4 kpa (240kg/m2) has been considered; which is typical for high-rise buildings, corresponding to maximal wind gust (duration of three seconds) with a return period of fifty years. wind load has been implemented in the fe model as surface pressure on plate elements (fig. 3c). normal to plane displacements of top and bottom transoms is linked to simulate the shear transfer (from one panel to the panel above) at stack joint position (fig. 3d). a parametrical study varying the stiffness of the adhesive, starting from an elasticity modulus of 1 mpa (typical for structural silicones) to 1500 mpa (typical for rigid epoxies) have been studied. furthermore, different bracket positions (distance from the restraint to the top edge of the panel) at 200mm, 400mm and 800mm have been investigated, to simulate various solutions that can occur during the facade design. parametrical study results on the influence of the adhesive elasticity modulus ea on tec-facade out-of-plane deflection fy can be seen in fig. 3a. due to the increase in the glass/gfrp sandwich effect, using stiffer adhesive, out-of-plane deflection is decreasing, reaching almost constant value for adhesives with elasticity modulus higher than 200 mpa (once the sandwich effect reaches 100% – glass and gfrp act as rigid component: further increase of adhesive stiffness would not lead to further improvement. the serviceability limit here is settled to be 18.3mm which for spanning length greater than 3000mm correspond to the allowable deflection of l/300+5 concerning the (cwct, 2005). figure 4b is showing the influence of the adhesive elasticity modulus ea on the principal tensile stresses in the glass. it can be noted that principal tensile stresses in the glass are increasing slightly with higher modulus adhesives, and more evidently when the bracket is moving to the central span (800mm) then toward the ends (299mm). here, allowable tensile stress limit for glass is according the standard astm e1300-09a (astm 2009), where for float annealed glass strength limit on the edge is 18.8 mpa. 24 a. tomasi et al. / tec – thin environmental cladding a b fig. 4. fe results – influence of elasticity modulus of adhesive ea on (a) out-of-plane mullion deflection fy (b) principal tensile stress in glass σt. deformation shapes of the facade mullion are highly dependent on mullion restraint/bracket position. as can be seen in fig. 5a, with the bracket placed close to mullion end, bigger rotation happens at mid span, while for bracket position at 800mm from the mullion end it happens close to fixing point. this has direct influence on principal tensile stresses in the glass, because glass through the adhesive is forced to follow the mullion deformation. maximum stress on glass occurs at glass edge, for which reason chamfering and polishing of glass edges is strongly recommended. for cost and safety reason (post breakage behaviour), annealed float glasses can be considered and an upper bound has been defined for the adhesive elasticity modulus. 4. gfrp/glass composite behaviour based on the obtained numerical results, an adhesive screening phase took place. the following requirements have been defined of primary relevance: elasticity modulus between 10 and 100 mpa, elongation at failure higher than 200%, elasto-plastic or hyper-elastic behaviour, relatively fast curing time, high moisture and uv resistance, mechanical stability at extreme temperatures (from −20◦c to +80◦c) as well as compatibility with gfrp substrate. literature screening, data sheet collection and manufacturer interviews have been done, considering at the early stage many adhesive types (acrylics, silicones, epoxies, polyurethanes and ms polymers). with the collaboration of the manufacturer, preliminary tensile and shear tests on the most promising adhesives were done. the following adhesives have been considered as the most promising: • mono-component acrylic adhesive (cured by uv exposure) • bi-component acrylic adhesive (fast curing) • high modulus structural silicone tensile test, performed in accordance to iso 527-1 (iso, 2012), on mono-component acrylic adhesive shows very good stability in the defined temperature range, maintaining a high deformation at failure also at −20◦c (fig. 6). although having elongation at break higher than 300%, due to elasto-plastic material behaviour, with limited elastic phase and large ductility, this adhesive was considered as a a. tomasi et al. / tec – thin environmental cladding 25 fig. 5. fe results – influence of bracket position (from left, 500mm, 850mm and 1200mm below the stack joint) on (a) deformation shape (b) distribution of principal tensile stress in glass. 26 a. tomasi et al. / tec – thin environmental cladding fig. 6. tensile test of mono-component acrylic at (a) −20◦c, (b) +23◦c and (c) +80◦c. risk due to irreversible elongation that can occur due to differential thermal expansion between gfrp and glass. furthermore, thin design thickness (1.5mm), has been considered as disadvantage, due to difficulty of thickness control during the production. the bi-component acrylic showed good adhesion to glass/gfrp substrates as well; however, due to quite low glass transition temperature at around 50◦c, mechanical properties drop immediately, reaching at 80◦c values comparable with standard structural silicone, which can be seen from the dma test results on fig. 7a. similar elasto-plastic behaviour with its disadvantages explained earlier, can be noted from the tensile test in fig. 7b. as a conclusion, the high modulus structural silicone has been chosen. it proved good adhesion performance, compatibility with the gfrp substrate and the typical stability and durability of silicones. however, elasticity modulus of 2.5 mpa, is lower than what was defined in the preliminary requirements. this has no influence on glass stresses because the diagram is almost flat in that region (fig. 4b), but the panel deflection limit is exceeded (fig. 4a). it has therefore been chosen to reinforce the gfrp structure and to consider, for the worst cases (maximum wind load and maximum panel dimensions), an alternative bracket system which allows fixing the panel bracket at 400mm below the panel-to-panel joint. it can be noted that the slightly increase of silicone stiffness (from standard structural to high modulus structural silicone) has in this particular case a big influence in reducing the panel deflection. long-term durability tests have successfully been performed with structural silicone, fulfilling requirements prescribed by etag002 part 1 (etag, 1999). figure 8 is showing the failure modes of the peeling tests and tensile tests. in both cases, a cohesive failure of the adhesive can be noted. gfrp substrate demonstrated to be compatible with silicone; several surface pre-treatments were tested before suitable cleaner and primer were selected. a. tomasi et al. / tec – thin environmental cladding 27 fig. 7. test results of bi-component acrylic adhesive (a) dma test (b) tensile test at 23◦c. fig. 8. failure mode (a) peeling test (b) tensile test. 5. acoustic analyses if structural performance can be accurately determined upfront through simulations, the same cannot be said for acoustics. acoustic software allows evaluation of sound insulation values of relatively simple systems with tolerances not lower than 2db. moreover, good experience is required in order to properly set software parameter and to consequently understand the results. since acoustic behaviour of gfrp was not known, it has been chosen to proceed via testing. while final gfrp profiles were not available (test done in pre-production stage), a similar system has been built with existing pultruded profiles: actual profiles have been obtained bonding different gfrp parts with an epoxy adhesive (thin, stiff adhesive joints), while for bonding the laminate skin, 28 a. tomasi et al. / tec – thin environmental cladding fig. 9. acoustic test (a) specimen set-up (b) direct sound insulation test results. a medium modulus acrylic adhesive has been chosen. figure 9 shows the acoustic test set-up as well as results from the tests. four configurations have been tested, changing panel joint sealing. last configuration (tec4) represents the insulation of the facade with the igu shielded (hence, sound insulation of mullion and spandrel areas). results show very good improvements given by the float glass application: direct sound insulation value passed from 36db to 40db. the improvements given by the glass is much more evident at low frequencies: performance with correction for traffic noise (second parameter between brackets) passed from 26db to 36db. considering that 3db of difference in sound level corresponds to a double sound intensity, this can be considered a huge improvement. as a conclusion, the float glass skin added on the new design proved to satisfy acoustic and structural requirements. a test done at the end with the real pultruded profiles confirmed the results estimated, achieving very high sound insulation even with the limited wall thickness. 6. glass/gfrp lamination in order to maintain the characteristic aesthetics defined at an early stage of development, gfrp plates have to be added externally to the glass skin. given the limited thermal expansion coefficient of pultruded gfrp, which is similar to float glass, because high percentages of reinforcement fibres are used, development focused immediately on industrial glass/glass lamination processes. the following interlayer materials have been considered and tested: • standard autoclave lamination with pvb interlayer: not applicable because of elevated temperatures required by the process (140◦c) which is not sustainable by the gfrp; • vacuum bag lamination with pvb, eva, pu and sentryglass: purpose to study the adhesion properties of the interlayer and influence of a low oxygen atmosphere on gfrp. at process a. tomasi et al. / tec – thin environmental cladding 29 fig. 10. vacuum bag lamination process (a) delamination after pummel test (b) colour degradation at different temperature. temperature of 95◦c gfrp, preliminary pummel test shows promising results for pu and eva interlayers, but gfrp yellowish. at the process temperature of 80◦c, the gfrp did not deteriorate in colour, but adhesion problems between interlayer and gfrp occurs (fig. 10); • cold lamination processes using resins: three products have been considered: a 2-component polyurethane, a mono-component acrylic (uv cured) and a 3-component acrylic. the latter demonstrates better performances than the others (durability and adhesion) and has been considered for the next steps of the project. the technical feasibility of the lamination of large gfrp plates using a liquid product had to be tested. indeed, gfrp plates with a thickness of 3mm are not perfectly flat, due to residual stresses in the structure, caused by the simultaneous pulling and cooling actions during the process (fig. 11a). a full size production test has been internally manufactured to check whether the resin, once cured, can flatten the gfrp. tests proved that the hydrostatic pressure rising in the resin due to the weight of the glass alone (hence, glass pane placed on top of gfrp plate during curing) is sufficient to flatten the gfrp (fig. 11b). 7. fire analyses further aspects considered in tec-facade design are the fire reaction properties of the final product. the gfrp material, if applied with a 4mm thickness, is classified classb-s3-d0 (reference target for tec-facade) defined by tests previously performed according to en 13501-1 (en, 2007). however, decreasing the gfrp thickness from 4 to 3mm negatively influences the fire reaction property. indeed, the material burns quicker than in the thicker version and the peak of heat release rate exceeds the threshold set for class b classification. cone calorimeter tests have been performed to better understand how the heat generation is influenced by the thickness of the gfrp, by the presence of the back glass pane and by the contribution of the laminating resin (that is also a combustible material). pre-tests showed that a laminate composed by a 3mm gfrp plate and a 4mm glass (conservative choice) has performance very similar to a 4mm gfrp plate alone. indeed the glass helps reducing 30 a. tomasi et al. / tec – thin environmental cladding fig. 11. 3mm gfrp plate flatteners (a) before lamination (b) after lamination. the temperature at the burning area (inertia and heat conduction). on the other hand, the glass did not give any advantage if laminated to a 4mm gfrp plate. indeed, the total amount of combustible material (gfrp and laminating resin) is too high, resulting in an excess of total heat released by the material. optimal performance is given by a balance between low burning rate (which require higher mass) and low total heat released (which require lower mass). sbi corner tests have been performed in the final configuration (fig. 12), hence with a surface exposed to flames composed by a 3mm gfrp plate laminated with 1.5mm of laminating resin to a 10mm thick glass pane (a cavity of 60mm has been provided at the back, according to the real application conditions). class b-s3-d0 according to en 13501-1 has been achieved, confirming the estimation made through the cone calorimeter pre-tests. 8. conclusions this paper describes the holistic functional role of glass in the newly developed tec with regards to its acoustic, structural, aesthetics and fire behaviour. the acoustic insulation of the tec-facade has been improved by using two measures: surface weight increase and facade skin decoupling. these allowed the achievement of the imposed requirements, but as a consequence, made facade design more difficult. a. tomasi et al. / tec – thin environmental cladding 31 fig. 12. sbi corner test performed with the final facade configuration. as can be seen, with materials of limited combustibility (class b) there is very limited fire spread in vertical direction and none in horizontal direction. in particular, decoupling of internal and external skins, mandatory due to acoustic insulation, influenced the overall structural behaviour of tec-facade, making the gfrp weak to resist the imposed loads. this led to study the possibility of composite action of structurally bonded glass/gfrp. parametric numerical analyses demonstrate high influence of adhesive stiffness on gfrp mullion deflection and slight influence on principal tensile stresses in glass. based on obtained parametrical results and laboratory testing, the suitable adhesive has been selected: a high modulus structural silicone, which met all the imposed requirements, allowing for a stable and durable composite action between glass and gfrp. in order to maintain the desired aesthetics (showing the translucent gfrp on both sides), gfrp plates have been laminated to the added glazed skin. several interlayer materials as well as lamination techniques have been investigated and tested. gfrp material, not sustainable for high temperature applications, did not allow for application in standard autoclave lamination and vacuum bag processes. cold lamination process using 3-component acrylic demonstrated best performances in durability and adhesion and has been considered for the application. testing concerning the fire reaction properties of the tec-facade showed that glass helps reducing the temperature closed to the burning area (thermal inertia and heat conduction). added glass allowed a slight reduction of gfrp plate thicknesses, hence a maximization of facade light transmission, without increasing the burning rate. 32 a. tomasi et al. / tec – thin environmental cladding references astm e1300-09a (2009). standard practice for determining load resistance of glass in building. west conshohocken, pa: astm international. cwct (2005). standard for systemized building envelopes. bath, united kingdom: centre for window and cladding technology. en 13501-1 (2007). fire classification of construction products and building elements – part1: classification using test data from reaction to ire tests. brussels, belgium: cen – european committee for standardization. etag002-1 (1999). guideline for european technical approval for structural sealant systems (ssgs) – part 1: supported and unsupported systems. brussels, belgium: eota – european organization for technical approvals. fiberline composites (2002). fiberline design manual. middelfart, denmark: fiberline composites a/s. iso 527-1 (2012). plastics – determination of tensile properties – part 1: general properties. genève, switzerland: international organization for standardization. straus7 (2004). finite element analyses system straus7 release 2.3.3. g+dcomputing. from city’s station to station city 017 journal of facade design & engineering volume 9 / number 1 / 2021 effects of phase change materials on heat flows through double skin façades thomas wüest 1*, lars o. grobe 2, andreas luible 2 * corresponding author 1 lucerne university of applied sciences and arts, institute of civil engineering ibi, switzerland, thomas.wueest@hslu.ch 2 lucerne university of applied sciences and arts, institute of civil engineering ibi, switzerland abstract the potential of exemplary organic and inorganic phase change materials (pcms) as façade integrated storage is tested. the impact of two pcms on heat flows is assessed in comparison with water and concrete. the simulation-study employs a transient modelica simulation model of a test cell featuring the solar energy balanced façade (sebf). it is shown that, when compared to water, pcms of identical volume change the seasonal energy balance in winter and summer by only ± 4%. other than water, the pcms maintain this effect even if the storage volume decreases. due to spatial constraints, this can support the integration of thermal storage in façade design considerably. preliminary results indicate that designing thermal storage in façades with pcms must not only consider the latent heat storage capacity, but also take into account the combined effects of latent heat capacity, melting point, conductivity, and dead load. the application of pcms promises to foster the integration of the technology of sebf into façades, but the necessary deliberate selection of, and design with, pcms requires further research. keywords thermal storage, passive solar façade, trombe wall, phase change materials, solar energy balanced façade 10.7480/jfde.2021.1.5408 018 journal of facade design & engineering volume 9 / number 1 / 2021 1 introduction façades greatly affect energy demand and the level of comfort that can be achieved in a building. both targets are addressed by a novel hybrid façade system, that integrates a controlled solarthermal collector and storage into the transparent and opaque zones of double-skin-façades (dsf). the adaptive seasonal and daily management of solar gains of the solar energy balanced façade (sebf) (wüest & luible, 2018) improves the energy balance through the passive use of solar energy. the sebf reduces the transparent façade area, and instead introduces an opaque parapet which functions as thermal storage and mitigates daily energy flux variations. the second skin structure protects shading devices that are essential for the efficient and reliable management of solar gains absorbed by the thermal absorber, and admitted through the transparent areas. the sebf employs the functional principles of the trombe wall (tw) (hu, he, ji, & zhang, 2017). tws are passive solar façade systems, which store and redistribute heat. a typical tw combines a solid wall, acting as thermal storage, with external glazing to exploit the greenhouse effect to form a solar collector. openings, often equipped with fans, allow for an air exchange between the attached space and the cavity of the tw. several studies suggest that tws have a high potential to reduce energy demand in buildings, e.g. by 50% (quesada, rousse, dutil, badache, & hallé, 2012) and up to 69.7% (zhang et al., 2020). the main drawbacks of tws are i) the massive structure, ii) poor insulation, and iii) unbalanced performance for winter and summer (hu, he, ji, & zhang, 2017). a lightweight, ventilated tw element (lohmann & santos, 2020) showed up to 27% savings on heating energy demand; summer conditions were not considered. a thermal simulation model of the sebf, including an approximation of vertical heat flows between the opaque and transparent areas, has been developed in modelica. modelica is an object orientated, equation-based language that can describe physical systems in various domains (wetter, 2009). it supports transient thermal modelling and allows it to be combined with a customised control strategy for seasonal adaptive solar gain management. the model was validated against measurements on a test cell installation (wüest, grobe, & luible, 2020). the sebf aims to turn the fundamental overheating susceptibility of dsfs (manz & frank, 2005; balocco, 2002) into a means to improve the energy balance of buildings without active heat exchange. it aims to passively reduce heat losses in winter, and to control solar heat gains in summer. preliminary studies (wüest & luible, 2018; wüest & luible, 2019) confirmed the fundamental design of the sebf, and demonstrated the suitability of concrete and water as materials for the thermal storage. nevertheless, both materials are difficult to integrate into lightweight façades. concrete has a high dead load. water implies the risk of leakage and freezing. latent heat storage techniques lend themselves as an alternative to these problematic materials (biswas, 2016; vukadinović, radosavljević, & đorđević, 2020). this research compares the performance of a sebf element with thermal storage employing either water, concrete, or a set of phase change materials (pcms) by studying annual simulations under identical conditions. 019 journal of facade design & engineering volume 9 / number 1 / 2021 2 model development for this study, the storage tank, made of a high absorbing aluminium tank within the dsf cavity, was simulated with different materials (gelled water, concrete, and two pcms). in line with previous studies, all simulations were conducted in the modelica modelling environment and were performed at intervals of one minute. 2.1 the solar energy balanced façade one-dimensional heat transfer elements from the modelica standard library, e.g. modelica. thermal package, and its heattransfer sub-package were employed the sebf was modelled as two one-dimensional heat flows through its transparent (a) and opaque (b) areas. these fluxes were coupled via a vertical heat exchange element (c1) within the air cavity (see fig. 1). this approach has been validated experimentally (wüest, grobe, & luible, 2020) fig. 1 modelica model scheme for the four main components of the sebf system the modelling of convective heat flows on surfaces (external and internal) and through cavities was based on en 15099. to simulate unsteady (transient) heat transfer, material layers were divided into n equidistant conductors, and n+1 masses. each conductor represents 1/n of the layer thickness. the n+1 masses represent the surfaces with two times 1/(2n) of the total material layer mass and n-1 times core masses of 1/n of total material layer mass between the conductors (wüest & luible, 2019). this approach was applied with n = 6 to model the stack of storage materials and insulation forming the parapet. glass panes and venetian blinds were simplified with n = 2 and only one centred mass. air gap models a3 and b3 stand for the two different states without and with shading. the corresponding heat fluxes were modelled by one in the unshaded, and two convective and radiative conduction elements in the shaded case (acc. iso 15099 section 8.3.2.2 and 8.4.3.1). 020 journal of facade design & engineering volume 9 / number 1 / 2021 the elements in fig. 1 are: external solar heat transfer coefficient (shtc, a1 and b1), external glass pane (a2, b2), air gap with shading (a3 and b3 independent from each other), triple glazing unit (tgu, a4), storage mass (b4), insulation (b5), internal shtc (a5, b6), and vertical bi-directional heat flow element (c1). 2.2 heat capacity of phase change materials (pcms) modelling pcms in building simulation is challenging. the modelica standard library is not designed for dynamic heat flow elements. consequently, modelling heat transfer through pcms with its irregularity (reflected by the heat storage capacity parameter), meant that the standard mass element had to be modified. the varying enthalpy due to the phase transition is described by a continuous temperature-dependent function (halimov, lauster, & müller, 2019). for this research, the heat capacity during the melting and freezing process was approximated by a standard distribution. it was centred at the melting point μ = t melt . the standard variation σ was set to 1/6 of the melting range δt melt (to include 99.73% of latent capacity within the melting range δt melt ). its integral was scaled by the latent heat storage capacity (c lat ). adding the sensible specific heat capacity (c sens ) leads to the dependent heat storage capacity (c (t) ) as described in formula 1. 𝑐𝑐(") = 𝑐𝑐$%&$ + 𝑐𝑐'() ∙ * +,∙.∙/ !"#$%& ' 0 ( ∙ 𝑒𝑒 1 )"*"#$%&+ ( (∙!"#$%& ' . ( ( 1 ) from the innumerable variants of pcms, two exemplary storage materials from rubitherm were used: high density salt-water pcm (sp29eu) and organic pcm (rt25hc) as shown in table 1. fig. 2a illustrates the temperature dependent heat storage capacities (c (t) ), and fig. 2 b and c the heat storages per kg and l respectively. it is evident that the two pcms differ mostly by density, which accounts for the greater heat capacity per volume of sp29eu. in this research, only the effects of latent heat storage of pcms were taken into account. the influence of varying heat conductivities in solid and liquid states, as well as volume effects, were neglected. the thermal conductivity within the storage material layer is of minor influence and would lead to extended simulation time. a refined model taking account of varying heat conductivity could be realised in parallel to the described model. table 1 exemplary pcms evaluated in this research, source: https://www.rubitherm.eu/en/productcategories.html pcm id pcm melting point t melt [°c] melting range δt melt [k] sensible specific heat capacity c sens [kjkg-1k-1] latent heat storage capacity c lat [kjkg-1] density (solid) ρ [kgm-3] pcm a(30/3) sp29eu 30 3 2’000 170’000 1525 pcm b(25/5) rt25hc 25 5 2’000 200’000 825 021 journal of facade design & engineering volume 9 / number 1 / 2021 fig. 2 sensible and latent heat storage of pcm, concrete and water according table 1 as a) function of temperature according to formula 1, b) heat storage per kg, and c) heat storage per litre 3 experiment 3.1 configuration of the solar energy balanced façade and evaluated storage materials the simulations only consider one sebf element, 1.35 m x 2.9 m. the sebf was configured so that its external layer is an 8 mm thick single pane of glazing (τ = 0.823, ρ = 0.076). the inner layer is horizontally divided into a parapet area of 1.5 m², and a transparent area covered by the tgu of 1.9 m². the depth of the cavity was set to about 150 mm in front of the tgu, and 100 mm in front of the storage. the shading was implemented by means of highly reflective venetian blinds (ρ = 0.75). the thermal storage tank consists of two 3 mm aluminium sheets with 20 mm or 40 mm of filling material (table 2). the external surface of the tank has low reflectance and is highly absorbent (ρ = 0.25). the properties of the evaluated pcms (pcm a and pcm b ) were derived from the products reported in table 1. they differ in terms of density, conductivity, and latent heat capacity. table 2 relevant properties of the evaluated storage materials material thickness t [mm] density ρ [kgm-3] conductivity λ [wm-1k-1] specific heat capacity c sens [jkg-1k-1] latent heat capacity c lat [jkg-1] thermal effusivity b [jk-1m-2s-1/2] water gel 20 /40 981 0.35 4’183 1198 air 20 /40 var. var. concrete 20 /40 2400 2.1 1’000 2245 pcm a 20 /40 1525 0.5 2’000 170’000 1235 pcm b 20 /40 825 0.2 2’000 200’000 575 022 journal of facade design & engineering volume 9 / number 1 / 2021 3.2 variations of storage materials the selection of the pcms is challenging due to the wide range of properties of available materials and products, which by no means is represented by the selection of the two pcms (table 1). the selection of the two pcms is rather regarded as an example by which to evaluate the effect two different types—organic and high density—on the performance of the sebf‘s thermal storage. the effectivity of pcms is mainly defined by their melting temperatures. to activate the latent heat storage, variants of the pcm types were chosen with melting points low enough to be reached on cold and sunny winter days, but high enough to be discharged on summer nights. in the simulation experiment, each of pcm a and pcm b was evaluated assuming three different melting points: 25°c, 30°c, and 35°c. in the following passages, pcm types (a or b) and their variants (defined by t melt and δt melt ) are indicated as subscripts. a pcm of type a with t melt = 35°c and δt melt = 5 k would therefore be referred to as pcm a(35/5) . the simulations assume an initial maximum depth of the storage material of 40 mm. this is motivated by the identical geometric configuration of the sebf evaluated in a precedent study, the results of which shall be compared to wüest, grobe, and luible (2020). in addition, the effect of decreasing the thickness by of 50% to = 20 mm was analysed. water and concrete were modelled as reported by table 2. in parallel with the previous research, all results were compared to the case of an empty storage tank (material “air”). this allows the effects of the thermal mass of the storage to be isolated from those of other solar-optical mechanisms. 3.2.1 shading control to make use of solar gains in cold periods but prevent overheating during warmer periods, a customised control of the shading devices in the transparent and opaque zones was employed. the control enters three predefined modes based on the mean average external temperature over the last 24 hours t 24 (wüest, grobe, & luible, 2020). if t 24 is lower than 12°c, the system is in heating mode and maximises solar gains. if t 24 is higher than 15°c, the sebf is in cooling mode and minimises gains. the range between 12° and 15° activates ‘free floating’ mode, avoiding gains through the storage, and moderating direct gains through the tgu. the set points of the modes are listed in table 3. in all three modes, night-time losses are controlled by application of a threshold of 25 w/m2 to monitored global vertical irradiance e v . table 3 global irradiance set points for shading control response heating mode t 24 ≤ 12°c free float mode 12°c < t 24 < 15°c cooling mode 15°c ≤ t 24 close shading of tgu if e v > 350 w/m2 if e v > 350 w/m2 if e v > 150 w/m2 close shading of tgu at night always always never close shading of storage if e v < 25 w/m2 if e v > 25 w/m2 if e v > 25 w/m2 close shading of storage at night always never never 023 journal of facade design & engineering volume 9 / number 1 / 2021 3.3 boundary conditions in all simulations, internal air temperature was set to 22°c. influences of occupancy were not considered. the outdoor conditions were given by a standard design year for zürich (ch) on an hourly basis from meteonorm (meteotest ag, 2018). all parameters, such as temperature, wind speed, façade irradiance, and sun position were interpolated linearly from hourly values. 3.4 evaluation variables the effect of varying storage materials was evaluated by four variables that were solved by the simulation: – the maximum surface temperature of the storage tank, – the energy balance of the sebf element for each season, – short-term effects such over the course of the day, and – the dead load introduced into the façade by the storage material. the maximum surface temperature of the storage tank was expected to be the warmest point within the sebf element. because of its non-ventilated dsf structure, the façade tends to overheat (manz & frank, 2005). therefore, the maximum temperature within the element acts as an indicator for thermal loads on the sebf’s component. high temperatures could, for example, accelerate ageing processes or lead to fogging (outgazing of plasticisers and condensation on the glass surface). effects on the energy balance, as the primary design target of the sebfs, were evaluated as the key output of the simulation. therefore, the heat flow at the inner façade surface is regarded to evaluate the needed heating or cooling demand in the interior. to reduce complexity, the thermal balance of an sebf element was evaluated seasonally for winter (january, february, december), spring (march, april, may), summer (june, july, august), and autumn (september, october, november). to evaluate the intended delay of passive solar gains, as the secondary design target, one winter and one summer day were analysed in detail. the dead load imposed by building materials significantly affects the structural design of curtain walls. traditionally, the weight of glass accounts for the largest proportion of the dead load on façade elements. a typical tgu for façade applications consists of about 21 mm glass (comprising three panes of 8 mm, 5 mm and 8 mm), corresponding to a dead load of 57 kg/m2. this is the reference for the storage tank, where 29 kg/m2 is contributed by the containing aluminium sheets (~10 mm) and insulation (70 mm) alone. 4 results 4.1 temperatures table 3 shows that the shading control is effective in that the storage tank only receives solar irradiance when the external 24-hour mean temperature t 24 is below 12°. since this effectively blocks irradiance on hot days in summer, the maximum surface temperatures of the storage occur 024 journal of facade design & engineering volume 9 / number 1 / 2021 only from september until april. in summertime (june – august), they rarely surpass 40°c for the 40 mm tank, while the storage of reduced thickness of 20 mm reaches 47°c, and the empty (airfilled) tank 53°c. fig 3 distils these findings by showing only the daily maximum temperatures t max on the storage tank in its 40 mm, 20 mm, and air configurations. the maximum temperatures for the entire year are summarised in fig. 4. all configurations, except of air, reach higher temperatures when the thickness of storage material is reduced. the air element reaches the highest temperatures, up to 88°c, whereas the 40 mm water element, as proposed in the initial design of the sebf, is significantly cooler at approximately 63°c. the variants of pcm b with a thickness of 40 mm achieve similar results to 40 mm of water. with pcm a , the maximum temperatures are significantly lower. even the 20 mm configurations of pcm a(30/3) and pcm a(35/3) are at the same level as a water-filled tank of twice the volume. fig. 3 maximum tank surface temperature tmax of 40 mm, 20 mm and air configurations, in °c fig. 4 maximum annual tank surface temperatures tmax for all storage material configurations, in °c 025 journal of facade design & engineering volume 9 / number 1 / 2021 4.2 energy balance for each season the energy balance at the internal façade surface for each season due to heat conduction for all storage variants is summarised in table 4. in addition, the direct solar heat gains are reported, which are identical for all elements. all configurations achieve similar results, especially in winter and summer. higher differences occur at intermediate climate conditions in spring and autumn. table 4 energy balance for each element and season in [kwh] water air concrete pcm a 25/3 pcm a 30/3 pcm a 35/3 pcm b 25/5 pcm b 30/5 pcm b 35/5 dir. sol. gain 40 mm winter -29.0 -32.2 -29.6 -29.3 -29.1 -28.6 -30.3 -30.1 -30.2 28.8 spring 5.8 2.5 5.3 5.63 6.1 6.7 5.00 5.2 5.2 52.9 summer 13.6 15.5 13.6 13.0 13.4 13.6 13.4 13.6 14.0 35.8 autumn -4.1 -5.6 -4.4 -4.5 -4.0 -3.9 -4.9 -5.02 -4.7 37.7 20 mm winter -30.2 -32.3 -31.5 -29.4 -29.8 -29.20 -30.6 -30.8 -30.7 spring 4.9 2.3 3.7 5.7 6.1 6.27 5.2 5.0 4.8 summer 13.9 15.4 14.3 13.0 13.5 13.9 13.5 13.7 14.2 autumn -4.7 -5.8 -5.4 -4.8 -4.3 -4.2 -5.01 -5.2 -5.0 table 5 shows the percentage deviation relative to the 40 mm water tank for winter and summer periods. as reported by table 4, only the empty tank leads to significantly higher heat losses and gains (11% to 14%). table 5 energy balance for each element an season in [kwh] water air concrete pcm a 25/3 pcm a 30/3 pcm a 35/3 pcm b 25/5 pcm b 30/5 pcm b 35/5 40 mm winter 0% 11% 2% 1% 0% -1% 4% 4% 4% summer 0% 14% 0% -4% -2% 0% -2% 0% 3% 20 mm winter 4% 11% 9% 1% 3% 1% 5% 6% 6% summer 2% 13% 5% -5% -1% 2% -1% 0% 4% 4.3 short-term energy balance the short-term energy balance is presented for two sample days each for both winter (fig. 7) and summer (fig. 8). the corresponding exterior conditions are illustrated by fig. 5 and fig. 6. the two days in february represent the coldest two-day period (nights below -7°c), whereas the two days in july represent the warmest two-day period (peak 32.7°c). 026 journal of facade design & engineering volume 9 / number 1 / 2021 fig. 5 external conditions (air temperature, solar irradiance façade) 9th – 10th february according meteonorm fig. 6 external conditions (air temperature, solar irradiance façade) 24th – 25th july according meteonorm both fig. 7 and fig. 8 confirm the short-term behaviours of what table 4 and table 5 indicate as seasonal effects: all storage variants act very similarly. heat is mostly stored and released within 24 hours. therefore, only 40 mm water and 40 mm air variants were highlighted within the figures. a first analysis of those graphs reveals the following main findings: – all parapet heat flows are slightly undulated and close to zero, – all tgu heat flows are extremely volatile, because they react immediately to solar irradiance, – higher solar irradiance increases the differences in heat flows, – due to the low temperature and solar irradiance in winter (9th -10th february), the storage is almost ineffective, leading to nearly identical heat flows through all configurations, and – the effects of pcms are reflected by plateaus of constant heat flow in summer at values decreasing with melting temperature, e.g. 2 w, 4 w, and 7 w for melting temperatures of 25°c, 30°c, and 35°c => 7 w respectively. 027 journal of facade design & engineering volume 9 / number 1 / 2021 fig. 7 two-day period winter (9th – 10th february) heat flows fig. 8 two-day period summer (24th – 25th july) heat flows 4.4 dead load the effect of the evaluated storage materials on the dead load of the sebf is evaluated in comparison to a transparent parapet comprising a tgu with a typical glass mass (21 mm) of 57 kg m-2. the additional mass of 29 kg m-2 is taken into account for the supporting structure required for the storage tank (aluminium and insulation). table 6 reports the dead load per unit area corresponding to the evaluated storage materials compared to a tgu. 028 journal of facade design & engineering volume 9 / number 1 / 2021 table 6 storage materials dead load material thickness t [mm] density ρ [kgm-3] mass m [kgm-2] additional tank mass m tank [kgm-2] total mass m tot [kgm-2] to reference tgu (57 kgm-2) [kgm-2] water gel 20 981 19.6 29 48.6 -8.4 40 981 39.2 29 68.2 +11.2 concrete 20 2400 48.0 29 77 +20 40 2400 96.0 29 125 +68 pcm a 20 1525 30.5 29 59.5 +2.5 40 1525 61 29 90 +33 pcm b 20 825 16.5 29 45.5 -11.5 40 825 33.0 29 62 +5 5 conclusions as expected, with increasing thermal mass the tank surface temperature and therefore the overheating risk decrease. the tested high density pcm (pcm a ) limited the maximum temperatures on the storage tank, indicating the potential to limit the risk of overheating and thermal stress on façade components. concrete and pcm b had adverse effects. with all evaluated storage materials, surface temperatures covered a wide range from approximately 0°c to 70°c. under such extreme fluctuations, the beneficial effects of the latent heat storage capacity of pcms are not fully leveraged due to its low sensitivity. it has to be noted that some pcms might become in stable at the high temperatures that can occur within a non-ventilated dsf, e.g. up to 76.4°c in the studied configurations. the differences in overserved effects on seasonal and short-term energy balances by the tested storage materials were low. this holds true even with significantly different sensitive and latent heat storage capacities. this can be explained by effects of the thermal effusivity b, which is a measure of a material’s ability to exchange and store thermal energy. regarding the formula of thermal effusivity , a high influence of thermal conductivity appears. compared to water gel, concrete, for example, has a 42% lower volumetric heat capacity (ρ*c), but almost twice its thermal effusivity. consequently, concrete has a lower heat capacity but higher exploitation. for pcm a and pcm b, effusivity differs by a factor of approximately 2 due to their significantly different conductivities (see table 2). the particularly high dead load introduced into the sebf by concrete as a storage material based on high density is problematic. this drove the motivation to rely on water, with its outstanding specific heat capacity, in the first implementations of the sebf, although the integration of a liquid into a façade element was expected to be challenging. the dead load introduced by pcm a due to its high density seems to be a problem at first glance. on further observation, table 6 shows that 20 mm pcm a is competitive against 40 mm water gel and, therefore, a good option to reduce weight and enhance thermal performance. the dead load of pcm b in 40 mm is not problematic, but, due to its low thermal effusivity b, the volume is poorly exploited. 029 journal of facade design & engineering volume 9 / number 1 / 2021 in this comparison, the pcm a of 20 mm shows the highest potential for the application in terms of dead load and thermal effects. to sum up, the application of pcm as thermal storage material within non-ventilated dsfs is possible and justified. nevertheless, the results of this research indicate that the main benefit of pcms is not their effects on energy balance, but rather space and weight saving within the construction when compared to other storage materials. the choice should be carried out carefully and not just be based on sensitive and latent storage capacity. density and conductivity (thermal effusivity) could decide between success and failure in a particular application. as mentioned, pcm a seems promising for this application. however, further investigation on the right combination of its properties (λ, ρ, c sens , c lat ) for a dsf storage application could further improve thermal and structural aspects. for future development of the sebf or similar elements, two conclusions are made: a) pcm could enhance thermal performance, but non-pcm materials could also contribute to improved energy balance; and b) the choice of a suitable pcm needs an in-depth evaluation to find the right material. furthermore, besides the criteria in this work, other criteria as processability, availability, durability, price and so on should be considered. acknowledgements the authors gratefully acknowledge the funding provided by the swiss national science foundation (snf), grant no. izcnz0-174562). references balocco, c. (2002). a simple model to study ventilated façades energy performance. energy and buildings, 34, pp. 469-475. https:// doi.org/10.1016/s0378-7788(01)00130-x biswas, d. (2016). nano-based phase change materials for building energy efficiency*. start-up creation, pp. 183-211. https://doi. org/10.1016/b978-0-08-100546-0.00009-1 halimov, a., lauster, m., & müller, d. (2019). validation and integration of a latent heat storage model into building envelopes of a high-order building model for modelica library aixlib. energy and buildings, 202, p. 109336. https://doi.org/10.1016/j. enbuild.2019.109336. hu, z., he, w., ji, j., & zhang, s. (2017). a review on the application of trombe wall system in buildings. renewable and sustainable energy reviews, 70, pp. 976-987. https://doi.org/10.1016/j.rser.2016.12.003 iso 15099. (2003). thermal performance of windows, doors and shading devices detailed calculations. geneva: iso copyright office. lohmann, v., & santos, p. (2020). trombe wall thermal behaviour and energy efficiency of a light steel frame compartment: experimental and numerical assessments. energies, 13, p. 2744. https://www.mdpi.com/1996-1073/13/11/2744 manz, h., & frank, t. (2005). thermal simulation of buildings with double-skin façades. energy and buildings, 37, pp. 1114-1121. https://doi.org/10.1016/j.enbuild.2005.06.014 meteotest ag. (2018). meteonorm v7.3.3. bern switzerland. quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012). a comprehensive review of solar façades. opaque solar façades. reviews, renewable and sustainable energy, 16, pp. 2820-2832. https://doi.org/10.1016/j.rser.2012.01.078 vukadinović, a., radosavljević, j., & đorđević, a. (2020). energy performance impact of using phase-change materials in thermal storage walls of detached residential buildings with a sunspace. solar energy, 206, pp. 228-244. https://doi.org/10.1016/j. solener.2020.06.008 wetter, m. (2009). modelica-based modelling and simulation to support research and development in building energy and control systems. journal of building performance simulation, pp. 143-161. https://doi.org/10.1080/19401490902818259 wüest, t., & luible a. (2019). trombe curtain wall façade. powerskin conference proceedings, p. 313. retrieved from https://books. bk.tudelft.nl/index.php/press/catalog/view/isbn_9789463661256/786/679-3 wüest, t., & luible, a. (2018). solar energy balanced façade. façade 2018 adaptive! proceedings of the cost action tu1403 adaptive façades network final conference, pp. 183-194. wüest, t., grobe, l. o., & luible, a. (2020). an innovative façade element with controlled solar-thermal collector and storage. sustainability, 12, p. 5281. https://doi.org/10.3390/su12135281 zhang, l., hou, y., lui, z., du, j., xu, l., zhang, g., & shi, l. (2020). trombe wall for a residential building in sichuan-tibet alpine valley – a case study. renewable energy, 156, pp. 31-46. https://doi.org/10.1016/j.renene.2020.04.067 030 journal of facade design & engineering volume 9 / number 1 / 2021 from city’s station to station city 041 journal of facade design & engineering volume 6 / number 2 / 2018 how to analyse the performances of innovative variable diffusivity membranes integrated within prefabricated timber facades: computer-based modelling and experimental analysis riccardo pinotti1,2, stefano avesani2, francesco babich2, andrea gasparella1, alice speranza3 1 free university of bozen, piazza università 5, bozen, italy 2 institute for renewable energy, eurac, viale druso 1, bozen, italy 3 rothoblaas srl, via dell’adige 2, cortaccia, italy abstract vapour barriers and retarders are often needed to improve the hygro-thermal performance of the building envelope. their use is particularly important in prefabricated timber façades, especially when critical boundary conditions occur. in the literature, very little is known about the actual performance of complete envelope packages that integrate these membranes, since most previous studies focused on the analysis of single components. however, considering the growing interest and use of such timber facade elements, an analysis of the performance of integrated membranes is needed in order to improve the material function curves available in the datasheets to enable the correct design of the whole wall structure. thus, the novelty of this work lies in the validated analysis of a building envelope sample that integrates membranes with a variable vapour diffusivity. the focus of the paper is more related to the experimental set-up and particular attention has been paid to the development of a relatively simple testing procedure to analyse the behaviour of such integrated membranes. the study seeks to investigate the behaviour of an envelope component integrating a hygro-variable membrane and a breathable membrane by using computer simulation and experimental facilities. a thermo-hygrometric analysis of the element has been performed in delphin, and an experimental methodology is presented, aiming to validate the numerical model, measuring the temperature and relative humidity in different layers. two sets of boundary conditions have been accurately chosen as they are critical for the building component in terms of thermal and humidity transmission. results show very good agreement for one test condition. for the second condition, the measurement uncertainty was greater. one possible reason for this was the presence of condensate in the measurement box frame caused by the first test run. the experimental set-up developed is a relatively easy-to-replicate layout for the validation of similar complex packages. compared to previous studies, the experimental set-up used in this research is simpler and less expensive. keywords hygro-variable membrane, hygro-thermal analysis, delphin, timber façade, relative humidity measurement doi 10.7480/jfde.2018.2.2083 042 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction humidity can enter buildings in different ways: infiltration (caused by rain drainage or due to problems in wall-integrated supply ducts), capillary action (due to rising water from the ground), water vapour stored in materials during the building construction phase, and vapour generated by the occupants. high levels of humidity combined with low interstitial temperature within the building envelope can cause problems such as superficial and internal condensation, reduction of the insulation capacity of materials, aesthetic degradation, and mildew growth (lucas, adelard, garde, & boyer, 2001). hence, humidity levels not only influence the performance of materials, but can also become a problem for the comfort and health of occupants. therefore, it is crucial to carefully analyse the vapour diffusion within a building envelope in order to avoid these inconveniences and to preserve both the building integrity and the wellbeing of the occupants. in cold climates, and during wintertime, problems with excessive humidity are usually caused by the poor ventilation of indoor spaces. the vapour produced within the building moves through the building envelope and condenses near colder layers. in order to prevent this behaviour, the humidity level near the wall surface should be reduced, for instance using a vapour retarder membrane on the warm side of the wall. on the other hand, in hot and humid climates, the main source of vapour can be the outdoor air. in these conditions, condensation problems may occur near the inner layers of the wall, especially if the indoor environment is cooled. a possible solution to this issue is the use of breathable materials within the envelope, in order to let the humidity flow through the wall build-up, avoiding moisture being trapped in the material. vapour barriers and retarders are fundamental to control vapour diffusion through the building walls and therefore to regulate the hygrometric behaviour of the whole structure. previous research on in-situ existing wall and small single-material specimens have already been done (litti, khoshdel, audenaert, & braet, 2015) (guizzardi, derome, vonbank, & carmeliet, 2014) (campbell, mcgrath, nanukuttan, & brown, 2017), but few examples on more realistic multi-layer building samples that integrate variable permeability membranes have not been found in the literature. thus, this study aimed at investigating the behaviour of an envelope component that integrates a hygro-variable (“smart” vapour barrier) membrane named clima control 80 (rothoblaas, 2017) and a breathable membrane named traspir75 (rothoblaas, 2017), by using computer simulation and experimental facilities. the experimental set-up developed is a relatively easy-to-replicate layout for the validation of similar complex packages. 043 journal of facade design & engineering volume 6 / number 2 / 2018 2 methodology 2.1 experimental set-up in this work, the behaviour of a building envelope sample (0.5m x 0.5m), composed of the layers listed in table 1, has been analysed through numerical modelling and experimental investigation. the layout was defined in collaboration with a local supplier of components for timber constructions to ensure the selection of a realistic configuration. hence, the choice of testing the layer composition that is reported in table 1 has been agreed together with the company, in order to fulfil the requirements of the most hot and humid regions around the world, such as asia or south america. the traspir75 membrane is commonly adopted in façade packages similar to the one analysed in this study because of its air and water tightness combined with a high vapour permeability, allowing the drying out of the humidity stored within the wall. moreover, instead of using a vapour barrier in the inner layers to reduce the amount of humidity entering the façade from the inside, it has been decided to adopt the clima control 80. in fact, if this membrane is exposed to high humidity levels, it transforms from a vapour barrier into a breathable product, guaranteeing that the structure remains dry. material name thickness density specific heat thermal conductivity vapour diffusion resistance equivalent air layer thickness (sd) total uvalue indoor [mm] [kg/m³] [j/(kg*k)] [w/(m*k)] [mm] [w/(m²*k)] layer 1 gypsum fibre board 12.5 1133.35 1228.37 0.34 16.83 0.21 0.23 layer 2 wood fibre insulation board 60 150 2000 0.04 3 0.18 layer 3 clima control 80 membrane 0.2 400 1700 0.20 1000÷25000 0.2÷5 layer 4 wood fibre insulation board 100 150 2000 0.04 3 0.3 layer 5 osb board 18 630 1880 0.13 280 5.04 layer 6 traspir75 membrane 0.3 250 1800 0.30 67 0.02 outdoor table 1 layer composition and characteristics 044 journal of facade design & engineering volume 6 / number 2 / 2018 the clima control 80 is a particular type of vapour barrier that can adapt its vapour diffusion resistance based on the surrounding relative humidity. in particular, the more it increases, the lower membrane sd is obtained (table 2). table 2 clima control 80 membrane relation between surrounding rh and sd-value ee08 by e+e sensors (rhà0÷100% ± 2%; tà-40÷80°c ± 0.2°c) have been placed between specimen layers for temperature and relative humidity monitoring. fig. 1 shows the sensors’ position within the materials. in particular, four internal sensors have been embedded into small cavities inside the wood fibre insulation boards: position 1 is at gypsum – insulation (60mm) interface; position 2 is at insulation (100mm) – clima control 80 interface; position 3 is at osb – insulation (100mm) interface; position 4 is at clima control 80– insulation (60mm) interface. two different boundary conditions were created on the different sides of the specimen during the analyses. on one side, the temperature and relative humidity level were maintained by a monozonal climatic chamber. on the other side, it was decided to use a dummy climatic chamber made from wood, which is very well insulated for both thermal and vapour diffusion purposes. this box was made of osb panels (18mm thick), covered internally with pressed insulation material (styrodur 2500c, 60mm thick). during the two performed tests, the box was plugged by the specimen on one side, as shown in fig.2, and the whole block was inserted into the main climatic chamber. the desired temperature inside the box was set using a thermostat connected to a heating coil, while the relative humidity of the air was set using salt solutions. in this way, it was possible to impose a thermal and vapour flux from one side of the tested component to the other. 045 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 1 sensors’ position inside the specimen fig. 2 specimen closing the box 2.2 boundary conditions in order to analyse the behaviour of the component under critical temperature and high humidity conditions, two tests were undertaken. in the first one, typical hot and humid conditions for the outdoor environment were used. in particular, the temperature was set to such a high value (fig. 3) to take into account the possible effect of direct sun irradiation on the façade. in the second test, typical cold and very humid winter conditions were used (fig. 4). fig. 3 boundary conditions test 1 (hot/humid summer outdoor conditions) fig. 4 boundary conditions test 2 (cold/humid winter outdoor conditions) the duration of each test was determined after considering the necessary time it would take to reach the thermal and humidity flux steady-state conditions in the specimen, accepting a difference of 0.1 w/m² (heat flux) and 0.1 g/m²h (humidity flux) between entering and exiting fluxes, respectively. these values were assessed through a preliminary simulated numerical model. finally, it was decided to let each test run for almost 15 days. in the following figures (fig. 5, fig. 6), the monitored boundary conditions for the second test are reported. conditions for test 1 are not shown because no relevant differences with designed conditions were present. 046 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 5 climatic chamber temperature (left) & rh (right) test 2 fig. 6 box temperature (left) & rh (right) test 2 3 results in this section, the results of the monitoring campaign are compared with those calculated by the numerical model built in delphin 5. all the results for temperature and relative humidity are referred to specific positions (namely 1, 2, 3, and 4) within the specimen. these positions are presented in fig. 1. the presented values obtained with the model were reached following a calibration process on some uncertain parameters, mainly related to the humidity transfer function of those materials whose technical sheet was not available. 047 journal of facade design & engineering volume 6 / number 2 / 2018 3.1 test 1 – hot/humid outdoor conditions in this test, nacl solution has been used within the box to generate the desired humidity rate (rh=70%). in fig. 7 and fig. 8, the monitored and measured trends for temperature and relative humidity at each material interface are presented. fig. 7 relative humidity trends – measured data (thick line) & modelled data (thin line) – test 1 fig. 8 temperature – measured data (thick line) & modelled data (thin line) – test 1 048 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 9 presents the comparison between measured (both calibrated and not) and calculated relative humidity, after having reached stationary conditions for vapour flux across the specimen. fig. 9 comparison between rh values measured & modelled, after stationary conditions (averaged on last 5 hours) – test 1 table 3 and table 4 present the standard deviation and the absolute value between calculated and monitored values for each sensor in the final hours of the simulation (after stationary conditions occurred). rmse(rh1) rmse(rh2) rmse(rh3) rmse(rh4) 0.18 1.11 2.56 2.38 table 3 root mean square error rh values test 1 mae(rh1) mae(rh2) mae(rh3) mae(rh4) 0.17 1.11 2.56 2.38 table 4 mean absolute error rh values – test 1 3.2 test 2 – cold/humid outdoor conditions in the second test, mgcl 2 solution was used in order to create the desired humidity conditions within the box (rh=40%). in fig. 10, fig. 11, fig. 12, and table 5, the results of the comparison between monitored and calculated data are reported for test 2. it can be noticed that complete steady state conditions have not yet been reached. 049 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 10 relative humidity trends – measured data (thick line) & modelled data (thin line) – test 2 fig. 11 temperature – measured data (thick line) & modelled data (thin line) – test 2 050 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 12 comparison between rh values measured & modelled, after stationary conditions (averaged on last 5 hours) – test 2 rmse, mae (rh1) rmse, mae (rh2) rmse, mae (rh3) rmse, mae (rh4) 10.1 4.89 4.61 8.39 table 5 root mean square error & mean absolute error rh values test 2 4 discussion in both tests, the measured temperature values are in agreement with the model results. this is due to minor uncertainties relating to the heat transfer process. the only small issues regarding the temperature are caused by the climatic chamber`s difficulty in maintaining conditions around 0°c in a stable way during test 2. on the other hand, the relative humidity results (especially in test 2) show more discrepancies between the model and the reality. possible explanations for this are as follows. firstly, it is noticeable from fig. 5 and fig. 6 that the relative humidity boundary conditions, both in the box and in the climatic chamber, have quite low stability: for the box, this can be due to a nonoptimal dosage of the salt solution. regarding the instability of rh level in the climatic chamber, the main problem is related with the too low operative temperature set in the machine (~ 0°c). although the results of the first test are in good agreement with the simulation, another source of errors in the tests can be related to the sensor positioning inside the specimen. in fact, while rh simulation results from delphin represent the moisture contained in material`s pores, the e+e sensors that have been used in this experimental activity measure the humidity level in the small air cavity in the material in which they are inserted. another cause of uncertainty that is likely to have affected the second test is the inadequate estimation of drying time. some humidity, trapped in the sample from the previous test, may have slightly influenced the results of the second test. 051 journal of facade design & engineering volume 6 / number 2 / 2018 finally, it should be taken into account that the physical properties’ functions of all the materials used in the model, in particular those of less known materials within our specimen (e.g. osb layer and vapour retarders), can themselves present small uncertainties. in the future, this kind of study and experimental setup would allow for the reduction of uncertainties related to the material function by undertaking a step-by-step test and calibration of each layer. 5 conclusions it can be concluded that, in the first test, there is a good match between simulation and calculated data, with a maximum difference in stationary condition (value assumed as the average in the last 5 simulation hours) of 4% for rh and 1°c for temperature. the less conclusive agreement of the second test is likely to have been caused by the abovementioned possible reasons. thus, considering the overall results of the performed analyses, it can be concluded that the modelling of the smart vapour barrier was successful. overall, the methodology applied to this study, albeit with a limited budget, revealed itself to be a solid approach for the investigation of thermal and hygrometric phenomena in a building envelope sample. further analyses could investigate the hygro-thermal behaviour of the samples, using a double climatic chamber in order to set more stable boundary conditions across the specimen. moreover, a comparison between different sensor typologies could be done (e.g. dimension, accuracy, output typology), in order to consolidate this approach to measuring within envelope components. future studies should extend these analyses to more complex façade systems to investigate eventual problems related to the integration of components such as ventilation machines, pv modules, or solar thermal panels. acknowledgements the authors are grateful to valentino diener and giordano miori from eurac research for their technical support in the experimental work. this work is part of the research activities of the project 4rineu, funded by the european union’s horizon 2020 research and innovation programme under grant agreement no 723829. reference campbell, n., mcgrath, t., nanukuttan, s., & brown, s. (2017). monitoring the hygrothermal and ventilation performance of retrofitted clay brick solid wall houses with internal insulation: two uk case studies. case studies in construction materials, 7, 163-179. doi:10.1016/j.cscm.2017.07.002 guizzardi, m., derome, d., vonbank, r., & carmeliet, j. (2014). hygrothermal behavior of a massive wall with interior insulation during wetting. building and environment, 89, 59-71. doi:10.1016/j.buildenv.2015.01.034. litti, g., khoshdel, s., audenaert, a., & braet, j. (2015). hygrothermal performance evaluation of traditional brick masonry in historic buildings. energy and buildings, 105, 393-411. doi:10.1016/j.enbuild.2015.07.049 lucas, f., adelard, l., garde, f., & boyer, h. (2001). study of moisture in buildings for hot humid climates. energy and buildings, 3(4), 345-355. doi:10.1016/s0378-7788(01)00115-3 rothoblaas. (2017, september 29). rothoblaas catalogue. retrieved from https://www.rothoblaas.it/cataloghi-rothoblaas https://doi.org/10.1016/j.cscm.2017.07.002 https://doi.org/10.1016/j.buildenv.2015.01.034 https://doi.org/10.1016/j.enbuild.2015.07.049 https://doi.org/10.1016/s0378-7788(01)00115-3 https://www.rothoblaas.it/cataloghi-rothoblaas journal of facade design and engineering 2 (2014) 109–122 doi 10.3233/fde-140013 ios press 109 connecting through the reinforcement – design, testing and construction of a folded reinforced glass structure paulo l. carvalhoa,∗, paulo j. s. cruza and frederic a. veerb aschool of architecture, university of minho, guimarães, portugal bfaculty of architecture, delft university of technology, delft, the netherlands submitted 3 november 2014 revised 27 november 2014 accepted 14 december 2014 abstract. a reinforced glass folded structure has been developed using an innovative connection method. the concept relies on extending the reinforcement outwards from the laminated glass and using it to transfer a significant part of the load. the goal is to accomplish a glass element with high stiffness, connected by using a discrete almost invisible and easily assembled/disassembled mechanism. this paper addresses the main issues regarding the design and fabrication of a 90◦ folded structure, the experimental investigation of the out-of-plane compressive response and the construction of a full-scale prototype (2,95m high and 5,5m long) at the campus of the university of minho. it is demonstrated that the system offers both structural and aesthetical advantages. it combines a specific aesthetic, deriving from its hybrid character, with a considerable amount of out-of-plane compressive strength before and after failure. keywords: reinforced glass, connection detail, folded structure, prototype 1. introduction the detail of glass connections has received significant contributions in the recent years due to important technological improvements concerning the capacity to efficiently bond glass to metal. it not only has a significant impact on the structural behaviour of the solutions but also opens several new fields of design research (o’callaghan, 2007; bagger, 2010; o’callaghan & bostick, 2012; puller, 2012; neugebauer, 2013). among the several solutions available to the designers, embedded connections, in which the metal part is bonded to at least two glass panes with stiff interlayers such as sentryglas�, are of particular interest. both glass and metal elements tend to lose part of their individual identity fusing into a unified hybrid component. in this case, the relationship between design and fabrication becomes increasingly intricate, mutually influencing each other towards a consistent balance between aesthetics and performance. 2. connecting through the reinforcement concept reinforcing glass is an effective method to increase the post breakage behaviour of glass (feirabend, 2010; louter, 2011). once broken, the glass carries the compressive loads while the metal bridges the ∗corresponding author: paulo l. carvalho, school of architecture, university of minho, campus de azurém, guimarães, portugal. tel.: +351 253 510 500; e-mail: paulo.carvalho@arquitectura.uminho.pt. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:paulo.carvalho@arquitectura.uminho.pt 110 p.l. carvalho et al. / connecting through the reinforcement fig. 1. concept of extending the reinforcement outwards of the laminate to become the basis for the connection. tensile loads together with a suitable polymeric interlayer. this interesting structural concept may be enhanced if the reinforcement plays an active role in connecting the glass elements. to achieve this, it is proposed to extend the reinforcement outwards of the laminated glass (fig. 1). the purpose of connecting through the reinforcement is the resulting ability to visually dematerialize the connection element. due to the inevitable opaqueness of metal, instead of seeking to minimize its size, it is proposed to dematerialize it, replacing it by a thin perforated steel plate embedded in the laminate (carvalho, 2014). similar to the concept of reinforced glass, the connection relies for its mechanical behaviour on the combination of the significant compressive strength of glass and the significant tensile strength of steel. besides extending the reinforcement to the outside of the laminate, the glass-to-glass contact must be intermediated by a suitable intermediary layer that can deform under compression. the effectiveness of the reinforced glass connection system relies on the combination of three load transfer mechanisms (fig. 2): – adhesive, between glass and steel perforated plate by means of at least two adhesive interlayers; – mechanical, between the perforated steel plate and polycarbonate, using steel bolts and nuts; – contact between glass and polycarbonate edges with the aid of silicone for sealing purposes. fig. 2. scheme of the three main materials that compose the investigated connection and the correspondent three load transfer mechanisms. p.l. carvalho et al. / connecting through the reinforcement 111 the combination of the three-load transfer mechanisms will allow for an even load transfer. in the end, the distributed stress and the visual dematerialization become conceptually congruent. 3. design premises and prerequisites the hybrid condition of the connected through the reinforcement element, with the main materials embedded in one compact solution makes the distinction between glass and connection irrelevant. visually speaking, the “reinforced glass is the connection". with this main premise in mind, a series of fundamental design requirements were identified. the capacity to achieve a smooth optical transition between the glass elements is desired. the joints are also considered critical points in terms of transparency. therefore, the connection design seeks to reduce the joint width as much as possible, without compromising its own transparency or the mechanical properties. the several materials that compose the connection were integrated as much as possible within the laminate thickness in order to preserve the reflective integrity of the glass surface. it will also be beneficial in terms of maintenance to prevent the accumulation of water or dirt on any prominent profile. the ductile failure behaviour achieved with this design (carvalho, cruz & veer, 2012, 2013) must be preserved. the materials making the connection must keep their characteristics that allow for effective and even load transfer. dimensional tolerances must allow for fabrication tolerance and the inevitable temperature changes. simplicity of the design solution is essential in order to allow considerable constructional flexibility. this means that the design solution must allow for the development of several glazing configurations with minor adaptations to the base detail. the capacity to pre-fabricate the glass module as much as possible, leaving for the construction site just the mechanical assembly, is mandatory. additionally, both the work in execution and the assembly work must be kept as simple as possible. only then is it possible to meet the required high standards of quality without major changes on the production structure. it will also improve the future acceptance of the solution for real applications. if a glass panel needs to be replaced, the required disassembly must also be as simple as possible. finally, the detail must provide good water tightness as well as space for an efficient sealing of the interlayer along the protruding steel boundary. 4. the technological challenge of laminating glass with protruded elements the task of integrating a metal piece inside a laminated glass unit requires the application of specific production techniques. contrary to standard laminated glass, the inclusion of a third piece in between the glass panels creates uneven contact, demanding an efficient removal of the air inside to achieve a good final result. for the current investigation, the only method available for the fabrication of the specimens was the silicone blanket system (fig. 3). it is a lamination method targeted for industrial use, known to offer very good productivity at low cost. compared to the alternative vacuum bag system, the main advantage is the simplicity of the preparation work and the avoidance of waste resulting from the process, since the silicone blanket is re-usable. however, it also revealed during the investigation that the elasticity of the silicone blanket tends to apply a considerable force during the vacuum stage that causes displacement of the glass 112 p.l. carvalho et al. / connecting through the reinforcement fig. 3. view of the stacked table structure (left), silicone blanket covering the table (centre) and detailed view of the sealing mechanism performed by the steel angles on the edges of the tables (right). panes. although this displacement was just a few millimetres, which for a normal laminated glass application is usually not problematic, for the investigated technique it is fatal. this lamination process is considered more suited for large glass panes where the self-weight acts as a retention mechanism to the imposed stress by the silicone blanket. additionally, it was clear that this displacement tendency was accentuated by the asymmetrical disposition of the glass panes. when it is vacuum pumped, the silicone blanket tends to concentrate the elements as vertically aligned as possible, preventing staggered placement. during fabrication a simple timber shim was used to fix and protect the metal plate from being deformed. fixing the metal to the timber profile was found to be insufficient since the glass was free to move aided by the very low friction coefficient of the sg interlayer. the initial prototypes revealed some dimensional inaccuracies after the lamination process, requiring the investigation of different solutions to prevent it without compromising the premises. the design development of the connection detail, described in detail in the next section, provided until a certain stage considerable inputs to it, however the inherent complexity of most of the initial design versions revealed inhibitive for accomplishing a satisfactory result. the conviction was that the solution should be simplified as much as possible. to do so, it was decided that the lamination problem needed to be solved outside the scope of the connection design. a method to accurately place and fix the protruded steel plate in the correct position during the lamination process was developed. it consisted of the placement of an auxiliary profile to which the protruded steel was fixed. changing the thickness of the interlayer from 1,52mm to 0,89mm matched it with the steel thickness. only then it was possible to fix the plate and assure its correct position before and after lamination. the auxiliary profile used to fix the protruded steel plate was also responsible for fixing the staggered glass panels in the correct position during lamination. passing a belt over the auxiliary profile and tensioning it before the application of pressure, preserves the relative position between the two glass panes and the protruded steel (fig. 4 right). for the rest of the protruded insert length, unprotected by this profile, it was decided to use timber profiles, made by using carpentry techniques (fig. 4 left). p.l. carvalho et al. / connecting through the reinforcement 113 fig. 4. auxiliary profiles in position functioning as a gauge before (left) and after (right) tensioning of the belt. 5. design of connection detail 5.1. initial version the initial version of the connection design used polycarbonate bars to function as a shim mechanism. to allow for symmetry, the glass panels are staggered at a distance of 10mm. the folded reinforcement layer follows this reconfiguration and is placed with its folding line coincident with the lower edge of the upper glass panel. the geometrical configuration of the pieces is designed in order to execute a shim mechanism both on the upper and lower contact (fig. 5 left). a timberaluminium prototype of this connection design was made (fig. 5 right). the adhesive mechanism is simulated, using bolts and nuts to link the aluminium plates to the timber panels. this prototype allowed to empirically test the effectiveness and mechanical capacity of the solution. it was possible to understand that an excessive fragmentation of the intermediate layer (future polycarbonate bar) was prejudicial for the load transmission. as a consequence of the expected, although reduced, deformation of the elements that compose the interface, disintegration of the connection mechanism was easily achieved. fig. 5. detail (left) and timber and aluminium prototype (right) of the initial version of the connection design. 114 p.l. carvalho et al. / connecting through the reinforcement fig. 6. detail (left) and prototype (right) of the intermediate version of the connection design. 5.2. intermediate version the intermediate version of the connection design focused on the three problems arising from the previous designs, namely the tendentiously large width of the joint, the inefficient tensioning mechanisms between glass edges and polycarbonate, and the unintended displacement of elements during lamination process. the polycarbonate bars’ geometry was developed to a new configuration based on four vertically aligned bars with symmetrical positioning (symmetry as a design principle to equalize pressure during tightening). the idea was to use the outer bars to create structural contact with the glass edges and effectively perform the load transmission, and the inner bars to aid in the lamination process and assembly tasks. these inner bars would be applied on the individual glass panels before the lamination process by means of a transparent double sided bonding tape. this tape would fix the reinforcement layer to the polycarbonate bar, and also to the glass panel, contradicting the tendency of movement during lamination. the bonding tape was chosen for its ease of application, good adhesion to several materials, capacity to adapt different configurations and for being fully transparent. the staggering displacement between the glass panels was increased to 13mm. it enabled to vertically align the glass edges. the position of the reinforcement layer towards the glass panels was adjusted. the folding edge was moved from the upper glass edge to the lower glass edge. it enabled to increase the available thickness to embed the upper polycarbonate bar without overtaking the upper limit of the glass edge. both the adjustment of the glass-to-glass and reinforcement-to-glass relative position allowed for stabilization of four equal contact surfaces on each side, further refining the symmetry of the system (fig. 6 left). laminating with a fixing element revealed visible improvements in terms of dimensional accuracy. although not exactly at the same position as before lamination, the staggered position of the glass panes and the relative positioning of the metal layer were acceptable at a first evaluation. when testing the assembly of all pieces, it though revealed to be a difficult task. the steel bolt had to penetrate six layers of material (four polycarbonate hollowed bars and two steel perforated plates). a considerable looseness of the holes diameter was necessary to accomplish the bolt’s passage. when tensioning, it became clear that the reduced displacement during lamination had significant impact on the final cohesion of the parts. once in place, the several layers of polycarbonate bars, although smoothly polished, revealed a visual blurring; it was due to the significant number of overlapping surfaces. a total of 8 transparent surfaces comprised the connection, which when crossed by light result in a significant reflection, visually impairing the results (fig. 6 right). p.l. carvalho et al. / connecting through the reinforcement 115 fig. 7. detail (left) and prototype (right) of the final version of the connection design. 5.3. final version the final version comprises two symmetrical polycarbonate bars with a simple configuration. one side of the bar presents a bevelled geometry defining the 90◦ fold to be in contact with the glass edge on the upper side, and with the surface of the lower glass pane on the lower side of the connection. this geometry allows for some dimensional tolerance since there is an adaptive deformation capacity of the elements when being tightened. the other side of the polycarbonate bar presents an orthogonal configuration to be inserted in the reduced width joint, close to the reinforcement layers. in the inner gap between the polycarbonate bar and the glass it is proposed to apply silicone for sealing the sg interlayer boundary (penetrated by the protruded reinforcement) as well as on the polycarbonate-toglass contact to provide efficient water tightness to the system (fig. 7 left). this design solution proved to work efficiently during the assembly and tightening operations. since the main compressive loads are transmitted on the superior glass edge, this contact showed considerable consistency. finally, the capacity to see through the polycarbonate bar was thoroughly achieved (fig. 7 right). 6. out-of-plane compressive tests an experimental investigation on the mechanical behaviour of a folded reinforced glass component was desired to evaluate its capacity to be efficiently applied in facades and coverings. without any known reference to test this type of structure, the test setup was designed with the practical application in mind. the out-of-plane compressive behaviour was tested relating to a real application in which wind and other instant distributed loadings are expected. the application of load should be as well distributed as possible, although restricted by the specific folded geometry of the specimen. the capability for handling the specimen during tests, without any mechanical help was considered important for the completion of the tests. for this reason the length of the prototype was limited to 1m. although significantly smaller compared with the final prototype to be built, it was considered to be an element large enough to study the behaviour of such type of structure. 6.1. specimen description the out-of-plane compressive test specimens have a 90◦ folded geometry with a total length of 1m, comprising two symmetrical parts with 0,5m of width connected at the centre (fig. 8). each 116 p.l. carvalho et al. / connecting through the reinforcement fig. 8. diagram of out-of-plane compressive test folded glass specimen (left) and view before test (right). fig. 9. folded glass out-of-plane compression test setup, front view (left) and lateral view (right). part is made of two rectangular shaped pieces of annealed glass of 10mm thick, laminated with two sheets of 1,52mm thick sg interlayers to which a 1mm thick aisi 304 stainless steel perforated plate is semi-embedded. it has the same length as the glass and is 52mm wide; 32mm of this is embedded in the laminate. the steel plates were previously cleaned with isopropyl alcohol to remove oil and dust and assure good adhesion. two polycarbonate bars with bevelled edges are placed on the upper and lower part of the connection to realize full contact between the parts once firmly screwed with m5 (8.8 iso 4017) steel bolts, passing through the aligned perforated steel plate holes, with steel nuts and rings. five bolts were used in each specimen, at a distance of 200mm from each other. one of the specimens exhibited a small flaw close to the centre of the lower edge that had some influence on the results. 6.2. test setup the folded specimens were placed horizontally and supported on the four corners. a contact length of 100mm was ensured in each corner and a cylindrical nylon profile was used as an intermediary between the laminated glass lower edge and the steel base. due to the expected rotation of the glass panels during the test, the cylindrical shape allows for movement. this profile was previously machined to accommodate the laminate (fig. 10 centre). the lateral displacement of the specimen was constrained, using steel profiles fixed to the main frame. some deformation was allowed, using 50mm thick rubber blocks in between the nylon and lateral steel break. the specimens were loaded, using a dynamic loading jack of 300 kn with a ‘i’ steel profile to uniformly distribute the load along p.l. carvalho et al. / connecting through the reinforcement 117 fig. 10. folded glass test setup general view (left), detailed view of the lower specimen support (centre) and top contact between steel frame and polycarbonate joint (right). fig. 11. load-displacement curves of the out-of-plane compression tests on folded glass specimens (left) and detailed results (right). the upper edge (fig. 10 right). the contact is done directly to the polycarbonate bar that composes the joint. the load was applied at a constant displacement rate of 2mm/minute and the deformation was measured vertically on the upper joint and horizontally at the lateral edges (horizontal). 6.3. test results in fig. 11 the load-displacement curves of the three folded glass specimens tested are shown. it shows the vertical displacement of the folded specimen measured on the top joint of the specimen. 118 p.l. carvalho et al. / connecting through the reinforcement fig. 12. general view of specimen #1 (left), #2 (centre) and #3 (right) after test. fig. 13. close-up view of severe cracking of glass at a lower support of specimen #1 (left), isolated cracking of glass near a lower support of specimen #2 (centre) and shell shaped cracking of glass in the upper joint of specimen #3 (right). the three curves are very close to each other showing that the tests are quite reproducible. at the beginning of the test, all three specimens began to deform, opening the folded angle and compressing the rubber layers. at around 80 kn of load the first crack appeared on the specimens, starting at one of the lower supports. in the case of specimen #1 the first cracks appeared in both laminated glass panes at almost the same time, slowly developing along the glass body (fig. 12 left). despite the failure of glass, the specimen was capable of taking considerable additional loading. at 109 kn a second series of cracks started close to the lower support on the opposite side (fig. 13 left). the damage appeared more severe than the previous damage due to the increased load and the asymmetrical deformation already visible on the specimens. these continued to carry the increasing load with increased glass cracking, particularly close to the lower supports. around 130 kn the fragmentation of the glass close to the initial failure was so severe that it locally disintegrated, which considerably reduced the force. specimen #2 also exhibited its first cracking around 85 kn, starting close to a lower support but in only one of the laminated glass panes (fig. 12 centre). it stayed stable until 126 kn when the other glass pane of the laminate broke showing several simultaneous flaws. also in this case, despite the cracking p.l. carvalho et al. / connecting through the reinforcement 119 of both glass panes, the specimen was able to carry a further load increase. it happened around 160 kn, when the glass showed increasing fragmentation, some pieces of which were projected. this occurred in a very localized area, which suffered high stresses (fig. 13 centre). however, specimen #2 did not disintegrate. the considerable deformation of the specimen influenced the application of load with the unexpected result that the steel frame started to deform, which prevented any further increase in load. this phenomenon is clearly seen in the load-deformation curve where a soft ‘plastic’ curve is seen at the top. specimen #3 distinguished itself from the previous two by showing premature cracking of glass starting at the upper edge, close to the point of load introduction (fig. 12 right). although the polycarbonate interface prevented direct contact between glass and steel, possibly flaws remaining from the original glass cutting may have triggered it. the cracks developed very slowly, exhibiting a shell-like shape growing in the horizontal direction along the glass body (fig. 13 right). at around 120 kn of load a simultaneous cracking of four glass panes occurred. this happened close to the two supports on the same side of the specimen, in both laminated panes. it continued to carry an increasing load resulting in new crack branches that developed from both supports. at 130 kn a third main breakage occurred, away from the support area, caused by the pre-test damage mentioned. it meanwhile continued to carry further increases in the load causing considerable deformation of the specimen, until around 150 kn, when similarly to the previous test, the steel frame started to deform, which prevented further increased loading. 7. full-scale prototype a full-scale prototype of folded reinforced glass was built for the international conference on structures and architecture july 2013 held at the university of minho in guimarães. the chosen site was located on the east side of the azurém campus, flanked by the main entrance hall and the main auditorium. it is a rectangular shaped garden, measuring 20m × 5m covered with grass. besides the strategic location, this specific site was chosen for the possibility to bury the auxiliary structure and thus achieve a more dramatic display of the system. the prototype is composed of two folded geometry parallel glass walls. each wall is composed of seven individual plates, connected by using the developed connection system. all the individual plates measure 2950mm × 735mm, with the exception of the first, which measures only 1053mm high in order to show the detail clearly. the total assembly comprises 14 panels defining an area of 5,5m long and 2,6m width (fig. 14). each individual plate is composed of: heat-strengthened glass (2 × 12mm), sg foils (3 × 0,89mm), stainless steel perforated plate (1,2mm) (in certain panels where the steel perforated plate covers the whole area, only two sg foils were used). one of the walls is fully ‘reinforced’, exhibiting glass panels with perforated sheets in its entire surface, while the other exhibits a freer arrangement, alternating zones with and without reinforcement. the purpose is to clearly show the flexibility of the designed connection system in creating structures. the ability to drastically reduce the amount of embedded perforated plate is created by the excellent behaviour of the adhesive mechanism studied during the experimental investigation (cruz, veer & carvalho, 2011). the fact that the polycarbonate bar of the joint has a reduced width of only 16mm, and exhibits a polished and recessed surface in relation to the glass plane, contributes to the desired optical fading of the joint. in the traditional systems, the joint is where the emphasis happens, which prevents a subtle definition of the transparent planes that compose it. 120 p.l. carvalho et al. / connecting through the reinforcement fig. 14. graphic representation of the prototype. fig. 15. placement of the panels in position, using a crane (left) and verification of the angle between panels (right). the assembly work was done with the help of a 30-m arm crane, in order to lift the panels, move them to the correct position and allow a slow and controlled descent (fig. 15 left). the first wall to be assembled was the fully reinforced wall. it was decided to start with the middle panels and build outwards; this in order to reduce and dissipate any relative positioning error. however, this precaution was found to be not necessary and the second wall was assembled in a single row, from one edge to the other. it became clear during the assembly that a ‘learning by doing’ process was happening. the first panels took several hours to fix in the position, while the last were placed and fixed in less than 20 minutes. the adjustment of the panels in a 90◦ position to the clamping profiles was at a certain point difficult, further delaying the process (fig. 15 right). however, in a real building application the aim will be to avoid this type of assembly and simplify the process. at the end, the system delivers a specific aesthetic deriving from its essence, in other words, what materializes is also that what reinforces and structurally connects. the different materials that compose it are characterized by a certain degree of transparency that, once superimposed, offer a complex game of different degrees of transparencies (fig. 16). by changing the angle of observation and lighting conditions, the solution shows its architectural potency. the folded geometry ensures high stiffness, allowing it to be self-supporting both in facades and roofs. p.l. carvalho et al. / connecting through the reinforcement 121 fig. 16. different points of view of the finished folded reinforced glass prototype. 8. conclusions this paper describes the final stage of a research programme focussed on the possibilities of connecting glass panels using embedded reinforcement. the concept was discussed and the materials needed to accomplish it were described with particular emphasis on the design and fabrication issues. the desire to develop a discrete connection solution with folded geometry at 90◦ created several technological problems that had to be solved. the out-of-plane compressive response was experimentally tested and results were quite reproducible. all the specimens showed considerable strength before and after breakage of several glass panes. in fact, the breakage of glass did not affect the increasing of load, which in some cases almost doubled after initial failure. only one of the specimens showed severe breakage on one of the lower supports compromising the integrity. the others showed considerable overall deformation, which did not threaten the integrity of the glass element but prevented further load increase. a full-scale prototype was built to evaluate the architectural possibilities of a folded structure, connected by using the investigated connection system. it comprises two parallel walls of 2,95m high, disposed to invite people to experience the complex vision of different degrees of transparencies, determined by the level of superimposition, angle of observation and lighting conditions. acknowledgments this work is funded by feder funds, through the operational programme competitiveness factors compete, and portuguese funds, through fct foundation for science and technology, under the project ptdc/ecm/116609/2010, “s-glass: structural performance and design rules of glass beams externally reinforced”. references bagger, a. (2010). plate shell structures of glass. studies leading to guidelines for structural glass. phd thesis, technical university of denmark. 122 p.l. carvalho et al. / connecting through the reinforcement carvalho, p. l. l., cruz, p. j. s., & veer, f. a. (2012). connecting through reinforcement – experimental analysis of a glass connection using perforated steel plates. challenging glass 3 – conference on architectural and structural applications of glass. delft university of technology, delft, the netherlands. carvalho, p. l. l., cruz, p. j. s., & veer, f. a. (2013). reinforced glass connection – concept, test and detail. 2nd international conference structures and architecture icsa 2013, guimarães, portugal. carvalho, p. l. l. (2014). (de)materializing detail. technology, structure, design. development of a reinforced glass connection technique, university of minho, portugal (doctoral thesis). cruz, p. j. s., veer, f. a., & carvalho, p. l. l. (2011), perforated steel plate to laminated glass adhesive properties, gpd 2011 – glass performance days international conference, tampere, finland, 219-223. feirabend, s. (2010). steigerung der resttragfähigkeit von verbundsicherheitsglas mittels bewehrung in der zwischenschicht. universität stuttgart, germany. louter, c. (2011). fragile yet ductile – structural aspects of reinforced glass beams. delft university of technology, delft, the netherlands. neugebauer, j. (2013). stainless steel fabric as a connection system for bomb blast glass. in j. belis, c. louter & d. mocibob (eds.), cost action tu0905, mid-term conference on structural glass (pp. 493-498). london, united kingdom: taylor and francis. o’callaghan, j., & bostick, c. (2012). the apple glass cube: version 2.0. in f. bos, ch. louter, r. nijsse, & fred veer (eds.), challenging glass 3 (pp. 57-65). delft, the netherlands: ios press. o’callaghan, j. (2007). an all glass cube in ny city. in glass performance days, proceedings of the 10th international conference (pp. 98-101). tampere, finland. puller, k. (2012). untersuchung des tragverhaltens von in die zwischenschicht von verbundglas integrierten lasteinleitungselementen. universität stuttgart, germany. from city’s station to station city 021 journal of facade design & engineering volume 6 / number 2 / 2018 characterisation of the thermal performance of a novel roof ridge solar hot water system e. fuentes, j. salom catalonia institute for energy research, adrià de besòs, barcelona, spain. abstract the demand for the development of novel systems for the on-site integration of renewable energy in buildings is increasing, in order to reduce the energy consumption resulting from domestic hot water, heating, and cooling usages. within this context, the development of efficient solar collectors for domestic hot water production that can benefit from being integrated in the architecture of buildings is highly relevant.in this study, a novel solar collector device with a tube-in-tube concept, which integrates domestic hot water storage and an absorber in a single unit, is tested under the standard iso 9459-5. the thermal performance of the collector is evaluated by means of the so-called dst (dynamic system testing) method that allows its annual energy efficiency to be predicted under different climate condition scenarios. the study concluded that three collector modules in series can provide a high annual dhw energy coverage of 62-70 % for southern european climates and in the range of 30-40% for central and northern european climates. along with its compactness and efficient design, which allow easier architectural integration on roof ridges, an additional advantage of the system is that its cylindrical geometry makes it possible to rely on a significant surface for full diurnal radiation absorption, independently of solar orientation. with the objective for this new development to be technically and economically competitive compared to available solar domestic hot water systems (sdhw), it is currently in pre-production phase and ready to enter the market in 2018. keywords domestic hot water, solar collector, iso 9459-5, dynamic testing, sdhw, roof ridge doi 10.7480/jfde.2018.2.2082 022 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction new policies and mandates are being increasingly introduced to implement renewable energy technologies for reducing the carbon footprint derived from energy use, including the introduction of solar thermal systems to meet demands for domestic hot water (cte-he4). however, the design of current solar systems is yet to be improved in order to enhance their efficiency and integration in the architectural design of buildings (munari & roecker, 2007). some of the limitations of current solar thermal cells are that they require a southern orientation on tilted roofs and that they compete for space with photovoltaic (pv) cell installations and areas of roof windows. on the other hand, novel solar systems technologies are needed that are more easily integrated within the architectural design of buildings and that provide higher thermal performance. this study describes a new design that addresses some of the limitations indicated above. the solar system presented is a ridge-integrated cylindrical solar water heater that is independent of the space orientation, and is therefore suited to both tilted and flat roofs. it is based on a tube-in-tube concept that integrates the solar collector absorber and the water storage tank within the same design. specially designed for single family houses, the collector has minimal visual impact due to its integration on roof ridges, it does not compete for space with pv cells due to its compact design, and it does not require extra space for water storage. the current work presents the results from testing the energy performance of a single solar collector module using the dst (dynamic system testing) method for rating solar systems as described in the iso 9459-5 standard (iso 9459-5). estimations of annual energy performance were extrapolated to 3 module units, and evaluated under different climatic conditions and dhw tapping profiles using a simulation model in trnsys calibrated with the collector model parameters determined from the dst method. according to the results of the present study, the energy provided by 3 modules can meet more than 50% of the energy consumption for a single-family home. the system performance was evaluated with typical profiles, in terms of volumes and draw-off duration, as defined in standards for testing dhw production devices (en16147). the performance is dependent on the climate, the number of modules, and the tapping profile. high contributions of dhw energy coverage of 62-70% were found for southern european climates, with lower contributions of 30-40% for central and northern european climates. 2 solar system description the module that was tested is a solar collector in which the absorber and the water storage tank are integrated within the same unit (see figs. 1 & 2). the system consists of a tube-in-tube design in which one tube is assembled inside the other. the inner tube stores the hot water while the outer tube is provided with a special layer thanks to which the absorbed solar energy is efficiently converted into heat. the space between the tubes is under very low pressure, enabling the water to evaporate and condense in the inner tube, which is filled with tap water. when the outer tube is heated by the solar energy, the water evaporates. the evaporated water condenses and, in this way, transfers the heat from the outer tube to the inner one, where the hot water is stored. the system is installed on the ridge of the dwelling roof, as modules in series (fig. 2). depending of the number of modules connected, the storage can reach as much as 130l. due to the tubular design of the modules, the system performance is almost independent of the orientation of the dwelling. another advantage of this system is that an additional hot water storage volume in the dwelling is not needed. 023 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 1 cross section schematic of solar collector module fig. 2 cad representation of the installation of solar collector module on a ridge rooftop fig. 3 outdoor installation of the sunridge module used during testing the system has been developed within an innoenergy innovation project (knowledge innovation community of european institute of technology), which supports the developments of products with a relative high trl (technology readiness level), from about trl=6. this product has been in development since the end of 2014 and will be introduced to the market in 2018 as a competitive product. 3 set up for system testing the first prototype of the system (so-called econok system) and the final improved prototype (sunridge module) were tested following procedures in the iso 9459-5 standard (iso 9459-9). performance test methods for solar domestic hot water (sdhw) systems provide designers, manufacturers, installers, and users the necessary information to compare thermal performance among solar systems. the dst method is used to predict the long-term annual performance from a set of short-term measurements described in the iso standard. in this study, two prototype versions of the module were tested at the semi-virtual energy integration laboratory (seilab) at the catalonia institute for energy research (irec). this laboratory is equipped with facilities that are suitable for testing the dynamic performance of energy components and their optimal integration within the building environment. a single module of the econok and sunridge prototypes with a length of 1.4m and internal water volume of 35l, were tested in tarragona (north eastern spain), in march, 2016 and july, 2017, respectively. these months were chosen for the reason that, at the latitude at which the tests were performed, the requirement for total daily solar radiation levels is met from march to october. among other improvements related to easier installation and integration, the sunridge system differs from the first prototype version, econok, in design characteristics that enhance solar absorption and reduce the overall heat losses. the collector module (fig. 3) was attached to a roof test bench that allowed the thermal characterisation of the system by means of the following elements: – flowmeter: used to characterise the flow rate through the collector (cflow) – 2 pt100 temperature sensors: placed at the inlet of the collector (tin) and outlet of the collector (tout), respectively. an additional pt100 sensor was placed on the cold water by-pass line. – meteorological information: outdoor temperature (tamb) and global horizontal solar radiation were measured during the experiments. 024 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 4 laboratory schematic for testing the solar collectors the inlet and outlet pipes of the collector were connected to the laboratory hydraulic circuits as indicated in the schematic in fig. 4. the outlet of the collector was connected to a laboratory thermal test bench that controls the flow rate during dhw extractions. a water tank of 1000l was kept at the desired mains water temperature conditions (10°c) by means of the continuous recirculation of cold water provided by an external cooling circuit. a by-pass system was connected to the collector so that the cold water could be diverted to the test bench when there was no need for hot water extraction from the collector. cold water is circulated through the by-pass line before an extraction event to ensure that the inlet cold water was kept at the required set point temperature at the beginning of each water draw-off. in the dst method, a mathematical sdhw model is used to predict energy performance from the available experimental data. the measuring data are obtained from a series of short outdoor tests on the sdhw system. the data obtained is used together with the sdhw mathematical model in order to identify model parameters, which characterise the behaviour of the sdhw system tested. in order to predict the annual thermal performance for a selected climate condition and daily tapping profile, a sdhw computer model configured with the identified parameters was used. three types of measurement sets comprise the dst testing method: the so-called s-sol tests (a and b sequences) and the s-store test. the aim of tests a is to obtain information about collector performance at high efficiencies. the aim of tests b is to acquire information about the store heat losses and collector array performance at low efficiencies. the s-store test sequence is intended to identify the overall store losses. test sequences had to comply with a set of conditions for the test days to be valid. the following table summarises the deviations of the variables to be controlled during the tests, which comply with requirements defined in the iso standard. besides the above, the requirement of a minimum of 12 mj/m2 solar radiation per day for a test day was met for the experiments conducted. 025 journal of facade design & engineering volume 6 / number 2 / 2018 mains water temperature water flow (2 lpm) water flow (10 lpm) standard tests standard tests standard tests ±3k -1.5 k/+2.4 k ±0.5 ±0.3 ±1 ±0.35 table 1 mean deviation of control variables during laboratory tests in comparison with the iso standard requirements fig. 5 evolution of inlet temperature (tin), outlet temperature (tout), ambient temperature (tamb), global radiation and volume flow rate through collector during the a test sequence. fig. 6 evolution of inlet temperature (tin), outlet temperature (tout), ambient temperature (tamb), global radiation and volume flow rate through collector during the b test sequence. 026 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 7 evolution of inlet temperature (tin), outlet temperature (tout), ambient temperature (tamb), global radiation and volume flow rate through collector during the s-store sequence. 4 results figs. 5 and 6 show the evolution of the inlet (tin) and outlet (tout) temperatures of the collector, the water flow rate (cflow), the ambient temperature (tamb), and the global solar radiation during three valid test a and test b days for the sunridge prototype. during a water draw-off, a sharp drop in the inlet temperature was observed as a result of the activation of the by-pass and the circulation of cold water through the collector. on the other hand, at the start of each extraction, the outlet temperature increased slightly and then dropped as the cold water circulated through the tubing. maximum temperatures achieved during water extractions for a type experiments were in the order of 35°c. the highest thermal power obtained correlated with the peak of solar radiation, with a maximum of 14kw. for the b sequence (three valid days), similar behaviour was seen, with higher temperatures achieved than for the a days (maximum of 50°c). regarding the released heat, a maximum of 25kw at peak radiation conditions was obtained. the test sequence s-store is illustrated in fig. 7. this test consisted of two valid b days, after which the collector was covered to cool down for 36 hours. similar to the b test days above, in the s-store test, the maximum temperatures were close to 50°c. the dst model parameters obtained from the testing were used as inputs for a simulation model that was built in the software trnsys in order to evaluate the performance of a set of collectors in different climates and under different tapping profiles. this allowed for estimations of yearly energy production for several modules for both prototypes to be provided. calculations were done for cycles s (36l), m (100l), l (199l), xl (325l), and xxl (420l) for different european climates. the names s, m, etc. of the cycles represent tapping profiles as defined in testing standards for water heaters and heat pumps, depending on the total daily consumption of standard buildings (en16147:2011). the climatic conditions used for the calculations were de bilde, athens, stockholm, wurzburg, and barcelona, as shown in fig. 8 (left). as expected, better performance is obtained for the southern latitude countries, where the solar radiation is higher throughout the year. 027 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 8 left: annual delivered power for different daily consumption profiles for econok and sunridge modules. right: increase in the delivered power by the sunridge prototype with respect to the econok prototype. increase in power (%) = (power sunridge-power econok) x100/((power sunridgepower econok)/2) this last figure also illustrates the improvements achieved with the second prototype with respect to the first version, thanks to the lower energy losses achieved in the new prototype. the sunridge prototype showed an increase in the energy delivered, from 5 – 22%, depending on the climatic conditions and water consumption profile. the design differences between the econok and the sunridge module are based on a change in the water circulation design inside the collector and the material of the absorber. in the econok module, the inlets and outlets for water circulation are on the same side of the tube while in the sunridge module the inlet and outlet are placed on the opposite edges of the tubing. in addition, the absorption material in the sunridge module has a spectral selective coating material that was not used in the first prototype. table 2 summarises the predictions of the fraction of annual energy that would be covered by 3 solar collectors in series for a single-family home with dhw demand of 100l/day (cycle m), which corresponds to an annual energy demand of 7.62 gj/year. a significant improvement is achieved with the second prototype in terms of energy coverage thanks to the reduction in the heat losses. for warm climates, the energy coverage can reach values up to 70%, while in higher latitudes the energy coverage can be as high as 39%. the table also includes values of collector performance calculated as the ratio between the annual dhw energy produced and the annual solar radiation in % (parameter prototype namecollect). this value of collector performance ranges from 21.7 to 29% for the sunridge collector, with an improvement of up to 3% with respect to the first prototype version. the results of the study indicate that this novel system can provide significant energy savings, however, an auxiliary heater will still be necessary in order to meet the annual energy requirements and raise the water temperature to the levels required for avoiding legionella issues. no additional water storage will be required with several modules since 3 units provide a storage volume of nearly 100l. the system has successfully passed stagnation tests (with empty and filled storage), demonstrating that it meets the required standards to guarantee good performance under a variety of environmental and usage conditions. 028 journal of facade design & engineering volume 6 / number 2 / 2018 parameter de bilde stockholm wurzbug barcelona athens econok-dhw 28.9 31.4 34.5 53.8 65.8 sunridge-dhw 32.2 35.6 38.7 62.1 70.5 econok-collect 20.7 19.2 19.8 23.9 27.0 sunridge-collect 23.0 21.7 22.2 27.7 29.0 table 2 fraction of dhw annual energy coverage (%) (prototype-dhw) and solar collector efficiency (%) (prototype-collect) provided by 3 collector modules for a single family home consumption profile (100 l/day, cycle m) at different european climate zones. 5 summary and conclusions a new type of solar collector prototype has been tested for energy performance according to guidelines in the standard iso 9459-5. long term prediction performance has been compared for 2 prototype versions of the system for different climatic conditions using european standards tapping profiles. the evaluation of the performance of the second version of the prototype indicates an improvement in the thermal energy efficiency of 5-22%, depending on the climatic conditions and water consumption profile. the improved performance is a consequence of lower heat storage losses achieved with design modifications compared to the first prototype version. an annual coverage of the dhw energy demand of 62-70% was found for southern europe climates and between 30-40% for central and northern european climates. the compactness and the high performance of this system makes it highly competitive as a novel renewable energy technology when compared to other sdhw products on the market, since with a similar cost to current designs it provides good performance and easier integration than standard solar panels. references cte-he4 (2010). código técnico de la edificación, 30 de septiembre de 2010. he4 contribución mínima de agua caliente sanitaria. [technical guidelines for buildings, he4 minimum contribution of solar energy for domestic hot water production]. spainmunari, m.c. & roecker, c. (2007). towards an improved architectural quality of building integrated solar thermal systems (bist), solar energy, 81 (9), 2007, 1104-1116. iso 9459-5:2007, solar heating – domestic water heating systems – part 5: system performance characterization by means of whole system tests and computer simulation. en16147:2011 heat pump with electrically driven compressors. testing and requirements for making of domestic hot water units. acknowledgements this development is supported by innoenergy a knowledge innovation community through the funding of the kic sunridge project. innoenergy is supported financially by european institute of technology. in this project the following parties are in cooperation: monier, sme company that produces of roofs and tiles tno, research institute in the netherlands, irec, catalonia institute for energy research rtb de beijer, innovative sme company artenergy, sme company from city’s station to station city 163 journal of facade design & engineering volume 6 / number 3 / 2018 the role of geometry for adaptability: comparison of shading systems and biological role models susanne gosztonyi1,2 * corresponding author 1 competence center building envelope, institute of civil engineering, lucerne university of applied sciences and arts engineering and architecture, technikumstrasse 21, 6048 horw, switzerland, susanne.gosztonyi@hslu.ch 2 architectural facades and products research group, design of construction, architectural engineering and technology, tu delft, julianalaan 134, 2628 delft, the netherlands abstract dynamic shading systems represent the majority of realised adaptive façades. it seems that geometrically complex kinetic solutions have increased in recent years, mainly due to the use of parametric design tools and digital production. in most shading systems, however, geometry rarely plays a guiding role in the design. the kinetic mechanisms are confined to linear or planar geometries. geometry plays an important role in biological organisms, because it is the decisive factor for efficiency and growth. their growth patterns could provide new insights for dynamic shading designs. for this, spatial morphology criteria for shading systems were identified to obtain criteria directly related to geometry. these were supplemented by criteria on kinetic mechanisms. then, biological analogies that correlate geometrical structures with adaptability were sought. using biomimetic methods, particularly from functional morphology, principles in growth patterns were analysed and compared to shading systems. it revealed that the restriction to space, location, and material-inherent properties does not affect the solution diversity, but follows an evolutionary objective: plants, for example, use ingenious geometrical structures to allow adaptation, mainly over lifetime but also dynamically. whether these principles can be applied to the design of dynamic shading systems is then discussed. the aim of the paper is to provide impulses for further studies on adaptive shading systems that focus on the innovative use of space with greater flexibility in motion. the overall premise of the paper is to demonstrate the applicability of biomimetic methods for architectural engineering. keywords adaptive facades, shading systems, biomimetics, geometry, growth pattern, kinetic mechanisms, spatial morphology doi 10.7480/jfde.2018.3.2574 164 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction dynamic shading systems are of particular interest in the framework of energy efficiency strategies in buildings, because the cooling energy demand raises continuously. the research study ‘cost efficient solar shading solutions in high performance buildings’ mentions that “dynamic solar shading leads to mean cooling energy savings of more than 36% when averaged across all glazing types and climate conditions in europe”; and it could increase to 65-70% for south-west orientated facades in central europe (hutchins, 2015). thus, dynamic shading systems seem to be one of the key measures for drastically decreasing the cooling energy consumption of buildings in europe. however, recent evaluations show that implemented measures with regard to shadings do not show the desired effect (hutchins, 2015; werner, 2016). since there are few studies on the causes, one can only speculate. one obstacle to effectively operating dynamic shading systems may be the conflict of shading versus visual comfort (view out, use of daylight). this affects the energy consumption of artificial lighting during shading periods. the conflict might be solved by the — currently somewhat neglected — design of shading systems. conventional products show mainly linear and planar geometries with limited adaptive morphology. since there are few alternatives that are economically feasible and promise a certain robustness, the potential of the geometry of shading surfaces is yet to be explored. a closer look at the geometrical characteristics of dynamic shading systems raises questions, two of which are discussed in this paper: what role do geometrical patterns play in current shading systems? and, how do spatial morphology criteria and geometrical forms influence the flexibility of adaptation? ensuring the best possible functionality and adaptability by using geometrical growth patterns is an essential requirement of biological evolution. the systematic search for analogies in nature could show potentials, particularly for the second question, and enable a design shift away from the neglected geometry to innovative shading geometries. the aim of the paper is to present geometrical patterns of conventional shading systems and draw a link to biological role models that deal with surface optimisation strategies through geometry. the goal is also to illuminate the role of geometrical forms for energy efficiency in this context and to stimulate further studies as to whether spatial designability influences functionality. the paper begins in section 2 with a description of the applied methodology to identify various geometrical and functional mechanisms and continues in section 3 with the categorisation of parameters of shading systems that are linked to spatial morphology, in order to deal with the first question. section 4 deals with the potentials linked to geometrical forms and functions in nature in order to demonstrate the link between geometry and performance optimisation. it also briefly discusses some principles of the identified geometrical peculiarities in order to determine a possible transfer to dynamic shading systems, which addresses the second question. in the conclusion, a hypothesis is put forward in relation to a re-design strategy for dynamic shading systems based on geometrical patterns, which might overcome the conflict between performance and visual quality. 165 journal of facade design & engineering volume 6 / number 3 / 2018 2 methodology the use of biomimetic methods to identify biological potentials for advanced building design is a trend that has been increasing for several years. within the field of adaptive façades, optimisation investigations on daylight and shading components by applying biomimetic principles are a central topic. studies on shape morphing solar shadings by fiorito et al. (2016) and pesenti, masera, fiorito, & sauchelli (2015) can be cited as exemplary. while many activities focus on the development of new material composites (lienhard et al., 2011) or design recommendations in a ‘biomimetic’ manner (menges, 2012; al-obaidi, ismail, hussein, & abdul, 2017), very few studies are targeted at employing biomimetic methods for re-designing or upgrading existing material and system solutions. this work aims to contribute to this objective by presenting some biomimetic principles for the re-design of geometrical forms for effective dynamic shading systems. as an initial step towards understanding the role of geometry in the adaptive functionality of shading systems, spatial morphology criteria and kinetic patterns of conventional shading systems were developed. these were then assigned to different shading types in order to classify geometrical and motion-related parameters. in the next step, biological role models, showing geometrical and functional dependencies for the given context, were searched by applying the biomimetic analogy method. to understand the relations between patterns/shape, functions, and behaviour of the role models, a combination of methods from functional morphology, the ‘structure-form-(behaviour)function’ model (sartori, pal, & chakrabarti, 2010), and underlying physical laws are applied. it is assumed that patterns and forms in nature follow the laws of physics and thus can be (roughly) explained with mathematical formulae (cohen, reich, & greenberg, 2014). some conclusions about these relations were drawn in this work. while it is already a complex process to understand and abstract biological ‘structure-behaviour-function’ relationships, some go even one step further towards identifying generic design patterns (cohen et al., 2014). this intention is also a motivator for this work, which, so far, is only presented as a hypothesis in this paper. 3 spatial morphology of shading systems dynamic shading systems represent the majority of adaptive façade systems according to case studies in the cost “adaptive façade network” (cost tu1403, 2018) (loonen, trcka, costola, & hensen, 2013) (aelenei, aelenei, & vieira, 2016). in addition to the many functions that a dynamic shading system must fulfil with regard to aesthetic, visual, thermal, or structural requirements, its adaptability is the most critical task – more so than with any other façade component. in the design phase, however, shading systems are primarily regarded as an intangible factor for overheating or solar gains evaluation. in energetic building performance evaluations, they are considered as a static value or a range of static values representing worst, best, and standard cases. their optical properties (transparency, reflectance, emissivity), their influence on daylight quality (daylight transmittance, glare protection, visual quality), and their control strategies are taken into account by global data. the role and performance impact of the specific geometry of an element, as well as its related kinetic patterns, is not considered. few studies have been found during the literature survey for this work that focus on specific physical characteristics related to the (static) geometry of shading elements in order to enable better energy performance (fiorito et al., 2016) (cohen et al., 2014) (pesenti et al., 2015). 166 journal of facade design & engineering volume 6 / number 3 / 2018 3.1 parameters for spatial morphology whenever kinetic movements of façade components occur, certain geometrical and mechanical parameters are taken into account to allow a change of state. scale, size, and positioning of individual components, as well as the spatial extension and modularity of the system, are some of these parameters that have to be considered when designing adaptive (kinetic) shading systems. some respective criteria were identified from the analysis of the case studies in cost “adaptive façade network” (cost tu1403, 2018) and further developed for a first draft of spatial and kinetic criteria of adaptive facades (gosztonyi, 2015). they are summarised as “spatial morphology criteria” (fig. 1). one such criterion is the ‘physical impact’, which describes the geometrical appearance of the system, such as planar, linear, or polygonal patterns of the surface, and their changing appearance in the several adaptation states. this also describes the kinetic motion along defined axes (one or multi-axial). the second criteria, ‘repetitive structures’, describes the geometrical form itself and the modularity of the elements. while most elements are usually standardised (e.g. strip fins, planar textiles), there is no standard solution for freeform and curved elements. parametric design considerably supports the development of freeform geometries in order to achieve higher motion flexibility (and performative optimisation) (barozzi, lienhard, zanelli, & monticelli, 2016). the third criteria, ‘spatial versality’, is linked to the adaptation mechanisms and its space requirements. being mounted on guiding rails, hinges, or brackets, shading elements cause a spatial intrusion into the third dimension by folding, wrapping, rolling, and shifting, among others. the mechanisms define the kinetic morphology of the system and determine the coverage pattern of the façade surface. this criteria also describes the space that is needed for the motion, which is critical for the choice of the solution. fig. 1 spatial morphology parameters for shading systems: (a) ‘physical impact’ deals with the visual kinetic patterns of the shading system in various adaptation states, (b) ‘repetitive structure’ with the geometrical properties (size, scale, form of the element), and (c) ‘spatial versality’ with the mechanisms and need of space for motion. these parameters describe the geometrical design of the system (images retrieved from thermocollect, pinterest.com). 167 journal of facade design & engineering volume 6 / number 3 / 2018 3.2 categorisation of shading systems to make the geometrical characteristics of conventional shading systems visible, they are categorised according to their assembly types, orientation and motion, material properties, position relative to the façade, and the already mentioned spatial morphology criteria, as shown in table 1. these parameters are considered to have a direct link to geometrical constraints, although there are other criteria that might indirectly influence the geometry (e.g. comfort requirements, climatic situation, economic constraints). types façade orientation preferences position preference material motion “spatial morphology criteria” physical impact spatial versality repetitive structure overhangs, fins, shelfs south exterior all fixed static; planar, laminar appearance horizontal expansion; space need is high one unit brise-soleil, louvres east, west exterior all fixed (with moveable or fixed slats) semi-static; laminar appearance horizontal; space need is medium to high one element (repetitive) awnings all exterior textile, aluminum, plastic fixed, moveable framed, homogenous, planar appearance horizontal, sloped; space need is medium to high one unit roller, shutters all exterior steel, aluminum, plastics, glass moveable laminar, planar appearance horizontal, vertical; space need is medium to high one element (repetitive) venetian blinds all exterior, interstitial, interior aluminum, metal, wood, glass, plastic, textile moveable laminar appearance horizontal, vertical; space need is minimal one element (repetitive) blinds, screens all exterior, interstitial, interior aluminum, metal, wood, plastic, textile moveable planar, circular, polygonal appearance vertical; space need is minimal one element (repetitive) drapes, curtains, blackout screens all interior (seldom exterior) textiles, plastic moveable planar appearance vertical; no space is need one unit table 1 categorization of conventional shading systems: identified parameters that provide spatial information or influence on geometry 168 journal of facade design & engineering volume 6 / number 3 / 2018 the constructive characteristics of shading systems are further classified by structural frame types (if not self-supporting), suspension systems (guide rails, hinges, brackets), and kinetic actuators (hydraulic, electric). positioning relative to the building skin can be either external, internal, or interstitial, whereas the choice defines the spatial expansion bandwidth and performance efficiency. besides being the best choice for thermal protection, external shading devices provide the most complex geometries and also higher structural and durability requirements, due to the exposure to climatic conditions and aesthetic visibility. interstitial systems are less demanding in terms of spatial and structural requirements, but cause complicated maintenance requirements when they are moveable. for example, in closed-cavity façades, there is no maintenance option after being installed. thus, the whole element must be exchanged in case of malfunction. 3.3 kinetic patterns folding, rolling, shifting, etc. are kinetic movements that require certain geometrical arrangements. shading systems mainly use laminar (fold, flap) or planar (roll, shift) geometries to allow oneor twodimensional motion. this approach limits the flexibility of shade vs. non-shade areas, and increases the conflict between shading and visual quality tasks. either one or the other will not perform well, because the surface is shaded either too much or too little in relation to actual needs. polygonal shapes, on the other hand, allow higher flexibility to cover precisely defined areas and allow the use of planar structures to enable a multi-directional motion (fig. 2). fig. 2 geometrical forms and motion types: planar geometries (a) and laminar geometries (b) move generally in one or two dimensions, using a oneor two-axial mechanism. three-axial mechanisms need more flexible forms, resulting in polygonal geometries (c) or, at least, rectangular geometries (d) allowing free motion towards three-axial mechanism. (images retrieved from flickr.com, pinterest.com, wikipedia.com). this observation suggests that geometrical forms of shading systems seem to be directly related to motion-related criteria. the more flexible the form, the more flexible is the adaptation mechanism, and respectively, the motion pattern, and vice versa. the identified criteria of this observation are summarised in table 2. 169 journal of facade design & engineering volume 6 / number 3 / 2018 kinetic motion type geometrical form coverage of space direction of motion adaptation system kinetic mechanisms fold (e.g. blinds) planar, laminar form polygonal form linear, grid coverage 2-dimensional 3-dimensional guiding rails (in various positions), hinges, racks, racks with hydraulic actuator (into z-axis) roll (e.g awnings) planar form planar coverage 1-dimensional 3-dimensional brackets, cords, reel shift (e.g. screens) planar, laminar form polygonal form planar, grid coverage 1-dimensional guiding rails wrap or lift (e.g. curtains) planar (flexible) form planar coverage 1to 2-dimensional guiding rails, cords, reel flap (e.g. rotating screens) planar, laminar form polygonal form linear, grid coverage 2-dimensional hinges or brackets, fixed in rails (rotation point) table 2 kinetic motions of shading systems: selection of most applied motion types and their related criteria for adaptability and geometry 3.4 motion into third dimension as mentioned, the complexity of the kinetic mechanisms increases with the complexity of the geometry of the components. the kinetic façade of the al-bahr tower in abu dhabi (attia, 2015) is a representative example of a complex, multi-directional folding mechanism. inspired by the design of the arabic mashrabiya, the architect developed origami-like shading “umbrellas” that fold radially via a linear actuator into the third dimension (like the opening of a blossom). planar ptfe triangle units are steered by hydraulic actuators that “progressively open and close once per day in response to a pre-programmed sequence” (ctbuh, 2018). there are a few examples that use e.g. planar forms, such as the shifting panels of tessellate™ by the initiative ‘adaptive building initiative’ of the a. zahner company, or the lenses of the arab world institute in paris by jean nouvel, to generate hexagonal geometries and patterns. very few examples allow motion into the third dimension using rectangular geometries, such as e.g. the wind veil façade project in gateway village by ned 170 journal of facade design & engineering volume 6 / number 3 / 2018 kahn. finally, newer solutions liberate themselves from geometrical and kinetic mechanisms and create a motion into the third dimension through their material-inherent properties, such as e.g. the biomimetic "materialsystem hygroscope" designed by achim menges and steffen reichert or the use of shape-memory alloys (smas). all of these examples (also shown in fig. 3) have a more or less deep impact on the spatial morphology. fig. 3 grid patterns and kinetic motion in complex systems: (a) hydraulic umbrella shades of al bahr tower fold threedimensional, (b) the tesselate ™ concept shifts decorative metal sheets into changing grid patterns, (c) the institut du monde arabe uses a complex photo lenses-like system, (d) the wind veil façade of ned kahn allows wind to play with freely moving metal sheets, and (e) the adaptive biomimetic wood veneer hygroscope from achim menges and steffen reichert is able to bend automatically according to air humidity change. (images retrieved from flickr.com, pinterest.com, wikipedia.com). it might also be of interest to mention that some complex shading geometries are derived from local climatic conditions and related socio-cultural relations: grid-like, repetitive patterns, such as the mashrabiya in the islamic culture, are more frequent in regions with higher demands on privacy and higher solar radiation (subtropical, tropical, arid climate) than in cooler climatic zones. grid-based forms also leave a constant shading pattern due to their frame structure if it is not fully removeable. the adaptation degree is limited to the element within the grid. this will not be addressed in more detail in this paper. however, it is interesting that these patterns are based on geometrical formulae described by mathematical rules. these are seen as “universal law” in nature (cf. stankov, 2018). 4 growth geometries in nature according to the works by thompson (1945), “on growth and form”, and to more recent publications from ball (2009), morphological and physiological adaptation has its causality in mathematical problem-solution. it is widely accepted that growth and form developing processes in nature use the laws of physics, whether inanimate or animate bodies. nature deals with geometrical optimisation to allow growth at any time and any direction. thus, applying mathematical analysis helps to understand patterns in nature (cp. turing rd model) (kondo & miura, 2010) and might also support the understanding of adaptation mechanisms. it shall be noted that morphological processes in biology are strongly connected to chemical agents and triggers, and influences are difficult to describe solely with mathematical formulae (morrison, 1987; ball, 2009). the basic geometrical form (starting from the molecular level) in biological morphologies is a gridbased shape, based on circular or polygonal units. together with the basic form, certain growth patterns, such as spiral and sequential growth, allow the biological system to develop and adapt its form. thus, to understand the adaptation mechanisms of biological organisms, the understanding of their basic geometrical form is necessary. in this section, examples of biological seemingly static growth patterns are presented to discuss their growth principles. although these patterns are not directly associated with kinetic motion, they provide insights into the optimisation of surface geometries for (possible) multi-dimensional adaptability the goal of kinetic systems. 171 journal of facade design & engineering volume 6 / number 3 / 2018 the second part of this section then presents some kinetic mechanisms in nature and their possible relation to geometrical forms. it should be noted that geometrical forms, growth patterns, and kinetic mechanisms are not necessarily combined in one organism, but may be combined later in a technical solution. 4.1 universal law in nature two general questions guide the search for biological role models in the context described above: do geometrical forms play a role in adaptations of biological organisms? and, if so, how do geometrical forms support adaptability? shape is crucial for survival and adaptation to local conditions. a good example of this is the ecogeographical rules; these rules state that related species have developed different characteristics depending on the geographical region in order to adapt evolutionarily to the respective climatic conditions. this can affect the body volume (bergmann’s rule) or the relative size of the extremities (allen’s rule). carl bergmann suggested that the surface area to body volume ratio of animals correlates directly with the temperature of the region. mammals and birds in cold regions are usually larger than in warm regions to efficiently maintain or to release body heat (encyclopaedia britannica, 2017). large bodies have a smaller area to volume ratio. the allen’s rule, as a corollary rule to the bergmann’s rule, states that warm-blooded animals in colder regions have shorter protruding body parts relative to their body size than those in warmer regions for the same thermo-regulating reason (encylopedia.com, 2018). furthermore, animals living in regions of higher humidity have darker pigmentation than those living in drier regions, which is stated by the gloger rule (allaby, 2018). these rules are found in any evolutionary adapted animal, as well as in plants. although these examples are not dynamic in the sense of the paper, it can be assumed that certain geometrical forms and evolutionary growth patterns also support dynamic adjustments. in the search for these principles, especially in plants (which are unable to move and need to adapt to various local changes and impacts), it has become apparent that particularly geometrical patterns of surfaces facilitate dynamic adaptation. thus, the analogy search is divided into the investigation of basic geometrical forms (basic growth patterns) and dynamic adaptation mechanisms (kinetic mechanisms). 4.2 basic growth patterns in nature surface structures and their subsystems are decisively responsible for the control of environmental impact. their biological patterns, applying geometrical principles such as the golden ratio, platonic bodies, and sequential growth, allow differentiated and adaptable morphologies. figures of pentagonal symmetry and with a high repetitive pattern, in particular, are closely linked to growth. for example, the geometrical arrangements of seeds, branches, leaves or petals using the golden ratio allow not only optimisation of the surface area to the solar exposure, as shown in the sunflowers (fig. 4, a), but also enable kinetic (folding) mechanisms, as shown by the fern leaf (fig. 4, d). the golden ratio defines herein the geometrical basis for the ability to change, which is enabled by growth patterns such as the fibonacci sequence. the mathematical connection between the golden ratio and the fibonacci sequence is shown in the golden spiral, a proportional growth of φ in a rectangular pentagon (see (c) in fig. 4), which appears in the static structure of the sunflower blossom and also in the dynamic rolling function of the fern leaf. at first glance, the sunflower does 172 journal of facade design & engineering volume 6 / number 3 / 2018 not appear to be a suitable role model for the investigation, since the surface of the sunflower is static and oriented towards maximum solar radiation harvesting in a confined space an opposite intention to the goal of shading systems. the basic geometrical form, however, allows spatial expansion; the individual seeds of the flower could be multiplied into three-dimensionality within the condensed area due to their polygonal structure. this polygonal surface also corresponds well to the promising examples of complex kinetic shading systems. the aim could be to enlarge the shading surface without consuming more façade surface area – voronoi, tessellation, tangram geometries, and origami patterns could serve as a possible mathematical transfer path. in addition, these geometries allow multi-directional motion, as the fern leaf shows (fig. 4, d). shading systems would have more flexibility for individual shading of the surface if this approach were used instead of the conventional one. fig. 4 biological role models to demonstrate geometrical forms for the optimisation of surfaces and for growth patterns (images retrieved from www.greatmathsteachingideas.com, pinterest.com, wikipedia.com). 4.3 kinetic mechanisms in nature the screening of the biological database of the bioskin project (gosztonyi, gruber, judex, brychta, & richter, 2013) revealed that dynamic adaptation and geometrical form optimisation are not always to be found in one role model. for example, adaptive biological organisms that cannot move change their properties ‘passively’ through inherent structure-material characteristics. these can respond dynamically to environmental changes by changing their properties or effects to the environment, e.g. by structural colours, photonic crystals. one example of this kind of adaptation is the dynastes beetle (see left in fig. 5). other ‘active’ adaptations are achieved by kinetic mechanism activated through physiological or biophysical processes, such as e.g. folding or curling processes initiated by the turgor pressure, as applied in the mimosa pudica (see right in fig. 5). kinetic mechanisms are not necessarily related to geometry but influence its morphology. folding or rolling mechanisms seem to be the most commonly applied adaptation mechanism for shading systems. this also applies for biological role models – insofar as they have been investigated in this work. however, a refined approach must be applied by using detailed abstractions of the search questions and by combining role model functions. 173 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 5 adaptation mechanisms in nature: dynastes beetle (left) presents a static adaptation by photonic crystals that change its colour according to a changing humidity level. the mimosa pudica (right) adapts fast if contact occurs due to the turgor pressure, an osmotic flow of water through cells (images retrieved from wikipedia. sketches: s. gosztonyi, & s. richter). 5 conclusion to answer question one, regarding the role of geometrical patterns in shading systems. geometrical patterns do not play a major role in conventional shading systems; it seems that the goal of covering the façade area with simple or maximum area-covering forms is of utmost importance. more complex geometrical forms, such as circular or polygonal geometries, are found in vernacular shading systems and have become more popular today due to the digitalisation in design and production. it is assumed that a further development of the polygonal geometries for shading systems could lead to a better interaction between visual comfort and shading function, because the shaded area can be more specifically defined. a follow-up study to this assumption is currently in development. the second question, about the influence of spatial morphology criteria and geometrical form on the flexibility of adaptation, has not yet been fully answered. some technical solutions have been studied and it has been proven that more complex geometrical forms are more closely related to a higher flexibility of kinetic motion. in polygonal forms, the kinetic mechanism allows any movement into the third dimension, but simple kinetic mechanisms, such as folding and rolling mechanisms, also allow this expansion. the investigation of biological role models and their adaptation mechanisms supports the hypothesis that polygonal surface geometries (whether at micro or macro level) are the basis for flexible dynamic motions. these geometries enable the multidirectional ‘growth’ of a system. a possible transfer link between biological principles and a technical solution could be the application of mathematical models, such as the voronoi, tessellation, tangram geometries, and origami patterns. the adaptation patterns in nature have so far only been touched upon and will be a core topic for further studies in order to search for further answers to the second question. 5.1 next steps the purpose of future studies is to continue the above-mentioned investigations and to develop prototypes using certain mathematical models in order to create multi-directional kinetic shading systems that do not use more space but shade more flexibly. furthermore, the assumption will examined that the visual quality and shading efficiency improve equally if the shaded area of a façade is defined by a grid. 174 journal of facade design & engineering volume 6 / number 3 / 2018 acknowledgements the paper is based on earlier work of the author on biomimetic potentials for energy efficiency of façades and the development of design criteria for adaptive façades in a short-term scientific mission of the cost tu1403 “adaptive façade network”. the categorisation of dynamic shading systems was created on the basis of the database of the working group 1 in cost tu1403. the author would therefore like to thank the members of the cost tu1403 for the information and framework conditions to carry out this work, as well as the tu delft architectural façades and products research group (af&p) for the valuable discussions, which were beneficial to the general work related to this topic. references aelenei, d., aelenei, l., & vieira, c. p. (2016). adaptive façade: concept, applications, research questions . energy procedia 91, pp.269-275. allaby, m. (2018). gloger’s rule. retrieved from oxford reference a dictionary of zoology. retrieved from http://www.oxfordreference.com/view/10.1093/acref/9780199233410.001.0001/acref-9780199233410-e-3667 al-obaidi, k. m., ismail, m. a., hussein, h., & abdul, a. m. (2017). biomimetic building skins: an adaptive approach. j. renew. sustain. energy review 79, pp.1472-1491. attia, s. (2015). evaluation of adaptive facades: the case study of al bahr towers in the uae. adv build skins (economic forum, 2015), pp.1254 1262. doi:10.5339/connect.2017.qgbc.6 ball, p. (2009). shapes. nature’s patterns. a tapestry in three parts. oxford: oxford university press. barozzi, m., lienhard, j., zanelli, a., & monticelli, c. (2016). the sustainability of adaptive envelopes: developments of kinetic architecture. procedia engineering 155, pp.275-284. cohen, y., reich, y., & greenberg, s. (2014). biomimetics: structure function patterns approach. j mech des 136 (1), 111108. doi:10.1115/1.4028169 cost tu1403. (2014-2018). cost tu1403 – adaptive facades network. retrieved 05 28, 2018, from cost tu1403 adaptive facades network: http://tu1403.eu ctbuh. (2018, 04 07). al bahar towers external automated shading systems. ctbuh innovation award 2012. retrieved from council on tall buildings and urban habitat annual award. retrieved from http://www.skyscrapercenter.com/building/ id/9129 encyclopaedia britannica. (2017, 05 16). bergmann’s rule. retrieved from encyclopaedia britannica. retrieved from https://www. britannica.com/science/bergmanns-rule encylopedia.com. (2018, 04 16). allen’s rule (originally published by oxford university press 2004). retrieved from encylopedia.com a dictionary of ecology. retrieved from https://www.encyclopedia.com/earth-and-environment/ecology-and-environmentalism/environmental-studies/allens-rule fiorito, f., sauchelli, m., arroyo, d., pesenti, m., imperadori, m., masera, g., & ranzi, g. (2016). shape morphing solar shadings: a review. j. renew. sustain. energy review 55, pp.863-884. gosztonyi, s. (2015). adaptive façade – which criteria are needed? facade 2015 computational optimisation (proceedings, hs owl, detmold), pp.84 95. gosztonyi, s., gruber, p., judex, f., brychta, m., & richter, s. (2013). bioskin research potentials for biologically inspired energy efficient façade components and systems. vienna: nachhaltig wirtschaften bmvit. hutchins, m. (2015). high performance dynamic shading solutions for energy efficiency and comfort in buildings. executive summary. abingdon, uk: es-so. kondo, s., & miura, t. (2010). reaction-diffusion model as a framework for understanding biological pattern formation. review. science 329 (5999), pp.1616-1620. doi:10.1126/science.1179047 lienhard, j., schleicher, s., poppinga, s., masselter, t., milwich, m., speck, t., & knippers, j. (2011). flectofin: a hingeless flapping mechanism inspired by nature. bioinsp. biomim. 6, 045001. loonen, r., trcka, m., costola, d., & hensen, j. (2013). climate adaptive building shells: state-of-the-art. renewable and sustainable energy reviews, pp.483-493. menges, a. (2012). biomimetic design processes in architecture: morphogenetic and evolutionary computational design. bioinspir. biomim. 7, 015003. morrison, p. (1987). reviewed work: on growth and form: fractal and non-fractal patterns in physics, edited by h. eugene stanley and nicole ostrowsky. scientific american 256 (1), 26-27. retrieved from http://www.jstor.org/stable/24979292 pesenti, m., masera, g., fiorito, f., & sauchelli, m. (2015). kinetic solar skin: a responsive folding technique. energy procedia 70,pp. 661 672. sartori, j., pal, u., & chakrabarti, a. (2010). a methodology for supporting “transfer” in biomimetic design. artif. intell. eng. des. anal. manuf. 24 (4), 483-505. doi:10.1017/s0890060410000351 stankov, g. (2018, 02 12). the universal law of nature. retrieved from stankov’s universal law press: http://www.stankovuniversallaw.com/the-universal-law-of-nature/ thompson, d. (1945). on growth and form. cambridge university press: ny. werner, s. (2016). european space cooling demands. energy (110), pp.148 156. from city’s station to station city 001 journal of facade design & engineering volume 6 / number 3 / 2018 post-occupancy evaluation for adaptive façades shady attia1*, alessandra luna navarro2, miren juaristi3, aurora monge-barrio3, susanne gosztonyi4, zein al-doughmi5 * corresponding author 1 sustainable building design lab, department uee, faculty of applied sciences, university of liege, belgium, shady.attia@uliege.be 2 university of cambridge, uk, 3 universidad de navarra, school of architecture, spain 4 lucerne university of applied sciences and arts, engineering and architecture, switzerland 5 cardiff university, uk abstract post-occupancy evaluation is a valuable method of generating information on the performance of adaptive building façades in relation to users. this evaluation technique involves both procedural methods, such as soft-landing, and empirical measuring, such as environmental monitoring or self-reporting techniques including surveys. several studies have been carried out in recent decades to identify the most appropriate methods for occupant comfort, well-being, productivity, satisfaction, and health assessments in workplaces. post-occupancy evaluation of adaptive façades can, however, be a challenging task and information on this topic is still scarce and fragmented.  the main contribution of this paper is to bring together and classify the post-occupancy evaluation methods for adaptive façades and suggest a framework for their holistic evaluation. specific recommendations for improving current standards and guidelines are outlined here to enhance occupant satisfaction and environmental conditions in workplaces for future design projects. finally, we discuss various ongoing trends and research requirements in this field. keywords advanced façades, user interaction, measurement, surveys, criteria, framework, indoor comfort doi 10.7480/jfde.2018.3.2464 002 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction as we continue to innovate and build energy efficient and advanced façades that are automated, we are looking forward towards the optimisation of the overall work, living, and learning experience indoors. traditionally, post-occupancy evaluation (poe) was used to assess the users’ experience in relation to outdoor and indoor environments. however, poe for adaptive façades (af) (loonen, trčka, cóstola, & hensen, 2013)due to a growing demand to satisfy more ambitious environmental, societal and economical performance requirements. the application of climate adaptive building shells (cabs requires a specific approach of obtaining feedback about users’ experience and building performance in use. poe for af includes investigating occupants’ interactions with the envelope and the overall building performance regarding energy efficiency, indoor environmental quality (ieq), and occupants’ satisfaction, well-being, and productivity. this paper is part of the cost action tu 1403 on af and aims to provide an overview of existing and expected poe assessment methods. as part of work group 3, previous work has introduced adaptive façades systems assessment (attia favoino, loonen, petrovski, & monge-barrio, 2015; attia et al., 2019) and reviewed case studies (attia & bashandy 2016; attia, 2017; bilir & attia, 2018) of adaptive façades in which poes were performed. however, there is a lack of comprehensive poe for af that provides both qualitative and quantitative assessment and more importantly, involves users, designers, and building operators. the nature of adaptive façades that are able to adapt to changing climatic conditions on a daily, seasonal, or yearly basis requires different assessment and evaluation methods. the transient and dynamic behaviour of those façades make them a particular building technology that is novel and without precedent in terms of systematic assessment frameworks and approaches. therefore, in this paper we present a short introduction to af and poe. then, we present a brief literature analysis of three poe projects for af, assessed to show the challenges and requirements of af assessment. this includes summarising and comparing key poe assessment. in section 4, we propose an initial assessment framework and a discussion on the direction for future poe in section 5. 2 background of adaptive façades and post-occupancy evaluation a major challenge in respect to afs is the evaluation of their responsiveness to climate and occupants needs. the defining characteristics of af systems is their dynamic adaptability and multiusability of their components. some of them take over certain tasks to change the thermal, visual, or hygienic comfort situation. the influence of, for example, dynamic measures for thermal comfort on the user’s perception requires target criteria other than standardised comfort models (en iso 7730:2005) (ashrae, 2013). the topic “thermal sensation and perception of humans”, including the phenomenon alliesthesia (de dear, 2014) – a physiological approach on how pleasant or unpleasant stimuli can influence the thermal comfort perception of humans – needs to be introduced. this could lead to a “‘responsive’ standard that acknowledges the richness of human-environmental interaction and the potential for less energy-intensive design” (de dear, 2011). the dynamic behaviour of adaptive façades requires the continuous or high frequency data gathering from occupants to capture their response to transient changes in the properties of adaptive façades. adaptive façades can have different effects on occupants depending on the initial and final state of their adaptive process, as well as on the velocity and frequency of change. for instance, occupant response to automatic shading controls significantly changes if the system is lowering or raising the shading devices (reinhart & voss, 2003). bakker, hoes-van oeffelen, loonen, and 003 journal of facade design & engineering volume 6 / number 3 / 2018 hensen (2014) also showed that less frequent, discrete transitions in façade configuration are more acceptable to users than smooth transitions at a higher frequency. traditional poe methods do not allow real-time data gathering or transient assessment of adaptive buildings, and occupants are usually asked to “remember” their comfort state in surveys or interviews (buratti & ricciardi, 2009) or to record their comfort state in diaries, thereby undoubtedly losing important information on dynamic environmental changes and their effect on users. occupant satisfaction with personal control of, and interaction with, adaptive façades is also a time-dependent feature. examples of this are the changing levels of user acceptance for automatic strategies and expectations for personal control with time. in this sense, ball and callaghan (2012) presented an “adjustable autonomy system”, in which levels of control were gradually increased as the user gained confidence in using the interactive system. another challenge of af assessment is related to the time of assessment. poe comes at a late stage of the façade’s delivery process. poe starts with the operation stage, at the end of commissioning of newly or renovated buildings. as shown in fig.1, the life cycle of af is long and does not require an on-off poe, but rather a continuing poe, at least for the time required to assess the range of the façade´s adaptability. the nature of af requires that poe are adapted to become transient and frequent to match the control strategies, trace occupants response actions and the af response or action. the automatic control of af and users’ response is, in many cases, conflicting (bilir & attia). from one side, building operators control building systems to ensure good ieq and achieve energy efficiency, and on the other hand, building occupants are seeking localised control of their specific working, living, or learning environment. the conflict between the local and global spatial ieq and manual versus automated control of af makes the poe difficult. as learned from several case studies of af (bilir & attia, 2018) there is a lack of comprehensive poe to cater for af and empower users while assuring control by building operators during the af’s life cycle. this conflict requires continuous feedback and flexible building management systems and control software. historically, operators are responsible for the control of building systems. however, the awareness about well-being and occupant’s feedback, and the proliferation of low-cost sensors and interactions devices, requires a modern approach to manage this complex problem. the operation of af requires that users are central and that a building management system (bms) does not only respond to the operators. there is a need to create a balance between running the façades actuators and responding to user’s needs. fig. 1 adaptive façade life cycle 004 journal of facade design & engineering volume 6 / number 3 / 2018 3 current poe methods there are several extensive literature reviews that investigated poe (preiser, 1995, 2005; leaman & bordass, 2001; bordass & leaman, 2005; meir, garb, jiao, & cicelsky, 2009; pati & pato, 2013; kim, de dear, candido, zhang, arens, 2013; galatioto, leone, milone, pitruzzella, & franzitta, 2013; li, froese, & brager, 2018). preiser (1995) classified three levels of poe: 1) indicative, 2) investigative, and 3) diagnostic. this classification focused on grouping poe methods based on their purpose. however, the most common classification of poe methods is based on grouping them as follows (li et al., 2018): – subjective or qualitative methods: 1) occupants surveys, 2) interviews, and 3) walkthroughs. – physical quantitative methods: 1) ieq in situ measurements and 2) energy and water audits and monitoring based on our literature review, we identified poe methods that follow a systematic methodology to examine the overall performance of the building. table 1 provides a brief comparison of the three existing poe methods that were strongly present in the practice. poe method year country aspects evaluated 1 post-occupancy review of building engineering (probe) building use studies (bus) 1995 uk bus occupant survey, benchmarking against an existing database of case studies (leaman & bordass, 2001) 2 center of built environment (cbe) building performance evaluation (bpe) toolkit 2003 us occupant ieq satisfaction survey with a score card report generation tool. cbe thermal comfort tool to calculate thermal comfort according to ashrae standard 55 (zagreus, huizenga, arens, & lehrer, 2004) 3 performance measurement protocol 2010 us energy and water use and ieq. comprises three levels of evaluation. three levels— basic (indicative), intermediate (diagnostic), and advanced (investigative) (ashrae, 2010). 4 ashrae 55 comfort survey 2001 us comfort conditions are measured based on a survey (ashrae, 2013). table 1 comparison of current poe methods used frequently in practice based on our review of poe methods and their suitability for af evaluation in relation to user satisfaction, we identified emerging limitations inherent in the current poe methods. these limitations, related to poe for af, can be summarised under the following points: – current poe methods do not allow real-time data gathering and transient assessment, which are fundamental to capturing and verifying the dynamic performance and degree of responsiveness of adaptive façades. – current methods focus on comfort in relation to the occupant’s response and control. they are unable to assess the interaction between the user and the af in transient terms. – current poe methods do not identify the moment of dissatisfaction. rather, they provide an overall assessment based on a seasonal or annual evaluation and do not allow for the capturing of the effects of af change at a specific time. 005 journal of facade design & engineering volume 6 / number 3 / 2018 – researchers and building experts cannot associate or distinguish occupants’ interaction and behaviour from the overall environmental impact of af, likewise in relation to bms. – most of the time, poe outcomes are not fed back to inform the operator. the feedback loop is linear and not circular. simultaneously, there is a lack of continuous feedback that would allow occupants to respond to energy efficiency or comfort improvement measures during hours of operation. closing the information loop is also fundamental to allowing a dynamic poe, which is crucial to train and adjust af control strategies in order to meet or predict actual occupant demands. – researchers and building experts don’t have a benchmark for af to compare with the traditional poe of buildings database. the majority of poes are heavily customised to better assess the building behaviour, but this essential in af, since they are generally innovative envelopes designed with a specific purpose. from our current review, we can state that there is, at present, a knowledge gap and a challenge in assessing af using poe methods. there is a serious need for poe methods that can assess the engagement and overall well-being and productivity of occupants. there is a need to redesign poe methods that focus on the interplay between technology, the user in the physical space, and building operator. at last, since af are generally new systems and materials, poe (following previous assessment and validation of the adaptive system itself) will provide a further support for their implementation in the building sector. 4 future post occupant evaluation method for adaptive façades assessment in this section, we present a framework for future poe for af and suggest a user interface (ui) for a dynamic online use. furthermore, we suggest some key recommendations for future poe for af. we identified the main components that future poe of af should incorporate, based on our literature review and experience with poe, which was performed for three af case studies (attia & bashandy, 2016; attia, 2017; bilir & attia, 2018). additionally, as part of tu 1403 cost action, in work group 3 we developed a façade assessment framework for dynamic post-occupancy evaluation. as shown in fig. 2, the proposed framework allows multiple users, mainly occupants and operators, to share the management and control of the indoor environment and the adaptive façades technology. in this sense, the framework allows instantaneous feedback involving the users and operators in a dynamic and integrated way. our framework suggests transforming poe into a dynamic and interactive process. the developed framework focuses on energy savings, maintenance savings, control strategies, and productivity and user experience. the framework depends mainly on a central control point that connects users and operators through bms. future poe should be based on a platform that receives direct and continuous feedback from the indoor environment, and likewise from the façade system. with the help of bms, it is expected that a predictive model control with overriding control by the users can better assess the situation as frequently as the adaptability of the façade suggests, and perform a continuous automated poe assessment. it must always be kept in mind that the active interaction of the user is only accepted as and when necessary, since users prefer to be comfortable and feel productive without being aware of the controls, only interacting occasionally (buckman, mayfield, & b.m. beck, 2014). 006 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 2 adaptive façade framework for dynamic post-occupancy evaluation next, we developed a scheme in the form of a dashboard with a ui that can be used by smart devices or personal computers. the idea of this dashboard scheme is to encourage future studies and research in the area of poe to embrace instant feedback. historically, the loop of cause and effect was distant in time. however, the advances in it and sensor technology requires a revolutionary approach for poe. as shown in fig. 3, the ui provides real time feedback for comfort and energy performance (right). at the same time, the ui allows for the interaction between users and the building operator (left) through alarm messages or modification requests. the satisfaction of users in relation to the façade performance can be directly reported to facility managers. in this sense, users maintain better control on their indoor environment and their façade’s adaptive technology. we expect that such a ui is the front end of a complex bms and platform that integrates advanced control, intelligent algorithms, and actuators that allow the active management of the façade response, thereby providing value to occupants, building operators, and building owners. lastly, based in our experience of the cost action tu1403, we would like to recommend a series of new questions to be added to future poe surveys as they relate to a building with af (attia, bilir & safy, 2018). as li et al. (2018) conclude in their review, occupant satisfaction is the most common focus and occupant surveys the most frequently used method in poes. the following recommendations should be included in surveys: 007 journal of facade design & engineering volume 6 / number 3 / 2018 – are you aware of the adaptability of your façade? – are you comfortable with the adaptability? – are you satisfied with your ability to control your façade? – how often would you like your façade to change? – do you think that your façade contributes to the improvement of the thermal characteristics of your workplace/space? – do you think that your façade contributes to the improvement of the luminous characteristics of your workplace/space? – do you think that your façade contributes to the improvement of air quality in your workplace/space? – do you think that your façade contributes to ensuring a satisfactory acoustic environment in your workplace/space? fig. 3 adaptive façade control and feedback dashboard 5 discussion and conclusion there is a market trend for health and well-being within our architectural, engineering, and construction (aec) industry (attia, 2019). as we continue to humanise the experience of our working, living and learning places, afs are advanced and dynamic systems that have the potential to support life quality and people’s well-being and productivity in a resource-efficient manner. in this paper, we reviewed the current literature and identified the need for continuous monitoring and interactive control to benchmark the effectiveness of af. we found that several challenges and implications that have been previously reported in literature hinder the use of poe for af. most importantly, there is a very little uptake of poe from the façade industry and an imbalanced focus on the aesthetic aspects of af. af requires a closed loop of dynamic and instantaneous feedback to address the complexity of ieq, hvac systems performance, and occupant satisfaction. poe should be able to assess the availability of a range of user or operator control choices and their effectiveness in relation to hvac and af system characteristics. different control objectives in buildings with af can also work in opposition 008 journal of facade design & engineering volume 6 / number 3 / 2018 to each other. building operators and owners require tools and user interfaces that can locate and report upon occupants experience behind facades. there is a need for tools that empower users and help to solve those potential conflicts in af operation and interaction between occupants, façade systems, and other hvac components. therefore, there is a serious need to use test facilities and simulation-based approaches that can help building operators to test, compare, and improve poe methods and, consequently, optimise af supervisory control strategies based on a variety of metrics. novel and effective poe methods for af are also fundamental to allowing optimal façade responsiveness in time and, potentially, providing a means for enabling the modelling of predictive control strategies. lastly, the future of poe of af should be based on user experience. user experience is a key factor in the success of poe methods and a fundamental step towards the successful uptake of af in the construction industry. future research, therefore, should focus on developing novel metrics to capture user experience of af. our findings can be useful for researchers in identifying new and industry-relevant research areas and for practitioners to learn from empirically investigated challenges in poe, and base their improvement efforts on such knowledge. identifying and investigating the overlaps underline the importance of these challenges, and can also help in finding other research areas, not only for enhancing poe for af, but also for bms and control software quality in general. it also makes it easier for practitioners to spot, better understand, as well as find mitigation strategies for poe for af, through learning from past experiences and developments in the area of user experience and feedback quality. acknowledgements the authors appreciate the survey respondents’ valuable comments and feedback. the author would like to gratefully acknowledge cost action tu1403 “adaptive façades network” for providing excellent research networking. the authors would like to acknowledge the support of work group 3 members. references ashrae (2010). performance measurement protocols for commercial buildings. american society of heating refrigeration and air conditioning engineers. inc., atlanta. ashrae (2013). 55: thermal environmental conditions for human occupancy. american society of heating refrigeration and air conditioning engineers. inc., atlanta. attia, s., favoino, f., loonen, r., petrovski, a., & monge-barrio, a. (2015). adaptive façades system assessment: an initial review. 10th conference on advanced building skins, 3-4 november, 1265-1273, bern, switzerland. attia, s. & bashandy, h. (2016). evaluation of adaptive façades: the case study of agc headquarter in belgium. in belis, bos, & louter (eds.) challenging glass 5 – conference on architectural and structural applications of glass.  ghent university, belgium, isbn 978-90-825-2680-6. attia, s. (2017). evaluation of adaptive façades: the case study of al bahr towers in the uae. shaping qatar’s sustainable built environment, hamad bin khalifa university press, volume 2, issue 6, p1-13. attia, s. (2018). challenges and future directions of smart sensing and control technology for adaptive façades monitoring. next façades cost action tu1403 adaptive façades network conference, lucerne university, 26-27 november 2018. lucerne, switzerland. attia, s., bilir, s., & safy, t. (2018) adaptive façades performance assessment: interviews with façade experts, sbd lab, liege university, belgium. isbn: 9782930909097. retrieved from https://orbi.ulg.ac.be/handle/2268/213736. attia, s., bilir, s., safy, t., struck, c., loonen, r., & goia, f. (2018) current trends and future challenges in the performance assessment of adaptive façade systems. energy and building, in press.  ball, m., & callaghan, v. (2012, june). explorations of autonomy: an investigation of adjustable autonomy in intelligent environments. in intelligent environments (ie), 2012 8th international conference on ieee. (pp. 114-121). bakker, l. g., hoes-van oeffelen, e. c. m., loonen, r. c. g. m., & hensen, j. l. m. (2014). user satisfaction and interaction with automated dynamic facades: a pilot study. building and environment, 78, 44-52. bilir, s., attia, s. (2018). performance evaluation of adaptive façades: a case study with electrochromic glazing. next façades cost action tu1403 adaptive façades network conference. lucerne university, 26-27 november 2018. lucerne, switzerland. 009 journal of facade design & engineering volume 6 / number 3 / 2018 bordass, b., & leaman, a. (2005). making feedback and post-occupancy evaluation routine 1: a portfolio of feedback techniques. building research & information, 33(4), 347-352. buckman, a. h., mayfield, m., & b.m. beck, s. (2014). what is a smart building? smart and sustainable built environment, 3(2), 92–109. https://doi.org/10.1108/sasbe-01-2014-0003 buratti, c., & ricciardi, p. (2009). adaptive analysis of thermal comfort in university classrooms: correlation between experimental data and mathematical models. building and environment, 44(4), 674-687. de dear, r. (2011). revisiting an old hypothesis of human thermal perception: alliesthesia. building research information, 39 (2), 108-117. doi: 10.1080/09613218.2011.552269. de dear, r. (2014). thermal counterpoint in the phenomenology of architecture a psychophysiological explanation of heschong’s “thermal delight”. keynote speech, plea 2014, 16 – 18 december 2014. ahmedabad, india. en iso 7730:2005. (2009). ergonomics of the thermal environment -analytical determination and interpretation of thermal comfort using calculation of the pmv and ppd indices and local thermal comfort criteria. iso standard. galasiu, a. & veitch, j.. (2006). occupant preferences and satisfaction with the luminous environment and control systems in daylit offices: a literature review. energy and buildings, 38(7), 728–742. http://doi.org/10.1016/j.enbuild.2006.03.001 galatioto, a., leone, l., milone, d., pitruzzella, s., & franzitta, v. (2013). indoor environmental quality survey: a brief comparison between different post occupancy evaluation methods, advanced material research. 864–867, 1148–1152. doi:10.4028/www. scientific.net/amr.864-867.1148. kim, j., de dear, r., candido, c., zhang, h., & arens, e. (2013). gender differences in office occupant perception of indoor environmental quality (ieq), building and environment 70, 245–256. doi:10.1016/j.buildenv.2013.08.022. leaman, a., & bordass, b. (2001). assessing building performance in use 4: the probe occupant surveys and their implications, building research information 29, 129–143. doi:10.1080/09613210010008045. li, p., froese, t. m., & brager, g. (2018). post-occupancy evaluation: state-of-the-art analysis and state-of-the-practice review. building and environment, 133: 187–202. loonen, r. c., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483-493. meir, i. a., garb, y., jiao, d., & cicelsky, a. (2009). post-occupancy evaluation: an inevitable step toward sustainability. advances in building energy research, 3(1), 189-219. pati, d., & pati, s. (2013). methodological issues in conducting post-occupancy evaluations to support design decisions, health environments research and design journal. 6, 157–163. doi:10.1177/193758671300600312. preiser, w. (2005). building performance assessment—from poe to bpe, a personal perspective, architectural science review. 48 (2005) 201–204. doi:10.3763/asre.2005.4826. preiser, w. (1995). post-occupancy evaluation: how to make buildings work better, facilities. 13 (1995) 19–28. reinhart. cf, & voss, k. (2003) monitoring manual control of electric lighting and blinds. lighting research technology 35(3):243260. zagreus, l., huizenga, c., arens, e., & lehrer, d. (2004). listening to the occupants: a web-based indoor environmental quality survey. indoor air. 14, 65–74. doi:10.1111/j.1600-0668.2004.00301.x. journal of facade design and engineering 1 (2013) 97–104 doi 10.3233/fde-130002 ios press 97 nano-textured polymers for future architectural needs cees w.m. bastiaansen, albert schenning, michael debije and dirk j. broer∗ department of functional organic materials and devices, eindhoven university of technology, eindhoven, the netherlands received: 6 june 2013 accepted: 7 november 2013 abstract. the rapid developments in molecular sciences like nanotechnology and self-organizing molecular systems generate a wealth of new materials and functions. in comparison to electronics the application in architecture remains somewhat underexposed. new functionalities in optics, responsive mechanics, sensing and adjustable permeation for gases and water might add to new opportunities in providing for personal comfort and energy management in houses and professional buildings. with a number of examples we demonstrate how complex but well-controlled molecular architectures provide functionalities worthwhile of being integrated in architectural designs. optical coatings are capable of switching colors or reflectivity, creating possibilities for design but also for the control of thermal transmission through windows. they respond to temperature, light intensity, or both. selectively-reflective thin polymer layers or paint pigments can be designed to switch between infrared and visible regions of the solar spectrum. coatings can be designed to change their topology and thereby their appearance, of interest for in-house light management, or just for aesthetic appeal. plastic materials can be imbued with the property of autonomous sun tracking and provided morphing behavior upon contact with moisture or exposure to light. many of these materials need further developments to meet the requirements for building integration with respect to robustness, lifetime, and the like, which will only be accomplished after demonstration of interest from the architectural world. keywords: functional polymers, responsive coatings, smart materials, liquid crystal networks, architectural coatings 1. introduction nanotechnology based on organic materials started a revolution with respect to new functionalities for applications in electronics, communication and medical technology. only recently have new developments in the field of organic nanotechnology found their way to the world of civil engineering and architecture. in this respect one can imagine new solutions for the creation of green, energyneutral buildings, of in-house climate control and solutions to enhance personal comfort in houses and professional buildings like offices and hospitals. table 1 shows a wish list that was generated from discussion sessions between architects from the delft university of technology and polymer engineers from the eindhoven university of technology (klein, 2011). it is the objective of this chapter to provide solutions for the desired features presented in table 1 by means of well-constructed organic or polymer materials with organizational control down to the molecular level. by utilizing a combination of top-down and bottom-up structuring technologies, it is ∗corresponding author: prof. dr. dirk j. broer, professor at eindhoven university of technology, chemical engineering & chemistry, department of functional organic materials & devices (sfd), helix building sto 0.34, den dolech 2, 5612 az eindhoven, the netherlands. tel.: +31 40 247 5875, mob: +31 6 51662354; e-mail: d.broer@tue.nl. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:d.broer@tue.nl 98 c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs table 1 list of features fitting in the concept of the future envelope for buildings feature effect dynamic color surfaces changing color in order to maximize or minimize energy gain; switch between heat absorption and heat rejection oxygen control sensing and regulation of o2 versus co2 concentrations; surfaces that actively turn co2 into o2 electricity surface uses sun and air currents to produce electricity; eventually to be used for autonomous operating building elements clean/protected surface that always stays clean and is protected from pollution; surface that can be cleaned remotely sun/view window optimizing direct, diffuse or no light transmission, while maintaining unobstructed views; switchable windows adaptive daylight roof designed to meet lighting requirements by responding to exterior conditions, e.g. by morphogenesis air/sound filter air and sound should be selectively filtered through the surface. unwanted smells and sounds are kept out self-healing surfaces and constructs are self-healing after damage or aging structural integrity surfaces able to allocate different degrees of stiffness locally; soft and pleasing if desired, strong and tough when needed possible to create complex molecular architectures that provide functions corresponding to many of the concepts that are developed for future building projects. with top-down structuring technologies we refer to the formation of (often) periodic structures down to the micrometer level by means of photolithographic processes, (inkjet) printing, photo-replication or embossing, to name some techniques. for bottom-up structuring, we are utilizing the ability of particular molecules to self-organize into complex, three-dimensional structures which can be permanently fixed by a polymerization into a densely crosslinked polymer network. an overview of various aspects of liquid crystal networks can be found in (broer, crawford, & zumer, 2011). for this purpose we often use liquid crystal monomers (an example is shown in fig. 1) which may be aligned into complex morphologies using surface interactions, external electric or magnetic fields, or molecular properties including hydrogen-bridge formation or chirality. based on their molecular structure, we are able to make the materials response to light in both their color and shape (yamada, 2008), (van oosten, 2008, 2009) to create adjustable reflection with respect to wavelength and light polarization (broer, 1995), and to form nanoporous membranes with well controlled, adjustable pore dimensions (luengo gonzalez, 2008). 2. liquid crystals and liquid crystal networks: molecular architectures in polymer systems we all know liquid crystals are used as the light switching medium in flat panel television and mobile phone displays. the principle of light switching is based on changing the state of polarization of transmitted light. this technique can also be successfully utilized in building components such as switchable windows and switchable door panels. for instance, glass panels with these properties have been brought to the market by a number of suppliers under names privacy window, privacy glass, e-glass, and smart glass, among others, and are in most cases based on an electro-optical switching between a transparent and a scattering state of a liquid crystal polymer composite. what c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs 99 fig. 1. principle of photopolymerization of reactive liquid crystal monomers. the monomers can be controlled to align uniaxially, twisted, splayed, tilted or in a molecular helix (bottom-up) and can be applied locally by lithography or printing (top-down) to form a solid plastic film with control over the molecular positioning and orientation. is less well-known is that liquid crystals can be polymerized into plastic networks that preserved the molecular order of the liquid crystals in a solid matrix. these so-called ‘liquid crystal networks’ exhibit extraordinary properties. for instance, as low molecular weight components in displays they are able to control the state of polarization of light, and thus the transmission of light. when the orientation of the molecules in the network is brought into a helix with a pitch of the order of the wavelength of light, they start to reflect parts of the light spectrum. this feature is illustrated in fig. 2. by controlling the pitch of the helix, the reflection band can range from ultra violet through the visible region to the infrared region of the light spectrum. this property can be used to protect against harmful light, change the appearance of an object, or control heat uptake or heat rejection when applied at the exterior of a building or a window. it is also used to create efficient light concentrators for solar energy panels with freedom in both shape and color. in addition they are used to provide sensing functions by using changes in color or reflection band upon exposure to, among other gases, an excess of co2 (han, 2010). the pitch can be controlled by the presence of chiral molecules, and adjusted accurately by the concentration of chiral units. external triggers change the ‘helical twisting power’ of these chiral additives in a reversible way, enabling switching between reflection wavelengths. when the reflection wavelength is in the (near) infrared part of the spectrum, the material is transparent and colorless but performs its function by reflecting climate-affecting radiation. as shown in fig. 2, the helicoidally ordered networks can be processed as continuous films but can also be ground into smaller particles which can be added to paint as a pigment. in general, the formation of helicoidal or even more complex structures in these liquid crystal polymers can be controlled down to the molecular level. one can roughly say that by applying structure through techniques like molding, printing and lithography, the production time and production costs scale inversely with the dimension size of the structure. this makes production of larger surfaces using these 100 c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs continuous film with pattern reflective pigment spectral switching by either ∆t, hv, gases, ph, chemicals fig. 2. liquid crystal polymer networks with a helicoidal molecular order reflect light following bragg’s reflection rules. the reflection wavelength can be switched with a variety of triggers. the polymers can be applied as a continuous layer or as pigment added to a polymer binder. small elements costly. to overcome this time-dimension paradox, self-organization of molecules helps to make these accurately-controlled structures available for large area applications in architecture. the display industry already smoothed this path in their production of optical films, which are used to improve television performance, basically defect-free by reel-to-reel processes. 3. morphing plastics the degree of molecular alignment of a liquid crystal network, usually defined by the order parameter quantifying the distribution of the rod-like molecular units along a common orientation (director), is currently 0.6 to 0.7. upon decreasing this order parameter, a uniaxially ordered polymer film tends to decrease its length in the direction along the molecular director and to expand in the two perpendicular directions. this property manifests itself during temperature cycling in a negative coefficient of thermal expansion along the director. by modulating the order parameter via external triggers, such as light or exposure to specific liquids or gases, liquid crystal networks can change their geometrical dimension and shape. these dimensional changes can be further tuned by employing non-unidirectional director profiles such as a molecular orientation following a twisted or a splay/bend configuration. these orientations force the plastic to bend in a controlled way. by utilizing even more c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs 101 fig. 3. an azo compound undergoes geometrical changes upon exposure to uv light. when integrated in a liquid crystal polymer network, this reaction changes the degree of order which, in turn, induces reversible geometrical changes. when the molecules are brought in an azimuthal orientation this process induces shape changes in a flat film leading to conical or anti-conical geometries which are experimentally verified. the depicted film was made by carlos sanchez (university of zaragoza), and the deformation was modeled by mark warner9. complex director designs, deformations can be made even more complex (modes, 2011). for example, flat constructs with an azimuthal directors around a defect structure deform into a cone or into an anti-cone depending on whether the order parameter increases or decreases (fig. 3). an elegant way to change the order parameter is to introduce photo-sensitive groups into the liquid crystal network. most well-known are azo moieties that reversibly change their conformation upon absorption of light (fig. 3). typically the azo compounds absorb light in the uv part of the spectrum. when exposed to 360nm light they cause a liquid crystal network to deform in a manner dictated by its director pattern. when exposed with visible light they deform back into their initial shape, a process which also occurs spontaneously, albeit more slowly, in the dark via thermal processes. the chemistry behind liquid crystal network formation leaves much freedom for incorporating special properties. rather than having the plastic morph in response to light or heat, one can also make them sensitive for water or water vapor, special chemicals, changes in ph, or other stimuli (harris, 2005, 2006). control over the local molecular alignment will make it possible to switch between complex, pre-determined geometries. as an alternative to shaping the geometry of a film or an object, one can use similar processes to morph the surface topology of a film or coating (sousa, 2006). this is based on similar processes, changing the order parameter of the liquid crystal networks in patterned structures. figure 4 demonstrates a coating consisting of patterned chiral-nematic in an isotropic liquid crystal sea. the surface 102 c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs fig. 4. a patterned surface coating containing both chiral nematic and isotropic regions can be made by local polymerization at two different temperatures. decreasing the order parameter in the chiral-nematic area by raising the temperature makes them expand more than the isotropic area thus amplifying a surface relief structure. topology alters through reversibly changing the order parameter. in this case, the surface structures were formed by temperature changes, but similar effects have been demonstrated using light-induced actuation. as an example of an application, the surface relief can be utilized to control the distance between two plate elements, thereby controlling heat transfer and thermal insulation. 4. nanoporous plastics the liquid crystal networks discussed so far are based on nematic liquid crystal order. in the nematic phase, on average the molecules are oriented in the same direction (which might rotate in case of the chiral systems) but they don’t have a strict positional order. it is also possible to polymerize the liquid crystal monomers from a smectic phase. in the smectic phase, the monomeric entities adapt, positional ordering such that they are organized in layers in addition to their directional order. the thickness of these layers is equal to the length of the molecules, or even smaller in the case where the molecules interdigitate. typically, this forms polymer films with periodic structures with lengths on the order of 3 nanometers. these smectic polymer networks are used to make thin, thermally stable polarizer films, for example. normally, monomers that form liquid crystal networks consist of atoms that are completely connected by strong covalent bonds. but chemistry also allows us to build in these monomers a weaker, so-called secondary bond. this weaker bond can be easily broken selectively, such as by temperature or through contact with and alkaline or acidic solution. an example of secondary bonds are those based on hydrogen bridges which are stable at moderate temperatures and ph conditions but open by treatment with a ph >9 buffer or by heating to 160◦c. through this modification the smectic c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs 103 hv ∆ or ph well-defined pores fig. 5. schematic representation of the formation of nanoporous plastic by polymerization of a smectic liquid crystal containing weak bonds. a two-dimensional porous structure is formed after breaking the weak bonds, e.g. by dipping in a moderately alkaline solution. the layers are kept together by fully covalent molecules. the cryo-tem picture shows the nanopores after they are filled with barium ions for contrast. layers in the polymer network can break-up into separated layers (fig. 4). the integrity of the film is maintained by the presence of a small concentration of fully covalent smectic monomeric entities. these entities determine to what extent the smectic layers may separate, and their length determines the dimensions of the pores that are formed, which are typically in the range of 1 nanometer. the nanoporous polymers are being designed for size-exclusion filters and, can be made to ‘breathe’ air or liquid by peristaltic motion. 5. conclusions and outlook the combination of top-down structuring techniques like printing, lithography or embossing combined with bottom-up technologies such as self-organization of organic molecules has proven its value for applications in the world of electro-optics. in particular, the optics of flat panel television display has benefitted from liquid crystal technologies where molecular organization is controlled over large surface areas. it has now become worthwhile to study whether the extraordinary properties of liquid crystal networks can be used in the architectural world. the well-controlled molecular organization leads to autonomously changing properties in response to changes in the environment. chiral networks can reversibly switch from transparent to reflective states, either in the visible or in the infrared part of the spectrum, enabling in-house climate control without compromising properties of window transparency or façade appearance. autonomous morphing of films or coating surfaces can be used to control light capture by buildings and solar cells or to regulate heat transfer and thermal insulation. nanoporous polymers may contribute to regulation of oxygen and co2 levels in buildings while chiral networks may for instance indicate the presence of excessive co2 concentrations. however, these materials are new in architecture and civil engineering applications and need to prove their worth with respect to durability under harsh outdoor conditions. new concepts need to be developed, for integration in familiar architectural concepts such as paints, glass coatings and panels. but these new concepts can also be considered for more active elements for indoor textiles that react to climate changes or sun position, thus connecting outdoor conditions with in-house appearance. 104 c.w.m. bastiaansen et al. / nano-textured polymers for future architectural needs references broer, d. j., lub, j., & mol, g. n. (1995). wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient. nature, 378(6556), 467-469. broer, d. j., crawford, g. p., & zumer, s. (2011). crosslinked liquid crystalline networks – from rigid polymer networks to elastomers, crc press taylor & francis group, new york. yamada, m., kondo, m., mamiya, j.-i., yu, y., kinoshita, m., barrett, c. j., & ikeda, t. (2008). photomobile polymer materials towards light-driven plastic motors. angew chem int ed, 47, 4986-4988. macromolecules, 41(22), 8592-8596. han, y., pacheco, k., bastiaansen, c. w. m., broer, d. j., & sijbesma, r. p. (2010). optical monitoring of gases with cholesteric liquid crystals. j am chem soc 132, 2961–2967. harris, k. d., bastiaansen, c. w. m., lub, j., & broer, d. j. (2005). self-assembled polymer films for controlled agent-driven motion. nano letters 5(9), 1857-1860. harris, k. d., bastiaansen, c. w. m., & broer, d. j. (2006). a glassy bending-mode polymeric actuator which deforms in response to solvent polarity. macromol rapid commun, 27, 1323-1329. klein, t. (2011) outcome of brainstorm meetings with participants of delft university of technology and eindhoven university of technology. luengo gonzalez, c., bastiaansen, c. w. m., lub, j., loos, j., lu, k., wondergem, h. j., & broer, d. j. (2008). nanoporous membranes made of hydrogen bridged smectic networks with nanometer transverse pore dimensions. advanced materials 20(7), 1246-1252. modes, c. d., bhattacharya, k., & warner, m. (2011). gaussian curvature from flat elastic sheets. proc r soc a 467, 1121-1140. sousa, m. e., broer, d. j., bastiaansen, c. w. m., freund, l. b., crawford, g. p. (2006). isotropic islands in a cholesteric sea: patterned thermal expansion for responsive surface topologies. adv mat 18(14), 1842-1845. van oosten, c. l., bastiaansen, c. w. m., & broer, d. j. (2009). printed artificial cilia from liquid-crystal network actuators modularly driven by light. nature materials 8(8), 677-682. van oosten, c. l., corbett, d., davies, d., warner, m., bastiaansen, c. w. m., & broer, d. j. (2008). bending dynamics and directionality reversal in liquid crystal network photo-actuators. from city’s station to station city 034 journal of facade design & engineering volume 6 / number 3 / 2018 possibilities and challenges of different experimental techniques for airflow characterisation in the air cavities of façades emanuela giancola1*, m. nuria sánchez1, matthias friedrich2, olena kalyanova larsen3, alessandro nocente4, stefano avesani5, francesco babich5, francesco goia4 * corresponding author 1 ciemat, department of energy, energy efficiency in buildings unit, spain, emanuela.giancola@gmail.com 2 hafencity university hamburg, germany 3 aalborg university, denmark 4 norwegian university of science and technology, norway 5 eurac research, institute for renewable energy, italy abstract ventilated façades are applied in both new and existing buildings. it has been claimed that these components help to reduce energy use in buildings and improve occupant comfort. however, their performance strongly depends on the airflow passing through the cavity. in order to characterise and to model the behaviour of the ventilation and its effectiveness, the components need to be tested in the laboratory, as well as under real dynamic weather conditions. despite the steadily growing research in this area, there are few studies with conclusive results about the reliability of existing experimental procedures for characterisation of airflow in the ventilated cavities. the aim of this paper is to describe and review recent state of the art experimental assessments for the airflow characterisation in ventilated cavities. the paper starts with a short introduction on the potentialities and limitations of different experimental methodologies, and continues with a detailed classification and description of the most relevant monitoring techniques for airflow in air cavities of façades that have been developed in recent years. keywords façade characterisation, experimental techniques, airflow monitoring, tracer gas, velocity profile, ultrasound, pressure difference, piv, ldv, temperature profile & heat flux doi 10.7480/jfde.2018.3.2470 035 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction a comfortable and hygienic indoor climate is one of the fundamental requirements expected of the building envelope. one method of coping with the need to reduce cooling loads through the envelope is to make use of a natural or mechanically induced airflow in the cavity of the façade (lee, sang, yeo, & kim, 2009). even if there are many synonyms for the term ‘ventilated façade’ such as active façade, double envelope, rain screen, or double skin façades (dsf) which correspond (more or less) to different configurations, all of these terms refer to a building envelope system characterised by a ventilated layer. currently, innovative building elements perform one or more of the several functions that a building envelope is required to do, but the assessment of their effectiveness is a complex task. the functions of façade systems to be tested are now more numerous (and more complex) than those traditionally assessed through conventional metrics (such as the u-value, or the g-value). in particular, when it comes to ventilated façades, the assessment of the performance is often connected to the assessment of the airflow rate. however, the on-site (and laboratory) characterisation of the airflow in a ventilated façade is not a trivial task. although the european standard, en 16211-2015 ‘ventilation for buildings measurement of air flows on site – methods’, provides a description of the air flow methods and outlines how measurements are performed to achieve the stipulated measurement uncertainties, the implementation of these methods in a ventilated façade is not straightforward. there is therefore a clear need to define a robust and repeatable procedure for characterising the performance of ventilated façade, which goes hand in hand with the need to develop suitable facilities for research, development, and testing of façades (cattarin et al., 2018; goia, schlemminger, & gustavsen, 2017). the performance of the ventilated façade has been evaluated both numerically and experimentally in multiple studies (lópez, jensen, heiselberg, ruiz de adana santiago, 2012; sanjuan, suarez, gonzalez, pistono, & blanco, 2011; suarez, joubert, molina, & sanchez, 2011). the experimental evaluations have been developed in different scenarios: real buildings, outdoor test cells, and indoor laboratories. despite the differences between numerical predictions and experimental data, all results demonstrate a marked reduction in summer thermal loads due to the induced ventilation airflow. in-depth experimental analysis of this system will enable the updated model to better reproduce the façade energy savings and air quality conditions inside the building. following the classification proposed by cattarin, causone, kindinis, and pagliano (2016), the assessment of façade systems may be performed by means of three main types of test rig: outdoor real-scale facilities, outdoor test cells, and laboratory indoor facilities. the major constraints of field measurements are: a) the complexity of isolating a single variable (serra, zanghirella, & perino, 2010); b) the difficulty in comparing the measured data with other available data sets, due to the unique architectural features of each real-scale building and the boundary conditions; c) the complexity of achieving a high level of instrumentation and control necessary for accurate performance assessment (strachan & vandaele, 2008). instead, the tests carried out in controlled laboratory conditions give the possibility to carefully check the most influential parameters, such as ambient temperature, heating of the outer skin of the façade, relative humidity, and air velocity, as well as the possibility to test the influence of each parameter individually. laboratory experiments are carried out under steady state or, where appropriate, dynamic boundary conditions with predefined test sequences. the effects of one or more meteorological conditions are sometimes imitated through dynamic programs, but these cannot fully reproduce the complex interactions of pure stochastic processes typical of real climate, as well as of some characteristics of the outdoor boundary conditions (such as, for example, the geometrical component of solar irradiation on the façade). over the last decades, all types of mentioned experimental cells/facilities have contributed to 036 journal of facade design & engineering volume 6 / number 3 / 2018 the present state of the art in façade characterisation. however, different experimental methods for characterisation of the airflow in ventilated cavities can be used, depending on the geometry of the ventilated façade, peculiarities of the experimental cell, type of airflow, equipment at hand etc. the determination of the airflow in the naturally ventilated cavities is a key and challenging issue. the influence of airflow, however, has not been studied to the same extent. the lack of an overview of different established procedures for collection of experimental data for naturally ventilated cavities (dama, angeli, & kalyanova larsen, 2017) is the main reason for the present state of the art. measuring and predicting airflow are difficult tasks due to the stochastic nature of the wind. as reported by perino et al. (2008), one of the main problems of uncertainty in the estimation of the airflow is determined by the wind conditions and by the thermal behaviour of the façade. an increase of airflow rate in the cavity will reduce the temperature difference between the exterior and the air in the cavity. as a result, the airflow rate will diminish. this ‘self-regulating’ interaction is reported by saelens (2002). the number of existing experimental methods for estimation of airflow rate in the built environment is limited to the following: tracer gas measurements, velocity profile method, and ultrasound measurement of velocity, as well as the use of models with measured pressure differences across the opening (hitchin & wilson, 1967) and the temperature profile along the ventilated cavities. furthermore, the laser-based non-intrusive experimental techniques of laser doppler velocimetry (ldv) and particle image velocimetry (piv) (sánchez, sanjuan, suárez, & heras, 2013) are applied to determine indoor airflow behaviour. the air change rate of naturally induced airflow is significantly different in occupied spaces compared to façade cavities. however, there are no experimental methods specifically developed for ventilated cavities, and thus the traditional ones for occupied spaces are used. the scope of this paper is therefore to raise awareness about this problem and call for comparative investigations on existing experimental techniques, with a particular focus on naturally ventilated cavities, as well as on the development of specific guidelines for this purpose. 2 classification and review of existing experimental techniques for airflow characterisation in the air cavities of façades the intention of this section is to provide the reader with a comprehensive understanding of the experimental techniques for airflow characterisation of ventilated cavities, their possibilities and limitations. the key features of each technique are summarised in table 1 at the end of this section. 2.1 tracer gas measurements tracer gas measurements for determining airflow rates in buildings are frequently applied (laussmann & helm, 2011). there are three established tracer gas techniques that can be found in the literature: decay, constant concentration, and constant emission (etheridge, 2011). looking at the applicability of each of these techniques in ventilated cavities, a constant emission method is normally used, although there are a number of limitations that contribute to high uncertainty of the results obtained using this method. by recording the concentration of tracer gas in a defined volume (e.g. a room) and considering the background concentration, as well as the tracer gas supply, the air change rate can be calculated. the tracer gas is initially assumed to be equally distributed 037 journal of facade design & engineering volume 6 / number 3 / 2018 throughout the whole space. however, this assumption is limited in ventilated spaces as there will be a lower concentration near the fresh air supply and exhaust openings, and a higher concentration in the deeper part of the room (horizontal gradient) (larsen, 2006). in contrast to naturally ventilated occupied spaces, the application of the tracer gas method with constant emission in a ventilated façade brings additional uncertainty to the experimental estimation of naturally induced airflow. in the first place, this is caused by the stochastic behaviour of the wind and therefore the irregular dilution of the tracer gas, but the uncertainty is further increased by the lack of research within the field. until now, there have been no clear guidelines established with regard to the application of tracer gas methodology in ventilated façades, since the effect of positioning the emission source within the ventilated cavity, as well as the number and location of tracer gas dilution measurement points on measurement accuracy remain unknown. marques da silva, gomes, and moret rodrigues (2015) tested different positions for tracer gas emission and concentration sampling points. overall, the results show no clear tendency, as the airflow in the cavity is highly dynamic. the knowledge about flow dynamics of cavities is low and therefore no preferable position was found. kalyanova, jensen, and heiselberg (2007) found that tracer gas emission near the supply air opening of a cavity can cause a ‘wash-out effect’ where the gas is flushed out near the opening before it can mix with the cavity air, resulting in an inaccurate (too high) airflow rate. other sources of inaccuracies are found due to reverse flow and recirculation effects. nevertheless, the tracer gas method is one of the best available options, due to the lack of good alternatives, relatively simple installation of sensors, and minimal required instrumentation. 2.2 velocity profile the measurement of the air velocity is a means by which to determine the airflow rates and to estimate the surface convective heat transfer coefficient. both these two variables are very relevant for the calculation of the façade air cavity performances and therefore the direct measurement of the air velocity is of great interest. nevertheless, the air velocity spatial differences can vary substantially in the three-dimensional field, depending on the air cavity geometry and on the airflow regime. consequently, the air velocity field is difficult to characterise or generalise by physical or empirical equations for all façade air cavities. the air velocity profile method allows for the assessment of the façade air cavity performance through the measurement of the air velocity at some characteristic points. 2.2.1 experimental setup different types of anemometers can be used for punctual determination of air velocity. these devices must be able to detect high frequency fluctuations in transient air flow. the hot-wire and the hotsphere anemometer are the most frequently used instruments in façade-related applications (belleri, avantaggiato, & lollini, 2017; lópez et al., 2012; manz, schaelin, & simmler, 2004; mateus, pinto, & graça, 2014; park, augenbroe, messadi, thitisawat, & sadegh, 2004), mainly because of their fast responses and velocity range between 0-5 m/s. the definition of the experimental layout in terms of number and position of the anemometers across and along the air cavity is a trade-off between reducing their number, lowering the air channel obstruction, and increasing the measurement points to better appreciate the velocity variation. the sensors must be located at a reasonable distance from any obstruction. one further relevant factor to be considered in defining the experimental setup 038 journal of facade design & engineering volume 6 / number 3 / 2018 is the influence of the solar radiation on the temperature-based measurement principle (e.g. hotsphere) as discussed by jensen, kalyanova, and hyldgaard (2007). an example of experimental setup in an outdoor test bench can be found in kalyanova et al. (2007). 2.2.2 main challenges encountered in design and operation the main challenge in applying this method is the reduction of the cavity cross section due to the probes, cables, and fixing system, as well as deficiency of the method in detecting the direction of the air streams (upward or downward). larsen (2006) performed a measurement of the velocity profile in a naturally ventilated, wide cavity, where it is documented that due to high velocities in the boundary layer, a large number of measurement points in the boundary layer are necessary in order to build an accurate velocity profile. accordingly, the measurement accuracy becomes a trade-off between a number of measurement points and the disturbances that are introduced into the experimental domain. furthermore, the presence of upward and downward air streams poses problems in the design of the experimental set-up. jensen et al. (2007) investigated a method to determine the flow direction by using two hot sphere anemometers. however, determination of flow direction at one point is not enough for an accurate estimation of the whole cavity airflow, as in the case of two-directional flow occurrences. a second relevant challenge is the choice of probes features. the uncertainty of the hot-wire measurement system normally varies depending on the inverse of the velocity module. on the contrary, for naturally ventilated cavities, high accuracy at low velocity is requested. finally, the experimental layout must be designed starting from the specific façade geometry and expected airflow regime. consequently, the design of a good experimental layout is very challenging as it would require the extensive use of cfd simulations. 2.2.3 limitation of the experiment the main limitation of the experiment is the need for a detailed analysis of the airflow characteristics, due to the high 3d variability of the air velocity field. consequently, a punctual measurement of the air velocity carries very limited information on the air cavity airflow regime. as a result, both the number and the location of the sensors need to be optimised. 2.3 ultrasound measurement of velocity the use of sound waves in the ultrasound range is a well-established technique for the measurement of (volumetric) flows in ducts and pipes and for the measurement of wind velocity (2d, 3d) in the field of environmental monitoring. though the principles on which this measurement technique is based have been known for a long time (suomi, 1957), the development of sensors for hvac applications is rather recent (strauss, weinberg, & kopel, 1996) with ongoing research activities in the field of device development (raine, aslam, underwood, & danaher, 2015). this class of measurement techniques, which makes use of the interaction of (ultrasonic) sound waves with the moving fluid to determine the average velocity along the path of the sound 039 journal of facade design & engineering volume 6 / number 3 / 2018 wave (cuerva & sanz-andrés, 2000) is primarily based on two alternative concepts, to which correspond two different devices: the doppler effect ultrasonic flow meter and the transit time ultrasonic flow meter. 2.3.1 doppler shift flow meter these devices are based on the measurement of the frequency shift between a sound wave and its reflection caused by the particles in the flow. the flow rate is analytically determined, knowing the thermophysical properties and state properties, based on the doppler effect equation, by processing the signals from the transmitter and the receiver. it is necessary that the fluid under analysis is able to reflect ultrasonic waves due to small bubbles of gas (in the case of a liquid) or the presence of eddies in the flow stream. 2.3.2 transit time flow meter these devices are based on the contemporary emission/reception of two identical sound waves between two couples of emitter/receiver, where one emitted/received soundwave travels downstream and the other upstream of the direction of the fluid flow. in the case of a still air mass, the transit time in each direction is identical, while under a flowing volume the downstream sound wall travels faster than the upstream one, and the difference between the two velocity values increases with the flow rate. the transmitter analytically calculates the average velocity of the flow rate based on the difference in the transit time across the two sound paths. 2.3.3 main challenges encountered in design and operation the use of ultrasonic principle for airflow monitoring presents several advantages: a) it can handle a very wide range of velocity, under different flow regimes; b) it is non-intrusive and does not influence the fluid flow; c) one sensor measures the average velocity across a section of the façade/ duct, and multiple directions can be measured if more sensors are installed; d) because of the use of the difference between two velocity values, the procedure is independent from the temperature and pressure conditions of the fluid. when it comes to limitations and challenges, it is worth mentioning that the accuracy of this technique is reduced with very low air velocity. accuracy is also reduced for cavities that are too deep or too thin, but the technique seems to be well suited to measuring airflows in cavities in the approximate range of 0.1m to 0.5m. when more sensors are installed in the same cavity, and are close to each other, different frequencies for each sensor might be necessary to avoid incorrect readings. the implementation of this technique in a ventilated façade is not an established procedure and there may be challenges that are unknown at present, and which will be experienced only after several tests with this technique have been carried out. 040 journal of facade design & engineering volume 6 / number 3 / 2018 2.3.4 limitation of the experiment the accuracy of this measurement method is relatively good and can be in the range of 2 -5% of the measured values. however, in the case of extremely low velocity (range of 10-2 m/s) the uncertainty can become far higher, and almost in the range of the measured values. research activities are definitely necessary to deepen the applicability of this technique to façade systems due the poor literature in the field. requirements in terms of developed flow regime are to be investigated, due the lack of standardised procedures for the application of ultrasonic sensors. 2.4 measured pressure difference in theory, a pressure difference across an opening of naturally ventilated façade reflects the airflow rate induced by wind and buoyancy. more exactly, a relationship between the pressure difference and the airflow passing through the opening can be expressed as an equation (1) where q is air flow (m3/h), δp is pressure difference (pa), and a, b are empirically obtained coefficients. coefficients a and b depend on the shape of the opening itself and therefore the resistance that it initiates into the air passage. bpaq δ⋅= this theory is used as a background for the pressure difference methodology for airflow measurement of naturally induced airflow, which is comparable to the measurement of the airflow in mechanically ventilated spaces using an orifice method. the method includes two stages: the calibration of the opening and the actual measurement. in contrast to the relatively accurate orifice method, little progress was made with regard to the pressure difference method, in terms of accuracy evaluation, as well as in terms of its practical application. at present, there is only one known example of this method application. in kalyanova et al. (2007) it is used in the full-scale outdoor test facility ’cube’, where the results of this method are found to be disappointing for the natural airflow, but relatively successful when the cavity is mechanically ventilated. the main finding of this work is that the method is very sensitive to the positioning of the surface pressure and reference measurement, to the fluctuations in wind direction and wind speed. thus, further research is needed to establish a suitable methodology for the pressure difference measurement in a naturally ventilated cavity as it gives strong inspiration for finding a way to cope with the extremely high wind fluctuations and thereby fluctuations of the airflow. 2.5 particle image velocimetry (piv) piv is an optical method of flow visualisation and quantification of instantaneous velocity fields, measuring two velocity components in an area of analysis by adding small tracer particles. different suitable materials and particle generators are used. seeding particles are selected to ensure acceptable flow tracking and adequate light scattering efficiency. however, determining optimal particle size is more critical in turbulent flows and high-speed gas flows since the particle’s motion 041 journal of facade design & engineering volume 6 / number 3 / 2018 is more complex to treat. 2d-piv has become a common technique used in research studies based on piv, though a more complex piv setup based on stereoscopic flow field analysis (stereo-piv) has been used increasingly in recent years (sánchez, giancola, suárez, blanco, & heras, 2017). the latter enables the measurement of the out of plane velocity component. currently, the new concept of volumetric velocimetry (tomo piv) enables the measurement of the three velocity components in a volume. 2.5.1 experimental setup – a constructed ventilated façade model is simplified but designed considering the basic structure of real ventilated façades with the three main components: an exterior layer creating open joints, an inner layer, and an air chamber created between both coatings. a heating mat system is installed and well-adhered to the outside of the outer layer. – laboratory indoor facilities: – a fully-equipped lab with a double cavity pulsed laser, charge-coupled device cameras generating sets of images downloaded onto a pc, a laser pulse synchroniser acting as an external trigger to control the whole system, and a six jet atomiser which generates tracer particles. – additionally, temperature sensors and an infrared thermographic camera are used to perform different temperature measurements. 2.5.2 main challenges encountered in design and operation the piv technique requires a strong initial investment in human and financial resources. the equipment is sophisticated and quite expensive, as is its regular maintenance and upgrading. additionally, laboratory personnel must be specialised in handling piv equipment and its complex methodology. the design of the façade model presents multiple challenges. it is important to ensure the versatility and easy assembly of the model, dividing it into multiple components that can be easily exchanged. several challenges are faced because the experimental evaluation of the ventilated cavity is performed in laboratory conditions. experimental limitations are mainly linked to the specific requirements of the piv technique, the dimensions of the laboratory, and the reproduction of real environmental conditions. a major challenge of the experiment is to emulate the effect of incident solar radiation on the panels. regarding the operation of experimental piv measurements, the limited size of the measurement area of the ccd cameras does not enable the capturing of the airflow evolution inside the ventilated camera in a single experimental run. 042 journal of facade design & engineering volume 6 / number 3 / 2018 2.5.3 limitation of the experiment a major limitation of this technique is to simulate wind effect on airflow inside the ventilated cavity. solar heat load effects are also critical with respect to façade performance. solar radiation outdoor conditions are usually reproduced in the laboratory by using heating mats over the exterior layer. 2.6 laser doppler velocimetry (ldv) the ldv or laser doppler anemometry (lda) is a laser-based optical method for velocity measurement in transparent or semi-transparent fluids. invented by yeh and cummins (1964), it is based on the doppler shift in a laser beam scattered by a particle (added as seeding or normally present in the flow). 2.6.1 experimental setup a monochromatic laser beam is divided in two in a bragg cell, a device that uses the opto-acoustic effect to introduce a frequency shift in one of the beams. each beam is then separated into three colours and each addressed to the probe. here, a lens focuses the beams which collide in the measurement volume where the interference creates a series of fringes. when a particle crosses the fringes, it will scatter the light with a doppler frequency proportional to its velocity in the three spatial directions. since the frequency shift generates a known motion in the fringes, it is possible to resolve the sign of the velocity in each direction. as with all the optical techniques, the ldv is not invasive (moureh, tapsoba, & flick, 2009). a laser blade is not necessary, therefore the optical access is easier and there are no flare problems introduced by the blade sides (wuibaut, bois, el hajem, akhras, & champagne, 2006). 2.6.2 main challenges encountered in design and operation the velocity is measured directly, with a linear response, and there is no need for calibration procedures. the accuracy is very high, due to the small dimension of the measurement volume, and the system has a better signal-to-noise ratio if compared to piv since it does not require the analysis of artificially created images. nevertheless, ldv measures one point at the time while piv can reveal the global structure of the flow, which is useful in the research of flow mechanism. special care needs to be taken regarding the accuracy of the traverse system for the placement of the probe and the optical setup (zhang & eisele, 1995). this assumes an even higher importance is given to 3d measurements, where one of the three couples of beams is focused by a second probe. in addition, the location of the measurement volume must be carefully evaluated, and it must be taken into account the possible influence that curved surfaces have on the reflection of the laser beam (eisele, zhang, casey, gulich & schachenmann, 1997). typical application of ldv is in flow research such as aerodynamics of vehicles, water flow measurements, spray and combustion characterisation, as well as in automation, and lately in 043 journal of facade design & engineering volume 6 / number 3 / 2018 hemodynamics. the technique was successfully applied by bhamjee, nurick, and madyira (2013) to the study of airflow in ventilated windows in cases of forced and natural flow. 2.7 temperature profile and heat flux method. real scale facility real-scale facilities realise a large variety of full scale façade prototypes built on a large-scale test building (marinosci, semprini, & morini, 2014) or in a real building (fantucci, marinosci, serra, & carbonaro, 2017; giancola, sanjuan, blanco, & heras, 2012). the enthalpy discharge linked to the airflow within the ventilated cavity directly affects the reduction of the heat flux across the inner wall; for this, the measure of the temperature profile is a means by which to quantify this effect. the data presented in literature indicates that for fixed weather conditions, to guarantee the lower thermal load entering the building, the measured temperature within the cavity must be the lower maximum value. 2.7.1 experimental setup in literature, the following variables are measured along the façade: air temperature in the middle of the ventilated cavity at different heights; surface temperature of the external layer and surface temperature of the insulated façade at different heights; velocity and direction of the wind next to the façade; heat fluxes at the interior and at the exterior surfaces of the inner mass wall; relative humidity and global radiation on the horizontal surface; global and infrared radiation on the façade surface; ambient temperature. sensors are placed on the centreline of the façade at different heights above the floor. the surface temperature sensors are placed inside the ventilated cavity and are exposed to occasional handling as well as to climatic conditions. consequently, openwire thermocouples of cooper-constantan type (t) are used. the surface temperatures inside the ventilated cavity are measured using bare sensors covered with a reflecting tape to prevent possible errors due to surface-to-surface radiation heat fluxes. the heat flux through the internal mass wall is measured with plane fluxmeters. the heat flux is measured at the inside surface and at the exterior surface of the inner mass wall. 2.7.2 main challenges encountered in design and operation the main challenges encountered in design and operation are the accuracy of the sensors and the measurement chain, as the assumptions included in the monitoring aim affect the overall uncertainty of the performance indicators. experiments carried out in real-scale facilities are usually designed to evaluate the overall energy performance of the building. in addition, real-scale facilities present many more limitations which restrict their field of applicability. 044 journal of facade design & engineering volume 6 / number 3 / 2018 2.7.3 limitation of the experiment a major limitation is due to longer testing periods than steady-state laboratory tests. real-scale facilities are exposed to the external environment conditions that affect material degradation for which they require continuous maintenance and care. another limitation is the lack of a standardised procedure. technique physical prin­ ciple application type of measure­ ments intrusive measured physical quantity usual accuracy technical complexity investment real scale test cell labo­ ratory yes no tracer gas (constant emission) conservation of mass (air and tracer gas) x x x single points or volume x concentration of tracer gas to obtain air ventilation rate low low low velocity profile temperature-based measurement sensors x x x single points x air velocity low low low ultrasound a) doppler shift flow meter b)transit time flow meter a) frequency shift between sound wave and its reflection b) transit time in fluid flow of identical sound waves upstream vs downstream x x x plane x the interaction of ultrasonic sound waves with the moving fluid to obtain air velocity medium medium low pressure difference relation between the pressure difference and the airflow passing through an opening x x single points x air pressure difference to obtain airflow rate low low low piv cross-correlation between consecutive images of laser light scattered by tracer particles x plane or volume x particle images displacement over a given time interval between consecutive two laser pulses to obtain air velocity field very high high extremely high ldv doppler shift in a laser beam scattered by a particle x1 x1 x single points x doppler frequency of scattered light proportional to particles velocity high high extremely high >>> 045 journal of facade design & engineering volume 6 / number 3 / 2018 temperature profile and heat flux method enthalpy discharge x x x x ambient and surface temperature heat flux relative humidity global radiation on the horizontal surface global and infrared radiation on the facade surface low low low table 1 key features of each experimental techniques for airflow characterization of ventilated cavities 1 if the amount of naturally occurring seeding particle (i.e. dust) is sufficient 3 conclusions the experimental evaluation of the airflow rates in ventilated façades is proven not to be a straightforward task. until now, the experimental methods developed specifically for characterising ventilation in buildings and airflow in ducts were applied for measurements on ventilated façades. this is one of the reasons why there are few studies with conclusive results about the reliability of existing experimental procedures for the characterisation of airflow in the ventilated cavities. there is, therefore, a need to raise awareness on the lack of knowledge and robust procedures to assess the airflow in a ventilated façade. this paper has therefore the intention of stimulating: – further testing of existing methods in order to evaluate their accuracy for application with ventilated cavities; – the development of common guidelines and generally acknowledged measurement procedures for application with ventilated façades; and – the development of new methods suitable for application with ventilated cavities. in this paper, a description and a review of the recent state of the art of the experimental assessments for the airflow characterisation in ventilated cavities was presented. the classification is based on laboratory tests, as well as under real dynamic weather conditions. the tests carried out in a controlled laboratory give the possibility to carefully check all the most influential parameters. the number of existing experimental methods for the estimation of airflow rate in the built environment is limited to the following: tracer gas measurements, velocity profile method, and ultrasound measurement of velocity, as well as the use of models with measured pressure differences and the temperature profile along the ventilated cavities. furthermore, the laser-based non-intrusive experimental techniques of laser doppler velocimetry (ldv) and particle image velocimetry (piv) are applied to determine indoor airflow behaviour. potentialities and limitations as well as a detailed classification and description of such monitoring techniques for airflow in façades air cavities have been reported. considering the applicability of each of the tracer gas measurement techniques in ventilated cavities, a constant emission method is normally used. until now, no clear guidelines have been established with regard to the application of tracer gas methodology in ventilated façades, as the effect of 046 journal of facade design & engineering volume 6 / number 3 / 2018 positioning the emission source within the ventilated cavity, as well as number and location of tracer gas dilution measurement points on measurement accuracy remain unknown. overall, the results show no clear tendency as the airflow in the cavity is highly dynamic. the measurement of the air velocity is a means by which to determine the airflow rates and to estimate the surface convective heat transfer coefficient. the air velocity profile method allows the assessment of the façade air cavity performance from the measurement of the air velocity at some characteristic points. the main limitation of the experiment is the need for a detailed analysis of the airflow characteristics, especially in airflows with high 3d variability of the air velocity field. the use of the ultrasonic principle for airflow monitoring presents several advantages, for example: it’s non-intrusive and does not influence the fluid flow. the accuracy of this measurement method is relatively good and can be in the range of 2 5% of the measured values. research activities are definitely necessary to advance in their effective implementation to façade systems, paying special attention to the requirements in terms of developed flow regime. currently, the results of pressure difference method application to a ventilated cavity are successful in mechanically ventilated cavities but differ considerably in natural airflows. the main finding of this work is that the method is very sensitive to the positioning of the surface pressure and reference measurement, and to the fluctuations in wind direction and wind speed. thus, further research is needed to establish a suitable methodology for the pressure difference measurement in a naturally ventilated cavity, as it gives strong inspiration for finding a way to cope with the extremely high wind fluctuations and thereby fluctuations of the airflow. a major limitation in real-scale facilities is the testing time, longer than normal testing periods in laboratory. also, resulting from its environmental exposure, material rapidly becomes degraded and ongoing maintenance is required. all previous techniques have the common drawback of measuring over single points or single sections instead of a field or a volume to characterise the three-dimensional behaviour of the air velocity field. consequently, previous measurement methodologies need to be further developed, determining optimal points of measurement set up to balance low intrusiveness in airflow with adequate spatial resolution. the non-intrusive ultrasound piv on the contrary, allows detailed analysis of the airflow variability. however, the important initial financial investment and the technical complexity might be an obstacle to their widespread application. a major limitation of this technique is the simulation of wind effect on airflow inside the ventilated cavity. solar heat load effects are also critical with respect to façade performance. acknowledgements the authors acknowledge the networking opportunities given by the cost action tu1403 “adaptive facades network”. 047 journal of facade design & engineering volume 6 / number 3 / 2018 references belleri, a., avantaggiato, m., & lollini, r. (2017). ventilative cooling in shopping centers’ retrofit: the mercado del val case study. energy procedia 111, pp. 669-677 bhamjee, m., nurick, a., & madyira, d. m. (2013). an experimentally validated mathematical and cfd model of a supply air window: forced and natural flow. energy and buildings 57, pp.289-301 cattarin, g., causone, f., kindinis, a., & pagliano, l. (2016). outdoor test cells for building envelope experimental characterisation a literature review, renewable and sustainable energy reviews 54, pp.606-625 cattarin, g., pagliano, l., causone, f., kindinis, a., goia, f., carlucci, s., & schlemminger, c. (2018). empirical validation and local sensitivity analysis of a lumped-parameter thermal model of an outdoor test cell. building and environment 130, pp. 151–161 cuerva, a., & sanz-andrés, a. (2000). on sonic anemometer measurement theory. journal of wind engineering and industrial aerodynamics 88 (1), pp.25-55 dama, a., angeli, d., & kalyanova larsen, o. (2017). naturally ventilated double-skin façade in modeling and experiments. energy and buildings 144, pp.17–29 eisele, k., zhang, z., casey, m. v., gulich, j., & schachenmann, a. (1997). flow analysis in a pump diffuser—part 1: lda and ptv measurements of the unsteady flow. journal of fluids engineering 119(4), pp.968-977 etheridge, d. (2011). natural ventilation of buildings: theory, measurement, and design. hoboken, new jersey: john wiley & sons, pp. 428 european standard, (2015). ventilation for buildings measurement of air flows on site – methods. ns-en 16211, european committee for standardization, brussels, belgium fantucci, s., marinosci, c., serra, v., & carbonaro, c. (2017). thermal performance assessment of an opaque ventilated façade in the summer period: calibration of a simulation model through in-field measurements. energy procedia 111, pp.619-628 giancola, e., sanjuan, c., blanco, e., & heras, m. r. (2012). experimental assessment and modelling of the performance of an open joint ventilated façade during actual operating conditions in mediterranean climate, energy and buildings 54, pp.363-375 goia, f., schlemminger, c., & gustavsen, a. (2017). the zeb test cell laboratory. a facility for characterization of building envelope systems under real outdoor conditions. energy procedia 132, pp. 531-536 hitchin, e. r., & wilson, c. b. (1967). a review of experimental techniques for the investigation of natural ventilation in buildings. building science 2, pp.59-82 jensen, r. l., kalyanova, o., & hyldgaard, c. e. (2007). on the use of hot-sphere anemometers in a highly transient flow in a double-skin façade. proceedings of roomvent 2007. finvac ry, helsinki, finland kalyanova, o., jensen, r. l., & heiselberg, p. (2007) measurement of air flow rate in a naturally ventilated double skin façade. proceedings of roomvent 2007. finvac ry, helsinki, finland larsen, t. s. (2006). natural ventilation driven by wind and temperature difference. (dce thesis). department of civil engineering, aalborg university, aalborg, denmark laussmann, d., & helm, d. (2011). air change measurements using tracer gases. in mazzeo, n. (eds.) chemistry, emission control, radioactive pollution and indoor air quality, pp. 365-406. intechopen, rijeka, croatia doi: 10.5772/18600 lee, s., sang, h. p., yeo, m. s., & kim, k. w. (2009). an experimental study on airflow in the cavity of a ventilated roof. building and environment 44, pp.1431–1439 lópez, f. p., jensen, r. l., heiselberg, p., & ruiz de adana santiago, m. (2012). experimental analysis and model validation of an opaque ventilated facade. building and environment 56, pp. 265-275 manz, h., schaelin, a., & simmler, h. (2004). airflow patterns and thermal behavior of mechanically ventilated glass double façades. building and environment 39 (9), pp. 1023-1033 marinosci, c., semprini, g., & morini, g.l. (2014). experimental analysis of the summer thermal performances of a naturally ventilated rainscreen façade building. energy and buildings 72, pp.280-287 marques da silva, f., gomes, m. g., & moret rodrigues, a. (2015). measuring and estimating airflow in naturally ventilated double skin facades. building and environment 87, pp.292-301 mateus, n. m., pinto, a., & graça, g. cd. (2014). validation of energyplus thermal simulation of a double skin naturally and mechanically ventilated test cell. energy and buildings 75(0), pp.511-522 moureh, j., tapsoba, m., & flick, d. (2009). airflow in a slot-ventilated enclosure partially filled with porous boxes: part i–measurements and simulations in the clear region. computers & fluids 38(2), pp.194-205 perino, m., serra, v., zanghirella, f., issoglio, r., marques da silva, f., & gomes, m.g. (2008) performance evaluation of advanced integrated façades in laboratory facilities. proceedings of 29th international aivc conference, kyoto, japan park, c. s., augenbroe, g., messadi, t., thitisawat, m., & sadegh, n. (2004). calibration of a lumped simulation model for double-skin façade systems. energy and buildings 36, pp. 1117-1130 raine, a.b., aslam, n., underwood, c.p., & danaher, s. (2015). development of an ultrasonic airflow measurement device for ducted air. sensors 15, pp.10705-10722 saelens, d. (2002). energy performance assessment of single storey multiple-skin facades. (doctoral thesis) laboratory for building physics, department of civil engineering, catholic university, leuven, belgium sánchez, m. n., sanjuan, c., suárez, m. j., & heras, m. r. (2013). experimental assessment of the performance of open joint ventilated façades with buoyancy-driven airflow. solar energy 91, pp.131-144 sánchez, m. n., giancola, e., suárez, m. j., blanco, e., & heras, m. r. (2017). experimental evaluation of the airflow behaviour in horizontal and vertical open joint ventilated facades using stereo-piv. renewable energy 109, pp.613-623 sanjuan, c., suarez, m. j., gonzalez, m., pistono, j., & blanco, e. (2011). energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade, solar energy 85 (9), pp.1851-1863 serra, v., zanghirella, f., & perino, m. (2010). experimental evaluation of a climate façade: energy efficiency and thermal comfort performance, energy and buildings 42, pp.50-62 048 journal of facade design & engineering volume 6 / number 3 / 2018 strachan, p.a., & vandaele, l. (2008). case studies of outdoor testing and analysis of building components. building and environment 43, pp. 129–42 strauss, j., weinberg, h., kopel, z. (1996). u.s. patent 5,583,301: ultrasound air velocity detector for hvac ducts and method therefor suarez, c., joubert, p., molina, j. l., & sanchez, f. j. (2011). heat transfer and mass flow correlations for ventilated facades, energy and buildings 43, pp.3696-3703 suomi, v.e. (1957). sonic anemometer, exploring the atmosphere’s first mile, vol. 1. new york: pergamon press, pp.356–366 wuibaut, g., bois, g., el hajem, m., akhras, a., & champagne, j. (2006). optical piv and ldv comparisons of internal flow investigations in shf impeller. international journal of rotating machinery, vol. 2006 yeh, y., & cummins, h. (1964). localized fluid flow measurements with an he–ne laser spectrometer. applied physics letters, 10(4), pp.176–178 zhang, z., & eisele, k. (1995). off-axis alignment of an lda-probe and the effect of astigmatism on measurements. experiments in fluids 19(2), pp.89-94 journal of facade design and engineering 1 (2013) 73–84 doi 10.3233/fde-130001 ios press 73 lightweight bonded acrylic facing at the vitra vsl factory matthias michela,b,∗ and holger techenb,c adelft university of technology, faculty of architecture and the build environment, architectural engineering & technology, julianalaan, the netherlands bimagine structure, diesterwegstr., frankfurt am main, germany cfrankfurt am main university of applied sciences, faculty of architecture, nibelungenplatz, frankfurt am main, germany received first submission: 2009 received upgraded version: 16 august 2013 accepted: 23 october 2013 abstract. acrylic glass is omnipresent in the industrialised world; but as a building material most architects, facade planners and engineers are still unfamiliar with this material. in most cases it is applied as a substitute for glass which leads to inappropriate joints and fixtures. during the years of the path toward the digital era, the authors were in the fortunate position to be involved in several unconventional glass and acrylic glass projects. on the basis of their most recent project, the facade of the vitra vsl factory by sanaa architekten, they describe the development of a facade for which they chose acrylic glass not as a substitute for glass but rather as a conscious material choice. since the entire facade is it was possible to apply the manufacturing technology of deep-drawing, allowing for very thin wall thicknesses. keywords: facade design, structural analysis, building envelopes, cladding, acrylics 1. acrylic glass is more than a mere substitute for glass in the building environment, acrylic glass plays a subordinate role, and is mostly integrated into products that as plug-and-play solutions do not require in-depth planning and material discussions. building envelopes consisting of acrylic glass are rare. and for these occasional facade projects acrylic glass is used as a ‘substitute’ for glass; the goal is to achieve the appearance of glass but using glass itself seems impractical; processing is too expensive and/or elaborate. these projects usually feature complex surface curvatures with no or very few equal surface sections. thus, acrylic glass is used as a substitute for glass, and the deviating qualities and characteristics are put up with. the joining methods for these solutions are borrowed from building with glass; using clamps and point fixtures, for example. the error of thinking you are dealing with a glass made off plastic harbours the risk of constructing in a manner that does not comply with the material used. the authors’ opinion is that a thorough knowledge of the material in question is critical. it offers a broader range of architectural expression if used as an independent material and is thus integrated into the design process. the façade of the vitra vsl factory is an example; acrylic glass with an opaque background is used to create a curtain, literally; and the image of the curtain is reflected by the slender shape and the method of construction and fixtures. ∗corresponding author: matthias michel, e-mail: michel@imagine-structure.eu. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:michel@imagine-structure.eu 74 m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory the sensitivity of the material requires thorough knowledge of the material properties – as they are described in the following during the early concept phase. façade panels should be mounted in a way that avoids stress concentration. in order to achieve this with the vitra façade, the panels were mounted with a glue joint on the rear side for the first time without providing for mechanical protection. rather, the authors developed a redundancy system with which defects resulting from potential failure of the primary support structure can be detected from the outside. the fragile façade material and the technique of glue joining combine into a harmonised technology which will be described in the following. 2. working with acrylic glass the correct material name for acrylic glass is polymethylmethacrylate, abbr. pmma. it belongs to the group of thermoplastic materials and consists of a dense mesh of arbitrarily braided long molecular chains. the individual molecule has the shape of a loose ball of yarn. each ball is penetrated by the neighbouring ball. however, the rigidity of this inner structure loosens with increasing temperatures (ehrenstein, 1999). besides its transmissive properties and easy processability, an important advantage of the material is the possibility to reshape it under temperature – even into two-dimensionally curved geometries. its problematic reputation in the building industry results from material characteristics that are atypical for many other building materials. a brief description of these characteristics is provided in the following. – relaxation: just like most thermoplastics acrylic glass tends to creep. under long-term stress component deformation increases according to the viscoelastic behaviour of pmma. – structure: despite good short-term strength and impact resistance, prolonged and regularly reoccurring stress must be kept below the level at which crazes, i.e. micro defects in the form of detanglement of the molecules occur at increased rates. when crazes are present, the inhomogeneity of the structure can cause cracks with according notch stress. a large number of crazes also increases the tendency to creep (ehrenstein, 1999). – joining: load application and joining require special constructive consideration and high quality workmanship. hereby, edge smoothness of boreholes and cut edges is very important for the load-bearing capacity. – chemical resistance: acrylic glass is sensitive to alcohols and other solvents. it tends to stress fracture corrosion. – temperature: one remarkable material property is the large expansion coefficient. at 70*10−6 k−1 it is six times higher than that of steel and approximately ten times higher than that of glass. this results in special requirements regarding zero-stress installation of components. in case of load-bearing building parts the temperature load cases play an important role in the calculation. and the mechanical properties are temperature dependent; a factor that needs to be considered during trial conceptualisation and the layout of statically undetermined systems. – uv light resistance: contrary to common belief acrylic glass is completely uv proof. often, other non uv proof translucent plastics are mistaken for acrylic glass. – reaction to fire: similar to most other translucent plastics acrylic glass is normally inflammable; however without flaming droplets and burning almost entirely pollutant-free. – production: since manufacturers add different auxiliaries such as surfactants to the base material, material properties vary. this means that it is necessary to retrieve a binding product and project m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory 75 specific material data sheet at the beginning of the project. tests and trials have to be conducted repeatedly. – pmma is not included in the buildings rules lists. there is no building-related standardisation. a dimensioning concept can be derived from (bauüberwachungsverein, 2010). the list of the most important properties shows that the commonalities with glass are limited to transparency and surface appearance. in practice, the properties atypical for building materials make it difficult for the planner or architect to assess feasibility or plausibility of a façade concept that uses acrylic glass. how can the feasibility of an acrylic glass project be assessed without pertinent experience? the authors’ findings gained from previous projects condense into a model-like comparison with a well known building material. considering constructive and process-related aspects in addition to the mechanical properties, birch or beech veneer plywood closely resembles acrylic glass. roughly described, acrylic glass and plywood have similar design strength, even though the specific weight of acrylic glass is twice that of plywood and its stiffness is a third less. despite these deviations the comparison of plywood and acrylic glass has proven to be applicable for all of our projects, and it is useful to validate a concept: if it is conceivable in plywood it can also work with acrylic glass (michel, 2010) (bauüberwachungsverein, 2010). 3. facade of the vitra vsl factory, weil am rhein for the façade of the distributing warehouse project for vitra ag in weil am rhein by japanese architects sanaa, the authors developed a structural design with which they participated in a tender: a round building, approximately 150m in diameter, 12m high, is to be enveloped by a light shell with an immaterial appearance in the form of a white undulating curtain. the exterior walls of the industrial building feature only few gates, entrances and windows; the light curtain and the rounded shape soften the harshness of the building. technical details were to be kept absolutely invisible. the tokyo designers and local partners chose a thermoplastic material for the façade plane. the architects’ execution concept envisions a small number of differently shaped vertical panels which, arranged in an intelligent sequence create a pseudo-random wave formation. the distance between waves lies between 15 and 40cm, and the wave depth is approximately 20cm. there were no fire protection requirements for the facade since it forms a separate shell in front of a mostly enclosed steel concrete wall. thus, off the shelf co-extruded acrylic glass was the material of choice; one transparent layer and one white imbued layer are melted together, creating a white surface with a transparent coating. the visual effect of the material is appealing; mainly due to the glossiness of the transparent coat. in order to underline the desired abstract effect and to ensure material appropriate mounting, the panels of the shell are glued onto the substructure without visible fixtures, and the shell is positioned in front of the near windowless reinforced concrete exterior wall. 3.1. acrylic glass technology according to german building code a façade construction made of acrylic glass requires approval on a case-by-case basis, in the course of which its stability and durability are tested, e.g. by trials, and an operating manual is compiled for the user. 76 m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory fig. 1. acrylic glass as a substitute for glass: joints with point fixtures on the acrylic glass shell of the bmw hourglass (photo: photodesigner). in their function as engineers, the authors were able to accompany the façade project through the approval process up until its execution and completion by the general contractor, and develop it further in a team that already worked on projects such as kunsthaus graz and the nordkettenbahn innsbruck. the façade panels have no horizontal joints but are manufactured as 12m long components that are suspended. the material thickness is very low (few millimetres) in relationship to the panel dimensions. deformation takes place in a custom built oven by deep drawing at approximately 160◦c. the 6.5mm thick semi-finished part is mounted into a clamp frame on the edge of the cnc-milled and hand polished positive mould and heated. applied vacuum sucks the hot acrylic glass into the mould (fig. 5). the deep drawing process locally reduces the initial thickness to some extent. this entire process takes place inside the oven. deformation and cooling off are computer controlled, and are monitored from the outside. subsequently each panel is tempered to relax stress or tension in the material created by the cooling process. a 5-achsis mill is used to trim around all sides to create the open upper and lower edges of the façade; an important feature to achieve the abstract appearance of the façade (fig. 6). the above mentioned high thermal expansion also has an effect on the manufacturing process. it is easily conceivable how much a life of its own the 12m long and few millimetres thick blank takes on during production when it is heated by 140◦k, deformed, cooled off and finally relaxed at 80◦c. therefore an important characteristic of this building project – as of most others that include m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory 77 figs. 2, 3. study of the structure and façade visualisation (images: sanaa, nkbak). fig. 4. completed façade section (image: michel). free-formed elements – is that it would not be feasible without computer-assisted production but that material specific knowledge and experience determine ultimate failure or success. on the building, the panel is subjected to wind loads of approximately 1kn/m² and temperature fluctuations of ±40◦k. for the top mounted 12m long panel this results in contraction or elongation of 35mm at the lower end. according to the early concept which was then further developed and fine-tuned with the team of façade execution planning the panel’s rear side is glued onto an aluminium substructure which in turn is joined to the insulated reinforced concrete wall with gliding fixtures. these consist of horizontal latch profiles that are hooked into wall-mounted hooks. on the profiles are so-called glue fixtures onto which the acrylic glass panels are mounted at the indentations between the wave crests. between the latches with the glue fixtures the panel hangs free because the undulating form provides sufficient stiffness. only the edges, which are partially flattened, are designed to be supported with a stiffening 78 m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory figs. 5, 6. deep drawn panel in the oven – wave section made of co-extruded acrylic glass (images: michel). profile to avoid deformation. glue fixtures are hooked onto these as well; cutting the panels’ edge span widths in half (figs. 7, 8). the panel’s own weight is dissipated at the top end. conforming to the visual appearance of a curtain it is suspended from the parapet level, and horizontally fastened in regular intervals to withstand wind loads. this method of vertical mounting is an important feature that allows for the low thickness of the material. the relaxation behaviour of the material and warm temperatures would eventually cause a bottom-mounted panel to take on a progressively warped shape. our first concept envisioned a lip along the upper edge of the panel, and the panel being suspended from this lip. design aspects, however, caused us to vote for a hard cut edge, eliminating the possibility of simple mechanical mounting. thus, the chosen solution involves glued fixtures on the rear side, also used to dissipate the panels’ own weight, and foregoes mechanical mounting. 3.2. gluing method the conception and dimensioning of the glue fixtures is determined by the chosen gluing method. the adhesive must allow for large thicknesses to accommodate the different radii of the glue fixtures and the panel’s curved shape. and it must allow for the different temperature expansion of aluminium and acrylic glass. it must be uv and heat resistant, and must be sufficiently strong under push and pull loads. double sided adhesive tape was neglected because of its low thickness and the resulting large number of different curvatures for which appropriate glue fixtures would have had to be produced. the best suited adhesive material found was polyurethane glue. however, in-depth research highlighted that its uv resistance under acrylic glass cannot be completely guaranteed at this time because m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory 79 figs. 7, 8. illustration of panel substructure – panel prototype during trial assembly without insulation (images: imagine, michel). building scientific research is focussed on gluing glass, not acrylic glass. since the uv permeable spectral ranges of glass and acrylic glass differ, no secured inference on the durability can be drawn form these studies. the planning timeframe for this particular project did not permit project based research. therefore, structural-glazing sg silicone is used for all glue fixtures of this project. it fulfils most of the mentioned requirements, but its suitability for long-term push loads as they occur in this application is limited. on the other hand, the existence of a technical approval guideline for sg silicones (etag002) facilitates further steps as it describes testing procedures for the certification of sg silicones and the according methods of dimensioning. even though etga002 is limited to sg façades, i.e. façades with linear glued glass panes, it does provide tests which can be drawn upon for an approval for this particular project as is required for the described project (hagl, 2007). the testing program is set up in two phases. during the first phase tests are conducted with small samples under uv influence, and in cold and hot storage as they are defined according to etag 002. these tests are designed to show the properties of the silicone-pmma glue joint referring to adhesion and cohesion. in our case, there were no significant limitations in comparison to the target values for glued joints of aluminium and glass. cohesion was sufficiently researched during the eta certification process for the silicone material; it, too, offers small risk of surprise, which means that compliance with the values of the etag is to be expected for these kinds of tests. 80 m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory the second testing phase involves pull tests on pull holders with the original geometry (length 10cm to 30cm) (fig. 9). for dimensioning glued joints, etag provides for a partial safety factor of 6, with which the test results are attenuated (hagl, 2007). with a 5% fractile of the breaking force of more than 10kn for the longest sample the gluing method proves suitable to absorb wind loads. inconsistent silicone filling become clearly visible when breaking the glued joint. thus, the tests are also useful in terms of quality assurance to generate a processing guideline and a qm concept, as was done in this case with the executing parties under accompaniment of the authorised expert. since closely defined tests do not lend themselves to research the resistivity of the thin and planar acrylic glass in a practical manner and not all special circumstances of the glue joints can be represented by testing, we have created thorough finite element models of each panel type and the most important special geometric situations in ansys. these finite element models exactly depict panel, glue joint and sub-structure of a panel section. to determine the material parameters of the hyperelastic material law, the setup of the pull tests was modelled and recalculated. upon concordance of test and calculation the design resistances were derived from the generated tension. since etag 002 is based on the simplified approach of even stress distribution it can be expected that, in certain areas, the fe calculated stresses lie above the design resistance values of the guideline, as was the case here (hagl, 2007) (figs. 10–12). whereas testing provided only good results in terms of adhesion, the research related to stress crack corrosion on acrylic glass narrowed the range of silicone candidates. with plastics, stress cracks occur under mechanical stress if the plastic material is simultaneously subjected to specific solvents. as a general rule, solvents and plasticizer-containing products that are to be in contact with acrylic glass must be tested for compatibility. with silicone, volatile components whose concentration is only critical during processing can corrode acrylic glass if pull stress is present. hereby, at the time of processing, it is not the tension from external impact that is important but rather embedded tension that results from thermal deformation without subsequent tempering. the design of the glue joints on the sub-structure strictly follows the type of stress incurred (figs. 7, 14). the glue mounts mounted in a row onto the latch profiles accommodate horizontal wind loads only. at the upper end there are additional load-bearing mounts that carry the own-weight of the panel. they are suspended from the uppermost latch profile which is rigidly coupled to the reinforced concrete wall on consoles. as mentioned before silicone is not an optimal adhesive for permanent push loads because this type of joint tends to creep. therefore, the allowable stress for such a glue joint is low and requires large glue areas. (etag limits this type of stress to accordingly qualified products for class iv. prerequisite is successful completion of a 90-day test at 55◦c and 90% humidity. the product of the partial safety factors versus permanent push loads results in a factor of 60). the vertical load mounts are therefore large. they are made of angled sheet metal; the curvature matches that of the individual wave shapes. due to their length, the vertical load mounts are designed as a three-membered sprocket chain, so that they do not add rigidity under wind loads and absorb push loads only. 3.3. redundant fixtures despite 90 days of push tests as part of the approval phase, the option of a mechanical safety device was still on the table because no results of long-term material specific research are yet available. however, the goal is to avoid visible mechanical protection at the upper edge of the panel. m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory 81 figs. 9, 10. test: gluing the pull holder and according fe model (images: michel). figs. 11, 12. recalculation of the silicone tension for the pull test from fig. 9 (eccentric load) tension distribution glue joint in the panel model (images: imagine). therefore the substructure is designed such that a redundancy is achieved in case of failure of the push glue joints. in this unlikely case the glue joints would detach the load mounts, and the panel together with the remaining substructure and the hereon mounted glue mounts would slide downward. this means that the panel with the remaining latch profiles falls a few centimetres into the wall-mounted hooks whose original purpose is to dissipate wind loads. in this case all glue joints 82 m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory fig. 13. fe simulation mount failure (image: imagine). figs. 14, 15. functionality of the redundant push glue joints (green=intact; red=failure) (images: imagine). m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory 83 figs. 16, 17. rear side of a panel during assembly – completed façade section (images: michel). of the pull mounts temporarily participate in carrying the weight of the panel (figs. 14, 15). the upper end of the panel is also no longer protected against wind suction, and is held as a cantilever arm from the second row of mounts. specific detailing makes it possible that in case of failure all edge mounts remain effective so that only the waves in the middle section of the panel have to serve as cantilever arms (fig. 13). panel failure can easily be detected from the outside because it hangs a few centimetres lower than the adjacent panels. thus, timely identification and repair are possible. assembly is very easy; a support frame is used to erect a panel, then, after removing the support frame the panel is hooked into the mounts on the building shell (fig. 16). in april 2013, the project with its 280 panels and just under 6000m² was completed after one and a half years of planning and execution. 4. conclusion the project report shows that the gluing method allows using large yet filigree plastics components on façades that could not be realised with mechanical fixtures. gluing is particularly predestined for plastics materials that feature low firmness or tend to relaxation deformation under long-term loads. however, the report also allows conclusions about the scope of the engineering constructive expenditure necessary to reach the goal of constructing a panel that is simply glued to a wall. it was helpful that already established standards, even though applied in other areas, of gluing with structural glazing silicone could be applied to acrylic glass, even if with some restrictions. more research needs to be done; in particular related to uv stability of polyurethane glue joints with acrylic glass because the permeability of glass and pmma differ. the broader the knowledge in this field, the more we can expect a willingness to create plastic façades without mechanical fixtures on buildings of large vertical dimensions if other technical issues do not limit the size to that of low buildings. the only other remaining limiting factor will then be possible demands with regards to flammability of the material. in the documented project, acrylic glass is not reduced to a substitute for glass but is considered an independent building material with unique characteristics; particularly as they result from joining transparent and white-opaque materials. 84 m. michel and h. techen / lightweight bonded acrylic facing at the vitra vsl factory project imprint: – client vitra ag, weil am rhein – architects sanaa, tokyo and nkbak, frankfurt – general contractor stabag ag, direktion ao metallica, m. pagitz – acrylic glass panel production ktec gmbh, radstadt – building authority landesstelle f. bautechnik, tübingen/stuttgart – authorised expert dipl.-ing. heinz pfefferkorn – façade execution planning gbd zt, dornbirn – testing laboratory gbd lab, dornbirn – inspection engineer bernhard strasser, bad säckingen – structural and façade engineering imagine structure gmbh, frankfurt references bauüberwachungsverein. (2010). büv empfehlung: tragende kunststoffbauteile im bauwesen [tkb], berlin. ehrenstein, g. (1999). polymer-werkstoffe: struktur eigenschaften anwendung. (pp. 99-102). munich: hanser verlag. hagl, a. (2006). die innovation – kleben. stahlbau, 75, 508-520 hagl, a. (2007). bemessung von strukturellen silikon-klebungen. stahlbau, 76, 569-581 knippers, j., cremers, j., gabler, m., & lienhard, j. (2011). construction manual for polymers + membranes, munich: birkhäuser verlag. michel, m. (2010). acrylic facades, three case studies. in u. knaack, & t. klein (eds.), the future envelope, 3, (pp. 1 -12). amsterdam: ios press. from city’s station to station city 049 journal of facade design & engineering volume 6 / number 3 / 2018 thermal assessment of glass façade panels under radiant heating: experimental and preliminary numerical studies chiara bedon1*, marcin kozłowski2,3, dániel honfi4 * corresponding author 1 university of trieste, italy, chiara.bedon@dia.units.it 2 lund university, sweden 3 silesian university of technology, poland 4 rise research institutes of sweden, sweden abstract nowadays, glass is increasingly being used as a load-bearing material for structural components in buildings and façades. different structural member solutions (such as panels, beams, columns) and loading conditions were the subjects of several research studies in recent years. most of them, however, were typically limited to experimental testing and numerical simulations on glass elements and assemblies at room temperature. thermo-mechanical investigations, inclusive of the temperature-dependent behaviour of visco-elastic interlayers used in laminated glass solutions, as well as the typical thermo-mechanical degradation of glass properties in line with temperature increase, in this regard, are still limited. such an aspect can be particularly important for adaptive façades, in which the continuous variation of thermal and mechanical boundary conditions should be properly taken into account at all the design stages, as well as during the lifetime of a constructed facility. given the key role that thermo-mechanical studies of glazing systems can pe use of glass in façades, this paper focuses on finite element (fe) numerical modelling of monolithic and laminated glass panels exposed to radiant heating, by taking advantage of past experimental investigations. in the study discussed herein, being representative of some major outcomes of a more extended research project, one-dimensional (1d) fe models are used to reproduce the thermal behaviour of selected glass specimens under radiant heating, as observed in the past experiments. given the high computational efficiency but very basic assumptions of 1d assemblies, a critical discussion of experimental-to-numerical comparisons is then proposed for a selection of specimens. keywords monolithic glass, laminated glass, thermal loading, radiant heating, experimental testing, finite element (fe) numerical modelling doi 10.7480/jfde.2018.3.2477 050 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction façade systems, both traditional and innovative (i.e. adaptive façades that have been gaining widespread attention recently), are subjected to a multitude of boundary and loading conditions over their lifetime, including thermal and mechanical variations. in this regard, the system behaviour, especially for adaptive façades, should be properly assessed by giving careful consideration to several loading combinations, since these are responsible for degradation phenomena at material, component, and assembly level (bedon, 2017; bedon et al., 2018c). this is especially true when considering structural issues for façades in general, where both full-scale experiments and finite element (fe) numerical models that are able to capture the actual material and assembly behaviours are often required. in this paper, the thermo-mechanical performance of glass façade panels is the subject of preliminary investigation, via experimental testing and simplified fe numerical methods. glass, as commonly known, is largely used in engineering applications as a structural material, especially in the form of laminated sections composed of multiple glazing layers, bonded together by thermoplastic foils (see for example (haldimann, luible, & overend, 2008; feldmann et al., 2014)). however, major issues in the design of structural glazing assemblies are represented by the high sensitivity of glass and common bonding layers to temperature variations. in recent years, several research studies have focused on the thermal and optical assessment, or energy performance evaluation, of several types of glazing assemblies and solutions, both at the component and at the whole building level (see for example fang, eames, & norton, (2007); ghosh, norton, & duffy (2016); ghoshal & neogi, (2014); li, li, zheng, liu, & lu (2015); aguilar et al. (2017); parra, guardo, egusquiza, & alavedra (2017), etc.). from a structural point of view, the thermal performance of glass elements and systems directly reflects upon their load-bearing capacity, due to the intrinsic material properties (see bedon (2017), for example, for a state-of-the-art review on load-bearing structural glass systems under fire). the key role of appropriate thermo-mechanical investigations is further enforced in the case of adaptive façades, rather than in traditional static curtains, where glass components could be further affected by continuous variations in both the thermal and mechanical boundary conditions (favoino, jin, & overend, 2014; hasselaar & looman, 2007; aldawoud, 2017; baumgärtner, krasovsky, stopper, & von grabe, 2017; etc.). on one side, given a daily or accidental/extreme temperature variation, thermal shock phenomena occurring after moderate/high thermal gradients can cause changes that exceed the strength of the glass, leading to opening and propagation of cracks, with loss of the structural integrity (cuzzillo & pagni, 1998; tofilo & delichatsios, 2010; etc.). glass systems, due to the typically low material resistance and low thermal conductivity (haldimann et al., 2008), are particularly vulnerable to failure from thermal shock (see for example fig. 1(a)). at the same time, the degradation of mechanical properties of the glass brought about by high temperature (modulus of elasticity, tensile resistance, etc.) could severely affect the structural performance of such assemblies (see fig. 1(b)). on the other hand, the well-known temperature/ load-time dependent behaviour of viscoelastic interlayer and/or rubbery materials that are commonly used to bond together the glass panels, as well as to join the glass panels to the structural background (see (haldimann et al., 2008)) can have crucial effects even under operational conditions. at the design stage, the combination of these multiple phenomena should be hence properly accounted for, for safe design purposes. 051 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 1 glass systems under thermal loading: (left) examples of thermal shock phenomena in a glass window and (right) experimental variation with temperature of the modulus of elasticity of ordinary glass (figure adapted with permission from bedon (2017) plots are derived from different investigations) experimental research studies have been focused, in the last few years, on the assessment of the thermo-mechanical performances of glass windows, façade systems, small-scale components subjected to high temperature scenarios and/or fire loading. thermo-mechanical investigations on glazing systems, however, are indeed still limited in number and structural typology. an increasing interest is captured especially by the performance of structural glass components under fire, due to safety purposes. while full-scale experimental studies are still rare in the literature (see bedon (2017) for an overview), numerical modelling can represent a robust tool and support for designers, as a further extension of time consuming and expensive testing. key input parameters and possible limits in the same fe method, however, should be properly taken into account. bedon, pascual agullo, luna-navarro, overend, & favoino (2018b) numerically investigated the thermal and structural performance of glass-to-gfrp sandwich façade modular units, under ordinary thermal conditions and wind pressure. although limited to specific loading configurations, the study suggested the importance of coupled thermal and mechanical simulations for a given glazing system, which should be preferably taken into account to optimise its overall performance. bedon & louter (2018) numerically investigated the load-bearing response of laminated glass plates under standard fire iso curve and imposed mechanical loads. the numerical analyses generally gave evidence of a close correlation with test results, even suggesting the extension of the same study to a wide set of thermal-to-mechanical loading ratio conditions. at the same time, the mechanical restraints were found to have a crucial role in the overall performance of the same glass plates, both from the thermal and numerical points of view. in this paper, numerical simulations are carried out in abaqus (simulia) and proposed for both monolithic and laminated glass specimens under radiant heating, by means of geometrically simplified but computationally efficient one-dimensional (1d) assemblies. in doing so, the major advantage is taken from available experimental results, for small-scale glazing samples under thermal loading. comparative results are then critically discussed, based on selected experimental results, in order to emphasise the potential and possible criticalities of the fe method. the research outcomes partly summarised in the paper are derived from two short-term scientific missions (stsms) by the involved authors (wg2 “structural” task group members), which has been approved and financially supported, throughout 2018, by the eu-cost action tu1403. 052 journal of facade design & engineering volume 6 / number 3 / 2018 2 summary of the reference experimental campaign the numerical analyses presented in this paper are based on the results of past experimental studies. debuyser et al. (2017) carried out an experimental campaign that aimed to assess the thermal behaviour of annealed monolithic or laminated glass samples (annealed glass plies bonded together with pvb and sgp interlayers). in the reference study, glass panels with nominal dimensions of 185mm × 285mm were mounted in a supporting frame and exposed to radiant heating. see fig. 2. fig. 2 radiant panel tests (debuyser, 2015; debuyser et al., 2017) (left); heat flux meters mounted on the frame, (right) testing of a sample sample # glass thickness / build-up [mm] interlayer thickness t2 10 (mg) t4 15 (mg) t5 6+10+6 (lg) 0.76 mm (pvb) t12 6+10+6 (lg) 0.76 mm (pvb) table 1 overview of selected test specimens, according to (debuyser, 2015; debuyser et al., 2017) key: mg= monolithic glass, lg= laminated glass the distance between the radiant panel and the typical glass specimen was about 450mm. all of the experiments started at an imposed air flow and gas flow of 8l/s and 0.475l/s, respectively. however, a time dependent decrease of the same gas flow (and hence of the corresponding heat flux) was generally observed during the tests. see fig. 3. besides measuring the surface glass temperature (based on a set of thermocouples mounted both on the exposed and unexposed sides of each sample), the heat flux was also continuously monitored throughout the tests. this was done by means of gardon gauge type sensors, which use a differential thermocouple as a transducer to measure the temperature difference between the centre and the circumference of a thin circular foil disc. the latter is bonded to a circular opening in a cylindrical heat sink. see fig. 2(left). moreover, within the same experimental study, additional test measurements of thermal properties such as conductivity, diffusivity, and volumetric heat capacity of glass and pvb/sg interlayers were carried out. further details of the tests results can be found in debuyser (2015). 053 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 3 measured heat flux (at the side of the samples) for the selected tests specimens t2, t4, t5, and t12 fig. 4 schematic representation of 1d heat transfer models (abaqus). typical view of (above) monolithic and (below) laminated glass specimens from the total number of 16 specimens discussed in debuyser, (2015) and debuyser et al., (2017), four sets of glass samples were selected for the current study. in table 1, the samples labelled as t2 and t4 represent monolithic glass panels with a thickness 10mm and 15mm, respectively. specimens t5 and t12 are both laminated glass samples, composed of two external, 6mm thick glass plies each side of a central 10mm ply (0.76mm is the nominal thickness of the bonding pvb foils). fig. 3 presents the measured heat flux at the side. from the figure, slightly declining values of the heat flux for all specimen can be observed. the phenomenon is strictly related to the specifics of the radiant panel, in which constant heat flux was very difficult to obtain. moreover, for the specimens t4 and t5 sudden drops of the heat flux at approx. 500s and 900s can be seen. these drops were caused by the unintentional overheating of the radiant panel, which was followed by the shutting off of the device. however, in these cases, the radiant panel was powered on again and the tests were continued. 3 numerical simulations the main aim of the numerical study summarised herein was to simulate the thermal behaviour of monolithic and laminated glass panels subjected to the assigned heat flux histories. in doing so, a one-dimensional (1d) heat transfer modelling approach was initially chosen, due to the wellknown computational efficiency, in order to assess the potential and possible limits, compared to more advanced and refined (but time-consuming) full solid 3d models. the numerical results and comparisons discussed herein, in this regard, represent some major outcomes of an extended investigation. in the fe study, the set of samples summarised in table 1 was numerically analysed by taking into account the experimental heat flux measured during the tests. in accordance with the adopted 1d modelling approach, given a glass sample under thermal loading, the absorbed energy is conducted through the monolithic glass or through the different layers of 054 journal of facade design & engineering volume 6 / number 3 / 2018 the glass laminates, thereby causing a temperature increase within the materials. to describe this absorption and conduction, several temperature dependent thermal properties of both the glass and the interlayers are required, namely the specific heat capacity, the density, and the thermal conductivity. the temperature increase within the specimen is then estimated as the net result of the absorbed heat flux and the cooling heat fluxes. therefore, the radiative and convective heat transfer to the environment are modelled at both the exposed surface and the back surface. the radiation to the environment requires the surface emissivity as input, and is assumed to be lumped at the surfaces of interest. the convection is then calculated as a function of the convective heat transfer coefficient. as the convective heat transfer coefficient is dependent on the nodal temperatures calculated within the given fe model, a user subroutine is required to describe its evolution (see section 3.1). the basic assumption of such a fe modelling strategy is that three-dimensional effects (i.e. in-plane heat flux variations or in-plane temperature gradients, in combination with the imposed through-the-thickness heat flux) can be disregarded. the experimental-to-numerical comparisons are therefore reliable as long as the test temperatures from the past samples are measured in the central part of the glass panels, where the in-plane temperature gradient is negligible (see section 2). careful consideration was given to the post-processing, especially for the fe analysis of the amount and evolution of temperature on the specimens’ surfaces (both exposed and unexposed to the imposed heat flux), as well as the influence of heat flux history on the glass thermal response, including the temperature gradient dt in the thickness of each sample, throughout the simulation time. the latter aspect (manually calculated as the difference of temperature at the exposed and unexposed nodes) is a particularly important parameter because it is directly related to potential failure of glass due to thermal shock phenomena. 3.1 model description a one-dimensional (1d) heat transfer model, analogous to the modelling approach presented in debuyser (2015) and debuyser et al. (2017), and further developed in bedon, honfi, & kozłowski (2018a), was created using the commercial computer software abaqus (simulia). the typical 1d model consisted of 2-node, one-dimensional diffusive heat transfer elements (type dc1d2, from abaqus element library), (fig. 4). temperature dependent thermal properties of glass and interlayers, such as conductivity and specific heat, were taken from references in literature (tong, 1994; cardenas, leon, pye, & garcia, 2016; debuyser, 2015). an optimal emissivity coefficient equal to 0.97 was then assumed for the glass surfaces, based on previous sensitivity studies (bedon et al., 2018a). to define the thermal boundary conditions between the external glass nodes and the surrounding environment, a fortran script user-subroutine was used. this involved a convective heat transfer coefficient dependent on the varying temperature of the exposed and unexposed nodes, as also described in detail in debuyser (2015), debuyser et al. (2017), and bedon et al. (2018a). in doing so, long-wave radiative phenomena and related effects were neglected. while long-wave exchanges typically have a key role in energy balance simulations in buildings and require advanced numerical tools (see for example stefanizzi, wilson, & pinney (1990) miller, thomas, kämpf, & schlueter (2015) etc.), they are mostly negligible and conventionally disregarded for thermomechanical engineering simulations on ordinary, soda lime silica glass systems (tong, zhu, guo, & ma, 2002; wang, wang, & li, 2013; wang & wang, 2016; etc.). an initial ambient temperature of 20°c was applied to the typical fe model. in the simulations, additional physical constants were also taken into account, such as the stefan-boltzmann constant (5.67 × 10-8 w/m2k-4) and the absolute zero temperature (-273°c). the thermal exposure was finally simulated by applying a concentrated heat 055 journal of facade design & engineering volume 6 / number 3 / 2018 flux to the exposed node of each 1d model, by taking into account for each sample the heat flux histories from the experiments. sections 3.2 and 3.3 present some selected results for the monolithic and laminated glass samples object of investigation. there, continuous lines are conventionally used to represent the temperature evolution at the exposed node of each specimen (“exp”, in the following), while dashed lines are used for the unexposed node (“unexp”). 3.2 numerical results for monolithic glass specimens fig. 5 presents a comparison of numerical and experimental results for the 10mm thick monolithic glass specimen (t2, according to table 1). in the past experiment, the specimen t2 was exposed to a constant heat flux slightly decreasing over time, (fig. 3). such a thermal loading typically resulted in a rather stable increase of temperature over time in the thickness of the sample(fig. 5(a)). in general, the fe model proved to offer a rather acceptable agreement with the corresponding test measurements. however, the numerical results were found to partly overestimate the experimental predictions. a close correlation was observed especially at the beginning of the temperature history (≈300s), at both the exposed/unexposed nodes. for the following instances of thermal exposure, the average scatter was found to grow to a maximum of 12%, which may be related to the input parameters assumed herein, and in particular the thermo-physical properties of glass. fig. 5(b), in this regard, shows a point-by-point comparison between the experimentally and numerically estimated temperatures, on both the exposed and unexposed faces of the t2 specimen. input data are taken from selected time instants (0-1500s) and proposed in normalised form. given the linear trend of both the series of dots, it is possible to notice that through the full thermal exposure the numerical data estimate the experiments in the average value of 10% and 5%, at the exposed and unexposed nodes, respectively. the experimental-to-numerical correlation, in addition, has mostly a linear trend for both the reference control points, with major scatter in the range of 130180°c only (approx. 300-500s). additional experimental-to-numerical results are finally proposed, for the same t2 sample, in fig. 5(c), in the form of temperature gradient dt in the thickness of glass. as expected, the numerical results were observed to generally overestimate the experimental calculations, up to approx. 50% in some exposure intervals. such a scatter, resulting from cumulative effects depicted in fig. 5(b), was found to have a mostly uniform trend for the full time history. this effect could be caused by the input thermal parameters of glass, as previously highlighted, but also by possible defects of the measurement methods, as partly described by bedon et al. (2018a). in terms of thermal cracking for the examined specimen, both the experimentally and numerically calculated temperature gradients dt are presented in fig. 5(c) and lie below the allowable value of 45°c that the pren thstr:2004 document recommends for annealed glass panels with polished edges (10mm the nominal thickness, as in the case of the t2 sample), to prevent thermal shock. 056 journal of facade design & engineering volume 6 / number 3 / 2018 a b c fig. 5 comparison between experimental and numerical (abaqus) results for the monolithic t2 sample: (a) temperature history and (b) point-by-point temperature comparisons for selected time instants, with (c) calculated temperature gradient, as a function of time glass type limit values (°c) as-cut or arrissed smooth ground polished float or sheets ≤12mm thick 35 40 45 float 15mm or 19mm thick 30 35 40 float 25mm thick 26 30 35 patterned 26 wired patterned or polished wired glass 22 heat strengthened 100 tempered 200 laminated smallest value of the component panes table 2 allowable temperature gradients, according to pren thstr:2004 provisions for clarity of presentation, table 2 reports the actual reference values for different glass types and treatments, according to the same pren thstr:2004 provisions. no major cracks due to thermal 057 journal of facade design & engineering volume 6 / number 3 / 2018 loading were observed during the experiment, even if limited damage propagation was noticed close to the sample supports, at the edge of the t2 panel, after 18min (≈1000s) of exposure). according to fig. 5(c), such a time instant corresponds to a limited temperature gradient (≈22°c), hence suggesting further studies are necessary. with the 15mm thick, monolithic specimen t4 being taken into account with a difference from the t2 sample represented by the nominal thickness and the assigned radiant heating (fig. 2, “t4 exp” plot) the temperature estimations proposed in fig. 6 were obtained. a b c fig. 6 comparison between experimental and numerical (abaqus) results for the monolithic t4 sample: : (a) temperature history and (b) calculated temperature gradient, as a function of time, with (c) experimentally observed cracks in the sample as in the case of the t2 specimen, the numerical results were generally observed to slightly overestimate the experimental data (fig. 6(a)). for the fe node exposed to the heat flux, much better correlation was found, especially at the beginning of the temperature history (up to 600-700s), rather than at the later stage of the analysis (where, in any case, the fe temperature values exceed fewer than 10% the corresponding test results). in the case of the unexposed node, for the whole simulation 058 journal of facade design & engineering volume 6 / number 3 / 2018 time, the fe results were indeed found to overestimate the experimentally measured temperatures, by approximately 10%. for both the exposed and unexposed nodes, after ≈500s of thermal exposure, a drop in temperature can be also clearly observed, which was caused by a sudden shutting off of the radiant panel. such a phenomenon is more evident on the exposed surface, while the unexposed node due to the thermal inertia of glass volume is less sensitive. fig. 6(b) shows the temperature gradient δt as a function of time, as calculated from the past experimental data and obtained from the numerical analysis of the t4 specimen. as shown, the numerical results were observed to underestimate the experiments until approximately 1200s of exposure, while, subsequently, the fe predictions overestimate the experiments. the measured and simulated δt was much larger than the allowable temperature gradient recommended by the pren thstr:2004 provisions (i.e. 40°c for 15mm thick polished panels, see table 2) and was typically associated in the reference experiments – with severe cracks in the specimen, (fig. 6(c)). since the numerical gradient of fig. 6(b) also rises up to ≈22°c, given the mechanical properties of glass (i.e. fig. 1(b), etc.) and its sensitivity to temperature variations, it is also expected that the thermomechanical analysis of the same sample typically requiring a time-temperature scenario for the fe model nodes as a key input parameter could also result in crack propagation. 3.3 numerical results for laminated glass specimens  two selected laminated glass specimens were then taken into account from the full set of experimental samples, with the reference cross-section schematised in fig. 7. fig. 7 reference layered section for the laminated t5 and t12 specimens, with evidence of numerical control points fig. 8, in this regard, presents a comparison of numerical and experimental results for a laminated glass specimen composed of two 6mm plies and a middle 10mm ply (t5, see table 1). even moving from a monolithic to a layered cross-section, as also observed for the t2 and t4 results, much better agreement with the experiments was observed for the early stage of the fe analysis on the t5 assembly (600-700s). this included the evolution of temperature at both the exposed an unexposed nodes. during the reference experiment, similarly to the t4 sample, a sudden shutting off of the radiant panel took place at approx. 950s of exposure. such an accident can be clearly perceived in the temperature histories shown in fig. 8(a). given the presence of multiple glass layers compared to the t4 monolithic sample it is, in any case, possible to notice that the temperature records were affected on the exposed surface only, with mostly null effects on the back face of the laminated assembly. worth noting, finally, is that after 700-800s of exposure, the fe model generally proved 059 journal of facade design & engineering volume 6 / number 3 / 2018 to underestimate the test results for the exposed node (up to -15% their scatter at the final stage of the analysis). conversely, the temperature evolution was overestimated for the unexposed glass layer (up to +30%, at the end of the analysis). a similar result could be derived both from the input thermal properties of glass and interlayers, as well as from the basic assumptions of the 1d models presented herein, hence suggesting the need for further extended investigations. a b c fig. 8 comparison between experimental and numerical (abaqus) results for the laminated t5 sample: (a) temperature history; (b) numerically calculated temperature gradient for all the laminate plies, as a function of time and; (c) absolute / total gradient for the t5 sample, as obtained from the test and the fe model additional comparative calculations were then carried out by taking into account the fe temperature gradient over time, through the thickness of the sample. fig. 8(b) presents temperature gradients for each ply, as obtained from the t5 numerical simulation. at the time of the experimental investigation for the laminated specimens no additional thermocouples were mounted within the thickness of the given sandwich section, but only at the external faces of the samples. consequently, no direct dt comparisons can be carried out for each glass ply, and the fe results in fig. 8(b) can offer qualitative feedback only. as shown, the exposed surface (“l-6” plot) heats up the fastest, and an high temperature gradient (> 30°c) is predicted at an early stage of the simulations (≈130s). 060 journal of facade design & engineering volume 6 / number 3 / 2018 the middle, 10mm thick glass ply (“m-10”) shows a dt evolution in time of lower magnitude, due to the protective contribution of the exposed layer (≈ 30°c the maximum dt prediction), as well as a certain delay in the temperature increase (≈250s). the middle layer itself insulates the unexposed glass ply, which shows limited dt values (in the order of 50% the other plies, “r-6” plot) and a totally different evolution in time. in fig. 8(b), the sudden drop of dt for the exposed l-6 ply can still be observed at approx. 950s of exposure, in accordance with the experimental findings. with regard to the examination of the middle and unexposed glass layers, mostly null effects can be perceived from the same heat flux drop. for laminated glass systems (table 2), the thermal fracture is conventionally ensured insofar as the allowable nominal gradient for the weakest ply is not exceeded, according to the existing recommendations. in the case of the t5 laminated assembly (as well as the t12 specimen discussed in the following chapters), the experimental crack was observed to initiate in the range of 230-260s of exposure. in this regard, the absolute/total gradient for the sample of investigation can offer some further feedback on its overall performance. in fig. 8(c), the absolute variation is shown, over time, for the temperature measurements at the exposed and unexposed nodes (i.e. assuming a fully monolithic performance for the laminated cross-section). compared to the reference nominal value of 45°c (table 2), it can be observed that the experimental cracks were typically observed to propagate for absolute thermal gradients in the order of 60-70°c. the actual effect of the intermediate pvb foils for the nominal layered section (fig. 7), however, requires further extended investigations. in fig. 9, finally, comparative results are shown for the t12 laminated specimen, having the same geometrical and mechanical properties of the t5 sample (fig. 7). in accordance with earlier observations, a close agreement with the experiments was observed, especially for the early stage of the fe analysis (600-700s, with less than 10% of scatter). at approximately 600s of exposure, however, a marked increase of experimental temperatures for the exposed node can be observed. such an effect can be justified, most probably, by the detachment of the aluminium foil shielding the thermocouple of interest from direct radiation. a mostly linear trend was, in fact, recorded for the heat flux (fig. 3), thereby excluding possible abrupt variations in the loading condition. for the same reason, a mostly stable temperature increase was numerically predicted for the exposed node, hence resulting only in an apparent mismatch between the compared curves (15-20% their scatter at the final stage of the experiment). in the case of the unexposed node, similarly to the t5 specimen, the fe analysis generally overestimated the corresponding test data, with increasing scatter towards the final phase of thermal exposure (+30%, after 1500s of testing). the close correlation with t5 observations (fig. 8(a)) can be considered as a suggestion for possible 1d modelling limitations, requiring the use of more refined fe models able to capture the actual performance of the laminated specimens. with regard to the temperature gradient in each glass layer that were taken into account for the t12 assembly (fig. 9(b)), a close qualitative correlation with the t5 numerical predictions can again be perceived. some minor variations in figs 8(b) and 9(b) are related to the different input thermal loading for the t5 and t12 samples (see fig. 3). accordingly, the exposed glass ply is subjected to > 25°c of thermal gradient in the first 150s of exposure. the middle glass layer suffers a lower and delayed dt increase (-15% the exposed ply, after 300s of thermal loading). the unexposed glass layer, finally, is mostly protected by the other assembly components. a maximum dt in the order of 10°c can be noticed only after 1500s of exposure. 061 journal of facade design & engineering volume 6 / number 3 / 2018 a b c fig. 9 comparison between experimental and numerical (abaqus) results for the laminated t12 sample: (a) temperature history, (b) calculated temperature gradient for all the laminate plies, as a function of time and (c) absolute / total gradient for the t5 sample, as obtained from the test and the fe model in fig. 9(c), finally, a qualitative agreement can again be observed between the t12 and t5 fracture performance, insofar as the layered cross-section of fig. 7 is roughly approximated to an equivalent, fully monolithic section. in this regard, further investigations are required to capture the contribution of common interlayer foils for the overall thermo-mechanical performance of glass laminates, both from a pure thermal point of view, as well as in terms of mechanical efficiency, given the limited resistance and shear stiffness of these films to high temperatures. the effect of simplified numerical approaches for laminated sections under radiant heating and fire, in particular, needs to be assessed in terms of experimental data. 062 journal of facade design & engineering volume 6 / number 3 / 2018 4 summary and conclusions in this paper, some selected results from a research collaboration between the involved authors were presented, as obtained during two short-term scientific missions (stsms) approved and financially supported in spring 2018 by the eu-cost action tu1403 “adaptive façades network”, as well as from the excellent networking activity that still follows the same stsms. during the joint research project, special care was given to the development of a reliable thermo-mechanical model for monolithic and laminated structural glazing at elevated temperatures. further studies would require the calibration of several input features, especially those being the key influencing parameters of both thermal and mechanical aspects of relevance on the overall structural performance of these systems. as a first step, one-dimensional (1d) models were used to study the thermal behaviour through the cross-section of glass plies. the typical heat transfer fe analysis was validated against previous experimental investigations, where glass specimens with several geometrical and mechanical features have been exposed to radiant heating, resulting in some cases in breakage of the panels. although the adopted fe modelling approach includes several simplifications first of all the lumped thermal performance of the glass plates object of analysis interesting conclusions can be drawn and used in the future developments of more refined thermo-mechanical models. one major challenge in the fe assessment of the structural performance of glass systems under thermal loading is that the available information on the temperature dependence of various thermo-physical and mechanical material properties in the literature is scarce. therefore, experimental testing is generally highly valuable. however, testing glass samples and assemblies at high temperatures is commonly challenging, time-consuming and costly. furthermore, the measurements themselves can include difficulties and uncertainties (see for example bedon et al. (2018a)). it requires careful planning to decide how the temperatures and heat fluxes are measured to obtain the relevant information for the validation of the numerical models. in terms of numerical modelling, 1d assemblies are typically associated with a well-known computational efficiency, but also to marked simplifications in their input features and expected results. in this regard, compared to more detailed two-dimensional (2d) or three-dimensional (3d) numerical models, 1d systems are not able to account for several key aspects for thermo-mechanical simulations, such as: – thermal boundary effects (i.e. thermal exposure of the samples faces to the assigned thermal load) – mechanical boundary effects, namely represented by the temperature distribution and evolution in time, in the contact regions between the glass specimens and the supports (and/ or the test setup components, etc., typically consisting of different materials with specific thermo-mechanical features) – size effects, being the 1d (and 2d) models intended to predict the temperature distribution in a given ideal section of the specimen under investigation, i.e. disregarding edge effects and other local phenomena the sensitivity of similar 1d thermal models is then further emphasised insofar as the mechanical performance of the same systems is analysed under the effect of elevated temperatures. at the current stage, for example, the actual role and fe modelling assumptions for common interlayer foils used in laminated glass assemblies from both a thermal and mechanical point of view still requires further studies. these aspects are currently under investigation, as an extension of the ongoing research study. 063 journal of facade design & engineering volume 6 / number 3 / 2018 acknowledgements the eu-cost action tu1403 “adaptive façades network” (2014-2018, http://www.tu1403.eu) is gratefully acknowledged for providing excellent research cooperation between the involved authors. part of the research study discussed in this research paper has been financially supported by the eu-cost action tu1403 in the form of short-term scientific mission grants for the involved authors (dr. kozłowski visiting university of trieste, italy, and dr. bedon visiting rise, sweden). in addition, cost is acknowledged for facilitating fruitful networking between the authors and international experts. references aguilar, j.o., xamán, j., olazo-gómez, y., hernández-lópez, i., becerra, g., & jaramillo, o.a. (2017). thermal performance of a room with a double glazing window using glazing available in mexican market. applied thermal engineering, 119: pp.505-515 aldawoud, a. (2017). assessing the energy performance of modern glass façade systems. matec web of conferences, 120, paper id: 08001, doi: 10.1051/matecconf/201712008001 baumgärtner, l., krasovsky, r.a., stopper, j., & von grabe, j. (2017). evaluation of a solar thermal glass façade with adjustable transparency in cold and hot climates. energy procedia, 122: pp.211-216 bedon, c. (2017). structural glass systems under fire: overview of design issues, experimental research, and developments. advances in civil engineering, volume 2017, article id 2120570, 18 pages. retrieved from https://doi.org/10.1155/2017/2120570 bedon, c., & louter, c. (2018). thermo-mechanical numerical modelling of structural glass under fire preliminary considerations and comparisons. proceedings of challenging glass conference, vol. 6, pp. 513-524, https://doi.org/10.7480/cgc.6.2173 bedon, c., honfi, d., & kozłowski, m. (2018a). numerical modelling of structural glass elements under thermal exposure. the 3rd international electronic conference on materials sciences. doi10.3390/ecms2018-05241 bedon, c., pascual agullo, c., luna-navarro, a., overend, m., & favoino, f. (2018b). thermo-mechanical investigation of novel gfrp-glass sandwich façade components. proceedings of challenging glass conference, vol. 6, pp. 501-512, https://doi. org/10.7480/cgc.6.2172 bedon, c., zhang, x., santos, f., honfi, d., kozlowski, m., arrigoni, m., figuli, m., & lange, d. (2018c). performance of structural glass façades under extreme loads – design methods, existing research, current issues and trends. construction and building materials, 163: pp.921-937 cardenas, b., leon, n., pye, j., & garcia, h.d. (2016). design and modeling of high temperature solar thermal energy storage unit based on molten soda lime silica glass. solar energy, 126. pp.32–43 cuzzillo, b.r, & pagni, p.j. (1998). thermal breakage of double-pane glazing by fire. journal of fire protection engineering, 9(1): 1-11 debuyser, m., sjöström, j., lange, d., honfi, d., sonck, d., & belis, j. (2017). behaviour of monolithic and laminated glass exposed to radiant heating. construction and building materials, 130. pp.212–229 debuyser, m. (2015). exploratory investigation of the behaviour of structural glass in fire. (master’s dissertation) ghent university, belgium. feldmann, m., kasper, r., abeln, b., cruz, p., belis, j., beyer, j., colvin, j., & et al. (2014). guidance for european structural design of glass components – support to the implementation, harmonization and further development of the eurocodes. in p. dimova & d. feldmann (eds) report eur 26439, joint research centre-institute for the protection and security of the citizen. doi: 10.2788/5523. fang, y., eames, p.c., & norton, b. (2007). effect of glass thickness on the thermal performance of evacuated glazing. solar energy, 81(3). pp.395-404 favoino, f., jin, q., & overend, m. (2014). towards an ideal adaptive glazed façade for office buildings. energy procedia, 62. pp.289298 ghosh, a., norton, b., & duffy, a. (2016). measured thermal & daylight performance of an evacuated glazing using an outdoor test cell. applied energy, 177 pp.196-203 ghoshal, s., & neogi, s. (2014). advanced glazing system energy efficiency approach for buildings a review. energy procedia, 54. pp.352-358 li, d., li, z., zheng, y., liu, c., & lu, l. (2015). optical performance of single and double glazing units in the wavelength 337-900 nm. solar energy, 122. pp.1091-1099 haldimann, m., luible, a., & overend, m. (2008). structural use of glass. iabse, isbn 978-3-85748-119-2 hasselaar, b., & looman, r. (2007). the climate adaptive skin, the integral solution to the conflict between comfort and energy performance. proceedings of cib world building congress 2017, pp. 1115-1125 miller, c., thomas, d., kämpf, j., & schlueter, a. (2015). long wave radiation exchange for urban scale modeling within a co-simulation environment. proceedings of cisbat 2015, september 9-11, 2015, lausanne, switzerland, pp.871-876 parra, j., guardo, a., egusquiza, e., & alavedra, p. (2015). thermal performance of ventilated double skin façades with venetian blinds. energies, 8: 1882-4898, doi: 10.3390/en8064882 pren thstr:2004. (2004). glass in buildings thermal stress capitulation method, cen, brussels, belgium simulia. (2018). abaqus v. 6.14 computer software and online documentation, dassault systems, providence, ri, usa stefanizzi, p., wilson, a., & pinney, a. (1990). internal long-wave radiation exchange in buildings: comparison of calculation methods: review of algorithms. building services engineering research and technology, 11(3): pp.81-85 tong, t.w. (1994). thermal conductivity 22. lancaster, pa, usa: technomic publishing company, ltd.,, isbn 1-56676-172-7 tong, s.t., zhu, l.b., guo, w., & ma, f.d. (2002). numerical simulation on thermal radiation of low emissivity glass surface. journal of building materials, 5: pp.60-65 064 journal of facade design & engineering volume 6 / number 3 / 2018 tofilo, p., & delichatsios, m. (2010). thermally induced stresses in glazing systems. journal of fire protection engineering, 20(2): pp.101-116 wang, t.-p., & wang, l.-b. (2016). the effects of transparent long-wave radiation through glass on time lag and decrement factor of hollow double glazing. energy and buildings, 117. pp.33-43 wang, t.-p., wang, l.-b., & li, b.-q. (2013). a model of the long-wave radiation heat transfer through a glazing. energy and buildings, 59 pp.50-61 copyright notice authors hold their copyright without restrictions. from city’s station to station city 029 journal of facade design & engineering volume 6 / number 2 / 2018 development and prototyping of an integrated 3d-printed façade for thermal regulation in complex geometries maria-valentini sarakinioti1, thaleia konstantinou1, michela turrin1, martin tenpierik1, roel loonen2, marie l. de klijn-chevalerias2, ulrich knaack1 1 faculty of architecture and the built environment, tu delft, the netherlands 2 department of the built environment, tu eindhoven, the netherlands abstract currently, several research projects investigate additive manufacturing (am) technology as a possible construction method for future buildings. am methods have some advantages over other production processes, such as great freedom of form, shape complexity, scale, and material use. these characteristics are relevant for façade applications, which demand the integration of several functions. given the established capacity of am to generate complex geometries, most existing research focuses on mechanical material properties and mainly in relation to the load-bearing capacity and the construction system. the integration of additional aspects is often achieved with post processing and the use of multiple materials. research is needed to investigate properties for insulation, thermal storage, and energy harvesting, combined in one component and one production technology. to this end, the research project “spong3d” aimed at developing a 3d-printed façade panel that integrates insulating properties with heat storage in a complex, mono-material geometry. this paper gives an overview of the panel development process, including aspects of material selection, printing process, structural properties, energy performance, and thermal heat storage. the development process was guided by experiments and simulations and resulted in the design and manufacturing of a full-scale façade element prototype using fdm printing with petg. the project proved the possibility of the integration of functions in 3d-printed façades, but also highlighted the limitations and the need for further developments. keywords additive manufacturing, 3d-printing, petg, heat storage, thermal insulation, façade module doi 10.7480/jfde.2018.2.2081 https://www.tue.nl/en/university/departments/built-environment/the-department-of-the-built-environment/staff/ 030 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction additive manufacturing methods provide great freedom of form compared to traditional methods (strauss & knaack, 2016). nowadays, designers and engineers can freely create complex designs in shapes that traditional production processes could not provide. furthermore, additive processes allow access to the inner part of the product, thus enabling integration of multiple design domains to realise multi-functionalities (yang & zhao, 2015), with no additional cost for the increased complexity (gao et al., 2015). in additive manufacturing, regardless of their degree of complexity, objects are fabricated following the same procedure. this capability provides the design with very large geometric design freedom (quan et al., 2015). complexity in form is observed in the façade, which is one of the most challenging parts of a building. this complexity can be attributed to the multifunctional nature of the component that controls the indoor environment of a building (strauss & knaack, 2016). moreover, the growing demand for low energy consumption and comfort have given the façade an important role in the overall building concept. it must not only be extremely well insulated but also adaptive in order to positively modulate the interior climate. this ultimately has a positive effect on the use of energy. the façade becomes an integral part of the climate concept and building services components can be integrated into it (klein, 2013). an increasing interest in the application of advanced building envelope solutions can be seen both in research activities and in industrial developments (favoino, goia, perino, & serra, 2016; loonen, trčka, cóstola, & hensen, 2013). the potential of 3d printing technology to generate complex geometries for integrating multiple materials and functions should be investigated in this respect. the potentials of 3d printing technology for façade construction has been investigated by a few studies, such as the one from peters (2016), paoletti (2017) and de witte et al. (2017). however, there is still research to be done to define the performance limits, especially for insulation and building physics to be combined in one component and one production technology. the focus of most research has been on the mechanical material properties, and mainly in the relation to the load-bearing capacity (labonnote, rønnquist, manum, & rüther, 2016). it is necessary to explore and determine the boundaries of those functions that can be integrated within a façade component. in this context, the present research focuses on the potential and limits of integrating multiple functions in one façade component with 3d printing production technology. the main objective of this research is to demonstrate that with 3d printing technology it is possible to create monomaterial façade components that integrate multiple functions. this hypothesis is tested by creating a façade panel that regulates the temperature inside the building with the use of its thermal insulation and heat storage properties. this paper presents the design and evaluation of the façade panel. four research phases are presented. in the first one, samples were designed and 3d printed based on symmetrical cellular structures. in the second phase, samples were designed and 3d printed based on elongated and asymmetrical structures; and a broad range of heat flux and temperature measurements were conducted. in the third phase, channels for water circulation were designed, 3d-printed and tested for water tightness. finally, the most promising design principles were scaled up into larger prototypes. 031 journal of facade design & engineering volume 6 / number 2 / 2018 2 methodology the proposed façade panel has two main functions: thermal insulation and adaptable heat storage. the heat storage consists of the two external layers that are 3d printed: the water based liquid and the water tank. according to the different seasons and time of the day, the water is placed on the inside or outside of the façade to absorb or release heat, as shown in fig. 1. to circulate the water, two reversible pumps are used, connecting the two external layers with the water tank. the façade thus acts as a cooling device in summer and as a heating device in winter. along with the heat storage function, the panel needs to provide thermal insulation as part of the external envelope. a research through design methodology was used to test if the above façade concept can be manufactured as a 3d-printed mono-material panel. research through design is a methodology in which design alternatives, samples and prototypes are being developed in an iterative process, in which evaluations (e.g. simulations and measurements) lead to input for the development of the next generation of design alternatives, samples, and prototypes. the main tools used for these evaluations were theoretical models and literature, physical experiments, heat transfer simulations and structural optimisation. multiple samples were produced for the different parts of the façade with variation in different parameters such as: layer height, infill percentage, speed, extrusion width, and temperature. fig. 1 façade panel principle: water circulation in a cooling season (a) in a heating season (b) the designs of the first samples were created using software called rhinoceros 3d and grasshopper, in order to produce the geometry of the thermal insulation and the heat storage system. the 3d printing process that was selected was fused deposition modelling. fdm technology and polymer filament is easily accessible to anyone who wants to 3d print. in addition, thermoplastics are relatively lightweight, have low thermal conductivity, and some of them are recyclable. transparent petg was chosen due to its higher solar transmittance for the external layer. moreover, petg has higher stiffness and strength than pla for instance, and it is 100% recyclable. 3 phase 1: polyhedra configuration in the first phase, the preliminary research of polyhedra structures and their potentials in thermal insulation were considered. the part that was designed and tested first was the external layer that will integrate water for heat storage. the main parameters used to evaluate the specific configuration were the ability to 3d print this configuration, the 3d printing time, the water tightness, the ability to design and print larger components in short timeframes, and the structural performance. the material that was used was pla. 032 journal of facade design & engineering volume 6 / number 2 / 2018 a b c d fig. 2 printing samples of the polyhedral configuration, showing problems of the printed surfaces. closed cellular structures are known to have relatively good thermal performance (ashby, 2006). the cells are designed to have a low surface area, like a foam structure, which creates potential for cellular structures that have a relatively low ratio of solid to gas. previous investigation into the 10mm cells showed that small panels made with these cells have relatively low effective thermal conductivity, 0.044 w/mk (sarakinioti, 2016). the limiting factor in reducing the effective thermal conductivity is the minimal wall thickness needed from the manufacturing technique. the process that followed was to 3d print the first samples of this configuration using fdm production technology. the objective was to test the properties of the 3d printed surface in terms of water tightness and surface quality. the samples were 3d printed with a nozzle of 0.4 mm. the length of time taken for printing was an issue and a solution needed to be investigated to produce the larger components. furthermore, the specific configuration of polyhedral structures resulted in some surfaces being horizontal and collapsing during the printing process, as shown in fig. 2. 4 phase 2: elongated cells configuration based on the conclusions drawn during phase 1, the next phase of the research was set, aiming to improve the use of material, printing speed, and total production time. in particular, the approach based on two layers was revised, leading to re-thinking and re-designing the façade panel with different configurations. 4.1 cells design considerations the results received from the investigation of the phase 1 helped to design further configurations that require less material, shorter printing time, and have good thermal performance. considering the two different functions of the façade component (heat storage and thermal insulation), the two parts of the façade were investigated separately. the inner core needed air cavities to act as insulation, whereas the external layer needed channels that integrate water for heat storage and circulation. the air cavities were stretched in all directions except the direction of the heat transfer which was kept between 10-20mm, while the water channels were re-designed based on structures driven by fluid dynamics, using streams and channels that would circulate the water faster and with lower pressure drop from the bottom of the layer to the top (fig. 3). 033 journal of facade design & engineering volume 6 / number 2 / 2018 a b fig. 3 cross section of the panels, showing the elongated cells in the insulation core and the channels in the external water layer. in addition, the cells were initially designed with surfaces that are connected to enclose air inside them. the sides of the cells were regular and created sharp edges at the points at which they are connected. in the final design, the configurations have curved edges and sides. as a result, the extruder follows a continuous path while extruding material, the travelling time of the extruder is reduced, the printing speed could be increased as vibrations were lowered, and, therefore, the total printing time was reduced. 4.2 thermal conductivity properties first, the possible options for the insulation layer were designed, 3d printed, and tested for their thermal conductivity. for the thermal tests, a 1 m3 box made of polystyrene was built and a hole of 20cm x 20cm in which to place the samples was created in one side. a lamp was placed inside the box to heat up the environment. the tests of the heat flux that is transferred from one side of the sample to the other took place during the summer months. in one typical summer day the mean temperature of the office was 28.7°c. on that day the temperature inside the box was 54°c, while outside was 28.7°c. this means that the difference was 25.3°c. a similar difference in temperature was also measured in the temperatures of the surfaces. the thermal resistance of the sample was determined by measuring the temperature inside the box, on the surface of both sides of the sample, and of the external environment using thermocouples, and by measuring the heat flux through the sample with a heat flux plate (hukseflux hfp01) on each side of the sample. only the long-term average results were used once a steady-state situation had been reached. the results for the different samples, as shown in table 1, showed a relatively high thermal conductivity. the reason for this is the amount of material needed due to the production process; the porosity of the samples is rather low. the values of the thermal conductivity of the samples were similar and therefore the configuration with the shortest printing time was selected. the selected configuration was flexible and not stiff for a façade panel. therefore, the configuration was deformed by keeping the main principle of the smooth curves and inserting connection points in some parts of the surfaces to increase the structural stiffness. 034 journal of facade design & engineering volume 6 / number 2 / 2018 design 1 2 3 4 nozzle size mm 0.4 0.4 0.4 0.4 wall thickness 1.2mm 0.8 mm 1.2mm 1.2mm material petg colour petg transparent petg transparent petg transparent mass kg 0.533 0.286 0.417 0.693 volume ratio solid /gas 0.2 0.08 0.23 0.23 thermal conductivity λ 0.101w/mk 0.094w/mk 0.104w/mk 0.109w/mk table 1 different configurations that were tested for thermal conductivity 5 phase 3: external layer configuration and first large-scale printing in the third phase, the focus was on the external layer but also on the design of the large components. the design with the most promising performance, highest printing speed, potentials for uniform water flow, and lowest pressure drop was selected and further 3d printed. several tests for water tightness were undertaken. moreover, for the larger component, the two layers were designed in one object and the final design was 3d printed for the first time. 5.1 external layer the configuration of the external layer is inspired by natural configurations that transfer fluids such as blood vessels, veins in leaves, and 3d bionic structures. the external layer, where the liquid flows, requires water-tightness and the channels in this layer require a hydrodynamic shape to allow for minimal pressure drop and uniform flow. based on this concept, channels of different diameter were incorporated in the external layer (fig. 4). fig. 4 printed segment of the external layer 035 journal of facade design & engineering volume 6 / number 2 / 2018 a b fig. 5 water circulation test in the external layer several samples with different configurations were tested for flow resistance and water tightness, using a water pump and a hose that connects the input of the sample with the output of the pump (fig. 5). preliminary testing of the samples showed that the surface had micro-holes between the layers, caused by discontinuities and high speeds in the printing process. an epoxy coating (epoxacast 690) was applied both on the inside of the channels and on the external surface of the component to ensure water tightness. 5.2 thermal performance in parallel to the design and manufacturing process, the potential for energy saving was investigated for different climates. the potential in the heating season was assessed by analysing correlations between solar irradiance and ambient temperature in tmy weather files for 14 cities. similarly, the potential in the cooling season was investigated by comparing the difference between daytime air temperature and night-time sky temperature. from this study, it was found that spong3d has more potential for cooling than for heating, because there are several warm climates in which the nighttime sky temperature drops significantly below the thermal comfort zone, and can thus be used as a heat sink for nocturnal cooling. the added value of spong3d is most pronounced in composite climates with at least a moderate need for both heating and cooling. madrid, los angeles, and cape town are examples of such cities. the climate analysis showed that, in these cities, there is potential for natural heating or cooling with spong3d on 75% to 80% of the days in a year. in all three climates, spong3d has potential throughout almost the entire cooling season and for around half of the heating season (sunny days). this result indicates that spong3d can have a significant impact on reducing heating and cooling energy demand, provided that its operation is tuned to the resources that are available in the ambient climate. a dynamic simulation study was carried out to further quantify the performance potential of spong3d. to this end, a trnsys model of a reference office zone was developed with hydronic systems in the exterior (solar collector) as well as interior (radiant system) layers of the façade. these heat-exchanging surfaces were coupled with a storage tank with controlled fluid flow to represent the daily and seasonal behaviour of spong3d. the spong3d system was only modelled on the south facing façade – it was assumed that the office space was part of a bigger building, and that other 036 journal of facade design & engineering volume 6 / number 2 / 2018 0 10 20 30 40 50 60 70 low high low high low high low high _ low high low high low high low high amsterdam madrid los angeles cape town _ amsterdam madrid los angeles cape town a nn ua l e ne rg y de m an d [k w h/ m 2] low lowhigh high low thermal insulation rc = 3 m2k/w high thermal insulation rc = 7 m2k/w additional demand without spong3d heating demand with spong3d additional demand without spong3d cooling demand with spong3d fig. 6 predicted energy demand for a reference office space in four different cities. walls, floor, and ceiling are adjacent to rooms with similar thermal conditions. among the many design variables that were studied, it turned out that tank volume, solar absorptance, and fluid flow rates have the greatest influence on the resulting performance of spong3d. the case study results (fig. 6) show that application of spong3d can lead to significant reductions in energy demand, particularly for cooling. percentage-wise, the energy-saving effect is similar for both a moderate and a highly insulated building envelope system. this is an encouraging finding for the viability of the spong3d concept, as it shows that the thickness of the insulating middle layer can be kept at modest values and still lead to satisfactory performance. 6 phase 4: large scale prototypes the project aimed not only to design the panel but also to produce a 1:1 scale prototype for a proof of concept. two different prototypes were designed for the final production. the first one was a doublecurved panel. the second one was a straight panel. both prototypes faced several challenges, of which two are the most relevant. 6.1 large scale 3d printing the first challenge regarded the identification of a proper number of layers for the large-scale prototype, based on achieving a balance between printing time and stiffness of the printed surfaces during printing. the second challenge regarded the stabilisation of the prototype during printing, to avoid warping during the cooling process of the extruded material, and to avoid any shifting of the prototype on the print bed. firstly, a double-curved panel with water channel sizes of 5mm-15mm was prototyped. the printing process stopped at a lower height than expected. the final size of the produced panel was (55x30x30cm). the process of prototyping the double-curved panel allowed for the drastic improvement of the production of the straight panel with regard to both of these main challenges. regarding the number of layers, for the double-curved panel, the external channels were designed to be printed with one layer only, to minimise the use of material and time. because the channels were not stiff and not thick enough, the extruder could put pressure on the channels and therefore could easily cause a displacement of the channels and deformation of those parts. regarding the 037 journal of facade design & engineering volume 6 / number 2 / 2018 a b fig. 7 the double-curved prototype, showing the displacement of the printed layers; cross section (a) and frond view (b). stabilisation of the prototype during printing, every surface adhered to the print bed with a narrow base (a large thin printed surface). the surface that connected the model to the print bed was minimised and each surface would act independently during the printing process causing vibrations of the model and higher possibilities to deform (figs. 7a and 7b). based on this experience, decisions were taken to print the straight prototype with multiple layers for the external channels. moreover, the decision was taken to sufficiently enlarge the surface that connected the model to the print bed. 6.2 final prototype the final product is a large-scale prototype, which basically constitutes the proof of concept. the design was adjusted to address the problems identified in printing during the previous phase. the diameter of the channels was increased from 5mm to 20mm for the small channels and from 15mm to 40mm for the big channels. moreover, a second wall in the channels was inserted to make them stiffer. a single base for all the parts of the component was designed. these settings and some irregular parts of the design caused the creation of small surface cavities in the transitional areas between the large and smaller channels. the large panel had a rectangular shape. the internal layer, which provides the thermal insulation properties, was designed according to the elongated cells principle (fig. 8a) developed and measured in phase 2. the thermal conductivity λ was measured to be 0.1 w/(m·k), which means that the panel with a thickness of 33cm insulation, has a thermal transmittance coefficient u value of 0.30w/(m2·k). the overall prototype dimensions are 1500x500x360mm. due to limitations of the printed size, the panel was printed as two components. the two models interlocked vertically and consisted of a free-standing component (fig. 8b). the purpose of this model was to demonstrate the feasibility of printing such a panel, and it is not a fully functioning system. the water circulation system was not incorporated at this stage. nevertheless, the possibility of water circulation in the 3d-printed channels was tested in the previous development phase, with a working prototype. the printing process of the large component was successful except for some small cavities in the surface. the total printing time for the panel was 512 hours. 038 journal of facade design & engineering volume 6 / number 2 / 2018 7 discussion the manufacturing of the multifunctional façade panel was proven to be feasible, as the production of a 1:1 scale prototype indicates. such a development, although still experimental, provides a positive outlook of the possibilities of additive manufacturing to integrate functions and improve the energy efficiency of buildings. the steps of the development process provided insights into the issues that may be encountered during the manufacturing of larger building components and how they can be addressed, as well as the opportunities offered by complex geometries, not only for appearance but also for performance of the building. however, it needs to be clarified that considerable further developments are required to lead it towards a marketable façade system, which are, on the one hand, related to the manufacturing process, the function, and the performance of the system and, on the other hand, to the system implementation. regarding the manufacturing process, which is the main innovation of the study, a number of issues need to be addressed. firstly, the water circulation system should be integrated and tested in the panel and the design of the channels should be improved for better water circulation and heat storage. most importantly, the 3d printing time is long for complex geometries and multiple integration of performances, despite trying to reduce the time through proper design. however, there is a continuous development of larger scale printers/chambers and techniques that can shorten the 3d printing time. finally, further investigation needs to be done in relation to the feasibility of the production of large scale components in terms of time and costs. a b fig. 8 the final full-size prototype: side view of the bottom part (a) and the full, freestanding prototype (b) 039 journal of facade design & engineering volume 6 / number 2 / 2018 furthermore, an in-depth analysis and testing on the structural behaviour of the 3d printed material are required, especially when considering extreme thermal conditions and durability, as well as long term testing on creep. the testing is also related to the investigation in 3d printed polymers and the selection of the most efficient polymer. 3d printing technology provides the designer with an opportunity to create elements that are tailored to the overall shape of the façade and the placement on the building, which allows the design of the façade to adapt according to customised requirements. further investigation needs to be done to find the correct direction for implementing this customisation potential. moreover, the component should be investigated in terms of its potential to be part of a façade that can integrate areas with transparency, translucency, and opacity. to further develop the proof of concept into a façade product, investigation needs to be done on the potentials for compatibility with existing façade systems and components, such as gaskets sealants, infills, connectors etc., in addition to the 3d printed product’s assembly and connection to the main structure and other building components. finally, compliance with building regulations, such as structural load, building physics, daylight, fire safety etc. needs to be proven and ensured, before the façade panel can be considered as a product. 8 conclusion this paper discusses the development process of a mono-material, multifunctional, adaptive façade panel, which controls the heat exchange between the indoor and the outdoor environment, while testing the potential of additive manufacturing for its production. the panel concept is based on an inner core with highest possible thermal resistance and two outer layers where water accumulates heat from solar gain or indoor sources, circulates, and releases it on the other side. the intention was to create a component that is relatively lightweight and is recyclable. the plastic filament is a material that can be easily accessible to anyone who wants to 3d print. the research work was structured in four phases. during the first phase, samples were designed and 3d printed based on symmetrical cellular structures; structural performances were measured on specimens and simulated. in the second phase, samples were designed and 3d printed based on elongated and asymmetrical structures; a broad range of thermal measurements for heat-flux were conducted under different thermal conditions. in the third phase, channels and tanks for water circulation were designed, 3d printed, and tested for water tightness. finally, in the fourth phase, the most promising design principles were further implemented in the design and manufacturing of a one-to-one scale prototype. the construction of the prototype constitutes the proof of the concept, as it demonstrated that it is possible to print a façade panel with complex geometry, which incorporates a heat storage system and thermal insulation properties. moreover, the thermal simulations showed that, even though the system is not capable of replacing conventional heating and cooling systems entirely, it still shows promising predictions in reducing the heating and cooling demand of buildings and thereby reducing the costs for heating and cooling energy as well as associated greenhouse gas emissions, especially when implemented in a location with a composite climate requiring a moderate need for both heating and cooling. 040 journal of facade design & engineering volume 6 / number 2 / 2018 the described façade panel is an experimental approach to prove that integrating functions in additively manufactured building components is possible. such approaches have the potential to provide the needed energy efficiency, while accommodating design requirements for complex geometries. nevertheless, several developments, investigations, and tests should be performed in terms of a marketable façade system, including structural, construction, and safety requirements, along with expanding design and performance possibilities. as additive manufacturing technologies rapidly advance, the design and production of integrated products will be, on one hand, facilitated and, on the other hand, will set the requirements for those technologies. acknowledgements this research proposal was funded by 4tu federation and co funded by tu delft and tu eindhoven for the development of project spong3d (2016-2017). this project is the result of the collaboration of researchers from tu delft and tu eindhoven. alongside the authors, jan hensen and student remco van woensel from the department of the built environment and section of building physics and services at tu eindhoven were arno pronk, patrick teuffel, and students eline dolkemade, arthur van lier, rens vorstermans from the section of structural design. from tu delft came researchers paul de ruiter, milou teeling, and mark van erk from the chair of design informatics. the production of the large-scale prototypes and the samples of the thermal tests were developed in kiwi solutions by dick vlasblom. reference ashby, m. f. (2006). the properties of foams and lattices. philosophical transactions of the royal society a: mathematical, physical and engineering sciences, 364(1838), 15-30. doi:10.1098/rsta.2005.1678 de witte, d., de klijn-chevalerias, m. l., loonen, r. c. g. m., hensen, j. l. m., knaack, u., & zimmermann, g. (2017). convective concrete: additive manufacturing to facilitate activation of thermal mass. journal of façade design and engineering(1), 107-117%v 105. doi:10.7480/jfde.2017.1.1430 favoino, f., goia, f., perino, m., & serra, v. (2016). experimental analysis of the energy performance of an active, responsive and solar (actress) façade module. solar energy, 133, 226-248. doi:http://dx.doi.org/10.1016/j.solener.2016.03.044 gao, w., zhang, y., ramanujan, d., ramani, k., chen, y., williams, c. b., . . . zavattieri, p. d. (2015). the status, challenges, and future of additive manufacturing in engineering. computer-aided design, 69, 65-89. doi:http://dx.doi.org/10.1016/j.cad.2015.04.001 klein, t. (2013). integral façade construction: towards a new product architecture for curtain walls: tu delft. labonnote, n., rønnquist, a., manum, b., & rüther, p. (2016). additive construction: state-of-the-art, challenges and opportunities. automation in construction, 72, part 3, 347-366. doi:http://dx.doi.org/10.1016/j.autcon.2016.08.026 loonen, r. c. g. m., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483-493. doi:https://doi.org/10.1016/j.rser.2013.04.016 paoletti, i. (2017). mass customization with additive manufacturing: new perspectives for multi performative building components in architecture. peters, b. (2016). solar bytes pavilion. in d. reinhardt, r. saunders, & j. burry (eds.), robotic fabrication in architecture, art and design 2016 (pp. 326-337). cham: springer international publishing. quan, z., wu, a., keefe, m., qin, x., yu, j., suhr, j., . . . chou, t.-w. (2015). additive manufacturing of multi-directional preforms for composites: opportunities and challenges. materials today, 18(9), 503-512. doi:http://dx.doi.org/10.1016/j.mattod.2015.05.001 sarakinioti, m. v. (2016). the spongy skin: the potentials of am methods in cellular structures. delft university of technology, delft. retrieved from uuid:8e9de23d-4c31-4eff-bcec-131a80df08ee strauss, h., & knaack, u. (2016). additive manufacturing for future façades. journal of façade design and engineering, 3(3-4), 11. doi:10.7480/jfde.2015.3-4.875 yang, s., & zhao, y. f. (2015). additive manufacturing-enabled design theory and methodology: a critical review. the international journal of advanced manufacturing technology, 80(1), 327-342. doi:10.1007/s00170-015-6994-5 from city’s station to station city 095 journal of facade design & engineering volume 6 / number 2 / 2018 hybrid numerical and experimental performance assessment of structural thermal bridge retrofits garay martinez, roberto1* tecnalia, sustainable construction division abstract a methodological approach to the multi-dimensional heat transfer assessment of building envelopes is performed. the proposed method focuses on thermally weak points in envelope-structure junctions and the assessment of envelope retrofit alternatives. thermal performance in these spots is seldom assessed in energy audit processes, although it is one of the main heat loss paths in many insulated façade solutions. an envelope-slab junction case is presented, where multi-dimensional heat transfer occurs. this paper proposes a methodology that allows for a hybrid experimental and numerical performance assessment in such circumstances. a numerical model is calibrated against experimental data, which is then modified to reflect various envelope retrofit solutions. several possible analysis procedures are proposed, based on the capacities of transient thermal models. key words building envelope; experimental assessment; thermal bridge; finite element; envelope retrofitting doi 10.7480/jfde.2018.2.2206 096 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction 1.1 heat transfer in building envelopes & envelope retrofitting according to the sustainable building and climate initiative of the un (unep, 2016), and other sources (doe, 2008; pérez-lombard, ortiz, & pout, 2008; eu, 2002; eu, 2010), buildings are responsible for 40% of the global primary energy consumption. within buildings, envelopes – roofs, façades, and glazed areas represent the main heat transfer path from a building to its environment. ecofys (2007) studied the techno-economical optimum insulation level for building envelopes. in this study, optimal thermal transmittance levels were found to be substantially below the current insulation levels in building envelopes. thus, there is great potential for heat flux reduction by incorporating additional insulation to building envelopes. once the decision to renovate a building is taken, incorporating additional thermal insulation is a robust solution, as it increases the overall thermal resistance of the building envelope. commonly, this measure is the first energy efficiency measure taken in most buildings, and combined with other energy efficiency measures, provides for a medium to long-term return of investment (roi). there are various technical solutions such as external thermal insulation composite systems (etic), ventilated façades, cavity wall insulation, and internal insulation systems. the basic approach of all these systems is to improve the thermal transmittance of the wall, by means of the addition of insulation materials. in this context, the thickness of the insulation material and its insulation capacity are key variables (elguezabal & garay, 2015). 1.2 thermal bridges. relevance & calculation procedures commonly, retrofitting design decisions are made based on one-dimensional performance of insulation systems. multi-dimensional heat transfer paths such as window sills, slab-façade junctions, balconies, etc. are disregarded. these items account for a relevant share of the heat loss coefficient of a building envelope. several sources, such as asiepi (2010), show that the relevance of thermal bridges within the heat balance of a building is up to 30% of heating energy loads due to these elements. this ratio is considered to increase for highly insulated buildings. the correct design and improvement of junction details is estimated to reduce the same ratio to 15%. for this reason, adequate thermal bridge calculation methods for highly insulated buildings are needed (kuusk, kurnitski & kalamees, 2017). one of the reasons to avoid multi-dimensional heat transfer in the assessment procedure of a building envelope retrofit lies in the complexities of numerical models and the lack of robust experimental procedures to conduct such assessments. regarding numerical models, multi-dimensional heat transfer codes such as therm (lbnl, 2018) are freely available to designers, but, to the authors knowledge, these are seldom applied in construction projects. when related to the on-site experimental assessment of the thermal performance of architectural junctions, standard methods such as en iso 9869-1:2014 cannot be applied and only qualitative assessments can be made by means of methods like infrared imaging. 097 journal of facade design & engineering volume 6 / number 2 / 2018 in garay, uriarte, and apraiz (2014), numerical and experimental works were conducted over a 2-dimensional thermal bridge. in this work, it was observed that steady-state numerical models did not correctly match the dynamics of the thermal bridge. this same source showed that for cases with unknown thermal properties, models failed to correctly identify the steady-state and transient aspects of thermal bridges. in recent dates, works including that by o’grady, lechowska, and harte (2017) have studied the possibility of integrating thermal imaging as a quantitative source of information for thermal bridge assessment. however, their applicability is yet to be further demonstrated. 1.3 experimental processes for building envelope assessment experimental heat transfer assessment procedures in building envelopes have traditionally been focused on one-dimensional heat transfer assessment. in fact, it is common to find instructions to avoid the influence of thermal bridges in experimental setups within standardised assessment procedures. experimental procedures for on-site assessment heat transfer in buildings are standardised under en iso 6946:2007 and astm c1155 – 95 (2013). although these standards integrate transient assessment methods, they are primarily focused on steady-state performance metrics. their most common implementation is performed by means of averaging processes, which filter out the dynamics of building envelopes. within the research community, there is an increasing awareness of the need for transient assessment methods, which has led to specific transient methods (gutschker, 2008, strachan & vandaele, 2008, naveros, bacher, ruiz, jiménez, & madsen, 2014). in any case, all of this experience remains in the one-dimensional domain. atsonios, mandilaras, kontogeorgos, and founti (2017) apply en iso 9869-1:2014 and astm c1155 – 95 (2013) procedures over datasets from field experiments on building envelopes with various levels of insulation, at different times of the year. for each case, the required campaign length is identified. in some cases, it is impossible to obtain satisfactory results from steady-state methods, while in others, campaign lengths up to 20 days are required. transient methods perform substantially better, delivering robust results in 5 to 10 days. the methodology presented in this paper does not intend to be a substitute for experimental methods to assess one-dimensional heat transfer. it will complement existing procedures with a novel system for the assessment of multi-dimensional heat transfer, which, to date, is outside the scope of experimental procedures. 1.4 goal and limitations of the proposed methodology in this paper, a hybrid numerical and experimental procedure is proposed to assess the present thermal performance of an architectural junction. the procedure allows building envelope retrofit systems to be assessed. ultimately, this allows for a more detailed assessment of the thermal performance of a retrofitting intervention. 098 journal of facade design & engineering volume 6 / number 2 / 2018 the methodology is illustrated by means of a 2-dimensional façade-slab junction. the presented calculation method is also suitable for 3-dimensional heat transfer. the 2-dimensional heat transfer case is presented as it gives a greater representation of the performance gap illustrated in this section. singular 3-dimensional thermal bridges are known to be less relevant in terms of heat transfer, but critical in terms of cold spots, and potential locations for mould growth. users trying to replicate this methodology for repetitive 3-dimensional thermal bridges such as cladding anchors may experience difficulties in doing so. for these cases, users are encouraged to deal with this phenomena by means of pseudo-2d models. specific adaptations of the present methodology may need to be developed. reference to multi-dimensional heat transfer in architectural junctions may be found in atsonios, mandilaras, kontogeorgos, and founti (2017). 2 thermal assessment methodology thermal bridges are construction details in which multi-dimensional heat transfer occurs. as such, heat flux in these locations cannot be measured directly by means of heat flow meters. the proposed assessment method bases its assessment of the heat flow across architectural junctions on several localised temperature and heat flow measurements. point measurements are used to calibrate a transient numerical thermal model. once calibrated, the model can be used to provide an accurate assessment of the heat transfer of the architectural junction under examination. the impact of envelope retrofit alternatives on the heat transfer across the junction can be calculated with the calibrated model. in table 1, the method is presented as a stepped approach. step activity 1. monitoring of present state define the location of sensors monitorisation campaign, ~1month 2. calibration of thermal model construction of a numerical model optimisation of thermal properties of materials 3. evaluation of retrofitting alternatives parametric study of retrofit possibilities vs performance indicators table 1 thermal assessment sequence. the goal of this methodology lies in the identification of thermal and geometrical properties of an already constructed architectural junction based on insufficient data. although geometrical details are commonly known in buildings constructed in the last 60 years, many thermal properties remain unknown and their determination is commonly performed by means of bibliographical research. this method allows for the determination of critical information in the assessment of thermal bridges such as the effective thermal conductivity of insulation layers and air cavities, specific heat and density of concrete and brick constructions, etc. this proposed hybrid methodology is complemented by state of the art one-dimensional heat transfer assessment techniques as shown in fig. 1. 099 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 1 integration of assessment methods in a joint experimental campaign for on-site works 3 step 1: monitoring in this step, the geometrical detail is defined, and several spots are selected for the installation of sensors. commonly, 3-4 sensors are sufficient to provide a detailed thermal map of the architectural junction. in the selection of the sensor location, sensors should be located in such a way as to allow the mapping of the architectural detail in all its relevant internal surfaces. the particular location of sensors will depend on each architectural junction, and the feasibility of integrating sensors in some of the locations. garay, uriarte and apraiz (2014) used steady-state thermal models to identify suitable locations for sensor placement. by doing so, better experimental conditions are achieved. alternative processes such as thermal imaging are also possible means of identifying suitable sensor locations. the goal is to achieve spatial and transient representation of the measurement scheme: – spatial representation is achieved by means of sensor placement across the architectural junction, some of them mostly exposed to the external ambience, others in contact with indoor environment, and some of them in between. – transient representation implies that sensors are positioned in such a way that they are exposed to different dynamics. sensors in contact with insulation materials will deliver faster responses than those in contact with concrete and other capacitive materials. the number of sensors to be placed needs to be defined based on the scope of the selected assessment. considering that standardised one-dimensional heat transfer assessment procedures (en iso 9869-1:2014) incorporate ambient and surface temperature sensors and at least one heat flow sensor, this amount should be increased to achieve good representation. good practice should incorporate the following sensors: – 1 ambient temperature sensor for each of the boundary conditions of the thermal bridge – 1 surface temperature sensor for the coldest spot on each of the boundary conditions 100 journal of facade design & engineering volume 6 / number 2 / 2018 – 1 heat flux on each of the internal boundary conditions – 1 additional surface temperature sensor for planar systems not instrumented for en iso 6946:2007 – external weather conditions need to be measured, and comprise outdoor ambient temperature, wind speed & direction, and solar radiation over the façade. for the application presented, pt100 temperature sensors and phymeas heat flux sensors are identified as suitable devices. in fig. 2, a monitorisation scheme is proposed for a slab-façade junction. in this figure, meteorological sensors are not shown. fig. 2 monitorisation scheme of a slab-façade junction (left), distribution of sensors in a multi-storey setup (right) to facilitate the experimental process, the experimental campaign should be coordinated with the installation of other sensors for the one-dimensional assessment of the thermal performance of walls. this would allow for the common utilisation of data loggers. in the same figure, the monitorisation spots are redistributed, to allow for the installation of the data acquisition system within one floor of a multi-storey building. the presented experimental setup would only be valid in a multi-storey building in which boundary effects caused by foundations and roof can be neglected (i.e. central floor in a 7-storey building). in fig. 3, the detailed location of sensors in an architectural junction can be seen. fig. 3 location of temperature and heat flux sensors in an experimental assessment of the heat transfer in a façade-slab junction (model height: 2.77m) (garay martinez, uriarte arrien & apraiz egaña, 2015) 101 journal of facade design & engineering volume 6 / number 2 / 2018 depending on the existing boundary conditions (i.e. indoor-outdoor temperature gradient), the insulation level of the construction etc., the length of the monitorisation campaign may divert. however, it is reasonable to assume that a proper result can be achieved with experimental campaign lengths in the range of 3 to 5 weeks. in atsonios, mandilaras, kontogeorgos, and founti (2017), one-dimensional heat transfer assessment was performed by means of identification processes over various wall assemblies. different climatic conditions, envelope compositions, and assessment methods resulted in variable campaign length requirements to deliver a result. when transient methods were applied, campaign lengths in the range of 10-20 days were required to achieve good identification of the system. in the proposed methodology, longer experimental campaigns are required to properly address heat dynamics in massive elements, such as concrete slabs. in any case, the prescribed campaign length is still inductive. 4 step 2: calibration a thermal model of the architectural detail is constructed based on the available information relating to the junction. commonly tabulated data from sources such as en iso 6946:2007 and ministerio de fomento (2013) are taken to complete project-specific data. it should be considered that, in most cases, retrofitting projects are performed over relatively old buildings, with non-professional owners (e.g. individual owners/dwellers, not involved in the construction process), with only minimal architectural data available. the definition of architectural dimensions needs to cover the influence area where multidimensional heat transfer occurs. general criteria established in en iso 10211:2007 suggest that 1m of one-dimensionally homogeneous wall/slab length shall be modelled. the readers should consider that secondary criteria such as symmetry planes and wall thickness may modify this length. fig. 4 shows a thermal model of a façade-slab architectural junction. fig. 4 thermal model of an architectural junction. (garay marinez, 2017) 102 journal of facade design & engineering volume 6 / number 2 / 2018 boundary condition data from the monitorisation campaign is introduced in this model, and a transient thermal simulation is performed over the monitored period. the boundary conditions incorporated into the model are ambient temperature (on all surfaces) and solar radiation (for external surfaces only). thermal properties of materials and modelling assumptions are varied to minimise the observed error in output variables when compared with monitored spots in the physical junction within the monitored campaign. in fig. 5, output data from a calibrated model in voltra (physibel, 2009) is compared to experimental data taken from a façade-slab junction constructed in the kubik experimental building (garay, chica, apraiz, campos, tellado, uriarte, & sanchez, 2015). error minimisation needs to be achieved simultaneously in all point measurements. the comparison of the calibrated model against experimental data for all sensors installed in the junction can be found in garay, uriarte and apraiz (2014) and garay martinez, uriarte arrien, and apraiz egaña (2015). fig. 5 calibrated output signal on a thermal model. (garay, uriarte, & apraiz, 2014) (garay martinez, uriarte arrien & apraiz egaña, 2015) in garay, uriarte, and apraiz (2014) and garay martinez, uriarte arrien, and apraiz egaña (2015), the model was parametrised to incorporate thermal capacity and conductivity of materials (concrete, steel, polyurethane, and xps). it was found that minor tuning was required to identify the parameters of concrete. concrete density and conductivity were varied in a range of 2000-2400kg/ m3, and 2-2.6 w/mk respectively. the model best fit to experimental data was achieved with 2300kg/m3 and 2.2w/mk. this same model was found to be more sensitive to internal convective heat transfer phenomena. separate heat transfer coefficients were required for horizontal, vertical upward, and vertical downward heat transfer. additional coefficients were required for corner areas. each of these resulted in surface heat transfer coefficients in the range of 2.5-4 w/m2k, substantially lower than reference values in en iso 6946:2007. full details on the calibration process can be found in garay, uriarte, and apraiz (2014). 103 journal of facade design & engineering volume 6 / number 2 / 2018 at the end of this process, the thermal model is classified as “calibrated”, and can be used for later assessment of retrofitting alternatives. in tecnalia (2013), a visual inspection was used to identify the model that best fit the experimental data, but this process can be improved by using error minimisation techniques simultaneously over all measurement spots. from experimental data, the model was able to predict surface temperature within +0.2 ºc, as can be seen in fig. 4. the calibration is performed based on punctual sensor locations, none of which are sufficiently reliable as to fully represent the thermal performance of the architectural junction. however, considering the accordance of the calibrated model with experimental data, it is reasonable to accept that the calibrated thermal model can be used to predict the thermal performance of the full architectural junction. 5 step3: evaluation of retrofit alternatives the calibrated model from the previous section can be used to predict the thermal performance of architectural junctions targeted at various performance figures. the model itself is a transient thermal model, which can be used to perform both transient and steady-state calculations of the architectural junction for various purposes such as the following: – calculate thermal bridge coefficients and temperature factors of various alternative designs, based on calculation criteria and boundary conditions in en iso 10211:2007, but with calibrated thermal parameters for the baseline junction – calculate the overall coupling coefficient of the building envelope under standard en iso 13790:2008 – calculate the transient thermal response of the architectural junction under harmonic boundary conditions similar to en iso 13786:2007 and that described in garay martinez, riverola, and chemisana (2017). – obtain transfer functions and response factors of the architectural junction by procedures, as described by martín, flores, escudero, apaolaza, and sala (2010). – obtain equivalent one-dimensional thermal models for its integration into energy simulation programs by means of system identification techniques, stochastic procedures, etc. as proposed by gacía gil (2008). – perform heat transfer analysis of the architectural junctions for the verification of energy savings in energy performance contracts by means of ipmvp (evo, 2012) or equivalent methods. overall, the proposed models allow for a detailed assessment of the architectural junction, with many relevant output parameters, which should be defined on a case-by-case basis, along with the particularities of each project from its many perspectives (architectural constraints, expected performance levels, engagement of contractors in the final performance, etc.). in the following paragraphs, a case study on the assessment process for building energy retrofits is presented. the model for the façade-slab section presented in fig. 3 is taken as baseline, and façade retrofit is performed by means of a ventilated façade system. this system is a closed joint ventilated façade cladding system (ulma architectural, 2018) based on vertical profiles and point anchors to the edge of concrete slabs (garay martinez, 2017). the presented study was performed by means of multi-dimensional modelling of the junction. the ventilated façade model is parametrised to incorporate insulation thickness as the main 104 journal of facade design & engineering volume 6 / number 2 / 2018 variable. anchor thickness is a dependent variable, as this parameter is required to be increased when the cladding is separated from the façade to meet mechanical criteria. the suitability of each alternative is assessed by means of surface, linear, and point heat transfer, and temperature factor is obtained. fig. 6 shows the architectural detail and temperature field of the studied junction. fig. 6 architectural detail and thermal field in the cross-section of the slab-façade junction the u-value of the façade changes from 1.05 w/m2k (uninsulated) to 0.13 w/m2k (20 cm of insulation). the achieved insulation levels are compared with normative requirements in spain (ministerio de fomento, 2013). in fig. 7, the evolution of thermal transmittance and temperature factors is shown for varying thermal insulation levels. for the uninsulated case, there is a 12% surplus heat transfer due to the 2d heat transfer over the 1d study. when adding insulation over this junction, the 2d surplus heat is substantially mitigated. however, 3d heat transfer introduced by mechanical anchors becomes a relevant part of the heat transfer across the façade. this surplus heat increases from 16% (5cm) to 48% (20cm) when the façade is insulated externally. the surplus 3d heat transfer is stable in absolute terms for all cases, but its relative relevance increases substantially. fig. 7 temperature factors and thermal transmittance values. (arregi goikolea, garay martinez, riverola lacasta & chemisana villegas, 2016) 105 journal of facade design & engineering volume 6 / number 2 / 2018 the method results in a more precise assessment, where calculation errors due to 3d heat transfer are detected and corrected. as a result, the façade system is selected for compliance with the spanish requirement of overall façade u-value (0.25 w/m2k). in this correction, insulation thickness is increased from 10cm to 15cm of mineral wool. 6 conclusions with the increasing thermal performance levels required by national building codes in developed societies, steady-state thermal performance of one-dimensional sections of envelopes are not sufficient to guarantee the thermal performance of architectural envelopes. the need for detailed assessment is increasingly relevant in retrofitting projects, where architectural information and design alternatives face relevant constraints. under such schemes, advances in design and assessment procedures are necessary, particularly considering that thermal bridges in these junctions are major heat loss paths, and cold spots exist in which surface condensation and mould growth are more likely to occur. the proposed methodology provides a minimally intrusive methodology for the robust assessment of thermal performance of architectural junctions with many possible outcomes, which could be defined based on the requirements of each case. considering the rapid adoption of wireless technologies in the sensor and monitorisation market, it could be expected that the intrusiveness of the methodology could be further reduced by removing wires in the monitorisation process. acknowledgements this study has been partially developed within the bresaer research project. however, it reflects only the authors’ view and the european commission is not responsible for any use that may be made of the information contained in it. this project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no 637186 references arregi goikolea, b., garay martinez, r., riverola lacasta, a., & chemisana villegas, d. (2016). estudio de transferencia de calor en los puntos de anclaje a forjado de una subestructura de fachada ventilada. [study of heat transfer in the anchoring points to the slab of a ventilated facade substructure]. iii congreso de edificios de consumo de energía casi nulo (eecn); grupo tecma red. isbn 978-84-608-8686-0. asiepi (2010). wp4, an effective handling of thermal bridges in the epbd context final report of the iee asiepi work on thermal bridges astm c1155 95(2013) standard practice for determining thermal resistance of building envelope components from the in-situ data atsonios, i.a., mandilaras, i.d., kontogeorgos, d.a., & founti, m.a. (2017). a comparative assessment of the standardized methods for the in–situ measurement of the thermal resistance of building walls. energy and buildings 154, 198-206. doi:10.1016/j. enbuild.2017.08.064 doe (2008). 2008 buildings energy data book. retrieved from http://web.archive.org/web/20130215004243/http://buildingsdatabook.eren.doe.gov/docs/databooks/2008_bedb_updated.pdf (2018/05/08) ecofys (2007). u-values for better energy performance of buildings, eurima. retrieved from https://www.eurima.org/publications/13/143/ecofys-vii-u-values-for-better-energy-performance-of-buildings.html (2018/05/08) elguezabal, p., & garay, r. (2015). experiences when employing different alternatives for envelope upgrading. journal of façade design & engineering 3(1), 81-89. doi:10.3233/fde-150032 en iso 6946:2007. building components and building elements – thermal resistance and thermal transmittance – calculation method 106 journal of facade design & engineering volume 6 / number 2 / 2018 en iso 9869-1:2014 thermal insulation –building elementsin-situ measurements of thermal resistance and thermal transmittance – part 1: heat flow meter method en iso 10211:2007: thermal bridges in building construction heat flows and surface temperatures detailed calculations. en iso 13786:2007 thermal performance of building components. dynamic thermal characteristics. calculation methods en iso 13790:2008: energy performance of buildings. calculation of energy use for space heating and cooling. e.u. (2002). 2002/91/ec of the european parliament and of the council of 16th december 2002 on the energy performance of buildings e.u. (2010). 2010/31/eu of the european parliament and of the council, of 19 may 2010 on the energy performance of buildings (recast) evo, efficiency valuation organization (2012). ipmvp international performance measurement and verification protocol, volume 1. retrieved from http://evo-world.org/ (2018/05/08) garay, r., uriarte, a. & apraiz, i. (2014). performance assessment of thermal bridge elements into a full scale experimental study of a building façade, energy and buildings 85, 579–591. doi:10.1016/j.enbuild.2014.09.024 garay, r., chica, j.a., apraiz, i., campos, j.m., tellado, b., uriarte, a., & sanchez, v. (2015). energy efficiency achievements in 5 years through experimental research in kubik. energy procedia 78, 865-870. doi:10.1016/j.egypro.2015.11.009 garay martinez, r., uriarte arrien, a., & apraiz egaña, i. (2015). calibration procedures for multi-dimensional heat transfer models based on on-site experimental data. energy procedia 78, 3222-3227. doi:10.1016/j.egypro.2015.11.784 garay martinez, r. (2017) dynamic performance assessment of multidimensional heat transfer in buildings (doctoral dissertation of master’s thesis). retrieved from http://hdl.handle.net/10803/460756 (2018/05/24) garay martinez, r., riverola, a., & chemisana, d. (2017). disaggregation process for dynamic multi-dimensional heat flux in building simulation. energy and buildings 148, 298-310. doi:10.1016/j.enbuild.2017.05.029 gacía gil, a. (2008). modelado de puentes térmicos en la simulación térmica de edificios (master dissertation). etsii málaga. retrieved from https://es.scribd.com/doc/26313726/modelado-de-puentes-termicos-en-la-simulacion-termica-de-edificios (2018/05/08) gutschker, o. (2008). parameter identification with the software package lord. building and environment 43(2), 163–169. doi:10.1016/j.buildenv.2006.10.010 ietcc, cepco & aicia (2010). catálogo de elementos constructivos del cte [catalogue of construction elements of the spanish building technical code]. retrieved from https://www.codigotecnico.org/ images/stories/pdf/aplicaciones/ncatalog_infoeconstr/cat-ec-v06.3_marzo_10.pdf (2018/05/08) kuusk, k., kurnitski, j., & kalamees, t. (2017). calculation and compliance procedures of thermal bridges in energy calculations in various european countries. energy procedia 132, 27-32. doi:10.1016/j.egypro.2017.09.626 lbnl (2018). therm, two-dimensional building heat-transfer modeling. retrieved from https://windows.lbl.gov/software/therm (2018/05/08) martín, k., flores, i., escudero, c., apaolaza, a., & sala, j. m. (2010). methodology for the calculation of response factors through experimental tests and validation with simulation. energy and buildings, 42(4), 461–467. doi:10.1016/j.enbuild.2009.10.015 ministerio de fomento (2013). código técnico de la edificación (cte) [technical building code]. documento básico de ahorro de energía (db-he) [basic document, heat economy (db-he)]. naveros, i., bacher, p., ruiz, d.p., jiménez, m.j., & madsen, h. (2014). setting up and validating a complex model for a simple homogeneous wall. energy and buildings 70, 303–317. doi:10.1016/j.enbuild.2013.11.076 o’grady, m., lechowska, a., & harte, a. (2017). infrared thermography technique as an in-situ method of assessing heat loss through thermal bridging. energy and buildings, 135, 20-32. doi:10.1016/j.enbuild.2016.11.039 pérez-lombard, l., ortiz, j., & pout, c. (2008), a review on buildings energy consumption information. energy and buildings 40(3), 394-398. doi:10.1016/j.enbuild.2007.03.007 physibel (2009). voltra. computer program to calculate 3d & 2d transient heat transfer in objects described in a rectangular grid using the energy balance technique, version 6.3w. strachan, p.a., & vandaele, l. (2008). case studies of outdoor testing and analysis of building components. building and environment 43(2), 129-142. doi:10.1016/j.buildenv.2006.10.043 tecnalia (2013). estudio de investigación sobre eficiencia energética y viabilidad de la aplicación de fachadas ventiladas en soluciones de rehabilitación. [ research study on energy efficiency and viability of the application of ventilated façades in rehabilitation solutions.] anexo 4: caracterización dinámica de los elementos de frente de forjado [annex 4: dynamic characterization of the elements in floor slab edges]. retrieved from http://www.euskadi.eus/contenidos/ noticia/20130426_kubik/es_kubik/adjuntos/kubik3.pdf (2018/05/08) ulma architectural (2018). vanguard range. retrieved from http://www.ulmaarchitectural.com/en/ (2018/03/28) unep (2016). sustainable building and climate initiative. retrieved from http://www.unep.org/sbci (2016/04/28) from city’s station to station city 57 journal of facade design & engineering volume 6 / number 1 / 2018 numerical investigation of capabilities for dynamic self-shading through shape changing building surface tiles robert joseph zupan1, dale clifford2, richard beblo3, john brigham4 1 university of pittsburgh, pittsburgh, pa, united states of america 2 california polytechnic state university, san luis obispo, ca, united states of america 3 university of dayton research institute, dayton, oh, united states of america 4 durham university, durham, united kingdom abstract a concept for a smart material morphing building surface tile that would utilise adaptive surface wrinkle patterns to improve solar interaction is explored. the effect of the wrinkle patterns is numerically investigated in the context of an objective to reduce solar irradiance entering buildings by changing the shape of the surface (i.e. surface topography) so that the façade is self-shading, thereby reducing energy costs of the building for temperature control. a generally applicable algorithm was utilised, and is presented to quantify the area of an arbitrarily shaped/oriented surface that is in shade for any given date/time and geographic location. numerical case studies are shown that utilise the self-shading algorithm to evaluate the capabilities of various wrinkle patterns, both static and dynamically changing, to self-shade a building surface over the course of a day. the results indicate that a morphing wrinkle pattern can substantially increase the amount and duration of surface area in shade over time in comparison to any static (non-morphing) patterns, although it is noted that changing the surface pattern results in a trade-off in the energy cost. furthermore, it is shown that as the location of the proposed tile on the building changes, the optimal wrinkle pattern changes as well. keywords morphing, building envelope, self-shading, cactus tile, adaptive structures, wrinkle, smart material doi: 10.7480/jfde.2018.1.1781 58 journal of facade design & engineering volume 6 / number 1 / 2018 1 introduction buildings that can adaptively respond to fluctuating environmental conditions have the proven potential to increase occupant comfort and significantly decrease energy consumption and carbon emissions (hughes & dhannu, 2008; z. sun, wang, & ma, 2011). moreover, with commercial buildings alone accounting for over 40% of energy consumption in the united states (energy, 2010), environmentally responsive building technologies for reducing energy consumption are a particularly promising area of collaborative and multidisciplinary research. the main consumers of energy in commercial buildings are lighting, ventilation, and heating and cooling (energy, 2010). therefore, in an effort to increase energy efficiency, many building technologies being developed focus on optimally regulating these main consumers using a variety of dynamic control systems for lighting (bryans & jump, 2001; eckel et al., 1999; guillemin & morel, 2001; hughes & dhannu, 2008), ventilation (ayata, çam, & yıldız, 2007; gavan, woloszyn, kuznik, & roux, 2010; hagentoft, sasic kalagasidis, nilsson, & thorin, 2008; z. sun et al., 2011), and cooling (aldawoud, 2013; inoue, 2003; ip, lam, & miller, 2010). these technologies are widely used in commercial buildings, and recently, in an effort to further increase energy efficiency, researchers have been focusing on utilising the building envelope (capeluto & ochoa, 2014; sauchelli, lobaccaro, masera, & fiorito, 2013; zhou & chen, 2010), which includes the façade, roof, and windows. technologies that utilise the building envelope, such as the façade, have been shown to affect the energy demand of commercial buildings (chwieduk, 2003; yan liu et al.; sadineni, madala, & boehm, 2011; sozer, 2010). these technologies have the ability to affect ventilation, interior lighting, and wind drag by utilising varying porosity (chang, 2006; karava, stathopoulos, & athienitis, 2007), light filtering (aldawoud, 2013), and varying surface texture (lignarolo, lelieveld, & teuffel, 2011). the present work focuses on environmentally responsive exterior shading. controlling how the building interacts with sunlight has the potential to reduce both lighting and heating and cooling costs. examples of exterior building components that focus specifically on interaction with light that have been proposed include external louvres and awnings/overhangs (dubois & arch, 1998; hashemi, 2014; jones, 1980; nielsen, svendsen, & jensen, 2011; palmero-marrero & oliveira, 2010), photovoltaic panels (di vincenzo, kesten, & infield, 2010; l. sun, lu, & yang, 2012; wang & hsu, 2010; yoo & manz, 2011), and building surface skins (r. m. barrett & m. r. p. barrett, 2016; r. m. barrett & r. p. barrett, 2016a, 2016b; cilento, 2012; dewidar, mohamed, & ashour, 2013; fiorito et al., 2016; nagy et al., 2016). of the adaptive building envelope technologies in existence and under development, many utilise smart materials to facilitate the adaptive behaviour. a recent example is the proposed use of hygromorphic materials to create an adaptive structure concept that is inspired by the opening and closing of the surface elements of conifer cones (holstov, bridgens, & farmer, 2015). alternatively, barrett and co-workers (r. m. barrett & m. r. p. barrett, 2016; r. m. barrett & r. p. barrett, 2016a, 2016b) have been developing a device that is likely most similar to the concept proposed herein, in terms of both objective and mechanism. barrett et al. have proposed a smart material building covering, called ``thermadapt”, that would change shape based on thermal loading throughout the day. to achieve the surface morphing the building coverings take advantage of coefficient of thermal expansion (cte) mismatch. the building covering has an outer layer with a lower cte than the inner layer, therefore when the temperature rises the inner layer expands first, causing the building covering to curve outwards, altering the shape of the exterior into a self-shading configuration. the inner layer would also be the first to contract in cold weather, causing the building coverings to curve inwards, providing insulation by trapping air pockets under the coverings. this building covering technology has been shown to lead to peak temperatures 10 14° f lower than ambient 59 journal of facade design & engineering volume 6 / number 1 / 2018 peak temperatures. it also performs well in cold temperatures, showing similar temperatures as standard insulation while using much less material. on a larger scale, capeluto investigated the effects of utilising a larger portion of the building envelope for self-shading purposes and showed similar positive results (capeluto, 2003). this work proposes a building surface tile that (similarly to barrett’s work) self-shades the surface to affect the solar irradiance entering the building. in contrast, the approach for self-shading is based upon a technology that was previously explored by clifford, referred to as ``cactus tiles” (carnegie mellon university). the cactus tile is a bio-inspired form for a building surface that is based on the skin of a cactus, which has extensive wrinkling that functions to self-shade the cactus and prevent it from overheating in the sun. this effect was explored previously by ehleringer (ehleringer, mooney, gulmon, & rundel, 1980). preliminary tests showed that on a 100° f day on a south-facing façade, a wrinkled building surface was 30° f cooler than a surrounding unshaded brick surface, substantially lowering thermal transfer to the building interior. however, this concept was initially envisioned to be static (i.e. the wrinkle pattern of the building surface would be the same year-round), and would not account for changes in the angle/direction of sunlight or changes in the self-shading (i.e. cooling) demand throughout a day and a year. as such, the current study intends to explore the potential of a morphing cactus tile that has a dynamically changing wrinkle pattern, which could control the level of self-shading provided by the façade based on the time of day and/or season. the key difference between the proposed technology and the technologies developed by barrett (r. m. barrett & m. r. p. barrett, 2016; r. m. barrett & r. p. barrett, 2016a, 2016b), nagy (nagy et al., 2016), and others is that the proposed technology has a continuous deformable surface while the others simply have discrete elements with a limited range of motion. therefore, it is expected that the proposed technology will have a design space with a higher number of solutions, potentially leading to greater efficiency or functionality. to evaluate the potential of this morphing cactus tile concept and to motivate future efforts to develop the technology, a set of numerical case studies were investigated regarding the self-shading capability of various static and dynamic cactus tiles surface topographies. first, it was necessary to identify and utilise an algorithm to calculate the portion of a surface that is self-shaded for any feasible (wrinkled) tile shape, geographic location, and environmental conditions, which is elaborated upon in the following. then, the case studies are presented, which examine the potential self-shading capabilities of various basic wrinkle pattern possibilities throughout a given day. lastly, concluding remarks and future directions to further explore the proposed technology are discussed. an important note is that the potential benefit of having non-flat tile shapes is through the control of how heat is transferred to the building. specifically, note the fourth-order relationship between thermal radiation and temperature in contrast to the linear relationship of thermal convection and conduction to temperature (howell, menguc, & siegel, 2010). for example, if a fixed amount of solar energy is focused on a surface area that is 50% smaller than the original area of the surface, the resulting temperature of the exposed surface area will be doubled. due to the fourth-order relationship between temperature and thermal radiation this leads to the thermal energy being radiated from the tile to increase by a factor of 16, which in turn reduces the amount of the finite thermal energy being conducted into the building. in this simplified example, radiation and convection are assumed to be wholly exterior phenomena. 60 journal of facade design & engineering volume 6 / number 1 / 2018 2 algorithm for quantification of self-shading of an arbitrary surface most applications requiring quantification of shaded area of a surface have largely focused on structured shapes, such as overhangs, awnings, and louvres (dubois & arch, 1998; hashemi, 2014; jones, 1980; nielsen et al., 2011; palmero-marrero & oliveira, 2010; pongpattana & rakkwamsuk, 2006; yanda & jones, 1983). as such, the algorithms utilised therein are not applicable to arbitrarily shaped surfaces as are considered here for the morphing building surface tile concept, and a more generalised algorithm is needed. rendering technology, such as radiosity (cohen & wallace, 2012; hanrahan, salzman, & aupperle, 1991), uses algorithms capable of calculating the area of shaded area of arbitrarily shaped surfaces. therefore, an algorithm, similar to those used in such technologies, was modified to be applicable to a shape defined by a finite element mesh. the shading quantification algorithm utilised assumes that the surface to be evaluated be defined by a standard finite element-type mesh, based upon a set of nodes and their connectivity as elements. then, all that is necessary is to determine the state of shading of each element, using a form of backwards ray tracing, and numerically integrate over the mesh to determine the total area that is self-shaded for the given arbitrarily shaped surface. as such, the algorithm utilised to quantify the self-shading of an arbitrary surface can be described as follows: input: nodal coordinates and element connectivity of mesh description of surface, latitude, longitude, and elevation of the surface location, date, and time. 1 calculate azimuth and zenith angles of the sun’s position relative to the surface (as detailed in the following). 2 loop over each element in the mesh, and for each element: a determine the centroid of the element. b determine the direction (vector) of a solar ray that passes through the centroid. c determine if any other element in the mesh intersects the solar ray vector (i.e. shading the current element). d if the solar ray vector intersects any other element (between current element and sun), then add the current element area to the total shaded area sum. a critical element of any approach to shading quantification (including the algorithm utilised herein) is calculation of the solar position at the point in space and time on the surface of interest. there are numerous methods to determine solar position, characterised by the zenith (or elevation) and azimuth angles (blanco-muriel, alarcón-padilla, lópez-moratalla, & lara-coira, 2001; grena, 2008; reda & andreas, 2004). the algorithm used in the present effort is the solar position algorithm (spa) developed in reda and andreas (2004). spa was chosen because of the relatively high accuracy compared to other options, with maximum uncertainties in the calculated angles of ±0.0003°. as detailed in reda and andreas (2004), spa requires the location (latitude and longitude), date, time, and spatial and temporal properties of the location (elevation, average annual temp/pressure, etc.), and puts out several solar position measures, including those required for the self-shading quantification algorithm herein of the zenith and azimuth angles. the unit vector of the solar ray can then be characterised using the zenith and azimuth angles as: (1) where γ s is the elevation angle (degrees), a s is the azimuth angle relative to south (degrees), north is positive x, west is positive y, and height is positive z. 57 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 32 example of the self-shading estimate from the algorithm presented herein for four different levels of mesh refinement (a d), with shaded elements in black and exposed elements in red. a final component to an algorithm such as this is to perform a mesh convergence analysis to ensure that the mesh utilised to produce the final result is sufficiently small for the result to be accurate. as shown in figure 1 for an example self-shading calculation, the algorithm would be repeated with sequentially more refined meshes until the shading estimate stops significantly changing. as can be seen in the example, the estimates provided by figure 1(c) and figure 1(d) are significantly more similar than either of the prior estimates, and it is likely that the estimate in figure 1(d) is sufficiently converged. to verify and validate the accuracy of the self-shading algorithm, examples of shading geometries, their locations, and orientations from pongpattana and rakkwamsuk (2006) were evaluated. the first shape considered was the overhang shape shown in figure 2, oriented west at 13:07 on march 10th, 2004. the algorithm was verified by first performing a mesh dependency study, and then comparing to the analytical solution found by projecting the overhang through the solar angle onto the vertical wall and calculating the shaded area using simple geometry. the algorithm converged to a value differing from the analytical solution by 0.1% using 3584 elements, confirming the algorithm converges within acceptable limits. finally, to validate the present algorithm, converged estimates of surface shaded area were calculated for two additional test cases from the work in pongpattana and rakkwamsuk (2006) that had corresponding experimental measurements. the two test cases considered were the overhang case with the same location and orientation from the verification test and a double vertical fin device oriented west at 13:36 on march 14th, 2004, as shown in figure 3. the experimental result reported (pongpattana & rakkwamsuk, 2006) for the overhang case was 75% shaded area, in contrast to the 73% predicted by the shading quantification algorithm. additionally, the shading quantification algorithm estimated a self-shaded area of 17% for the conditions of the double vertical fin case, while the experimental results were 19%. with both sample cases resulting in differences between the experimental and numerical estimates of shading of only 2%, the shading quantification algorithm was considered to have calculated the shaded area of the objects within acceptable limits. 58 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 33 meshed representation of the west-facing overhang surface example. fig. 34 mesh representation of the double vertical fin surface example. 59 journal of facade design & engineering volume 6 / number 1 / 2018 3 self-shading numerical case studies several case studies were numerically investigated with respect to shading potential of the proposed building surface tile concept involving various scenarios for potential tile shapes, orientation, location, and time of day. overall, these case studies were based upon a single shape-changing tile, covering a 1m by 1m plane area, on a conceptual surface located in phoenix, arizona (lat: 33.45, long: -112.07) throughout the day of july 4th, 2015. it was assumed that the tile would be rigidly held on two of the four edges (e.g., top and bottom or left and right), and with shape changing only occurring through changes in the out-of-plane surface position. it was further assumed that the tile would operate similarly to the previously mentioned technology being developed by barrett et al. (r. m. barrett & m. r. p. barrett, 2016; r. m. barrett & r. p. barrett, 2016a, 2016b), with the tile being comprised of a smart material, and the shape change therefore being achieved by deformation of the tile material (rather than mechanical components such as hinges). to address potential material limitations at this stage of the development (i.e. without having selected an optimal smart material for device construction), each shape change considered was normalised for comparability between shapes such that the surface area change with respect to a flat surface was fixed at 150% (i.e. the total surface area of each deformed shape considered was fixed at 1.5 m2). although the distribution of this deformation would likely become important in future development, this 150% overall deformation is well within the limits of many smart materials, such as shape memory polymers (mather, luo, & rousseau, 2009) (commonly shown to exceed 200% recoverable deformation). furthermore, the normalisation allows each shape change to be fairly compared in the sense that the total deformation is equivalent. for simplicity, three basic wrinkle patterns were considered, which were chosen somewhat arbitrarily, but based upon the previously discussed concept of a “cactus” tile (carnegie mellon university). the three basic wrinkle patterns considered (in addition to the undeformed tile) are shown in figure 4, and are described by the equation for the out-of-plane surface position, z, as a single sin-wave overhang: (2) a continuous unidirectional sin-wave: (3) and a continuous bidirectional sin-wave: (4) where x and y are the in-plane coordinates of the plate, and a and p are the amplitude and period of the sin-wave, respectively. 60 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 35 mesh-based surface representation of the (a) single sin-wave overhang, (b) continuous unidirectional sin-wave, (c) continuous bidirectional sin-wave, (d) undeformed tile shapes to quantify the shading potential of each tile scenario over time, the 10-hour period of daylight on the specified date was considered and the surface shading for each tile scenario was calculated each hour on the hour for two conceptual locations on the building, a west-facing vertical wall and a south-facing roof (45°). in the following, the self-shading potential of these three basic wrinkle patterns is examined individually by evaluating the amount of self-shading for a set of variations (e.g., changes in shape amplitude, etc.) for each basic shape form over the specified 10-hour period. next, the basic wrinkle patterns are compared to one another in terms of the shading capability. an important note is that this first set of tests considered the tiles to be static (i.e. non-morphing) to evaluate the potential of the shapes. next, the potential self-shading of a morphing tile is evaluated by considering the capability of the tile to morph between the wrinkle patterns, and considering both shading throughout the day as well as a measure of complexity for the morphing process. finally, a comparison is made between the results of the two conceptual locations on the building. it should be noted that mesh convergence was confirmed for all shapes. 61 journal of facade design & engineering volume 6 / number 1 / 2018 3.1 shading potential of the individual basic wrinkle patterns the percentage of each tile shaded was determined over the specified day for three variations of each shape form for the vertical west wall. the variations in the three basic wrinkle patterns were generated by modifying the values of a and p in (2), (3), and (4). table 1 shows all nine parameter combinations of the basic wrinkle patterns considered. again, note that the shape variations all maintained the specified total surface area constraint (i.e. increasing the period of the shapes required increasing the amplitude to maintain the 1.5 m2 surface area). figure 5 compares the percentage of the tile surface area shaded over time for the variations considered for the overhang basic shape form. the overhang with a period of 0.125 m provides the highest percentage of self-shading until approximately 13:30, then the overhang with a period of 0.250 m temporarily provides the highest shading until approximately 14:30. the overhang with a period of 0.500 m provides the highest percentage from 14:30 to 17:00, at which point the periods of 0.125 m and 0.250 m provide approximately equivalent shading for the remainder of the day. figure 6 shows the analogous results for the unidirectional sin-wave. the unidirectional sin-wave with period of 0.125 m provides the highest percentage of self-shading for the entirety of the day except between 15:00 and 16:00 hours, when the period of 0.250 m temporarily provides more shading. finally, figure 7 shows the percentage of the tile shaded over time for the three different parameter combinations of the bidirectional sin-wave shape. similar to the unidirectional sin-wave, the period of 0.125 m provides the highest self-shading for the first part of the day, until approximately 15:30, and once again the three shape variations remain approximately equivalent for the remainder of the day. generally, as the period decreased (i.e. the more ``wrinkly” the shapes became) the amount of self-shading increased. however, with the limitation on the total surface area, there is a point of diminishing returns when decreasing the period size (which increases the number of shadows), since the amplitude must correspondingly decrease (which decreases each shadow height). although not shown here for brevity, additional tests indicated that this point of diminishing returns was approximately the amplitude of 0.125 m for the examples herein. shape: p (m) a (m) overhang 0.125 0.1550 0.250 0.1740 0.500 0.2150 unidirectional sin-wave 0.125 0.0342 0.250 0.0675 0.500 0.1350 bidirectional sin-wave 0.125 0.0975 0.250 0.1850 0.500 0.3656 table 18 the period (p) and amplitude (a) for each of the variations of the wrinkle patterns considered. 62 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 36 percentage of the surface that is self-shaded with respect to the time of day for variations of the period length (p) of the single sin-wave overhang shape. fig. 37 percentage of the surface that is self-shaded with respect to the time of day for variations of the period (p) of the unidirectional sin-wave shape. fig. 38 percentage of the surface that is self-shaded with respect to the time of day for variations of the period length (p) of the bidirectional sin-wave shape. fig. 39 percentage of the surface that is self-shaded with respect to time of day for the overhang (o), unidirectional (u), and bidirectional (b) shapes with period length of 0.125m. fig. 40 percentage of the surface that is self-shaded with respect to time of day for a morphing tile (m) and the overhang (o), unidirectional (u), and bidirectional (b) shapes with period length of 0.125m. 63 journal of facade design & engineering volume 6 / number 1 / 2018 figure 8 shows the shading over time for the three basic wrinkle patterns with a period of 0.125 m (i.e. the period with the highest total shading over the day for each). the unidirectional sin-wave had the highest percentage of shading for a large portion of the day, with the exception of 14:00 and after 16:30 when the overhang shape provided the highest amount of self-shading. more importantly, a general observation from figures 5-8 is that none of the basic wrinkle patterns or parameter combinations had a higher amount of self-shading than the others throughout the entirety of the day. in other words, a tile that is able to morph between such shapes and parameter combinations throughout the day would be able to self-shade more of the surface over a longer period of time compared to any static shape considered. 3.2 shading potential of a dynamic tile shape to begin evaluation of the potential of a morphing tile, figure 9 shows the optimal combination of the shapes considered in the previous section in terms of maximising the total shaded area over the day by allowing the tile shape to change every hour compared to the highest total shading static versions (i.e. unchanging) of the basic wrinkle patterns. the morphing tile shades the surface significantly more over the day than any one of the static shapes, with a total percentage of surface-hours of shade (i.e. area under the shaded percentage vs. time curve) of 7.50 compared to the next highest static shape, the unidirectional sin-wave of 7.18. however, an important aspect that has not been discussed up to this point is that there is expected to be an energy (or other) cost to morph a surface tile from one shape to another. therefore, the optimised morphing process shown in figure 9, which requires five shape changes, may ultimately have more cost than benefit compared to a tile that changes shape less often in certain circumstances. again, it should be noted that without further developments of the proposed technology the morphing cost cannot be predicted precisely. depending on the smart material/structure mechanism used, this cost could include energy for activation or actuation of the structure/material or simply the complexity of the build needed to achieve the functionality. to provide a perspective, it is assumed herein that the cost of performing a shape change is directly proportional to a measure of the difference between the two subsequent tile shapes during the morphing process. again, noting that the hypothetical mechanism for the morphing tiles herein would be deformation (as through the use of a smart material), the amount of deformation (i.e. shape change) is expected to be a reasonable estimate of the relative input energy required to morph. as such, the following metric of the difference between two tile shapes was utilised: (5) where z i and z j are the out-of-plane surface positions of the shapes at the ith and jth hour, respectively. the difference metrics for all possible combinations of the shapes considered are displayed in table 2. 64 journal of facade design & engineering volume 6 / number 1 / 2018 overhang unidirectional sin-wave bidirectional sin-wave period (m) p = 0.125 p = 0.250 p = 0.500 p = 0.125 p = 0.250 p = 0.500 p = 0.125 p = 0.250 p = 0.500 overhang p = 0.125 0.00 7.01 7.41 4.64 5.40 5.36 5.37 4.93 4.71 p = 0.250 0.00 6.23 5.82 4.50 2.98 4.41 3.18 5.41 p = 0.500 0.00 5.75 1.73 5.71 2.30 5.14 5.53 unidirectional p = 0.125 0.00 3.07 1.52 3.82 1.78 0.43 p = 0.250 0.00 2.98 1.56 2.61 2.88 p = 0.500 0.00 3.51 0.79 1.32 bidirectional p = 0.125 0.00 2.99 3.55 p = 0.250 0.00 1.51 p = 0.500 0.00 table 19 difference metric, calculated from (5), for each combination of shapes considered in the case studies. note: the matrix presented in this table is symmetrical. considering both maximisation of the percentage of self-shading and minimisation of the cost of morphing over an entire day, selecting the optimal morphing tile can now be thought of as a multiobjective design problem as follows: (6) with (7) and (8) where s(t) is the percentage of shading over time and β is a user-defined parameter to weigh the relative importance of the two competing objectives. to interpret the trade-off between b-1 and c of this morphing tile, the pareto front was generated for the present example by varying the total number of shape changes over the day (and thus controlling the morphing cost). figure 10(a) shows the value of the two design objectives for each point on the pareto front and figure 10(b) shows the corresponding sequence of shapes obtained from the shading maximisation. the pareto front shows that there is a substantial trade-off between the morphing cost (i.e. amount of shape change) and the amount of self-shading provided throughout the day. in particular, there is a nearly linear trade-off between the cost-benefit of 0 to 3 shape changes over the day (especially 1 to 3 shape changes). alternatively, the increase in the amount of shading decreases significantly after 3 shape changes, with almost negligible differences in the total shading with 3 shape changes compared to 5 shape changes, but with a substantial increase in the shape change cost. an important reminder, however, is that this design solution set is for one particular tile location/position on a conceptual building, and it is expected that not only different geographic locations, but even different positions on the same building would yield different optimal morphing tile systems, as considered in the following. 65 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 41 (a) the total shape change metric (b) with respect to the inverse of the total percentage of shading over the day (a) and (b) the sequence of shapes for each point on the morphing tile pareto front. 3.3 shading potential for a different morphing tile location to evaluate how a tile’s location on the façade might affect the morphing scheme, an additional set of numerical tests were carried out. this second set of tests considered a tile at the same geographic location and date as the previous tests, but on a south-facing roof façade angled at 45° (in contrast to the west-facing vertical wall considered previously). for the roof façade, the self-shading throughout the day was calculated for the same basic wrinkle patterns (and variations) that were previously considered. additionally, for this roof example, each shape form and period variation was considered with the entire surface rotated in plane by 90°. note that these 90° variations were also tested for the wall example, but not shown/discussed previously since they were all considerably less effective in self-shading the surfaces compared to the original orientations (which is a relatively intuitive result based on the expected path of the sun with respect to a west-facing vertical surface). figure 11 shows the percentage of self-shading with respect to time for the optimal variation (i.e. period value and rotation producing the highest total shading) of the three basic shapes on the roof façade. there are significant differences between the self-shading provided by each shape (and parameter variation) for the roof example compared to the wall. first, while the period of 0.125 m provided the most self-shading for all shapes on the wall, the period of 0.500 m provided the most self-shading for the overhang shape on the roof. in addition, the change in the percentage of shading over time for each shape (i.e. the shading curve) is substantially different for the roof compared to the wall, with the beginning of the day having considerably more differences between the shapes for the roof case. also of note is the fact that the original orientations overall provided higher shading compared to their 90° rotation counterparts, but the 90° rotations provided the highest self-shading at specific hours. in particular, the bidirectional sin-wave shape had the highest amount of selfshading at both the beginning and end of the day for the roof, when considering no rotation, but never had the highest self-shading for the vertical wall. the total percentage of surface-hours of shade was also much higher for the bidirectional sin-wave shape, indicating that it provides a higher shading on average. 66 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 42 percentage of the surface that is self-shaded with respect to time of day for the overhang shape with period of 0.500 m (o), the unidirectional shape with period of 0.125 m (u), and the bidirectional shape with period of 0.125 m (b), all with the original orientation. fig. 43 gantt chart representing the shape sequences for the west wall and south roof façades that maximise the total self-shading over the day. (it should be noted that shapes that were not optimal at any times are not taken into account in the legend.) 67 journal of facade design & engineering volume 6 / number 1 / 2018 to further compare the two building locations, figure 12 shows the combination of the shapes that maximises the total shaded area over the day by allowing the tile shape to change every hour (not considering the cost of the shape changes) for the roof example compared to the analogous combination previously shown for the wall example. in particular, the optimal combination for the roof was considerably more dynamic, with eight shape changes between seven shape variations, which is a change almost every hour, compared to just five changes between five variations for the wall. in addition, maximising the self-shading through morphing had a significantly larger effect on the roof, with the optimised shape change sequence yielding 5.47 total percentage of surfacehours of shade, which is 24.6% greater than the highest value for any of the static shapes, compared to the 4.5% increase provided by morphing for the wall. it should be noted that even though the bidirectional sin-wave was the best static shape in terms of total percentage of surface-hours of shade, it never provided the highest self-shading of all variations considered at any specific hour, so it does not appear in figure 12. if there was a limitation on the number of morphs, the bidirectional sin-wave would most likely be in the optimal scheme. 4 conclusions and future directions a concept was presented and numerically investigated for a morphing building tile that would reduce solar irradiance entering buildings by changing the surface topography so that the building envelope is self-shading, thereby reducing energy used by internal temperature control. numerical case studies were used to evaluate the capabilities of this concept with three basic wrinkle patterns, both static and dynamically changing, to self-shade a building surface over the course of a day. the results indicated that a morphing surface can provide an increase in the total percentage of surface hours of shade as large as 24.6%, in comparison to the highest performing static (nonmorphing) shapes. however, it was noted that there is an expected trade-off in the mechanical cost to change the surface topography. for example, it is expected that there will be a point in the design where the increase in self-shading from further morphing will not be worth the energy required to achieve morphing (e.g. through material activation and/or mechanical actuation if using a smart material component). in particular, the results showed a substantial reduction in the improvement to the self-shading after three shape changes for the designs considered. yet, a significant change in self-shading behaviour depending upon the tile location was observed, in terms of both total shading capability and the benefits of morphing. therefore, separate design optimisations, such as that presented, would likely be beneficial for tiles that would be located in different geographic locations and positions/orientations on the building surface. although the results presented show promise for a morphing, self-shading tile technology to significantly improve solar interaction for a building envelope, there are several considerations moving forward with development. one critical consideration is the full effect that this technology will have on the energy demand of a building. this physical process needs further investigation to truly understand the potential of the technology proposed herein and those that are similar. another consideration is the choice/development of a smart material solution out of the multitude of existing concepts to facilitate the controllable surface topography. for example, two smart materials that are potentially applicable are shape memory allows (smas) (duerig, melton, & stöckel, 2013; fremond, 1996; jani, leary, subic, & gibson, 2014; morgan, 2004) and shape memory polymers (dietsch & tong, 2007; lendlein & kelch, 2002; yanju liu, du, liu, & leng, 2014; mather et al., 2009). both smas and smps allow for large strain, which is expected to be a requirement of this morphing tile concept. also, the shape memory effect of both smas and smps could aid in achieving different tile configurations 68 journal of facade design & engineering volume 6 / number 1 / 2018 with a reduced actuation requirement. another consideration is that throughout the presented work the design only considered a finite number of tile topographies. as such, future work to explore not only optimisation of the morphing scheme, but also the surface topography of the tile could lead to improvements to both efficiency and effectiveness. acknowledgements the authors gratefully acknowledge the financial support of the national science foundation through award no. 1536797. references aldawoud, a. (2013). conventional fixed shading devices in comparison to an electrochromic glazing system in hot, dry climate. energy and buildings, 59, 104-110. ayata, t., çam, e., & yıldız, o. (2007). adaptive neuro-fuzzy inference systems (anfis) application to investigate potential use of natural ventilation in new building designs in turkey. energy conversion and management, 48(5), 1472-1479. barrett, r. m., & barrett, m. r. p. (2016). thermally adaptive building coverings: theory and application. paper presented at the asme 2016 conference on smart materials, adaptive structures and intelligent systems. barrett, r. m., & barrett, r. p. (2016a). thermally adaptive building covering field test. procedia engineering, 145, 26-33. barrett, r. m., & barrett, r. p. (2016b). thermally adaptive building coverings inspired by botanical thermotropism. paper presented at the proceedings of the asme 2016 conference on smart materials, adaptive structures and intelligent systems (smasis). blanco-muriel, m., alarcón-padilla, d. c., lópez-moratalla, t., & lara-coira, m. (2001). computing the solar vector. solar energy, 70(5), 431-441. bryans, d. j., & jump, l. b. (2001). lighting control subsystem for use in system architecture for automated building: google patents. capeluto, i. g. (2003). energy performance of the self-shading building envelope. energy and buildings, 35(3), 327-336. capeluto, i. g., & ochoa, c. e. (2014). simulation-based method to determine climatic energy strategies of an adaptable building retrofit façade system. energy, 76, 375-384. carnegie mellon university. thermal masonry. retrieved november 6th, 2017, from https://cmubiologic.weebly.com/cactus-tile. html chang, w. (2006). effect of porous hedge on cross ventilation of a residential building. building and environment, 41(5), 549-556. chwieduk, d. (2003). towards sustainable-energy buildings. applied energy, 76(1), 211-217. cilento, k. (2012). al bahar towers responsive facade/aedas. archdaily,’september, 5. cohen, m. f., & wallace, j. r. (2012). radiosity and realistic image synthesis: elsevier. dewidar, y., mohamed, n., & ashour, y. (2013). living skins: a new concept of self active building envelope regulating systems. paper presented at the advancing the green agenda; technology, practices and policies conference–buid. di vincenzo, m. c., kesten, d., & infield, d. (2010). assessment of performance of building shading device with integrated photovoltaics in different urban scenarios. paper presented at the sustainable energy technologies (icset), 2010 ieee international conference on. dietsch, b., & tong, t. (2007). a review-: features and benefits of shape memory polymers (smps). journal of advanced materials, 39(2), 3-12. dubois, m.-c., & arch, m. (1998). awnings and solar protective glazing for efficient energy use in cold climates. paper presented at the renewable energy technologies in cold climates’ 98 conference, may. duerig, t. w., melton, k., & stöckel, d. (2013). engineering aspects of shape memory alloys: butterworth-heinemann. eckel, d. p., batko, t. j., walter, m. r., rose, w. j., donlon, b. p., & zeichner, d. a. (1999). motion sensing system with adaptive timing for controlling lighting fixtures: google patents. ehleringer, j., mooney, h., gulmon, s., & rundel, p. (1980). orientation and its consequences for copiapoa (cactaceae) in the atacama desert. oecologia, 46(1), 63-67. energy, r. (2010). energy efficiency trends in residential and commercial buildings. fiorito, f., sauchelli, m., arroyo, d., pesenti, m., imperadori, m., masera, g., & ranzi, g. (2016). shape morphing solar shadings: a review. renewable and sustainable energy reviews, 55, 863-884. fremond, m. (1996). shape memory alloy shape memory alloys (pp. 1-68): springer. gavan, v., woloszyn, m., kuznik, f., & roux, j.-j. (2010). experimental study of a mechanically ventilated double-skin façade with venetian sun-shading device: a full-scale investigation in controlled environment. solar energy, 84(2), 183-195. grena, r. (2008). an algorithm for the computation of the solar position. solar energy, 82(5), 462-470. guillemin, a., & morel, n. (2001). an innovative lighting controller integrated in a self-adaptive building control system. energy and buildings, 33(5), 477-487. hagentoft, c.-e., sasic kalagasidis, a., nilsson, s., & thorin, m. (2008). mould growth control in cold attics through adaptive ventilation. paper presented at the proceedings of the 8th symposium on building physics in the nordic countries, copenahgen denmark. 69 journal of facade design & engineering volume 6 / number 1 / 2018 hanrahan, p., salzman, d., & aupperle, l. (1991). a rapid hierarchical radiosity algorithm. paper presented at the acm siggraph computer graphics. hashemi, a. (2014). daylighting and solar shading performances of an innovative automated reflective louvre system. energy and buildings, 82, 607-620. holstov, a., bridgens, b., & farmer, g. (2015). hygromorphic materials for sustainable responsive architecture. construction and building materials, 98, 570-582. howell, j. r., menguc, m. p., & siegel, r. (2010). thermal radiation heat transfer: crc press. hughes, r. f., & dhannu, s. s. (2008). substantial energy savings through adaptive lighting. paper presented at the electric power conference, 2008. epec 2008. ieee canada. inoue, t. (2003). solar shading and daylighting by means of autonomous responsive dimming glass: practical application. energy and buildings, 35(5), 463-471. ip, k., lam, m., & miller, a. (2010). shading performance of a vertical deciduous climbing plant canopy. building and environment, 45(1), 81-88. jani, j. m., leary, m., subic, a., & gibson, m. a. (2014). a review of shape memory alloy research, applications and opportunities. materials & design, 56, 1078-1113. jones, r. e. (1980). effects of overhang shading of windows having arbitrary azimuth. solar energy, 24(3), 305-312. karava, p., stathopoulos, t., & athienitis, a. k. (2007). wind-induced natural ventilation analysis. solar energy, 81(1), 20-30. lendlein, a., & kelch, s. (2002). shape-memory polymers. angewandte chemie international edition, 41(12), 2034-2057. lignarolo, l., lelieveld, c., & teuffel, p. (2011). shape morphing wind-responsive facade systems realized with smart materials. paper presented at the adaptive architecture: an international conference, london, uk, march 3-5, 2011. liu, y., du, h., liu, l., & leng, j. (2014). shape memory polymers and their composites in aerospace applications: a review. smart materials and structures, 23(2), 023001. liu, y., liu, j., yang, l., hou, l., wang, m., & qiao, y. cooling effect for integrated application of phase change envelopes and night natural ventilation in western china. mather, p. t., luo, x., & rousseau, i. a. (2009). shape memory polymer research. annual review of materials research, 39, 445-471. morgan, n. (2004). medical shape memory alloy applications—the market and its products. materials science and engineering: a, 378(1), 16-23. nagy, z., svetozarevic, b., jayathissa, p., begle, m., hofer, j., lydon, g., . . . schlueter, a. (2016). the adaptive solar facade: from concept to prototypes. frontiers of architectural research, 5(2), 143-156. nielsen, m. v., svendsen, s., & jensen, l. b. (2011). quantifying the potential of automated dynamic solar shading in office buildings through integrated simulations of energy and daylight. solar energy, 85(5), 757-768. palmero-marrero, a. i., & oliveira, a. c. (2010). effect of louver shading devices on building energy requirements. applied energy, 87(6), 2040-2049. pongpattana, c., & rakkwamsuk, p. (2006). efficient algorithm and computing tool for shading calculation. songklanakarin journal of science and technology, 28(2), 375-386. reda, i., & andreas, a. (2004). solar position algorithm for solar radiation applications. solar energy, 76(5), 577-589. sadineni, s. b., madala, s., & boehm, r. f. (2011). passive building energy savings: a review of building envelope components. renewable and sustainable energy reviews, 15(8), 3617-3631. sauchelli, m., lobaccaro, g., masera, g., & fiorito, f. (2013). smart solutions for solar adaptive façade preliminary studies for an innovative shading device. paper presented at the xix iahs world congress,, milan, italy. sozer, h. (2010). improving energy efficiency through the design of the building envelope. building and environment, 45(12), 25812593. sun, l., lu, l., & yang, h. (2012). optimum design of shading-type building-integrated photovoltaic claddings with different surface azimuth angles. applied energy, 90(1), 233-240. sun, z., wang, s., & ma, z. (2011). in-situ implementation and validation of a co 2-based adaptive demand-controlled ventilation strategy in a multi-zone office building. building and environment, 46(1), 124-133. wang, y.-j., & hsu, p.-c. (2010). analytical modelling of partial shading and different orientation of photovoltaic modules. iet renewable power generation, 4(3), 272-282. yanda, r., & jones, r. (1983). shading effects of finite width overhang on windows facing toward the equator. solar energy, 30(2), 171-180. yoo, s.-h., & manz, h. (2011). available remodeling simulation for a bipv as a shading device. solar energy materials and solar cells, 95(1), 394-397. zhou, j., & chen, y. (2010). a review on applying ventilated double-skin facade to buildings in hot-summer and cold-winter zone in china. renewable and sustainable energy reviews, 14(4), 1321-1328. from city’s station to station city 010 journal of facade design & engineering volume 6 / number 3 / 2018 possibilities and constraints for the widespread application of solar cooling integrated façades alejandro prieto1*, ulrich knaack1, tillmann klein1, thomas auer2 * corresponding author 1 delft university of technology, faculty of architecture and the built environment, department of architectural engineering + technology, architectural façades & products research group, the netherlands, a.i.prietohoces@tudelft.nl 2 technical university of munich, department of architecture, chair of building technology and climate responsive design, germany. abstract cooling demands in buildings have drastically increased in recent decades and this trend is set to continue into the near future, due to increasing standards of living and global climate change, among the most relevant factors. besides energy consumption, the use of refrigerants in common vapour compression cooling technologies is a source of concern because of their environmental impact. hence, there is a need to decrease cooling demands in buildings while looking for alternative clean technologies to take over the remaining loads. solar cooling systems have gained increased attention in recent years, for their potential to lower indoor temperatures using renewable energy under environmentally friendly cooling processes. nonetheless, their potential for building integration has not been fully explored, with the exception of scattered prototypes and concepts. this paper aims to address these knowledge gaps by presenting the results of the phd research project ‘coolfaçade: architectural integration of solar cooling technologies in the building envelope’. the research project explored the possibilities and constraints for architectural integration of solar cooling strategies in façades, in order to support the design of climate responsive architectural products for office buildings, driven by renewable energy sources. this paper explores different aspects related to façade integration and solar cooling technologies, in order to provide a comprehensive understanding of current possibilities for façade integration, while drafting recommendations based on identified barriers and bottlenecks at different levels. keywords solar cooling, integrated façades, façade design, renewables, barriers doi 10.7480/jfde.2018.3.2468 011 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction energy demands for cooling have increased drastically in recent decades, due to societal and economic factors such as higher standards of living and affordability of air conditioning, as well as environmental aspects such as temperature rise in cities in what is known as urban heat islands, and global climate change (santamouris, 2016). total energy projections for the coming decades show that energy consumption will keep rising, mostly driven by fast-growing emerging economies (bp, 2016; doe/eia, 2016), and cooling energy demands are expected to follow this trend (jochem & schade, 2009; oecd/iea, 2015). as an example, yearly sales of room air conditioning units are expected to grow by 10-15%, going from 100 million worldwide in 2014, to an expected 1.6 billion by 2050 (montagnino, 2017). the first course of action in tackling this situation should always aim to reduce energy consumption through saving measures and the application of passive design strategies in buildings. nonetheless, this is often not enough to avoid mechanical equipment altogether, particularly in the case of office buildings in warm climates, which are characterised by particularly high cooling demands (qi, 2006). in this regard, solar cooling technologies have been increasingly explored, as an environmentally friendly alternative to harmful refrigerants used within vapour compression systems, while also being driven by solar thus, renewable energy. the principles behind some of these technologies have been researched for over a century, reaching mature solutions and components, and being recognised as promising alternatives to commonly-used air-conditioning units (goetzler, zogg, young, & johnson, 2014). nonetheless, application in buildings remains mostly limited to demonstration projects and pilot experiences (balaras et al., 2007; henning & döll, 2012). recently, façade integrated concepts have been explored as ways to promote widespread application throughout the development of multi-functional building components (avesani, 2016; ibañez-puy, martín-gómez, bermejo-busto, sacristán, & ibañez-puy, 2018; prieto, knaack, auer, & klein, 2017a; xu & van dessel, 2008). however, while these are regarded as relevant and promising standalone concepts, further research is still needed to assess the integration potential of diverse solar cooling technologies, and identify any barriers that must be overcome, in order to promote the widespread application of solar cooling components in the built environment. this paper aims to address these knowledge gaps by presenting the results of the phd research project ‘coolfaçade: architectural integration of solar technologies in the building envelope’, carried out by the main author under the supervision of the co-authors. as the title suggests, the research project explored the possibilities and constraints for the architectural integration of solar cooling strategies in façades, in order to support the design of climate responsive architectural products for office buildings, without compromising the thermal comfort of users. the underlying hypothesis was that self-sufficient solar cooling integrated façades may be a promising alternative to conventional centralised air-conditioning systems widely used in office buildings in warm climates. the research explored different aspects relating to façade integration and solar cooling technologies, in order to provide a comprehensive understanding of current possibilities for the development of architectural products. hence, different types of barriers were identified, corresponding to distinct aspects that need to be considered in the development of integrated concepts. these specific findings have been presented separately and discussed in detail in previous publications. so, this article presents a collated summary of all results, focusing on the general discussion of overall possibilities after accounting for key aspects for further development, and drafting recommendations based on the identified constraints at different levels. 012 journal of facade design & engineering volume 6 / number 3 / 2018 2 research strategy and methods the evaluation of the façade integration potential of solar cooling technologies was carried out considering two main families of parameters, targeting particular key aspects for the development and application of integrated façade concepts. therefore, selected solar cooling technologies were assessed in terms of (a) architectural requirements for the integration of building services within the façade design and development process, and (b) the potential climate feasibility of self-sufficient integrated concepts, matching current technical possibilities with cooling requirements from several climates under an holistic approach to climate responsive façade design. the basic strategy behind the research project is summarised in fig. 1. architectural requirements for facade integration of building services facade integration potential of solar cooling technologies climate feasibility of self-sufficient cooling facade concepts solar cooling technologies possibilities & constraints for facade integration fig. 1 research strategy and parameters for the assessment on the one hand, the response of the technologies to architectural requirements for façade integration was assessed qualitatively, based on a comprehensive review of the key aspects of each technology and their potential to overcome the main identified barriers for façade integration of building services. these barriers were previously identified and discussed by means of a survey addressed to experienced professionals in the fields of façade design and construction. the survey aimed to identify the main perceived problems relating to the façade integration of building services (prieto, klein, knaack, & auer, 2017), and the integration of solar collection technologies (photovoltaic panels and solar thermal collectors) (prieto, knaack, auer, & klein, 2017b), discussing specific barriers separately. the responses from the survey were interpreted using qualitative content analysis techniques and quantitative descriptive statistics, defining barriers relating to the design and construction process, as well as barriers relating to the products themselves. on the other hand, the feasibility of applying integrated façade concepts in several climates was evaluated through numerical calculations based on climate data and building scenarios simulated with specialised software (energyplus). the goal of this assessment was to check the theoretical feasibility of solar cooling façades as self-sufficient cooling units, matching solar availability at different orientations and locations, with the cooling requirements of a base scenario that consisted of a single office room in different climate contexts (prieto, knaack, auer, & klein, 2018). these scenarios considered several passive cooling strategies, such as shading, window-to-wall ratio, glazing type, and ventilation, as the first step of the assessment, obtaining optimised base scenarios for each orientation before integrating solar cooling technologies (prieto, knaack, klein, & auer, 2018). the assessment focused on five main solar electric and solar thermal technologies, based on widespread categorisations: thermoelectric, absorption, adsorption, solid desiccant, and liquid desiccant cooling (henning, 2007; prieto, knaack, et al., 2017a). given that cooling needs are the main driver of the research, the assessment focused exclusively on warm climates, ranging from temperate to extreme desertic and tropical environments. furthermore, discussion about design 013 journal of facade design & engineering volume 6 / number 3 / 2018 possibilities is constrained to the façade, leaving potential for further optimisation of cooling demands throughout building level strategies, which is outside the scope of the research project. coordina�on physical integra�on knowledge performance technical feasibility logis�cs responsibili�es durability & maintenance aesthe�cs cost �me others availability durability others knowledge process aesthe�cs informa�on technical complexity performance economy barriers for facade integration of solar collection technologies (pv & stc) barriers for widespread facade integration of building services te abs adssd ld solar cooling technologies climate contexts product integration barriers te: themoelectric cooling hot-arid (deser�c) tf: technical feasibility abs: absorp�on cooling hot-humid (tropical) pi: physical integra�on ads: adsorp�on cooling temperate-dry (mediterranean) d&m: durability & maintenance sd: solid desiccant + evap cooling temperate-humid (sub-tropical) p: performance ld: liquid desiccant + evap cooling a&a: aesthe�cs & availability a & a p d & m pi tf a&a p d&m p i tf a &a p d & m p i t f tf p i d& m p a &a tf pi d&m p a& a facade integration potential & c lim ate fe as ib ili ty so lar cool ing technologies se m i-i nt eg ra te d b uil din g pr oduc ts semi-integrated building products m odular plug & play system s integral building compone nts m od ul ar p lu g & p la y sy st em s fig. 2 chart of current possibilities and identified barriers for the development of solar cooling integrated façades 3 results and discussion the driving force behind the research project was the intention to test the limits of solar cooling integration in façades, showcasing current possibilities while identifying technical constraints and barriers to be overcome to achieve a widespread application of integrated façade concepts. in response to this task, an overview of the main outcomes of the research project is presented in fig. 2. this chart is regarded as a summarised panorama of the identified strengths and shortcomings of the assessed technologies in terms of façade integration; serving as a compass to guide further explorations and developments in the field. 014 journal of facade design & engineering volume 6 / number 3 / 2018 the chart comprises several types of barriers acting at different levels, around a core composed of the solar cooling technologies assessed throughout the research project. the widespread application of integrated solar cooling façades will therefore depend on successfully overcoming each particular set of barriers. the ring around the core of solar cooling technologies consists of the threshold for façade integration of these systems, under self-sufficient operation in different climate contexts. therefore, this ring shows distinct possibilities and constraints for each assessed technology, allowing them to be compared against each other. on the contrary, the barriers depicted outside of the circle are identified barriers for the development of solar cooling integrated concepts in general, applicable to all technologies; these consider barriers for further façade integration of solar collection technologies, namely photovoltaics and solar thermal collectors (lower left corner), and barriers for widespread façade integration of building services in general. the font size alludes to the perceived relevance of each barrier, based on how often it was mentioned by respondents of the survey (prieto, klein, et al., 2017). this comparative relevance only applies within each group of barriers separately, due to the nature of the assessment tool. 3.1 façade integration potential of solar cooling technologies the potential for façade integration of each solar technology is represented by the shaded area around the inner core of solar technologies, summarising the qualitative evaluation conduced in terms of their ability to overcome product related key issues derived from the façade integration of building services. technical feasibility, physical integration, durability & maintenance, performance, and aesthetics & availability were defined as these key issues, following analysis of the aforementioned expert survey conducted during the project. first, it is clear that, although some technologies fare better than others, no technology currently meets all criteria in all required aspects for the development of self-sufficient integrated façade products, so further research and development is needed, targeting specific aspects. table 1 shows the final recommendations for each technology in order to overcome current key barriers for façade integration. these were obtained from a qualitative evaluation that was presented in detail and discussed in a scientific article currently under review for publication (prieto, knaack, auer, & klein, n.d.). recommendations that are marked in bold within particular aspects were identified as having particular shortcomings in relation to each technology, advocating for more pressing efforts on those matters. further developments and exploration focused on the generation of integrated building products, or plug & play compact systems, are needed for all assessed technologies. at the same time, the fact that liquid desiccant cooling technologies have only been explored recently, as opposed to other thermally-driven systems about which there is more knowledge, puts them at a disadvantage in both the level of development and technical maturity, needing further research in most aspects to be up to date. for adsorption and solid desiccant cooling, the main current bottlenecks are related to the size of components and the generation of compact integrated systems. this also holds true for some compact desiccant units currently being developed, which still need to be field tested and thoroughly validated under different working conditions (finocchiaro, beccali, brano, & gentile, 2016; solarinvent, n.d.). finally, thermoelectric modules are regarded as a promising technology for the development of integral building components, and absorption-based compact units present interesting prospects for modular plug & play systems for façade integration. nevertheless, 015 journal of facade design & engineering volume 6 / number 3 / 2018 important performance barriers remain in the former, while further exploration of alternative working materials and testing of compact modular units are the main challenges for the latter. key product related issues for façade integration thermoelectric cooling absorption cooling adsorption cooling solid desiccant cooling liquid desiccant cooling technical feasibility prototype testing and experimental measurement of façade integrated concepts. further exploration and development of compact systems for façade integration. size reduction of components and exploration of alternative processes. development and validation of compact systems for façade integration. development and testing of compact units. physical integration standardize connections and components for development of architectural products. further exploration of plug & play integrated approaches to system design. exploration of integrated systems. exploration of integrated compact systems. exploration of alternative processes to simplify connections and increase compatibility. durability & maintenance testing of durability of te modules applied in building components over time and different climate conditions. exploration of noncorrosive working pairs and vacuum sealed compact systems. testing of compact adsorption systems over time and different climate conditions. testing of compact solid desiccant systems over time and different climate conditions. exploration and testing of alternative noncorrosive materials. performance increase cooling power of peltier modules, balancing adequate cop values. explore upscaled components. further development and testing of compact systems below 3kw. increase cop values of small scale systems. further development and testing of compact systems below 3kw for reliability of cop values. further development and testing of compact systems below 3kw for reliability of cop values. aesthetics & availability development of architectural products and integrated building components. development of plug & play systems for façade integration. size reduction of components for development of plug & play systems. size reduction and simplification of connections for development of decentralised ventilation systems. development and validation of compact integrated systems for future product development. table 1 recommendations for further development of solar cooling technologies for façade integration purposes 3.2 theoretical climate feasibility of selfsufficient solar cooling façades the inner ring shows the climate contexts where self-sufficient application could be theoretically feasible, based on the development of integrated concepts based on specific technologies. results from numerical calculations showed that the application of these concepts could be feasible on virtually all orientations, from every assessed location (prieto et al., 2018). even though these outcomes followed a theoretical approach, based on several assumptions and referential values, this fact is regarded as evidence that the application of self-sufficient solar cooling façades is not a far-fetched concept and could indeed be promoted following further technical developments to overcome previously identified barriers for façade integration. nevertheless, the self-sufficiency of these concepts is conditioned by important restrictions for façade design in most cases, seeking to optimise the solar input throughout lower panel tilt and bigger dimensions of photovoltaic/ thermal collector solar arrays in the building façade. these design constraints are more persistent in south and north façades, making east and west orientations more generally suited to solar cooling applications. 016 journal of facade design & engineering volume 6 / number 3 / 2018 with regard to the climatic application potential of the assessed technologies, there are clear research and development paths. although solar electric processes present advantages for façade integration, as previously discussed, their overall performance is a significant barrier, allowing for application in mild temperate dry climates, as a best case scenario, under medium to strong design constraints. solar thermal offers more possibilities, further research is recommended for the application of sorption-based concepts in temperate and hot-arid contexts with minor to medium constraints, depending on orientation and climate severity. finally, desiccant based units are recommended for warm-humid environments, both due to higher efficiencies associated with the technology, and particular general suitability to handle larger latent loads. in hong kong and singapore, west, east, and north applications are theoretically feasible with medium design constraints, but south applications are heavily hindered. the design constraints discussed refer to requirements for the optimisation of the solar array. however, basic design constraints remain for the application of all concepts, based on the collection technology needed to achieve the reference efficiencies used in the calculations. presently, building integrated solar thermal (bist) and photovoltaic (bipv) products such as coloured thermal collectors or transparent pv cells, especially designed to appeal to architects, have lower efficiencies than stateof-the-art basic systems with no ‘aesthetical considerations’. hence, further development of these technologies is needed to expand the general range of façade design possibilities. furthermore, the self-sufficiency of integrated concepts is conditioned to the use of passive strategies to lower cooling demands to a manageable amount. if these concepts are theoretically possible under important design constraints, their feasibility is downright impossible without being embedded within a climate responsive approach to façade design. 3.3 general barriers for façade integration of building services and solar collection technologies in general, barriers related to the overall process are perceived as more critical issues to solve than issues relating to the end product itself, to allow for widespread façade integration of building services. in particular, problems related to coordination of different professional areas are perceived to be the most relevant, which holds true in all three defined stages of façade development (design, production, and assembly). in terms of other frequently mentioned process related problems, lack of technical knowledge seems to be especially relevant during design and assembly stages, and less so during production. nonetheless, several logistical issues were identified in production and assembly stages, focusing on the lack of flexibility within the production chain, together with economic barriers during production for the construction of high quality components, aggravated by a common underestimation of cost projections during design phases. finally, other mentioned problems, which are minor in comparison, refer to undefined responsibilities and warranties throughout the overall process. in terms of product related problems, the physical integration of components seems to be the most relevant issue during both production and assembly stages. additionally, the inaccuracy of long term performance estimations and operational limitations of currently available systems were stated as relevant problems in the design stage. furthermore, other product related barriers that were identified, albeit with fewer mentions across all stages, are: the technical feasibility of integrated concepts; durability and maintenance; and aesthetics and lack of variety of available building services for integration. even though these problems do not seem to hold the same importance as 017 journal of facade design & engineering volume 6 / number 3 / 2018 process related aspects, they represent basic requirements and relevant challenges that must be overcome on the development path of components and systems for façade integration. regarding the particular integration of solar collection technologies in façades, economic issues were declared as the most pressing barrier to overcome. the cost of current systems, energy prices, and the lack of economic incentives were mentioned among key aspects within this barrier. secondly, grouped product related issues were perceived as a highly relevant barrier, based on the total amount of mentions. the disaggregated exploration of product related issues refers to performance, technical complexity of the systems, aesthetics, durability, and product availability. besides performance, aesthetics is a relevant perceived issue to be overcome, which makes sense considering the strong impact of solar collectors and pv panels on the external finish of the façade and thus, the outward appearance of buildings. 4 conclusions general results based on the assessment of current possibilities show that self-sufficient integrated façade concepts based on solar cooling technologies are still far from achieving widespread application. however, there is clear potential for further development of distinct integrated concepts, based on specific technologies, provided that we manage to overcome existing barriers and technical bottlenecks. the main recommendations for further research and development in the field follow the different types of barriers discussed in the paper, posing specific challenges for diverse disciplines. first, there is a need for further research on small-scale solar cooling systems and components, aiming to increase current efficiencies and simplify their operation. fundamental research on new working materials and alternative cooling processes derived from the main addressed principles would enhance the potential for application at a base technological level. furthermore, experimental and applied research at a system level is encouraged for all cooling technologies, in order to develop integrated building components, or modular compact plug & play units for façade integration purposes. fundamental research needs to be carried out by specialists, but the development of systems conceived for architectural integration would greatly benefit from a multidisciplinary approach, in order to tackle technology-specific challenges. similarly, the integration of solar collection technologies in façades needs to be further promoted. the technical optimisation of these systems is currently well on track, steadily achieving performance goals set by different technological roadmaps, whilst there is an increasing number of products conceived with ‘aesthetical considerations’ in mind. nonetheless, important economic restrictions remain in order to promote widespread application. recommended actions to mitigate this include the further manufacturing of cost-effective products for integration by system developers, technical improvements in electricity and heat storage technologies, and the exploration of new business models and subsidy schemes to incentivise their application. finally, further parallel actions are needed to push for the integration of building services and new technologies for high-performing façades in general. building technologies should be a central part of façade design education, striving for a basic understanding of technical aspects of building systems and façade requirements under an integrated design approach. moreover, the façade manufacturing industry should take the lead in the exploration of new production processes, simplifying logistical and coordination issues derived from the integration of several systems, 018 journal of facade design & engineering volume 6 / number 3 / 2018 under an integrated supply chain. furthermore, research on innovative business models for the management of façade systems could change the current value chain, generating new incentives for the development and application of high-performing façades under an environmentally conscious design approach. acknowledgements this paper is part of the ongoing phd research project titled coolfaçade: architectural integration of solar cooling technologies in the building envelope, developed within the architectural façades & products research group (af&p) of the department of architectural engineering + technology, delft university of technology (tu delft). the research project is being funded through a scholarship granted by conicyt, the national commission for scientific and technological research of chile (resolution n°7484/2013). references avesani, s. (2016). design of a solar façade solution with an integrated sorption collector for the systemic retrofit of the existing office buildings. (doctoral thesis). leopold-franzens-universität innsbruck, innsbruck, austria. balaras, c. a., grossman, g., henning, h.-m., infante ferreira, c. a., podesser, e., wang, l., & wiemken, e. (2007). solar air conditioning in europe—an overview. renewable and sustainable energy reviews, 11(2), 299-314. doi: 10.1016/j.rser.2005.02.003 bp (2016). bp energy outlook, 2016 edition. london, united kingdom. doe/eia (2016). international energy outlook 2016. washington d.c., usa: us energy information administration, us department of energy. finocchiaro, p., beccali, m., brano, v. l., & gentile, v. (2016). monitoring results and energy performances evaluation of freescoo solar dec systems. energy procedia, 91, 752-758. doi: 10.1016/j.egypro.2016.06.240 goetzler, w., zogg, r., young, j., & johnson, c. (2014). energy savings potential and rd&d opportunities for non-vapor-compression hvac technologies. usa: u.s. department of energy, office of energy efficiency and renewable energy, building technologies office. henning, h.-m. (2007). solar assisted air conditioning of buildings – an overview. applied thermal engineering, 27(10), 1734-1749. doi: http://dx.doi.org/10.1016/j.applthermaleng.2006.07.021 henning, h.-m., & döll, j. (2012). solar systems for heating and cooling of buildings. energy procedia, 30, 633-653. doi: 10.1016/j. egypro.2012.11.073 ibañez-puy, m., martín-gómez, c., bermejo-busto, j., sacristán, j. a., & ibañez-puy, e. (2018). ventilated active thermoelectric envelope (vate): analysis of its energy performance when integrated in a building. energy and buildings, 158, 1586-1592. doi: 10.1016/j.enbuild.2017.11.037 jochem, e., & schade, w. (2009). 2-degree scenario for europe policies and impacts. adam: adaptation and mitigation strategies: supporting european climate policy. karlsruhe: fraunhofer institute for systems and innovation research (fraunhofer-isi). montagnino, f. m. (2017). solar cooling technologies. design, application and performance of existing projects. solar energy. doi: 10.1016/j.solener.2017.01.033 oecd/iea. (2015). energy and climate change / world energy outlook special report. paris, france: iea international energy agency. prieto, a., klein, t., knaack, u., & auer, t. (2017). main perceived barriers for the development of building service integrated façades: results from an exploratory expert survey. journal of building engineering, 13, 96-106. doi: 10.1016/j.jobe.2017.07.008 prieto, a., knaack, u., auer, t., & klein, t. (2017a). solar coolfaçades framework for the integration of solar cooling technologies in the building envelope. energy, 137, 353-368. doi: 10.1016/j.energy.2017.04.141. prieto, a., knaack, u., auer, t., & klein, t. (2017b). solar façades – main barriers for widespread façade integration of solar technologies. journal of façade design and engineering, 5(1), 51-62. doi: 10.7480/jfde.2017.1.1398 prieto, a., knaack, u., auer, t., & klein, t. (2018). feasibility study of self-sufficient solar cooling façade applications in different warm regions. energies, 11(6), 1475. doi: 10.3390/en11061475 prieto, a., knaack, u., klein, t., & auer, t. (2018). passive cooling & climate responsive façade design exploring the limits of passive cooling strategies to improve the performance of commercial buildings in warm climates. energy and buildings, 175, 30-47. doi: 10.1016/j.enbuild.2018.06.016. prieto, a., knaack, u., auer, t., & klein, t. (n.d.). coolfacade: state-of-the-art review and evaluation of solar cooling technologies on their potential for façade integration. renewable & sustainable energy reviews, (under review). qi, c. (2006). office building energy saving potential in singapore. (master’s thesis). national university of singapore (nus), singapore. santamouris, m. (2016). cooling the buildings – past, present and future. energy and buildings, 128, 617-638. doi: 10.1016/j. enbuild.2016.07.034 solarinvent. (n.d.). freescoo / solarinvent s.r.l. http://www.freescoo.com. (accessed on april 11th 2018). xu, x., & van dessel, s. (2008). evaluation of an active building envelope window-system. building and environment, 43(11), 17851791. doi: 10.1016/j.buildenv.2007.10.013 from city’s station to station city 101 journal of facade design & engineering volume 6 / number 3 / 2018 opportunities and challenges for performance prediction of dynamic complex fenestration systems (cfs) giuseppe de michele1*, roel loonen2, hemshikha saini2, fabio favoino3, stefano avesani4, luca papaiz5, andrea gasparella1 * corresponding author 1 free university of bolzano/bozen, faculty of science and technology, giuseppe.demichele@eurac.edu 2 eindhoven university of technology, department of the built environment 3 politecnico di torino, tebe research group, department of energy 4 eurac research, institute for renewable energy 5 glass advisor abstract this article presents an overview of possibilities and points of attention for modelling the performance of dynamic cfs in building performance simulation software. following a detailed analysis of the unique requirements that are associated with modelling of cfs, a comparative study of the capabilities in different software implementations is presented. in addition, we present on overview of state-of-the-art approaches to obtain the necessary bi-directional scattering distribution functions (bsdf), involving experimental characterisation, databases, and component-level ray-tracing approaches. the second part of the paper provides a detailed discussion of a case study of a high reflective lamella system. this case study complements the review with hands-on information from a practical example and highlights the importance of developing models at the right level of complexity, taking into account the type of questions that the simulation intends to answer and the required accuracy level to do so. keywords complex fenestration systems (cfs), building performance simulation, bi-directional scattering distribution functions, reflective lamella, modelling complexity doi 10.7480/jfde.2018.3.2531 102 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction contemporary building facades are expected to be increasingly multi-functional. they should not only provide shelter and protection, but often simultaneously also take care of energy conservation, daylight admission, glare prevention, and mitigation of overheating. in response to these highperformance requirements, a growing interest in façades with light redirecting elements, or layers with light scattering properties, can be observed (appelfeld, mcneil, & svendsen, 2012; gong, kostro, motamed, & schueler, 2016; saini, loonen, & hensen, 2018; vera, uribe, & bustamante, 2017). examples include venetian blinds, glass frits, prismatic films, etc. unlike conventional glazing, these systems usually exhibit non-specular transmission. moreover, their transmission properties idiosyncratically depend on the position of the sun or wavelength of the incoming radiation. to distinguish these fenestration systems from specular glazing types, they are often referred to as complex fenestration systems (cfs) (fig. 1). a b c fig. 1 a) adaptive fritted glass, adaptive building initiative (http://www.hoberman.com/abi.html accessed in may 2018); b) igu cavity integrated solar shading lamellas (https://performanceglass.co.uk/pellini-blinds/venetian-blinds/ accessed in may 2018); c) prismatic pcm system, glassx (http://glassx.ch/index.php accessed in may 2018) the product development of such innovative façade systems can greatly benefit from inputs obtained using building performance simulation tools (loonen, singaravel, trčka, cóstola, & hensen, 2014). such computational tools can also aid in the adoption of cfs in new buildings, by providing opportunities for informed design decision-making from the planning phase itself. a number of requirements are associated with modelling and simulation of cfs, deriving from their physical characteristics: – the optical properties of cfs tend to have a high solar angle dependency. – many cfs involve scattering/diffusing layers that can be difficult to characterise. some cfs involve materials (e.g. phase changing materials, pcm) that have a special impact on the heat transfer characteristics of the façade component. – the three-dimensional shape of optical elements (e.g. lamellas) in cfs can introduce unconventional physical effects. – cfs are often part of an adaptive façade system, i.e. they can be controlled by varying the component’s physical properties to meet different performance requirements. in such situations, it is important to take appropriate façade operation strategies into account in the models (loonen, favoino, hensen, & overend, 2017). 103 journal of facade design & engineering volume 6 / number 3 / 2018 because of these considerations, standard simulation workflows might not always provide sufficient flexibility to carry out the task at hand, leading to the need for dedicated models at a higher resolution. when intending to use or develop such detailed models, one should be aware of potential pitfalls and other points of attention. however, in academic literature and software manuals, there is very little attention given to best-practice advice and practical considerations for performance prediction of buildings with cfs. the objective of this article is, therefore, to compile and present an overview of possibilities and points of attention for modelling the performance of dynamic cfs in building performance simulation tools. following a detailed analysis of the unique requirements that are associated with modelling of cfs, a comparative study of the capabilities in different software implementations is presented. in addition, we provide an overview of state-of-the-art approaches to obtain the necessary bi-directional scattering distribution functions (bsdf) for quantification of optical properties of cfs, involving experimental characterisation, databases, and component-level ray-tracing approaches. a case study of a high reflective lamella system is discussed in detail, to complement the review with hands-on information from a practical example. 2 advanced simulation models for cfs 2.1 requirements due to their intrinsic properties, cfs influence the characteristics of transmitted solar radiation (visible and non-visible). as a result, the main challenges for accurate performance prediction of cfs using building performance simulation (bps) tools lies in: – achieving an appropriate representation of two-dimensional angular dependency (on solar geometry and/or control of the components) of optical properties of the fenestration system, such as visible transmittance; – achieving an appropriate representation of two-dimensional angular dependency (on solar geometry and/or control of the components) of thermal properties of the fenestration system, such as solar heat gain coefficient; – implementing a control logic (either intrinsic or extrinsic) during the simulation run-time in the case when cfs are part of an adaptive façade system. this is because when façade properties vary over time, the amount of solar radiation that enters the zone also varies, leading to a different thermal response of the space; – taking into account the interactions between both visual and thermal physical domains to predict the performance of such fenestration elements in an appropriate way. 2.2 definition of bsdf data the most commonly-used way of representing the two-dimensional angular dependency of solar properties (transmission and reflection) of cfs is via bi-directional scattering distribution functions (bsdf). the bsdf method was proposed by klems (1994) to calculate solar transmission of multilayered cfs through matrix multiplication. in this method, the front and back hemisphere of the cfs layer is discretised into 145 patches; for each of these, optical properties are specified depending 104 journal of facade design & engineering volume 6 / number 3 / 2018 on azimuth and altitude angles. the bsdf dataset-containing file describes the transmission (bidirectional transmission distribution function, btdf) and reflection (bi-directional reflectance distribution function, brdf) properties of a complex glazing system by a 145 x 145 matrix, according to incident and outgoing angles using klems’ angle basis (fig. 2). fig. 2 a bsdf file describes the directional transmission and reflection for many different combinations of ingoing and outgoing directions. (image by christian kohler, lbnl) 2.3 review of modelling of cfs in bps tools generally, bps tools only offer the possibility of considering one domain at a time, either thermal or visual. as far as bps tools that allow the evaluation of whole building energy performance by means of thermal networks (i.e. energy plus, trnsys, esp-r, etc.) are concerned, the main practice, while considering the thermal and optical properties of a complex fenestration system, is to use calculation algorithms according to iso 15099 for the layer-by-layer heat transfer. in the iso 15099 standard, the analytical algorithms for the optical modelling are restricted to simplified models, developed for planar or curved blinds that behave as ideal diffusers. for this reason, the algorithm relative to the optical modelling has been fully replaced with bsdf data in most of these bps tools: 1 energyplus: since version 7.2, the bsdf functionality has been part of energyplus as one of the optical representations of fenestration systems. this implementation relies on the strong integration between energyplus and berkeley lab window (or window) (berkeley lab, 2007) software that allows the export/import of .idf files. the construction:complexfenestrationstate (us department of energy, 2010) can be controlled during simulation run-time, by making use of the ems (energy management system) functionality. additionally, energyplus offers the possibility to define specific external schedules for solar transmission and solar absorption of cfs. in recent work, hoffmann, lee, & clavero (2014) used this approach for pre-calculating the two schedules, for different shading systems, by using radiance. the bsdf function is also integrated in comfen, a user-friendly interface to the energyplus/radiance engines. 105 journal of facade design & engineering volume 6 / number 3 / 2018 2 esp-r: esp-r is the only tool with an in-house developed model for complex fenestration systems, based on the agsl shading model; not bsdf. it is aptly named the cfs functionality (lomanowski & wright, 2012)the complex fenestration construction (cfc. alternate property sets for different fenestration/shading states can easily be changed using tmc control or the bcvtb-esp-r control functionality (hoes, loonen, trčka, & hensen, 2012). an alternative approach called the “black-box model” was developed by kuhn, herkel, frontini, strachan, & kokogiannakis (2011), who proposed a new methodology for the evaluation of solar transmission of the complex system. this approach simplifies the modelling part and does not require many measurements. the model takes as input measured or analytically derived total solar energy transmittance (gtot), total solar transmittance (te tot), and total solar reflectance (re tot) for different angles of incidence. then, the model introduces the radiant and convective effect of solar heat gains into the energy balance of the building through a two-layer approach. other applications of the model are reported in (frontini, kuhn, herkel, strachan, & kokogiannakis, 2009). 3 fener: fener is a dedicated tool that was developed to ease the modelling and simulation of cfs systems (bueno, wienold, katsifaraki, & kuhn, 2015). it combines radiance with a reduced-order rc network approach for thermal calculations on a time step basis. one of the specific strengths of fener is its flexibility for implementing shading control algorithms, based on, for example, daylighting variables such as illuminance and glare, thermal variables such as indoor temperature and energy load, or weather variables such as wind and solar radiation. 4 trsnsys: a new trnsys type for daylight performance prediction with bsdf systems has recently been developed at eurac research (typedlt) (de michele, filippi oberegger, & baglivo, 2015). these daylight predictions can be coupled with the multi-zone thermal model (type 56). since version 18, bi-directional thermal properties, according to iso 15099, can also be calculated in trnsys’s thermal building model (hiller & schöttl, 2014). the main and most accurate bps tool that allows evaluation of the performance of the built environment in the visual physical domain (adopted as a calculation engine in many interface softwares) is radiance (ward, 1989). in particular, the multi-phase matrix-based methods (e.g. threephase and five-phase) are useful in the present context, because of the possibility for annual evaluations (subramaniam, 2017). all the matrix methods use common input data to describe the light passing through the cfs, which is the bsdf. differently from the thermal analysis, in the daylighting analysis it is also possible to employ high resolution bsdf (tensor tree resolution) mainly for glare analysis and the calculation of the annual sunlight exposure (ase) index (ies, 2012). 3 sources of bsdf data 3.1 complex glazing database bsdf files (in xml format) for a variety of window materials and daylighting systems can be obtained from the lbnl complex glazing database (cgdb). this resource contains more than 100 systems, such as shading devices and materials (e.g., venetian blinds, roller shades, drapes, cellular shades, shade fabrics, etc.), light redirecting materials (e.g., prismatic films, etc.) and scattering glazing (e.g., diffuse glass, glazing frits, decorative glass, etc.). with this database, customised multilayer glazing systems for different configurations of a façade system (e.g. shades up, down, and/or 106 journal of facade design & engineering volume 6 / number 3 / 2018 tilted) can be created using the berkeley lab window program. from this program, data files can be exported for use in a number of whole building performance simulation programs. especially when innovative fenestration systems are considered, it may happen that the optical behaviour of the fenestration system is not yet available in the cgdb. in this scenario, two options are available to obtain the necessary bi-directional optical data: i) experimental characterisation or ii) modelling/simulation. 3.2 measurements photometric equipment, such as a goniophotometer, is needed to characterise the angular transmission and reflection properties. this equipment is available in only a few research labs around the world. experimental characterisation is practically possible for small-scale cfs with homogenous scattering properties. 3.3 simulations for macro-scale cfs with complex geometry e.g. louvres and specular blinds, the incident light source of photometric equipment cannot sufficiently take into account variations in cfs. bsdf files for such systems can be created by applying radiosity or ray-tracing algorithms on a geometrical model of the shading system or daylighting device in window or tracepro/genbsdf, respectively. for a simplified geometrical model and lambertian systems, window can be used, while for complex geometries and/or non-lambertian surfaces, tracepro or genbsdf should be used. tracepro is a commercial software with a 3d cad-based graphical user interface for design and analysis of optical and illumination systems. genbsdf, part of the radiance daylight simulation suite, is a free and open source tool that generates a bsdf file from a radiance or mgf scene description (mcneil, jonsson, appelfeld, ward, & lee, 2013). 3.4 combine measurement and simulations it is possible to extend the simulation method with detailed measured data of the shading material. for this specific application, the opaque material reflectance can be characterised either through a spectral curve (fig. 3 a) or brdf (figs. 3 b and c). the spectral curve method is a hemispherical measure of the material reflection, which is reliable for materials that behave or can be assumed as lambertian; while the brdf is suitable for materials whose reflection presents complex behaviour (e.g. high reflective or retro-reflective material) and an angular distribution is required. in order to use the spectral data within window and radiance, it has to be integrated on the solar and visible ranges. the integrated values are directly set in the window material library, while in radiance, they are adopted by using the plastic material. then, the material has to be applied to a geometry that represents the shading device. regarding the brdf, such data can be used in radiance by applying the bsdf material to a 3d geometry using genbsdf to generate an xml file of the shading device. this xml file can then be imported in window as a shading layer where it can be directly joined to the glass layers in order to generate the bsdf of the whole system. 107 journal of facade design & engineering volume 6 / number 3 / 2018 a b c fig. 3 a) is an example of spectral reflection curves of an opaque white material; b) and c) are two measured brdf respectively on klems and tensor-tree base. 4 performances evaluation of highly reflective lamellas in this section, the modelling and simulation of a cfs is presented. the shading system is a curved lamella characterised by a highly reflective coating. in order to show the impact of correct modelling of the shading system on the energy and daylighting results, two models of the same shading system are compared. 4.1 modelling of the shading system the shading device is a curved commercial blind produced by pellini screenline®. the blind is coated with a highly reflective layer. two fixed shading configurations with 15° and 30° blind tilt angles have been considered in these simulations. geometrical dimensions are reported in fig. 4 and table 1. blind width 16 mm blind thickness 0.2 mm pitch 12 mm blind tilting 15 30 ° raise 1 mm fig. 4 3d geometry of blind table 1 blind dimensions 108 journal of facade design & engineering volume 6 / number 3 / 2018 the configuration of the two blinds has been modelled using both simplified and detailed approaches. simplified approach. the shading model was completely generated within the berkeley lab window software. the coating behaviour was assumed as lambertian in order to use the material definition of window. in particular, the measured spectral data of the coating have been imported into window as shade materials. table 2 shows the integrated reflectance values of the coating and the emissivity used as material for the blinds. the blind geometry has been precisely reproduced using the built-in functionality of window to generate custom horizontal venetian blinds; fig. 5 shows the lamellas’ definition within the shading layer library. conductance 100 w/m k thickness 0.2 mm integrated values solar reflectance front 0.901 solar reflectance back 0.840 visual reflectance front 0.959 visual reflectance back 0.823 emissivity front 0.150 emissivity back 0.450 fig. 5 blinds geometry in window table 2 coating characterisation within window detailed approach. the shading modelling is performed using angular measured data and radiance. the highly reflective coating has been characterised by means of measured angular reflectance values (brdf) for visible and near infrared wavelengths. the brdfs were applied as material to the 3d geometry of the blinds within radiance. the function genbsdf was then used to describe the geometry of the shade and its complex coating in the form of bsdf, with klems resolution, for solar and visible spectrum. additionally, a third bsdf for the infrared (ir) wavelength was created using the plastic material of radiance and assuming as coefficient of reflection the complement to 1 of the emissivity front and back in table 2. this last step was required in order to evaluate the hemispherical emissivity front and back and the infrared transmission of the system. finally, an xml file that collects all the previous information was generated and imported into window. fig. 6 workflow of the detailed modelling procedure of the shading system 109 journal of facade design & engineering volume 6 / number 3 / 2018 the shade models were then coupled with the glass layers within window in order to compose the complete system (fig. 6). the fenestration consists of a triple-pane insulating glazing (8-29-6-168.8). glass layers 1 and 2 are monolithic float glass, while layer 3 is laminated float glass; their sizes and thermal properties are listed in table 3. on faces 3 and 5, a low-emissivity coating is placed. the cavities are filled with a gas mixture containing 90% argon and 10% air. the thermal and optical characteristics of the glazing system are: u-value 0.72 w/m2 k, shgc 0.527 and visible transmission 0.6. the shading system is located in the first cavity, 29 mm. no window frame is considered. property glass1 glass2 glass3 thickness (mm) 8.0 6.0 8.8 solar transmittance 0.797 0.618 0.573 solar reflectance front 0.074 0.247 0.245 solar reflectance back 0.074 0.186 0.137 visual transmittance 0.892 0.893 0.882 visual reflectance front 0.082 0.044 0.043 visual reflectance back 0.082 0.048 0.048 ir transmittance 0.000 0.000 0.000 emissivity front 0.837 0.037 0.037 emissivity back 0.837 0.837 0.837 conductivity 1.000 1.000 0.757 table 3 thermal and optical properties of glazing panes 4.2 impact at façade level: simple procedure against advanced characterisation fig. 7 bsdfs visible transmission front for the fenestration system with blind titled at 15°. the graphs show the 145 outgoing values of visible transmission for an incident ray normal to the system and at 30° of elevation (yellow patch on the first left image). a first comparison can be done by observing the differences between the angular visible transmission of the cfs in fig. 7. in particular, the case with blinds tilted at 15° with an incident ray of azimuth angle 0° and elevation angle of 30° is shown. the difference between the models is evident; the detailed approach is able to reproduce the inter-reflections that occur between the blinds due to the high reflectivity of the coating. the angular transmission values are higher, green and 110 journal of facade design & engineering volume 6 / number 3 / 2018 yellow patches in the right graph of fig. 7, compared to the simplified model, which underestimates the light transmission. in fact, the hemispherical front transmission values are very different: 35% for the detailed model against 17.6% for the simplified model. similar behaviour has been found for other incident angles and blind configurations. another relevant consideration can be made, based on the angular solar heat gain coefficient (shgc). fig. 8 shows the shgc values for 145 incident angles, for the cases analysed. looking at the polar graphs, it can be observed that in general the detailed model has greater values of shgc in the upper part of the hemisphere compared to the simple model. therefore, the simple model will underestimate the solar gain in the thermal simulation. simplified model detailed model 1 5 ° ti lt a n g le 3 0 ° ti lt a n g le fig. 8 angular dependent shgc 4.3 impact on reference room: simple procedure against advanced characterisation the previous paragraph has reported the differences between the simplified and detailed approach at the façade level. in this part, the two modelling approaches for cfs have been evaluated at a reference room level, using trnsys18 with the new cfs module based on iso 15099 for the energy part, and the daylighting coefficient method of radiance for the daylighting analysis. the model used is a single zone of dimensions 3.3 m x 8 m x 2.7 m, located in bolzano (46.467° n 111 journal of facade design & engineering volume 6 / number 3 / 2018 and 11.33° e), italy. the zone consists of a south facing external façade with a window-to-wall ratio (wwr) of 50% (fig. 9). in order to underline the effect of the façade on the energy balance, all the surfaces are assumed adiabatic except for the south-façade, and internal gains are not considered. fig. 9 3d model and main dimensions of the zone the opaque element transmittance and reflectance are reported in table 4. the set point for cooling is 20 °c, while for heating it is 26 °c. the heating and cooling system has unlimited power and is always on. infiltration is always set to 0.40 ach. the daylight availability is calculated over a grid located at 0.8 m from the floor and with a resolution of 0.5x0.5 m. surface u [w/m2 k] reflectance wall 0.51 0.50 roof/ceiling 0.32 0.85 floor 0.39 0.25 ground 0.20 table 4 thermal and optical characteristics of the opaque elements results are reported for the two cases analysed, blinds always deployed at 15° and at 30° tilt angle. the comparison between simplified and detailed modelling is done on the annual ideal energy load and annual daylight distribution. a / 15° tilt angle b / 30° tilt angle heating [kwh/m2y] cooling [kwh/m2y] heating [kwh/m2y] cooling [kwh/m2y] simplified 13.1 10.1 simplified 17.1 7.4 detailed 7.1 21.1 detailed 8.6 12.6 difference -46% 108% difference -50% 69% table 5 ideal heating and cooling demand for shadings deployed at 15° (a) and 30° (b) 112 journal of facade design & engineering volume 6 / number 3 / 2018 table 5 shows ideal heating and cooling load for the two blind tilt angles. the differences between simplified and detailed are relevant in both cases, especially for the cooling demand with blinds at 15°, where the difference rises to 108% (table 5a). the detailed model considers the effective reflection of the material blind and then it accounts for a higher solar gain as also shown in the images in fig. 8. both in summer and winter seasons, the solar gains through the detailed model are higher than the simplified model. in fact, the heating demand is overestimated by 46%. similar trends are found for the lamellas at 30°. the cooling difference is reduced to 70% since the more closed position of the blinds reflect more energy when the sun is around the solstice period. the difference in heating load is slightly higher (50%) because the detailed model accounts for the inter-reflected direct radiation, which is more relevant for the 30° tilt angle case. a / 15° tilt angle b / 30° tilt angle simplified detailed difference simplified detailed difference udi-n 19.1 17.5 -8% udi-n 24.4 19 -22% udi-s 26.5 24.1 -9% udi-s 28.3 23.7 -16% udi-a 48.2 46.9 -3% udi-a 44.6 49.2 10% udi-x 6.2 11.5 85% udi-x 2.7 8.2 204% da_300 54.5 58.5 7% da_300 47.3 57.3 21% sda_300/50 58.3 62.5 7% sda_300/50 62.8 71.8 14% table 6 daylighting indicators for shadings deployed at 15° (a) and 30° (b). values are averaged over the sensor grid. simplified model detailed model 1 5 ° ti lt a n g le 3 0 ° ti lt a n g le fig. 10 udi-exceed (illuminance > 3000 lux) distribution over the sensors grid 113 journal of facade design & engineering volume 6 / number 3 / 2018 table 6 shows the average sensor grid values of selected annual daylighting indices for the two cases. the indicators used for the comparison are useful daylight illuminance (udi) (mardaljevic & nabil, 2005), daylight autonomy (da) and spatial daylight autonomy (sda) (reinhart, mardaljevic, & rogers, 2006). for the latter two indices, the illuminance threshold was set to 300 lux. regarding the daylighting results, the main differences were found for the case with 30° tilting. this is explained by the fact that the more the blinds are closed, the greater is the influence of the inter-reflection caused by the specularity of the blind coating. in particular, the simple model underestimates the percentage of hours in which the illuminance values exceed the 3000 lux (udi-x) of the 204%. additionally, in the case of 15° tilt, the greater difference is for the udi-x, 85%. the main differences are related to the udi-x, also because this index is primarily evaluated on the sensors close to the window (fig. 10), and then close to influence of the shading system. in general, the trend agrees with the expectation; the detailed model reduces the indices of illuminance below a threshold value (i.e. udi-n and udi-s, illuminance values below 100 lux, and between 100 and 300 lux respectively) from 8% to 22% for the cases with 15° tilt and 30° tilt respectively, while increasing the indicators of illuminance higher than a threshold (i.e. da, sda), from 7% to 21%. this result clearly shows that the detailed model allows for a major quantity of light entering the space. simplified model detailed model 1 5 ° ti lt a n g le 3 0 ° ti lt a n g le fig. 11 udi-autonomous (300 lux < illuminance < 3000 lux) distribution over the sensors grid 114 journal of facade design & engineering volume 6 / number 3 / 2018 another important consideration concerns the udi-a (i.e. illuminance level between 300 lux and 3000 lux). in both cases, the difference related to this index is small (-3% and 10%), but looking at the udi-a distribution over the grid in fig. 11, the distribution of the highest udi-a values, for the detailed model, shifts almost one metre inside the room. this means that with the detailed model the light penetrates deeper into the room, and that the simple model would not be able to highlight this effect. 5 conclusions this article has reviewed several challenges and opportunities of modelling the performance of cfs in building simulation software. first, the unique simulation requirements were identified in terms of physical phenomena and the intended purpose of the simulation task. then, a detailed overview of cfs models in state-of-the-art bps tools was given, complemented by a description of bi-directional scattering distribution functions, and how such input data can be obtained. the second part of the article has demonstrated these concepts in a practical case study with a highly-reflective lamella system. the main take-home message of the case study relates to the complexity level of the simulation models, showing that this should always be carefully chosen with respect to the characteristics of fenestration system and the type of questions the simulation study should address. acknowledgements this study has been developed in the framework of the research activities of the project facecamp n. itat1039, funded by european regional development fund and interreg ita aut programme. thanks to pellini spa for the support providing technical information and detailed measurements of their shading system. references appelfeld, d., mcneil, a., & svendsen, s. (2012). an hourly based performance comparison of an integrated micro-structural perforated shading screen with standard shading systems. energy and buildings, 50, pp.166–176. http://doi.org/10.1016/j. enbuild.2012.03.038 berkeley lab. (2007). lbnl window 7. retrieved from http://windows.lbl.gov/software/window/window.html bueno, b., wienold, j., katsifaraki, a., & kuhn, t. e. (2015). fener: a radiance-based modelling approach to assess the thermal and daylighting performance of complex fenestration systems in office spaces. energy and buildings, 94, pp.10–20. http://doi. org/10.1016/j.enbuild.2015.02.038 de michele, g., filippi oberegger, u., & baglivo, l. (2015). coupling dynamic energy and daylighting simulations for complex fenestration systems. in building simulation applications 2015 2nd ibpsa-italy conference. bolzano-bozen. frontini, f., kuhn, t. e., herkel, s., strachan, p., & kokogiannakis, g. (2009). implementation and application of a new bi-directional solar modelling method for complex facades within the esp-r building simulation program. eleventh international ibpsa conference,pp.936–943. retrieved from http://ibpsa.org/proceedings/bs2009/bs09_0928_935.pdf gong, j., kostro, a., motamed, a., & schueler, a. (2016). potential advantages of a multifunctional complex fenestration system with embedded micro-mirrors in daylighting. solar energy, 139, pp.412–425. http://doi.org/10.1016/j.solener.2016.10.012 hiller, m., & schöttl, p. (2014). modellierung komplexer verglasungssysteme in trnsys. bausim pp. 387–394. hoes, p., loonen, r. c. g. m., trčka, m., & hensen, j. l. m. (2012). performance prediction of advanced building controls in the design phase using esp-r, bcvtb and matlab. in proceedings of building simulation and optimization. loughborough, uk. hoffmann, s., lee, e. s., & clavero, c. (2014). examination of the technical potential of near-infrared switching thermochromic windows for commercial building applications. solar energy materials and solar cells, 123, pp.65–80. http://doi.org/10.1016/j. solmat.2013.12.017 ies, i. e. s. (2012). approved method: ies spatial daylight autonomy (sda) and annual sunlight exposure (ase). iso 15099:2003: (2003). thermal performance of windows, doors and shading devices — detailed calculations. klems, j. h. (1994). a new method for predicting the solar heat gain of complex fenestration systems ii. detailed description of the matrix layer calculation. ashrae transactions, 100(1), pp.1065–1072. http://doi.org/citeulike-article-id:10521068 kuhn, t. e., herkel, s., frontini, f., strachan, p., & kokogiannakis, g. (2011). solar control: a general method for modelling of solar gains through complex facades in building simulation programs. energy and buildings, 43(1), pp.19–27. http://doi. org/10.1016/j.enbuild.2010.07.015 115 journal of facade design & engineering volume 6 / number 3 / 2018 lomanowski, b. a., & wright, j. l. (2012). the complex fenestration construction: a practical approach for modelling windows with shading devices in esp-r. journal of building performance simulation, 5(january), pp.185–198. http://doi.org/10.1080/19 401493.2011.552735 loonen, r. c. g. m., favoino, f., hensen, j. l. m., & overend, m. (2017). review of current status, requirements and opportunities for building performance simulation of adaptive facades†. journal of building performance simulation, 10(2). http://doi.org/10.108 0/19401493.2016.1152303 loonen, r. c. g. m., singaravel, s., trčka, m., cóstola, d., & hensen, j. l. m. (2014). simulation-based support for product development of innovative building envelope components. automation in construction, 45. http://doi.org/10.1016/j.autcon.2014.05.008 mardaljevic, j., & nabil, a. (2005). the useful daylight illuminance paradigm: a replacement for daylight factors. lux europa, berlin, pp.169–174. mcneil, a., jonsson, c. j., appelfeld, d., ward, g., & lee, e. s. (2013). a validation of a ray-tracing tool used to generate bi-directional scattering distribution functions for complex fenestration systems. solar energy, 98(pc), pp.404–414. http://doi.org/10.1016/j. solener.2013.09.032 reinhart, c. f., mardaljevic, j., & rogers, z. (2006). dynamic daylight performance metrics for sustainable building design. leukos journal of illuminating engineering society of north america, 3(1), pp.7–31. http://doi.org/10.1582/leukos.2006.03.01.001 saini, h., loonen, r. c. g. m., & hensen, j. l. m. (2018). simulation-based performance prediction of an energy-harvesting façade system with selective daylight transmission. in proceedings of icae2018 international congress on architectural envelopes. san sebastian, spain. subramaniam, s. (2017). daylighting simulations with radiance using matrix-based methods. retrieved from https://radiance-online.org/learning/tutorials/matrix-based-methods us department of energy. (2010). energyplus engineering reference: the reference to energyplus calculations. us department of energy, (c), pp.1–847. http://doi.org/citeulike-article-id:10579266 vera, s., uribe, d., & bustamante, w. (2017). optimization of a fixed exterior complex fenestration system considering visual comfort and energy performance criteria. building and environment, 113(june), pp.11–14. http://doi.org/10.1016/j.buildenv.2016.07.027 ward, j, g. (1989). the radiance lighting simulation and rendering system. 21st annual conference on computer graphics and interactive techniques, pp.459–472. retrieved from http://radsite.lbl.gov/radiance/papers/sg94.1/siggraph1994a.pdf from city’s station to station city 089 journal of facade design & engineering volume 7 / number 1 / 2019 bio-inspired transparent microfluidic platform as transformable networks for solar modulation mark e alston1, uta pottgiesser2 ,ulrich knaack3 1 school of engineering, architecture and the built environment, university of nottingham, mark.alston@nottingham.ac.uk 2 professor of interior architecture – faculty of design sciences, university of antwerp 3 professor of design of construction department of architectural engineering + technology, tu delft abstract the glazed envelopes on buildings play a major role in operational energy consumption as they define the boundary conditions between climate and thermal comfort. such a façade is viewed as an uncontrolled load that sets the operational performance requirements for artificial lighting and aircooling mechanical systems. this is in contrast to nature, which has evolved materials with the ability to learn and adapt to a micro-environment through self-regulation using materials that are multifunctional, formed by chemical composition in response to solar load. leaf vasculature formations are of particular interest to this paper. through leaf maximisation of daylight capture, the total leaf area density and angular distribution of leaf surfaces define the tree structure. this paper will define an approach to simulate nature to advance a microfluidic platform as a dynamic nir absorber for solar modulation: a transformable network of multi-microchannel geometry matrix structures for autonomous transparent surfaces, for real time flow management of conductivity. this is realised through active volumetric flows within a capillary network of circulation fluidics within it, through it, and out of it for energy capture and storage, the cycle of which is determined through precise management of heat flow transport within a material. this advances transparent façades into an energy system for heat load modulation nested to climate and solar exposure, which is demonstrated in this paper. keywords microfluidic, thermal transport, absorber, solar, geometry matrix, bio-ins doi 10.7480/jfde.2019.1.2785 090 journal of facade design & engineering volume 7 / number 1 / 2019 1 introduction transparent glass façades play a major role in operational energy consumption, as they define the boundary between climate and human comfort. this determines operational performance requirements in setting mechanical systems (air-conditioning, artificial lighting demands) in the consumption of primary energy. the conductance of opaque materials for solar modulation is well researched and advanced analytically, and such materials are well established as energy collectors. these materials outperform optic materials and there is nothing of note to add in terms of this research. optic materials’ performances, however, are energetically weak in terms of visible transmittance, and have limited solar energy modulation efficiency and high transmission temperature. as these façades suffer from long-wave solar radiation inputs leading to the overheating of internal spaces and increased cooling demand loads, façades are viewed in terms of transmission loss. this is due to a resulting inflow heat load that is dependent on the overall solar transmittance ‘g’, including any solar shading. the technical challenges in providing thermal and visual comfort in buildings with large expanses of floor to ceiling glazing are significant. large areas of glazed façades orientated to the east, west, or south (or north if in the southern hemisphere) (hens, 2011) will experience the thermal impact of overheating due to sunlight and harsh solar glare. energy demands in buildings bring together a range of complex relationships between the climate, individuals, and their perception of comfort. the glazed boundary layer has been determined to date by impact energetic flows focused on high energy consumption, lighting lumens/watts, hvac, and plug-in end-use loads. passive systems of low zero carbon (lzc) technologies have been employed as active responses to solar radiation and these measures have been determined by: ventilated, double-skin, kinematic shading system, hybrid, nanocoating thin film reflective, pcm membranes, vacuum insulation, electrochromic, and transparent insulation materials. the minimisation of operational energy building use and maximisation of generated energy in order to reduce greenhouse gas emissions is an aim of the european directive 2010/31/eu (2010). however, this strategy requires material component elements to respond in real time to yearly, seasonal, and hourly changes in climatic and microclimatic conditions. this multiscale design approach would enable materials to react to external influences and change their thermal behaviour and functionality accordingly (knaack, klein, bilow, & auer, 2014). lzc technologies do not currently adopt such characteristics of integrated material functionality. the challenge is to progress from the static boundary conditions of steady state theory to the characterisation of non-steady states, and this is determined by thermal (energy) flow. government targets are making considerable demands on energy reduction targets within an increasingly uncertain climate. these facts all converge on a clear need for a solution that is more compliant than the current state of the art. there is a greater need to measure and understand the nature of thermal transfer effects at the material level for real time responsive conductance measures. nature’s use of matter and energy is a dynamic relationship that is achieved at differing scales through material diversity (species) and material connectivity (chemical compounds). nature assembles materials at a formation level to actively manage the composition of a microenvironment that obeys the rules of minimum energy loss and minimum effective power output. leaves are of particular interest to this research proposal, in terms of management of fluidics through absorption (photosynthesis). this research uses a leaf-like model to progress experimental absorption testing to establish proof of concept. 091 journal of facade design & engineering volume 7 / number 1 / 2019 this paper demonstrates this as an early assessment prototype that could be established at a larger scale for greater comprehensive performance evaluation. this absorption approach is not used in the current state of the art for fully glazed buildings, in which directly transmitted natural light often needs to be controlled by shading or reflection in order to avoid glare problems and unfavourable distribution of light within a room. various coatings and pigments are available for glazing to reduce the transmission of solar radiation near infrared irradiation in buildings. reflecting metal oxide layers are most frequently used for this purpose. by reflecting solar infrared irradiation, heat gains inside the building are avoided, but incident energy is also lost when heat gains would be favourable. furthermore, this reflective solution will be absorbed by other structures around the building and thus would contribute to urban heat island effects. research has been undertaken to introduce fluidics into windows, using the fluidglass technology (stopper, boeing, & gstoehl, 2013) as an absorption solution rather than a reflective one. this work used a triple glazed unit with two fluidic chambers acting as absorption layers by fluid depths of 2mm. the glazing panes that formed this assembly to create the overall unit is composed of 6mm ( 3 in total ) clear glass and two low-e coated 6mm panes. this study utilised the cavity void between glass panes by filling the void with water for conductivity absorption of solar radiation. the research demonstrated optimised results in a dyed metal particle anti-freeze fluid, demonstrating a reduction in cooling demand energy of 39%.this was achieved through the fluid volume in active flow within the two chambers absorbing solar radiation. the introduction of anti-freeze eliminates the possibilities of freezing at low night-time temperatures. further research highlighted a similar approach however the fluidic window generated warm water that was used for heating applications (chow, chunying, & zhang, 2011). the use of water gave higher conductivity for effective window cooling designs in warm climates (chow & chunying, 2013). water flow in the experiments was set at 200 ml/min with the greater efficiency gained in higher incidence of solar radiation for working efficient conditions. however, the lack of fluidic flow management within the free-flowing volume resulted in flow turbulence and water movement uncontrolled by gravity. this also impacted on water thermal fluidic expansion through solar radiation heat transfer and glazing deflection of the water under gravity. this presents challenging issues for full volume chamber fluidics, which remain unresolved. it was observed that the water chamber reduced the indoor temperature to 26.14°c in comparison to convection double glazed air-filled unit of 37.72°c on the summer solstice (lopez & gimenezmolina, 2012). however, the difference in temperature, through natural heating buoyancy, created a temperature variation in the liquid volume. this variation in temperature heating and decay increased the thermal expansion issues and diminished control of the liquid volume for solar absorption optimisation. the volumetric weight of the liquid within the assembly is also significant when applied to floor-to-ceiling glazed façade engineering, which further reduces the effectiveness of the application. the research presents a microfluidic-based platform as a method to advance solar modulation, not through a reflective approach of current practice but through an absorption solution, as a leaf-like model. this experimental work is exploratory in nature, as it is established within a laboratory environment to make an early assessment for proof of concept. the paper presents possible methods of integration within an envelope fabric that can possibly be scaled up for advances in envelope design. however, this manuscript does not set out to demonstrate a 092 journal of facade design & engineering volume 7 / number 1 / 2019 comprehensive performance evaluation, but rather presents the next stage in which the process is scaled up for manufacturing. this introduced method uses an ir absorbing fabrication process and characterisation method with a vascular heat transport system. using fl uidics in capillary channels as heat sinks within a material, we can modulate volumetric fl ows rates in the material to manipulate the material and fl uid thermal transfer. using active fl uids in fl ow within a material will enable the removal of material thermal stresses, as a material absorbs solar radiation. 2 leaf-like model leaves sync in real-time with the pattern changes of solar radiation (feugier, mochizuki, & iwasa, 2005; blonder, violle, bentley, & enquist, 2011). each leaf reacts and responds to variations in wind direction and orientation, and they adjust their surface exposure to harvest solar gain (fig. 1). fig. 1 leaf solar model fig. 2 illustrates how the network can continue to supply fl uid fl ow even when the main central leaf fl uid structure has been damaged (denoted by the central punched hole, (katifori, szollosi, & magnasco, 2010). this single leaf unit acts within a transformable daylight capture system: the tree canopy— through distribution of the leaf surfaces for day light capture. this is determined by canopy volume, total leaf area density, and the angular distribution of leaf surfaces, to form the tree structure. this approach to solar orientation and absorption of light energy by biochemical processes are responsive measures and a dynamic system of solar radiation. leaves use embedded microfl uidic channels as a means to harvest solar energy through fl uidic volume fi lled networks . this formation of capillaries is determined through precise control of channels’ geometries set within a material determined by a rule-based hierarchical pattern for the transportation of active liquids. this approach of a leaf-like model could advance materials that are thermally functional to act as a nir stop band through an absorption approach. 093 journal of facade design & engineering volume 7 / number 1 / 2019 these networks of micro-channels are called vascularisation patterns. this is a highly refi ned energy reaction system for enhanced material properties of chemical energy fl ow. vasculature patterns are linked to material scale, leaf size, and species in the formation of the channel network (dengler & kang, 2001). these closed loop exothermic networks are subject to fl ow resistance and fl ow rate, thus enabling signifi cant regulatory roles with tolerances given to damage and water stress conditions (fig. 2). the leaf fl uid network structure (illustrated by the yellow colour, fig. 3) exhibits a diminishing order of vein size, as all classes of veins contract in size distally from the main fl uidic (central) stem vasculature channel (turing, 1952). this distribution network is defi ned by hierarchical scaling that conforms to rules of minimum energy loss, minimum eff ective fl uidic power fl ow rates, and minimum pressure drop, determined through pressure equalisation by diminishing fl ow pressure variation across the network. this can be simulated through resistance circuit theory (oh, lee, ahn, & furlani, 2010), by the regulation of fl uidics that is achieved by unifi ed fl ow rate regulation and thermal dynamics of the fl uid within the capillaries. an experimental device was fabricated to assess and validate the concept within a laboratory environment. sensors and actuators connected to the device gave active measures in regulating fl ow rate and absorptivity in setting steady state energy capture and storage. a thermal transport system was determined by energy load – unload processes to maintain a steady state liquid temperature for solar modulation. 2.1 experimental multi microchannel device a plant closed loop vasculature can be analysed through simulation to generate optimum succession sequencing of a multi microchannel network, such as a leaf-like model. a device demonstrated this iterative approach to obtain a fl ow parabolic profi le for a fully developed fl ow rate, to advance a multiple channels’ network defi ned by hydrodynamic control of fl uids. this optimisation work of capillary succession of channels achieved an accuracy of 1 micron in the capillary geometry formation. fig. 3 cfd illustrates a unifi ed fl ow rate to enhance uniform absorption of solar radiation at high temperature. 094 journal of facade design & engineering volume 7 / number 1 / 2019 the device demonstrated this functionality by ir absorption that is dependent on solar radiation at high temperature (alston & barber, 2016). cfd illustrates the optimisation of fl ow rate within the multi microchannel network to achieve unifi ed planar extensional fl ow across a planar device (fig. 3). nir absorption is characterised by heat fl ow determined through the temperature diff erence between input and extract liquid, coming from the fl ow circuit (alston, 2017). the fl uid in this network circuit increases in temperature in a non-linear way as a result of solar radiation hitting the surface of the polymer (fig. 4). fig. 4 temperature gain in k for absorption rates as a function of fl ow rate (nestle, pulbere, & alston, 2018) fig. 4 illustrates the temperature diff erence between fl uidic feed-in to the network and the extract temperature of the fl uid coming from the microchannel fl ow circuit. the nir power (w/ m2) determines the thermal profi le of liquid temperature rise (delta t) by passage through the microfl uidic network. by changing the fl ow rate, we change the temperature increase of the fl uid in steady state fl ows. this contribution proposes that a microfl uidic-based platform will advance nir control by low transition temperature polymer using an absorption fl uid approach. the geometry of the channels is determined by systematic resistance networking of multi micro-channel succession, inspired from biology, specifi cally a bifurcated leaf formation. the input and extract microchannels (manifolds) play a primary role for feed-in fl uidics for the network’s longitudinal channels. simulation (cfd) have been undertaken to focus on successive channel widths, and to develop a hierarchy that emulates leaf vascular principles as a closed loop network. flow input and export channels to accommodate and distribute incoming fl uidic fl ow into the network defi ne this optimised sequence of channel widths. successive channel widths will increase in relation to increasing fl ow path length that is determined roughly by the square root of fl ow to channel path length. the optimised channel sequence in the polymer device was set at longitudinal channels at an equal spacing pattern formation of 15.575mm, with channel widths of: r0-2.0mm, r1-2.3mm, r3-2.6mm, r4-2.8mm, and the outermost channel r4-3.0mm. this channel geometry sequence is a hierarchical pattern defi ned by setting the value of resistance that is emulated by all channels. this was determined by the central channel r0 (target channel) to evaluate all others. this is a leaf-like absorption model which refl ects the control of fl uidics within capillary channels for unifi ed fl ows across a network at low-pressure drop. this method follows 095 journal of facade design & engineering volume 7 / number 1 / 2019 the leaf vein formations by the principle of all veins diminishing in size distally from the fluidic input supply. this approach does not use or try to emulate leaf photosynthesis chemical solution fluidics due to formulation fluid complexity. analysis of leaf-like model advances laminar nonturbulence flows at low flow rate for heat transfer. to optimize heat transfer by volumetric fluidic flow through uniform distribution across the network, the external face of the device, as a scale up, would comprise: 6mm low-e coated glass planitherm one to act as a weather facing material and fire protection to form the initial solar control layer. bonding of this pane to the polymer-based material allowed direct lateral heat flow transfer into a liquid, which was observed by experimental testing as indicated in fig. 4 results. the synthetic polymer material overall depth is 10mm (formed by two panes). channel depth within a network composite material (2mm) reduces the associated weight that is currently associated with volume chamber fluidic windows. thermal functionality is determined through optimum lateral heat transport flow with a minimum amount of fluid volume in network channels at a low flow rate of 1ml/min. results indicated that water temperature rise occurred through the passage of a fluid within the network by absorbed solar heat gain from a heated polymer surface. experimental results demonstrated that input distilled water temperature at 10 0c was heated to an output temperature of 140c by the polymer heating up through the passage within the capillary network. the solar load applied to the device was 1000 w/m2. this energy gain was distributed within the device through the top polymer pane absorbing 210w/m2, the fluid absorbing 707w/m2, and the lower polymer pane 83w/m2. if this device was to be scaled up to a façade area of 10m2, 200 litres of water would be obtained at this temperature. increased solar loading would amplify water temperature rise in the network and associated water output storage temperatures. in principle, polymer microfluidics act as ir stop bands through absorption that is modulated by fluidics to manage the thermal stress that would occur if heat storage was not removed within the material. this is achieved by a reactive response to changing solar intensity environments that is managed by flow rate ml/min and temperature rise, delta t, measurements. flow rate sensors and temperature sensors, thermocouples, are established practices within the automotive and aerospace industries, and measure the above parameters. these sensors were used in the experimental device set-up, to actively modulate temperature by fluidics to regulate high temperature thermal issues. this contribution, however, does not address the visible part of the electromagnetic spectrum. static solar shading or translucent materials would have to be incorporated within the façade design to avoid unwanted glare. the encouraging proof of concept determined a polymer acting as an infrared ir stop-band block at high temperatures through a material ability to lower its phase transition temperature. the functionality of the device is defined by heat flow transport within a composite polymer for enhanced thermal conductance. application to the real world of the experimental device would need to consider others factors that cannot be replicated within a laboratory setting, which depends on building scale and geometry orientation connected to fluidic management. 3 synchronizing modular solar geometry current transparent façade technology considers a glass building to be one surface, notwithstanding that this one surface is comprised of a number of assembly components, frames, mullions, waterproofing gaskets, and drainage channels. the entire glass envelope in a capillary composite glass material could not be treated as one entity, as the vascular network will have a significant 096 journal of facade design & engineering volume 7 / number 1 / 2019 resistance to fl ow. pumping pressures need to be controlled for solar modulation, as this function would be outweighed by the pumping energy demands within the network. if the façade was broken into segments per fl oor level to work collectively to form the emergent façade, this would enhance fl uidic fl ow regulation that is contained within a fl oor layer, (fig. 5). fig. 5 building level layering approach each level acts as a photoactive layer to create the planar surface. this layer-by-layer approach, using gravitational pull to infl uence and manage fl uidic fl ow, will reduce energy power demand, by avoiding the pumping of fl uids through continuous vertical surfaces over multiple fl oors. this approach contains the fl uid to a zonal (fl oor) level to manage energy load shift in segments for recirculation. this load shift moves the active fl uids in a fl owing circuit into thermal storage tanks. by using the structural fl oor zone, the reservoir feed in and extract fl ows will be determined through fl uidic temperature rise in relationship to time, t. extract reservoirs, at the structural fl oor zone, will remove fl uid at higher temperature (fig.6), from the network for energy download heat exchange. this energy removal cycle is determined by heat transport fl ow within the network system that is regulated by hydrodynamic and thermal sensors in connection to a defi ned material datum temperature. by modulating volumetric fl ow rate ml/min in the networks, we are able to manipulate heat gain at fl uid / polymer interfaces, using energy transfer (thermal storage, and electrical – energy conversion from peltier devices) to monitor temperature. this process would advance energy syncing to user energy demands for consumption profi ling to each and every fl oor (fig. 6). 097 journal of facade design & engineering volume 7 / number 1 / 2019 fig. 6 section through a building layer with a capillary glazed system fig. 7 section through a building layer with a capillary glazed system 098 journal of facade design & engineering volume 7 / number 1 / 2019 fig. 6 illustrates a repeatable multi-module approach to forming a nir fi ltering network to create a geometry system. as each module is independently regulated, this acts as a stop band block to boundary conditions as a performance metric of conductivity, convection, and thermal radiation. this collective approach simulates the structure of a tree by synchronizing network geometry in response to solar radiation intensity. the density of the geometry modules would increase or decrease depending on solar modulation and the requirement need for optical clarity (fig. 7). this nir regulation, by absorption, will heat up the polymer through solar high radiation that will introduce increased thermal stresses within the material. these stresses are managed using precise hydrodynamics control of the microfl uidic platform networks. the control of a fl uid in volume networks, in comparison to full chamber fl ooding, enables greater management of thermal absorption to enhance the cooling of the polymer. with increasing solar radiation loading, the module cell geometry spatial separation will become fi ner and fi ner as the unit spacing distances reduce to form an overall coherent, maximised-density module pattern. conversely, as solar radiation decreases, the spacing module pattern will increase to a point at which solar-energy modulation is not needed. this will result in geometry distinctions between building surfaces in a capillary glazing approach method that will be defi ned by surface function response, as illustrated in fig. 8. the geometry confi guration is a radiant balance between atmosphere and thermal comfort, using module cells that are synchronised to solar radiation load, as a thermal conductance system. a repeatable cell pattern will form the surface of the façade by the deployment of multiple units. module geometry transformations are set against a changing environmental background aligned to synchronising nir fi ltering (figs 8 & 9). fig. 8 multiple parallel-aligned module cells for maximised low transition temperature fig. 9 module cells with spatial separation for reduced solar radiation load figs 8 & 9, illustrate multiple parallel-aligned module cells as a heat seeking system that uses the optimisation parameters of visible transmittance and transition temperature for air conditioning reduction demands. it works as a collective unit that is synced to the user energy demand vector (heating, cooling) through building management monitoring. figs. 8 & 9 show this geometry systemised solution. however, using nir absorption glazing as a heat transport method could be optimised for enhanced areas for transparency and functional properties. 099 journal of facade design & engineering volume 7 / number 1 / 2019 3.1 geometry transformation of module cells each module cell is individually autonomous, and would be regulated through fl ow control measures in relation to fl uidic temperature increase within multiple network patterns. these cells are aligned vertically to block ir solar radiation at high temperatures. this gives attractive properties for each cell acting as an ir radiation stop band within a collective formation to maintain low pumping pressures and unifi ed fl ow at each network entrance of multiple entrance points. these are diffi cult issues to resolve in maintaining equalised fl ow distribution for solar absorption. this method would enable operational performance cell tracking through delta t to detect solar radiation properties and the parameters for transition temperature. this approach would advance variant distribution patterns (fig. 10) to transform transparent façades. fig. 10 transformable module cells networks for solar modulation this distribution system is a dynamic envelope that is nested to performance change by the hour, season, and weather conditions. 100 journal of facade design & engineering volume 7 / number 1 / 2019 4 conclusion global government targets for energy consumption reduction, and increasing uncertainty in the climate, all convey on a clear need for a solution for façades that has greater compliance to such standards. this has been determined to date through thin film reflecting metal oxide layers, electrochromic, and transparent insulation materials. these approaches also reduce the visual contact and optical benefits of view, colour, and light intensity that cannot not be underestimated for human well-being. the research demonstrates a material microfluidic platform of multiple cell geometry as a nir absorption solution to enhance solar modulation properties, by employing the strategy of fluidics to control and manage multiple microfluidic based module cells to drive the assembly formation of a fully glazed façade as a stop band block for infrared ir solar radiation. the regulation and management of a material is advanced through multiple networks in response to high solar radiation, by changing the synchronising geometry pattern that enhances distributed nir filtering to create autonomous optic material surfaces for adaptive performance. acknowledgements colleagues within basf se advanced materials and system research, germany, who are referenced within this paper. thank you to dr. robert barber for his support and advice from scientific research facilities council, uk government facility srfc, daresbury, uk, and to our reviewers who have given formative feedback to progress this paper. references alston, m.e. (2017). optimal microchannel planar reactor as a switchable infrared absorber: (invited manuscript) mrs advances. nanomaterials. vol. 2, 14, pp. 783-789 alston, m.e. & barber, r. (2016). leaf venation as a resistor to optimize a switchable ir absorber. scientific reports 6, 31611; doi: 10.1038/srep31611. blonder, b.,violle, c., bentley, l. p., & enquist, b. j. ( 2011). venation networks and the origin of the leaf economics spectrum. ecology letters. 14, pp. 91–100. chow, t.t., chunying, l., & zhang, l., (2011) thermal characteristics of water-flow double-pane window. international journal of thermal sciences, 50: 140-148. chow, t.t., & chunying, l., (2013) liquid-filled solar glazing design for buoyant water-flow. building and environment 60: 45-55. dengler, d., & kang, j. (2001). vascular patterning and leaf shape. plant biology, elsevier science. 4, pp. 50-56. european parliament directive 2010/31/eu (2010). european parliament and of the council of 19 may 2010 on the energy performance of buildings (recast), official journal of the european union 53 (2010) 13. feugier, f.g., mochizuki, a., & iwasa, y. (2005). self organization of vascular systems in plant leaves: inter-dependent dynamics of auxin flux and carrier proteins. journal of theoretical biology . 236, pp. 366–375. hens, h. (2011). applied building physics, boundary conditions, building performance and material properties. berlin: ernst + sohn. katifori, e., szollosi, g.j., & magnasco, m.o. (2010). damage and fluctuations induce loops in optimal transport networks, physical review letter, prl 104,04874. knaack, u., klein, t., bilow, m., & auer, t. (2014). façades, principles of construction. basel: birkhäuser verlag gmbh. lopez, t., gimenez-molina, c (2012) influence of double glazing with circulating water chamber on a thermal energy saving in buildings, energy and buildings, 56, pp.56-65 nestle, n., pulbere, s., & alston, m.e. (2018), possible benefits of capillary flow glazing in translucent wall elements, facade 2018 – adaptive!, proceedings of the cost action tu1403 – adaptive facades network final conference, luzern. oh,k.w., lee, k.,ahn, b., & furlani, e.p. (2010). design of pressure-driven microfluidic networks using electric circuit analogy. lab on a chip.12, pp.515-545. stopper, j., boeing, f., & gstoehl, d., (2013) fluidglass façade elements: influences of dyeable liquids within the fluid glass façade, energy forum on solar building skins, bressanone. turing, a. m . (1952). the chemical basis of morphogenesis, philosophical transaction of the royal society of london, series b, biological science. 237, 641,pp 37-72 . copyright notice  author(s) hold their copyright without restrictions. journal of facade design and engineering 2 (2014) 67–83 doi 10.3233/fde-150016 ios press 67 applications for curved glass in buildings jürgen neugebauer∗ university of applied sciences – fh-joanneum, graz, austria submitted 15 september 2014 revised 3 december 2014 accepted 22 december 2014 abstract. in the last years an increase of the number of building projects with built-in curved glass can be observed. the applications can principally be curved monolithic glass, laminated safety glass or insulated glass. this fact makes it absolute of interest to make more investigations in this field. the investigations can be focused on e.g. the process of the bending of the glass to bring it into a certain shape, or the very difficult topic of pre-stressing it. the state of the art of the production process of such glass shows some different ways to produce curved glass. the most used way is to bend the glass at a high temperature of more than 550◦ celsius. another kind of curved glass can be achieved in combination with the laminating process. with the cooling down at the end of the laminating process the interlayer becomes stiff enough to hold the shape by activated shear forces between the glass layers. another possibility is to produce flat glass and bend it while mounting the glass. the question how to pre-stress curved glass is on the very first beginning of investigations. all these different processes are on the first view very easy but very difficult in the detail. keywords: cold bent glass, hot bent glass, curved insulated glass 1. introduction the tendency to use curved glass in buildings shows an increasing number of realized outstanding projects like the ‘nordkettenbahn’ in innsbruck (austria), designed by the famous architect zaha hadid. the new cable railway substituted the old railway, which connects the city of innsbruck with the area hungerburg located in the north of the city. this cable railway consists of four stops – ‘congress’ next to the city centre, ‘löwenhaus’, next at the riverside of the river inn, ‘alpenzoo’, which is on the half way up to the final station at the very steep hillside and the ‘hungerburg’ which is the final destination on the very top. the principle glass geometry follows the raster of the steel grid, which is the main structure in the inner of the glass sculptures. all of the different glass elements are free form bent glass, as can be seen in fig. 1 below. with a special stainless steel supporting system, which is glued to the surface, the glass elements were assembled to the steel substructure. 2. cold bent glass by assembling 2.1. general a possibility is to produce the glass planar. the demanded shape is achieved during the mounting of the glass. the simple process can very simply be described as a glass which shall be fixed with ∗corresponding author: j. neugebauer, university of applied sciences – fh-joanneum, graz, austria. e-mail: juergen.neugebauer@fh-joanneum.at. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:juergen.neugebauer@fh-joanneum.at 68 j. neugebauer / applications for curved glass in buildings fig. 1. ‘nordkettenbahn’ station hungerburg, innsbruck (strabag). four glass fittings, as shown in fig. 2 below. the glass, e.g. a laminated safety glass, is produced as a planar panel. in the first step all four glass fittings are in the same plane surface. if one of these glass fittings is moved out of this plane surface, a curved surface of the glass occurs. the maximum curvature is limited by the bending strength of the used glass. 2.2. tgv railway station, strasbourg one wonderful example for such technique with cold bent glass by assembling, is the tgv railway station in strasbourg, see in fig. 3 below. the laminated safety glass was produced planar and these big glass panes were mounted at the building site into their demanded curved shape. fig. 2. cold bent glass by assembling. j. neugebauer / applications for curved glass in buildings 69 fig. 3. example for cold bent glass by assembling (fildhuth, 2014). fig. 4. cold bent glass by lamination. 3. cold bent glass by lamination 3.1. general another kind of curved glass can be produced in combination with the laminating process. the glass layers are fixed in there demanded shape in special moulds. the typical laminating process will be started. at a temperature of approximately 120◦c and a pressure of 1 bar up to 12 bar (depends on the kind of process) the interlayer, e.g. polyvinylbutyral (pvb), will become soft and the glass layers will be connected. during the cooling down at the end of the laminating process the interlayer becomes stiff enough to hold the shape by shear forces which are activated between the glass layers. when releasing the laminate from the production jig (e.g. the mould) the bending energy stored in the laminated glass is partially released. this effect can be described by the following 70 j. neugebauer / applications for curved glass in buildings fig. 5. pavilions at eiffel tower (leduc, 2012). two phenomena. a quasi-instantaneous, elastic spring back and a continuous long-term spring back, i.e. the decreasing relaxation of the laminate related to the viscoelastic interlayer properties including time and temperature (fildhuth, 2014). 3.2. eiffel tower one excellent project, as shown in fig. 5, is the replacement of the gustave eiffel and ferrié pavilions with two new pavilion housings and exhibition space, along with the facade renovation of the restaurant 58, designed by moatti & rivière architecture et scènographie. the glass panels are insulated glass units consisting of toughened laminated safety glass on the external public side and a single tempered glass on the inner side. the traditional method to fabricate the glass as a hot bent fig. 6. viscosity behaviour of glass at high temperature. j. neugebauer / applications for curved glass in buildings 71 glass was not appropriate because of two reasons. firstly the costs for all different geometries would have been too high and secondly such a double curved glass is not producible as a toughened glass by heat treatment (leduc, 2012). 4. hot bent glass 4.1. general the most used procedure is the bending of glass at a high temperature with more than 550◦c. in this case there is a change of the viscosity in connection with decreasing of the young´s modulus at such high temperatures. due to the gravity effect, the softer glass sags down into special designed moulds and gets its demanded geometry. the process of the deformation of a flat glass pane at high temperatures is the opposite process of the very old technique of glass blowing. the glass is again and again brought in a furnace on a temperature of over 600◦c, in order to form it by blowing to a cylinder. the ends of the cylinder are cut. in the second step the glass is reheated in another furnace and made flat. at temperatures above the temperature of transformation of approximately 550◦c, the glass becomes softer. a physical description of the procedure is possible with the viscosity. the viscosity designates the strength, which is required, in order to shift two parallel surfaces in a certain distance with a certain speed. one recognizes the meaning of the viscosity, if one regards the formation of a fig. 7. hot bent glass products. 72 j. neugebauer / applications for curved glass in buildings glass. melted glass is a liquid and differs from the solid state by the fact that the bonds between the molecule particles are missing. if individual particles move, then the bonds between these molecule particles must be blown up. the energy needed to blow up bonds between the molecule particles is supplied by thermal energy (neugebauer, 2007). the higher the temperature is, the more bonds between the particles are blown up and the glass matrix becomes softer. this effect results in more curvature in the glass and smaller bending radii. at a temperature of approximately 800◦c it is possible to produce such extreme curved glass with e.g. a corrugated or trapezoidal shape as shown in fig. 7 below (neugebauer, 2013). 4.2. vivarium maria hof an example of a special application of curved laminated safety glass is the glass tunnel for the ‘vivarium’ in maria hof in austria, shown in fig. 8 below. it is a tunnel made from glass for an aquarium containing piranhas. the tunnel with a total length of 6m was built with five cylindrical curved glass sections. each cylinder has a radius of approximately 2.0m, a width of 1.2 and an arc length of 3.6m. the lower edge of the glass lies in a water depth of 3.0m. for this very high load of (30 kn/m²) a laminated safety glass, consisting of 4 × 12mm bent glass, was needed. due to the big differences of the loads between the lower and the upper edge a stainless steel sub-construction for the stabilisation of the glass elements was used. this project very impressively demonstrates how the thickness of the glass can be reduced by activating membrane forces in the glass due to a special supporting system of the glass at the lower and the upper edges (neugebauer, 2007). 4.3. jcdecaux an extraordinary transformed glass was used for a project in london. a tower for advertising for the company – jcdecaux in brentford, 1000 great west road, middlesex, united kingdom tw8.9 – was planned by the famous architect lord norman foster; see in fig. 9 below. the tower for advertising is situated directly at the exit road to the airport heathrow in brentford. with its height of fig. 8. glass tunnel ‘vivarium’ maria hof, austria. j. neugebauer / applications for curved glass in buildings 73 fig. 9. view of the jcdecaux tower london. approximately 29m the tower has a triangular ground plan form. the lengths of all sides of the triangle are approximately 6m. the tower has two large light boxes for advertising at the sides arranged to the road. a structural steelwork is situated in the interior on which the cladding is fastened. the structural steelwork consists of three vertical columns in the corners and horizontal girders at each glass gap with a vertical clearance of approximately 3m. with diagonals the steel construction became stiffer due to the wind loads. the whole tower was shrouded in this corrugated glass. the corrugated glass with an average thickness of 8mm has a wave-length of 76mm and a difference between valley and peak of 20mm (neugebauer, 2007). 4.4. joanneumsviertel, graz a very interesting project, finished at the end of 2011, is the new entrance of the museum quarter ‘joanneumsviertel’ in the historical centre of graz in austria. this project is so important, because it shows different kinds of curved glass applications like curved laminated glass and curved insulated glass. the museum was founded in the year 1811 and on the occasion of the 200 year jubilee the government of styria had decided to renovate this museum. the complex of the museum’s building consists of two wings (museum of natural science and the museum of modern art) of the existing structure. for the connection of those old parts of the museum the architects – eep architekten, graz, austria and nieto sobejano arquitectos, madrid, spain – designed the new entrance between them. the visitors of the museum can reach the biggest cone designed as the museum entrance via a specially designed public place. the picture below (fig. 10) shows this new entrance with an escalator marked with an arrow. the basement with a depth of approximately 10m was excavated and the two levels were covered with wide spanned reinforced concrete slabs. an architectural challenge of this project was to bring daylight into these two lower floors. the concept of the architects was to let natural daylight flow into the basement via vertical funnels. these funnels have the form of small round courtyards with different diameters of up to approximately 16m. laminated safety glass and insulated glass were used for the cladding of these conical-shaped funnels. the cones have a central axis which are inclined up to 15◦ from the vertical. for this reason the inclination of the glass panes vary from the vertical position to an inclined position of up to 30◦ from 74 j. neugebauer / applications for curved glass in buildings fig. 10. new entrance – ‘joanneumsviertel’ (neugebauer, 2012). fig. 11. production of conical shaped insulated glass. the vertical. two of the six cones interpenetrate and another one is posed on its top and situated in the centre of a larger one (neugebauer, 2012). 4.4.1. production of conical shaped insulated glass the usual way to produce the conical shaped glass is by the process of a high temperature of more than 550◦ celsius and with the usage of gravity. in this case there is a change of the viscosity in connection with decreasing of the young´s modulus at such high temperatures. due to the gravity effect the softer glass sags down into special designed moulds and gets its demanded geometry. the procedure of the production of such conical shaped insulated glass (igu) can be described with the following steps. the first step was to cut out the glass to the demanded geometry and the edge treatment. after this first step the glass had to be brought into the furnace for the bending process. the bent glass must be laminated, if needed. the final step was the assembling of the insulated glass unit, by usage of soft-spacers for the edge sealing. the laminated safety glass and insulated glass were glued onto the stainless steel sections. j. neugebauer / applications for curved glass in buildings 75 fig. 12. cones 1 & 2 (neugebauer, 2012). 4.4.2. cone 1 & 2 cones 1 & 2 are the cones with an interpenetration located on the northern part of the public place. cone 1, with a diameter of approximately 9m, extends into the first basement level and cone 2, with a diameter of approximately 6m, extends into the second basement level (see in fig. 12 below). for the balustrade, laminated safety glass with a total thickness of 24mm, which consists of 2 × 12mm conical-curved annealed glass panes, was used. the cladding in the basement levels consists of insulated glass with conical-curved 12mm glass on the outer side, a 16mm space, and laminated safety glass, which consists of 2 × 8mm conical-curved annealed glass on the inner side. a special detail is the interpenetration of these two cones. the guarantee of the tightness against the rain for the parabolic curve of the interpenetration was a difficult part, as well as the geometrical challenge which had to be solved. the gap between the glass panes of the different cones was covered with a specially formed stainless steel profile with approximately the same u-shaped cross section as used for the stainless steel handrails of the balustrade. 4.4.3. cone 3 & 4 cone 3 has its larger diameter on the upper side in comparison with cone 4, which was posed on its top and has the larger radius on its bottom edge. the smaller cone number 4 is situated in the centre of cone 3, which is larger. for the balustrade, laminated glass with a thickness of 24mm consisting of two 12mm conical-curved annealed glass panes was used. the cladding in the first basement level was made for both cones of insulated glass with conical-curved 12mm glass on the outer side, a 16mm space, and a laminated safety glass, which consists of 2 × 8mm conical-curved annealed glass on the inner side. the glazing in the second basement level, which is used as a depot for the exhibits, was designed as laminated safety glass with a total thickness of 24mm. cone 6 is equal to cone 3 (neugebauer, 2012). 76 j. neugebauer / applications for curved glass in buildings fig. 13. cones 3 & 4 (neugebauer, 2012). the picture of fig. 13 shows the view from the inner of the museum in the first basement level through the insulated glass units of cone 3 to top off the insulated glass of cone 4. the top of cone 4 is covered with an elliptical, but flat, insulated glass pane. a very slender steel construction positioned in the gaps between the conical glass units carries this insulated glass of the top and is supported on the concrete slab. 4.4.4. cone 5 the biggest cone – cone 5 – with a diameter of approximately 16m was designed as the new entrance for the visitors of the museum. via an escalator the people reach the first basement level and enter the museum through a sliding door. the central axis of this cone is inclined by approximately 15◦ from the vertical and for this reason the inclination of the glass panes vary from a vertical position up to an angle of 30◦ from the vertical (near to the escalator). for the balustrade, laminated safety glass with a total thickness of 24mm, which consists of two 12mm conical-curved annealed glass panes, was used. in the balustrade, a gap for installation of the escalator was positioned (see fig. 14 below, left). 4.4.5. principle concept of glazing the special boundary condition of the great deformation of the wide span concrete slabs of more than 30mm (for the long-term deformation) causes the special structural system of all the cladding. the glass panes of the balustrade had to be stacked on the insulated glass of the level below. this means that the lower glass has to carry the vertical loads, e.g. the dead loads of the glass above. to keep the distance between the upper and the lower glass level, synthetic blocks were used. the calculations made during the design process showed that the additional stresses due to the dead load of the upper glass were not very high and in this case absolutely acceptable. at the lower edge of the insulated glass pane the dead load is supported by steel consoles, which were mounted on the concrete slab. j. neugebauer / applications for curved glass in buildings 77 fig. 14. cone 5. fig. 15. principle concept of glazing (neugebauer, 2012). for the horizontal loads, the glass panes were glued to stiff stainless steel ring sections, which were discretely supported at their ends. these hinged supported systems transfer the horizontal loads, e.g. wind or human impact, to the concrete slabs and guarantee the freedom of vertical movements of the concrete slabs (see in fig. 15 above). 4.4.6. design of cones all the different cones were designed with a finite element model which covers all glass panes. the loads were defined with dead loads, wind loads and horizontal loads due to human impact. for the design of the balustrade in the public area a horizontal load of h=3.0 kn/m was used. this high level of the load is based on the possibility of a big gathering of people. beside these mechanical loads the 78 j. neugebauer / applications for curved glass in buildings fig. 16. finite elements model of cone 5 (neugebauer, 2012). fig. 17. modell of the berlin state library (neugebauer, 2009). climatic loads in the insulated glass units were taken into account. these climatic loads include the difference in temperature (summer and winter), the difference in the meteorological air pressure and the difference in the altitude (between the production site and the building site). all these internal and external loads were superposed in the finite element model (neugebauer, 2012). 4.5. state library, berlin the thirteen storeys old building complex of the berlin state library with a length of 170m and a width of 107m is located in the heart of berlin. for the, in the second world war partly destroyed, a call for tenders was realized by the city council of berlin. the famous german architect hg merz won j. neugebauer / applications for curved glass in buildings 79 fig. 18. concept of the double skin facade (neugebauer, 2009) and view of the outer skin on the site (right), cross section. this architectural competition. the cubical structure of the new reading room was situated between two courtyards in the middle of the old building complex. the design concept of the architect was to bring as much daylight as possible into the reading room. so glass was used for the facade (neugebauer, 2009). the vertical cladding of the reading room was designed as a double skin facade. for the outer, secondary skin of the double skin facade, 8mm heat-formed toughened glass panes with a lot of small dents, according to a special design of the architect, were used. the depths of these dents were designed up to approximately 20mm. for the outer skin with a size of 2834 m² 1215 glass elements were produced. all glass elements were glued and sealed into aluminium frames. these aluminium frames were mounted at the outer side of the vertical steel structure of the facade. the space in between the two skins is used for maintenance. the distance between the inner and the outer skin given by the architect is approximately 1m. the gangway for the maintenance staff is covered with a steel grid, and runs in each storey through cut-outs of the vertical steel-beam (see in fig. 18, below). the inner, primary skin consists of insulated glass units with a 2 × 6mm laminated safety tempered glass on the outer side, and 8mm heat formed toughened glass with a lot of small dents on the inner side. the depths of these dents at the inner skin are up to approximately 10mm. all insulated glass units were glued and sealed into aluminium frames. the aluminium frames were mounted at the inner side to the vertical steel structure of the facade. for the inner skin with a size of 1560 m² 661 insulated glass units were produced. the picture in the middle in fig. 18 shows the outer skin during the process of assembling. the picture shows very well the nature of the cladding with the hot deformed glass. 5. pre-stressing in some cases, e.g. for glass with boreholes, pre-stressed glass is demanded. two techniques are currently available. the first possibility is thermal pre-stressing and the second is chemical treatment. a special technique is the production of e.g. front shields for cars. very special and expensive ceramic moulds are used for such glass. this is only possible because such moulds are used for many thousands of front shields. for thermally heat treated glass there is a geometrical limit. only for cylindrical shaped glass a thermal pre-stress is possible. 80 j. neugebauer / applications for curved glass in buildings 5.1. thermal pre-stressing at the market big machines are available to thermally strengthen and bend the glass in the same step of the process. at first the flat glass is heated up to approximately 550◦c. the hot glass is moved to the next part of the machine. in this part the glass will be bent and pre-stressed. instead of stiff roles a flexible kind of chain is used to bend the glass. depending on the direction of these chains with regards to the axis of the machine the glass can be defined as b-shaped or c-shaped. after bending of the glass the surface is blown off with air. in principle the pre-stress process is the same as for flat glass. thermally pre-stressed cylindrical shaped glass comes out at the end of the machine, see in fig. 19 below. 5.2. chemical pre-stressing chemically strengthened glass is a type of glass that has increased strength as a result of a postproduction chemical process. when broken, it still shatters in long pointed splinters similar to float glass. for this reason, it is not considered a safety glass and must be laminated if a safety glass is required. however, chemically strengthened glass has typically a six to eight times higher bending strength in comparison to annealed glass. the glass is chemically strengthened by a surface finishing process. glass is submerged in a bath containing a potassium salt (typically potassium nitrate) at 300◦ up to 400◦c. this causes sodium ions (na) in the glass surface to be replaced by potassium ions (k) from the bath solution, see in fig. 20 below. these potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. this replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension. the surface compression of chemically strengthened glass may reach up to 690 mpa, see in fig. 20 below. fig. 19. machine for thermally pre-stressed glass (neugebauer, 2013). j. neugebauer / applications for curved glass in buildings 81 fig. 20. principle of chemical strengthening. chemical strengthening results in a strengthening similar to toughened glass. however, the process does not use extreme variations of temperature, and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. this differs from toughened glass, in which slender pieces can be significantly found (karlson, 2010). 6. failure in various projects curved glass with defects can be observed. these failures can be less critical optical ones or very critical mechanical ones. 6.1. optical failure an example of a failure in the production method is the cylindrical shaped heat strengthened glass of a shop window found in graz. this glass is mounted with glass fittings at the substructure of the masonry. figure 21 below shows this window with the distorted glass. these distortions are the so called roller waves in the glass. the reason of these waves is the too high adjusted temperature in the furnace during the heat treatment. due to the too high temperature the glass is too soft and the waves can occur (neugebauer, 2013). 6.2. mechanical failure in bolsward, the netherlands, the ‘broerekerk’ (church of the brothers), founded before the 13th century was completely ruined by a fire in 1980. in 2004 works were started to pre-serve the ruin, and it was decided to fit a new roof, designed mainly from steel and glass, onto the old church, see in fig. 22. cylindrical shaped heat-treated laminated safety glass was used for the glass of the roof. while assembling the glass on the roof a glass breakage of some glass elements could be observed. after investigation of this problem the failure could be described. it was a failure happening while bending and pre-stressing the glass. the flat glass panel is placed on top of the mould and put into the furnace. as the glass is heated up, it sags into the mould until it just touches the heat resisting fabric. the deformed panel is not moved in the mould, neither the mould is moved in the airflow. 82 j. neugebauer / applications for curved glass in buildings fig. 21. shop window with optical failures. fig. 22. church ‘broerekerk’ in bolsward (the netherlands) with broken glass panels. the panel is directly cooled by cold air from the top. but from the bottom the air is hindered by the steel mould and the resisting fabric. at the supporting point no cold air reaches the hot glass pane and zones of tension instead of pressure occur on the surface of the glass. these cause an opposite effect of the pre-stress of the glass and results in much less bending strength (niderehe, 2008). to avoid such failures it is absolutely necessary to have the detailed knowledge about the behaviour of glass during the process at high temperature. there is a strong relationship between the size and the mass of the glass on the one hand and the duration and temperature of the production process on the other hand. only with the combination of the right duration and temperature it is possible to produce a good quality. 7. summary in a lot of realized projects around the world one can find curved glass in many different applications with a big number of different geometries. these applications can principally be curved monolithic j. neugebauer / applications for curved glass in buildings 83 glass, laminated safety glass or insulated glass. our focus was on the process of bringing the bended glass into a certain shape and the very difficult topic of pre-stressing it. the most used way is to bend the glass at a high temperature of more than 550◦ celsius. another kind of curved glass can be achieved in combination with the laminating process. with the cooling down at the end of the laminating process the interlayer becomes stiff enough to hold the shape by activated shear forces between the glass layers. another possibility is to produce flat glass and bend it while mounting the glass. references fildhuth, t., knippers, j., bindji-odzili, f., baldassini, n., & pennetier, s. (2014). recovery behaviour of laminated cold bent glass – numerical analysis and testing. in challenging glass 4 & cost action tu0905 final conference (p. 113). crc press. karlsson, s., jonson, b., & stålhandske, c. (2010). the technology of chemical glass strengthening – a review. glass technology-european journal of glass science and technology part a, 51(2), 41-54. leduc, n., raynaud, j., & baldassini, n. (2012), project for the eiffel tower: constructive geometry. in challenging glass 3. conference on architectural and structural applications of glass, faculty of civil engineering and geosciences, delft university of technology, june 2012 (p. 93). ios press. neugebauer, j. (2009). a design concept for bent insulated glasses for the reading room of the berlin state library. proceedings of glass performance days. neugebauer, j. (2012). big daylight funnels made of conical shaped insulated glass, engi-neered transparency. international conference at glasstec. düsseldorf, germany. neugebauer, j. (2013). applications for curved glass. in j. belis, chr. louter, & d. mocibob (eds.). cost action tu0905 mid-term conference on structural glass (p. 69-75). boca raton, fl: crc press, taylor & francis group. niderehe, s. & eekhout, m. (2008). spontanious glass breakage in hot bent, heat strengthened, laminated glass panels, in challenging glass, conference on architectural and structural applications of glass, faculty of civil engineering and geosciences, delft university of technology (p. 513). ios press. strabag, www.strabag.at www.strabag.at from city’s station to station city 019 journal of facade design & engineering volume 6 / number 3 / 2018 smart and multifunctional materials and their possible application in façade systems miren juaristi1*, aurora monge-barrio1, ulrich knaack2, tomas gómez-acebo3 * corresponding author 1 universidad de navarra, school of architecture, spain, mjuaristi@alumni.unav.es 2 faculty of architecture and the built environment, tu delft, the netherlands 3 universidad de navarra, tecnun school of engineers, spain abstract today’s society needs to face challenging targets relating to environment and energy efficiency, and therefore the development of efficient façade systems is essential. innovative concepts such as adaptive building façades might play a role in the near future, as their dynamic behaviour could optimise the performance of a building. for their successful development, a balance between sophistication and benefit is necessary and the implementation of smart and multifunctional materials in building envelopes could be the key, as they have the ability to repeatedly and reversibly change some of their functions, features, or behaviours over time in response to environmental conditions. however, these materials were predominantly developed for use in other fields, and there is a lack of specific technical information to evaluate their usefulness in façade engineering. the aim of this paper is to collect the critical information about promising responsive materials for use in the design of adaptive façades, in order to help designers and technicians in decision-making processes and to scope possible future applications in façades. investigated materials were analysed from the building science standpoint; their weaknesses and threats in the built environment were highlighted, and their technical feasibility was examined through the study of their availability in the current market. keywords responsive, autoreactive, intelligent, adaptive, design, innovation doi 10.7480/jfde.2018.3.2475 020 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction architecture and façade engineering are usually considered to be “conservative” fields in relation to the application of innovative materials. the complexity of requirements that they have to meet, as well as their interdisciplinary nature, make it difficult to achieve paradigm changes. however, new challenges, such as nzeb targets and low-carbon-based economies, put pressure on the development of new design approaches and strategies. thus, the implementation of smart and multifunctional materials might be more achievable than it may have seemed some years ago. these materials can respond reversibly and intelligently to changes in the surrounding environment without any external actuators, and this could be useful when designing adaptive, responsive, or intelligent façades, as the robustness of complex mechanisms is a critical issue (loonen, trčka, cóstola, & hensen, 2013). broadly speaking, the best known responsive materials are smart materials, highly engineered materials that can modify their function or behaviour (addington & schodek, 2005). addington and schodek (2005) distinguish two types of materials according to the way they react. type 1 materials change in one or more properties in direct response to a variation in the surrounding environment. some of these materials are already being applied in building technology, such as electrochromic windows (addington & schodek, 2005; gavrilyuk, tritthart, & gey, 2007; granqvist, 2014), which change their surface emissivity when there is a change in the voltage field (see section 3). on the other hand, type 2 materials react by transforming one energy form to an output energy in another form. for instance, electroactive materials transform electrical energy into mechanical energy and vice versa (madden, 2008). additionally, some new composite materials also present multifunctional properties, as they were designed to have a desired multiple response, and are referred to as multifunctional materials (mm) in this paper. at this point, it is important to note that multi-ability, or multi-function, has a different denotation than the concept of adaptability, as different objectives can be fulfilled consecutively and not only concurrently (loonen et al., 2013). thus, mms are non-homogenous materials in shape and/ or composition, and if their anisotropy is properly controlled, they behave differently according to the external conditions (reichert, menges, & correa, 2015). for example, thermobimetals comprise two sheets of differing metals alloys which, as they are laminated together, expand at different rates causing the bending of the component as a response to a temperature gradient (adriaenssens et al., 2014; lópez, rubio, martín, croxford, & jackson, 2015). this bending could create various desired but not concurrent façade morphologies, and the different geometries might enhance the performance of a façade component. for instance, the cladding of a ventilated façade could be as closed as possible in the insulation mode (function 1) or have an open-joint configuration in the heating-dissipation mode (function 2) (juaristi, monge-barrio, sánchez-ostiz, & gómez-acebo, 2018). furthermore, in recent years, the development of complex software and innovative manufacturing processes allow for greater control of the structural composition of the materials, which make possible the design of multi-functional and multi-property elements. these materials, designed by computational techniques, are also known as information materials (kretzer, 2017) but their possible application in façade technologies is still unclear and they were not addressed in this paper. the general consensus about multi-functional or adaptive materials is that they are often too sophisticated and therefore expensive (kretzer & hovestadt, 2014), and that even so, their service life is too short. however, there is a lack of technical information to establish this assumption as true for each sm or mm. this paper studies not only the common characteristics of a material family, but goes further in the analysis of specific materials. it enables the detection of potential materials 021 journal of facade design & engineering volume 6 / number 3 / 2018 for the building industry and the determination of whether they would perform properly in façade applications. to foresee this possibility, first the possible roles of a material in a dynamic façade element were proposed and their design potential and limitation analysed (section 3). secondly, different properties predetermining the dynamic performance of the material were explained (section 4) and, to conclude, the importance of knowing specific physical properties of these materials was highlighted and further areas of research were suggested (section 5). 2 methodology this paper collected technical information about smart and multifunctional materials applied not only in the façade industry but in any field, as long as their operational scenarios and scales of adaptability matched with façade requirements. the criterion for the inclusion of adaptive materials in this review was the operational scenario, the fatigue life, and their scale of adaptability (defined in section 4), according to their possible roles (section 3). for instance, thermochromic materials with potential uses in external claddings were only included when they perform at ambient temperature and when their fatigue life is longer than the service life of the façade element. when considering materials with kinetic responses to be applied in movable double skins, only materials with reactions of a magnitude of centimetres were considered. the results shown in this paper were obtained from scientific papers, open access material databases (materia,n.d.-a; materiability, n.d.-a) and from market products information (dynalloy, inc., n.d.; fraunhofer institute for applied polymer research iap, n.d.; kanthal, n.d.; lcrhallcrest, n.d.; qcr solutions corp, n.d.; smart films international, n.d.). scientific papers were particularly interesting for identifying different adaptive materials and understanding their dynamic operation. however, when scoping possible innovative roles, online multidisciplinary databases and market available products were especially valuable as their information helped to analyse design potential and limitations. 3 possible application of sm/mm in adaptive façade elements 3.1 smart windows the application of sm/mm in smart windows provide two types of dynamic performance: shading and climate control (fig. 1). those smart materials used for a shading reaction are the most developed and are mainly available in the market as part of window components (addington & schodek, 2005; granqvist, 2014; lampert, 2003; mlyuka, niklasson, & granqvist, 2009). at the present time, these materials can be implemented as thin films (gavrilyuk et al., 2007; granqvist, 2014; mlyuka et al., 2009; seeboth, ruhmann, & mühling, 2010; smart films international, n.d.), directly in the glass using nanotechnology in the chemical composition (granqvist, 2014; materia, n.d.-b; seeboth et al., 2010) or as inks, pigments or dyes (qcr solutions corp, n.d.; seeboth et al., 2010). the main families of technologies that provide a shifting surface colour are electrochromics, thermochromics, and photochromics, and their differentiation factor is the input by which their 022 journal of facade design & engineering volume 6 / number 3 / 2018 response is activated. electrochromics react to a change in the voltage field (gavrilyuk et al., 2007; granqvist, 2014); thermochromics change their colour at a set temperature (kretzer, 2017; ma & zhu, 2009); and photochromics change when they are exposed to uv radiation (zhang, lee, mascarenhas, & deb, 2008). similarly, in thermotropics, the change of the light scattering properties at certain operational temperatures is caused by a phase separation process at molecular level (seeboth et al., 2010). fig. 1 responsive materials that could be used in smart window components and meaningful design features. 3.2 opaque adaptive façade components even if the application of responsive materials in opaque façade components is less developed than in transparent façade components, there is a wide range of possible roles for which they could be used. firstly, they could be used in exterior claddings to modify the surface temperature to optimally control the thermal performance of the outer skin (ma & zhu, 2009). experimental assessments were made for materials that change their colour at a specific temperature (thermochromics), but as electrochromics or photochromics modify their solar heat radiation factor, this effect might also modify the surface temperature of façades, and therefore, their thermal performance. all of these materials are smart materials, which means that the available colour range depends on their chemical composition, and as there are, as yet, few chemical structures providing this responsive performance, there are few colour options for each material family (fig. 2). besides, most of the time these colours are vivid, which could be a challenge when applying them in some urban contexts. anyway, to really evaluate the application of these materials in responsive façade elements, the holistic behaviour of the system needs to be considered, as it is illogical to try to collect solar thermal energy throughout the cladding if the envelope completely blocks thermal flux. 023 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 2 responsive materials that could be used in opaque exterior claddings and meaningful design features. to tackle this problem, components made by sm or mm could be applied in the intermediate façade layer as embedded devices that control heat flow (fig. 3). phase change materials are good examples of responsive thermal control materials and, while they are no longer widely used in the building environment, the current scientific research is quite advanced and shows that they have high potential for reducing heating and cooling energy demand (cabeza, castell, barreneche, de gracia, & fernández, 2011). fig. 3 responsive materials that could be used in intermediate layers and meaningful design features. lastly, interior cladding could also have an adaptive reaction that would be useful in controlling the hygrothermal conditions of the interior environment (fig. 4). materials that have high humidity absorption, such as natural porous materials or hydrogels, could be used not only to achieve a suitable level of humidity in the air, but also to cause an evaporative cooling effect (markopoulou, 2015). this behaviour could be integrated directly in the material used in the interior surface (maeda & ishida, 2009; watanabe, fukumizu, & ishida, 2008) or as devices (raviv et al., 2014). 024 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 4 responsive materials that could be used in opaque interior claddings and meaningful design features. 3.3 movable or kinetic skins reactive materials with a kinetic or shape-changing ability might also have a broad field of application in movable skin façades (fig. 5). the most developed role is shading, as these materials modify their dimension when an external stimulus exists, such as temperature rise or solar radiation incidence. they could trigger the motion of the outer skin when they are incorporated in the external surface (adriaenssens et al., 2014; fiorito et al., 2016), when they are placed at joints, or as external actuators in flexible components. there are already some built examples (laughlin & howes, 2012), however, mechanical actuators are more widely used than those that are embedded in materials (loonen et al., 2013). fig. 5 responsive materials that could be used in movable double skins and meaningful design features. 025 journal of facade design & engineering volume 6 / number 3 / 2018 furthermore, materials that modify their dimensions in reaction to different inputs could be used to provide automatic ventilation, as was demonstrated in a functional prototype which responded to humidity (reichert et al., 2015). although several materials with a kinetic response were found in the literature review, such as co2 responsive polymers (lin & theato, 2013), hydrogel actuators (markopoulou, 2015), or materials responding to temperature changes (adriaenssens et al., 2014; fiorito et al., 2016; lópez et al., 2015), their possible façade performance was not explored. even so, their potential to provide automatic ventilation according to these environmental inputs looks promising. the main drawback of these possible new roles is that if responses were self-induced, it would make it impossible to override adaptation in contrast with the electrical input. for that reason, climate and use conditions should be considered with an overall perspective to determine if the construction of auto-reactive air dampers would be suitable for energy and comfort requirements. lastly, these kinds of materials could also enhance the thermal behaviour of ventilated opaque façades (juaristi et al., 2018). they could open or close the air cavity between the outer and inner skin depending on the exterior temperatures and wind conditions. the convective movements occurring in the cavity could be enhanced or blocked to control thermal dissipation. 3.4 design potential and limitations one of the most challenging tasks when trying to face the dissemination gap between different scientific fields and façade engineering was to learn how these materials look. each sector has its application scale, roles, and restrictions, and usually, sms and mms are manufactured in such a way that they are not adequate for the built environment, making it even harder to envision their potential new uses. in order to boost the implementation of innovative materials in façades, technology applicators need to understand the determining factors of each material and the detection of the following design parameters is essential: – available colours (fig. 1 fig. 5) – possible geometries due to material family and type of façade elements (fig. 1 fig. 5) – thickness – width – length – assembly method – manufacture process such information was found for electrochromics (granqvist, 2014; smart films international, n.d.), thermochromics (materia, n.d.-b; materiability,n.d.-b; mlyuka et al., 2009; qcr solutions corp, n.d.; smart films international, n.d.), photochromics (lcrhallcrest, n.d.; reichert et al., 2015), thermotropics (seeboth et al., 2010), shape memory alloys (dynalloy, inc.; fiorito et al., 2016; madden, 2008), electroactive polymers (fiorito et al., 2016; jiang, kelch, & lendlein, 2006; madden, 2008; markopoulou, 2015; samatham, kim, & dogruer, 2007) and hydrogels (materiability, n.d.-c), as they are currently commercialised products and manufacturers provide useful information for design considerations. besides, materials belonging to the same product family usually have some similar characteristics, especially regarding the possible geometries. for instance, thermochromics and electrochromics come mainly as rectangular rolls and sheets (materia, n.d.-b; smart films international, n.d.), whereas self-shaping materials are most often manufactured as strips (dynalloy, inc., n.d.; kanthal, n.d.; materiability, n.d.-d; fiorito et al., 2016; jiang et al., 2006), wires (dynalloy, 026 journal of facade design & engineering volume 6 / number 3 / 2018 inc., n.d.; fiorito et al., 2016), beams (adriaenssens et al., 2014) and sheets (fiorito et al., 2016; samatham et al., 2007). little information was found in relation to assembly methods and manufacture process, and more research should be undertaken to get this information, which would be necessary in order to foresee more design options beyond those commercially available. 4 dynamic operation and adaptive materials when classifying smart and multifunctional materials in families, the common feature is the dynamic operation that they are able to provide (addington & schodek, 2005). accordingly, in this section, we detected and grouped specific materials and studied the relevant properties that enable an understanding of their adaptive performance. based on the parameters that loonen et al. established as key factors for climate responsive façade elements, it is essential to find accurate information about the mechanism of actuation of each material, the range and velocity of adaptation, the type of control, the operational scenario and their fatigue life (loonen et al., 2013). 4.1 control of visible light and solar transmittance. reversible colour change responsive materials can enhance thermal performance and/or daylight by switching visible transmittance and/or solar heat gain coefficient. nowadays, there are several electrochromic, thermochromic, photochromic, and thermotropic products available on the market, mainly for smart windows. fig. 6 shows visible transmittance and solar transmittance range for some of these products. colour is of great importance, as the lighter it is, the more daylight is provided and the less solar transmittance is blocked. if the purpose of the material is to boost indoor natural light, the use of some of these electrochromic products is questionable, as their highest value doesn’t reach 40% visible light transmission. indeed, if the aim is to control thermal performance, then solar transmittance is the key parameter to look at in this graph. furthermore, when applying electrochromic materials, a balance should exist between the required electrical current and the obtained energy saving; oxide films look like a promising solution to meet this purpose (fig. 7). finally, some commercialised thermochromics (qcr solutions corp, n.d.; smart films international, n.d.) and photochromics (lcrhallcrest, n.d.) that are available on the market don’t have a suitable life-span for façade engineering, as uv degrades them quickly. the reason why this does not happen in switchable windows may be due to the fact that the glass blocks the majority of uv radiation, which could extend the service life of the component. 027 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 6 visible transmittance and solar transmittance of some electrochromic, thermochromic, and thermotropic products available on the market 028 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 7 required voltage to activate some meaningful electrochromic materials 4.2 reversible heat flow direction thermoelectrical materials, such as bi2te3 based compositions, create a hot and cold junction when an electrical input is applied and a temperature difference occurs between the two faces of the material as a result. if these materials were applied in façades, the direction of thermal flux could be controlled to obtain the desired effect (the enhancement of energy exchange or the insulation) (addington & schodek, 2005; ibañez-puy, bermejo-busto, martín-gómez, vidaurre-arbizu, & sacristán-fernández, 2017). 4.3 electrically activated mechanical displacement (and the converse) electroactive materials, also known as piezoelectric materials, can produce a mechanical displacement when an electrical current is applied and conversely, the material can produce an electrical signal when a mechanical displacement occurs, as molecular structures are electrically polarised when a stress force is applied to the material (kornbluh, 2008; madden, 2008; samatham et al., 2007). this could be applied in kinetic façade components, and there are already some built examples, such as the shapeshift project, which employed a siliconeand acrylic-based dielectric elastomer to trigger the motion of the skin (rossi, augustynowicz, georgakopoulou, & sixt, n.d.). 029 journal of facade design & engineering volume 6 / number 3 / 2018 the determinant property when analysing the suitability of these materials for innovative adaptive façade system application was the required electrical current (control) to achieve a reversible deformation (quantitative value of the response). from fig. 8, it can be seen that a large amount of energy is necessary to produce a meaningful shrinkage deformation, which makes questionable the use of piezoelectric and electroactive polymers if the aim is to reduce the energy demand of the building. fig. 8 required electrical current to achieve certain level of shrinkage deformation for some electroactive materials 4.4 reversible expansion and possible bending electroactive polymers can also reversibly bend due to the electrical charges. the current generates the attraction of the opposing charges and the repelling forces between equal charges, and as a result, the thickness of the polymer contracts while it expands in length. moreover, there are several materials that could provide this bending reaction, for instance, shape memory polymers and shape memory alloys reversibly change their shape (in one or more directions) when they reach an operational temperature; this is due to thermal-elastic transformations at molecular levels. apart from the aforementioned smart materials, there are some multifunctional materials that were designed to achieve the desired bending under specific environmental conditions (table 1). thermobimetals, for example, are composed of two metal sheets that have different coefficients of thermal linear expansion, which makes the composite material bend when it is exposed to a temperature gradient (adriaenssens et al., 2014). bi-layer hygromorph composites are also based on the same principle, but their differences in the coefficient of linear expansion are based on their capacity to absorb ambient humidity (reichert et al., 2015). 030 journal of facade design & engineering volume 6 / number 3 / 2018 smart material family specific material multifunctional material family specific material electroactive polymers silicone and acrylic based dielectric elastomer(rossi et al., n.d.), polyvinylidene fluoride (pvdf) (madden, 2008) thermobimetal astm tm2 bimetal, ni-fe alloy (adriaenssens et al., 2014) (containing 41%, 37% and 36% nickel) (kanthal, n.d.) shape memory polymers polymer resin systems, polystyrene (markopoulou, 2015; materia, n.d.-b; matweb material property data, n.d.), composite from multiple photocurable methacrylate based copolymer network (ge et al., 2016) hygromorphs bi-layer composites ex. composite mixture of glass fibre, epoxy bonding and maple wood (reichert et al., 2015) shape memory alloy ni-ti alloy (55%-56% nickel and 44%-45% titanium) (dynalloy, inc., n.d.; madden, 2008) table 1 autoreactive materials which have the ability to reversibly expand and bend for the consideration of their possible application in façade engineering, the main restriction was their adaptability range. in the reviewed literature, we found several materials in other fields that change their shape by bending, but as we already explained in section 2, we only compiled those that deformed by at least at a centimetre. 4.5 moisture absorption hydrogels (table 2) are well-known hygro-expansive materials, as they have the ability to dramatically increase their volume when they absorb moisture (raviv et al., 2014). they could constitute actuator devices for kinetic façades that aim to respond to humidity variations, which might have some benefits in the hygrothermal performance (markopoulou, 2015). smart material family specific smart material multifunctional material family specific multifunctional material hydrogel hydrophilic uv curable polymer(raviv et al., 2014), polymers of hydroxyethyl, insoluble polymers of acrylate, insoluble polymers of acrylamide, insoluble polymers of polyethylene oxide(markopoulou, 2015) hydrothermally solidified soil bodies sepiolite clay, allophane (emile, 2002; watanabe et al., 2008), earth ceramics natural porous materials cedar (emile, 2002), silica gel, mixture of gibbsite and clay material(watanabe et al., 2008), mesoporous material derived from kaolinite, mesoporous material derived from metakaolinite(maeda & ishida, 2009) table 2 autoreactive materials triggered by ambient humidity in addition, the academic literature on moisture absorption and passive cooling has revealed the emergence of new multifunctional materials that could control the hygrothermal conditions of the indoor environment (maeda & ishida, 2009; watanabe et al., 2008). inspired by the suitable performance of natural porous materials in humid and hot climate conditions, hydrothermally solidified soil bodies were developed in such a way that the porosity of their micro-structure 031 journal of facade design & engineering volume 6 / number 3 / 2018 could enable the self-regulation of the water content in the surrounding air through capillary condensation. however, as far as the authors know, there are no experimental validations that prove this assessment and they should be instigated in order to discover the potential of these promising multifunctional materials for their application on interior claddings. 5 discussion and conclusion 5.1 meaningful physical properties adequate performance of the façade must be ensured by each element of the system. sms and mms need to provide a suitable adaptive reaction while accomplishing traditional façade requirements regarding safety, economy, and comfort. furthermore, materials need to behave optimally during their whole service-life. the physical properties of such materials determine whether they can perform properly in the proposed role. for instance, some of these materials might be required to fulfil structural performances, so their structural properties, such as compressive strength, bending strength, or elastic modulus need to be appropriate. if they were to perform kinetic work, then performed work, elastic modulus, and fatigue life would be fundamental. finally, depending on their position in the façade system and the relationships with the other building materials, fire resistance and fire containment, rain and water-vapour resistance, thermal properties, and fenestration properties would be required. nevertheless, as we are not expecting the same behaviour for a coating and for a metal sheet, firstly, the material family needs to be detected, so that we can determine what kind of performance can be demanded from a particular type of material. after that, according to their performance in the whole façade system, specific properties are sought. in general, commercially available adaptive façade materials provided the meaningful physical properties in their data-sheets, but when analysing materials used in other fields, it was not possible to find out this technical information. 5.2 towards promising new roles most of the smart and multifunctional materials shown in this paper were sophisticated raw materials, highly engineered or designed. however, they could be used to develop simple façade products as, by applying them, it wouldn’t be necessary to make intricate details including complex electronics or mechanical actuators. still, there are few examples in architecture that include smart or multifunctional materials, and even fewer built façades. thus, it was difficult to get accurate, essential technical information regarding specific façade requirements and meaningful dynamic operation parameters. nowadays, smart glazing is technologically the most advanced and new roles beyond are yet to be explored. moreover, a wide variety of multifunctional materials could be created, inspired by promising reactions of smart materials. as their complexity comes mainly from the design process instead of their raw material availability or engineering process, overcoming this intellectual challenge could inspire a great opportunity to achieve new functionalities in architecture. 032 journal of facade design & engineering volume 6 / number 3 / 2018 5.3 further research overall, more experimental assessments are needed in order to get indispensable information regarding design characteristics, dynamic operation, and physical properties, as, so far, only a few responsive materials available in the building industry make their technical information available. furthermore, assessments should be done at building scale, as these materials might behave unexpectedly and the value of their reaction might be non-linear at different scales (kolarevic, 2014). last but not least, more information on suitable operational scenarios and optimal scales of adaptability would help us to establish a greater degree of accuracy on this matter. acknowledgements this paper is the output of the short term scientific mission entitled “the-state-of-the-art of adaptive and multifunctional materials”, funded by cost action tu1403 “adaptive façade network”. it was developed as part of the working group 1 and within the façade research group (frg) of the department of architectural engineering + technology, delft university of technology (tu delft). it is also part of the ongoing phd research project titled “adaptive opaque façades: a design and assessment method”, funded through a scholarship granted by asociación de amigos of the universidad de navarra. references addington, d. m., & schodek, d. l. (2005). smart materials and new technologies : for the architecture and design professions. amsterdam: elsevier, architectural press. adriaenssens, s., rhode-barbarigos, l., kilian, a., baverel, o., charpentier, v., horner, m., & buzatu, d. (2014). dialectic form finding of passive and adaptive shading enclosures. energies, 7(8), 5201–5220. http://doi.org/10.3390/en7085201 cabeza, l. f., castell, a., barreneche, c., de gracia, a., & fernández, a. i. (2011). materials used as pcm in thermal energy storage in buildings: a review. renewable and sustainable energy reviews, 15(3), 1675–1695. http://doi.org/10.1016/j.rser.2010.11.018 dynalloy, inc. (n.d.). retrieved march 23, 2018, from http://www.dynalloy.com/tech_data_ribbon.php emile, i. (2002). soil-ceramics (earth), self-adjustment of humidity and temperature. encyclopedia of smart materials. wiley. fiorito, f., sauchelli, m., arroyo, d., pesenti, m., imperadori, m., masera, g., & ranzi, g. (2016). shape morphing solar shadings: a review. renewable and sustainable energy reviews, 55, 863–884. http://doi.org/10.1016/j.rser.2015.10.086 fraunhofer institute for applied polymer research iap. (n.d.). retrieved march 23, 2018, from https://www.iap.fraunhofer.de/ content/dam/iap/en/documents/fb2/solardim_eco_fraunhofer-iap.pdf gavrilyuk, a., tritthart, u., & gey, w. (2007). photo-stimulated proton coupled electron transfer in quasi amorphous wo3 and moo3 thin films. philosophical magazine, 87(29), 4519–4553. http://doi.org/10.1080/14786430701561516 ge, q., sakhaei, a. h., lee, h., dunn, c. k., fang, n. x., dunn, m. l., … qi, h. j. (2016). multimaterial 4d printing with tailorable shape memory polymers. scientific reports (vol. 6). http://doi.org/10.1038/srep31110 granqvist, c. g. (2014). electrochromics for smart windows: oxide-based thin films and devices. thin solid films, 564, 1–38. http:// doi.org/10.1016/j.tsf.2014.02.002 ibañez-puy, m., bermejo-busto, j., martín-gómez, c., vidaurre-arbizu, m., & sacristán-fernández, j. a. (2017). thermoelectric cooling heating unit performance under real conditions. applied energy, 200, 303–314. http://doi.org/10.1016/j.apenergy.2017.05.020 jiang, h., kelch, s., & lendlein, a. (2006). polymers move in response to light. advanced materials, 18(11), 1471–1475. http://doi. org/10.1002/adma.200502266 juaristi, m., monge-barrio, a., sánchez-ostiz, a., & gómez-acebo, t. (2018). exploring the potential of smart and multifunctional materials in adaptive opaque façade systems. journal of façade design and engineering; vol 6 no 2: icae2018 special issuedo 10.7480/jfde.2018.2.2216. kanthal. (n.d.). retrieved march 23, 2018, from https://www.kanthal.com/en/search/?q=bimetal kolarevic, b. (2014). adaptive architecture: low-tech, high-tech or both? in m. kretzer & l. hovestadt (eds.), alive : advancements in adaptive architecture. (p. 220). basel/berlin/boston: birkhäuser,. kornbluh, r. (2008). fundamental configurations for dielectric elastomer actuators. dielectric elastomers as electromechanical transducers. elsevier ltd. http://doi.org/10.1016/b978-0-08-047488-5.00008-3 kretzer, m. (2017). information materials. springer international publishing ag switzerland. http://doi.org/10.1007/978-3-31935150-6 kretzer, m., & hovestadt, l. (2014). alive : advancements in adaptive architecture. (nv-1 o). basel/berlin/boston : birkhäuser,. lampert, c. m. (2003). large-area smart glass and integrated photovoltaics. solar energy materials and solar cells, 76(4), 489–499. http://doi.org/10.1016/s0927-0248(02)00259-3 laughlin, z., & howes, p. (2012). material matters: new materials in design. united kingdom, europe: black dog publishing ltd. lcrhallcrest. (n.d.). retrieved march 23, 2018, from https://www.hallcrest.com lin, s., & theato, p. (2013). co2 -responsive polymers. macromolecular rapid communications, 34, 1118–33. http://doi.org/10.1002/ marc.201300288 033 journal of facade design & engineering volume 6 / number 3 / 2018 loonen, r. c. g. m., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483–493. http://doi.org/10.1016/j.rser.2013.04.016 lópez, m., rubio, r., martín, s., croxford, b., & jackson, r. (2015). active materials for adaptive architectural envelopes based on plant adaptation principles. journal of façade design and engineering, 3(1), 27–38. http://doi.org/10.3233/fde-150026 ma, y., & zhu, b. (2009). research on the preparation of reversibly thermochromic cement based materials at normal temperature. cement and concrete research, 39(2), 90–94. http://doi.org/10.1016/j.cemconres.2008.10.006 madden, j. d. w. (2008). dielectric elastomers as high-performance electroactive polymers. dielectric elastomers as electromechanical transducers. elsevier ltd. http://doi.org/10.1016/b978-0-08-047488-5.00002-2 maeda, h., & ishida, e. h. (2009). water vapor adsorption and desorption of mesoporous materials derived from metakaolinite by hydrothermal treatment. ceramics international, 35(3), 987–990. http://doi.org/10.1016/j.ceramint.2008.04.007 markopoulou, a. (2015). design behaviors ; programming matter for adaptive architecture. next generation building 1, 1, 57–78. http://doi.org/10.7564/15-ngbj17 materia. (n.d.-a). retrieved march 23, 2018, from https://materia.nl materiability. (n.d.-a). http://doi.org/http://materiability.com materiability. (n.d.-b). retrieved march 23, 2018, from http://materiability.com/portfolio/thermochromics/ materiability. (n.d.-c). retrieved april 9, 2018, from http://materiability.com/wp-content/uploads/2014/09/m_06.jpg materiability. (n.d.-d). retrieved march 23, 2018, from http://materiability.com/portfolio/thermobimetals/ matweb material property data. (n.d.). retrieved march 23, 2018, from http://www.matweb.com/search/datasheettext.aspx?matguid=da5f0f16f66446a38bce7b1ee4fe2c61 mlyuka, n. r., niklasson, g. a., & granqvist, c. g. (2009). thermochromic multilayer films of vo2 and tio2 with enhanced transmittance. solar energy materials and solar cells, 93(9), 1685–1687. http://doi.org/10.1016/j.solmat.2009.03.021 qcr solutions corp. (n.d.). retrieved march 23, 2018, from http://www.qcrsolutions.com/site/home___qcr_solutions_corp.html raviv, d., zhao, w., mcknelly, c., papadopoulou, a., kadambi, a., shi, b., … tibbits, s. (2014). active printed materials for complex self-evolving deformations. scientific reports (vol. 4). http://doi.org/10.1038/srep07422 reichert, s., menges, a., & correa, d. (2015). meteorosensitive architecture: biomimetic building skins based on materially embedded and hygroscopically enabled responsiveness. cad computer aided design, 60, 50–69. http://doi.org/10.1016/j. cad.2014.02.010 rossi, d., augustynowicz, e., georgakopoulou, s., & sixt, s. (n.d.). shapeshift. retrieved march 23, 2018, from http://caad-eap. blogspot.com.es samatham, r., kim, k. ., & dogruer, h. . (2007). active polymers: an overview. in j. k. kwang & s. tadokoro (eds.), electroactive polymers for robotic applications (pp. 1–36). london: springer. seeboth, a., ruhmann, r., & mühling, o. (2010). thermotropic and thermochromic polymer based materials for adaptive solar control. materials, 3(12), 5143–5168. http://doi.org/10.3390/ma3125143 smart films international. (n.d.). retrieved march 23, 2018, from http://smartfilmsinternational.com/solar-glass/#thermo_glass_ download watanabe, o., fukumizu, h., & ishida, e. h. (2008). development of an autonomous humidity controlling building material, 19–29. zhang, y., lee, s. h., mascarenhas, a., & deb, s. k. (2008). an uv photochromic memory effect in proton-based wo3 electrochromic devices. applied physics letters, 93(20), 10–12. http://doi.org/10.1063/1.3029775 journal of facade design and engineering 1 (2013) 17–29 doi 10.3233/fde-130006 ios press 17 a bpmn-based process map for the design and construction of façades eleanor vossa,∗, qian jinb and mauro overendb aglass and façade technology research group, department of engineering, university of cambridge, cambridge, uk bglass and façade technology research group, university of cambridge, cambridge, uk received: 12 august 2013 accepted: 20 november 2013 abstract. process mapping can lead to significant efficiency and quality improvements in construction engineering and is an ideal basis for developing it support tools. the increasing complexity and multidisciplinary nature of façade design and construction suggest that a process map would be beneficial in this sector of the construction industry, but it has received limited attention to date. this paper presents a verified process map of the façade design and construction process. the map is the first of its kind to represent, in detail, the whole process relevant to all façade types, from commencement of the façade consultant’s and contactor’s participation, to the end of their involvement. the paper describes the process by which the mapping notation was selected, followed by the development and verification of the process map, including testing in two independent research projects. the buildingsmart’s bpmn notation is found to have superior system features and comprehensibility for this application and the resulting process map is easy to interpret and verify by industry experts. the trialling of the map in the two research projects indicate that the map is a useful tool for assessing process improvements in the façades sector. keywords: process map, façade design process, project management 1. introduction façade design and construction is a relatively new sector of the construction industry, as is the role of the professional façade engineer (karsai, 1997). the sector is complex in terms of the range of materials employed, geometries involved, performance requirements and its collaborative multidisciplinary nature. these conditions suggest that a process map would be a very useful aid to researchers in this field. 1.1. the benefits of process mapping the main benefits of process mapping include: increasing understanding of actor’s roles and activities (klotz et al., 2008); aiding identification of strategic, process and it requirements (aouad et al., 1999); or forming the basis for it systems (tzortzopoulos et al., 2005). mapping the ‘as-is’ situation in the planning, design and construction of sustainable buildings, increases the understanding of the process by 5–27% and leads to efficiency and quality improvements ∗corresponding author: eleanor voss, glass and façade technology research group, department of engineering, university of cambridge, trumpington street, cambridge, uk. e-mail: ev236@cam.ac.uk. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:ev236@cam.ac.uk 18 e. voss et al. / a bpmn-based process map for the design and construction of façades klotz et al. (2008). this agrees with the findings of tzortzopoulos et al. (2005), whose list of benefits to the construction industry includes supporting education and development of it systems. 1.2. process mapping in the façades sector there has been limited mapping of the façade design and construction process. much of what does exist focuses on specific façade typologies or provides a low level of detail. however, some of the work conducted to date has been used to develop the map presented in this paper. klein (2013) produced a high-level map showing the current involvement of stakeholders during different project stages and a set of maps showing a revised process for a new approach to façade design. in addition, a hierarchical set of façade design and construction process maps were developed in the claddiss project (pavitt & gibb, 2003) by eliciting knowledge from industry through interviews and questionnaires (pavitt & gibb, 1999). these maps are presented in a tabulated format and capture the key stages, decisions and management issues related to the interface between the façade and other building components. in both research projects the maps fulfil the purpose for which they were intended, but they either work to a very high level of abstraction (and low level of detail) or describe an ideal (or proposed) process. pavitt’s and gibb’s work formed the basis of a set of maps developed subsequently by vaz et al. (2008). the maps use the integration definition for function modelling (idef0) notation and were produced to improve the understanding of the façade design process and to identify façade-structure interface issues. these maps mainly provide a high-level overview of the façade design and construction process, showingthemainprojectstages.however,amoredetailedmapfocusingonlyonthedesignofthefaçadestructure interface was also developed. in addition, the idef0 notation used by vaz et al. is significantly clearer than the tabulated format used in the claddiss project. it is therefore evident that the choice of mapping notation should be considered carefully as it affects comprehensibility. the idef0 notation was also used in the cimclad project to map the design-to-installation processes of rainscreen cladding (agbasi et al., 2003). the map is constructed from the perspective of consulting façade engineers and it was used for the development of a rainscreen cladding product model. although the process is mapped to a high level of detail, the work focuses on a single type of cladding. there are also several research projects on the mapping of the precast concrete cladding sector. karhu (1997) worked on mapping the precast cladding process in order to develop a product data model using the idef0 notation. gray and al-bizri (2007) proposed a knowledge-based-engineering (kbe) representation of design processes and illustrated the concept with a map of the precast cladding design process. the maps were intended to clarify the roles and responsibilities of the designer. finally, eastman et al. (2003) have developed several maps, using different notations, of the architectural precast design and fabrication processes. this body of work provides significant levels of detail, but the maps are limited to precast concrete cladding. 1.3. process map production 1.3.1. mapping notations the idef0 mapping notation has been used in several processes mapping research projects, both in the façades sector (kalian et al., 1997; vaz et al., 2008) and the wider construction industry (laitinen, 1999; dawood, 2002; pingtserng, 2004). the idef0 is a function modelling notation and is the standard initial step in the iso-step methodology (eastman et al., 2002). it aims to capture decisions, actions, e. voss et al. / a bpmn-based process map for the design and construction of façades 19 and activities of an organization or system. the notation comprises of hierarchical diagrams using ‘box and arrow’ graphics, the box representing functions and the arrows represent inputs, outputs, controls and mechanisms (idef, 2010). an alternative notation is the business process modelling notation (bpmn) which is used by buildingsmart to develop the ifcs (international foundation classes). the notation is based on a flowchart technique with graphical objects representing activities and flow controls describing their sequence. hierarchical modelling is supported through a plus sign indicating expansion in the activity object (wix & karlshoj, 2010). a range of bpmn concepts are expressed in ifc-compatible format, for example, a swimlane, or zone of the map where the activities relating to a single professional role are placed, is represented by ifcactorrole (wix & karlshoj, 2010). several process maps are now available through the buildingsmart website (buildingsmart, 2008), some of which include parts of the façade design process, for example, the ‘architectural design to building energy analysis’ map. in addition, eastman et al. (2010) have developed a process map of the precast façade design process using the buildingsmart – bpm notation. two main knowledge elicitation methods are frequently used in the development of these process maps: (1) an initial literature review to gather information about the design and construction process (kagioglou et al., 2000; gray & al-bizri, 2007), followed by; (2) an evaluation by industry members, through workshops or case studies, to either produce or enhance the map (aouad et al., 1999; kalian et al., 2004; gray & al-bizri, 2007). once completed, the accuracy of a process map may be quantified by means of interviews in which real-world case studies are compared with the original map (austin et al., 1999). 1.3.2. evaluation of mapping notations alternative mapping approaches may be analysed by means of the structure provided by kochikar and narendran (1994), who distinguished between modelling power and decision power, where: modelling power is the universe of system features that a notation can represent, and; decision power is the ease with which a map can be analysed. the features of each of these two characteristics are summarised by koskela (p.g. 506, 1995). the idef notation has been reviewed in several research projects (karhu, 2001; eastman et al., 2002; wynn, 2007), however, there is a lack of publications on evaluations of the buildingsmart-bpm notation. 1.4. scope of the paper and content to follow at present, there is no detailed façade design and construction process map that is applicable to all façade types. such a map would be useful for identifying and quantifying possible improvements to the façade design and construction process. for example, in the development of software applications that support façade designers/fabricators without disrupting the existing processes, or in assessing the impact of a new procurement system. therefore this paper presents a verified process map of the façade design and construction process from initial design to completion for use as a research resource. in section 2, the two principal mapping notations (bpm-buildingsmart and idef0) are evaluated and a preferred notation is selected. this is followed by a description of the iterative interview-approach used to develop the map, a description of the map itself, and a qualitative evaluation of its accuracy. the value of the map as a resource for further research and development in the façade industry is demonstrated in section 3. finally, section 4 presents conclusions and areas of further work. 20 e. voss et al. / a bpmn-based process map for the design and construction of façades table 1 comparison of idef0 and bpmn-buildingsmart notations sufficient number of system features represented idef0 – does not effectively model the information generated by, and used in, activities (eastman et al., 2002) (karhu, 2001) – cannot be used effectively to describe iterative activities which are a key characteristic of design processes (wynn, 2007) – uses a diagram for each actor, making it difficult to compare actor’s activities alongside other actors’ (karhu, 2001) bpmn – represents generation and use of information with the specific ‘information object’ (wix & karlshoj, 2010), see fig. 1 – can represent repeat or iterative activities by a loop or a circular arrow respectively (wix & karlshoj, 2010) – uses ‘swim lanes’ to easily identify which actor performs which activity (wix & karlshoj, 2010), allowing the whole process with every actor involved to be seen at once hierarchical modelling and ability to represent different levels of abstraction idef0 – uses activity box labeling and a whole process map plan to indicate how the levels link together bpmn – uses a plus sign in the activity box to indicate that the box expands to reveal further detail on a lower hierarchical level model verifiability idef0 – does not include an effective way to verify the accuracy of the map produced (eastman et al., 2002) – produces complex models that are difficult to understand (karhu, 2001), (idef, 2010), making verification through industry consultation difficult bpmn – lack of published research on the comprehensibility of the bpmn-buildingsmart notation in the construction industry and so it will be reviewed as part of this mapping project in section 2.5 fig. 1. example of (a) bpm notation and showing an activity and two information objects and (b) idef0 notation showing two activities and an information annotation. 2. production of the process map 2.1. selection of mapping notation buildingsmart’s adaptation of the bpmn is a strong candidate for the façade process map, as it supports an exchange schema (buildingsmart ifc) that is particularly suitable for the development of computer based support tools. however, this modelling notation has not been used previously for e. voss et al. / a bpmn-based process map for the design and construction of façades 21 façade design process mapping. it is therefore sensible to evaluate it against the idef0 notation which is a recognised modelling standard (by american national institute for standards and technology) and which has frequently been used for process mapping research projects. the comparison presented in table 1 evaluates these two mapping notations against each other by considering each of koskela’s (1995) characteristics of modelling methods, however since the primary purpose of this map is modelling power, the evaluation disregarded, but did not penalise poor decision power. the accuracy of the whole façade design and construction process map presented in this paper has been verified through interview based consultation with industry, hence it is essential that the maps are understood with ease so that errors and omissions can be identified. therefore comprehensibility is used to assess model verifiability. in addition, the purpose of this mapping project is to produce an ‘as-is’ representation of the current process, rather than any changes to it. therefore the criterion that the notation should be able to represent evolution of processes is neglected at this stage. 2.1.1. conclusions on mapping notations from this evaluation it is evident that the buildingsmart bpmn adaptation is at least as suitable as the idef0 notation for the production of a whole façade design and construction process map, and in some criteria, the buildingsmart bpmn out-performs idfe0. the areas where the idef0 notation under-performs in the context of this mapping project are: (1) available representation system features (particularly iterative activities and allocation of activities between industry professions), and; (2) model verifiability through review, which requires comprehensibility. hence, the bpmn-buildingsmart modelling notation was selected. 2.2. the process map development method following initial consultation with the potential interviewees the following five step method was developed and employed: 1. relevant information from existing façade-related process maps was extracted. the most relevant work was the cimclad process maps (kalian et al., 2004). the idef0 notation employed in the original map was difficult to understand (see section 2.1.3), therefore, the maps were reformatted in the bpm-buildingsmart notation. the maps produced by the claddiss project (pavitt & gibb, 2003) were not used further in this project as they represent the ideal rather than the ‘as-is’ activities and information exchanges. 2. further knowledge was elicited from industry to ensure the map represented the design and construction process for all façade typologies. this started with the production of a highly simplified process map followed by interviews with selected industry experts. the interviewees were asked to work through the process map, adding information and making corrections. four interviews were conducted, two with façade sub-contractors and two with façade consultants. 3. the two sources of information (from step 1 and 2) were merged to form the version 1.0 of the process map using the bpmn-buildingsmart notation. when combining the information from the interviews, no judgement was made as to the relative authority or accuracy of different sources, instead leaving this to the later steps of review (step 4) and accuracy verification (step 5). any process map element that was specified by any source was included, while clashes 22 e. voss et al. / a bpmn-based process map for the design and construction of façades table 2 results of map validation in step 5 interviews. (total number of map elements=169) interview (a) missing (b) redundant (c) modified (d) accuracy (%) # 1 16 5 5 88 # 2 24 27 11 79 # 3 12 31 9 86 # 4 14 5 10 87 average 17 17 9 85 in sequencing were resolved by representing these steps as ‘potentially concurrent’ on the map. 4. the map was reviewed by industry experts using a workshop format and the suggested modifications were incorporated. the workshop participants consisted of three façade consultants from a single firm. the revised process map (v1.1) consisted of a total of 169 elements. 5. the revised map (v1.1) was verified by a further set of four interviews with industry experts using a case study format. the interviews were conducted with four different façade consultants, none of whom had been involved in the previous steps of the mapping process. during each of these interviews, the map was compared to the process undertaken in a recent real-world façade design projects and inaccuracies and omissions were recorded (table 2). each façade project had a construction cost of over £15m and was either completed in 2012 or later. three categories of inaccuracies were identified: (a) missing, which represent the process elements that were needed to capture the project but were not in the map; (b) redundant, which represent those elements that should be removed from the map to make it reflect the real project; (c) modified elements indicating process elements that required minor modification to conform to the process undertaken on the real project. from these the accuracy, of the map which quantifies the overall ability of the process map to represent each project, therefore accuracy d=[(169–b–c))/(169+a–b)]∗100%. 2.3. the process map the process map is large, therefore only extracts are reproduced in this paper. the full map (v1.1) is available through the glass and façade technology research group website (gft, 2012), where it is possible to navigate electronically through the map hierarchy using hyperlinks. two extracts have been selected to show a range of detail. figure 2 shows the top level map that summarises the whole façade design and construction process and an expanded portion of activity a5. the top level map (fig. 2a) illustrates the whole project process, ends of key stages are identified through double lined circles, and the whole process end is indicated by a bold lined circle. the map shows the façade consultant’s activities in the top swimlane and the facade contractor’s process step in the bottom swimlane. the plus signs at the base of each activity indicate that further detail on this activity is available and can be accessed by the user. the expansion of the main activities (fig. 2b) includes details of the information used and produced in each sub-activity. for example, the construction information comments are generated by activity a5.3 review of construction information including coordination with other systems. e. voss et al. / a bpmn-based process map for the design and construction of façades 23 fig. 2. map v1.1 (a) top level map and (b) portion of activity a5 expanded. 2.4. evaluation of process map and notation the accuracy verification step identified some errors in the map. however, the majority of the map accurately represented the real projects used in the verification interviews (step 5) interviews. on average v1.1 of the map was 85% successful in capturing the real-world project. it was noted that the same inaccuracies were identified in more than one interview, for example, the construction and review of visual and performance mock-ups had not been included in the map of step 5. all four interviews identified these as ‘missing’. although further development and improvement of the map is underway, the validation by means of independent interviews indicates that the map provides a reasonably accurate representation of the façade design and construction process. 24 e. voss et al. / a bpmn-based process map for the design and construction of façades during the interviews and workshops the industry experts were able to quickly interpret the map, indicating the notation is easily understood, and the industry processes were represented with ease, indicating that the notation has a sufficient number of system features. this practical evaluation confirmed that the bpmn-buildingsmart notation was suitable for the mapping project, particularly with regards to the ability of mapping non-experts to interpret the map. 3. use of process map in research projects a process map, such as the one presented in this paper, is particularly useful for assessing whether the outcomes of a research project are compatible with current industry processes (incremental /evolutionary innovation) or whether it would be beneficial to alter the current industry processes in order to exploit the benefits of the research (revolutionary / disruptive innovation). the bmpn façade process map was deployed in two on-going research case studies, and the contributions of the map to the research projects are identified and discussed in this section. 3.1. research case study 1 the knowledge-based façade design development (kbfdd) research project aims to improve communication between construction industry domains thereby improving the process of designing, construction and operation of façades. the objective of the research project is to develop a prototype building envelope information model (beim) tool that supports the façade consultant – and indirectly the design team – as they devise the panelisation scheme of façades. the beim tool encourages the users to generate design options that are feasible and cost effective to manufacture by increasing the users understanding of the effects of downstream design constraints. the project uses a construction industry neutral bim digital format to improve communication of design information between members of the design team (voss & overend, 2012). the process map described in section 2 played three key roles in the research project: firstly, the process map described in this paper provided the researcher with an overview of the façade industry, placing the project in context. the map was used as graphical summary of the actors, information exchanges, and roles in the sector. secondly, the map was used to identify patterns of use of specific information objects. for example, by identifying the repeated use of information objects that are not project-specific, a system can be developed to capture, store, and make accessible these elements of information for re-use in subsequent projects. in addition, by assessing when these information objects are used, the beim tool has been developed to encourage users to update the recycled information. for example, activity a1.5.1 (shown in fig. 3) requires knowledge of size restrictions on façade panels to assess the panelisation scheme proposed by the project architect. this information is owned by the consultant and not specific to the project. the same information is used again for activity a1.5.2, a development rather than a review task. the information stored by the consultant can be updated during activity a4.6.1 when the same type of information, this time owned by the contractor, but communicated to the consultant, is used to review the design. finally, the map provided the resource necessary to develop a tool that could be deployed in industry without disrupting the current process i.e. the tool will support and not hinder current practice. for example, the map enabled the researcher to ensure that the information required by the tool will be available at the stages in the design process that the tool is to be used. the beim tool can be e. voss et al. / a bpmn-based process map for the design and construction of façades 25 fig. 3. map v1.1, portion of activity a1.5 expanded to show sub-activities a1.5.1a, a1.5.1b and a1.5.2 and the information required and produced during these activities. 26 e. voss et al. / a bpmn-based process map for the design and construction of façades employed from activity a1.5, when the consultant has the appointment brief which documents the proposed panelisation scheme for the consultant to review. however, it is clear that the panelisation scheme becomes more detailed during the design phases so the beim tool must be able to handle a range of levels of scheme detail. 3.2. research case study 2 the second case study used the process map to develop a whole-life value (wlv) based facade design and optimisation tool. the optimisation tool is developed for early design stage. it is accounts for the three major design objective values: (a) functional value (indoor environment quality, structural serviceability and safety, and durability); (b) financial value (initial capital cost, operating cost, replacement cost, and disposal cost); (c) environmental sustainability value (resources consumption and emission to ecosystem). the evaluation of the three objective values is performed using a simulation engine consisting of 3rd party software and specially developed matlab scripts. this optimisation tool is still under-development, and some preliminary results can be found in jin et al. (2011, 2013). the bpmn façade process map was used: (1) to assess whether the design optimisation tool is compatible with the current façade design process, i.e. whether the information available at early façade design stage is sufficient to run the design optimisation tool and whether the outputs from the design optimisation tool provide sufficient information for the subsequent design stages, and; (2) to identify how the current façade design process captured in the process maps could be modified to make better use of the design optimisation tool. the ‘façade consultant’ swim lane encompassing activity a2 and activity a3 (fig. 2) is relevant in this case. according to activity a2 (shown in fig. 4) of the process map, the thermal performance criteria (e.g., u-value and g-value) are provided by the building services engineers for activity a2.x when initial thermal performance calculations are carried out by façade consultants to identify façade options that satisfy these criteria. this map shows that this activity is iterated until the proposed design satisfies the requirements set by both building services engineers and façade consultants. the building energy simulation module in the design optimisation tool can be deployed here, however, it performs a multiobjective optimisation i.e. the two tasks performed by the building services engineers and the façade consultants are considered simultaneously rather than separately. the process map indicates that all other input variables and constraints required for the optimisation tool (e.g. building geometry, location, orientation, plant size and plant type will be available at this stage from the riba (royal institute of british architects) stage c report. the output from the optimisation tool will also be timely as it can be included in the riba stage d report. therefore, the wlv façade design optimisation tool is partly compatible with the current design process and the process map successfully identified a problem that would arise when using the optimisation on real-world projects unless changes are made to the optimisation tool or to the current design process. a similar process was undertaken to assess the compatibility of the optimisation tool with the structural analysis (activity a3.1) and detailed thermal analysis (activity a3.2), but are not reported in detail here for brevity. this showed that the inputs and outputs for the design optimisation tool are compatible with the current design process. 4. conclusions and further work this work included a theoretical and a practical evaluation of two candidate process mapping notations from which buildingsmart-business process mapping notation emerged as the more suitable e. voss et al. / a bpmn-based process map for the design and construction of façades 27 fig. 4. map v1.1, portion of activity a2 expanded showing the information required and produced during initial thermal performance calculations. notation for the façades sector. the theoretical evaluation, using the characteristics of modelling notations, indicated that the bpmn was more suitable than the idef0 notation to model the façade design and construction process. the practical evaluation, through use of the process map in interviews with construction industry members, concludes that the notation can represent a sufficient number of system features, and is sufficient comprehensible to enable rapid verification through industry consultation. this paper also presented the first ever detailed bpmn-based process map of the whole facade design-construction process from commencement of façade consultant and contractor participation to the end of their involvement. this map combines previous mapping efforts and recent developments in this sector elicited from industry. the map is intended as a research tool that captures existing construction practices thereby providing the research community with an increased understanding of the construction industry and a basis for the development of design theories. the full map v1.1 is freely available to view through the gft website (gft, 2012). on average version 1.1 of the map successfully captured 85% of the processes undertaken in each of the four real façade projects used in the verification step interviews. however, the sample size is likely to be too small to make significant further generalisations. the verification step identified 28 e. voss et al. / a bpmn-based process map for the design and construction of façades some inaccuracies in the map that are being addressed at the time of writing to produce version v1.2, however, further testing will be required to verify the modifications. in addition, the map is limited by the sample size in both the map construction and map verification steps. in particular, the verification step (step 5), was limited to participants from façade consulting firms and none from contracting firms. this is likely to cause the design stages of the map to be more detailed and accurate than the construction stages. furthermore, the map has been produced from the perspective of the façade consultant and façade sub-contractor roles. this limits the map’s documentation of interaction of the core façade design and construction team with other project roles, for example the structural engineer. both of these issues could be addressed through further development work, but currently this is not planned. finally, the selection process for the notation deliberately neglected ability to represent process evolution. therefore if the map is used in future research to consider ongoing modifications of processes, further work would be required to verify the suitability of the notation. the process map was successfully trialled in two independent research projects that involve the development of knowledge-based systems for the facades sector. the map was found to be very useful for assessing the interaction between the proposed knowledge-based system and the current façade design-construction processes. the process map was successfully used in case study one to detect the presence of non-project specific actor knowledge throughout the design process, thereby assessing whether knowledge based technologies would be effective. the process map was used successfully in the second case study to verify the availability of information required for the proposed design optimisation tool. in this latter case, the use of the process map highlighted a shortcoming in the current division of scope and responsibility between designers that may inhibit façade design optimisation. acknowledgments thanks are due to focchi ltd, permasteelisa uk ltd, and the façade team at ramboll uk ltd for their input into the map construction phases. additional thanks are due to wsp group, arup, newtechnic and wintech for their contribution to the map verification phase. we would also like to acknowledge ramboll uk ltd and the epsrc for funding this research project. references agbasi, e., mcwilliams, r., stevens, w., stapleton, j., anumba, c. j., & gibb, a. (2003). cimclad, rais report, organisational-level issues bearing on cim implementation: facade engineer’s perspectives. whitby bird and partners report. aouad, g., cooper, r., kagioglou, m., & sexton, m. (1999). the development of a process map for the construction sector. in cib working groups w65/w55. south africa. austin, s., baldwin, a., li, b., & waskett, p. (1999). analytical design planning technique: a model of the detailed building design process. design studies, 20(3), 279-296. buildingsmart. (2008). information delivery manual overview. retrieved july 25, 2010, from http://iug.buildingsmart.com/idms/overview dawood, n. (2002). development of automated communication of system for managing site information using internet technology. automation in construction, 11(5), 557-572. doi:10.1016/s0926-5805(01)00066-8 eastman, c., lee, g., & sacks, r. (2002). a new formal and analytical approach to modeling engineering project information processes. in cib w78 (pp. 125-132), aarhus, denmark. eastman, c. m., jeong, y.-s., sacks, r., & kaner, i. (2010). exchange model and exchange object concepts for implementation of national bim standards. journal of computing in civil engineering, 24(1), 25. doi:10.1061/(asce)0887-3801(2010)24:1(25) eastman, c., sacks, r., & lee, g. (2003). development and implementation of advanced it in the north american precast concrete industry. precast concrete, 1-16. http://iug.buildingsmart.com/idms/overview e. voss et al. / a bpmn-based process map for the design and construction of façades 29 gft (2012). glass and facade technology research group. retrieved from www.gft.eu.com gray, c., & al-bizri, s. (2007). modelling trade contractor information production. architectural engineering and design management, 3(1), 39-48. idef (2010). www.idef.com. retrieved september 22, 2011, from www.idef.com jin, q., overend, m., & thompson, p. (2011). a whole-life value assessment and optimisation model for high-performance glazed facades. in proceedings of international conference of building simulation, sydney, australia. jin, q., & overend, m. (2013). a prototype whole-life value optimization tool for facade design. journal of building performance simulation. doi:10.1080/19401493.2013.812145 kagioglou, m., cooper, r., aouad, g., & sexton, m. (2000). rethinking construction: the generic design and construction process protocol. engineering construction and architectural management, 7(2), 141-153. doi:10.1046/j.1365-232x.2000.00148.x kalian, a., watson, a., agbasi, e., anumba, c., & gibb, a. (2004). modelling the building cladding attainment processes. business process management journal, 10(6), 712-723. karhu, v. (1997). product model based design of precast facades, royal institute of technology, stockholm, sweden. karhu, v. (2001). a generic construction process modelling method, royal institute of technology, stockholm, sweden. karsai, p. (1997). facade procurement: the role of the facade consultant. in icbest ’97, bath, uk. klein, t. (2013). integral facade construction. rotterdam: design: sirene ontwerpers. doi: http://dx.doi.org/10.7480/a%2bbe.vol3.diss3 klotz, l., horman, m., bi, h. h., & bechtel, j. (2008). the impact of process mapping on transparency. international journal of productivity and performance management, 57(8), 623-636. kochikar, v. p., & narendran, t. t. (1994). on using abstract models for analysis of flexible manufacturing systems. international journal of production research, 32(10), 2303-2322. doi:10.1080/00207549408957069 koskela, l. (1995). on foundations of construction process modelling. in cib workshop on computers and information in construction. stanford, california. laitinen. (1999). model based construction process management. durability of building materials and components, 8, 2844-2863. pavitt, t. c., & gibb, a. g. f. (1999). managing organizational interfaces in the cladding supply chain: initial results from expert interviews. in w. hughes (ed.), 15th annual arcom conference, 2, 15-17. liverpool, uk. pavitt, t. c., & gibb, a.g.f. (2003). interface management within construction: in particular, building facade. journal of construction engineering and management, 129(1), 8. doi:10.1061/(asce)0733-9364(2003)129:1(8) pingtserng, h. (2004). developing an activity-based knowledge management system for contractors. automation in construction, 13(6), 781-802. doi:10.1016/j.autcon.2004.05.003 tzortzopoulos, p., sexton, m., & cooper, r. (2005). process models implementation in the construction industry: a literature synthesis. engineering, construction and architectural management, 12(5), 470-486. vaz, d., al bizri, s., & gray, c. (2008). the management of the design of modern curtain wall cladding systems. in arcom 24th annual conference (pp. 759-768). cardiff, uk. voss, e., & overend, m. (2012). a tool that combines building information modeling and knowledge based engineering to assess façade manufacturability. in abs 2012, graz. wix, j., & karlshoj, j. (2010). buildingsmart: information delivery manual guide to components and development methods. wynn. (2007). model-based approaches to support process improvement in complex product development. university of cambridge. www.gft.eu.com www.idef.com www.idef.com http://dx.doi.org/10.7480/a%2bbe.vol3.diss3 from city’s station to station city 047 journal of facade design & engineering volume 9 / number 1 / 2021 potential of façade-integrated pvt with radiant heating and cooling panel supported by a thermal storage for temperature stability and energy efficiency mohannad bayoumi faculty of architecture and planning, king abdulaziz university, saudi arabia abstract hybrid photovoltaic/thermal (pvt) systems combine electric and thermal energy generation and provide noiseless operation and space-saving features. as the efficiency of photovoltaic (pv) panels increases at low surface temperatures, this paper suggests combining the pvt panel with a radiant cooling and heating panel in one system. a thermal storage tank fluidly connects the heat-exchanging pipes at the back of the pvt system and radiant panel. the upper portion of the tank feeds the radiant panel and the lower portion of the tank is connected to the pvt system. the proposed device is expected to function in connection with a heat pump that feeds the thermal storage. using the dynamic thermal simulation software polysun, the performance of the proposed façade-integrated device was investigated while considering the surface temperatures and energy production in the moderate climatic condition of the city of munich. the results indicate a substantial impact on the efficiency of the pv module with an increase of up to 35% in the electricity production of the pv due to the lowered surface temperature. the obtained results contribute to façade-supported cooling/heating and electricity generation through the novel coupling and integration of pv, pvt, and radiant cooling elements. keywords photovoltaic/thermal systems, radiant cooling, building-integrated photovoltaic, façade, solar cooling 10.7480/jfde.2021.1.5442 048 journal of facade design & engineering volume 9 / number 1 / 2021 1 introduction façade-integrated energy generation essentially contributes to the increase in the energy efficiency of buildings. in such an unconventional local energy generation approach, remarkable savings are achieved through the reduced efforts and losses in the transportation and conversion processes. in building-integrated photovoltaics, photovoltaic (pv) cells absorb a significant portion of the irradiated energy. this results in a significant increase in the surface temperature of the pv panel. the increased cell temperature substantially reduces the overall module efficiency. the output performance of the pv decreases by 0.4-0.5% for each degree increase in the cell temperature (natarajan, mallick, katz, & weingaertner, 2011). this is calculated in comparison with the standard test conditions (stc), where 25°c and 1000 w/m2 are set as the standard values of ambient temperature and solar irradiance (g), respectively. furthermore, according to experiments, the stc parameters do not represent the real operating conditions of pv panels (razak et al., 2016). in an investigation by radziemska (2003), the increase in surface temperature led to a power deterioration of 0.65% per kelvin. obviously, in hot climates, this effect is more severe owing to the relatively higher ambient temperatures. despite low ambient temperatures in moderate climates, the absorbed solar irradiance leads to remarkable losses as it also increases the cell temperature (huld & gracia amillo, 2015). this effect is particularly noticeable in façade-integrated pv module, for example, a south-oriented vertical pv module in europe. another important factor that affects the module efficiency is wind velocity, which is associated with convective heat transport. huld and gracia amillo (2015) claimed that despite the clear impact of this factor, ambient temperature and solar irradiance are still the two primary parameters that affect the efficiency. this is particularly true in the case of crystalline siliconbased cells. obviously, the geographical location performs a role in the intensity of either factor and results in a fluctuation in module efficiency ranging from -15% to +5% (huld & gracia amillo, 2015). moreover, the surface cooling of the pv has an inverse effect and increases the electricity production efficacy (pathak, pearce, & harrison, 2012). therefore, associating cooling with façade-integrated energy provides the potential for a further increase in efficiency. this indicates a deviation from the fact that high cooling loads often occur during times of high solar irradiation. pv cells can be cooled by attaching pipes that circulate a fluid at the back of the pv absorber. this combination of two systems is called hybrid pv/thermal (pvt) technology (zhang, zhao, smith, xu, & yu, 2012). the fluid in the pipes absorbs the thermal energy from the heated surface and delivers it to another point for uses such as potable water heating. depending on the design of the system, water is usually used as a refrigerant owing to its higher thermal capacity. moreover, the generated thermal energy can also be used for other applications such as potable water, air, and room heating. kern and russell (1978) were the first to present this technology in 1978. basically, a pvt system includes the functions of both a pv panel and a solar collector. besides the increased space efficiency, it offers lesser installations and more cost reductions. according to a study by charalambous, maidment, kalogirou, and yiakoumetti (2007), a pvt system can produce more energy per unit area than a pair of pv panels and a solar collector located next to each other. generally, the generated heat in the pv surface that is absorbed by the fluid is of low quality for domestic hot water uses or room heating. therefore, several pvt domestic hot water generation systems are equipped with auxiliary heaters connected to the heat exchanger at the top of the hot water storage tank to compensate for the thermal energy deficit (aste, del pero, & leonforte, 2012). furthermore, heat can be used for cooling energy generation. prieto, knaack, auer, and klein (2017) discussed the potential of façade-integrated pvt systems for solar-assisted cooling based on the 049 journal of facade design & engineering volume 9 / number 1 / 2021 principles of solar cooling addressed by henning (henning and international energy agency, 2004). however, these approaches are limited by the outlet temperature of the fluid that comes out of the pvt system, as significantly high temperatures in the range of 90-110 °c are often required for the operation of the absorption or adsorption chiller. this issue is beyond the scope of this paper. radiant cooling and heating systems have garnered significant interest owing to their potential to achieve high thermal comfort, low energy demand, noiseless operation, and space-saving features. in these systems, water pipes are attached to the back of a radiating panel. the circulating chilled water is delivered through the pipe and it cools down or heats up the panel, which eventually exchanges heat with other surfaces primarily through radiation or convection (rhee & kim, 2015; stetiu, 1999). the radiating panels can be integrated into floors, ceilings, walls, or any room surface. however, the surface temperature of the panel must remain above the dewpoint temperature of the air in the room to avoid condensation on the surface. several studies explored different methods to eliminate the risk of condensation, which often occurs in the cooling scenario (bayoumi, 2018b; hindrichs & daniels, 2007; hong, yan, d’oca, & chen, 2017; seo, song, & lee, 2014; song & kato, 2004; vangtook & chirarattananon, 2007; zhang & niu, 2003). the potential of combining façade-integrated radiant cooling and the pvt system in one device was investigated (bayoumi, 2018a). the cooling water is supplied by a chiller to the radiant panel; the return water is supplied to the pvt system and the return water of the pvt is supplied back to the chiller. the simulation results of this investigation concluded that in the selected warm climate, an increase of 35% in power conversion efficiency was achieved when compared to a conventional pv system. in another study, a detailed calculation of the heat transfer process was presented in association with thermal comfort simulations in several locations (bayoumi, 2020). the results indicated a substantial impact of thermal comfort as well as energy generation efficiency. in both studies, the simulated façade device is attached to a heat pump or chiller that supplies the radiant cooling panel surface with cold water. further, a pipe links the return of the radiant cooling surface with the supply of the pvt element. this helps to cool down the surface of the pvt element using relatively cool water that has already cooled the radiant cooling surface. both studies were limited to the room cooling process, which recommends further exploration of the potential for developing such devices for both purposes, i.e., room cooling and heating. the potential of a façade-integrated solar heater with thermal storage was discussed by pugsley, zacharopoulos, mondol, & smyth (2020). it primarily focused on the domestic use of warm water. the results indicate significant potential for increasing the efficiency of the heat generation process as well as the utilisation factor. this is primarily owing to the potential for overnight water storage. however, the potential for radiant heating for room climatisation was not considered in the proposed solution. the present research suggests a combination of a façade-integrated pvt panel with a radiant cooling/heating panel and thermal storage located between both panels. the thermal storage is connected to a heat pump and supplies both panels with water. the analysis includes the impact of thermal storage on increasing the efficiency of the system through the reduced energy demand of the heat pump. 050 journal of facade design & engineering volume 9 / number 1 / 2021 2 methods the basic components of the proposed façade-integrated system are a pvt panel, 150 l thermal storage, radiant heating and cooling panel, and heat pump that can be attached to the system. the characteristics of the heat pump are outside the scope of this proposal. as illustrated in fig. 1, while the pvt is located on the external side, the radiating surface faces the interior space and the thermal storage is located between both panels. the inlet and outlet points that connect the system with the heat pump are located on the side of the thermal storage. this implies that thermal storage is the distributor of the hot and cold water to the panels. the system was modelled using polysun to assess the performance and functionality of the proposed system. fig. 1 left: axonometric drawing showing the radiant heating/cooling panel from inside; right: external view showing the pipes attached to the back of the photovoltaic/thermal (pvt) panel. a high level of transparency has been set to the pvt panel for illustrative purpose fig. 2 exploded axonometric drawing of the system 051 journal of facade design & engineering volume 9 / number 1 / 2021 the exploded axonometric drawing in fig. 2 illustrates the basic relationships between the components. it can be observed that the radiant panel is connected to the thermal storage through supply and return connections located in the upper part. in the lower part, the supply and return connections transfer relatively cold water to the pvt system. in this approach, the thermal storage works as a water exchange medium between the three elements: pvt, radiant panel, and heat exchanger. further, this concept benefits from the temperature stratification that occurs during thermal storage. thus, cold water always goes to the pvt element and the output warm water goes to the radiant panel in winter. in summer, the cooling is not significantly affected because the radiant cooling panel operates at temperatures in the range of 18-24 °c, which is one of the advantages of a so-called high-temperature cooler. fig. 3 left: layout of the simulated system; right: parameters of the controllers the image on the left in fig. 3 illustrates an overview of the simulated model using polysun. it can be observed that the system includes two main pumps attached to each panel in addition to the primary heat pump. the pumps are operated by output signals from a set of controllers that read input signals from the surface temperatures of the radiant and pvt panels. in addition, the water temperature on different layers of the tank sends input signals to controller-1. the table on the right of the figure lists the protocols and operation statuses of the three controllers. the simulation was performed for 8760 h and covers the four seasons of the year. according to the schematic diagram, the height of the water supply and return in each component, including the thermal storage, is 052 journal of facade design & engineering volume 9 / number 1 / 2021 crucial as it affects the thermal quality of the water. the listed configuration is the result of different arrangements and indicates the optimum outcome. the system includes a dc/ac inverter attached to the pvt system. however, the efficiency of the inverter is not considered in the context of this study, and the analyses were primarily limited to the dc output. the proposed system has been filed for patenting (us patent application number: 16999129). table 1 specifications of the photovoltaic/thermal module electrical data (1) typical power (pn) [wp] 245 open circuit voltage (voc) [v] maximum power voltage (vpm) [v] short circuit current (isc) [a] 8.74 maximum power current (ipm) [a] 8.17 module efficiency (η) [%] 15.5 maximum system voltage [v dc] 1000 reverse current load (lr) [a] 15 temperature coefficient pn (ϒ) [%/°c] -0.43 temperature coefficient vpm (β) [%/°c] -0.34 temperature coefficient ipm (α) [%/°c] 0.065 (1) stc condition: irradiance = 1000 w/m2, cell temperature = 25 °c thermal data in the case of pvt aperture area [m2] 1.59 thermal efficiency (2) (η 0 ) [%] 56 nominal thermal power (3) [w] 888 volume flow rate [l/m] 1.5-2.5 flow losses [mmh 2 o] 400-900 fluid volume [l] 0.9 coefficient α1 (2) [-] 9.12 coefficient α2 (2) [-] 0 effective thermal capacity [kj kg-1 k-1] 20 iam k0 at 50 °c (2) based on aperture area (3) pv off conditions referred to (tm-ta) = 0 specification cells [-] 60 poly-si thickness [mm] 156 electrical connectors [-] mc4 hydraulic connector [“] 1/2 female dimensions [mm] 1638 x 982 x 41 weight [kg] 27 053 journal of facade design & engineering volume 9 / number 1 / 2021 for the purpose of testing the practicality of the presented invention, a location that requires heating and cooling was selected. consequently, munich, germany, was selected as the standpoint for the simulations. an orientation to the south with an inclination of 90° was selected. the assessment focused on the surface temperatures of both the pvt system and radiating panel corresponding to external factors such as outdoor temperature and irradiance into the pvt module. one of the advantages of radiant heating systems is to provide heating with a relatively low surface temperature and thus less heating power. for this to work effectively, the temperature of the room surfaces should be homogeneous and within the human comfort zone. besides air tightness, this requires well insulated walls, floors, and ceilings, as well as effective radiant heating elements. in this study, the surface temperature was set to around 21°c. the quality of the room surfaces and their insulation properties, which obviously affect the room temperature, were out of the scope of this paper. more focus was given to the surface temperature of a single radiant heating/cooling element. obviously, more elements may be needed in a room depending on its heating load. the radiating panel comprises an aluminium sheet with a thermal conductivity of approximately 202 w/m.k and specific heat capacity (cp) of 871 j/kg.k. further, the selected size of the radiating panel is similar to the pvt module, which is 1 m × 1.65 m. the pipes of both panels are of the same size, and the water mass flowrate is also identical across the components of the system. however, the water mass flowrate to/from the heat pump and the storage tank can be varied according to the heating or cooling demand of the system. table 1 lists the specifications of the pvt module. 3 results and discussion an overview of the simulation results is shown in fig. 4. the average data over the course of one year are presented. the preliminary axis depicts the temperatures in degrees celsius and the secondary axis depicts the energy in watts. the light grey curve indicates the pvt dc electric energy production in watts and the black curve indicates the thermal energy exchange between the heat pump and the thermal storage in watts. the outdoor temperature is indicated by the yellow curve. the light blue curve depicts the surface temperature of the radiating panel that achieves the cooling and heating functions of the room. while the pvt panel temperature is indicated by the dark blue curve, the pink curve indicates the surface temperature of a typical pv panel under the same conditions. from the first glimpse, it can be noted that in summer, a difference of approximately 12 k is achievable between the surface temperature of the pv and the pvt panels. this suggests that the proposed system has succeeded in significantly cooling down the surface temperature. from the figure, it can be observed that the temperature of the radiating element is stable at approximately 21 °c and varies within a small range of 1-1.5 k. this reflects the impact of thermal storage on stabilising the temperature of the radiating panel. another advantage of the relatively cool water at the bottom of the tank is that the surface temperature of the pvt panel in summer is significantly lower than the outdoor temperature. in winter, the panel temperature is maintained above 0 °c when the outside temperature is below 0 °c to avoid the requirement for antifreeze solutions. however, this aspect can be further optimised using controlling schemes. from the results of other simulations, it can be noted that the surface temperature of a conventional pv panel reaches 52.8 °c in the selected location. in the proposed system, the maximum surface temperature reached 37 °c. this difference has significant advantages for energy production and performance enhancement. as described earlier, for every 1 k increase in the surface temperature of 054 journal of facade design & engineering volume 9 / number 1 / 2021 the pv panel above 25 °c, a reduction of 0.5% in the performance is expected. from the diagram, it is also clear that the pvt dc electric energy production increases in winter, spring, and autumn, when the altitude of the sun is relatively low. during these seasons, the radiating panel presents a surface temperature that is reliable for maintaining the mean radiant temperature of the room within the comfort range. the generated electric energy can be used to operate the electric-driven heat pump. fig. 4 general overview on the system performance over the course of one year fig. 5 illustrates a comparison of the simulation results for the energy production in dc between a conventional pv system and the proposed pvt system. the brown and orange curves represent the energy production using the pv and pvt systems, respectively. generally, higher electricity production is expected in autumn, winter, and spring owing to the low altitude of the sun on the south façade. it is notable that a substantial increase in production efficiency can be achieved using the pvt system. this is primarily owing to the cooling effect and the reduction in surface temperature caused by the integrated thermal storage tank. further, the difference in electricity production reaches to more than 35%. fig. 5 comparison in energy production in dc a detailed observation during a high-temperature summer period in the first two weeks of july is shown in fig. 6. it is important to notice the effect of ambient temperature on the surface temperature of the conventional pv and proposed pvt systems. the right axis presents the solar irradiance into the module area. a remarkable temperature increase was observed in the first case that exceeds the outdoor temperature by approximately 40%. conversely, the pvt system presents a reduced module surface temperature in most of the cases, which reaches up to 10% of the outdoor temperature. the surface temperature of the radiant cooling/heating panel is stable and marginally affected by the outdoor temperature. it moderately varies between 20 and 24 °c. however, there is a clear indication that the surface temperature of the pv module is significantly affected by the solar irradiance into the module area. it is notable that both curves are in correlation. 055 journal of facade design & engineering volume 9 / number 1 / 2021 fig. 6 detailed observation during a high-temperature summer period in winter, another detailed observation was performed around the beginning of february and the results are shown in fig. 7. a significant increase in the surface temperature of the conventional pv system is evident from the figure. as noted in the previous figure, unlike the conventional pv module, the surface temperature of the pvt system is less likely to be affected by solar irradiance. the surface temperature of the radiant cooling/heating panel is nearly stable at 20 °c despite the extreme drop in temperature that reaches up to -12 °c. fig. 7 detailed observation during a low-temperature winter period to provide a validation for the increased efficiency of the proposed and simulated system over conventional ones, two comparisons have been made against existing systems whose live statistical data are available online. both pv systems are located in munich. the characteristics of each system are outlined in table 2. table 2 table 2 framework of the reference systems system number [-] 1 2 system name [-] dahoam (pvoutput.org, 2020a) gb53 (pvoutput.org, 2020b) system size [kw] 4.176 4.5 number of panels [-] 16 20 panel capacity [wp] 260 225 inverter [-] samil solarlake 4500 tl-d sma sunnyboy 3600 tl maximum efficiency (inverter) [%] 97.6 97 orientation [-] south south west tilt [°] 37 42 056 journal of facade design & engineering volume 9 / number 1 / 2021 using the simulation software polysun, comparisons have been made between the proposed system and each of the reference systems. therefore, in each simulation the framework of the proposed system has been modified to match the reference case. these modifications were essentially applied to the panel capacity, inverter efficiency, orientation, and tilt angle. the comparisons shown in fig. 8 and fig. 9 depict the difference in the accumulated monthly ac output of the pv panels (primary axis) and the associated increase/decrease in module efficiency (secondary axis). the diagram in fig. 8 presents an overall increase in the pv output of the proposed system over the reference system-1. it is clear that in the cold months remarkable increases in efficiency can be noticed. this is basically attributed to the decreased surface temperature of the panels. this is notable as the radiant panels are used for heating and the upper part of the thermal storage supplies it with relatively warm water. therefore, the water coming out of the thermal storage and moving at the back of the panel doesn’t seem to negatively affect the efficiency of the module. further, a slight decrease in the output in february can be seen. this exception maybe attributed to the statistically collected climate data that are embedded in the simulation software and may have different data regarding the irradiance and cloudiness of the site in comparison to the real readings from the system. this obviously affects the simulated output. moreover, thanks to the proposed an increase in efficiency of up to 31% can be noted. -10% -5% 0% 5% 10% 15% 20% 25% 30% 35% 0 5 10 15 20 25 30 35 40 jan feb mar apr may jun jul aug sep oct nov dec ef fic ie nc y [% ] [k w h/ m on th ] reference system-1 proposed system increase/decrease in efficiency fig. 8 comparison between the proposed system and the reference system-1 a more remarkable increase in efficiency is noted in fig. 9, where the efficiency of the proposed system has improved by 47% over the reference case. generally, the electrical output of the pv panel has substantially increased over the course of the 12 months. this is of particular interest in summer as, despite the increased ambient temperature, the cooled water from the thermal storage helped reduce the surface temperature and thus increase the module efficiency. however, the increased module efficiency and thus pv output are amongst the advantages of the proposed system that combines radiant cooling, radiant eating, thermal storage, and energy generation with an increased efficiency in one façade-integrated device. 057 journal of facade design & engineering volume 9 / number 1 / 2021 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 0 5 10 15 20 25 30 35 40 jan feb mar apr may jun jul aug sep oct nov dec ef fic ie nc y [% ] [k w h/ m on th ] reference system-2 proposed system increase/decrease in efficiency fig. 9 comparison between the proposed system and the reference system-2 4 conclusion this paper has presented the potential of a façade-integrated device where the pvt panel is located on the external side and the radiant cooling/heating surface faces an interior space; moreover, a thermal storage tank is located between the panels. both elements are connected through thermal storage. a comparison with conventionally free integrated pvs was included. the results indicate the high potential of the proposed system over the conventional pv system in terms of reduction in surface temperature and increase in electricity production efficiency. as high cooling loads coincide with high façade irradiance, the proposed solution has practical applications for an integrated building envelope system. the system also helps save space as it combines energy generation with radiant cooling/heating and integrates this into the building envelope. the impact of the thermal storage was clear in stabilising the surface temperatures and eliminating the correlation between the solar irradiance and the surface temperature. furthermore, the cooling or heating energy can be directly generated using a heat pump that is connected to the thermal storage. the proposed system can be developed in different sizes and depths and can be incorporated into walls, roofs, or inclined surfaces at any angle. multiple elements can be connected in series to achieve higher cooling and heating loads. key advantages: – a year-long solution in locations where heating and cooling are required. – the integration of thermal storage reduces overheating. – increased efficiency for electricity, cooling, and heating. – a shorter reaction time owing to thermal storage. – supply of heating energy during both the day and night. – good façade insulation owing to the various layers of materials, including the thermally insulated storage tank. – no requirement for antifreeze as the thermal storage can supply warm water to the pvt element when required. 058 journal of facade design & engineering volume 9 / number 1 / 2021 – the entire system is a wall element that can be sized flexibly. the thickness of the system can vary, and the water tank can also have variable thicknesses. – a larger system can also include potable water production. further research and development can be conducted to convert the panel to a window element that is operable to allow fresh air intake during periods of pleasant temperature. instead of cooling pipes, capillary tubes can be attached to the surfaces to achieve higher performance and a slimmer design. references aste, n., del pero, c., & leonforte, f. (2012). optimization of solar thermal fraction in pvt systems. energy procedia, 30, 8–18. retrieved from https://doi.org/10.1016/j.egypro.2012.11.003 bayoumi, m. (2018a). façade-integrated pvt with radiant cooling panels for increased energy and space efficiency. in 13th conference on advanced building skins (pp. 749–758). bern: advanced building skins gmbh. bayoumi, m. (2018b). method to integrate radiant cooling with hybrid ventilation to improve energy efficiency and avoid condensation in hot, humid environments. buildings, 8(5), 69. retrieved from https://doi.org/10.3390/buildings8050069 bayoumi, m. (2020). extending the efficiency of facade-integrated pvt through coupling with radiant cooling. journal of civil engineering inter disciplinaries, 1(2), 5–16. henning, h.-m., & international energy agency. solar heating and cooling programme. (2004). solar-assisted air-conditioning in buildings : a handbook for planners. wien ; new york: springer. hindrichs, d. u., & daniels, k. (2007). plusminus 20°/40° latitude : sustainable building design in tropical and subtropical regions. stuttgart ; london: edition a. menges. hong, t., yan, d., d’oca, s., & chen, c. (2017). ten questions concerning occupant behavior in buildings: the big picture. building and environment, 114, 518–530. retrieved 1 april 2017 from https://doi.org/10.1016/j.buildenv.2016.12.006 huld, t., & gracia amillo, a. m. (2015). estimating pv module performance over large geographical regions: the role of irradiance, air temperature, wind speed and solar spectrum. energies, 8(6), 5159–5181. retrieved from https://doi.org/10.3390/ en8065159 natarajan, s. k., mallick, t. k., katz, m., & weingaertner, s. (2011). numerical investigations of solar cell temperature for photovoltaic concentrator system with and without passive cooling arrangements. international journal of thermal sciences. retrieved from https://doi.org/10.1016/j.ijthermalsci.2011.06.014 pathak, m. j. m., pearce, j. m., & harrison, s. j. (2012). effects on amorphous silicon photovoltaic performance from high-temperature annealing pulses in photovoltaic thermal hybrid devices. solar energy materials and solar cells, 100, 199–203. retrieved from https://doi.org/10.1016/j.solmat.2012.01.015 prieto, a., knaack, u., auer, t., & klein, t. (2017). solar coolfacades framework for the integration of solar cooling technologies in the building envelope. energy. retrieved from https://doi.org/10.1016/j.energy.2017.04.141 pugsley, a., zacharopoulos, a., mondol, j. d., & smyth, m. (2020). bipv/t facades – a new opportunity for integrated collector-storage solar water heaters? part 2: physical realisation and laboratory testing. solar energy, 206(february), 751–769. retrieved from https://doi.org/10.1016/j.solener.2020.05.098 pvoutput.org. (2020a). pvoutput-dahoam 4.176kw. retrieved 18 november 2020, from https://pvoutput.org/display.jsp?sid=31109 pvoutput.org. (2020b). pvoutput-gb53 4.500kw. retrieved 18 november 2020, from https://pvoutput.org/display.jsp?sid=75152 razak, a., irwan, y., leow, w. z., irwanto, m., safwati, i., & zhafarina, m. (2016). investigation of the effect temperature on photovoltaic (pv) panel output performance. international journal on advanced science, engineering and information technology, 6(5), 682. retrieved from https://doi.org/10.18517/ijaseit.6.5.938 rhee, k.-n., & kim, k. w. (2015). a 50 year review of basic and applied research in radiant heating and cooling systems for the built environment. building and environment, 91, 166–190. retrieved 20 june 2017 from https://doi.org/10.1016/j. buildenv.2015.03.040 seo, j.-m., song, d., & lee, k. h. (2014). possibility of coupling outdoor air cooling and radiant floor cooling under hot and humid climate conditions. energy and buildings, 81, 219–226. retrieved 11 july 2014 from https://doi.org/10.1016/j. enbuild.2014.06.023 song, d., & kato, s. (2004). radiational panel cooling system with continuous natural cross ventilation for hot and humid regions, 36, 1273–1280. retrieved from https://doi.org/10.1016/j.enbuild.2003.07.004 stetiu, c. (1999). energy and peak power savings potential of radiant cooling systems in us commercial buildings. energy and buildings, 30(2), 127–138. retrieved from https://doi.org/https://doi.org/10.1016/s0378-7788(98)00080-2 vangtook, p., & chirarattananon, s. (2007). application of radiant cooling as a passive cooling option in hot humid climate. building and environment, 42(2), 543–556. retrieved from https://doi.org/10.1016/j.buildenv.2005.09.014 zhang, l. z., & niu, j. l. (2003). indoor humidity behaviors associated with decoupled cooling in hot and humid climates. building and environment, 38(1), 99–107. retrieved from https://doi.org/10.1016/s0360-1323(02)00018-5 zhang, x., zhao, x., smith, s., xu, j., & yu, x. (2012). review of r&d progress and practical application of the solar photovoltaic/ thermal (pv/t) technologies. renewable and sustainable energy reviews, 16(1), 599–617. retrieved from https://doi. org/10.1016/j.rser.2011.08.026 p:\ios_press\finals\fde\fde 1(1-2)\web\fde0008.dvi journal of facade design and engineering 1 (2013) 1 doi 10.3233/fde-130008 ios press 1 preface journal of facade design and engineering we are delighted to announce the launch of the journal of facade design and engineering. it is the result of an ongoing discussion with colleagues from our discipline about the need for a scientific platform that focusses on the building envelope. since we founded the façade research group at the tu deft in 2005, we observed a growing demand for façade design and engineering in the building field. one reason obviously is the importance of the facade related to the energy consumption of buildings as well as the user comfort. this development is also reflected in the scientific field; with many questions yet to be answered. they concern technologies, methods and tools to reach the above mentioned energy savings of buildings. we need to investigate constructional, functional and formal developments of the building envelope and we need more fundamental and in-depth knowledge about façade building materials and structural aspects. this means façade design and engineering is a multidisciplinary field that touches many other scientific disciplines such as climate design, building physics, structural design, architectural design, process management, product development and many more. on top of that, we believe that the dissemination of science into practice and industrial innovations must be one of the main targets of the journal. when the european façade network was founded in 2009 (http://facades.ning.com), an international community was established that allowed us to start creating a new scientific platform: facade design and engineering is a peer reviewed, open access journal, funded by the netherlands organisation for scientific research nwo (www.nwo.nl). we see ‘open access’ as the future publishing model. but it certainly requires new financial models which we will have to explore in the coming years. however, primarily it is a great opportunity to attract a broad, open and lively audience and author community. the first issue is an important step. we received a lot of interesting paper contribution by different colleagues and the following issues are already planned. the final goal is to publish four 4 issues per year. while proceeding, we now face the challenging task to shape the scope of the journal in detail (which is currently rather broad) and we need your valuable contribution to it. the published content is one outcome of this activity; the other is the scientific discussion around our interesting discipline. may jfde become a valuable resource for professionals and academics involved in the design and engineering of the building envelope. we are looking forward to many interesting contributions and discussions! ulrich knaack, tillmann klein issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. http://facades.ning.com www.nwo.nl from city’s station to station city v journal of facade design & engineering volume 7 / number 1 / 2019 editorial the powerskin conference series is a biennial event organised cooperatively between tu münchen, tu darmstadt, and tu delft, which is already in its fourth edition, having started in 2017. this coming edition of powerskin has also been supported and organised with the support of rwth aachen. the conference addresses the role of building skins in accomplishing a carbon-neutral building stock. therefore, integrating the environmental dimension of material and construction into the design phase is increasingly essential. this is done primarily by considering the energy and emissions linked to the building fabric's fabrication and its ability for reuse and recycling. for this reason, the focus of the powerskin conference 2022 is the building fabric with its environmental potential to unlock. therefore the theme is: "build in stock – renovation strategies: inorganic, circular materials vs organic, compostable materials". this theme is discussed through the following sub-themes: 1 envelope: the building envelope as an interface for interacting between indoor and outdoor environments, new functionalities, technical developments and material properties. 2 energy: new concepts, accomplished projects, and visions for the interaction between building structure, envelope and energy technologies. 3 environment: façades or elements of façades which aim to provide highly comfortable surroundings where environmental control strategies, energy generation and/or storage are an integrated part of an active skin. this special issue of the journal of façade design and engineering dedicated to powerskin 2022 showcases the conference's most prominent and relevant papers, aiming to enhance their visibility for a larger audience. ulrich knaack editor in chief doi http://doi.org/10.47982/jfde.2022.powerskin.00 journal of facade design and engineering 1 (2013) 3–16 doi 10.3233/fde-130004 ios press 3 assessing facade value how clients make business cases in changing real estate markets alexandra den heijer∗ delft university of technology (tu delft), faculty of architecture and the built environment, department of real estate & housing, delft, the netherlands received: 23 july 2013 accepted: 5 november 2013 abstract. relevance: the value of any intervention in the built environment is most relevant for the stakeholders that are investing in it. for them all costs need to be balanced with benefits not necessarily directly financial, but adding value to the performance of the accommodated organization. business cases contain performance criteria like competitive advantage (branding the organization), productivity (optimally supporting users of the building), profitability (on organizational level) and sustainable development (monitoring the ecological footprint). in the changing real estate markets – from supply-driven to demand-driven and with increasingly higher vacancy rates – priorities in decisions about buildings have been shifting. purpose: this paper elaborates on how (a) the trends in real estate markets and (b) changing priorities in decision making affect the quality demand for buildings and their facades. design/methodology/approach: this paper is based on research of the real estate management chair in general (market analysis, transformation trends, conceptual frameworks) and more specifically on decisions about university campuses in the past 10 years (14 campuses and 57 buildings assessed). findings: this paper provides both conceptual frameworks to assess the (added) value of interventions in the built environment for the client and their (changing) priorities in the brief for buildings and their facades. keywords: performance assessment, life cycle, costs, sustainable development, buildings, integration 1. introduction the value of any intervention in the built environment is most relevant for the stakeholders that are investing in it. for them all costs need to be balanced with benefits, not necessarily directly financial, but adding value to the performance of the accommodated organization or individuals. in general the ‘return on investment’ in the built environment is measured in better performance. the question is how to measure (changed) performance and relate it to choices in design and engineering, and more specifically facades. ∗corresponding author: dr. ir. alexandra den heijer, delft university of technology (tu delft), faculty of architecture and the built environment, department of real estate & housing, julianalaan 134, 2628 bl delft, the netherlands. tel.: +31 15 278 4159; e-mail: a.c.denheijer@tudelft.nl. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:a.c.denheijer@tudelft.nl 4 a. den heijer / assessing facade value how clients make business cases in changing real estate markets fig. 1. assessing ‘facade value’, linked to performance criteria (den heijer, 2011, edited). 1.1. scientific approach: real estate management measuring the (presumed) impact of real estate interventions on performance is the basis of ‘real estate management’ as a scientific discipline. if real estate had no effect on performance, no society, organisation or individual would spend resources on it. ironically, it is easier to prove the negative effect of a dysfunctional facade on the productivity of employees or image of an organisation than to find evidence for a positive effect on performance. however, any evidence of the effect on performance will be important to real estate managers, designers and engineers of the built environment of the future. 1.2. four types of values this paper focuses on decisions about (portfolios of) buildings and their (presumed) effect on performance, either negative or positive. it uses conceptual frameworks from corporate and public real estate management (de vries, de jonge & van der voordt, 2008; den heijer, 2011) to operationalize ‘facade value’ linked to four performance criteria, see fig. 1. as illustrated in fig. 1, there are four types of performance criteria that organisations are focussing on, based on empirical studies and interviews with decision makers in public and private organisations (de vries et al., 2008; den heijer, 2011; van der schaaf, 2002). the four types of performance criteria connect demand side and supply side of the built environment, on strategic and operational level: competitive advantage (branding the organization), productivity (optimally supporting the users of a. den heijer / assessing facade value how clients make business cases in changing real estate markets 5 the building), profitability (costs and benefits on organizational level) and sustainable development (monitoring the ecological footprint). these criteria correspond with four types of values: strategic, financial, functional and energy value. these values interact and need to be balanced in each decision about the built environment. 1.3. assessing four types of facade value in accordance with this theory the following four questions – corresponding with the four value types – should be answered when assessing ‘facade value’: how does the building’s facade support the organisation’s identity and goals, adding to the competitive advantage (rank, distinctiveness) of the organisation and representing the ‘strategic value’ of facades? how does the building/facade affect life cycle costs, market value, adding to the profitability (costs and benefits on organizational level) and representing the ‘financial value’ of facades? how does the building/facade support user activities, improve user satisfaction, adding to the productivity (output versus input) of the organisation and representing the ‘functional value’ of facades? how does the building/facade reduce energy use, improve the technical condition (indoor climate), adding to the sustainable development (reducing the ecological footprint) of the organisation and representing the ‘energy value’ of facades? the strategic and functional value focus on (optimally supporting) the primary processes of the organisation – the demand side of the market; the financial and energy value focus on (reducing) the resources that are required for these processes – the supply side of the market. these four (facade) value types, the market changes that influence them (section 2), the way they interact (section 3), and how clients prioritize (section 4) will be subject of this paper. 1.4. research methods and structure paper research results from the real estate management chair in general are used to describe the changing market context in section 2: real estate market analysis in public and private sectors, trend reports and common real estate strategies. these results are based on document analysis, literature review and databases of buildings and their characteristics – location, age, functions, quality aspects (including the facade quality), costs and space utilization – making use of theories, concepts and conclusions from various phd research projects (de vries, de jonge, & van der voordt, 2008; den heijer, 2011; remøy, 2010; soeter, 2010). in sections 3 and 4 the different value types are operationalized and illustrated with evidence from (phd) research projects. some conclusions about “user preferences” – aspects that users consider ‘valuable’ – and “priorities of decision makers” are based on either interviews or questionnaires (remøy, 2010; den heijer, 2011). next to analysing these stated preferences (and priorities) the revealed preferences were also assessed, collecting transaction and vacancy data, and relating preferences to rent prices and costs of ownership (functional versus financial aspects). for assessing decisions about university buildings all fourteen dutch universities supplied project data (57 recent university projects) and portfolio data (14 university campuses). these databases are used to illustrate trends and to recognise patterns in decision making about buildings and their facades. this paper will 6 a. den heijer / assessing facade value how clients make business cases in changing real estate markets end with conclusions and recommendations for both the design product (section 5) and the design process (section 6). 2. the changing market context any decision about the built environment is influenced by developments and trends in the real estate market. this paper focuses on non-residential real estate. this market is currently (2013) characterised by high vacancy rates. this is illustrated with some figures from the dutch real estate market: more than 14% – representing 7 to 8 million m2 – of commercial offices are vacant and more than 7% of commercial retail floor area, about 2 million m2. both are likely to increase (rapidly) due to the on-going development of new office buildings and large-scale retail on the demand side and new ways of working and internet shopping on the demand side (heijnders, 2013; koppels, remøy, & de jonge, 2009; remøy, 2010). 2.1. most real estate is not commercial however, ‘commercial real estate’ – even though it dominates the news about the real estate market – only represents a relatively small part of the total non-residential real estate portfolio. the majority of buildings are either ‘corporate real estate’ (owned and used by industry) or ‘public real estate’ (schools, hospitals, government buildings, cultural facilities). the dutch non-residential real estate market consists of about 80 mln m2 commercial real estate (offices, retail – owned by private and institutional investors), 120–140 mln m2 ‘public real estate’ and more than 170 mln m2 ‘corporate real estate’ (soeter, 2010; heijnders, 2013). this excludes an estimated 250 mln m2 buildings for agricultural and other purposes (soeter 2010, based on historical data about sectoral investment, building permits and demographic trends). 2.2. transformation as leading trend since the economic crisis and the high vacancy rates ‘transformation’ has been the leading trend in the real estate market (remøy & wilkinson, 2012), both in an ‘upgrading’ and ‘downgrading’ sense. transformation usually refers to changing the function of the building – from office space to residential space for instance – in order to increase the profits per m2 (upgrading). value is added when the profits exceed the costs per m2. this can also be achieved by choosing a function that will decrease the costs (downgrading). in any case the owner of the building will search for a function that is still in demand. even the traditionally supply-driven market for ‘commercial real estate’ has become demand-driven, paying more attention to future use and users. investors that have traditionally focussed on ‘financial value’ are now expanding their view to ‘functional value’, ‘energy value’ and ‘strategic value’, because their potential tenants have plenty of alternative choices. 2.3. vacancy is primarily related to location, but facade quality also matters research (koppels & remøy, 2013) shows that vacant buildings in the (commercial) office market are most likely located in mono-functional, peripheral areas. certain ‘age groups’ are well represented in the vacancy rates, like buildings from the period 1980 to 1995 (remøy, koppels, & de jonge, a. den heijer / assessing facade value how clients make business cases in changing real estate markets 7 2009). these buildings are more likely to be vacant because of their location than because of their building characteristics. nonetheless, there also seems to be some evidence that the appearance of the building has an effect on structural vacancy. facade material and the facade’s technical quality are explicitly mentioned as aspects that matter when preventing vacancy or coping with vacancy (remøy, 2010). even for buildings that were built after 1995, poor facade quality significantly increases the odds of structural vacancy. research results also illustrate that it is easier to prove the negative effect of a low quality facade than the positive effect of a high quality facade. however, these results show that facade quality matters in vacancy rates and that the facade therefore represents a financial value for real estate owners. 2.4. user preferences: the building’s appearance is important apart from assessing vacancy rates – identifying revealed preferences of office users – there are also research projects that measure ‘stated preferences’: how office users themselves rank their preferences (which can differ from revealed preferences due to the effect of price). next to location aspects like the availability of parking, the appearance of the building and the space efficiency (flexible lay-out) had the highest scores in a so-called ‘office scan’ (remøy et al., 2009; remøy, 2010). the researchers stated that in times of crisis office tenants are “keeping up appearances”, referring to the importance of the building’s exterior. the building’s appearance was studied as several separate variables, including facade material, specific architecture, monumentality and building height. other research that compares the dutch and the australian office market, shows that – in adaptation projects – changing the appearance of the building has a relatively high added value compared to other interventions (wilkinson & remøy, 2011). this last conclusion again emphasizes the financial value of facades (extra investment in the facade versus added value per m2). 2.5. energy performance also affects vacancy because of the increased attention for sustainable development – there is a relation between vacancy and building characteristics for buildings dated from 1995 (until 2013). their energy performance plays a role in the vacancy rates: a better the energy performance is more attractive for tenants and consequently also leads to more cash flow. this relation illustrates the connections between ‘energy value’ on the one hand and ‘functional value’ and ‘financial value’ on the other hand. since the facade plays an important role in the energy performance of buildings, this highlights the energy value of the facade and – due to its impact on both productivity and vacancy – indirectly the functional and financial value of the facade. 2.6. demand-driven markets corporate and public real estate markets are much more ‘demand-driven’ than commercial real estate markets: their focus is on their primary processes (i.e. selling products, education, health care). real estate is just one of the resources to achieve the corporate or public goals. consequently, they will compare the effectiveness of an investment in their buildings with an investment in their personnel (den heijer & de jonge, 2012). owners of these buildings want to measure the effect of a new facade on their performance (productivity, rank, profit, footprint). if a smart facade can improve 8 a. den heijer / assessing facade value how clients make business cases in changing real estate markets their productivity with 5%, this can be worth extra investments, which should be made explicit in a business case. for owners and users of ‘corporate real estate’ and ‘public real estate’ assessing the influence of the building on performance has a longer history. their accountability to stakeholders or responsibility to achieve political goals forced them to. consequently, they are more likely to make business cases when they assess the (added) value of investments in their buildings. compared to commercial real estate owners they will put more emphasis on energy value, functional value and strategic value than on (only) financial value – also when assessing the added value of the facade. 2.7. decentralised budgets if one trend characterises the context of public real estate, it is the tendency to decentralise budgets to the users themselves (boards of schools, universities, hospitals, etc.). the advantage is that all performance criteria – strategic, financial, functional and physical (energy) – are covered by the same organisation, overseeing both the investment costs and the operating costs. the disadvantage is that the organisation needs expertise on all of these subjects to make ‘the right decision about a project’ by weighing different types of values. since this is even a complex matter to experts, this challenge can also lead to suboptimal decisions, like choosing a facade based only on the (lowest) initial costs. consultants, designers and engineers can support clients in the decision-making process by including all value types in their arguments for building (facade) solutions. 2.8. decreasing budgets in the past decades the decentralisation of budgets has usually been combined with ‘budget cuts’, which has forced organisations to be more efficient with their resources. this could lead to focussing on the lowest initial investment level (of a construction project), even if it doubles the operating costs or – worse negatively affects their productivity. nonetheless, the culture of assessing ‘life cycle costs’ and measuring the effect on performance has become more common. this is most established in sectors with large ‘public real estate’ portfolios, like government buildings (9–12 mln m2 in netherlands), schools and universities (40–45 mln m2) and health care facilities (roughly 55 mln m2), data (heijnders, 2013) from eib and vng. these are the types of clients that are most likely to make business cases for their decisions about transforming, adapting or adding buildings – to account for spending public resources and to demonstrate the added value to public goals. 2.9. strategy: more quality and less quantity as a consequence of market developments current trends in corporate and public real estate are reducing the footprint (both ecological and physical: in m2), considering transformation before adding new space, reconsidering ‘territory’ and encouraging ‘shared space’ (den heijer, 2011). this can be summarized in “trading quantity for quality of space” (less m2, higher quality). still, higher costs per m2 need to be compensated by higher benefits per m2. these trends in the (public) sectors are illustrated in the next sections, using university real estate as an example. a. den heijer / assessing facade value how clients make business cases in changing real estate markets 9 3. assessing value – financial versus functional value when assessing design and construction projects many clients still consider the investment costs the most important part of ‘financial value’. fig. 2 shows a selection of dutch campus projects (57 projects in the database, den heijer, 2011; den heijer, curvelo magdaniel & bentinck, 2013) in m2, investment level, use and goals. these projects were described to create references for new campus projects. the investment levels of these projects range from 1375 to 4160 euros per m2 gross floor area (gfa). the database also includes operating costs (maintenance, energy and cleaning), which are equally important as investment costs for assessing financial value. 3.1. focus on life cycle costs increasingly, clients are focussing on life cycle costs: the total costs of ownership (or rent) per m2. for the total campus these range from 70 to 130 euro per m2 gfa (data from 13 dutch universities, den heijer, 2011). case study research of school buildings also showed that the annual operating costs have a larger share in the life cycle costs than the annual capital costs of the initial investment in the building (de jong & arkesteijn 2013). the annual capital costs are influenced by what is considered the economic lifetime of an investment – 30 years is considered a standard depreciation period. a building with an investment level of 3000 d /m2 would roughly have annual capital costs of 100 d /m2, when not considering interest on the loan. however, components of the building could have a longer or shorter functionality, which could lead to new investments (replacements) after 15 years. any (facade) solution that extends the functional lifetime and postpones reinvestments can be worth its initial (extra) investment, which life cycle costs calculations could indicate. 3.2. measuring benefits per m2 in the annual operating costs the three larges components are ‘maintenance’, ‘energy’ and ‘cleaning’ costs. especially energy costs are increasing rapidly (estate management statistics, aude, united kingdom). in the uk more than 160 universities compare data about property costs (75 to 100 £/m2 m2 net internal area (nia) for the whole campus, excluding capital costs). at the same time uk universities collect data about the benefits per m2. they use key performance indicators (kpis) like “teaching income per m2 teaching space” and “research income per m2 research space”. in the same year teaching income ranged from 1400 to 2100 £/m2 nia and research income from 1100 to 2300 £/m2 nia. if an investment in a new facade concept could add 5% to the productivity (leading to profit) per m2 annually, the extra capital costs per m2 could be 55 £/m2 nia (considering the lowest income/m2). this can be capitalized to an investment budget. this is the type of business case clients should make for any design, construction or management decision. 3.3. functional value: hierarchy of needs the example above shows that financial value is very much dependent on functionality. functional value and financial value are closely connected, representing the benefits and costs of a project. for assessing the functional value maslov’s hierarchy of needs (1954) can be used as a conceptual framework. this classification of cumulative human needs is a useful tool for determining perceptual qualities that need to be realised and should be operationalized in the brief for a building (blyth & 10 a. den heijer / assessing facade value how clients make business cases in changing real estate markets fig. 2a. a selection of dutch campus projects, expressed in m2, euros, use and goals – codes refer to the project database – price level 2009 or 2012 (for projects after 2009), workplacem2 and % refer to usable floor area (ufa), investment costs are related to gross floor area (gfa) – for full project profiles, see dissertation (den heijer, 2011) and benchmark report (den heijer et al., 2013): http://managingtheuniversitycampus.nl/publications. http://managingtheuniversitycampus.nl/publications a. den heijer / assessing facade value how clients make business cases in changing real estate markets 11 fig. 2b. (continued) 12 a. den heijer / assessing facade value how clients make business cases in changing real estate markets fig. 3. hierarchy of needs – cumulative levels in quality demand, used for campus research: goals of projects: “plain & efficient” (covering healthy & safe), “meeting place” (adding social) and “representative” (adding attractive and inspiring) – applied in fig. 2. worthington 2001). maslov defined seven types of needs: (a) physiological needs, (b) safety needs, (c) social needs: need for contact, (d) esteem needs, to be valued and respected, (e) cognitive need: the urge to know and understand, (f) aesthetic needs: striving towards beauty and the perception thereof and (g) self-actualisation. for the brief of buildings these were translated to cumulative quality levels for creating a healthy, safe, social, attractive and inspiring built environment, see fig. 3. the essence of these cumulative needs is that they need to be fulfilled from bottom to top: just investing in an attractive physical environment without covering health & safety issues can have an adverse effect on both production and user satisfaction. in practice many campus managers have discovered that creating a (social) place to meet has been more functional – leading to more productivity – than creating a ‘landmark building’. this aligns with maslov’s theory. a landmark building usually adds to strategic value – adding to the reputation and attractiveness of the organisation. investing in (facade) aesthetics is also used for urban regeneration, adding to the reputation or image of an urban area (riccardo, van oel, & de jong, 2012). other research shows that investing in a healthy and safe environment has the best measurable effect on productivity (i.e. tno, 2007: assessing school buildings). again, finding evidence for the negative effect of an unhealthy indoor environment on productivity is much easier. business cases for new (facade) concepts – especially when assessing transformation projects – could also aim at ‘preventing future productivity loss’, as another way of ‘increasing productivity’. sustainable innovations can contribute to healthier and safer indoor environment, while contributing to sustainability goals at the same time. the energy performance of the built environment has gained importance in the past decade: many organisations have signed agreements to reduce the footprint, not just the ecological footprint, but also the physical footprint in floor area per user (den heijer & teeuw, 2011). priorities of policy makers have changed, influenced by market developments and changed needs of society, organisations and user groups. more emphasis on better use of scarce resources (energy, euros, m2) has changed the real estate strategies of many clients, which will be illustrated in the next section. 4. changing priorities of decision makers – strategies to reduce footprint interviews with decision makers about campuses in the past five years illustrated a change in strategy: from creating territory to creating shared space. the (very) low occupancy rates of classrooms, a. den heijer / assessing facade value how clients make business cases in changing real estate markets 13 lecture halls, laboratories and offices in combination with the high life cycle costs per m2 were the main incentive. the relative large footprint per student and academic staff member also negatively affected the benefits per m2: the production in terms of diplomas, publications, citations, patents etc. increasing flexibility while safeguarding identities of user groups has become the challenge on campus – balancing energy use and strategic value. 4.1. key performance indicators to measure effect reducing the footprint as a campus strategy has many positive effects on a university’s performance: it could reduce the costs, increase the benefits per m2, encourage social and intellectual interaction between different user groups and add to sustainability goals. to measure these effects (before and after the project) key performance indicators are used for sustainable development (from co2 omission to m2 per student), for productivity (occupancy rates, user satisfaction), profitability (life cycle costs as % of income per m2) and competitive advantage (importance of quality of life and facilities in the university’s reputation monitor). to illustrate the impact of facilities on competitive advantage: “in 2011 research showed that 36% of british students rejected a university due to the (poor) quality of its estate” (hedqf, 2012). strategic value is usually expressed in terms of ‘opportunity costs’: the cost of an alternative strategy to achieve the same goal. the campus projects that were introduced in fig. 2 will be assessed over time on these key performance indicators: are the project goals achieved and what can designers and other decision makers learn from that assessment? by analysing projects from many different public and private portfolios evidence is collected about the (added) strategic, functional, financial and energy value of (facade) solutions, which will support designers and engineers. based on evidence-based insights they can improve their products, aligning with the client’s demands in the (facade) brief. it will also contribute to better communication between the client, designers and engineers, while the key performance indicators combine variables from both the demand side (goals, users, needs) and the supply side (m2, quality, costs, energy use). 5. conclusions and recommendations propositions for the (facade) brief summarizing the previous sections, the following question should be answered to assess ‘facade value’: how does the facade influence the organization’s productivity, profit, identity and sustainability goals? this paper gave an overview of research projects that contribute to answering this comprehensive research question. notions from real estate management research can help to find more evidence-based answers to this question, to support facade designers and engineers in understanding the preferences and decision-making process of clients. some of them are expressed below, in conclusions about facade value and recommendations for the (facade) brief. – the facade brief should contain aspects that refer to all value types: financial value, energy value, functional value and strategic value. consultants, designers and engineers can support clients in the decision-making process by including all value types in their arguments for building (facade) solutions. 14 a. den heijer / assessing facade value how clients make business cases in changing real estate markets 5.1. financial facade value – arguments about how the facade adds to financial value should relate the initial investment costs to the annual capital costs (depending on the lifetime of the solution: the period until reinvestment is required) and the operating costs, demonstrating a life cycle costs approach. at the same time, these life cycle costs should be compared to life cycle benefits (production per m2). – architects and engineers should find solutions to extend the functional lifetime of their concepts, because this will add to the benefits per m2, decrease the annual capital costs per m2 and enable a higher initial investment level (larger budget for the project). – to increase profitability it can be a better strategy to improve space utilization and reduce floor area than to save costs on the quality of the building and its facade. 5.2. functional facade value – arguments about how the facade adds to functional value should include the influence of the facade on productivity – or on preventing productivity loss – and how the facade supports the changing user needs. – useful in the facade brief can be the notions from maslov’s theory, a cumulative approach to user needs: health, safety, social and aesthetic needs (in that order). focussing on aesthetics before focussing on health and safety issues negatively influences user satisfaction and functional value. even fulfilling social needs – like adding to the sense of community – precedes satisfying aesthetic needs, according to maslov’s hierarchy of needs. increasingly, organisations want to be open and transparent, show the production process and enable (social) interaction to encourage innovation – facade design should support this ambition. – with the increasingly shorter functional lifetimes of buildings – due to rapidly changing user needs and changing tenants of buildings – flexibility is an important strategy, because many buildings will be adapted or transformed during their lifetime. since the use of the building has to be flexible, the facade should not dictate fixed solutions for the floor plan. this also relates to the flexible floor plan as one of the highest-scoring user preferences. 5.3. energy facade value – arguments about how the facade adds to energy value should include the influence of the facade on the indoor environment and energy use. – related to (reducing) the energy value and adding to sustainability goals many clients are changing their strategies to ‘reducing their footprint’ and shared space, even though identity of users has become more import. trading quantity of space for quality of space (which adds to the identity of users) has become a popular strategy. in short: more focus on reducing the client’s footprint will save resources to invest in quality of space. 5.4. strategic facade value – arguments about how the facade adds to strategic value should refer to the image of the organisation and how its most important stakeholders (and clients) perceive the quality. for some types a. den heijer / assessing facade value how clients make business cases in changing real estate markets 15 of organisations a facade with a high aesthetic quality (implying relatively high initial costs) can even have a negative affect on the organisation’s image. – strategic value for commercial real estate owners can also be related to the long-term rentability of the building, preventing vacancy. research shows that the quality of the facade does have an impact on office vacancy rates and is ranked in the top 3 of user preferences. these research results show that facade quality matters in vacancy rates and that the facade therefore represents a financial value for real estate owners. – in general it is recommended to relate every design decision to the client’s performance criteria. that includes the process of defining the (facade) brief, in close collaboration with the client. this approach will also contribute to better communication between clients and designers. 6. new partnerships in the design process many conclusions and recommendations as stated above are subject of on-going research to collect more evidence and customised business cases to support them or elaborate upon them. project databases – like the campus databases – can be used to generate references to support design decisions. this requires data from both supply and demand side of the project. this encourages partnerships between designers and clients at an early stage, to improve so-called ‘value chains’. involving the client – the future owner/user of the building – in briefing and design decisions is one step, but giving them the evidence-based knowledge about how new (facade) concepts influence their performance is even more important for successfully implementing innovation in the built environment. for the future of the built environment demand and supply side should go hand-in-hand. research that explores the changing roles of designers also emphasizes that. this is all the more reason to share knowledge between the chairs of real estate management, design & construction management and design of constructions for future research. this paper can be considered as a first step in making this happen. acknowledgments the author wants to thank the staff of the facade research group at the faculty of architecture and the built environment of the tu delft for the opportunity to approach this subject from a different scientific perspective, both at the future envelope 7 “facade value” conference in june 2013 and in this paper. this is considered to be the starting point of more collective projects, which i look forward to. i want to thank my colleagues at the department of real estate & housing for providing their most relevant research material, which could be merged into a comprehensive overview of the knowledge that we could share on this subject, to improve the communication between client and designers/engineers – spending the scarce resources on the most effective interventions – and to collectively contribute to the quality of the built environment. references blyth, a., & worthington, j. (2001). managing the brief for better design, london, spon press [using maslov 1954 – hierarchy of needs]. de jong, p., & arkesteijn, m. (2013). “life cycle costs of dutch school buildings”, conference paper, eres 2013, vienna. 16 a. den heijer / assessing facade value how clients make business cases in changing real estate markets de vries, j. c., de jonge, h., & van der voordt, t. (2008). “impact of real estate interventions on organisational performance”, in journal of corporate real estate, vol. 10. den heijer, a. (2011). managing the university campus information to support real estate decisions, (dissertation/book), delft, eburon academic publishers. den heijer, a., & teeuw, p. (2011). “sustainable visions for the campus of the future”, at misbe 2011, management and innovation for a sustainable built environment, amsterdam, the netherlands. den heijer, a. & de jonge, h. (2012). “adding value – linking decisions and performance” in p. a., jensen, t., van der voordt, & c. coenen, (eds.), fm added value anthology, lyngby (denmark): polyteknisk. den heijer, a., curvelo magdaniel, f. & bentinck, s. (2013). dutch university campus projects (part 4:17 projects) hoi benchmark 2012-2013, (research report for 14 dutch universities). delft: tu delft. “estate management statistics” (2010). association of university directors of estates (aude), retrieved through aude website http://aude.ac.uk (directly: http://www.hesa.ac.uk). heijnders, l. (2013). vastgoed – problematiek en oplossingsrichtingen (report for the social and economic council of the netherlands – ser – in dutch: sociaal-economische raad), rijksgebouwendienst/tu delft, 2013. hedqf (2012). “higher educational facilities research” report by higher education design quality forum, hedqf, uk (google: “out of the box” aude 2012). koppels, p., & hilde r. (2013, forthcoming). “the amsterdam office market, (dataset, maps and preliminary conclusions)” – to be published in 2013, client: municipality of amsterdam. koppels, p. w., remøy, h. t., & de jonge, h. (2009). “the economic value of image” in propertynl research quarterly, 31-38. “performance in higher education estates”(2006) by association of university directors of estate (aude) estate management statistics (ems), annual report. remøy, h., koppels, p., & de jonge, h. (2009). “keeping up appearance” in real estate research quarterly, 8(3). remøy, h. (2010). out of office a study on the cause of office vacancy and transformation as a means to cope and prevent. phd thesis, tu delft, faculty of architecture. remøy, h. t., & wilkinson, s. j. (2012). office building conversion and sustainable adaptation: a comparative study, property management, 30(3), 218-231. riccardo, f., van oel, c. j., & de jong, p. (2012). “neighbourhood regeneration by facade redesign: a visual experiment on energy efficiency and aesthetics.” in the international journal of architectonic, spatial, and environmental design, 6(2), 57-80. soeter, j. p. (2010). bouwen voorraadeconomie 1960-2025, delft, vssd. tno, het effect van ventilatie op de cognitieve prestaties van leerlingen op de basisschool, (“the effect of ventilation on the cognitive skills of primary school pupils”), delft, tno bouw en ondergrond, 2007. van der schaaf, p. (2002). public real estate, challenges for governments, delft: dup science. wilkinson, s. j. & remøy h. (2011). “sustainability and within use office building adaptations: a comparison of dutch and australian practices” in prres 2011 conference proceedings, gold coast australia, bond university. http://aude.ac.uk (directly: http://www.hesa.ac.uk) from city’s station to station city v journal of facade design & engineering volume 10 / number 1 / 2022 editorial ulrich knaack 1, thaleia konstantinou 1 1 delft university of technology we are proud to publish a new issue of the journal of façade design and engineering. this first issue of volume 10 covers a broad range of themes such as adaptivity, automation, circularity, refurbishment, daylight, acoustics, and user interaction are addressed by peer-reviewed articles. we would also like to take this opportunity to share some changes in the jfde editorial team and publishing process. prof. tillmann klein is changing his role from editor-in-chief to editorial board member. tillmann klein has been a front-runner in open-access publishing since jfde was launched in 2013. together with ulrich knaack he was responding to a call from the netherlands organisation for scientific research (nwo) to develop new journal models. the editorial team is most grateful for its outstanding contribution to making jfde a reference point in the façade design and engineering field. the role is taken over by dr. thaleia konstantinou, who has been promoting the journal as managerial editor in the past years. as we go forward, we strive to ensure scientific quality as well as relevance for society and industry. jfde has revised the editorial process giving a larger responsibility to the editorial board to control the scientific rigour of our published articles. we have also introduced procedures to regularly extend the editorial board. it makes us very proud to have reached the 10th year of publishing with jfde, respected and valued by our authors and readers. thanks for this! and, of course: to be continued! ulrich knaack and thaleia konstantinou editors in chief doi http://doi.org/10.47982/jfde.2022.1.00 journal of facade design and engineering 1 (2013) 31–51 doi 10.3233/fde-130007 ios press 31 thermal and lighting perception in four fully glazed office buildings in santiago, chile claudio vásqueza,∗, felipe encinas pinoa, alejandro prieto hocesa and carlos aguirre nuñezb aschool of architecture, catholic university of chile, providencia, santiago, chile bschool of architecture, desarrollo university, san carlos de apoquindo, las condes, santiago, chile received: 15 october 2013 accepted: 21 november 2013 abstract. this paper is part of a general research project whose main objective is to establish a baseline for post-occupancy energy consumption and indoor environmental quality for office buildings in santiago, chile. this study aims at understanding how architectonical variables relate to, and can even determine, user comfort perception. thus, one-year continuous monitoring in several floors at four office buildings was performed and seasonal surveys were completed. survey participants were asked a series of questions regarding spatial orientation and comfort perception in their workspace. the data from the comfort survey and onsite measurements such as season of the year, case study, type of workspace and possibility of an outdoor view from the workstation were contrasted with the components obtained by a principal component analysis (pca). three components were selected from the pca, and three maps of perception (mp) were produced. these maps were then analyzed and interpreted so as to obtain information on the general perception of thermal and lighting comfort at workspaces within several office buildings in santiago. keywords: office buildings, thermal comfort, architectural design 1. introduction office building development in santiago has increased dramatically since the end of the 20th century. this growth has brought along significant technology transfer in construction methods and the subsequent sophistication of building components. however, high building technology standards do not necessarily correlate with good indoor environmental conditions or efficient energy consumption. quite the contrary, high energy requirements for cooling purposes and glare recurrently appear in office buildings in santiago. indeed, according to a sensitivity analysis conducted by pino, bustamante, escobar and encinas (2012), the factor that best correlates with energy demand (for both heating and cooling) in santiago, is the ratio of glazed surfaces versus opaque areas of the vertical envelope. within the office buildings here studied, window-to-wall ratios of 20% show energy demands below 40kwh/m2 (in the best case, as low as 25kwh/m2) while fully glazed façade buildings can reach up to 155kwh/m2. this is particularly significant in santiago, where building technologies originally associated to the improvement ∗corresponding author: claudio vásquez, school of architecture, catholic university of chile. 1916 el comendador str. providencia, santiago, zip: 7530091, chile. tel.: +56 9 92826305; e-mail: clvasque@uc.cl. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:clvasque@uc.cl 32 c. vásquez et al. / thermal and lighting perception in four fully glazed office of environmental or energy efficiency issues are used mainly for aesthetic reasons, failing sometimes to fulfill their primary intention (encinas, 2004). user behavior and building energy performance are highly correlated. the presence of elements such as scheduled indoor gains or ventilation (by means of manually controlled windows) can lead to variations in the energy consumption. various studies have proven that similar buildings can present large differences in thermal behavior, which suggests that user behavior exerts a strong influence (guerra, 2011). thermal comfort for those working in an office building depends on parameters such as orientation, outdoor view from workspace, type of workspace (open floor plan, individual or shared office), seasonal or personal characteristics. this is true even in buildings as those here described in which indoor air is mechanically conditioned and controlled. in the case of dwellings, for instance, andersen, toftum and olesen (2009) proposed that given the strong link between user behavior and energy consumption, thermal comfort needs to be characterized in order to forecast user interaction with building control mechanisms (window opening and closing, use of heating, use of artificial lighting, and use of solar shading). a similar situation occurs in office buildings, as showed by a survey on thermal comfort and workplace occupant satisfaction conducted in 16 german low energy office buildings which proved that indoor environment control by users and the perceived impact of their intervention strongly influence thermal comfort level (wagner, gossauer, moosmann, gropp, & leonhart, 2007). zhang & altan (2011) performed a comparative study on the occupant comfort between a conventional (mechanically conditioned) high-rise office building and an environmentally concerned building (including a south-facing atrium with the aim of driving the passive stack natural ventilation) in sheffield, uk. according to their study, there is a significant difference between both buildings in terms of their thermal and visual comfort. the conventional building was characterized by overheating, poor ventilation, glare and excessive solar radiation, which may be explained by the presence of extensive glazed surfaces on its envelope and the excessive dependence on its hvac system (evidenced by the difficulty to open windows, for example). this situation explains our interest in establishing a baseline of post-occupancy energy consumption and indoor environmental quality of office buildings in santiago, chile, by means of continuous monitoring of several floors in four office buildings and an online seasonal survey. 2. background from a climatic point of view, santiago’s warm temperate weather with a dry summer reminds us of california, north africa, the mediterranean zone and the persian gulf as described in köppen’s climatic classification. a short winter rainy period and a long dry season are characteristic. seasonal thermal oscillation reaches 13◦c (summer vs winter average temperatures), and daily thermal oscillations range between 14 and 16◦c (maximum vs minimum daily temperatures). the radiation level in the horizontal plane is between 1.380 and 1.666kwh/m2 year, equivalent to chile’s northern desert coastal zone. from a descriptive point of view, high standard office buildings in santiago exhibit the most innovative and complex façade solutions available in chile today. a review of 43 office buildings built in santiago between 1997 and 2010 revealed that 64% of the cases use curtain wall; 78% has an airtight façade solution (no operable windows); 66% has no solar protection and 58% has window to wall ratio (wwr) in the range between 75% and 100%. shape wise, most are slender prisms and have a low thermal mass (table 1). c. vásquez et al. / thermal and lighting perception in four fully glazed office 33 table 1 representative characteristics of office buildings in santiago building shape parameters facade system parameters more balanced less curtain nonstructural loadbearing mixed compact compact compact wall facade facade facade (mc) (bc) (lc) (cw) (nf) (lf) (mf) shape factor 65% 21% 14% facade type 65% 16% 5% 14% (sf) (ft) high average low 0%–24% 25%–49% 50%–74% 75%–100% proportion proportion proportion (hp) (ap) (lp) floor plan 33% 47% 20% window-to0% 9% 16% 75% proportion (pp) wall ratio (ww) thus, if we combine climatic and constructive features, the general hypothesis is that fully glazed office buildings in santiago do not respond adequately to their climate context, leading to indoor discomfort and the need for high energy consumption in order to achieve comfortable conditions. architectural design greatly determines building energy performance, and architects normally work in interdisciplinary teams to achieve effective results. despite these efforts, large discrepancies between what designs predict and what occupants perceive are observed. much research is conducted trying to define and calibrate mathematical models that can be transferred to software in order to reduce these discrepancies. however, user behavior is not usually considered. in their research on the impact of occupancy parameters in office building energy simulation, azar and menassa (2012) did include user behavior. their study included models representing a variety of sizes and climatic situations in the usa. the results revealed that energy use in buildings is significantly influenced by user’s actions and occupancy behavioral parameters (building size and weather conditions), two parameters that are not normally considered in calculation models. other authors as, macdonald, clarke & strachan (1999) stated that one of the main sources of uncertainty in the field of building performance simulation is user behavior. a third research paper posed that user behavior may also constitute one of the main reasons for the large and frequently observed discrepancies between the calculated or simulated energy performance and measured energy performance in real buildings (roetzel, tsangrassoulis, dietrich, & busching, 2010; olesen, 2011). similarly, yu, fung, haghighat, yoshino, & morofsky (2011) suggested that it is difficult to completely identify the influences of user behavior through simulation due to user’s behavior diversity and complexity. this idea was further expanded by hummelgaard, juhl, saebjörnsson, clausen, toftum and lankilde (2007) in a study in which user’s perception and indoor environment conditions in five mechanicallyand four naturally-ventilated open floor plan office buildings were characterized and compared. during the one-week study, indoor air quality was monitoring. results revealed that temperature and co2 concentration were the most variable features, varying in some mechanically-ventilated buildings more than in naturally ventilated cases. however, user perception did not differ greatly. this led the authors to consider that user perception may have been influenced by other building design features -lighting qualities, visual aspects, solar accessibility, etc.and management characteristics (autonomy 34 c. vásquez et al. / thermal and lighting perception in four fully glazed office in the use of light and ventilation or air conditioning operating features). these studies led us to believe that user comfort perception and its correlation to architectural design is an area on which further research needs to be performed. qualitative perception data is usually collected via surveys and always needs to be contrasted with air quality measurements. differences between perception and raw data will reveal the real performance of design, and can help us define some guidelines on office building management. the correlation between building maintenance management services for indoor environmental quality and user satisfaction was established in two surveys carried out in korea (kwon, chun, & kwak, 2011). the first survey was applied to seven cases. the study concludes that services maintenance and user satisfaction are correlated up to 60% and from that point on, the correlation is lost. the second survey selected two cases from the first group: one case with high services maintenance and low user satisfaction, and other with low maintenance management services and high user satisfaction. this second survey showed that user satisfaction was the same in both situations, despite the differences in adaptive opportunities (operable windows and personal thermostats). unexpectedly, users do not seem to value environmental adaptive opportunities, thus other aspects such as design or local habits need to be taken into account to assess user perception and satisfaction. the relationship between user perception and office building design needs to be further understood, and this paper aims to be a contribution in this direction. to do so, this paper presents a holistic approach to user comfort perception by means of a comfort perception data analysis in several office building in santiago, chile represented as maps of perception (mp) based on a principal component analysis (pca). 3. methodology cases were selected from an extensive office building review which only included office buildings in use for over two years. a total of 43 office buildings constructed in santiago between 1997 and 2010 were reviewed, and four cases were selected considering the main characteristics of the whole sample (table 1). three kinds of data were assessed in each case: indoor environmental parameters, energy consumption and comfort perception. the environmental parameters were used to compare indoor and outdoor conditions during occupied and unoccupied hours, and energy consumption measurements and comfort surveys were used to quantify operation cost and gain insight on user perception, respectively. this paper aims at comparing comfort perception (obtained from survey user’s responses) with on-site measurements. 3.1. case description the four study cases share constructive, normative and orientation features, being located very near each other (table 2; fig. 1). other similarities include floor surface and height, presence of a curtain-wall façade system and a high window-to-wall ratio (between 75% and 100% for all cases). regarding glazing, all cases consider tinted hardened glass as outer layer of a double glazing, with solar heat gain coefficients between 0,28 and 0,31. also, all offices consider internal shading devices (screens), operated by each user. centralized hvac systems (two chillers on top of the building), with fan-coils for air distribution within each office floor are also present in all four cases. in terms of management, case 1 is owned c. vásquez et al. / thermal and lighting perception in four fully glazed office 35 ta bl e 2 co m pa ri so n of st ud ie d ca se s st ud y ca se s bu ld in g ch ar ac te ri st ic s fl oo r ch ar ac te ri st ic s n ◦ g ro ss flo or fa ca de to ta l a na liz ed g ro ss flo or fl oo r ) fa ca de fa ca de /fl oo r fl oo r pl an sq ua re in te rn al of flo or s ar ea (m 2 ) ar ea (m 2 ) vo lu m e (m 2 ) flo or ar ea (m 2 ) vo lu m e (m 3 ar ea (m 2 ) ar ea pr op or ti on m et er s pe r ga in s/ flo or (w id th /l en gt h) pe rs on ar ea (k w h/ m 2 ) c. 1 23 21 .8 99 ,4 5 10 .4 97 ,2 0 76 .6 48 ,0 8 23 95 2, 15 2. 57 0, 81 35 2, 08 0, 37 0, 61 20 ,7 0 0, 24 c. 2 20 22 .8 46 ,6 0 9. 64 9, 88 77 .6 78 ,4 4 15 1. 14 2, 33 3. 02 7, 17 37 6, 06 0, 33 0, 71 18 ,4 2 0, 30 c. 3 20 23 .7 79 ,0 0 9. 02 4, 84 78 .4 70 ,7 0 6 1. 18 8, 95 3. 09 1, 27 35 5, 52 0, 30 0, 75 9, 99 0, 35 c. 4 24 24 .1 66 ,3 2 11 .3 83 ,2 0 82 .1 65 ,4 9 10 1. 00 6, 93 2. 66 8, 36 36 9, 68 0, 37 0, 64 8, 32 0, 46 36 c. vásquez et al. / thermal and lighting perception in four fully glazed office fig. 1. scheme of case study urban situation. entirely by one company, and thus, managed by one administration. the other three cases are rental buildings; therefore, management may vary between floors. this study analyzed floors individually for each building. 3.2. the instruments: survey and on-site measurements the questions of the comfort survey are divided into three comfort dimensions, namely lighting, temperature, and sound, and their respective indicators (table 3). in addition to these variables, specific on-site measurements were included for the purpose of comparison. a) control and segmentation: this variable registers demographical and location information for each survey participant. the demographical dimension includes indicators such as gender and age. location information registers workspace information such as orientation and spatial characteristics (private office, shared office, open floor plan). b) environmental comfort perception: this variable includes three dimensions: thermal, visual and acoustic comfort each with its own variables. this variable aims to identify user level of environmental satisfaction. also, it aims to characterize user perception regarding various attributes, architectural or adaptive, which affect thermal, visual and acoustic comfort. for example, regarding thermal comfort, the questionnaire asked about architectural attributes such as orientation or access to outdoor views. among adaptive attributes, survey participants were asked to rate options such as being able to open windows or control the hvac system –options that may not be available in their building. survey participants rated their comfort perception as “relevant”, “neutral” or “not relevant”. c) on-site measurements: with each survey, temperature and relative humidity measurements were registered with data loggers every 10 minutes. these data loggers were located so as to include all orientations and types of workspace (fig. 2). this information was used to establish the percentage of hours within a certain temperature range under work schedule (20–24◦c on autumn/winter and 23–26◦c on spring/summer months), thus generating a new indicator c. vásquez et al. / thermal and lighting perception in four fully glazed office 37 ta bl e 3 su rv ey ’s qu es ti on na ir e 38 c. vásquez et al. / thermal and lighting perception in four fully glazed office fig. 2. studied cases plan description. to estimate temperature sensation and compare it to perceived thermal comfort.1 also, daily maximum, minimum and average outdoor temperatures were included. the questionnaire was applied to 138 randomly selected users distributed in the four cases described above. all selected users answered simultaneously the one-week survey four times throughout the year. with a total population of 361 office workers, the chosen sample allows us to establish a 95% of confidence level and a 6.5% margin of error. answers were collected online and on the same week per season in all four cases. 1 for each of the questionnaires, the data from the nearest data logger was used, in order to assess thermal discrepancies between different zones of the floor. c. vásquez et al. / thermal and lighting perception in four fully glazed office 39 fig. 3. average external thermal measures by season. 3.2.1. descriptive analysis of the survey from a gender point of view, 73.2% (n=101) of the sample corresponds to male participants, and 26.8% (n=37) are women. per ages, 64.5% (n=89) are 18 to 35 years old, 34.8% (n=48) are 36 to 50 years old and 0.7% (n=1) are over 51 years old. in terms of workspace, 47.8% (n=66) of survey participants stated they work in an open floor plan, 37.7% (n=52) shared their office and 14.5% (n=20) had private offices. given that all offices included in this survey were mechanically conditioned by means of hvac system, a comfortable environment at least in terms of thermal comfort was to be expected. however, 27.5% (n=38) of the participants declared that they had not felt thermally comfortable in their offices during the previous month. furthermore, regarding specific thermal sensation, 28.3% (n=39) of the participants declared that they had felt either “hot” or “warm” at some point, 17.4% (n=24) stated having felt “cool” or “cold” and 54.3% (n=75) of the people expressed their perception of thermal comfort as “neutral”. the latter was expected to be the most extensive answer given the use of air conditioning. for visual and acoustic comfort indicators, 21.7% (n=30) of the interviewed subjects declared that they did not feel comfortable in terms of visual conditions, and just 10.1% (n=14) declared acoustic discomfort in their workspace. 3.2.2. descriptive analysis of the measurements maximum/minimum daily outdoor temperature measurements showed differences of up to 7.8◦c (autumn) and 12.4◦c (summer). differences on average temperature reached to 13.9◦c between 40 c. vásquez et al. / thermal and lighting perception in four fully glazed office fig. 4. average internal thermal measures by season. summer and winter, and 4.5◦c between autumn and spring. results are typical for santiago, where large thermal differences are characteristic (fig. 3). the results obtained per workspace and outdoor temperatures per surveyed week are shown in figs. 3 and 4. 3.3. methodology of statistical analysis: principal component analysis (pca) variables were analyzed using a principal component analysis (pca) to find the underlying structure of the data obtained from the survey and onsite measurements. pca is a statistic method capable of identifying inter-correlations among large numbers of variables, so as to infer the set of shared underlying factor or components (hair & anderson, 2005). the goal is to condense information contained in the original variables into a smaller number of variables (aka components) causing a minimal loss of information. this allows researchers to establish the underlying structure of the array of original data. for each analyzed variable, the pca calculates the pearson correlation coefficient of each component. only statistically significant variables are considered (table 4). each component generates a set of new orthogonal variables with continuous values which are then allocated in the array of original data. in this study, values ranged from −2.43 to +3.73 (table 5). variables were checked for information redundancy to ensure coherence between the resulting components and the theoretical framework of the study. four components were established which allow us to holistically understand user comfort perception in the case studies. c. vásquez et al. / thermal and lighting perception in four fully glazed office 41 table 4 pca components factors comp. 1 (ref) comp. 2 (alf) comp. 3 (tso) comp. 4 (rif) relevance given by users to orientation for thermal comfort 0,66 relevance given by user to daylight excess for their lighting comfort 0,63 relevance given by user to the lack of dayligth for their lighting comfort 0,68 relevance given by user to reflection for their lighting comfort 0,82 frequency that artificial lighting is used by users during last month 0,87 time of day when artificial lighting is used by users 0,90 relevance given by user to external noise for their acoustic comfort 0,67 relevance of internal noise for acoustic comfort 0,85 thermal sensation of past 30 days declared by user 0,65 0,50 percentage hours above 26◦c, measured during the week that survey was responded 0,71 table 5 pca components description value comp. 1 (ref) comp. 2 (alf) comp. 3 (tso) comp. 4 (rif) min −2,29 −2,70 −2,43 −2,30 max 1,87 1,12 3,73 2,54 average 0,00 0,00 0,00 0,00 3.3.1. maps of perceptions (mp) mps attempt to visually display the comfort perception revealed by users in the survey. given that each component allocates orthogonal factors to each variable of the array of original data, the variables can be graphed in correlation to the components. in addition, the factors of the original array can be grouped and graphed into families to reveal how the whole behaves in correlation with the components. in mps abscissas and ordinates correspond to pca resulting components. in this study the data families are, namely, season of the year; study case; orientation; and workplaces with/without outdoor view. 4. results a data base including survey responses and measurements was set up. nine data selected by pca analysis were used to create four components, each one suggesting an aspect of user comfort percep42 c. vásquez et al. / thermal and lighting perception in four fully glazed office tion. table 4 describes the factors comprised in each pca component. the closer a pca coefficient is to 1, the higher the correlation. positive values indicate a direct correlation with the component, whilst negative values indicate an inverse correlation. table 5 contains maximum, minimum and average values of the factors associated to every survey response by component. 4.1. component 1: relevance of external factors (ref) ref comprises directly related factors which are conceptually linked to user comfort perception (table 4). ref reflects the impact of the outside in indoor comfort, and thus survey questions for this component consider the incidence of outdoor environmental factors on user comfort perception, mainly lighting comfort. ref ranges from relevant to non-relevant. sun reflection received from neighboring buildings represents the highest score factor. all four cases are located in an urban context in which neighboring buildings are glazed. the survey indicated that both access as well as lack of daylight affected user comfort probably, being the latter a consequence of needing to use indoor screens as protection from the glare of other buildings. the relevance given by users to outdoor noise was rather surprising, particularly because these buildings have façade solutions which are soundproof and this factor was within the same range as the impact of natural lighting factors. 4.2. component 2: artificial lighting frequency (alf) alf comprises two factors involving the frequency in the use of artificial lighting: daily and monthly use (table 4). alf ranges from high frequency to low frequency in use of artificial lighting. the responses reflect the differences between types of workspaces, such as open floor plan offices or individual ones, as well as the ensuing management criteria and adaptive opportunities offered to users. the fact the pca analysis considered both artificial lighting and natural lighting components can be understood as a architectural consequence due to the issue of glare in glazed facades. this critical problem generates many management strategies that affect user comfort perception. for example, the use of multilayer screens with blackout curtains and venetian blinds is a typical internal shading solution that leads to an unnecessary use of artificial lighting. this situation grants high coherence to the generation of this component in a comfort perception survey. 4.3. component 3: thermal sensation and orientation (tso) tso describes thermal behavior at workspaces and comprises survey results as well as registered temperature data (table 4). this is the only component in which qualitative aspects provided by survey were merged with on-site measurement registers. in general, qualitative and quantitative data are processed separately but the pca analysis stated these were correlated and data was merged into one component: tso. tso ranges from warm to cold. two factors were extracted from the survey: last 30 days thermal sensation declared by users and the degree of relevance assigned by users to workspace orientation in their thermal sensation. the third factor was onsite measured temperature which was quantified as the percentage of hours in which the indoor temperature could be considered as not belonging to the comfort zone (above 26◦c). c. vásquez et al. / thermal and lighting perception in four fully glazed office 43 4.4. component 4: relevance of indoor factors (rif) rif quantifies indoor space comfort perception, based on indoor conditions, specifically the incidence of indoor noise on acoustic comfort and the thermal sensation of last 30 days (table 4). value differences and the absence of more factors weakened this component, and thus, it was not considered in the maps of perceptions discussion. 5. discussion: maps of perceptions (mp) mps were used to related pairs of components obtained by pca. the goal was to graphically and holistically represent user perception. axes represent positive and negative values for the various families of data obtained and onsite measurements for each family of data as a component valuation. the four quadrants allow for allocating the families of data so as to graph their behavior in relation to the components represented by each axis. in this study the families are: season, case, workspace type and possibility of outdoor view. season data corresponds to all the responses given by users per season in all the analyzed floors; case data is a separated view of every analyzed floor considering every season surveys together; the type of workspace is a segmentation data declared by users considering all analyzed floors together; and finally, the outdoor view options is a segmentation data too, that allow us to relate the component with the outdoor view options per season, considering all cases together. 5.1. map 1: general perception comfort (ref/tso) map 1 graphs the relationship among heat perception, outdoor lighting and acoustic comfort. these three factors account for the most common and easiest-to-ask aspects impacting on user comfort. tso’s scale ranged from hot to cold, with zero being a neutral thermal perception. ref’s scale ranged from not relevant to relevant, with zero representing indifference. the ideal situation would be a zero reading for both components. in the season analysis (fig. 5), seasons paired into three pairs: winter and summer, and summerautumn and winter-spring. the winter and summer pair shows the largest differences in the tso components and express the natural thermal difference in the cold and warm season. however, the other two pairs respond to different perceptions of the ref component: the ref components tend to be considered as relevant in winter-spring, and as less relevant in summer-autumn. season analysis brings to light the fact that general comfort perception is influenced by thermal aspects as well as outdoor factors such as light and noise. cases also grouped into two pairs (fig. 6) with different behaviors. the c4-c2 pair does not reveal any negative general comfort perception, while the c1-c3 pair does. floor plans in c4 (fig. 2) combine open floor plans facing north and south, and in c2 (fig. 2) floor plans combine all types of workspaces with open floor plan facing south and east. thus, these two cases are mixed floors. the c1-c3 pair shows an asymmetrical behavior, which could be explained by the cases’ plan distribution: c1 (fig. 2) is a low density floor with mostly single or shared offices -no open floor plan is considered-; while c3 (fig. 2) is organized as an outer ring of offices and meeting rooms, with an indoor open floor plan which is not does not provide an outdoor view. most users in both cases have no outdoor view. 44 c. vásquez et al. / thermal and lighting perception in four fully glazed office fig. 5. map 1: general perception comfort. season analysis. fig. 6. map 1: general perception comfort. case study analysis. the differences revealed by the case analysis in map 1 allow us to assert that floor distribution may contribute significantly to general comfort perception. comfort perception differences are irrelevant considering types of workspaces (fig. 7). the ref components tend to be less relevant among those working in open floor plan, perhaps because the depth of the floor does not provide for an outdoor view, particularly when blackout screens are c. vásquez et al. / thermal and lighting perception in four fully glazed office 45 fig. 7. map 1: general perception comfort. workspace analysis. fig. 8. map 1: general perception comfort. outdoor view analysis. chosen as a light control method anytime throughout the day. type of workspace is not relevant for general perception of comfort as can be seen in map 1. data on seasonal perception throughout the year was only provided by those at workspaces with an outdoor view (fig. 8). our analysis revealed an unexpected fact in summer: having an outdoor 46 c. vásquez et al. / thermal and lighting perception in four fully glazed office fig. 9. map 2: lighting comfort perception. case study analysis. view appears to be another key issue affecting user comfort perception. thus, floor distributions that create blind workspaces will have a low user comfort perceptions. 5.2. map 2: lighting comfort perception (ref/alf) map 2 explains lighting comfort given that alf includes data on the use of artificial lighting and ref the significance assigned to natural light for lighting comfort. ref’s scale ranges from important to not important and alf’s from high to low frequency use of artificial lighting. the analysis by case (fig. 9) reveals some significant issues. in three of the four buildings, users do not related the frequency of use of artificial lighting (alf) to outdoor factors (ref) which determine the need to use artificial light. this is the result of having protocols determining the use of artificial light regardless of users’ visual needs. c3 is the only case in which users grant significance to the ref component. in this case some workspaces provide an outdoor view and others are totally blind, two extreme situations emphasizing the relevance to outdoor factors. workspace analysis reveals significant differences (fig. 10). alf component is highly rated by open plan users and poorly rated by private offices users. in general, differences in alf rating are not associated to changes in the rating of the ref component, which can be interpreted as the fact that the use of artificial lighting is not perceived as an issue associated to outdoor factors which, in essence, should determine its use. protection from the glare caused by curtain wall façades seems to account for this need to use artificial light when natural light is available. outdoor view (fig. 11) clearly impacts on the alf component. for all seasons, high ratings were provided by users without outdoor view and low, even negative values, are provided by those with outdoor views. this seems logical; however, the ref component does not reveal this behavior. users c. vásquez et al. / thermal and lighting perception in four fully glazed office 47 fig. 10. map 2: lighting comfort perception. workspace analysis. fig. 11. map 2: lighting comfort perception. outdoor view analysis. with outdoor views rated the ref component positively in winter and negatively in summer and the opposite proved true for those without outdoor views. this proves the impact of having an outdoor view on lighting comfort perception. 48 c. vásquez et al. / thermal and lighting perception in four fully glazed office 5.3. map 3: thermal perception and use of artificial lighting (tso/alf) map 3 would reveal the relation between artificial lighting use (alf) and user thermal sensation (tso). the scales and contents for these components have been previously explained in maps 1 and 2. fig. 12. map 3: thermal perception and use of artificial lighting. season analysis. fig. 13. map 3: thermal perception and use of artificial lighting. case study analysis. c. vásquez et al. / thermal and lighting perception in four fully glazed office 49 fig. 14. map 3: thermal perception and use of artificial lighting. workspace analysis. seasons analysis (fig. 12) show pairs of cases sorted by thermal differences and natural lighting availability. the division between warm and cold seasons can be clearly seen. coldest seasons are near zero. warmer seasons generally show negative alf values and positive tso vales, especially on summer. as would be expected, seasons in which less artificial lighting is needed reveal positive tso values. cases behave in pairs and are analyzed in map 3 (fig. 13). the values for the c3-c4 pair are practically zero. the behavior of the c1-c2 pair is extremely different. c1 is a low density floor and users are highly dependent on tso component and not that dependent on alf, whereas c2 has no, or practically no open floor plan, a feature also true for c1. finally, from the workspaces analysis (fig. 14), workspaces and the alf component seem very dependent, whereas workspaces and tso seem practically independent. lowest use of artificial lighting was stated by single offices users and highest use by open floor plan ones. the lack of relation between both component when they are observed by workspace, explain a high influence of lighting management because all cases, as was explained before, have lighting schedules no related with any logic criteria. 6. conclusions 3 mp were produced based on three components obtained through pca, crossing survey results sorted by: season, case, type of workspace and ability or non ability to see outside. the mp express general perception of thermal and lighting comfort on workspaces within the analyzed office buildings. the conclusions were: a) thermal user perception does not follow the logic of seasonal thermal perception. all mp reveal show some aspect related whit light working together with thermal environmental situation. 50 c. vásquez et al. / thermal and lighting perception in four fully glazed office this emphasizes the need to consider workspaces lighting design to ensure satisfactory thermal perception. b) although the four cases correspond to typologically similar buildings, thermal comfort perception behaves differently in each one, and even more so lighting comfort. this demonstrates the need to coordinate the architectural design for workspaces and the building energy conception, a notion that is contradictory with the free floor design paradigm. c) comfort perception varies significantly among the type of workspace, namely, private office, shared office or open plan. this variation does not apply to thermal comfort but does impact heavily on lighting comfort. high frequency in the use of artificial lighting in the free floor area contrasts with the extremely low use in private offices. the use of artificial lighting is independent from outdoor factors but rather depends on how indoor lighting is managed in order to attain a general comfort perception. thus, comfort perception, particularly regarding lighting comfort is related with façade design, due to high luminance derived from high glazing ratio within the analyzed buildings. the need to avoid glare –and the ensuing solutions and problems-, was present in every analyzed mp. d) the main comfort issue in the analyzed office buildings, however, appears to be the impact of being able or not to see outside, a factor determined by thermal and lighting comfort. mps allowed us to observe a consistent correlation between lighting and thermal comfort perception and the possibility of seeing outside. this architectural challenge in office building design must be taken into consideration given that an inability to see outside has a significant impact on general comfort perception. based on these results, we foresee various research lines to work on. other types of office buildings need to be analyzed, starting by those in which there is an interest in improving comfort perception as well as those having a different kind of envelope strategy, given that curtain walls seem to negatively impact comfort perfection in high standard office buildings in santiago. acknowledgments this paper is part of a general research (fondecyt project #11100143) funded by the chilean government through conicyt, national committee for scientific and technological research, and sponsored by the center for sustainable urban development (cedeus) of the catholic university of chile (pontificia universidad católica de chile). references andersen, r. v., toftum, j., andersen, k. k., & olesen, b. w. (2009). survey of occupant behaviour and control of indoor environment in danish dwellings. energy and buildings, 41, 11-16. azar, e., & menassa, c. (2012). a comprehensive analysis of the impact of occupancy parameters in energy simulation of commercial buildings. energy and buildings, elsevier (in review). encinas, f. (2004). the technology transfer of double skin facades from europe to chile, an evaluation by means of cfd simulations, dissertation submitted in partial fulfilment of the regulations for the degree of master of science in renewable energy & architecture at the university of nottingham, nottingham. guerra santin, o. (2011). behavioural patterns and user profiles related to energy consumption for heating. energy and buildings, 43, 2662-2672. hair, j., & anderson, r. (2005). análisis multivariante. mcgraw hill. c. vásquez et al. / thermal and lighting perception in four fully glazed office 51 hummelgaard j., juhl p., saebjörnsson k., clausen g., toftum j., & langkilde g. (2007). indoor air quality and occupant satisfaction in five mechanically and four naturally ventilated open-plan office buildings. building and environment, 42, 4051-4058. kwon s., chun ch., & kwak r. (2011). relationship between quality of building maintenance management services for indoor environmental quality and occupant satisfaction. building and environment, 46, 2179-2185. macdonald, i., clarke, j., & strachan, p. (1999). assessing uncertainty in building simulation. building simulation 1999, kyoto, japan. olesen, b. w. (2011). can we meet the ventilation required in international standards in an energy efficient way? joint conference 32nd aivc conference and 1st tightvent conference: “towards optimal airtightness performance”. proceedings presentations, 789-818. brussels: aivc, air infiltration and ventilation centre. pino, a., bustamante, w., escobar, r., & encinas, f. (2012) thermal and lighting behavior of office buildings in santiago de chile. energy and buildings, 47, 441-449. roetzel, a., tsangrassoulis, a., dietrich, u., & busching, s. (2010). a review of occupant control of natural ventilation. renewable and sustainable energy reviews, 14, 1001-1013. yu, z., fung, b. c., haghighat, f., yoshino, h., & morofsky, e. (2011). a systematic procedure to study the influence of occupant behaviour on building energy consumption. energy and buildings, 43, 1409-1417. zhang, y., & altan, h. (2011). a comparison of the occupant comfort in a conventional high-rise office block and a contemporary environmentally-concerned building. building and environment, 46, 535-545. from city’s station to station city 67 journal of facade design & engineering volume 6 / number 2 / 2018 new biocomposites for innovative construction façades and interior partitions julen astudillo1, miriam garcía2, javier sacristán3, nayra uranga3, markku leivo4, michaella mueller5, inma roig6, sarka langer7, gianluca gemignani8, markku vilkki9, ger gijzen10, susana silva11, m.a. nuñez12, michał dąbek, marius sprenger14, alberto ortiz de elgea15 1 tecnalia research & innovation 2 ik4-lortek 3 acciona construcción 4 vtt 5 fraunhofer/fhr 6 aimplas 7 ivl, swedish environmental research institute 8 collanti concorde 9 conenor oy 10 unstudio 11 amorim cork composites 12 enar 13 bergamo technologie 14 sicc gmbh 15 visesa abstract osirys is a european research project where a holistic solution for façades and interior partitions ready to be applied in building retrofitting and new construction has been developed. the project uses biocomposites as the base material to define different products: a multilayer façade, a curtain wall, a window, and an interior partition. the biocomposites developed have different functionalities able to meet the strictest requisites of the european building codes in relation to fire and structural performance, improve indoor air quality through the elimination of vocs (volatile organic compounds) and microorganisms, increase thermal insulation, and increase the durability of construction elements. the new systems are lighter than traditional ones, leading to reductions in overall weight, thereby reducing implementation costs during both manufacturing and assembly processes, thanks to an industrialised concept that utilises modular elements. the project was developed with the collaboration of 18 european partners (5 research centres, 9 smes, 2 large industries, and 2 public bodies). the main activities were devoted to the establishment of requirements, the development of materials, the design of products, the integration of materials into products, the verification of properties by simulation and testing according to eu standards, the integration of products into real buildings, and economic and environmental assessment. the scope of this paper is to provide a general overview of the entire project work and results to demonstrate the feasibility of using biocomposites in envelope solutions with the aim of solving some of the main problems that exist in façade traditional solutions. the project finishes with the implementation of the developments in real buildings as prototypes; further research is required before industrial scale manufacturing of the systems can be launched into the market. keywords façades, curtain wall, biocomposite, multilayer façade, interior partition, windows. 68 journal of facade design & engineering volume 6 / number 2 / 2018 1 introduction nowadays, several types of façade systems are used (traditional systems like brick façades, and steel and aluminium curtain walls). however, various problems are associated with these façades, as listed below: – the use of steel and aluminium, materials with high thermal conductivity, demand the use of a different material to avoid thermal bridging. this requires the use of geometrically complex profiles and is therefore problematic for manufacture. new materials with better thermal attributes can improve this behaviour, with less complexity in geometry and manufacturability. – the use of fossil resources and high energy consumption in the manufacturing process must be reduced. new materials from natural resources, such as bio-based materials, can accomplish a more sustainable production process and a better use of natural resources (eventually achieving a better result in relation to a circular economy in which all the materials and processes are connected during their entire life cycle). – many materials that are now on the market have problems in relation to the generation of contaminants (like vocs, formaldehyde, fibres, and particulates). new materials that do not generate such contaminants are needed. moreover, active materials that can eliminate this type of hazardous components and other elements, like microorganisms emitted by other sources, are required. this requirement is produced by an increased concern for indoor air quality in buildings, a requirement that will soon become more stringent due to new standards and regulations that are in the process of being approved in europe. – it is necessary to avoid the use of water on construction sites and to reduce the amount of substandard work normally involved in the construction of a building. it is essential to explore new solutions that have a high component of industrialisation and a modular approach, that avoid the use of water on-site and improve the quality of the final work by increasing work completed in the factory and reducing these works during the installation process. this also leads to higher skills amongst the workers that participate in the entire process. additionally, new possibilities related to design in the envelope should be explored. the use of new materials with different manufacturing processes can lead to new solutions that work better for building envelopes, avoiding the constrains of the current materials. with the aim of solving the above-mentioned constraints, the osirys project was developed. the aim of the project was to design a holistic biocomposite façade solution with better performance than traditional solutions. the selection of the biocomposite materials was done according to their performance with regard to thermal conductivity, design possibilities, absence of hazardous materials, industrialisation capacity, and reduction of use of fossil fuel resources, in comparison to metals, the current most widely used material. however, the use of a new material with different characteristics involves a complete redesign of the product geometries and construction process. thus, the osirys project starts with the development of different biocomposite materials, progressing through the design and manufacture of biocomposite products, and finishes with the testing and installation of these products in real conditions. the project has been running for 4 years with the collaboration of 18 partners, of which there are 5 research centres, 9 smes, 2 industries, and 2 public bodies, from 11 eu countries. the consortium combines scientific and technological knowledge on materials and design to reach the goal of the osirys project. 69 journal of facade design & engineering volume 6 / number 2 / 2018 – the rtos (research and technology organisations) focused on the development of materials, the improvement of their characteristics, environmental assessment, and validation of performance through standardised testing. – the smes and industries up-scaled the laboratory-scale processes to semi-industrial processes to produce real-scale products, and developed the engineering and architecture to allow them to be installed in the demo buildings. – the public bodies were in charge of the demo activities by using the products in real buildings. ] the methodology followed to obtain the final results during the execution of the project is explained below: fig. 1 methodology to design and develop osirys products taking into account these requirements, the following materials and products were developed in the project: materials developed for use as components of the final products: – light foam biocomposite: to be used as a base material for the internal layer of the different components developed. with improved thermal characteristics and less weight. complying with building codes requirements. – natural insulator: to be used as an insulation component in the different systems. improving fire behaviour without losing thermal characteristics. – biocomposites with improved mechanical and fire performance: one of the problems of the composites is their poor mechanical performance when compared to steel and aluminium solutions. new developments using different types of resins and fibres must be explored to avoid this problem. moreover, the fire performance of these materials must be improved to reach the requirements of the european regulations. 70 journal of facade design & engineering volume 6 / number 2 / 2018 – biocomposites with good outdoor durability performance: to improve the outdoor performance of these materials to make them comparable with the existing systems on the market such as cladding elements for ventilated façades. – indoor multifunctional coating: to avoid the generation of contaminants and to improve active characteristics to eliminate the problematic components that are present indoors after construction. final components and systems, utilising characteristics of the new materials developed: – multilayer façade: an industrialised system with the aim of substituting traditional opaque façades (brick and concrete) for a lighter option, with better thermal, acoustic, and fire performance and with enough flexibility to be adapted to different building solutions. mainly built in the factory, with a kit concept to avoid works on site. – curtain wall and window: a biocomposite curtain wall and window, with less complex profiles, better thermal performance, and high aesthetic flexibility. moreover, this uses a modular and industrialised concept. – interior partition: a system to construct all the internal walls of a building with the new ecomaterials, without losing the beneficial qualities of traditional systems. with these new developments, the project offers a complete solution to be used in both envelopes and interior partitions, in many different types of buildings (mainly residential, as well as tertiary buildings such as commercial, administrative, and offices), overcoming the problems associated with current systems. the aim of this paper is to provide a general overview of the osirys project, to make readers aware of the innovations that can be achieved by using novel materials such as biocomposites for façades and interior partitions. so, the information provided in this article is not an exhaustive detail of each development and test, but intends to give general information and results about new possibilities when using materials other than steel and aluminium. 2 objectives for the development of the new materials and systems the development of such novel systems requires the fulfilment of several objectives. the most relevant ones are detailed below: – development of a new photocatalytic coating to be active under indoor illumination, with protection against fire, and coloured. it was estimated that a voc concentration reduction to <1mg/m3 and mould generation is can be reduced by a reduction factor of 5-6 logarithmic units in 12 hours. – development of a lighter system with weight savings of 70% compared to traditional brick and precast concrete walls. – development of an industrialised system to reduce implementation costs. assembly works were expected to be reduced significantly. – fulfilment of the main requirements from the european building codes related to mechanical resistance, thermal behaviour, fire, and acoustic performance, etc. (see tables in the article) – biomass feedstock was above 60% in the multilayer façade and almost 70% without including fastening devices. 71 journal of facade design & engineering volume 6 / number 2 / 2018 – assumed price increase of the system over traditional products: comparable price for the curtain wall, 32% increase for the multilayer façade, and 15-20% increase for the interior partition. 3 work performed in order to achieve these results, the osirys project was divided into different research activities that are explained as follows: 3.1 analysis of requirements the main objective of this work is to establish and quantify the requirements to be met by the developed materials and products as a basis for further developments to be achieved in the following works. in this regard, the following requirements have been considered and analysed: – characteristics that the materials must fulfil – design requirements for products to facilitate construction – regulatory requirements affecting the materials and products – market requirements focused on market drive forces – definition of the most common types of buildings where the new developments can be implemented results from lca studies most commonly show that raw material production has the greatest importance in the total environmental impact of a product. thus, in order to keep the environmental impact of a product as low as possible, the impact from raw material production should be minimised. materials based on biomass feedstock were considered for the replacement of existing petrochemical and other traditional materials. natural fibres such as wood, cork, flax, and jute, and bio-based biopolymers were considered for the development of new materials and products. a first set of guidelines to design the four products defined in the project (interior partition system, multi-layered wall system, window, and curtain wall) were established. this process considered both a prefabricated system combining all osirys elements and a site assembled hybrid system where the elements can be combined with commercial systems. each product is made by combining the different materials that were developed in the project according to their final function (i.e. the curtain wall comprises profiles, the interior partition includes profiles, insulation, and coating, etc.). each element that comprised the four novel products was defined, along with its function. in addition, reuse and recycling concepts were also considered in the design process. in accordance with construction product regulation (eu) nº 305/2011 (cpr), six basic requirements (br) for construction works were checked to ensure that they are designed and executed so as not to endanger the safety of persons, domestic animals, or property, nor damage the environment. after analysing the requirements for the four products in various european countries, very demanding osirys targets were set up. there is no specific regulation for indoor air quality, but various recommendations have been considered to minimise vocs emissions and the growth of microorganisms. osirys products can be used in different types of buildings. therefore, they were analysed for different building typologies (offices, residential, commercial, and culture/public), and were also related to building envelope types (multi-layered wall and curtain wall), while also considering the optimum u-value for each city/region of europe. 72 journal of facade design & engineering volume 6 / number 2 / 2018 3.2 development of highly bio-based multifunctional materials (to be used as the basis for the products development) within this work, several materials were developed, each one for a specific purpose. indoor multifunctional coating: the aim of this material was to decompose vocs and introduce antimicrobial properties on indoor surfaces. first of all, a suitable method was developed, based on methylene blue to evaluate the photocatalytic activity of pigment powders and coatings under indoor lighting conditions. the developed coating showed good photocatalytic performance based on the methylene blue method; activity performance was higher than other commercial products. a testing setup to analyse antimicrobial properties was optimised. results indicated that the new paints show very good antimicrobial activity against two selected microorganisms: aspergillus niger (mould) and sarcina lutea. to evaluate the fire performance of the coatings, cone calorimeter testing was carried out. the coatings did not ignite on aluminium substrates and did not contribute to the release of heat. when applied on organic substrates, the adhesion was good but it was necessary to add a fire retardant to the coating to reduce heat emission. customers and architects are interested in having a coating with a range of colours, and so the coloration capacity of the coating was evaluated along with the stability of the colour after three months in both standard indoor conditions and extreme humidity conditions. results showed that every colour is available in bright shades and remains stable over time. light foam biocomposite: the aim of this material was to develop a light foam biocomposite sandwich panel with good mechanical, thermal, and acoustic performance for use in interior walls. the surface panels that best promote mechanical and sound absorption properties were found to be porous wood fibre webs produced by a foam-laid forming technique. the core layer was formed by expanded biopolymer foam produced by extrusion to improve thermal and sound insulation properties. initially, fibre material, additives, process parameters of foam forming, pressing and drying processes, and manufacturing procedures for fibre web production was tested and optimised. the formulations with the best performances were optimised and several tests were conducted and compared to traditional gypsum board. lignin-based biopolymer foaming was possible at lab-scale, but up-scaling the process was unsuccessful. therefore, pla based biopolymer foam was optimised. pla foam layers of 10-15mm thickness were obtained with a density of 60 kg/m3 and thermal conductivity in the range of polystyrene. pilot scale layers were obtained. the thermal performance of pla foam was better than commercial xps. sample id thermal conductivity [w/mk] thermal diffusivity [mm2/s] specific heat per volume [mj/m3k] pla foam 05/15 0.037 0.449 0.083 xps (reference) 0.033 0.573 0.057 table 1 pla foam thermal performance 73 journal of facade design & engineering volume 6 / number 2 / 2018 natural insulator: cork materials were used as part of the sandwich panels to improve the thermal and acoustic performance of the final systems. although cork is a natural material with fire resistance properties, agglomerated cork with different binders could affect the fire performance of the material. the main objective was to improve the fire performance of the cork materials. to do so, different fire retardants were included in the agglomerated materials. samples were produced by block technology, cylinders, and double belt press. a small flame test was used as a screen process and the best formulations were tested in a cone-calorimeter to analyse the ignitability, the fire propagation, and smoke density. in general, the production line products showed worse fire reaction performance than laboratory ones. however, all the fire analysed retardants improved the fireretardant properties of the samples. biocomposites with good outdoor durability performance: following research on different coatings, six commercial coatings have been selected (four colour coatings and two transparent coatings) for evaluation in terms of their suitability for improving the outdoor durability of the biocomposites in their use as ventilated façade cladding. durability was assessed by testing the samples for 1000h in a quv device. in addition, the mechanical performance of the coatings was also evaluated. biocomposites with improved mechanical performance: to increase the mechanical performance of the biocomposites to be used as profiles, graphene functionalisation was tested. a roll mill was selected as the optimum dispersion method of graphene in bioepoxy resin. it was possible to include a minimum percentage of nanofiller in the system without a substantial increase in viscosity. afterwards, the pultrusion process at lab-scale was optimised as follows: – modifications to the pultrusion equipment to adapt to the curing cycle of the bioepoxy resin – processability evaluation of the bioepoxy resin with glass fibre and graphene – impregnation of the flax fibres by combining them with glass fibres, graphene, and a standard polyester resin – rebars with different formulations were obtained for mechanical testing – tensile tests of the biocomposites indicated that graphene does not improve mechanical performance of the materials and flax fibres did not reinforce as successfully as glass fibres. however, hybrid systems composed of flax fibres and glass fibres provided similar mechanical performance as glass fibres. biocomposites with good fire performance: thermoset biocomposites were protected by three methods: coatings, the use of mats impregnated in a liquid flame retardant, the use of graphene. however, dispersion problems in infusion type bioepoxy hindered the use of graphene as a fire retardant. in pultrusion bioepoxy, the graphene contributed somewhat positively to the fire-retardant effect. in terms of protective coatings, all of the coatings tested for improved fire performance were tested by single flame test. the best candidates were tested in the cone-calorimeter. the intumescent coating showed the best results. in the case of thermoplastic biocomposites, different fire retardants and natural fibres were tested. 74 journal of facade design & engineering volume 6 / number 2 / 2018 1 2 3 4 fig. 2 osirys products under testing (from top left, clockwise): fire resistance test for multilayer façade, acoustic test preparation, wind load test for the ventilated façade (external layer of the multilayer façade), kubik testing for thermal properties,) 3.3 integration of materials into components the overall objective of this part of the work is to combine the different materials developed previously into the final components. manufacture of sandwich structures: the different materials developed by osirys partners were characterised with respect to their thermal, mechanical, and viscoelastic properties, and combined to produce multilayer sandwich panels. overall, these multilayer sandwich panels performed well, 75 journal of facade design & engineering volume 6 / number 2 / 2018 indicating their potential for use as partition walls, interior separation panels, and cladding panels in buildings. their low density and the known properties of thermal insulation, acoustic insulation, and low hygroscopicity given by the configurations tested contribute to the value of these sandwich panels for the planned project demo cases in construction. evaluation of assembly properties: the aim was to develop an adhesive that can improve indoor air quality by achieving low voc emissions while offering a good bonding solution for the sandwich structure. adhesives provide some benefits over mechanical fasteners, like lower structural weight, lower fabrication cost, and improved damage tolerance. manufacturing and testing of lab-scale prototypes: different configurations were produced with a variety of previously developed sheets, core materials, profiles, coatings, and adhesives, and were assembled to provide a wide range of solutions for the case studies proposed: internal partition and multilayer façade. the multilayer façade was divided in 3 sections: interior finish, multi-layered core module, and exterior finish panel. in order to choose the most suitable configurations, the thermal conductivity of the internal partition and interior finish was tested. results indicated that the range of thermal insulation of the interior partition was similar to mineral wool and eps, and that of the interior partition was similar to the use of natural fibres. with these results and the design results, the final configurations for lab-scale prototypes were set up. performance test standard result comment structural safety impact resistance to soft body eota technical report tr001, section 2 e= 500j, 1200j permanent denting of the outer layer. no breakage or release of elements ok no breakage impact resistance to hard body eota technical report tr001, section 3 e= 5j, 10j no damage protection against noise acoustic insulation en iso 10140-2 r w = 54 db ok average with commercial products thermal insulation thermal transmittance theoretical calculus u=0.127 w/m2k for the complete system u=0.193 w/m2k without xps ok to targets defined by demo owners u tartu =0.14 u spain =0.25 air permeability air permeability en 12114 class 3 0.034 m3/hm2 at (+/-) 55 pa ok values according to low energetic consumption buildings water tightness water vapour transmission en iso 12572 and theoretical calculus µ cork = 40 µ wpc =1,700 s d,total = 40m (3) ok breathable and non-breathable layers fire safety fire resistance en 1363 ei90 ok to target ei60 defined in the project reaction to fire en iso 11925-2 bs2d0 ok to the target defined table 2 resume of the obtained test results on lab-scale sandwich systems 76 journal of facade design & engineering volume 6 / number 2 / 2018 3.4 design of construction systems for façades and interior partitions building typologies scenarios: several façade typologies of buildings in europe were studied and classified to set up a guideline to assist in the design of the new products. two façade groups were defined according to the main uses of buildings (residential or office buildings), and within each group several typologies were defined according to the construction method. thus, the main features and characteristics of each constructional solution have been studied, taking into account the climatic zone, the year of construction, and other parameters. conclusions drawn from the study are summarised below: – residential building conclusions: – the 67% of the european residential building stock that was built before 2005 comprises a heavy resistant wall (such as concrete or cored brick), often covered with an exterior overlay and an interior plaster, but with no air chamber and with no insulation. – almost 50% were constructed between 1946 and 1990 (post-war buildings) in southern europe (31.7 %) and in central europe (15%) and 15% are classified as old buildings – 28% of the residential buildings are composed of a heavy resistant wall with an unventilated air chamber filled with insulation. – most of these were constructed between 1946 and 1990, mainly in central europe – among the current building construction trends in south europe, 50% of the façade typologies are ventilated façades, which comprise a resistant wall, a discontinuous overlay, a ventilated air chamber, and insulation to the outside. – almost 40% of current residential buildings are composed of a heavy resistant wall with an unventilated air chamber that is filled with insulation. – office buildings conclusions: – among the current office building construction trends, 35% of the typologies constructed are stick-system curtain wall façades, which are the most commonly used construction solution in south europe and central europe. – this prevalence of this system is followed closely by the unitised curtain wall system. – double skin systems are becoming more common on the market due to the higher importance of energy efficiency and reduction in energy consumption (zero-energy buildings). – conclusions drawn from the study of the residential buildings have been taken into account in the design of the multi-layered façade solution and the biocomposites window design, while the conclusions obtained from the analysis of office buildings have been used for the design of the curtain wall product. conclusions for the osirys multi-layered façade geometric features floor to floor height (m) 3.0 free floor height (m) 2.5 – 2.6 % window / façade ratio 30% façade description thickness (cm) 35 40 cm (up to 2005) 23 29 cm (new trend) conclusions for the osirys curtain wall geometric features floor to floor height (m) 3.8 – 3.9 free floor height (m) 3.0 façade modulation (m) 1.8 façade description thickness (cm) 20 % with solar protection 20 % table 3 conclusions to be used in the design of osirys products 77 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 3 multilayer façade system components multilayer façade design: the developed multilayer façade system combines three main sections: internal finishing, core of the wall (multi-layered module), and external cladding. each section consists of several elements developed within osirys and combined together to create a fully functional building envelope. – the assessment process used to verify the different products of the osirys project comprises the following steps: – initial design of the system – structural and thermal simulations – laboratory prototype construction and testing – breathability, moisture, and presence of fungus and micro-organism simulations – final design of the product – computer simulations provided important information showing that the design of the multilayer façade system comprising novel elements developed within the osirys project fulfils necessary and crucial requirements like structural stability and proper thermal performance. hygrothermal performance analysis of the multilayer façade panel under northern climate conditions was also assessed and results indicated that a vapour resistant membrane (moisture barrier) in cold (northern climate) was necessary to avoid moisture accumulation in the structure. – with these results, the final design for the multilayer façade was provided element by element and also per system. the final designs also included the design of the connections between the osirys elements, and between osirys elements and non-osirys elements, to integrate the new products in the demo buildings. 78 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 4 curtain wall façade system components curtain wall and window design: different possibilities have been evaluated to find the best solution. the dimensions for prototyping are 3.8m high x 1.2m wide. the preliminary system design included the configuration and schematic definition of components, the study of the interactions of system components, and building systems and modularity. like the multilayer façade design, structural and thermal simulations were performed to validate the final design according to the defined requirements. for the final design the following aspects were considered: – the complexity of the profiles was reduced to avoid problems in manufacturing. a total of 3 profiles were designed for the curtain wall. – for the window frame, a pultrusion profile with an operable part was designed. – the final glass types were selected for the curtain wall and the window, and outdoor features for the tartu demo were decided. partition system design: after analysing the types of partition walls, it was decided to focus on a stud wall hybrid system, which is a fusion of a system assembled on-site and a prefabricated sandwich partition panel, which provides particular construction advantages. the partition wall is divided into two main components: structural profile and sandwich wall board. – structural biocomposite profiles are designed based on a standard, c-profile: aluminium structure with a width of 50mm, height of 60mm, and wall thickness of 5mm. – the second component of the partition wall is the sandwich wall board. different partition layers are combined together into one prefabricated panel mounted directly on to pultruded studs. the panel will ensure that all necessary properties that are to be fulfilled by partition walls are met. it will comprise bio-based acoustic and thermal insulation as well as a wall board that works as a wall lining. the width and cut edges of the panel are adjusted to the correct shape, and the distance between pultruded profiles is 400mm. 79 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 5 detail of the interior partition system laboratory prototype construction and testing: full size prototypes were manufactured and validated under european standards to verify that the osirys products can be used in the demo buildings without legal problems regarding their final behaviour onsite. a resume of the final results for the fourth systems can be found in the following tables. essential characteristic standard european standards minimum requirement osirys final result reaction to fire en 13501-1 and test under en 13823 b-s2-d2 b-s2-d0 fire resistance 1364-3 and en 1364-4. classification by en 13501 ei60 ei90 watertightness en 12865 150a 300a wind load resistance (services loads) etag 034 depending of the building (façade classified) 1800pa (p) 1400 (s) mechanical resistance etag 034 category iv category i horizontal point loads etag 034 impact resistance etag 034 airborne sound insulation en iso 140-3 depending of the building and the situation (33 db in some cases) 53 db thermal resistance en iso 10077-2 0,25-0,35 w/m2k 0,139 w/m2k table 4 multilayer façade results essential characteristic standard european standards minimum requirement osirys final result water permeability en 12155 r4 r7 wind resistance (service loads) en 12179 depending of the building (façade classified) 1200 pa (p) 1200 pa (s) self-weight l/500 or 3mm l/500 or 3mm airborne sound insulation en iso 140-3 depending of the building and the situation (33 db in some cases) (*) 40 dba thermal transmittance en 13947 depending of the building and the situation (2 w/m2k in some cases) (*) 1,04 w/m2k air permeability en 12153 a1 a2 table 5 curtain wall results 80 journal of facade design & engineering volume 6 / number 2 / 2018 essential characteristic standard european standards minimum requirement osirys final result water permeability en 1027 1a 3a wind resistance en 12211 600 (pa) 3000 (pa) self-weight l/500 or 3mm l/500 or 3mm airborne sound insulation en iso 140-3 depending of the building and the situation (33 db in some cases) (*) 40 dba thermal transmittance en iso 10077-2 depending of the building and the situation (1,2 to 2 w/m2k in some cases) (*) 1,15 w/m2k air permeability en 1026 class 1 class 3 table 6 window results essential characteristic standard european standards minimum requirement osirys final result reaction to fire en 13501-1 and test under en 13823 c-s2-d0 b-s2-d0 fire resistance 1364-3 and en 1364-4. classification by en 13501 ei60 ei90 resistance to dynamic loads etag 003 category 2a (residential) category 2b (offices) category 2a (residential) category 2b (offices) resistance to eccentric vertical loads etag 003 resistance to point loads etag 003 airborne sound insulation en iso 140-3 depending of the building and the situation (33 db in some cases) 47 db table 7 interior partition results 3.5 demonstration activities different activities were performed during the project to verify the final behaviour, in real conditions, of the developed products. these activities were divided into three main demos: demonstrator in kubik test building: the kubik facility (from tecnalia) is a three-storey high building with a basement floor in which hvac systems are located. an enclosed east-facing room on the ground floor is dedicated to the osirys project. a prototype of the osirys multi-layer façade was successfully prefabricated (wall core) and erected on site (wall core, external cladding, and internal part of wall), validating the construction process of the wall. together with the on-site assembly process, a number of sensors were installed, allowing an experimental campaign for the measurement of the thermal performance of the multi-layered wall. 81 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 6 multilayer façade installed in kubik test cell demonstrator in a real building: case study 1: public building in northern europe: the tartu demo building is a 250m2 stadium building located at mart reinik high school in the central area of tartu (estonia). the demo building consists of 4 dressing rooms, 2 shower rooms, 2 saunas, office space for teachers or referees, and a dressing room for the referees. the building will serve around 1200 students from 3 nearby schools. it will also be used by around 300 recreational users every week throughout the year. – firstly, the architectural design on tartu demo building was carried out. the technical design of the tartu demo building was finished after all details and joints between osirys products and the other building materials were agreed. the hvac design was also accomplished. fig. 7 floor plan of the tartu building 82 journal of facade design & engineering volume 6 / number 2 / 2018 – osirys systems (multilayer panel, interior panels, and curtain wall) were successfully installed quickly and simply, despite some on-site works being required due to the handcrafted nature of the products. thus, the feasibility of using osirys products in buildings was demonstrated, although optimisation of the industrialisation process and aesthetics is required for further commercialisation. fig. 8 external view of the tartu demo building. the front curtain wall was developed through the osirys project like the rest of the façades (which use the multilayer façade solution). the partitions inside the building also use the osirys developments. – demonstrator in a real building: case study 2: residential building in southern europe, promoted by the partner visesa (basque government): the demo building is located in a social housing block in san sebastian (north of spain). the construction plot is rectangular in shape (69 x 12m), with a slope of 4%, it consists of 2 underground floors + ground floor + 7 floors + attic and has 10 apartments per floor (total of 70 apartments). the apartment in which the osirys products would be installed was selected on the south-east corner, at second floor level. it comprises two bedrooms, bathroom, kitchen and living room. fig. 9 second floor of the san sebastian demo building with the osirys apartment highlighted – the installation of osirys products had to overcome barriers related to the inexperience of the construction team handling and working with this type of materials. so, to help the working teams with the installation of the systems, a guide for design and maintenance was prepared. 83 journal of facade design & engineering volume 6 / number 2 / 2018 fig. 10 render of the final solution for the san sebastian demo 3.6 monitoring results for all demo buildings indoor air quality: concentration of nitrogen dioxide and ozone were below the recommended guideline values for indoor air in the 3 demo sites and the indoor-to-outdoor ratios were < 1, indicating efficient removal of these compounds from outdoor to indoor air. tvoc measurements indicated that without ventilation it takes 7 weeks to reach a safe concentration of 300 µg/m3 as recommended in the guidelines. however, when slight ventilation of 23 m3/h (corresponding to approx. 0.77 air exchanges per hour) is applied, even the initial tvoc concentration decreases to 500 µg/m3 and the time required to reach the safe level is reduced to 5 weeks, approximately half of the time required by a commercial coating to reach the same safe level. according to the tvoc guidelines issued by the german federal environmental agency, painters will be exposed to indoor air of moderate quality. thus, ventilation is adequate to provide a safe workplace. on the other hand, it was also observed that without lighting tvoc concentration increased, showing the photocatalytic effect of the coating upon indoor lighting. thermal performance: a thermal transmittance u-value of 0.160 w/m²k was estimated for the plane areas of the multi-layer wall. the measured u-value keeps within the same range as expectations from numerical calculations, with a slight increase of 0.021 w/m²k, which is commonly found in physical measurements and can be attributed to workmanship issues or measurement errors, among others. the measured thermal performance confirms that the osirys multi-layer wall is a well-insulated assembly that can work in any european climate and complies with requirements of all national regulations. infrared images, together with the constant temperatures measured, showed that the multi-layer wall keeps a uniform temperature across its area, and the thermal bridging effect of the biocomposite profiles is very low. relative humidity measurements indicate no moisture accumulation. localised moisture spots dried out during this period, which is an indication of good drying capacity. life cycle assessment: the environmental performance of the multilayer façade, curtain wall, and interior partition was assessed. each product was defined according to its elements and materials, and the weight per m2 of wall and bio-content were quantified. despite there being no similar products to 84 journal of facade design & engineering volume 6 / number 2 / 2018 osirys on the market, the following benchmark products were used for comparison because of their extended use or similarity to osirys developments: – multilayer façade: brick wall façade. traditional façade system used throughout europe. – curtain wall: glass façades of fiberline composites. a product that could be comparable to osirys products due to the use of composite materials. – interior partition: knauf aquapanel indoor panels. gypsum panels with metal profiling. the environmental assessment included the analysis of these parameters: – biomass feedstock: the bio-content of the products increased from around 1% of benchmark products to above 30% and 40% in the curtain wall and interior partition, respectively, and to almost 70% in the multilayer façade with 0.14 w/m2k, corresponding to the materials developed in osirys (without glass or fastening devices). these results indicate the high value of the osirys products in relation to their use of natural materials. – embodied energy: because no multilayer façade comprising similar materials exists on the market, the wood-plastic fire-retardant panel included in the multilayer façade (that can be also found as fossil-based/traditional materials in the market) was chosen for comparison. results indicated that the embodied energy in the fire-retardant wood plastic composite (wpc) panel was 161 mj/m2, while in the conventional wpc it was 232 mj/m2. the osirys product was thus 31% lower in the content of embodied energy than the wpc. – the weight of the new systems has been significantly reduced in comparison to commercial benchmark products: 78% for the multilayer façade, 50% for the interior partition, and 10% for the curtain wall. 4 final conclusions the project is a first step to introduce new materials in the envelope sector, trying to solve some of the problems found in the traditional solutions or that are solved with more complex systems. the technical viability of using biocomposite materials on constructional elements to be applied in different european climates, assuring more comfortable buildings regarding energy efficiency and indoor environmental performance, has been assessed. the main result shows that new materials can be used in façades without major problems and that they can provide new characteristics and possibilities to the envelopes sector. three real examples of their good behaviour can be shown in the demos of the project. therefore, the demo buildings are a showcase for the developed novel products. the best results are obtained in terms of the thermal characteristics of the systems. the solutions developed are simpler than the traditional ones (easier for manufacture), with better thermal behaviour, with more flexibility to be adapted to different climate conditions without major changes, and with fewer problems related to condensation and corrosion. fire performance results are also in line with european requirements. moreover, the creation of a façade identity by using biocomposite materials allow differentiation of the building works. the environmental results are very promising as defined in the monitoring results. the project is now finished. some additional research is needed to improve both the mechanical characteristics of the materials and design to increase performance such as acoustic, air permeability, watertightness, and wind resistance. however, these problems can be easily overcome due to the 85 journal of facade design & engineering volume 6 / number 2 / 2018 good behaviour of the materials. the consortium is willing to continue working on improving the performance of the products in a further research project. as soon as industrialisation of manufacturing processes is completed, these novel products can be launched into the market. furthermore, the design process has considered the possibility of combining the different new elements with traditional systems to facilitate market penetration. likewise, some materials like the photocatalytic coating, the low-voc adhesive, and the insulation cork have already been commercialised. other products are under a patent process to be marketed. future publication will include a detailed explanation of the different activities developed in the project. acknowledgements this work was supported by the eu under the 7th framework programme of research, technological development, and demonstration project titled: “forest based composites for façades and interior partitions to improve indoor air quality in new builds and restoration” under grant agreement nº 609067. the development of this article and of this project would not have been possible without the collaboration of all consortium partners: fundación tecnalia, acciona infraestructuras, teknologian tutkimuskeskus vtt, fraunhofer-gesellschaft zur foerderung der angewandten forschung ev, research association of plastic and related materials (aimplas), ivl svenska miljoeinstitutet ab, tartu stad, tecnaro gesellschaft zur industriellen anwendung nachwachsender rohstoffe mbh, netcomposites limited, omikron-dokk muanyagiparo kft, conenor oy, van berkel & bos un studio b.v., amorim cork composites sa, enar envolventes arquitectónicas, bergamo tecnologie spzoo, visesa, sicc gmbh and collanti concorde srl. references the european flame retardants association. (january 2007). flame retardants frequently asked questions. retrieved from https://www.flameretardants-online.com/images/itempics/2/9/1/item_18192_pdf_1.pdf pauchard, v., grosjean, f., campion-boulharts, h., & chateauminois, h. (2002). application of a stress-corrosion-cracking model to an analysis of the durability of glass/epoxy composites in wet environments. composites science and technology, 62, 493-498. arbelaiz, a., fernández, b., ramos, j.a, retegi, a., llano-ponte, r., & mondragon, i. (2005). mechanical properties of short flax fibre bundle/polypropylene composites: influence of matrix/fibre modification, fibre content, water uptake and recycling. composites science and technology, vol. 65, no. 10, pp. 1582–1592. bos, h.l. (2004). the potential of flax fibres as reinforcement for composite materials. (master’s thesis). technische universiteit, eindhoven, the netherlands. doi: 10.6100/ir575360 ferm m, & rodhe, h. (1997). measurements of air concentrations of so2, no2 and nh3 at rural and remote sites in asia. journal of atmospheric chemistry 27, pp.17-29. bourmaud, a. & c. baley (2007). investigations on the recycling of hemp and sisal fibre reinforced polypropylene composites. polymer degradation and stability 92(6): pp. 1034-1045. kymäläinen, h.-r. & sjöberg, a.-m. (2008). flax and hemp fibres as raw materials for thermal insulations. building and environment 43(7): pp.1261-1269. international organization for standardization (2011). iso16000-3:2011. indoor air: part 3: determination of formaldehyde and other carbonyl compounds in indoor air and test chamber air. active sampling method. retrieved from https://www.iso.org/ swedish standards institute (2014). sis-ts 41:2014. determination of critical moisture level for mold growth on building materials (laboratory method). retrieved from www.sis.se european committee for standardization (2007). en15251 (2007). indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. retrieved from www.cen.eu british standards institution (1989) bs 476-6:1989 fire tests on building materials and structures. method of test for fire propagation for product. retrieved from http://www.standardsuk.com/ abadie, m.o. & wargocki, p. (september 2017) international energy agency. indoor air quality design and control in low-energy residential buildings. report on subtask 1: defining the metrics. iea ebc annex 68. aivc contributed report 17. retrieved from www.aivc.org journal of facade design and engineering 2 (2014) 3–18 doi 10.3233/fde-130012 ios press 3 exploratory experimental investigations on post-tensioned structural glass beams christian louter∗, jagoda cupać and jean-paul lebet steel structures laboratory (icom), school of architecture, civil and environmental engineering (enac), école polytechnique fédérale de lausanne (epfl), lausanne, switzerland submitted 20 june 2014 revised 8 october 2014 accepted 18 october 2014 abstract. the mechanical response of post-tensioned glass beams is explored in this paper. this is done through bending experiments on post-tensioned glass beam specimens with either mechanically anchored or adhesively bonded steel tendons by which a beneficial pre-stress is inflicted on the glass beams. in addition, reference beams with identical geometry but without tendons are tested. from the results of the bending experiments it can be seen that the post-tensioned glass beams reach higher initial fracture loads than the reference glass beams. furthermore, the post-tensioned glass beams develop a significant post-fracture reserve. from this it is concluded that post-tensioning a glass beam is a feasible concept, which provides increased initial fracture strength and enhanced post-fracture performance. keywords: structural glass, post-tensioning, beam, experiment, fracture, strength, redundancy 1. introduction this paper discusses exploratory experimental investigations into the mechanical response of posttensioned structural glass beams with either mechanically anchored or adhesively bonded steel tendons. it combines and adds to the results published earlier in (louter, pérez, jordan, & lebet, 2013) and (louter, cupać, & debonnaire, 2014). the main goals of post-tensioning a structural glass beam are 1) to augment the initial fracture strength of the glass beam and 2) to provide a significant post-fracture residual load-carrying capacity. the first goal of augmenting the fracture strength of the glass beam is envisioned by applying a favourable pre-stress in the glass beam. this pre-stress annuls the bending stress at the lower edge of the glass beam caused by external load and thereby increases the fracture strength of the glass beam. in the current research, the pre-stress is applied either by mechanically anchored or adhesively bonded steel pre-stressing tendons. the second goal of enhancing the post-fracture performance of structural glass beams is envisioned by the tensile capacity and ductility of the steel tendons. upon glass fracture, the tendons are expected ∗corresponding author: dr. ir. christian louter, steel structures laboratory (icom), school of architecture, civil and environmental engineering (enac), école polytechnique fédérale de lausanne (epfl), gc b3 505, station 18, lausanne ch-1015, switzerland. tel.: +41 21 6932427; fax: +41 21 6932868; e-mail: christian.louter@epfl.ch. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:christian.louter@epfl.ch 4 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams to bridge the crack(s) in the glass, thereby carrying the tensile forces. together with a compressive force in the (unfractured) top part of the glass beam, an internal moment capacity is generated, which enables the fractured beam to still carry significant load. this concept resembles the reinforced glass beam concept studied earlier by several authors (belis, callewaert, delincé, & van impe, 2009; correia, valarinho, & branco, 2011; cruz & pequeno, 2008; freytag, 2004; kreher & natterer, 2004; louter, belis, veer, & lebet, 2012; ølgaard, nielsen, & olesen, 2009; palumbo, palumbo, & mazzucchelli, 2005; speranzini & agnetti, 2014; weller, meier, & weimar, 2010). however, the post-tensioned glass beam concept aims at ‘active’ steel tendons which contribute to the initial fracture strength of the glass beam, as explained earlier, rather than ‘passive’ reinforcement sections which are mainly activated upon glass fracture only. the concept of post-tensioned glass beams has currently been explored in only a limited number of (research) projects (belis, louter, verfaille, van impe, & callewaert, 2006; bos, veer, hobbelman, & louter, 2004; jordão, pinho, martins, santiago, & neves, 2014; louter et al., 2014; louter, nielsen, & belis, 2013; louter, pérez, et al., 2013; schober, gerber, & schneider, 2004; weller & engelmann, 2014). despite their limited number, these projects clearly demonstrate the potential of post-tensioned glass beams. however, investigations are currently still in the preliminary phase and additional studies are needed. the current paper therefore aims to contribute to the knowledge on post-tensioned glass beams and investigates the mechanical response of such system. this is done through four-point bending tests on 1.5m long glass beam specimens. different series of beam specimens are tested namely 1) beams with mechanically anchored post-tensioning tendons integrated at the recessed top and bottom edge of the glass beams, 2) beams with pre-tensioned tendons adhesively bonded at the lower edge of the glass and 3) reference beams which are identical to the post-tensioned glass beams, but without post-tensioning tendons. the following sections describe the applied materials and specimens in detail. furthermore, the post-tensioning method and the four-point-bending procedure are explained. subsequently, the results of the four-point bending tests are provided, compared and discussed. finally, conclusions from the study are provided. 2. materials & specimens the cross-sections of the beam specimens tested in this study are provided in fig. 1. they can be separated in group mech (mechanically anchored) and group adh (adhesively bonded). all specimens consisted of three layers of annealed float glass (6-10-6mm) with polished edges, which were laminated by means of 1.52mm thick sentryglas® (sg) interlayer sheets. all edge polishing and lamination was executed by professional glass processors, though by different processors per group of specimens. the beams were 1500mm long and 125mm high. group mech consisted of one series of beam specimens with mechanically anchored posttensioning tendons (mech-pt) and one series of reference beams without tendons (mech-ref). for all specimens of group mech, the inner glass layer was recessed along both edges. the long edges of all glass layers were polished before lamination, whereas the short edges at the beam ends were polished after lamination to guarantee a perfect alignment of the glass where the post-tensioning forces were introduced, see section 3.1. the applied tendons were stainless steel threaded bars m8, grade 1.4301, integrated at the recessed beam edges. c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 5 fig. 1. cross-section of the different beam specimens tested in this study; mech=mechanically anchored; adh=adhesively bonded; pt=post-tensioned; ref=reference (without tendons); all dimensions are given in mm. group adh consisted of one series of beam specimens with pre-tensioned tendons bonded at the lower edge of the beams (adh-pt), and one series of reference beams without tendons (adh-ref). for all specimens of group adh, both the long and the short glass edges were polished after the lamination process. the tendons were stainless steel solid sections, grade 1.4301, with cross-section dimensions of 3*25mm. the tendons were pre-tensioned following the method described in section 3.2 and were subsequently bonded to the lower edge of the glass by means of a two-component epoxy adhesive, 3m scotch-weld dp490 (‘3mtm scotch-weldtm epx epoxy adhesive dp490 datasheet’, 1996), at a targeted adhesive thickness of 0.1mm. for each beam series, the estimated second moment of area iy is provided in fig. 1. this is calculated following the rules of steiner, according to equation 1: iy = n∑ i=n ( bi · h3i 12 · ei eglass + z2i · bi · hi · ei eglass ) (1) where: bi =width of the considered component in the section hi =height of the considered component in the section ei =e-modulus of the considered component in the section eglass =e-modulus of glass zi =distance of the centroid of the considered component to the overall neutral axis (n.a.) 6 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams the overall neutral axis, here defined as the distance zt from the top edge of the beam, can be determined using equation 2: zt = ∑( zt,i · bi · hi · eieglass ) ∑( bi · hi · eieglass ) (2) where: zt,i =distance of the centroid of the considered component to the top edge of the beam in equation 1 the second moment area (bi · h3i /12) of the individual components within the crosssection and the product of their area (bi · hi) and the squared distance zi of their central axis to the overall neutral axis are summed. furthermore, the aspect ratio between the e-modulus ei of the considered component in the cross-section and the e-modulus eglass of glass, which is the dominant material within the cross-section, is taken into account. for the calculation of iy of the different beam series the following e-moduli have been adopted: eglass =70.000 mpa (en 572-2, 2004), etendon =200.000 mpa (en 1993-1-4, 2006) and esg =493 mpa (stelzer, 2010). it should be noted that for the calculation of iy (according to equation 1) the bevels at the polished glass edges are ignored and that nominal dimensions are used. furthermore, it is assumed that the unbonded m8 threaded bars don’t contribute to the inertia iy of the beam series mech-pt. reversely, for beam series adh-pt full composite action is assumed between the glass and the tendon, and as such the tendons are fully incorporated in the calculation of iy. 3. method 3.1. post-tensioning method for series mech-pt the post-tensioning method applied for beam series mech-pt is illustrated in fig. 2. the posttensioning forces were applied at the beam-ends by means of steel end-pieces at which the steel tendons were anchored. to prevent excessive peak stresses, intermediary aluminium sheets were provided between the steel end-pieces and the short ends of the glass beam. furthermore, as indicated in section 2, the short ends of the glass beams were polished after the lamination process to guarantee perfect alignment of the glass layers, and thus to prevent excessive peak stresses. on one end of the beam, see fig. 2 (left), the tendons were extended by means of hydraulic jacks connected to a manually operated hydraulic pump. on the other end of the beam, see fig. 2 (right), the force in the tendons was monitored by means of load cells through which the tendons were fed. once the required post-tensioning force was obtained, the tendons were anchored with screw nuts, and the hydraulic jacks were released and removed. to be able to monitor the forces in the tendons during the bending tests, the load cells remained in position. for the mech-pt beams, the applied compressive pre-stress (σgl,p) can be calculated according to equation 3: σgl,p = − p agl (3) where: p=applied pre-load (total of both tendons, as provided in table 1) agl =cross-section area of the glass beam (=2550 mm2 for the mech-pt beams) c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 7 fig. 2. post-tensioning method applied for the beam series with the mechanically anchored post-tensioning tendons (mechpt). for beam specimen mech-pt-#2, the strain in the glass during the post-tensioning procedure was monitored by means of strain gauges. for this, a total of four strain gauges were provided at mid-span of the beam. three gauges were provided at side a (gauges 1, 2 and 3) and one gauge at the opposite side b (gauge 4), see fig. 3. gauges 2 and 4 were located at the neutral axis of the beam, whereas gauges 1 and 3 were positioned 5mm from the upper and lower edge of the glass, respectively. 3.2. post-tensioning method for series adh-pt the post-tensioning method applied for beam series adh-pt is illustrated in fig. 4. the tendons were placed in a steel u-section rig, anchored by a bolt at one side (see fig. 4a) and tensioned at the other side (see fig. 4b). during tensioning, local strains in the tendons were measured at both ends via strain gauges, see fig. 4a and b, from which the force in the tendons was derived. subsequently, the two-component epoxy adhesive was applied on the tendons, and the glass beams were positioned on top, see fig. 5. the adhesive was left to cure for at least three days before releasing the tendons. the beams were tested in four-point bending seven to ten days after pre-stressing. for the adh-pt beams, the applied pre-stress at the top (σgl,t,p) and bottom edge (σgl,b,p) of the glass can be calculated following equations 4 and 5, respectively: σgl,t,p = − p aeq + p · e iy · zt (4) σgl,b,p = − p aeq − p · e iy · zb (5) where: p=applied pre-load (as provided in table 1) aeq = agl + n · at equivalent cross-section area of the beam agl =cross-section area of the glass beam (=2750 mm2 for the adh-pt beams) 8 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams fig. 3. strain gauges applied at specimen mech-pt-#2; (a) cross-section view; (b) side view of the beam at mid-span. fig. 4. post-tensioning method applied for the beam series with the pre-tensioned adhesively bonded tendons (adh-pt). (a) tendon anchorage by means of bolt in temporary tensioning rig; (b) tendon tensioning mechanism. at =cross-section area of the steel tendon (=75 mm2 for the adh-pt beams) n = etendon/eglass e =eccentricity of the applied tendon pre-load p to the neutral axis (see fig. 6) c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 9 fig. 5. applied method for bonding the pre-tensioned tendons to the lower edge of the glass beam (series adh-pt). fig. 6. post-tensioning force balance on adh-pt beam series. zt =distance from the neutral axis to the top glass edge (see fig. 1) zb =distance from the neutral axis to the bottom glass edge (see fig. 1) the first parts of equations 4 and 5 account for a compressive pre-stress which can be envisioned by shifting the applied pre-load p to the neutral axis over distance e, see fig. 6. the second parts of the equations account for the bending stress resulting from an upwards-bending moment p·e inflicted by the tendon. 3.3. four-point bending procedure after post-tensioning, the beams were tested in four-point bending, see fig. 7. for this, a custommade support frame was mounted on a universal tension-compression machine (zwick 500 kn). the support span amounted to 1.4m with a load span of 0.4m. lateral supports were provided at a centre span of 0.55m. a fixed displacement rate of 1mm/min was applied until glass fracture, after which the displacement rate was augmented to 2mm/min and 5mm/min, respectively, to reduce test duration. overall, the duration of a single test amounted to about 30–45 minutes. during the test, the applied force (f) and machine displacement were measured and recorded. 10 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams fig. 7. schematic representation of the applied four-point-bending configuration. the applied bending stress either at the top (σgl,t,f ) or bottom (σgl,b,f ) glass edge is calculated by equations 6 and 7, respectively: σgl,t,f = f 2 ·s2·zt iy (6) σgl,b,f = f 2 ·s2·zb iy (7) where: f =applied force s2 =distance between support and load point (see fig. 7) 4. results the results of the four-point-bending tests are provided in table 1 and figs. 8-17. table 1 presents the pre-load as applied by the post-tensioning tendons, the fracture load and the maximum post-fracture load recorded during the four-point bending test and the post-fracture reserve. the latter expresses the maximum post-fracture load as a percentage of the initial fracture load. figures 8–11 provide the force-displacement curves resulting from the four-point-bending tests. all beam series show initial linear elastic response until fracture of the glass occurs. after initial glass fracture, the reference beams show a significant drop in load and a limited post-fracture reserve, see figs. 9 and 11. the post-tensioned beam series, however, demonstrate significant post-fracture reserves and maximum post-fracture load values higher than the initial fracture loads, see figs. 8 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 11 table 1 results of the four-point-bending tests spec. no. pre-load fracture load maximum post-fracture load post-fracture reserve p ffracture fpost−fracture fpost−fracture / ffracture [kn] [kn] [kn] [%] mech-pt-#1 49.6∗ 12.0 16.6 139 mech-pt-#2 50.6∗ 11.9 15.4 129 mech-pt-#3 49.9∗ 11.6 16.8 144 mean 50.0 11.8 16.3 138 mech-ref-#1 – 8.3 2.4 28 mech-ref-#2 – 7.3 2.5 35 mech-ref-#3 – 8.4 2.6 30 mean – 8.0 2.5 31 adh-pt-#1 23.9∗∗ 18.1 35.0 193 adh-pt-#2 28.3∗∗ 20.0 35.1 175 adh-pt-#3 28.2∗∗ 22.3 36.6 164 mean 26.8 20.1 35.6 178 adh-ref-#1 – 7.8 2.6 33 ahd-ref-#2 – 9.7 2.8 28 mean – 8.8 2.7 31 ∗total of the two tendons. the applied force was equal in both tendons. ∗∗corrected values from (louter et al., 2014). and 10. figure 12 provides a comparison of typical force-displacement curves of the different beam series. figures 13–15 show typical crack patterns observed for the different beam series. the cracks in the glass originate from the lower (tensile) edge of the glass beams and propagate upwards. the reference beams typically show a localized v-shaped cracking pattern, see figure 13. the post-tensioned beams of both series mech-pt and adh-pt show distributed cracking and a distinct zone where ultimate compressive failure of the glass occurred, see figs. 14 and 15. the post-tensioned beams of series adh-pt typically exhibit debonding of the tendons, see fig. 15. figure 16 shows the results of the strain gauge measurements performed during the post-tensioning procedure of specimen mech-pt-#2. figure 17 shows the force in the tendons of specimen mech-pt-#3 during the four-point bending procedure. 5. discussion 5.1. series mech; glass beams with mechanically anchored tendons from the results of series mech-pt and mech-ref, the following is observed. firstly, it can be seen that the post-tensioned mech-pt beams reach higher initial fracture loads than the mech-ref reference beams. whereas the reference beams reach an average initial fracture load of 8.0 kn, this amounts to 11.8 kn for the mechanically post-tensioned beams, see table 1. 12 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams fig. 8. force-displacement curves of series mech-pt, post-tensioned beams with unbonded m8 tendons. fig. 9. force-displacement curves of series mech-ref, reference beams without tendons. fig. 10. force-displacement curves of series adh-pt, post-tensioned beams with adhesively bonded tendons. fig. 11. force-displacement curves of series adh-ref, reference beams without tendons. fig. 12. comparison of typical force-displacement curves of all beam series. c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 13 fig. 13. typical localized cracking in reference beams. fig. 14. typical distributed cracking in mech-pt specimens (tendons not shown on picture). fig. 15. typical distributed cracking in adh-pt specimens and significant yielding of tendon. fig. 16. stress plots derived from strain gauge measurement during post-tensioning of specimen mech-pt-#2. this increase in initial fracture strength of the mechanically post-tensioned beams is explained by the compressive pre-stress applied by the post-tensioning tendons. figure 18 provides an overview of the stress distribution along the height of the mechanically post-tensioned mech-pt glass beams at the verge of initial glass fracture, based on mean values 14 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams fig. 17. force measurements in the tendons of specimen mech-pt-#3 made during the four-point bending test. fig. 18. stress distribution in the glass of mech-pt beams at the verge of initial glass fracture, based on mean values. provided in table 1. the figure consists of three parts, namely (i) the applied compressive pre-stress as calculated from equation 3, (ii) the applied bending stress at the point of initial glass fracture as calculated from equations 6 and 7, and (iii) the resulting stress distribution in the glass which is a sum of (i) and (ii). it can be seen that the resulting tensile (fracture) stress (iii) at the lower edge of the glass amounts to 43.5 mpa. this value is similar to the tensile fracture stress of the mech-ref reference beams, which amounts to 42.8 mpa, as can be calculated by equation 7. taking into account the known and rather significant scatter in glass strength, it can be concluded that post-tensioning the glass beams has been successful and that the observed increase in beam strength is indeed likely to originate fully from the applied compressive pre-stress. for specimen mech-pt-#2 an attempt was made to record the pre-stress applied during the posttensioning procedure by means of strain gauges bonded at either side of the beam, as explained in c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 15 section 3.1 and fig. 3. from the strain gauge measurements provided in fig. 16, it can be seen that the pre-compressive stress at side a reaches higher values than at side b. at a total pre-tensioning load of 50 kn, a pre-compressive stress of −31.5 mpa is observed at side a, whereas this amounts to about −8.5 mpa at side b. this indicates that the specimen is – to a limited extent – bending along its weak axis during the post-tensioning procedure. this bending may originate from a misalignment of the tendons in the recessed notches, i.e. the tendons are not perfectly centred, and in fact it was observed during some attempts of the post-tensioning procedure that re-positioning of the tendons in the recessed edges could reduce the stress differences between side a and b. in addition, it is assumed that initial imperfections, i.e. global bow of the beams (belis, mocibob, luible, & vandebroek, 2011), may have enlarged this effect. since only one specimen was instrumented with strain gauges, it could not be observed to what extent this effect also occurred in the other specimens. further studies will focus in more detail on this effect, thereby also determining the lateral deflection of the beam specimens during the post-tensioning procedure. however, it should be noted that all gauges on side a indicated the same stress level, thus uniform loading over the full height of the beam was obtained. averaging the pre-compressive stress at side a and b results in a mean pre-compressive stress of (−31.5–8.5)/2= −20 mpa, which is indeed very close to the calculated pre-stress of −19.6 mpa, as provided in fig. 18. secondly, it can be seen that the post-tensioned mech-pt specimens develop a significant postfracture resistance. whereas the reference beams demonstrate a single crack in the glass and a post-fracture reserve of only about 30% of their initial fracture strength, the mechanically posttensioned beams show repetitive and distributed cracking of the glass and reach about 140% of their initial fracture strength, see table 1. for the reference beams, the post-fracture load-carrying mechanism is originating from the sg interlayer sheets (belis, depauw, callewaert, delincé, & van impe, 2009). in the reference beams, fracture of the glass occurs in all three glass layers at the same location. at that location the sg interlayer sheets are the only ‘intact’ components able to transfer tensile forces over the crack in the glass. together with a compressive force in the top part of the beam, a post-fracture load-carrying mechanism is generated. however, the stiffness of the interlayer is relatively low, which results in limited post-fracture strength of the beams. furthermore, under continued loading, the interlayer starts to deform plastically, which causes a plastic hinge in the beam and a gradual reduction of the post-fracture load, as can be seen from figs. 9 and 11. for the mechanically post-tensioned beams (mech-pt) the post-fracture load-carrying capacity is generated by the lower tendon that bridges the crack(s) in the glass. a tensile force in the tendon and a compressive force in the top part of the glass beam generate an internal moment capacity that allows the fractured beam to still carry load. under continued loading the fractured beam is even able to carry load beyond its initial fracture load, which causes additional cracks in the glass to occur. this increasing post-fracture load-carrying capacity of the post-tensioned beams is associated with a gradually increasing tensile force in the lower tendon and a gradually decreasing force in the upper tendon, as can be seen from fig. 17. due to yielding of the lower tendon, the beams show a ductile post-fracture response. in addition, progressive cracking of the glass further contributes to a decrease in post-fracture stiffness of the beams. collapse is finally caused by complete failure of the compression zone in the glass, see fig. 14. whether this is caused by overstressing of the glass or by local instability of the compression zone could not be observed during the bending experiments and needs further study. 16 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams fig. 19. stress distribution in the glass of adh-pt beams at the verge of initial glass fracture, based on mean values. 5.2. series adh; glass beams with adhesively bonded tendons from the results of series adh-pt and adh-ref, the following is observed. firstly, it can be seen that the post-tensioned beam specimens adh-pt reach higher initial fracture loads than the reference beams adh-ref. the reference beams fracture at an average load of 8.8 kn, while this amounts to 20.1 kn for the post-tensioned adh-pt beams, see table 1. this difference in initial fracture strength is again explained by the beneficial pre-stress inflicted by the post-tensioning tendons. figure 19 shows the stress distribution in the post-tensioned adh-pt beams at the verge of initial glass fracture, thereby making use of equations 4-7 and applying mean values as provided in table 1. the tensile fracture stress (iii) at the lower glass edge amounts to 36.1 mpa. this value is similar to the fracture stress of the adh-ref reference glass beams, which amounts to 38.4 mpa as can be calculated from equation 7. this again indicates that the glass strength is similar for the adh-pt and adh-ref beams, which means that the post-tensioning method is successful and that the increase in beam strength of the adh-pt beams is likely to originate fully from the beneficial pre-stress inflicted by the tendons. secondly, it can be seen that the post-tensioned beams develop a significant post-fracture resistance, whereas this is largely absent for the reference glass beams. the post-fracture load-carrying capacity amounts to about 180% of the initial fracture load for the series adh-pt and about 30% for the reference beams, see table 1. for the series adh-pt post-tensioned beams the post-fracture load-carrying capacity is, similarly to the series mech-pt post-tensioned beams, generated by a tensile force in the tendon and a compressive force in the upper zone in the glass. the transfer of forces between the tendon and the glass fully relies on shear in the epoxy adhesive bond. this adhesive bond is sufficiently strong to transfer the shear load between the tendon and the glass and even enables the tendon to reach the yielding point. however, under continued loading gradual adhesive failure and debonding of the tendon – from mid-span outwards – is observed. final beam collapse is associated with full debonding of the tendon – from mid-span to one of the beam ends – and explosive failure of the c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams 17 glass compression zone, see fig. 15. which phenomenon occurs first could not be observed during the experiments and will need to be investigated in future research. finally, it is observed that the adh-pt beams reach higher post-fracture strength levels than the mech-pt beams, see fig. 12. this is explained by the larger cross-section area of the tendons that are applied in the adh-pt beams than in the mech-pt beams. this allows the tendons in the adh-pt beams to carry higher forces than the tendons in the mech-pt beams. a higher tensile capacity of the tendons implies that the beams can reach higher post-fracture loads due to an enhanced internal moment capacity. 6. conclusions in this study, bending experiments have been conducted on post-tensioned glass beams and reference glass beams. from the results of the bending experiments it is concluded that post-tensioning structural glass beams, by means of mechanically anchored or adhesively bonded tendons, is a feasible concept which provides increased initial fracture strength and enhanced post-fracture performance. the post-tensioned glass beams reach initial fracture loads which amount to 150–230% of the initial fracture load of the reference glass beams. this is due to the pre-stress applied by the tendons, which annuls the tensile bending stress at the lower glass edge and thus augments the fracture strength of the beam. furthermore, the post-tensioned glass beams develop a significant post-fracture reserve and reach post-fracture load levels which amount to 140–180% of their initial fracture load. this post-fracture reserve is generated by the tendons that successfully bridge the cracks in the glass and provide a post-fracture load-carrying mechanism. it should be noted, however, that the results presented here are merely exploratory. more in depth studies into post-tensioning glass beams are required. especially the concept of post-tensioning glass beams with adhesively bonded pre-tensioned tendons requires specific attention. thermal expansion differences between the tendon and the glass may cause significant stress in the adhesive layer and thus need to be addressed. furthermore, permanent stressing of the adhesive due to the pre-stressing tendon may cause creep in the adhesive layer and thus reduction of pre-stress over time. for this, it might well be that a solution can be found in combining the concept of adhesively bonded tendons with an additional mechanical anchor. these and various other aspects will be investigated by the authors in near-future studies. acknowledgments the swiss national science foundation is gratefully acknowledged for funding the present research through snf grant 200021 143267. additionally, the contribution of msc students arno pérez, thibaut jordan and mathieu debonnaire in performing part of the tests for this research is gratefully acknowledged. finally, the cost action tu0905 structural glass – novel design methods and next generation products is gratefully acknowledged for providing a research network on structural glass. references 3mtm scotch-weldtm epx epoxy adhesive dp490 datasheet (1996). retrieved october 02, 2014, from http://catalogue.3m.eu/ en eu/eu-mro/3m adhesives/structural adhesive 2 part duo-pack epoxy/scotch-weld%e2%84%a2 epx∼epoxy adhesive∼dp490/ep oxy adhesive http://catalogue.3m.eu/en_eu/eu-mro/3m_adhesives/structural_adhesive_2_part_duo-pack_-_epoxy/scotch-weld%e2%84%a2_epx~epoxy_adhesive~dp490/epoxy_adhesive http://catalogue.3m.eu/en_eu/eu-mro/3m_adhesives/structural_adhesive_2_part_duo-pack_-_epoxy/scotch-weld%e2%84%a2_epx~epoxy_adhesive~dp490/epoxy_adhesive 18 c. louter et al. / exploratory experimental investigations on post-tensioned structural glass beams belis, j., callewaert, d., delincé, d., & van impe, r. (2009). experimental failure investigation of a hybrid glass/steel beam. engineering failure analysis, 16(4), 1163-1173. doi:10.1016/j.engfailanal.2008.07.011 belis, j., depauw, j., callewaert, d., delincé, d., & van impe, r. (2009). failure mechanisms and residual capacity of annealed glass/sgp laminated beams at room temperature. engineering failure analysis, 16(6), 1866-1875. doi:10.1016/j.engfailanal.2008.09.023 belis, j., louter, c., verfaille, k., van impe, r., & callewaert, d. (2006). the effect of post-tensioning on the buckling behaviour of a glass t-beam. in international symposium on the application of architectural glass isaag (pp. 129-136). belis, j., mocibob, d., luible, a., & vandebroek, m. (2011). on the size and shape of initial out-of-plane curvatures in structural glass components. construction and building materials, 25(5), 2700-2712. doi:10.1016/j.conbuildmat.2010.12.021 bos, f., veer, f., hobbelman, g., & louter, c. (2004). stainless steel reinforced and post-tensioned glass beams. in icem1212th international conference on experimental mechanics. correia, j. r., valarinho, l., & branco, f. a. (2011). post-cracking strength and ductility of glass–gfrp composite beams. composite structures, 93(9), 2299-2309. doi:10.1016/j.compstruct.2011.03.018 cruz, p. j. s., & pequeno, j. (2008). timber-glass composite beams: mechanical behaviour & architectural solutions. in challenging glass (pp. 439-448). en 1993-1-4. eurocode 3-design of steel structures part 1-4: general rules supplementary rules for stainless steels (2006). en 572-2 (2004). glass in building – basic soda lime silicate glass products – part 2: float glass. freytag, b. (2004). glass-concrete composite technology. structural engineering international, 14(2), 111-117. doi:10.2749/101686604777 963991 jordão, s., pinho, m., martins, j. p., santiago, a., & neves, l. c. (2014). behaviour of laminated glass beams reinforced with pre-stressed cables. steel construction, 7(3), 204-207. doi:10.1002/stco.201410027 kreher, k., & natterer, j. (2004). timber-glass-composite girders for a hotel in switzerland. structural engineering international, 2, 149-151. louter, c., belis, j., veer, f., & lebet, j.-p. (2012). structural response of sg-laminated reinforced glass beams; experimental investigations on the effects of glass type, reinforcement percentage and beam size. engineering structures, 36, 292-301. doi:10.1016/j.engstruct.2011.12. 016 louter, c., cupać, j., & debonnaire, m. (2014). structural glass beams pre-stressed by externally bonded tendons. in glasscon global (pp. 460-469). louter, c., nielsen, j. h., & belis, j. (2013). exploratory experimental investigations on post-tensioned structural glass beams. in the 2nd international conference on structures and architecture icsa 2013 (pp. 358-365). louter, c., pérez, a., jordan, t., & lebet, j.-p. (2013). post-tensioned structural glass beams – experimental investigations. in cost action tu0905 mid-term conference on structural glass (pp. 277-284). ølgaard, a. b., nielsen, j. h., & olesen, j. f. (2009). design of mechanically reinforced glass beams: modelling and experiments. structural engineering international, 19(2), 130-136. doi:10.2749/101686609788220169 palumbo, d., palumbo, m., & mazzucchelli, m. (2005). a new roof for the xiiith century “loggia de vicari” (arquá petrarca pd italy) based on structural glass trusses: a case study. in glass processing days (pp. 434-435). schober, h., gerber, h., & schneider, j. (2004). ein glashaus für die therme in badenweiler. stahlbau, 73, 886-892. speranzini, e., & agnetti, s. (2014). strengthening of glass beams with steel reinforced polymer (srp). composites part b: engineering, 67, 280-289. doi: 10.1016/j.compositesb.2014.06.035 stelzer, i. (2010). high performance laminated glass. in challenging glass 2 (pp. 467-474). weller, b., & engelmann, m. (2014). deformation of spannglass beams during post-tensioning. in challenging glass 4 & cost action tu0905 final conference (pp. 285-294). weller, b., meier, a., & weimar, t. (2010). glass-steel beams as structural members of façades. in challenging glass 2 (pp. 517-524). journal of facade design and engineering 1 (2013) 53–71 doi 10.3233/fde-130005 ios press 53 comparison and development of sustainable office façade renovation solutions in the netherlands michiel ritzena,b,∗, bertold vd meijdena, ronald roversb, zeger vroonb,c and chris geurtsa,c aeindhoven university of technology, the netherlands bribuilt/zuyd university of applied sciences, the netherlands ctno, the netherlands received: 13 august 2013 accepted: 14 november 2013 abstract. environmental, commercial and societal developments in the netherlands stimulate the environmental improvement of the existing office building stock. in the netherlands, about 15% of all office area was vacant in 2012, and the majority of offices have a relative poor energy performance. to measure the improvement, different assessment tools are applied. these tools either focus on one aspect, such as operational energy, and result in a specific outcome such as mj/m2, or these tools combine different aspects, such as energy and materials, through a weighted system and result in a generic outcome, such as ‘excellent’. in this research, the relation between assessment outcome and actual environmental impact is investigated of both types of tools, by reflecting the outcome of the tool to the carrying capacity of a system. the relation is investigated through a comparison of the energy and material aspect of three office façade renovation solutions using four different assessment tools. using a tool in which energy and material impact is related to the carrying capacity, current energy focused optimization might lead to a sub optimization of actual environmental impact. to illustrate this, a calculated façade solution is presented with minimal environmental impact based on carrying capacity. keywords: façades, building materials, office buildings, environmental impact 1. introduction between 1990 and 2005 global final energy consumption increased by 23% and co2 emissions increased with 25% ((iea), 2008). this consumption is expected to grow with another 45% between 2002 and 2025 (ko & widder, 2011). 20% to 40% of this global energy consumption is consumed in the built environment (pérez-lombard, ortiz, & pout, 2008), for more than 86% based on fossil fuels ((useia), 2011). between 1995 and 2005, extraction of fossil fuels increased with 24% (bruckner, giljum, lutz, & wiebe, 2012). to lower overall energy consumption in the built environment and to lower dependency on fossil fuels, it is agreed within the eu that by the end of 2020 all new buildings are nearly zero-energy buildings, and that by the end of 2018 all new buildings occupied and owned by public authorities are nearly zero-energy buildings (nzebs) (eu, 2010). nzeb means that the building has a very high energy performance and that the low amount of energy required should be generated to a very significant extent from renewable sources, on-site or nearby, having a ∗corresponding author: ir. michiel ritzen, eindhoven university of technology/zuyd university of applied sciences, the netherlands. e-mail: m.j.ritzen@tue.nl. issn 2213-302x/13/$27.50 © 2013 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:m.j.ritzen@tue.nl 54 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands connection to the grid to cope with seasonal differences (agentschapnl, 2012; eu, 2010; torcellini, pless, deru, & crawley, 2006). reaching the target of nearly zero energy depends only on improving the energy efficiency in the operational phase of the building. this requires adding material to the building for thermal insulation, building services and energy generation products. consequently, the realization of a less energy consuming built environment is largely depending on an increase of material consumption, and collateral increase of construction material extraction, resulting in an increase of the material related impact compared to the energy related impact (ramesh, prakash, & shukla, 2010). worldwide, extraction of construction minerals increased between 1995 and 2005 with 30% (bruckner et al., 2012). besides improvement the energy performance of new buildings, improvement of the energy performance of existing buildings is increasingly being realized, amongst others in the office sector. the dutch office market, consisting of 52.2 million square meters, had a vacancy percentage of 14.6% in 2012 (zadelhoff, 2013), corresponding with 7.62 million square meters. as the market situation of office buildings in the netherlands is not in equilibrium, renters have a wide variety of real estate to choose from, and are in the position to select offices with a high energy performance. the average energy label of the 10% of offices in the netherlands that have an energy label is e (agentschapnl, 2010a). this label corresponds with an operational energy performance of 1.49gj/m2·a for heating and cooling, lighting and hot tap water. besides this market development, the dutch government agreed that the dutch government itself, responsible for around 20% of office space occupation in 2010 (agentschapnl, 2010b), only rents offices with minimum energy label c since 2010, which results in a higher energy performance of buildings (agentschapnl, 2010a). already a number of ngo’s and companies have joined this agreement, and it is expected that more organizations will join this government agreement in the framework of corporate social responsibility (csr). as a result, many offices are renovated to improve their energy label to a minimum of c. in these renovations, the façade is often replaced to improve the operational energy performance of the building. currently, a wide range of tools is available to calculate the operational energy performance of buildings, such as the dutch standard energy performance calculation program ((nen), 2010), and vabi 114 (vabi, 2013). fig. 1. energy label distribution in the dutch office market 2010 (agentschapnl, 2010a). m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 55 but assessment tools, used to measure the environmental impact of buildings, should take both the energy aspect and the material aspect in such a way into account that the necessary insight is created in the total burden. examples of these tools are breeam, leed and the dutch greencalc+. in these tools, aspects such as energy and materials are combined with aspects such as management through a weighted combination of indicators (häkkinen, 2012). these tools have a number of advantages, such as the distinction in the level of sustainability of a building compared to other buildings, providing a communication tool, encouraging stakeholders to define sustainability requirements, and providing a vehicle for policymaking (tran, 2009). however, in these tools energy and materials account for different shares in total building performance outcomes and different categories are applied. the different categories are divided into different subcategories and the grading of the subcategories depends on different quantitative and qualitative parameters. the parameters are based on performance and evaluation, while different system boundaries are used and different levels of detail are applied. resulting in an outcome in which the level of sustainability for comparable buildings differs due do the different aspects and weighting (häkkinen, 2012). according to iwaro et al, the measurements and the weights that should be given to the criteria are unresolved issues (iwaro, mwasha, williams, & zico, 2014). the outcomes in the end might show how the energy and/or material situation has improved, but create a dilemma with regard to what the connection is between the outcome of the assessment tool and the actual environmental impact of the building. considering the material aspect, it is often only translated in embodied energy: the amount of energy necessary to process raw materials, modify materials and transport materials. energy, embodied in buildings, may account for up to 60% of total life cycle energy (dixit, fernández-solı́s, lavy, & culp, 2012). façades may account of up to 26% of total building embodied energy (thormark, 2007; yohanis & norton, 2002). the embodied energy in materials can be seen as a ‘rebound effect’ of energy performance improvement, and has in current practice a negative impact on the calculated operational energy performance improvement in household heating and cooling (herring, 2009). the same can be expected in office buildings. by calculating material consumption using only the embodied energy, the operational energy aspect and the material aspect are translated in a corresponding quantity; energy. for instance, belgian residential low energy buildings with a primary energy consumption for heating of ca. 900mj/m3 building volume over 30 years have a total embodied energy of 1400mj/m3 building volume over 30 years, which is higher than the energy consumption for heating (verbeeck & hens, 2010). for embodied energy calculations various definitions, methodologies and system boundaries are used (dixit et al., 2012). an example of the latter is that there is a distinction between methodologies in which only the amount of fossil based energy is part of the calculation as it is ‘added’ to the product, and methodologies in which the total amount of embodied energy, both fossil based and renewable based, which comes from ‘natural sources’ such as the sun, is calculated. besides the different calculation methodologies, most results have uncertainties due to temporal, spatial and technical circumstances (location, weather, societal and energy generation), which are in many cases not shown in databases underlying the calculation tools (dixit et al., 2012). a third aspect of embodied energy calculations is the varying system boundary of the calculation. all these aspects have resulted in databases which face the problem of incomparability and variation (dixit et al., 2012). besides these considerations, all embodied energy methods do not take into account the actual availability of resources, both renewable and non-renewable. as the extraction of construction materials increased significantly, it is worth exploring a method to be able to assess 56 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands energy and materials equally. due to the increase of material consumption and due to the associated increase of raw materials extraction more and more land is needed, with a negative impact on amongst others ecological systems, biodiversity and the reflectiveness of earth (planetary albedo). in addition, raw materials which are necessary for the production of the building materials, such as copper, do not have an infinite stock. besides land use required for extracting raw materials, there is also land needed for generating non-renewable and renewable energy to convert the raw materials in building materials or components and transportation of these building materials and components. as we have a limited amount of land and potential productivity of this land, it seems logical to base our consumption pattern on the land available for production and extraction of (building) materials, generation of energy, water production and food production. in future, land necessary to produce renewable energy might compete with land necessary for food production and material production, which may lead to other choices in the design and realization of buildings (rovers, de flander, gommans, & broers, 2011). consequently, sustainability needs to be based on what can be generated and consumed in equilibrium within the system, implying an indicator based on the carrying capacity necessary to materialize and operate a function, instead of on impact calculations without any relation with the system itself (rovers, 2010). the carrying capacity is the maximum persistently supportable load of a system (catton jr, 1986), and can be indicated by the amount of land necessary to sustain the functioning of the system and the time this land is necessary, embodied land. to calculate the embodied land, an assessment tool called maxergy is under development (rovers, 2010). the embodied land of a product (in m2·year) indicates the amount of land needed for the extraction of raw materials, the growth of materials, the generation of power, the recuperation of land, etc. in the dutch situation (rovers, 2010, 2011; v. rovers et al., 2011). the aim of the tool is to generate insight in the interaction of the energy and material aspect in buildings and relate the total impact of these impacts to the carrying capacity. in the methodology section this tool is further explained. in this research, the relation between building environmental assessment tool outcomes and actual environmental impact is investigated. the energy and material aspects of three office façade renovation solutions are compared by using four different assessment tools in relation to the carrying capacity. a comparison is made between the situation before and after renovation of a south facing simulated office space with the different façade renovation solutions, covering the energy use of the building (operational energy), materials of the façade (embodied energy) and the related land use (embodied land) and compared with the outcome of a generic tool. based on the comparison, a façade renovation solution with lowest environmental impact on carrying capacity has been calculated. 2. methodology the environmental impacts of three façade renovation solutions, realized in the netherlands, have been investigated through a comparison of the outcomes of four different assessment tools before and after renovation. for this research, a south orientated office space has been simulated in front of which different façade renovation solutions have been placed. the south facing façade has been selected because it is the building component which has the biggest effect on the annual cooling and heating load of a building in the netherlands. the simulated space has a width of 8m, a length of 8m and a height of m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 57 7m. the simulated space consists of office spaces divided over two floors, because office spaces are the most relevant spaces in the buildings and the design of one of the selected projects is based on two floors. the selection of the three office façade renovation projects was based on availability and completeness of data and drawings and the sustainability ambitions in greencalc+. fig. 2. impression of the selected façade renovation solution of the dhv office building (source: vd meijden). fig. 3. impression of the selected façade renovation solution of the wnf office building (source: vd meijden). 58 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands fig. 4. impression of the selected façade renovation solution of the central post office building (source: vd meijden). the following façade renovation solutions were selected: • dhv office, amersfoort; renovation during which the façade was totally replaced and the interior was preserved largely. • wnf office, zeist; renovation during which the façade was totally replaced and the building was partially demolished, stripped and refurbished. • central post, rotterdam; renovation of an existing post office, during which the façade was partially replaced. for each simulated space the same technical installations were applied for a relevant comparison of the influence of the façades on assessment tool outcomes. in all cases, only the material and energy aspect of the façade was taken into account. building structure and architecture, services, lighting, interior components, economic, societal and user behaviour were out of the scope of this research to generate in depth insight in the relation between the energy and the material aspect, although these other aspects might have a substantial influence on building performance and impact (stephan, crawford, & de myttenaere, 2012). for the calculations of the office façades a technical lifetime of 30 years (ebbert, 2010) was chosen as reference. energy and material aspects related to the pre-building phase as well related to the re-use phase and demolition phase were out of the scope of this research. in table 1 an overview is given of materials used in the different façade renovation solutions. table 1 overview of materials applied in the selected office façade renovation solutions dhv office aluminium curtain wall; double pane argon filled glazing wnf office wooden curtain wall, triple pane krypton filled glazing central post office aluminium curtain wall; double pane argon filled glazing m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 59 the office façade renovation solution with lowest environmental impact in terms of embodied land has been further optimized using maxergy, because this tool relates most closely to carrying capacity and assesses both the material and energy aspect and its interaction without weighting. the following assessment tools/databases have been applied: vabi, ice, greencalc+, and maxergy and will be further introduced in the following sections. 2.1. vabi 114 vabi 114 (vabi, 2013) is a dynamic building simulation program in which the annual heating and cooling load in mj can be calculated. vabi 114 generates in depth insight in the operational energy aspect in relation to the indoor climate. vabi 114 complies with national and international standards brl 9501, bestest, edr according to isso 54 and ashrae standard 140. in this research, the program has been applied to calculate the operational energy demand of the simulated space with different façades before and after renovation. the program only takes operational energy into account. other aspects, such as embodied energy, are not embedded in the program nor is the interaction between different aspects. 2.2. ice database in this research, the “inventory of carbon & energy” (ice) database of the university of bath (hammond, 2008) is selected to calculate the embodied energy of the different façade renovation solutions before and after renovation. the ice database has been selected because the data corresponds most closely to the dutch situation. the ice database is an inventory of the embodied energy of materials data, originating from life cycle analyses (lca’s), books and papers. in the embodied energy calculation there is no interaction with other aspects such as operational energy. 2.3. greencalc+ program greencalc+ (greencalc+, 2013) expresses the sustainability of a building in an environmental index. the environmental index of a building (milieu index gebouw mig) is based on a comparison of the environmental costs of material consumption, energy consumption, and water consumption with the environmental costs of a standard dutch building realized in 1990. greencalc+ has been applied to determine the overall building sustainability after renovation. by translating all aspects into one cost aspect they can be combined to one generic outcome and thus compared to other buildings. the determination of environmental costs of materials is based on cml-2, the lca method developed by the university of leiden, in combination with the eco-indicator ’99 method and the twin-model. the method of müller-wenk is used for the determination of transportation related noise disturbance. the determination of environmental costs is based on the dutch standards nen 2916:2004 and nen 5128:2004, complying with the dutch standard energy performance calculation. this calculation is through a lca translated into environmental costs. for office buildings, the determination of water consumption is calculated with the dutch ‘water performance standardisation’. this calculation is through a lca translated into environmental costs. although the impact of user mobility is calculated in greencalc+, it is not part of the generic outcome. the user mobility is determined for office buildings 60 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands fig. 5. floor plan of the investigated simulated south facing office space. fig. 6. section of the investigated simulated south facing office space. by a calculation in an adapted version of the software program vpl-kiss (greencalc+, 2013). between the different aspects in greencalc+ there is no interaction or interrelation. the standard reference building from 1990 has a value of 100 mig. when a building is more sustainable than the reference building from 1990, then the value becomes above 100 mig. buildings with a mig-value below 100 are less sustainable than a building realized in 1990. although this tool indicates the relative improvement of environmental impact of a building compared with other buildings and with a building in 1990, it does not indicate clearly the actual impact on the environment. m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 61 2.4. maxergy maxergy is a sustainability tool which expresses the energy and material impact of a project in the same physical quantity: embodied land. embodied land is the amount of space and time necessary to fulfil the energy and the material demand for a certain function in a certain environment. embodied land is expressed in m2·year (rovers, 2011). the embodied land of the different façade renovation solutions before and after renovation has been calculated using maxergy. the total embodied land (el) of a product is calculated using several databases as input the amount of new and recycled materials. the total embodied land calculation can be divided into direct embodied land (land and time required for the creation of a raw material), indirect embodied land (embodied energy converted into land and time) and operational energy (converted into land and time). the embodied land for a building consists of three components, a. el building, b. el materials and c. el operational energy, as indicated in fig. 7. a. el building indicates the land occupied by the building during its lifespan itself and can be directly derived from the design drawings in m2. fig. 7. schematic overview of the proposed embodied land calculation method for a combined environmental assessment of energy and materials in the built environment. 62 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands b. el for materials consists of two impacts: the primary impact indicates the time·land required to generate, produce and transport the material itself. the primary impact consists of the direct el for material generation, and indirect el for material production and transportation. 1. the direct primary el is calculated using a harvest database in which harvest/m2 are collected, depending on the origin of the material. the indirect primary el is based on the ice embodied energy database and through energy generation surface calculated (solar or fossil based) (hammond, 2008). the input needed to calculate both direct and indirect primary el is the mass of the building material (kg) and energy generating device efficiency. 2. the secondary impact indicates the time·land required to generate and produce the techniques and installations necessary to generate the materials; e.g. the photovoltaic panels required to generate the necessary embodied energy. the tertiary impact and other possible relevant impacts, such as operational transportation energy, are not taken into account (stephan, crawford, & de myttenaere, 2013). the input needed to calculate the secondary el is both the mass of materials used in the installations (kg) and installation efficiency (w/m2). c. el operational energy consists of the land necessary to generate the energy using solar energy (pv/ solar thermal) or fossil resources and the el necessary for the generation, production and transportation of the materials used for the energy generating devices. these tools and databases were selected to generate in depth insight in the operational energy and material aspect (vabi 114 and ice database), and to be able to compare these results with a widely used generic tool in the netherlands (greencalc+) of which the data of all cases was available, and to be able to relate this to the carrying capacity (maxergy). 3. results in the following section, the calculated results of the different applied assessment tools/databases; vabi 114, ice database, greencalc+ and maxergy are presented. in section 3.2.1 the results of vabi 114 and the ice database are combined. 3.1. vabi 114 figure 8 shows the results of the annual cooling and heating load of the simulated office space with different façade solutions before and after the renovation, calculated with vabi 114. the heating and cooling load of all three cases after the renovation is similar. the cooling load of the investigated south orientated space is in all buildings the largest energy factor both before and after the renovation. the renovation of the façade has mainly impact on the heat load of the building, which is strongly reduced after the renovation. this is achieved by increasing the rc value of the façade through the application of materials with higher values of thermal insulation and the application of double pane argon filled glazing or triple pane krypton filled glazing. 3.2. ice database figure 9 shows the results of all embodied energy calculations of the different façade solutions before and after the renovation. the embodied energy required for the façades of the dhv office m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 63 fig. 8. annual operational energy load consisting of heating and cooling of the simulated south facing office space with façade solutions before and after renovation. fig. 9. total embodied energy of the façade solutions before and after renovation. and the wnf office after the renovation is many times higher than the embodied energy of the façades before the renovation. after renovation, the dhv office has a new aluminium curtain wall with double pane argon filled glazing and the wnf office has new wooden curtain wall with triple 64 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands fig. 10. total energy performance of the simulated south facing office space with façade solutions before and after renovation, over 30 years. pane krypton filled glazing. the embodied energy required for the new façade of the central post building is relatively small compared to that of the other buildings. this is because this façade was only partly replaced and remained largely unchanged. furthermore, the embodied energy required for the original façade was already very high due to the large amount of the materials applied, such as concrete and steel. the existing aluminium façade with single glazing is replaced by a new aluminium façade with double pane argon filled glazing. 3.2.1. combination of vabi 114 and ice database in fig. 10 the results of total energy consumption calculations are shown (operational energy of the simulated office space and the embodied energy of the façade) before and after the renovation, for a total technical lifetime of 30 years. the results indicate that, for the investigated south facing office space, the cooling load has the largest energy impact, both before and after the renovation. it also shows that the heat load of the office space has decreased significantly after the renovation, which is the result of the improved thermal properties of the façades after the renovation. to achieve these improved thermal properties more embodied energy is required for the façades. in general the embodied energy of the façades increases after renovation, but the cooling load remains the largest energy demand for this south facing office space. 3.3. greencalc+ the results of all renovation projects in table 2 show a final score above 200 and all projects score both in the field of energy performance and material consumption an a, which is good. in the results there is no difference between the various projects in the field of energy performance and material m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 65 table 2 greencalc+ score of all renovation projects dhv office wnf office central post office material a a a energy a a a water e f g total 239 269 252 fig. 11. total embodied land in m2 of the simulated south facing office space with different façade solutions before and after renovation based on fossil fuels, over 30 years. consumption. the wnf office has the highest total score, because the building generates energy with photovoltaic (pv) panels. 3.4. maxergy figure 11 show the calculated embodied land of façade materials and the calculated embodied land of operational energy of the simulated office space when all energy required for the operational energy is based on fossil fuels. fossil fuels have a significant larger el that renewable fuels due to the large amount of land and large span of time necessary to generate these fuels (r. rovers et al., 2011). due to this, the embodied land of the operational energy is the determining factor compared with the embodied energy for the materials of the façade. only the results of the wnf façade solution show a different situation where the material use is the determining factor. this is because after the renovation operational energy in this building is generated by solar energy. figure 12 shows the embodied land calculations of the simulated office space with different façade solutions before and after renovation, over 30 years, when all energy is generated by solar energy (solar panels and solar collectors). in this calculation, the total embodied land for all solutions is much smaller than with a similar calculation using fossil fuels, due to the large time·land impact to 66 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands fig. 12. total embodied land in m2 of the simulated south facing office space with different façade solutions before and after renovation based on solar energy, over 30 years. generate fossil fuels. secondly, the embodied land for the façade materials is much greater than the embodied land for the operational energy. the embodied land of the operational energy is in most cases negligible compared to the embodied land of the façade materials. the results show that the wnf office façade solution after renovation scores very good in comparison with the other façades. this façade consists mainly of wood, a natural material with low embodied energy. natural (bio-based) materials score very well in the embodied land calculation, because these materials can grow back naturally by themselves, so a closed-loop system is created without adding energy. a closed-loop system for the materials is created when a material that is used as a building product has grown back within the lifetime of the façade, and all energy to realize the building product has been regenerated. the aluminium, concrete and steel that are used in the dhv and the central post office façade solutions are not bio-based and cannot grow back. these materials are however recyclable and partially reusable. the recycling percentages of these materials are not 100%, for aluminium it is for instance 94% (haas, 2002). according to the maxergy calculation a lot of energy is needed to win back the non-recycled percentage of these materials. 4. calculation of a façade renovation solution with lowest environmental impact on carrying capacity based on the results presented in section 3, the wnf façade solution has been further investigated and its environmental impact has been further minimized using maxergy. as indicated in the preceding sections, using only energy related calculations or using a generic assessment tool does not offer a comprehensive carrying capacity based indicator of the environmental impact of a building. analysis of the materials applied in the wnf façade solution (fig. 13) shows that most of the embodied land of the façade originates from non bio-based materials, such as steel and aluminium. m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 67 based on this analysis a comparison has been made between four façade solution versions to investigate the embodied land minimisation as a result of the interaction of the material and energy aspect. in fig. 14 an overview is given of these versions: a. the façade after renovation with a certain amount of material related el, mainly due to nonrenewable materials, and a certain amount of operational energy related el; b. minimisation of material related el while maintaining the same operational energy related el resulting in a façade in which the actual openings are maintained and thermal insulation is maintained, but all materials are 100% bio-based. c. minimisation of material related el, resulting in a façade consisting of a plywood sheet and no openings. d. minimisation of operational energy related el with high insulation values for the opaque façade components (rc=10m2·k/w). fig. 13. embodied land of different materials in the wnf façade solution in m2·year. fig. 14. total embodied land in m2 of the simulated south facing office space with different façade solution versions based on solar energy, over 30 years. 68 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands fig. 15. picture of the realized wnf façade renovation (source: vd meijden). in all versions the embodied land of the materials is still larger than the embodied land of the operational energy, indicating the importance of material consumption in this assessment method. even when the façade consists of only a minimal amount of bio-based materials (only a 30mm plywood sheet), the embodied land required for the operational energy is small. within the boundaries of the dutch building regulation, a minimisation of environmental impact of the façade renovation solution has been investigated. the dutch building regulations indicate the following for this office façade calculation: • insulation value for opaque façade parts rc 3.5m2 k/w. • u value transparent façade parts u 2.2w/k·m2. • 2m2 transparent façade surface per office floor. within these boundaries a maximum use of bio-based materials is investigated. non bio-based materials, like metals, need to be recycled as much as possible. the façade design consists for 93% of bio-based materials, in which the metal components have been replaced by fibre-reinforced composites. even the design of window placing is realized without metal components (fig. 16). resulting in a façade solution that needs a total of 304m2 embodied land for a lifespan of 30 years, which is a reduction of 70% compared to the actual wnf façade renovation solution. in addition, in the design is taken into account that the building components are easy to separate, increasing the possibilities for re-use and recycling. the collateral effect of this minimization of embodied land is a solution with disputable architectural quality, compared to the realized wnf office façade renovation design (fig. 16). 5. conclusions and discussion based on the comparison of the simulated south facing office with different façade renovation solutions and the calculation of a façade renovation solution with minimal environmental impact m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 69 fig. 16. picture of the calculated office façade renovation with minimal embodied land (source: vd meijden). based on carrying capacity, the following conclusions concerning operational energy performance, material performance and the related embodied land are drawn. • this research indicates that in the simulated cases operational energy efficient façade renovations result in a decrease of operational energy and an increase of embodied energy in the façade. assessment tools based either on one aspect such as operational energy or on only energy related aspects or resulting in a generic outcome do not generate insight to lower the actual total environmental impact. • in all cases, the cooling load is the largest energy part of total energy demand both before and after the renovation and the embodied energy of the façades is a small portion of the total energy demand, over a lifespan of 30 years, considering only a south facing façade. these results would presumably be different when the complete building would be taken into account and when other orientations of the façade would be investigated. • not only the amount of materials but also the choice of materials determines the embodied energy and embodied land of the façade. • in the case of the wnf façade solution, the building itself generates after the actual renovation to a high extend its own energy through photovoltaic (pv) panels on the roof. if in this case the operational energy would not be included in the calculation and the pv panels would be included in the material calculation, the total energy consumption of the building would consist solely of embodied energy. material consumption would in this case be the determining factor in environmental impact. • the embodied land calculations based on fossil fuels show in almost all cases that the operational energy is the determining factor compared to materials. an exception is the wnf office façade solution after the renovation because in this case the operational energy of the building is generated on site. in this situation, the material aspect becomes the determining factor in environmental impact. 70 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands • the calculated office façade renovation solution indicates that the amount of materials and the choice of materials determine the environmental impact in nearly all situations. bio-based materials, such as wood, score very well in this calculation because the low amount of embodied energy and renewability. further research is suggested to compare the façade versions using other tools, and base façade versions on these tools. • it can be concluded that in a combination of embodied and operational energy based on fossil fuels, the material aspect determines the environmental impact in the case of nzeb’s, such as the wnf building, emphasizing a tool in which the material aspect and energy aspect are non-weighted assessed, and the maxergy tool offers this possibility. based on this research project, the following conclusions concerning the maxergy tool are drawn to suggest further research in this direction. • the energy related embodied land calculation in maxergy is based on the surface of solar panels and solar collectors, which is necessary for generating electricity and heat. the results of the embodied land calculations are therefore highly dependent on the efficiency of the solar panels and solar boilers used for this. • operational energy is in an increasing number of buildings generated with renewable sources, but the majority of embodied energy is not. therefore, a comparison is made between the land use by means of fossil energy and solar energy. further research into impact by using the current energy mix (fossil fuels, nuclear and renewable energy) is recommended to generate insight in the actual energy related embodied land of materials. • an important part of the maxergy calculation is the recovery of raw materials such as metals. in many cases this is the decisive factor for the final result. for the recovery of metals for example a method is chosen, in which metal particles are filtered from seawater. this includes a number of assumptions. further research should be done on the recovery of metal particles. • as the maxergy tool aims at relating the combination of material consumption and energy performance of a building to the carrying capacity of a system it offers the possibility to generate insight in building performance from a perspective related to our planet. but as the tool is based on existing embodied energy data, the same discussions concerning availability of data, the bandwidth of results, etc. are relevant and further research should be conducted in order to generate more reliable outcomes related to the carrying capacity. references (iea), internation energy agency. (2008). worldwide trends in energy use and efficiency. (nen), nederlands normalisatie instituut. (2010). nen epw npr 5129+ a2:2010v2.2 energieprestatie van woonfuncties en woongebouwen rekenprogramma (epw). (useia), u. s. energy information administration. (2011). international energy outlook 2011: u.s. energy information administration. agentschap, n. l. (2010a). onderzoek naar het energieen co2-reductiepotentieel duurzaam inkopen van gebouwen, de rijksgebouwendienst als voorbeeld. agentschap, n. l. (2010b). rapport energiecijfers kantoren. agentschap, n. l. (2012). nationaal plan voor het bevorderen van bijna-energieneutrale gebouwen in nederland. bruckner, m., giljum, s., lutz, c., & wiebe, k. s. (2012). materials embodied in international trade – global material extraction and consumption between 1995 and 2005. global environmental change, 22(3), 568-576. doi: 10.1016/j.gloenvcha.2012.03.011 catton jr, w. r. (1986). the gaia atlas of planet management: for today’s caretakers of tomorrow’s world: n. myers (general ed.). journal of rural studies, 2(2), 170-171. doi: http://dx.doi.org/10.1016/0743-0167(86)90061-6 m. ritzen et al. / comparison and development of sustainable office façade renovation solutions in the netherlands 71 dixit, m. k., fernández-solı́s, j. l., lavy, s., & culp, c. h. (2012). need for an embodied energy measurement protocol for buildings: a review paper. renewable and sustainable energy reviews, 16(6), 3730-3743. doi: 10.1016/j.rser.2012.03.021 ebbert, t. (2010). re-face: refurbishment strategies for the technical improvement of office facades. (phd), delft university of technology, delft. eu directive 2010/31/eu of the european parliament and of the council of 19 may 2010 on the energy performance of buildings (recast) (2010). greencalc+. (2013). greencalc+ website. from http://www.greencalc.com/ haas, m. (2002). nibe’s basiswerk milieuclassificatie. häkkinen, t. e. (2012). sustainability and performance assessment and benchmarking of buildings final report. espoo: vtt. hammond, g. j. (2008). inventory of carbon and energy (ice). university of bath, uk. herring, h. s. (2009). energy efficiency and sustainable consumption (h.s. herring, steve ed.). hampshire, england: palgrave macmillan. iwaro, j., mwasha, a., williams, r. g., & zico, r. (2014). an integrated criteria weighting framework for the sustainable performance assessment and design of building envelope. renewable and sustainable energy reviews, 29(0), 417-434. doi: http://dx.doi.org/10.1016/j.rser.2013.08.096 ko, j., & widder, l. (2011). building envelope assessment tool for system integrated design. paper presented at the plea 2011, louvain-laneuve, belgium. pérez-lombard, l., ortiz, j., & pout, c. (2008). a review on buildings energy consumption information. energy and buildings, 40(3), 394-398. doi: 10.1016/j.enbuild.2007.03.007 ramesh, t., prakash, r., & shukla, k. k. (2010). life cycle energy analysis of buildings: an overview. energy and buildings, 42(10), 1592-1600. doi: http://dx.doi.org/10.1016/j.enbuild.2010.05.007 rovers, r. (2010). 0-material building: space time analyses. paper presented at the sb10, maastricht, the netherlands. rovers, r. (2011). maxergy and embodied land. in ribuilt (ed.). rovers, r., de flander, k., gommans, l., & broers, w. (2011). designing for only energy: suboptimisation. paper presented at the plea 2011, louvain-la-neuve, belgium. rovers, v., rovers, r., de flander, k., broers, w., houben, j., gommans, l., & sigwarth, s. (2011). maxergy, duurzaamheidsbereking op basis van landgebruik: ribuilt / zuyd university of applied sciences. stephan, a., crawford, r. h., & de myttenaere, k. (2012). towards a comprehensive life cycle energy analysis framework for residential buildings. energy and buildings, 55(0), 592-600. doi: http://dx.doi.org/10.1016/j.enbuild.2012.09.008 stephan, a., crawford, r. h., & de myttenaere, k. (2013). a comprehensive assessment of the life cycle energy demand of passive houses. applied energy, 112(0), 23-34. doi: http://dx.doi.org/10.1016/j.apenergy.2013.05.076 thormark, c. (2007). energy and resources, material choice and recycling potential in low energy buildings. paper presented at the cib conference sb07 sustainable construction, materials and practices lisbon, portugal. torcellini, p., pless, s., deru, m., & crawley, d. (2006). zero energy buildings: a critical look at the definition. paper presented at the aceee summer study, california. tran, h. t. (2009). measuring sustainability: carbon or land? paper presented at the inta-sega. vabi. (2013). vabi 114 website. from http://www.vabi.nl/producten/vabi-uniforme-omgeving/gebouwsimulatie verbeeck, g., & hens, h. (2010). life cycle inventory of buildings: a contribution analysis. building and environment, 45(4), 964-967. doi: 10.1016/j.buildenv.2009.10.003 yohanis, y. g., & norton, b. (2002). life-cycle operational and embodied energy for a generic single-storey office building in the uk. energy, 27(1), 77-92. doi: http://dx.doi.org/10.1016/s0360-5442(01)00061-5 zadelhoff. (2013). nederland compleet; kantoren en bedrijfsruimtemarkt. http://www.greencalc.com/ http://dx.doi.org/10.1016/j.rser.2013.08.096 http://dx.doi.org/10.1016/j.enbuild.2010.05.007 http://dx.doi.org/10.1016/j.enbuild.2012.09.008 http://dx.doi.org/10.1016/j.apenergy.2013.05.076 http://www.vabi.nl/producten/vabi-uniforme-omgeving/gebouwsimulatie http://dx.doi.org/10.1016/s0360-5442(01)00061-5 journal of facade design and engineering 2 (2014) 85–107 doi 10.3233/fde-140015 ios press 85 a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades jonas dispersyna,∗, manuel santarsierob, jan belisa and christian louterb alaboratory for research on structural models (lmo), department of structural engineering, faculty of engineering and architecture, ghent university, ghent, belgium bsteel structures laboratory (icom), school of architecture, civil and environmental engineering (enac), école polytechnique fédérale de lausanne (epfl), lausanne, switzerland submitted 19 september 2014 revised 29 october 2014 accepted 14 november 2014 abstract. the recent demand for architectural transparency has drastically increased the use of glass material for structural purpose. however, connections between structural glass members represent one of the most critical aspects of glass engineering, due to the fragile behaviour of this material. in that respect, research activities on adhesive point-fixings are currently on-going. the mechanical behaviour of adhesive point-fixings is affected by large nonlinearities, which are usually investigated by nonlinear finite element analysis (fea). this paper focuses on the geometrical and the material nonlinearities of adhesive point-fixings for glass structures. firstly, the nonlinear material behaviour of two selected adhesives are investigated by means of uniaxial tension and compression tests on the bulk material. the production of specimens, test methodology and displacement rate dependency are discussed. secondly, the nonlinear stress distribution occurring in the adhesive and the joint stiffness is investigated by means of nonlinear fea. the effects of several parameters on the mechanical behaviour of adhesive point-fixings, such as the connection dimensions and adhesive elastic properties, are studied. the adhesive stress-strain curves resulting from the experimental campaign show that the adhesives exhibit a large nonlinear behaviour. the results show that the stress and strain at failure reduce as the displacement rate is reduced. from the numerical investigations it is concluded that large nonlinearity involves the mechanical behaviour of adhesive point-fixing which cannot be neglected. the stress distribution within the adhesive deviates from uniform nominal stresses, even in case of simple load condition, with stress peaks up to four times higher than nominal stresses. keywords: structural analysis, glass facades, glass, steel, numerical model list of notation ασ, max [-] ratio between the maximum principal stress and the nominal stress ασ, mis [-] ratio between the von mises stress and the nominal stress σnom [mpa] nominal stress ∗corresponding author: jonas dispersyn, laboratory for research on structural models (lmo), department of structural engineering, faculty of engineering and architecture, ghent university, ghent, belgium. tel.: +32 9 264 54 84; fax: +32 9 264 58 38; e-mail: jonas.dispersyn@ugent.be. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:jonas.dispersyn@ugent.be 86 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades σmax [mpa] maximum principal stress σmis [mpa] von mises stress η [-] stress triaxiality p [mpa] hydrostatic pressure σ [mpa] stress tensor σ′ [mpa] deviatoric part of the stress tensor e [mpa] young’s modulus of elasticity of the adhesive v [-] poisson’s ratio of the adhesive d [mm] diameter of the metal connector t [mm] adhesive’s thickness εzz [-] longitudinal strain εyy [-] transversal strain εpeak [-] maximal strain εpeak [mpa] maximal stress εfail [-] strain at failure σfail [mpa] stress at failure 1. introduction the demand for architectural transparency has drastically increased the use of glass as a structural material. however, connections between structural glass members still represent one of the most critical aspects of glass engineering because of the fragile behaviour of the material. traditional systems to connect glass to the supporting substructure consist of linear supports, schematically depicted in fig. 1a. by using such systems, the transparency of the facade is highly reduced (haldimann et al., 2008). in contrast, the overall transparency improves significantly by using so-called point-fixings (siebert & herrmann, 2010; vyzantiadou & avdelas, 2004). these fixings typically consist of locally installed metal, of limited size, connecting the glass elements to the structure using bolts through the glass. this requires the glass panel to be drilled near the corners or edges, and subsequently to be tempered and bolted (siebert, 2006), as depicted in fig. 1b and c. a recent technology prevents drilling through the glass by using so-called undercut point-fixings, as illustrated in fig. 1d. these connections do not penetrate the insulation cavity at insulating glass units. however, with bolted connections, the glass is significantly weakened by the drilling process at the holes edges, which is where high stress peaks occur due to the local transfer of forces by the glass (beyer, 2007; feldmann et al., 2008; maniatis, 2006; mocibob & belis, 2010; overend 2005). the use of adhesive connections avoids this issue because the glass is directly bonded to the metal connector (see figs. 1e and 2). indeed, adhesive joints reduce high stress peaks by spreading the force over a larger area (dispersyn et al., 2014; overend et al., 2012; santarsiero et al., 2013; sitte et al., 2011; weller & tasche, 2005). the risk of thermal bridges and condensation is also strongly reduced. analytical prediction of the mechanical behaviour of adhesive point-fixings is rather complex and no analytical models are currently available. the structural response and stress distribution are indeed affected by severe nonlinearities. because of that, adhesive point-fixings are usually designed by means of nonlinear finite element analyses (fea). this work presents the results of two separated research activities performed at ghent university (belgium) and at epfl (switzerland), which focus on different aspects of the nonlinear behaviour of adhesive point-fixings. j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 87 fig. 1. schematic representation (cross-section) of three different ways to attached glass panels to the underlying structure with a) linear supports, b), c) and d) bolted point-fixings and e) adhesive point-fixings. a b fig. 2. transparent adhesive connection. adhesive material shows large nonlinearity in the stress-strain constitutive law. the stress-strain constitutive law of adhesive materials is usually obtained by experimental tests on the bulk adhesive material. the fabrication of specimens is thereby critical. the first part of the paper focuses on the nonlinearity of the adhesive material under uniaxial tensile and compressive forces. the experimental investigations of two selected adhesives (i.e. a soft ms-polymer and a stiffer two-component epoxy) are presented. the fabrication of the specimens, testing methodology and results are discussed. to determine the maximum stress that the adhesive can withstand, experimental investigations are performed measuring the load at failure. however, the stress distribution within the adhesive is strongly nonlinear, even in case of linear material and simple load conditions such as pure tensile and pure shear load. the maximum stress peak in the adhesive at failure is larger than the nominal stress (i.e. force divided by adhesive area). this is mainly due to the confined stress state of the adhesive 88 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades caused by the large ratio between diameter and adhesive thickness. accurate fea are therefore necessary to correlate the maximum force measured during the test to the maximum adhesive stress. the second part of this work presents a parametrical study on the effects of mechanical and geometrical parameters on the stress distribution in the adhesive and on the joint stiffness. in this part the adhesive material is assumed to be linear elastic, to investigate confinement effect nonlinearity separately from the material nonlinearity. 2. material nonlinearity this first section focuses on the material nonlinearity of adhesive materials for point-fixings. the hypothesis of linear elastic material is often not accurate enough for polymeric adhesive material used in point-fixing connections. these materials are indeed characterized by stress-strain constitutive laws that yield large nonlinear behaviour that need to be accounted for in the numerical simulation. to obtain an accurate stress distribution in the adhesive under large deformations, the nonlinear mechanical behaviour of the adhesive should be taken into consideration. this is done by implementing stress-strain curves in the fea. these stress-strain curves are usually obtained by uniaxial tensile tests and compression tests on bulk material of the adhesive (overend et al., 2011; santarsiero & louter, 2014). adhesive point-fixings will mainly be loaded in pressure or tensile by pressure of suction of wind load. for this reason only tensile is considered in this article. dead load of glass panel will be carried by mechanical self-weight support as it is usually done for structural sealant glazing systems, as described by etag 002 (etag 002). 2.1. materials and method 2.1.1. selection of the adhesives there are many types of adhesives on the market varying from relatively flexible and low-strength adhesives, such as silicones, to relatively stiff and high-strength adhesives, such as epoxies and acrylates. the latter are non-toughened, thermosetting adhesives. these kinds of adhesives are usually brittle materials that will fail at relatively small strains. these materials mostly yield sufficient accuracy when modelled with linear elastic behaviour, as long as the deformations and temperature are not too high. in contrast, other commercial adhesives are rubber toughened, such as silicones and ms-polymers. due to the rubber phase that occurs here, relatively large strains (>5%) can occur with large deformation prior to failure. two adhesives are selected, namely one relatively flexible and low-strength adhesive and one relatively stiff and high-strength adhesive. the selection is made based on earlier research. an extensive experimental programme on adhesives for structural applications with glass and metal connectors has been performed by researchers of ghent university and delft university of technology to help designers select potential adhesives based on specific environmental exposures and loading conditions. the project yielded an adhesive selection tool based on performance criteria (belis et al., 2011a, b). the selected adhesives are: – soudaseal 270 hs, a ms-polymer (modified silane) adhesive, a hybrid polymer adhesive with a base of polyurethane and silicone. this adhesive combines the advantages of a polyurethane adhesive and a silicone, which gives a strong and flexible bond. j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 89 – 3m scotch weld 9323 b/a: this two-component epoxy adhesive presented good properties on both tensile and shear strength. 2.1.2. tensile test samples young’s modulus and poisson’s ratio can be obtained by small-scale tests, commonly by means of uniaxial tensile tests on dumbbell specimens. the outcome of these small-scale tests can also be used to determine the nonlinear behaviour of the adhesive. in principle dumbbell specimens of adhesive products can be fabricated with a mould or with cutting dies. with the former method, the adhesive is poured and cured in a mould. to avoid bonding to the mould, the latter should be manufactured out of material with a low solid surface free energy. with the latter method, a layer of adhesive has to be made with a thickness of several millimetres. after curing, the dumbbell specimens can then be punched out of the film with a cutting device. however, this technique results in relatively quantities of waste material. in addition, it is problematic to make a layer with a continuous thickness. consequently, it is decided to work with a mould for the current study. the adhesive dumbbell specimens are fabricated at the university of luxembourg, by means of a non-sticky mould (dias et al., 2012), following the geometry provided in iso 527-2, see fig. 3 (iso 572-2). by means of the mould three samples were fabricated at the same time, as depicted in fig. 4. the mould is fabricated from polyoxymethylene (pom), a stiff material with a low solid surface free energy. after curing, the samples are stored at a constant temperature of 21◦c and a relative humidity of 45% without any uv-radiation in a climatic chamber. the curing process of the ms-polymer soudaseal 270 hs is moisture curing. since the chosen fabrication method implied sealing off the sample from air, extra water was mixed with the adhesive before injecting it into the mould to increase its moisture content. after contacting the manufacturer fig. 3. dimensions according to iso 572-2 (iso 572-2). fig. 4. mould for uniaxial tensile samples according to iso 572-2 type 1b. 90 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades fig. 5. dumbbell sample of the ms-polymer soudaseal 270 hs. fig. 6. dumbbell sample of the epoxy 3m scotch weld 9323 b/a. of soudaseal 270 hs, this extra amount of humidity was set to 5%. the polymer was sucked in the mould by using the vacuum pump to avoid air bubbles in the samples. an example of a dumbbell sample of the ms-polymer soudaseal 270 hs is given in fig. 5. in total 18 samples were fabricated. the mixing ratios by weight of the two components of the epoxy 3m scotch weld 9323 b/a are 27 to 100, respectively. the weighing of each component is done with a balance having an accuracy of 0.1 gram. the components are mixed manually in a specific teflon cartridge, which is then installed on the mould. however, the vacuum pump could not be used due to the low viscosity of the mixed adhesive. consequently it was pushed manually in the mould by means of a self-designed injector. the curing of the adhesive took place inside the mould. due to the high adhesion of the epoxy, an extra thin sheet of transparent pe film of 0.1 thickness was placed between the adhesive and the plexiglas® cover of the mould. an example of a dumbbell sample of the epoxy 3m scotch weld 9323 b/a is given in fig. 6. in total 22 samples were fabricated for testing. 2.1.3. compressive test samples in realistic situations the adhesive in an adhesive joint is mostly not only loaded in tension. complex stress distributions occur when the adhesive is loaded in several types of loading. hence, to take the compressive mode into account compressive tests are indispensable. compressive tests on bulk material are not as commonly executed as tensile tests, but the fea will be more accurate if experimental data is obtained for different load directions. with compressive tests on bulk material attention has to be paid to the compressive plates. the latter have to be parallel to each other and to avoid friction between the samples and the plates, lubricant or teflon plates can be used. the test samples can be typical blocks or cylinders (iso 604, astm d695). in this research the cylindrical compressive specimens are produced in a ptfe-mould with a diameter of 30mm and a height of 15mm, as depicted in fig. 7. during the removal from the mould, the upper surface of the epoxy samples was damaged. therefore, a 5mm layer was removed from the top of the samples, resulting in a height of 10mm. an example of a soudaseal 270 hs compression sample is given in fig. 8a and of scotch weld 9323 b/a in 8b. in total eight samples of each adhesive were selected for testing. after fabrication, the samples were also stored at a temperature of 21◦c and a relative humidity of 45% without uv-radiation. 2.1.4. tensile tests the tensile tests were performed on a universal electro-mechanic test machine instron 5800r (frame 4505 retrofitted with a digital controller 8800). a load cell of 10 kn was used for the test results j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 91 fig. 7. mould for uniaxial compression samples. a b fig. 8. compression sample of a) soudaseal 270 hs and b) scotch weld 9323 b/a. 92 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades table 1 theoretical tensile displacement rates and strain rates, calculated based on technical datasheets, for chosen time to failure time till failure soudaseal 270 hs scotch weld 9323 b/a displacement strain rate displacement strain rate rate [mm/min] [−/min] rate [mm/min] [10−3/min] 20sec. 253∗ 4.22 6 100 2min. 75 1.25 1 16.67 10min. 15 0.25 0.2 3.33 ∗theoretical value is 450mm/min, but 253mm/min was the maximum speed of the tensile testing machine. presented in this paper. the load and the displacement between the clamping devices were measured with respectively an accuracy of ± 0.023 kn and ± 0.02mm and registered at a frequency of 10hz until failure of the dumbbell sample occurred. the tests were performed at ambient temperature, i.e. 18.5◦c. a constant displacement rate v was applied on the mobile clamp, and this was comprised between 2 and 60mm/min according to the test. because of the viscoelastic behaviour of adhesives, it is also important to determine the ratedependency. this is done by performing the uniaxial tensile tests with three different displacement rates. the loading rate of each adhesive was established depending on the elongation at break. with the maximum deformation given in the technical datasheets of the adhesive and in the earlier research, the theoretical displacement rates can be calculated. due to time constraints, the three different time intervals to failure are set at 20 seconds, 2 minutes and 10 minutes. the applied displacement rates are summarized in table 1. since the maximum deformations given by the adhesives manufacturers are lower limits (characteristic values), the expected experimental time to failure is longer than calculated. 2.1.5. compressive test the compressive tests were performed on the same machine. the samples are placed between two parallel steel plates, as illustrated in fig. 9. minimal friction between the plates and the sample was obtained by spraying ptfe-spray on the plates. the load and the displacement between the plates are measured and registered at a frequency of 10hz till 10 kn was reached. fig. 9. compressive test on scotch weld 9323 b/a. j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 93 table 2 theoretical compression displacement rates and strain rates, calculated based on technical datasheets, for chosen time to failure time till failure soudaseal 270 hs scotch weld 9323 b/a displacement strain rate displacement strain rate rate [mm/min] [−/min] rate [mm/min] [10−3/min] 2min. 5 0.333 0.2 20 10min. 1 0.067 0.04 4 fig. 10. tensile stress-strain curves for different displacement curves for a) the ms-polymer soudaseal 270 hs and b) the epoxy scotch weld 9323. for the uniaxial compression test only two different rates will be conducted, because of the reduced number of fabricated samples. the time intervals for the compression test are 2 and 10 minutes. the displacementratescorrespondingtothesetimeintervalsaresummarizedintable2forthetwoadhesives. here they represent the time till 10 kn is reached, corresponding to the load limit of the load cell. 2.2. results 2.2.1. tensile test results the nominal tensile stress-strain curves are obtained by dividing the force by the initial cross section at the reduced section and the displacement by the initial testing length. figure 10 displays the mean curves of the samples for each displacement rate for the ms-polymer soudaseal 270 hs and for the epoxy scotch weld 9323 b/a. the main parameters with their values and standard deviations are summarized in table 3. the standard etag 002 refers to iso 527 to obtain the young’s modulus and is calculated with equation 1 (etag 002; iso 527). 94 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades table 3 main parameters and their values and the standard deviations between brackets of the tensile tests load rate εpeak σpeak εfail σmax et [mm/min] [−] [mpa] [−] [mpa] [mpa] soudaseal 270 hs 253 2.20 (0.321) 3.43 (0.269) 2.20 (0.321) 3.43 (0.269) 3.38 75 1.59 (0.458) 2.89 (0.439) 1.59 (0.458) 2.89 (0.439) 3.30 15 1.38 (0.305) 2.43 (0.414) 1.38 (0.305) 2.43 (0.414) 2.99 scotch weld 9323 b/a 6 0.049 (0.0074) 37.30 (1.674) 0.053 (0.0025) 36.28 (1.653) 1207 1 0.046 (0.0084) 32.62 (2.867) 0.047 (0.0076) 31.33 (1.406) 1146 0.2 0.044 (0.0059) 31.08 (2.189) 0.046 (0.0083) 28.80 (1.768) 1137 with εpeak =peak deformation, σpeak =maximal stress, εfail =deformation at failure and σfail =stress at failure. fig. 11. compression stress-strain curves for different displacement curves for a) the ms-polymer soudaseal 270 hs and b) the epoxy scotch weld 9323. et = σ2 − σ1 ε2 − ε1 (1) where et =young’s modulus in mpa; σ1 = the stress in mpa as measured on the deformation value of ε1 = 0.0005; σ2 = the stress in mpa as measured on the deformation value of ε2 = 0.0025. for the epoxy, the young’s modulus et is calculated with the proposed deformation values and for the ms-polymer with ε1 =0.05 and ε2 =0.25, due to the rapidly occurring large deformations. 2.2.2. compressive tests results the compressive stress-strain curves are obtained by dividing the force by the loaded surface and the displacement by the initial height of the samples. the mean stress-strain curves for each displacement rate are illustrated in fig. 11 for both the ms-polymer soudaseal 270 hs as for the j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 95 table 4 main parameters and their values and the standard deviations between brackets of the compression tests load rate εmax σmax et [mm/min] [−] [mpa] [mpa] soudaseal 270 hs 5 −0.66 (0.0142) −14.8 (0.0751) 8.42 1 −0.69 (0.00759) −14.9 (0.0511) 8.09 scotch weld 9323 b/a 0.2 −0.058 (0.0026) −14.19 (0.0189) 15.17 0.04 −0.057 (0.0071) −14.35 (0.336) 18.81 epoxy scotch weld 9323 b/a. the main parameters with their values and standard deviations are summarized in table 4. the young’s modulus et is calculated with equation 1. 2.3. discussion of the results the material nonlinearity of the adhesives is clearly depicted in the graphs. concerning the load rate dependency, for soudaseal 270 hs the stiffness in tension reduces by 2,4% when reducing the loading rate from 253 to 75mm/min and by 9,4% when reducing it from 75 to 15mm/min. for scotch weld 9323 b/a the stiffness is reduced by 5% and 0.8% when the displacement rate is reduced from 6 to 1mm/min and from 1 to 0.2mm/min, respectively. compared to the literature this decrease of stiffness is negligibly small (dias et al., 2014; lees & hutchinson, 1992; yu, crocombe & richardson, 2001). this is also visible on the stress-strain curves: the slopes of the curves are almost all the same (fig. 10). a noticeable aspect on the curves (fig. 10) and the main parameters is that the stresses and strains at the point of failure in tension reduce remarkably as the displacement rate is reduced. indeed, for scotch weld 9323 b/a the reduction of the strains is 11,3% and 2,1% and the reduction of the stresses is 13,6% and 8,1%, respectively for the displacement rate reduction from 6 to 1mm/min and from 1 to 0,2mm/min. for the soudaseal these reductions are respectively 27,7% and 13,2% for the strains and 15,7% and 15,9% for the stresses. the authors believe that this can be explained by the fact that defects have more time to grow through the specimens when the displacement rate is low. contrary to the expectations, when the displacement rate decreases the epoxy scotch weld 9323 acts stiffer in compression, whereas the ms-polymer acted more flexible. for scotch weld 9323 b/a the stiffness increases by 19% with a reduction from 0.2 to 0.04mm/min of the displacement rate. the young’s modulus is determined at the origin of the curves, where small imperfections at the surface have a great influence on the curve, due to the very stiff behaviour. because of that problem, the stiffness of scotch weld 9323 b/a in compression is determined by means of equation 1, using ε1 =0.03 and ε2 =0.04. the young’s modulus for a displacement rate of 0.2mm/min is 322.8 mpa and for a displacement rate of 0.04mm/min it is 345.1 mpa, a difference of 6.4%. this is also visible on the stress-strain curves: the slopes of the curves are almost identical (see fig. 11b). due to its softer behaviour, the stress-strain curve of the ms-polymer is less sensitive to small imperfections at the surface. therefore the difference in stiffness in compression can be calculated using equation 1, with standard values of ε1 and ε2. the stiffness decreases by 3.9% when the displacement rate is reduced from 5 to 1mm/min (see fig. 11a). as expected, the difference between the stiffness of the two selected adhesives is significant. it is almost a factor of 400. this difference was also observed at failure, the more flexible ms-polymer 96 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades failed at large strains while the epoxy failed at low strains. however the epoxy failed at a ten times higher force than the ms-polymer. 3. parametric study on the effects of mechanical and geometrical parameters 3.1. method in this section, the effects of geometrical and mechanical parameters are studied with nonlinear fea, modelling a point-fixing under tensile load. the adhesive of point-fixing under tensile load are subjected to high confined stress state due to large diameter-thickness ratio. to separately investigate the nonlinearity due to confinement effect from the material nonlinearity, in this section the adhesive material is assumed to be linear elastic. a parametric analysis is performed to investigate the effects of four key parameters: young’s modulus e, poisson’s ratio v, connector diameters d and adhesive thickness t. for each numerical analysis of the parametric study one parameter value is changing while the other parameters are kept at a reference value. the obtained results are then postprocessed to study the effects related to each parameter. in particular, joint stiffness and adhesive stress distributions under tensile force are investigated. to do so, two groups of curves are derived, namely: load versus relative displacement and adhesive stress versus normalized distance. maximum principal stress, von mises stress and stress triaxiality are computed. to quantify the stress peaks and the nonlinear stress distribution, two stress factors are here defined by equation 2, ασ,max and ασ,mis, as ratio between the actual nonlinear stress values and the nominal uniform stress, usually called engineering stress. therefore, these coefficients describe the deviation of the nonlinear stress distribution from the uniform nominal one due to nonlinearity. ασ,max = σmax σnom ; ασ,mis = σmis σnom ; η = − p σmis ; (2) with: σnom = f a ; p = − trace(σ) 3 ; σmis = √ 3 2 σ′ : σ′; (3) where in the previous expressions σnom is the nominal stress (force divided by adhesive surface area), σmax and σmis are respectively the maximum principal stress and the von mises stress, η is the stress triaxiality, p is the hydrostatic pressure, σ is the stress tensor and σ′ is the deviatoric part of the stress tensor. 3.2. numerical model the adhesive connection is numerically analysed by a three-dimensional model realised by means of the finite element software abaqus (simulia, 2011). the geometry consists of a circular metal connector (stainless steel aisi 316l) adhesively bonded/connected to a rectangular glass plate (300mm × 150mm × 19mm) (see fig. 12). the presence of the threaded hole in the metal connector is taken into account in the model. only half geometry of the connection is implemented due to the symmetry along the x-axis (see fig. 13a). half geometry is used rather then quarter since the same model is used also in further j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 97 stainless steel adhesive glass panel fig. 12. scheme of numerical model: multiple-point-constrains, displacement constrains and load application. a b fig. 13. a) mesh pattern of the numerical model. b) zoom of the mesh refinement at the adhesive level. analysis with shear load (not here presented). the load is applied to the metal connector by means of multiple-points-constraints between the bolt surface and a reference point of load introduction. the vertical displacements of the glass plate are constrained at the left and right side of the glass (see fig. 12). three-dimensional solid finite elements are used to mesh the model geometry. in particular, 20-nodes quadratic brick elements with reduced integration were used to perform the nonlinear numerical analysis (c3d20r, see fig. 14). the mesh was refined at the adhesive level to increase the accuracy of the stress field computation. for the same reason, an additional mesh refinement is also introduced at the edge of the adhesive, where the stress peaks are expected to occur (fig. 13b). static step-by-step numerical analyses are performed by means of the implicit abaqus solver. finite deformation theory is used in each step of the calculations. to perform the parametric study, an algorithm has been developed, which makes use of automatic scripts written in python language. given as input a set of parameter values, the algorithm automatically performs the creation of the model, equation solution, extraction of data from the results’ database and post-processing of data in final parametric curves. 98 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades a b fig. 14. a) 3d view of the numerical model. b) quadratic 20-node element (simulia, 2011). table 5 parametric analysis: set of parameters’ value (reference values in bold-italic based on sg and tssa at 23◦c) parameter symbol [unit] values young’s modulus e [mpa] 5, 10, 20, 60, 125, 200 poisson’s ratio v [-] 0.450, 0.470, 0.480, 0.490, 0.494, 0.498 1) diameter d [mm] 35, 40, 45, 50, 55, 60 thickness t [mm] 0.5, 0.8, 1, 2, 2.5, 3 1)(bennison & davies, 2008; callewaert, 2011). the results of a mesh and element study state that quadratic elements should be preferred to linear ones for convergence purpose. furthermore, regarding the stress distribution in the adhesive, it resulted that at least 5 elements through thickness should be used to model the adhesive thickness with sufficient accuracy. 3.3. results of the parametric study the effects of several parameters on the adhesive point connection behaviour are studied by parametric analysis. the effect of young’s modulus e, poisson’s ratio v, diameter of the metal connectors d and adhesive’s thickness t are here presented for tensile load condition. all the investigated values are collected in table 5. the reference values are indicated in bold. as described above, the results are presented by load-displacement curves and normalized stress distribution versus normalized distance. stresses are extracted in the middle of the adhesive layer, i.e. cutting the adhesive at half thickness. it should be mentioned that usually after the curing process the actual adhesive thickness might be different than the initial one. only the current thickness values have to be considered for the stress calculation. therefore, in the following paragraphs, with ‘thickness’ is meant the actual measured thickness value of the adhesive after the curing process. the first parametric analysis showed a large sensitivity of the results to the adhesive’s young’s modulus e, both in terms of stiffness and stress distribution (see figs. 15 and 16). because of that, two groups of parametric analyses were then performed: one for a lower value of e (indicated as ‘soft adhesive’, e=5 mpa) and one for a higher value of e (indicated as ‘stiff adhesive’, e=200 mpa). j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 99 fig. 15. stress distribution in the adhesive for different values of young’s modulus of the adhesive material: maximum principal stresses (a) and von mises stresses (b). fig. 16. load-displacement curve (a) and stress triaxiality distribution (b) for different values of young’s modulus of the adhesive material. 3.3.1. young’s modulus figures 15 and 16 depict the effect of the young‘s modulus e of the adhesive material. the investigated values are 5 mpa, 10 mpa, 20 mpa, 60 mpa, 125 mpa and 200 mpa. these range of values are defined according to the stiffness of the adhesive material investigated at epfl, i.e. sg, sentryglas, and tssa, transparent structural silicone adhesive. values are taken from (bennison & davies, 2008; callewaert, 2012; sitte et al., 2011). 100 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades fig. 17. load versus relative displacement for different values of poisson’s ratio. fig. 18. maximum principal stress distribution in the adhesive for different values of poisson’s ratio. 3.3.2. poisson’s ratio figures 17-19 depict the effect of the poisson’s ratio v of the adhesive material. the investigated values are 0.450, 0.470, 0.480, 0.490, 0.494 and 0.498, from bennison et al. (2008), callewaert (2012) and sitte et al. (2011) for sg and tssa. 3.3.3. diameter figures 20-22 depict the effect of the metal connectors’ diameter d. the investigated values are 35mm, 40mm, 45mm, 50mm, 55mm and 60mm. these values correspond to typical dimensions of bolted point-fixings commonly used in glass structures and facades. j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 101 fig. 19. von mises stress distribution in the adhesive for different values of poisson’s ratio. fig. 20. load versus relative displacement for different values of diameter. 3.3.4. thickness figures 23-25 depict the effect of the adhesive thickness t. the investigated values are 0.5mm, 0.8mm, 1.0mm, 2.0mm, 2.5mm and 3.0mm. these values correspond to a typical range of thickness used in adhesive connections for glass structures. 3.4. discussion of the results the collected numerical results demonstrate that large nonlinearity characterizes the behaviour of adhesive point-fixings, even in case of simple load conditions. both the stress factors (here defined as ratio between stress peaks and nominal stress) and the global stiffness of the connections result to be strongly dependent on the mechanical and geometrical parameters here investigated. this indicates 102 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades fig. 21. maximum principal stress distribution in the adhesive for different values of diameter. fig. 22. von mises stress distribution in the adhesive for different values of diameter. that experimental tests carried out to determine the maximum failure stress of the adhesive should always be performed together with an accurate calculation of the actual stress peak occurring in the connection during the test. figures 16, 17, 20, and 23 show that the connection’s stiffness is a function of both the poisson’s ratio and joint geometry. this is because, once subjected to longitudinal elongation εzz, the adhesive material tends to exhibit transversal deformation εyy due to the poisson effect (see equation 4 and fig. 26a). however, the boundary conditions of adhesive joints constrain this transversal contraction resulting in transversal deviatoric stresses, conversely to what occurs during uniaxial tensile tests on bulk material where the transversal deformations are totally free to occur (fig. 26b). these deviatoric shear stresses are higher close to the edge of the adhesive (see fig. 15b). j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 103 fig. 23. load versus relative displacement for different values of thickness. fig. 24. maximum principal stress distribution in the adhesive for different values of thickness. εyy = −v · εzz (4) it is also observed that this constraining effect, which is the cause of the nonlinearity of the stress distributions in the adhesive (see figs. 16 and 19), increases with higher values of diameter and lower values of thickness. this phenomenon can also be observed in figs. 17, 20, and 23, since the joint stiffness increases with higher value of the poisson’s ratio, larger value of diameter and lower value of adhesive thickness. furthermore, the numerical results demonstrate that the variation of both mechanical and geometrical parameters induces large variation of maximum values of the stress peak. this occurs for both the maximum principal stress (figs. 21 and 24) and the von mises stress (figs. 22 and 25). higher 104 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades fig. 25. von mises stress distribution in the adhesive for different values of thickness. fig. 26. a) transversal stresses in the adhesive connections due to the poisson’s effect. b) scheme of uniaxial tensile test on bulk adhesive material: free transversal deformation. stress peaks are observed in the case of stiff adhesive rather than soft adhesive (see for example fig. 24). in addition, the location of this value is also dependent on diameter and thickness values (see figs. 21 and 24). finally, remarkable differences in the stress triaxiality distribution were also observed between soft and stiff connection (see fig. 16a). more details on the effect of each parameter are listed below. in particular, the global stiffness of the connection: – increases with the young’s modulus (see fig. 16a), because the bulk adhesive material is stiffer; – increases with the diameter (see fig. 20), because when the diameter is larger the adhesive is more constrained against transversal contraction; – decreases with the adhesive thickness (see fig. 23), because larger thickness reduce the constrain effect; – increases with the poisson’s ratio (see fig. 17), because with larger values of poisson’s ratio the material tends to be incompressible, i.e. stiffer to volume changes. while the peak of maximum principal stress: – increases with young’s modulus (see fig. 15), because the bulk adhesive material gets stiffer and the effect of transversal constrain is enlarged; j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 105 – increases with higher poisson’s ratio (see figs. 18 and 19), because the material tends to be incompressible and therefore more rigid against volume changes which leads to a higher stress peak; – changes in general with diameter and thickness, with much larger variation in case of stiff adhesives than in case of soft adhesives (see difference between fig. 21a and 21b). – increases with higher d/t ratio, in case of stiff adhesives (see figs. 21 and 24), because at large diameter-thickness ratios, the adhesive is more constrained against transversal contraction. from figs. 20 and 23 it can then be derived that the stiff adhesive exhibits extremely small deformation during the tests, i.e. in the range of 10−3 and 10−2 mm. it is therefore recommended to include the glass deformability in the numerical simulations of this type of joints. this is because the hypothesis of rigid substrate, which is often adopted in literature for adhesive joints, may not be satisfied and thus it may lead to different stress distribution. in that regard, if one considers to apply the 5kn load (as it is in fig. 20a) to the glass panel only, and conservatively consider the glass as a beam fully constrained at the supports, the displacement can be estimated by p·l3/(k·e·j), where p is the load, l is the span, e is the glass young’s modulus, j is the glass moment of inertia and k is equal to 192 for concentrated load and to 384 for distributed load. this leads to displacements between 4·10−3 mm and 2·10−3 mm, which is comparable to the deformation exhibited by stiff adhesive material (see for example fig. 20a). furthermore, it is also observed that, for soft adhesive, an optimum value of diameter/thickness ratio is approximately around 45 [mm/mm] (see fig. 21b), when the other parameters are equal to the reference one. further works will focus on obtaining a semi-empirical analytical expression able to provide the value of the stress factor for certain adhesive and connection geometry. this will permit to evaluate the stress factor value, providing the connection dimension, adhesive thickness, young’s modulus and poisson’s ratio. this way the maximum load of the connection can be analytically calculated, given the maximum adhesive stress obtained from tests. alternatively, given the applied load, the stress peak in the adhesive can be analytically calculated. 4. conclusions firstly, the material tests here presented confirm that the adhesives behave nonlinearly under large deformations. the stiffness of the two selected adhesive does not depend on the displacement rate for the three tested displacement rates. it is also observed that stress and strain at failure in tension reduce as the displacement rate is reduced. in selecting a suitable adhesive for adhesive point-fixings several aspects should be considered together. flexible adhesives as ms-polymers and silicones will fail at large strains, which benefits to compensate differential thermal expansions. in contrast, stiffer epoxies and acrylates will fail at lower strains but have a higher failure load, which results in smaller and more visually appealing diameters. secondly, the parametric analysis here presented demonstrates that adhesive connections exhibit a quite complex behaviour with large nonlinearity that cannot be neglected. the stress distribution within the adhesive deviates from the uniform nominal stresses, even in case of simple load condition. the actual maximum stress peak results to be in general higher than the nominal one. in some cases the stress peak is more than 4 times higher than the nominal one. the joint stiffness is not only depending on the young’s modulus but also on the poisson’s ratio and diameter-thickness ratio. 106 j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades this is due to the high confinement state of the thin adhesive layer. the response of the connections appears to be dependent on both mechanical and geometrical parameters. these parameters have significant effects on both the stress distribution and the global stiffness of the joint. stiff adhesives yield larger stress peaks than soft adhesives. stiff adhesives are also more sensitive to parameter variation than soft adhesives. in conclusion, this work showed that large nonlinearity is involved when adhesive point-fixings are used. this means that (i) the nonlinear stress strain curve must be obtained for each adhesive for implementation in numerical modelling and (ii) accurate numerical simulation should be performed to calculate the stress peak occurring in the adhesive. acknowledgments the authors would like to thank the flemish government agency for innovation by science and technology (grant n◦ 121043) and the swiss national science foundation (grants 200021 134507 and 200020 150152) for funding the present research. in addition, the cost action tu0905 “structural glass – novel design methods and next generation products” is also acknowledged for facilitating and providing a useful research network for this study. the arcelormittal chair of steel and façade engineering at the university of luxembourg is gratefully acknowledged for the use of equipment to fabricate the adhesive bulk specimens. the materials science and engineering department of ghent university is acknowledged for the use of the testing equipment. also soudal (be) is gratefully acknowledged for providing the ms-polymer adhesive soudaseal 270 hs. references belis, j., callewaert, d., & van hulle, a. (2011a). bouwen met glas en adhesieven – praktische gids voor ontwerper en uitvoerder (in dutch), ghent university, ghent, belgium. belis, j., van hulle, a., out, b., bos, f., callewaert, d., & poulis, h. (2011b). broad screening of adhesives for glass-metal bonds. in 12th international conference on architectural and automotive glass (glass performance days 2011) (pp. 286-289). glass performance days. bennison, s. j., qin, m. h., & davies, p. s. (2008). high-performance laminated glass for structurally efficient glazing, innovative light-weight structures and sustainable façades, 1-12. beyer, j. (2007). ein beitrag zum bemessungskonzept für punktgestützte glastafeln (doctoral dissertation). technischen universität darmstadt, germany. callewaert, d. (2012). stiffness of glass/ionomer laminates in structural application. (doctoral dissertation). ghent university, belgium. dias, v., hechler, o., & odenbreit, c. (2012). determination of adhesives properties for non-linear numerical simulations of structural steel-glass connections. proceedings of challenging glass, 3, 195-207. dias, v., odenbreit, c., hechler, o., scholzen, f., & ben zineb, t. (2014). development of material law for silicone to simulate structural adhesive connections. engineered transparency handbook, 323-333. dispersyn, j., van der biest, t., & belis, j. (2014). experimental research on the failure of adhesive point-fixings between annealed and metal under uniaxial load. proceedings of challenging glass, 4 & cost tu0905 final conference, 339-346. en iso 572-2: determination of tensile properties of plastics. cen, 1996. etag 002 guideline for european technical approval for structural sealant glazing kits (ssgk), european organisation for technical approvals, 2005. feldmann, m., kasper, r., & pilsl, m. (2008). glass for structural applications – the development of the connection design. proceedings of challenging glass, 557-569. haldimann, m., luible, a., & overend, m. (2008). structural use of glass (vol. 10). iabse. lees, d. e., & hutchinson, a. r. (1992). mechanical characteristics of some cold-cured structural adhesives. international journal of adhesion and adhesives, 12 (3), 197-205. maniatis, i. (2006). numerical and experimental investigations on the stress distribution of bolted glass connections under in-plane loads (doctoral dissertation). technische universität münchen, germany. j. dispersyn et al. / a preliminary study of the nonlinearity of adhesive point-fixings in structural glass facades 107 mocibob, d., & belis, j. (2010). coupled experimental and numerical investigation of structural glass panels with small slenderness subjected to locally introduced axial compression. engineering structures, 32(3), 753-761. overend, m. (2005). optimizing connections in structural glass. proceedings of 2nd international conference on glass in buildings. overend, m., jin, q., & watson, j. (2011). the selection and performance of adhesives for a steel-glass connection. international journal of adhesion and adhesives, 31(7), 587-597. overend, m., nhamoinesu, s., & watson, j. (2012). structural performance of bolted connections and adhesively bonded joints in glass structures. journal of structural engineering, 139(12). santarsiero, m., carvalho, p., louter, c., & cruz, p. (2013). experimental and numerical investigations of metal-to-glass embedded connections with thin stainless steel plate. proceedings of cost action tu0905, mid-term conference on structural glass. santarsiero, m., & louter, c., (2014). the mechanical behavior of sentryglas® and tssa laminated polymers in cured and uncured state in uniaxial tensile test. proceedings of challenging glass, 4 & cost tu0905 final conference. siebert, b. (2006). anforderungen für ein berechnungskonzept für die bemessung punktgelagerter verglasungen, stahlbau, 75(8), 652-657. siebert, g., & herrmann, t. (2010). glazing with countersunk point fittings. proceedings of challenging glass, 3, 335-348. simulia (2011). abaqus 6.11 analysis user’s manual volume iv: elements. sitte, s., brasseur, m. j., carbary, l. d., & wolf, a. t. (2011). preliminary evaluation of the mechanical properties and durability of transparent structural silicone adhesive (tssa) for point fixing in glazing. journal of astm international, 8(10), 1-27. vyzantiadou, m. a., & avdelas, a. v. (2004). point fixed glazing systems: technological and morphological aspects. journal of constructional steel research, 60(8), 1227-1240. weller, b., & tasche, s. (2005). adhesive fixing in glass construction. proceedings of the 9th international conference on architectural and automotive glass (gpd), 267-270. yu, x. x., crocombe, a. d., & richardson, g. (2001). material modelling for rate-dependent adhesives. international journal of adhesion and adhesives, 21(3), 197-210. from city’s station to station city 71 journal of facade design & engineering volume 6 / number 1 / 2018 cladding materials in non-residential construction: choice criteria for stakeholder in the province of quebec samuel guy-plourde1, pierre blanchet2, michel de blois3, françois robichaud4 and costel barbuta5 1 department of wood sciences, industrial research chair in ecoresponsible wood construction, laval university, pavillon gene-h.–kruger, 2425, rue de la terrasse, québec, québec, g1v 0a6, canada. (corresponding author) email: samuel.guy-plourde.1@ulaval.ca 2 department of wood sciences, industrial research chair in ecoresponsible wood construction, laval university, pavillon gene-h.– kruger, 2425, rue de la terrasse, québec, québec, g1v 0a6, canada. email: pierre.blanchet@sbf.ulaval.ca 3 laval university design school, édifice la fabrique, bureau 3132, 295, boulevard charest-est – bureau 090, québec, québec, g1k 3g8, canada. email : michel.deblois@design.ulaval.ca 4 forest economic advisors, 298 great road, 01460, littleton, massachusetts, usa. email: frobichaud@getfea.com 5 fpinnovations, 319 rue franquet, québec, québec, g1p 4r4, canada. email: costel.barbuta@fpinnovations.ca abstract designing the façade of a building is a complex task due to the number of products that are offered, the multiple criteria to be considered, and the number of stakeholders involved. in this context, from a manufacturer point of view, it is critical to understand the decision-making process. this paper highlights three different categories of choice criteria (influence criteria, requirements, and issues) relative to the selection of cladding material for non-residential buildings. architects, contractors, subcontractors, and clients provide a systemic view of the situation, based on an analysis of the findings from a mixed-research method combining online survey and semi-structured interviews. the results show that the most important influencing criteria for cladding selection are the building type, client type, project context, personal experience, product reputation, and project delivery methods. for the architects, the most important requirements are, in order, performance, appearance, and good warranties. contractors and subcontractors look primarily for compliance with the delivery schedule, with an optimal deadline of fewer than three weeks. regarding installation, subcontractors seek speed of installation, system simplicity, easy coordination with envelope workers, and on-site product modulation. finally, the main issues with cladding are maintenance, the novelty of cladding systems, lack of construction details, and tendering process. a better understanding of the cladding selection process provides valuable insight to the manufacturer in order to provide the right information to satisfy a particular stakeholder’s need. keywords choice criteria, material selection, cladding, non-residential construction, construction stakeholders doi 10.7480/jfde.2018.1.1811 72 journal of facade design & engineering volume 6 / number 1 / 2018 1 introduction the process of selecting materials with respect to their function is a fundamental task for architects in the design process of a building. designing façades is particularly challenging. the choice must be carefully considered, as the façade becomes the visiting card of the building. the technical and aesthetic elements must be properly balanced “ to create the vocabulary for understanding and wielding material” (borden, 2009). cladding materials must meet three basic criteria; 1) to protect the enclosure from weathering; 2) to resist shocks; and 3) to satisfy the aesthetic appearance (herzog, krippner, & lang, 2007). the number of stakeholders involved in the decision-making process (client, architect, general contractor, and subcontractor) complicates the material selection process since each of them has specific stakes. this makes it difficult to reach a consensus in a multi-disciplinary team (šaparauskas, kazimieras, zavadskas, & turskis, 2011). successful product development and marketing campaigns are based on the solid identification of stakeholder’s needs and an understanding of the decision-making process. considering the number of competing materials, the variety of factors that drive the design and the number of stakeholders involved in the non-residential construction industry, the aim of this exploratory study is to outline which choice criteria are involved in decision-making for the exterior cladding materials during the construction process phases. to achieve this goal, choice criteria were analysed according to: 1) the criteria that influence the selection of cladding materials; 2) the requirements; and 3) the issues relative to the their selection. the study was undertaken in the province of quebec, canada. the proposed study is original since its fills the gap of scientific knowledge, which unifies multiple choice criteria, exterior cladding material, non-residential construction, and multiple stakeholders. ultimately, the results allow the different stakeholders, mostly the manufacturers, to understand the process of cladding material selection regarding the most important choice criteria. the following sections present a literature review, then the online survey and the interviews methods are covered with their underlying motivations, methodology, and results. finally, the discussion presents the most important choice criteria of the cladding construction process and presents opportunities for innovation. 2 literature review 2.1 cladding in non-residential construction cladding is the outermost layer of the envelope of a wall that encloses a building. it is the most visible part of a building and the most exposed to weather. the main function of cladding is to provide shelter from the elements. it protects the inner layers from weather elements. as a decorative function, cladding defines the building’s aesthetic. typically, the cladding is a non-loadbearing component. the cladding is designed to support its own weight, withstand temperature variations, and support loads of wind, snow, and impacts. it is usually used in conjunction with a structural frame for load transfer (gorse, johnston, & pritchard, 2012). in north america, commonly used cladding materials to construct the façades of non-residential buildings are stone, materials with mineral binders such as precast concrete, ceramic materials, glass, metals, timber, synthetic materials, render/plaster, cladding bent in mortar, and thermal insulation composite systems (hegger, auch-schwelk, drexler, & zeumer, 2006). each group has different sub-categories with different characteristics. therefore, the specifications and constraints of each project must be well analysed and understood by the decision makers in order to meet client’s objectives. 73 journal of facade design & engineering volume 6 / number 1 / 2018 it is important to distinguish the single household market from the non-residential market. nonresidential buildings include buildings designed for commercial, industrial, and institutional purposes. in building codes, large residential buildings are mostly treated like non-residential buildings, and as such can be included as part of the investigation. this study focuses on non-residential buildings since they are considerably different from residential buildings in terms of building size, materials, specifications, design, and cost (o’connor, fell, & kozak , 2003). from the manufacturer’s point of view, non-residential construction has a important economic potential as its value typically equates the residential market and tends to be less cyclical than the housing market (kozak and cohen 1999; o’connor et al., 2004). moreover, the non-residential market is strongly increasing due to densification of urban centers (fpac, 2013; unesco, 2010). 2.2 supply network and process the supply chain of cladding includes the client, the architect, the general contractor, the subcontractors, the distributor, and the manufacturer (fig. 1). because of its high complexity, this supply chain would be better described as a supply network (ledbetter, 2003). the nature of exchanges between stakeholders involves topics such as information, costs, production, services, and value (du, 2009). the traditional mode, also known as design-bid-build, is the most common method in the province of quebec. there are possible variations of the traditional model for delivery method and manufacturer structure (fig. 1). the traditional model relies on fixed contracts. first, the client awards the contract to professionals (architects and engineers) for the design stage. then, the client awards the second contract to the general contractor for the construction stage (for reading clarity, the term “contractor” will be used). stakeholders have well-defined roles in non-residential building projects. the client is generally the one who sponsors the real estate project. there are two types of clients: public and private. architects are the main decision-makers for the prescription of non-structural materials in non-residential buildings (garmston, pan, & de wilde, 2012). the contractor’s major role is to plan, coordinate, and supervise the work of the subcontractor. then, the subcontractor purchases the cladding system from a distributor and installs it. usually, the distributor is different from the manufacturer. the distributor can be a general building materials distributor such as a hardware store or a cladding specialised distributor. fully integrated manufacturer companies are only encountered in large and expensive contracts (ledbetter, 2003). 2.3 material selection criteria a non-exhaustive literature review revealed different strategies for the characterisation of the main choice criteria for cladding materials. major databases in construction sciences (compendex, sciencedirect, web of sciences) were searched using specific keywords (decisionmaking, material selection, building, façade, cladding, siding, criteria, and issues). because of their accurate description of very important building construction selection criteria, seven documents were considered major and relative to the subject under study. the studies, summarised in table 1, were analysed according to the four dimensions: the domain, the object, the subject, and the results. this method for analysing literature content was inspired by kassem and mitchell study (2015). the domains are the type of building analysed. it can be general buildings, single-home residential buildings, or non-residential buildings. the object can be as general as the material, or be specific to the façade function, or even more related to the exterior cladding. finally, the subject refers to 74 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 44 supply networks for exterior cladding and possible modifications for project delivery method and manufacturing structure (adapted from royal architectural institute of canada (2009)) the actors involved in the study. studies generally focused on architects since they are the main specifiers of materials in a construction project. other subjects included contractors, specialist subcontractors, clients, and engineers. the analysis of three dimensions showed a lack of scientific knowledge for the exterior cladding material specifically in the non-residential construction. table 1 also highlights other studies that consider the whole value network (client, architect, contractor, and subcontractor). 3 method to find relevant and effective answers/solutions to the diversity of problems observed and cited in the research questions, this study draws on the strengths of the instrumental collection of quantitative and qualitative research. this method allows a more exhaustive and differentiated knowledge of an observed phenomenon and problems herein. this mixed research method consisted of an online survey and semi-directed interviews (amaratunga, baldry, sarshar, & newton , 2002). chronologically, the online survey preceded the semi-structured interviews. it was a process that aimed to gradually deepen the subject under study. this study focuses on stakeholders from an east north american context (quebec, canada). the information gathered is based on a demanding northern climate characterised by a mix of cold and very cold climate zones (ashrae, 2007). although the results of this study are of greatest interest for north americans, they are also valuable for stakeholders in similar contexts. 75 journal of facade design & engineering volume 6 / number 1 / 2018 3.1 online survey architects are the main specifiers in the selection of a cladding material in non-residential construction. to better understand their principal choice criteria, an online survey was conducted. the online survey was selected as a first data collection method because it provides preliminary information quickly and inexpensively. only the architects were targeted for the online survey since they are the main specifier of exterior cladding materials. considering the exploratory nature of the survey, the architects were more easily reachable since they are all grouped under a professional body. the survey revealed that choice criteria were related to influence, requirement, and appearance during the cladding selection process. these results served as a basis for the development of the semi-structured interviews. damery et fisette (2011) kassem & mitchell (2015) wastiel & wouters (2011) architects hegger et al. (2007) akadiri & olomolaiye (2012) pan el al. (2012) singhaputtangkul et al. (2014) domain building x x x x x residential x x non-residential identified gap  object material    x x  x  x    façades x x cladding x  identified gap  subject client x x         architect x x x x x x engineer x x contractor x x x subcontractor identified gap table 20 literature review resumé according to the domain, the object, and the subject investigated authors choice criteria identified damery et fisette (2011) attribute influence appearance and performance product reputation and respondent’s first-hand knowledge of the product. less concerned with environmental record and service life kassem and mitchell (2015) issues influence inadequate knowledge of stakeholder and the late involvement of specialist façade cost, familiarity and past experience, and aesthetics wastiel and wouters (2011) selection context, manufacturing, material aspects and experience hegger, drexler, & zeumer. (2007) selection context of use, perception and ecological, economic, and technical properties akadiri and olomolaiye (2012) selection aesthetics, maintainability, and energy saving pan, dainty, and gibb (2012) decision cost coupled with time and quality singhaputtangkul, low, teo, & hwang (2014) selection appearance, initial cost, and health, safety, and security table 21 literature review results according to the type of choice criteria 76 journal of facade design & engineering volume 6 / number 1 / 2018 3.1.1 method to better identify important research themes and accurately represent stakeholder opinion, a high response rate was necessary. to maximise the response rate of the survey, the survey was designed according to the method devised by dillman, smyth, and christian (2014). completing the survey was simple and required little time (approx. 10 mins.) for the respondents. the questionnaire was devised in three parts. the first part collected information such as the profession and the region in which they practice in order to characterise the sampling. the second part aimed to understand the importance given by the respondents to different influences, requirements, and appearance criteria. finally, the third part was open-ended and asked the architects to identify reasonable expectations for the long-term durability and the maintenance cycle of exterior cladding material in non-residential buildings. the questionnaire was tested with an architect and an external research manager in business intelligence to ensure the criteria and questions were clear and relevant. to measure and understand the opinion of architects, multiple-choice questions were based on a balanced likert gradient scale (i.e. 1 = not important, 2 = somewhat, 3 = average, 4 = very, and 5 = extremely important). a free online-software (lime service) was used to conduct the survey. the architects were joined using a mailing list of the ordre des architectes du québec (oaq). an internet link was included in the bi-weekly newsletter sent to 3621 architects. participation in the online survey was based on willingness. the voluntary participation induced the assumption that the respondents were concerned with the research question. results were analysed using the ibm statistical package for the social sciences (spss) statistic 24 software program (international business machines corporation, usa). the study selected the severity index analysis (si) to rank the criteria for each category. the non-parametric procedure is more suitable than parametric statistics (mean, standard deviation, etc.) to rank the criteria and produce meaningful results (chen, okudan, & riley, 2010; johnson and bhattacharyya, 1996.; singhaputtangkul, low, teo, & hwang, 2014). equation 1 was used to calculate the si value for each criterion. where i = point given to each criterion by the respondent, ranging from 1 to 5; ω i = weight of each point; f i = frequency of the point i by all respondents; n = total number of responses (n = 67 in this study); and a highest weight (a = 5 in this study). chen et al. (2010) use the calculated si value to categorise the criteria into five importance levels: high (h) (0.8 ≤ si ≤ 1), high–medium (h–m) (0.6 ≤ si < 0.8), medium (m) (0.4 ≤ si < 0.6), medium–low (m–l) (0.2 ≤ si < 0.4), and low (l) (0 ≤ si < 0.2). 3.1.2 results sixty-seven architects completed the online survey. the response rate (1.72%) was not consistent with the literature (akadiri and olomolaiye, 2012; damery and fisette, 2001; singhaputtangkul et al., 2014) but the number of respondents is still higher than in some studies (damery and fisette, 2001; kassem and mitchell, 2015; singhaputtangkul et al., 2014). moreover, the number of architects 77 journal of facade design & engineering volume 6 / number 1 / 2018 number of responses architects sampling size response rate (%) region (%) montreal quebec south other 67 100 3621 1,86 35 11 9 12 table 22 characteristics of the respondents of the online survey choices criteria severity index importance level influence personal experience 0,675 h-m product reputation 0,651 h-m municipal regulations 0,618 h-m client’s choice 0,618 h-m technical data 0,573 m colleague’s experience 0,570 m manufacturer reputation 0,552 m building code 0,540 m cases studies 0,466 m manufacturer representation 0,412 m builder’s choice 0,293 m-l publicity 0,281 m-l requirement performance (long-term durability) 0,851 h appearance 0,806 h warranties 0,696 h-m design possibilities 0,687 h-m installation cost 0,645 h-m material cost 0,642 h-m environmental footprint 0,618 h-m external recommendations appearance durability over-time 0,764 h-m fit with other materials 0,731 h-m possibility of changing color 0,675 h-m texture 0,660 h-m color weathering 0,642 h-m constructive truth 0,621 h-m related to building function 0,576 m table 23 severity index (si) values and importance obtained for the different choice criteria performance criteria mean years (sd) long-term durability 40 (20) maintenance cycle 14 (8) table 24 average values for the reasonable expectations for the two performance criteri 78 journal of facade design & engineering volume 6 / number 1 / 2018 contacted represents the entire population of architects in the province of quebec. the aim of the survey was not to create a statistical database but rather provides insight into choice criteria. table 3 shows the general characteristics of the respondents. as shown in table 4, the online survey identified the product’s reputation and architect’s personal experience as being the most important influencing criteria. online survey results also show that municipal regulation is the third most considered influencing criterion tied to the client choice. results also revealed that architect’s three most important requirements in the choice of a cladding material are performance, appearance, and warranty. conversely, the environmental footprint is the least important criterion. with regard to performance, table 5 results indicated, on average, a 40-year period of long-term durability and a 14-year maintenance cycle to be reasonable expectations. appearance is the second most important requirement. appearance is a priority and serves the interest of the overall concept. the durability over time of the appearance is the most important concept. another important aspect of the appearance is the fit with the other materials. 3.2 semi-structured interviews semi-structured interviews were the second data source used to investigate the different stakeholders’ criteria for choice of an exterior cladding material during the construction process. interviewed stakeholders included clients, architects, contractors, and subcontractors. this method was used to put the results of the online survey into perspective. it enriches the understanding of the data, completes it and contributes to their interpretation (blanchet and gotman, 2007). the semi-structured interviews were used to structure the conversation on specific subjects as well as to obtain in-depth information by offering the respondents the freedom to express their opinions in their own words. 3.2.1 method the interview questions were identical for all the stakeholders: clients, architects, contractors, and subcontractors. as shown in table 2, it was important to have at least two participants for each group of stakeholders. the sampling includes stakeholders working in the province of quebec who had relevant experience in the specification, design, and construction process of exterior cladding. in order to properly represent industry practices, it was important to interview clients from the public and the private sectors. thirteen interviews were carried out with thirteen professionals, all from different companies. it was also important for participants to have been involved in the construction of different types of buildings. participants were contacted by email from an internet industry listing data bank (icriq) or because of their participation in a project whose construction site had been visited. participation was based on willingness. the interviews took place at the premises of the participants and lasted from 45 to 75 minutes. the interviews covered three topics. first, the questions were related to the role associated with different stakeholders in the decision-making process of choosing materials. then, the interview discussed the influence, requirement and issues at different stages of a project: client’s need, concept, technical design, call for tenders, distribution, installation, and maintenance. finally, the innovation potential of the cladding industry was discussed. interviews followed the approach advocated by oppenheim (2000) that consists of diminishing the interviewer’s contribution by minimising interactions while ensuring that the interviewee responds to the chosen theme. 79 journal of facade design & engineering volume 6 / number 1 / 2018 the interviews were recorded and transcribed. the content analysis of the interviews validated, qualified, and deepened the results of online surveys. the methodology used for content analysis followed the coding approach proposed by groat and wang (2002). the analysis of the semistructured interviews followed an iterative approach, which consisted of identifying the main themes, regrouping the data that had a similar meaning, synthesising the information, and finally interpreting the results according to themes and stakeholders. the software package n’vivo (qsr international pty. ltd. doncaster, australia) was used to facilitate data coding. when subsequent participant interviews revealed no new information, data saturation was reached, which marked the end of the analysis (mucchielli, 1996; pires, 1997). stakeholders position experience (years) architect associated architect 20 architect associated architect 20 architect associated architect 17 architect associated architect 20 general contractor project manager 8 general contractor president 30 general contractor project manager 20 subcontractor project manager 8 subcontractor project manager 10 subcontractor owner 10 client strategic planning advisor 12 client project manager 5 client construction supervisor 8 client architect 7 table 25 interview sample group: stakeholder, position in the enterprise and experience 3.2.2 results the results of the interviews made it possible to deepen several choice criteria and discover new criteria. table 7 summarises the results of the interviews with a brief description of each criterion. for the influence criteria, the interviews mainly allowed to the improvement of the context of the project criteria, and the discovery of the project delivery method criteria. one limit of the online survey is that requirement criteria are only ranked relative to the early design stage of the cladding construction process. the interviews allowed a better understanding by producing new requirement criteria and including other project development stages (call for tender, delivery, installation). finally, the issues criteria emerged exclusively from the interviews. issues criteria occur at different stages of the construction process and they will be covered in depth in the next section. 4 integrated results and discussion based on the findings from the online survey and refined by the semi-directed interview, this section presents a final categorisation of the stakeholder’s choice criteria for the cladding 80 journal of facade design & engineering volume 6 / number 1 / 2018 attainment process in a non-residential context. choice criteria were regrouped in three groups: influence criteria, requirement, and issues. this section ends by discussing innovation potential in the cladding industry. 4.1 influence criteria influence criteria are parameters that vary according to each project. they have a direct impact on the concept development stage and the material choice. the following section clarifies the most important influence criteria for the concept development: type of client, project context, and personal experience. choice criteria detail influence client type public, private or real estate developer, budget building type design parameters, use, and building code project delivery method different stakeholders have influence on the material choice personal experience highly correlated to product reputation product reputation related to reliability/constancy and history of the product requirement performance no absolute value. maintenance-free period of 25-year is acceptable appearance serves the overall concept. architects look for unique materiality warranty could “kill” a choice of cladding. crucial to understanding the warranty clauses information information data must be verifiable delivery schedule contractors and subcontractors expect honesty and rigor delivery delay less than 3 weeks represents a reasonable period supplier competition contractors and subcontractors seek for diversity for a type of material speed of installation products with low installation tolerance are cited as an example simple fastening systems avoid unique systems from a single company minimal coordination effort few workers and materials resources are preferred materials modulated on-site minimize the loss rate and offer modularity in case of breakage issues novelty of a product stakeholders do not want to play the role of “product tester” construction details caused by a lack of experience, time and budget cost war engendered by the rule of the lowest bidder manufacturer integration complicated because of the rule of the lowest bidder oversea/custom product longer delivery delay maintenance culture knowledge transfer. transmitted documents are not consulted manufacturer representative perceived like sellers instead of technical advisor table 26 refined and new criteria from the interviews with the stakeholders 81 journal of facade design & engineering volume 6 / number 1 / 2018 4.1.1 type of client the type of client influences the cladding selection process mainly by controlling the budget, dictating the needs, choosing the project delivery system, and, at the end of the project, by managing the maintenance cycle. the architect’s job to meet the client’s objectives through design strategies; according to architect #3, “a project is successful when the objectives are met”. since it is the client who dictates the requirements, his choices and expectations have a big influence in the selection of a cladding material. different types of clients have different priorities and often the cladding is targeted by cost reduction strategies in the early design stages. public clients may represent governments or municipalities, which have specific goals and performance expectations for institutional, industrial, or multi-residential buildings. public clients give precise guidelines on the life cycle through a programme. for cladding materials, there is generally no material selection but overall intentions. for example, the programme could require cladding with a 25-year maintenance-free warranty. public clients are mostly reluctant to experiment with new products and if they do, they tend to favour installation on small surfaces. they are looking for proven cladding systems. in some ways, private clients might be seen as more economically driven. for commercial and industrial buildings, the building is used to sell or produce goods or services. cost reduction is a high priority goal. the money invested in the cladding is not used for production or sale. the private client is greatly influenced by his own value, preconceived ideas, and by the prescriber’s expertise and preferences. for some businesses, branding is important and they make choices based on aesthetics, environmental aspects, and performance. private multi-residential builders are usually accustomed to the construction process and they work with overall intentions on a project depending on the price range of the dwellings. time and cost reduction are strongly considered. the tendering process depends on the type of client and the project delivery system. the project delivery method also has an influence on the project. in a traditional design-bid-build method, the general contractor does not give input. in a construction management or design-build project (fig. 1), the general contractor advises and enhances the design with recommendations based on assemblies, own experience, simplicity, construction details, and optimisation of the work sequence. the contractor has also a strong concern about cost reduction. the subcontractors have little to say in the choice of material. usually, they live with the choices of the architect and make sure to order the materials and install mouldings, furring, anchoring, and cladding. generally, the client pays little attention to maintenance and problems are identified too late. thus, the client type has an impact on the maintenance of the façades. generally, private clients think short term. on the other hand, public clients and seasoned clients who have expertise in the organisation are more concerned about the durability of the cladding. 4.1.2 project context the project context is subdivided in two criteria that influence the concept development stage: the physical context and the type of building. it is inspired from wastiels and wouters (2012). the physical context describes the site of the project. it refers to the environment of the building. cladding choice will be different for a project located in a city centre, near a beach, in the countryside, or near a forest. for a given area, municipal regulations (e.g. site planning and architectural 82 journal of facade design & engineering volume 6 / number 1 / 2018 integration programs), may require/prohibit certain types of cladding. as indicated by architects in the interviews, the type of building also greatly influences material choice. the needs are different according to building use and its volume. for instance, although a school and a law court can share the same volume/size, they do not share the same function and thus require different treatment. moreover, the national building code (nrc, 2010) may require the presence of non-combustible cladding in accordance with the following criteria: use, floor number, surface area, sprinklers, neighbouring buildings, and distance from public thoroughfares. 4.1.3 personal experience personal experience also strongly influences material selection in the concept development stage. it was noted that architects often work with the same products since they have to take responsibility for the final product’s performance. if the architects had a bad experience with a product, they will be reluctant to use it again. architects look for tested and standardised products in order to limit and transfer their responsibility. more than two-thirds of the interviewed stakeholders associate personal experience with the product’s reputation. in the interviews, contractors agreed with architects regarding the importance of the product’s reputation. a good product reputation is related to reliability/constancy and history of the product and the business supplier. reliability/consistency is important for three parameters: durability, , and physical properties. according to all the contractors interviewed, a good product will react well in a context where temperature variations can reach 60°c. 4.2 requirement criteria the requirements criteria define or represent what stakeholders ask from cladding products. based on the online survey and the interviews, six basic requirements were identified for exterior cladding in non-residential constructions. requirement criteria include sub criteria such as performance, appearance, warranties, information, delivery, and installation. 4.2.1 performance performance is the most important necessity for the architects. it is defined in terms of durability and maintenance. as most architects mentioned, cladding and wall performance is very important since deficiencies quickly lead to very high repair costs, which often result in significant monetary losses for the owner. the online survey results indicated an average 40-year lifespan for durability and a 14-year maintenance cycle to be reasonable expectations. on the other hand, when performance was discussed in the interviews with architects and general contractors, it appeared that it was impossible to quantify durability. there is no absolute value or minimum performance (in years) established by stakeholders. performance is a concept that must be adapted according to criteria such as the type of building, the client, and the budget. interviews identified a minimum maintenance-free period of 25 years to be acceptable. it corresponds to the amortization time or half time service life of the building in many cases. unanimously, the interviewed participants talked about system performance and not only cladding performance. this concept is based on the quality of construction details and the installation. thereupon, stakeholders have suggested manufacturers to increase not only cladding efficiency, but also the efficiency of systems. 83 journal of facade design & engineering volume 6 / number 1 / 2018 4.2.2 appearance appearance is the second most important requirement. appearance is a priority and serves the interest of the overall building concept. the durability of the appearance over time is the most important concept under the appearance requirement. there is a strong preference for materials that retain their aesthetic appearance. another important aspect of the appearance is the fit with the other materials. it can be explained by the fact that architects are looking for a unique materiality. architects look for products with great modularity and several architectural possibilities. the aesthetic flexibility of a material makes it possible to combine it well with the other materials. when architects think about materials and search on a company’s website, they quickly want to understand all the design possibilities of a product. furthermore, interviews revealed that architects do not consider the opinion of contractors and subcontractors on the aesthetic appearance of selected materials. 4.2.3 warranties warranties are the third most important need expressed by the architects. architect #2 mentioned that the warranty could kill a choice of cladding. the warranty concept is intrinsic to the durability of the material. architects also mentioned that they do not necessarily work with products offering the best warranties. a 50-year warranty nevertheless reassures architects. conversely, a 10-year warranty is, in most cases, unacceptable since one has to impose short maintenance cycles on the client. it is crucial for the architects to develop a good understanding of the warranty clauses (finish, material, labour, etc.). appropriate design details are therefore essential for the warranty to apply. finally, architects have to understand the limitations of the product itself as well as its installation requirements. in the interviews, architect #1 revealed that he would appreciate if warranties were adapted according to specific applications (e.g. façade orientation, colour, soffit, etc.). 4.2.4 information accessing product information is generally performed through two main avenues: manufacturers’ or distributors’ websites and manufacturing representatives. architects and contractors tend to increasingly rely on websites, as extensive in-house material libraries tend to disappear. regarding online research, architects stressed the necessity of being able to understand quickly all the visual and aesthetic possibilities of a product (project examples, layouts, shapes, colours, etc.). other important information relates to performance (guarantees, maintenance, durability, and physical characteristics), costs, delivery time, installation and technical data. technical data must be verifiable and comparable. thus, architects are asking manufacturers not to limit themselves to regulatory requirements, but to provide complete data for their products. for public clients, it is essential to present or offer product equivalences. private clients also appreciate this practice. for contractors and subcontractors, availability is a crucial factor. for projects aiming at environmental certifications, it is important to obtain thorough information on the provenance of the material. manufacturing representatives are specialists. they can aptly deepen architects’ knowledge on cladding assemblies. however, when architects seek information from manufacturers’ representatives, they often feel like they are dealing with sellers instead of technical advisors. they look for manufacturing representatives who are organised and knowledgeable about the proposed 84 journal of facade design & engineering volume 6 / number 1 / 2018 system. they seek to establish a trust-based relationship with a transparent representative. they expect representatives to educate them by identifying beneficial typologies as well as providing information on interaction with the envelope (i.e.: air barrier, vapour barrier, thermal bridges, moisture movement). during the interviews, common complaints concerned insistent representatives, too much information documents, and too many samples. 4.2.5 delivery delivery schedules must be respected as they have a direct impact on the final delivery of the building. contractors and subcontractors expect honesty and rigour in the delivery schedules. they require actual costs and delivery times to be able to make an effective plan of the budget and work sequence. contractors and subcontractors noted many delivery problems in the cladding industry. the delivery delay is important. respondents identified fewer than three weeks as a reasonable delay. the non-residential construction segment differs from the residential segment in the distribution of materials. non-residential construction uses more custom and overseas products. interviews indicated that customised products require longer delivery delay because of the length of the process: measurements at the site, sending data to the manufacturer, validations of drawings, approval, manufacture, painting, delivery. delivery times of over 14 weeks should be avoided. for overseas products, having to pay prior to delivery is a major irritant for subcontractors. 4.2.6 installation the installation stage mainly involves contractors and subcontractors. contractor #1 summarised the situation well by mentioning that at this stage, preoccupations are mainly on cost reduction and constructability. the results of the interviews have highlighted four requirements to define an easy installation: the speed of installation, the simplicity of the system, the coordination effort, and the possibility to modulate product on-site. first, the speed of installation is an important element. architects associate this notion with the price/surface covered ($/pi2). products with low installation tolerance are cited as an example of product with a low speed of installation. second, stakeholders are also looking for fastening systems that are simple, proven and familiar to installers. they try to avoid unique concepts or systems from a single company. they do not want installers to have to learn on the job in order to limit the errors, the time, and consequently, the construction cost. third, there is a need to minimise coordination efforts. there is a problem of communication between the different subcontractors involved in the construction of the envelope. products requiring few workers and few materials resources are preferred. subcontractor #2 mentioned a major problem with the envelope/cladding interaction. the cladding companies are not the same entities that deal with membranes installation. subcontractors do not handle envelope problems. the uniformity of the envelope depends on different trades and companies. for example, a subcontractor says that his workers always need to perforate the membrane for installing cladding, but that the membrane never gets repaired since they are never asked to do so. finally, according to the contractors and subcontractors, the best products can be modulated on-site (i.e.: cutting, joints lost, molding). these products minimise the loss rate and offer modularity in case of breakage. subcontractors appreciate being able to work and 85 journal of facade design & engineering volume 6 / number 1 / 2018 modify the products while trade workers are aboard the lifting equipment. errors are expensive on construction sites and product modularity reduces this impact. paradoxically, interviews with the subcontractors demonstrated an increased use of panel systems that are impossible to modulate onsite. these systems seem to be highly appreciated by architects, but not so much by subcontractors. 4.3 issues issues are problems that have been expressed by different stakeholders. issues have an impact on several stages of the construction process. regarding the cladding, the main issues discussed by stakeholders concerned product novelty, lack of construction details, lowest bidder rule, and maintenance. 4.3.1 product novelty new products and systems are perceived as problematic during the concept development phase as well as during the installation. stakeholders do not want to play the role of product testers since potential problems may have significant monetary consequences. when new products are specified, they search for projects that used those products before. architects seek more information and proceed to deep risk management analysis. there are informal communications with contractors and clients who previously used a product to get insight on installation, weathering, and potential future problems. contractors and subcontractors mentioned that it is common to verify the cladding installation on other buildings. when using a new product, architects want to transfer the responsibility to the manufacturer. they prefer to go with proven systems. in the end, a bold cladding choice remains the client’s and the architect’s responsibility. 4.3.2 lack of construction detail architects demand control over the process because it is their professional responsibility. they are primarily responsible for plans and specifications. moreover, architectural details are fundamental for adequate estimations. subcontractors will generally rely on the plans and specifications. as identified during the interviews, when subcontractors ask to modify the cladding choice, they do so primarily for three reasons: 1) anomalies in the specifications (e.g. molding, jointing, length, pledge, etc.); 2) identical product at lower costs; and 3) the product is not seen as a good product. contractors and subcontractors agreed on the general lack of details in plans and specifications. however, the lack of detail offers more freedom to the contractor. the downside is that contractors do not always have the knowledge to design good material junction details. subcontractors explain the lack of details in an architect’s plans by a lack of experience, a lack of knowledge of constructability, the copy of an old plan, and a lack of time or budget. it was noted that it is not the manufacturer’s responsibility to provide and ensure the material junction details. 86 journal of facade design & engineering volume 6 / number 1 / 2018 4.3.3 lowest bidder rules all stakeholders agreed that the way contracts are awarded is outdated. the rule of the lowest tenderer engenders a real price war. the focus is not on quality, but on price. different strategies take place to reduce the cost: cladding thickness reducing, fewer construction details, faster surface coverage, prefabrication, products substitution, use of established products. contractors and subcontractors want to have the choice of material supplier. they do not want to be tied to one cladding manufacturer. mainly for price reasons, they seek diversity of manufacturers for a type of material. another consequence of the lowest bidder rule is the silo effect. as architect #4 noticed, involving the manufacturer in the early stages of concept or technical design is sometimes more complicated because of the rule of the lowest bidder. this contravenes the tendering process. 4.3.4 maintenance lack of knowledge transfer has been pointed out during the interviews. there is a knowledge gap on how to use management tools and maintaining adequate documentation and « as built » plans. transmitted documents are often consulted or lost. thus, stakeholders mentioned that they prefer to use materials that do not require maintenance. architects, contractors, and subcontractors do not want to return to a site to perform maintenance because it costs time and money to the company. maintenance culture has to be developed. maintenance plans are required to increase building envelope life. leadership in energy and environmental design (leed®) projects are good examples that represent the implementation of maintenance and short, medium, and longterm investment plans. 4.4 innovation when asked if the cladding industry was innovative, most stakeholders argued that they see a lot of small products and/or process innovations. improvements mostly focus on performance, personalisation, productivity increase, and costs reduction. in terms of technologies, stakeholders mentioned some promising product examples: self-cleaning products, energy collectors, reactive glazing, integrated lighting, and dynamic façades. some stakeholders also addressed the issue of innovation in terms of how to design the building envelope. according to one contractor, in recent years, there has been a lot of development focused on mechanical systems, coordination, commissioning, and building structural elements. the importance of the envelope is often underestimated while having a critical impact on a building’s long-term durability. for stakeholders, innovation requires a more comprehensive understanding of the envelope, that is, thermal bridges, vapour barriers, air barriers, and moisture. manufacturers of cladding material have a role to play: they have to better understand the envelope as a whole. this is a general weakness of the industry. another contractor mentioned that plan details are too complex. professionals misunderstand the stakes of constructability on-site and make construction details too difficult to realise. better envelope understanding and design are essential to build more efficiently. manufacturers need to know how products are used in order to identify the best typologies. 87 journal of facade design & engineering volume 6 / number 1 / 2018 5 limitations of study given the exploratory nature of this research project, many limitations deserve to be emphasised. first, the regional nature of the study limits the scope of the results. it is difficult to extend the results to similar weather region since all stakeholders were from the same region: province of quebec, canada. secondly, the low response rate of participants is a limitation factor to consider for the online survey and the interviews. participation throughout the study was based on willingness, hence possibly introducing self-selection biases. in addition, sampling was not made at random. while these potential biases would hamper statistical representation of the population, they are acceptable within an exploratory study. however, it must be noted that results might have been different under different survey or investigation conditions. 6 conclusion in order to enhance the product development and marketing campaigns of cladding manufacturers, this paper highlighted stakeholder’s choice criteria in the selection of a cladding material for non-residential use in the context of quebec, canada. the results could also be used in contexts where construction procurement strategies are similar. surveyed stakeholders included clients, architects, contractors, and subcontractors. this article pointed out that the processes of selecting cladding material in non-residential construction is complex and mainly driven by influence criteria, requirements, and issues relative to the choice of a cladding material. the first objective was to identify which criteria influence the choice of a cladding material. results indicate that client type, building type, project context, and personal experience are factors that have the most impact influence on the material choice. the second goal was to determine the requirements regarding exterior cladding. performance was the most important criterion for the selection of cladding. performance is a broad concept and there is no definition for the minimum performance. however, stakeholders prefer a minimum maintenance-free period of 25-year or half the expected lifetime of the building. appearance was the second most important need for architects. architects are looking for modular products that keep their aesthetic condition over time. they want to be able to understand all the possible applications of a product as quickly as possible. warranties were the third most important requirement criterion for the architects. a warranty of less than 10 years is not desired as stakeholders are limited in the maintenance effort that they can transfer to the clients. the most important factor is to understand the warranty’s clauses and ensure good design details to make sure warranties apply. as for delivery, contractors and subcontractors are looking for delivery delays of fewer than three weeks. meeting deadlines is crucial because it has serious repercussions for a building’s delivery schedule. requirements for installation are the speed of execution, the simplicity of assembly, coordination, and possibility of modifying the product on-site. the results highlighted a maintenance problem in most buildings. finally, this paper examined the potential of innovation within the cladding industry. the most interesting innovation in the cladding industry would be a better understanding of the cladding’s interaction with the envelope and its position in the building. such progress by the manufacturers would ensure a building’s long-term durability. the findings of this exploratory project will enable the manufacturer to understand how stakeholders select materials in a non-residential market. it provides information on the desired attributes of a good exterior cladding product. ultimately, this study will serve as a point of dialogue between the stakeholders to ensure the construction of more efficient building envelopes. 88 journal of facade design & engineering volume 6 / number 1 / 2018 acknowledgment the authors are grateful to natural sciences and engineering research council of canada for the financial support through its irc and crd programs (ircpj 461745-12 and rdcpj 445200-12) (msc 1) as well as the industrial partners of the nserc industrial chair on eco-responsible wood construction (circerb). references akadiri, p. o., and olomolaiye, p. o. (2012). development of sustainable assessment criteria for building materials selection. engineering, construction and architectural management, 19(6), 666–687. amaratunga, d., baldry, d., sarshar, m., and newton, r. (2002). quantitative and qualitative research in the built environment: application of “mixed” research approach. work study, 51(1), 17–31. https://doi.org/10.1108/00438020210415488 ashrae. (2007). standard 90.1 normative appendix b building envelope climate criteria. the american society of heating, refrigerating and air-conditioning engineers. blanchet, a., and gotman, a. (2007). l’entretien (2e édition). paris: armand colin. borden, g. p. (2009). material precedent: the typology of modern tectonics. 97th association of collegiate schools of architecture, 92–101. chen, y., okudan, g. e., and riley, d. r. (2010). sustainable performance criteria for construction method selection in concrete buildings. automation in construction, 19(2), 235–244. damery, d. t., and fisette, p. (2001). decision making in the purchase of siding: a survey of architechts, contractors and homeowners in the u.s. northeast. forest products journal, 51(7), 29–36. dillman, d. a., smyth, j. d., and christian, l. m. (2014). internet, phone, mail, and mixed-mode surveys: the tailored design method (4th ed.). wiley publishing. drouin, m., blanchet, p., and beauregard, r. (2013). characterization of the design function in the appearance wood products for nonresidential buildings: a conceptual framework. the international journal of designed objects, 6(3), 1–16. du, q. (2009). integrated decision making int the cladding supply chain. university of bath. forest products association of canada (fpac). (2013). from surviving to thriving canada’s forest products industry in the 21st century. vancouver, canada. garmston, h., pan, w., and de wilde, p. (2012). decision-making in façade selection for multi-storey buildings. 28th annual conference of the association of researchers in construction management, arcom 2012, (september), 357–367. gorse, c., johnston, d., and pritchard, m. (2012). a dictionary of construction, surveying and civil engineering. oxford: oxford university press. groat, l., and wang, d. (2002). architectural research methods. new-york: john wiley and sons, inc. hegger, m., auch-schwelk, v., drexler, h., and zeumer, m. (2006). construction materials manual. basel: birkhäuser. hegger, m., drexler, h., and zeumer, m. (2007). basics materials. birkhäuser. herzog, t., krippner, r., and lang, w. (2007). construire des façades. lausanne: ppur. johnson, r. a., and bhattacharyya, g. k. (1996). statistics: principles and methods. new-york: wiley. kassem, m., and mitchell, d. (2015). bridging the gap between selection decisions of façade systems at the early design phase: issues, challenges and solutions. journal of façade design and engineering, 3(2), 165–183. kozak, r. a., and cohen, d. h. (1999). architects and structural engineers: an examination of wood design and use in nonresidential construction. forest products journal, 49(4), 37–46. ledbetter, s. (2003). communication in the cladding supply chain. façade design and procurement. april 2003, bath. mucchielli, a. (1996). dictionnaires des méthodes qualitatives en sciences humaines et sociales. paris: armand colin. o’connor, j., fell, d., and kozak, r. (2004). potential for increased wood-use in north american nonresidential markets, part ii builder/owner survey (project no. 3917). forintek canada corp., vancouver, canada. oppenheim, a. n. (2000). questionnaire design, interviewing and attitude measurement. bloomsbury academic. pan, w., dainty, a. r. j., and gibb, a. g. f. (2012). establishing and weighting decision criteria for building system selection in housing construction. journal of construction engineering and management, asce, 138(11), 1239–1250. pires, a. (1997). échantillonnage et recherche qualitative : essai théorique et méthodologique. in la recherche qualitative : enjeux épistémologiques et méthodologiques (pp. 113–169). montréal: gaëtan morin. royal architectural institute of canada. (2009). modes de réalisation des projets de construction. in institut royal d’architecture du canada (ed.), manuel canadien de pratique de l’architecture. šaparauskas, j., kazimieras zavadskas, e., and turskis, z. (2011). selection of façade’s alternatives of commercial and public buildings based on multiple criteria. international journal of strategic property management, 15(2), 189–203. singhaputtangkul, n., low, s. p., teo, a. l., and hwang, b.-g. (2014). criteria for architects and engineers to achieve sustainability and buildability in building envelope designs. journal of management in engineering, 30(2), 236–245. wastiels, l., and wouters, i. (2011). architects’ choices while selecting materials. materials and design, 34, 584–593. from city’s station to station city 132 journal of facade design & engineering volume 6 / number 3 / 2018 modelling envelope components integrating phase change materials (pcms) with wholebuilding energy simulation tools: a state of the art albert castell1*, marc medrano2, francesco goia3 * corresponding author 1 department of computer science and industrial engineering, university of lleida, lleida, spain, acastell@diei.udl.cat 2 department of computer science and industrial engineering, university of lleida, lleida, spain 3 department of architecture and technology, faculty of architecture and design, norwegian university of science and technology, trondheim, norway abstract building envelope systems that integrate phase change materials (pcms) are solutions aimed at increasing the thermal energy storage potential of the building envelope while keeping its mass reasonably low. building envelope components with pcms can be either opaque or transparent and can be based on different types of pcms and integration methods. in opposition to conventional building components, these elements present thermal and optical properties that are highly non-linear and depend to a great extent on the boundary conditions. such a characteristic requires the system development and optimisation process during the design phase to be carried out with particular care in order to achieve the desired performance. in this paper, a review of the existing modelling capabilities of different building energy simulation (bes) tools for pcm-based envelope components is reported, and the main challenges associated with the modelling and simulation of these systems through the most popular bes tools (among them, energyplus, ida-ice, trnsys, ies-ve, and esp-r) are highlighted. the aim of this paper is to summarise the evidence found in the literature of the latest development in the successful use of bes to replicate the thermal and optical behaviour of opaque and transparent components integrating pcms, in order to provide the community of professionals with an overview of the tools available and their limitations. keywords phase change materials, building envelope, modelling, simulation doi 10.7480/jfde.2018.3.2572 133 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction a phase change material (pcm) is a material that presents a change of its phase of aggregation within a desired temperature range and it is used to store and release thermal energy. latent heat presents higher energy densities compared to sensible heat, thus reducing the amount of required material and volume to store the same amount of energy. the pcm absorbs energy by changing its phase from solid to liquid, and releases that energy by changing its phase from liquid to solid. the use of pcm in building facades aims at reducing the indoor temperature fluctuations, delaying the air temperature peaks, and blocking the incoming radiation when used in transparent components. as a result, thermal comfort is increased, and/or energy consumption is reduced. 1.1 pcm in opaque components different materials and systems can be used to increase the thermal inertia in opaque building envelopes. when related to pcm, the main parameters to consider are as follows: material and thermphysical properties (cabeza, castell, barreneche, de gracia, & fernández, 2011); charging/ discharging method (navarro et al. 2016a and b); and integration system (navarro et al. 2016b). 1.2 pcm in transparent components an important feature of several pcms (among them, paraffin wax, salt hydrates) is that they are (partially) transparent to solar radiation. this property makes them suitable for integration not only in opaque components, but also in transparent components (silva, vicente, & rodrigues, 2016; vigna, bianco, goia, & serra, 2018). when coupled with transparent or semi-transparent components, the pcm becomes an integrated layer with the function of both thermal energy storage and solar shading (goia, perino, & serra, 2014). 1.3 peculiar thermophysical and optical phenomena of pcms subcooling subcooling (also called supercooling) happens when the pcm solidifies at a lower temperature than the solidification temperature (bony & citherlet, 2007). this phenomenon modifies the temperature range where the pcm will store/release the latent energy, and can significantly affect the behaviour and functionality of the pcm. in most simulations of pcm, the effect of subcooling is neglected. this is an acceptable assumption for low rates of subcooling, but it is problematic when subcooling reaches the order of magnitude of the driving temperature gradient between the heat transfer fluid and the storage (günther, mehling, & hiebler, 2007). hysteresis hysteresis happens when the solidification temperature is different from the melting temperature. subcooling is then a common cause of hysteresis (bony & citherlet, 2007; mehling & cabeza, 2008). hysteresis is also commonly neglected in modelling pcm-based components in software tools for 134 journal of facade design & engineering volume 6 / number 3 / 2018 building energy simulation (bes). both subcooling and hysteresis are considered to be two main sources of inaccuracy in modelling pcm-based components (kośny, 2015). convective heat exchange heat transfer by convection with pcm is different from ordinary convection. pcm can transport significant amounts of latent heat in the melting temperature range with comparatively little fluid movement and the density changes that drive convection are much stronger. in most simulations, the effect of natural convection is neglected, and is often included (both experimentally and numerically) (fantucci, goia, perino, & serra. 2018) in the conductive heat transport – i.e. an equivalent conductivity, which also accounts for the contribution of convection in liquid phase, can be used. pcm optical properties when a pcm is integrated in a transparent system, its optical properties become driving elements in the thermophysical behaviour of the system. the optical properties of pcm layers are highly dependent on the state of aggregation of the pcm: in solid/musky state, the pcm layer behaves like a highly diffusive material characterised by high scattering phenomena; in a fully liquid state, the behaviour shifts to that of a conventional transparent component, with dominating directto-direct transmission mode (goia, zinzi, carnielo, & serra, 2015). this dynamic feature leads to more complex information to be experimentally collected in order to describe the performance of these glazing systems. 1.4 aim of the paper the aim of this paper is to compare the available bes models capable of simulating pcm in building envelopes, as well as to summarise the evidence found in the literature of the latest development in the successful use of bes to replicate the thermal and optical behaviour of opaque and transparent components integrating pcms. the paper targets, in particular, the design professionals’ community, as well as graduate students and researchers who are currently approaching the modelling and simulation of pcm-based solutions with a limited background in the field. it is not an intention of this paper to deepen the reasons for adopting pcms in building, nor to report evidence of the effect of such implementations. readers interested in these topics can easily find innumerable literature review papers addressing these questions. on the contrary, this paper focuses on the overview of the bes tools available for modelling and simulating pcm-based envelopes, along with their potentials and limitations. 135 journal of facade design & engineering volume 6 / number 3 / 2018 2 simulation requirements phase change materials integrated in building components affect its thermal performance. thus, accurate modelling of pcm must be linked and performed in conjunction with the thermal simulation of buildings. the dynamics of melting and solidification involve a moving boundary that separates the two different phases with drastically different transport properties. moreover, the pcm behaviour is highly non-linear when changing phase, since its enthalpy (energy storage capacity) changes dramatically with temperature. therefore, numerical methods are required, and simplified techniques such as conduction transfer functions (ctf) are unsuitable (cabeza, 2015). some numerical models attempt to approximate the solution to simplified stefan problems. these so-called ‘strong formulations’ determine the moving solid-liquid boundary and the temperature profiles, and can be based on either fixed or variable grid methods (hu & argyropoulos, 1996). however, these models require too much computational effort for practical applications. therefore, the so-called “weak formulations” are commonly used to simulate the behaviour of a pcm system and to represent the absorption and release of energy. some of these formulations are the effective heat capacity method, heat integration method, source-based method, and the enthalpy method. nowadays, the effective heat capacity method and the enthalpy method are the most extended ones (voller, 1997). these methods (both weak and strong formulation) can be, and have been, applied to both pcm in opaque and in transparent/translucent building envelope systems. though it is reasonable to expect that the performance of these methods is independent from the application in an opaque or in a transparent/translucent system, it must be observed that dedicated investigations that compare them in the setting of a transparent/translucent building system have not yet been carried out. all the above-mentioned approaches have also been adopted for modelling pcm layers in the transparent building envelope. pcm simulation requires short time steps and fine discretisation of the physical domain in order to avoid numerical errors and/or phase-change jumping. moreover, special attention must be paid in the determination of the temperature-enthalpy curve. other physical phenomena can also be included in the models, such as convection heat transfer inside the pcm, subcooling of the pcm, and enthalpy hysteresis. when integrated in transparent or semi-transparent components exposed to solar radiation, nonlinearity is also seen in the optical properties of the pcm layer, which becomes an important variable in the simulation as they determine the interaction with the solar radiation – and ultimately most of solar energy intercepted by the layer. in general, the optical behaviour of these systems can be modelled with different degrees of accuracy, ranging from the solution of the full radiative heat transfer equation (ishimaru, 1978) with the 3-flux approximation (weinläder, beck, & fricke, 2005) by use of a scaling concept (mckellar & box, 1981), to modelling strategies that reduce the computational effort in the simulation by always treating the pcm layer as a non-diffuse medium (goia, perino, & haase, 2012; gowreesunker stankovic, tassou, & kyriacou, 2013; li et al. 2016, liu et at. 2016) – but incorporating the complexity of the optical behaviour in the solar coefficient used in the models. in any case, these optical properties must be temperature dependent. 136 journal of facade design & engineering volume 6 / number 3 / 2018 modelling based on raytracing techniques through the bulk material (and in the adjacent room) are mandatory when detailed daylighting analyses (both in terms of natural light distribution and of visual comfort) are to be carried out. in these cases (giovannini, goia, lo verso, & serra, 2017), the full set of optical properties for the solar range (i.e. the absorption coefficient and the scattering coefficient, which together give the extinction coefficient and the phase function, giving the probability that radiation with a certain propagation direction is scattered into a certain solid angle around the direction) is necessary. alternatively, the use of experimentally characterised (andersen, roecker, & scartezzini, 2005) bidirectional (optical) distribution functions (in the visible range) can represent a suitable alternative that reduces the simulation complexity by avoiding the modelling of the light ray paths within the bulk of the material. further assumptions on the optical properties of the pcm layer, supported by spectrophotometric analysis (goia et al., 2015), may lead to the consideration of a pcm layer with a thickness greater than a few millimetres as a perfectly diffusive material, when in solid state, and as a fully homogeneous and non-scattering material, when in liquid state. in such an approach, the modelling of the pcm layer can be carried out by considering it as a lambertian surface (in solid and musky state) and a conventional non-scattering material when in liquid state (giovannini et al., 2017). finally, control strategies can be crucial for the correct operation of pcm systems. for passive systems, no control strategy is directly applied, since the phase change process is controlled by the boundary conditions – and, therefore, possible control strategies must rely on the control of, for example, ventilation rate, or indoor air temperature. on the other hand, for active and hybrid systems, different control strategies can be applied, and their influence in the pcm behaviour is very important (de gracia et al., 2013; de gracia, navarro, castell, & cabeza, 2015a; de gracia, fernández, castell, mateu, & cabeza, 2015b). the need to adopt suitable control strategies is particular evident when pcms are integrated in transparent/translucent building envelope systems, since in these configurations the systems can easily receive more (solar) energy than the latent heat available. the control over the incoming radiation – either through shading systems (manz, egolf, suter, & goetzberger, 1997), prismatic glass panes (grynning, goia, rognvik, & time, 2013), or other dynamic layers such as, for example, thermotropic layers (bianco, cascone, goia, perino, & serra, 2017a; bianco, cascone, goia, perino, & serra, 2017b) – as well as of the discharge of the collected latent heat – for example, through a transparent ventilated cavity in which the pcm layer is installed (elarga, goia, zarrella, dal monte, & benini, 2016) is of great importance. 3 implementation of pcms modelling in building energy simulation tools there are many numerical models available, capable of simulating the inclusion of pcm in building envelopes. those most widely used are in trnsys (cf. section 3.1) and energyplus (cf. section 3.2), because they have, for a long time, been integrating direct methods for simulating pcm layers (at least in opaque components). however, there are also other software tools that have had for a long time (esp-r, cf. section 3.3), or have just recently added (ida-ice, cf. section 3.5), dedicated sub-routines that allow the simulation of pcm layers (again, in opaque components only) to be carried out in a rather straightforward way. 137 journal of facade design & engineering volume 6 / number 3 / 2018 the use of different simulation approaches, not based on dedicated sub-routines, has also been applied in other tools (e.g. ies-ve, cf. section 3.4) to replicate the performance of a pcm layer even if this is not directly modelled as such because of the limitations of the simulation environment. a crucial aspect is always the attention that must be paid to the physical phenomena considered, and the experimental validation (especially when the simulation strategy includes the use of non-validated, already implemented sub-routines). these kinds of models usually require the temperature–enthalpy curve, the thermal conductivity, and the specific heat of the pcm as input data. these models are application oriented and integrated in building energy simulation (bes) software, and are primarily developed for the application of pcms in opaque building elements. 3.1 overview of models most of the models that simulate the pcm behaviour in building envelopes analysed or presented in this paper are implemented in trnsys (59%), followed by esp-r (18%), energyplus (9%), ida-ice (9%), and ies-ve (5%) (fig. 1). fig. 1 pcm models implemented in bes software most of the analysed models are based on the enthalpy method or the effective heat capacity method (fig. 2). regarding the capability to simulate additional physical phenomena, most of the models do not include any additional capability. six of them are capable of simulating hysteresis; two are capable of simulating subcooling; and none is capable of simulating natural convection inside the pcm (fig. 2). 138 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 2 simulation method used and physical phenomena considered in pcm models 3.2 trnsys several pcm modelling efforts have been proposed for trnsys in the period 1991-2010 (ghoneim, klein, & duffie, 1991; stritih & novak, 1996; lamberg, jokisalo, & sirén, 2000; jokisalo, lamberg, & sirén, 2000; koschenz & lehmann 2004; ibañez, lázaro, zalba, & cabeza, 2005; ahmad, bontemps, sallée, & quenard, 2006; schranzhofer, puschnig, heinz, & streicher, 2006; kuznik, virgone, & johannes, 2010; dentel & stephan, 2010). most of these were 1-d models based on finite difference and enthalpy methods. they were compiled in the form of several types in trnsys (type58, type 204, type 232, type 101, type 241, and type 260, type 399). all were proposed for pcm walls or ceilings and only a few included validations with experimental data. only two included subcooling (type 204) or hysteresis effects (type 399). in 2014, (lu, liu, huang, & kong, 2014) developed a new type to simulate pcm in walls. the model is one-dimensional and uses the finite volume method. the apparent specific heat capacity method is used to simulate the pcm behaviour. hysteresis can also be taken into account; however, convection within the pcm cannot be simulated, nor phase change in microcapsules. although an experimental validation was performed, significant differences between the numerical and the experimental results were observed. in 2015, al-saadi and zhai (2015a) used different approaches to model pcm in walls: enthalpy method, heat capacity method, and heat source method, using as solvers the gauss-seidel and the tdma methods and also considering some correction steps. the model is one-dimensional and a fully implicit scheme is used with a spatial discretisation based on the finite volume method. after experimental validation, they concluded that only two schemes out of the eight developed could be considered as potential candidates for integrating into whole building simulation tool: the linearised enthalpy method with the iterative correction scheme and the hybrid correction scheme. 139 journal of facade design & engineering volume 6 / number 3 / 2018 later, al-saadi and zhai (2015b) validated the model with experimental data at building level, achieving good accuracy. in 2017, delcroix, kummert, and daoud (2017) presented a new model implemented in trnsys as type 3258, which was dedicated to modelling phase change materials in building envelopes. the model considered 1-d conduction heat transfer and used an explicit finite-difference method coupled with an enthalpy method to consider the variable pcm thermal capacity. the model included temperature-dependent thermal conductivity and pcm-specific effects like hysteresis and subcooling. the model was verified by comparing results with those of other numerical models, following the approach presented in haghighat et al. (2013) and johannes et al. (2011). results of the verification were successful. there is no evidence in the literature of the direct modelling of transparent systems incorporating pcms through trnsys. however, a matlab-based model of a pcm layer within a double skin façade has been coupled with trnsys to replicate the behaviour of this advanced façade solution (elarga et al., 2016; elarga, dal monte, andersen, & benini, 2017), though through the so called ‘ping-pong’ coupling that hensen (1999) implemented by means of type 155. furthermore, given the possibility to compile a deliberate type, it seems reasonable to expect that a dedicated type could be developed in the future, based on the different numerical models available in literature for pcm glazing systems, as explained in the simulation requirements section. 3.3 energyplus a new approach to simulate pcm in walls in energyplus was studied by barbour and hittle (2006). conduction transfer functions (ctf) were used to implement a numerical model for annual simulations, requiring less calculation capacity. the model was one dimension and was based on an ashrae toolkit. the model was validated with real data from previous experiments, but when implemented in energyplus, the simulations showed unacceptable errors when using pcm. in 2007, a new improved version of energyplus was presented, incorporating the capability to simulate pcm in building envelopes (pedersen, 2007) in 1-d. to solve the limitations of ctf to simulate pcm, a new implicit finite difference thermal model of building surfaces was incorporated into energyplus, making it possible to use temperature dependent thermal properties. the model simulates the performance of pcms using the enthalpy method. later on, in 2012, (tabares-velasco et al. 2012) presented a validation of the energyplus model to simulate pcm in walls. the procedure used was the one proposed by ashrae standard 140, consisting of analytical verification, comparative testing, and empirical validation. two bugs were identified and fixed, providing energyplus with a validated pcm model. in version 8.8 of energyplus, released in 2017, a dedicated module was first integrated in the simulation code to model hysteresis phenomena. however, such a sub-routine was only applicable to building envelope systems fully realised with pcms (i.e. not to multilayer walls), making it, in practice, of little or no use. this limitation has been overcome with the latest release of energyplus (v. 8.9) in march 2018 (energyplus, 2018). alternative approaches to model hysteresis with energyplus includes the use of the energy management system module in energyplus (goia, chaudhary, & fantucci, 2018) to impose a different enthalpy-temperature curve depending on the direction of the phase change (i.e. whether melting or solidifying). 140 journal of facade design & engineering volume 6 / number 3 / 2018 as far as the simulation of pcm layers in transparent components is concerned, the literature review reveals no example of models or approaches developed in the energyplus environment. the socalled conditioned cavity method (kendrick & walliman, 2007), which is explained in more detail in section 3.4, might be used in combination with ems functions and glass panes characterised by (controllable) dynamic optical properties, as a suitable strategy to carry out this modelling. however, the complexity of such an approach would probably be very high and some intrinsic limitations in energyplus might limit the verifications of the results too. preliminary attempts to overcome the current limitations of energyplus were made by one of the authors, by connecting an matlab/simulink based model with energyplus for co-simulation through the use of the external interface’s building controls virtual test bed (bcvtb) as well as the functional mock-up units (energyplus, 2015). however, in both cases the two coupled models solve the two sets of partial differential equations using a fixed synchronisation time step, which means that there is no iteration between the two simulation environments. this ‘ping-pong’ coupling may limit the reliability of the results for a system where inertial effects are crucial. given the open-source nature of energyplus, the implementation of custom model in the source code of the simulation environment is a feasible option. this might be a more suitable solution for expanding energyplus’s capabilities in simulating a transparent pcm-based layer than the co-simulation approach – though such an activity would require a significant programming effort resulting in the compilation of an entirely new code for the software. 3.4 esp-r in 2004, heim and clarke (2004) developed a modified esp-r program to simulate pcm-impregnated gypsum plasterboard. using control volumes, the effective heat capacity method and assuming equivalent homogeneous properties of pcm-gypsum composite, several temperatures were studied. unfortunately, the numerical model was not validated with real data and further macro-scale experiments are necessary. on the other hand, schossig, henning, gschwander, & haussmann (2005) developed an esp-r model to simulate micro-encapsulated pcm in gypsum wallboard and experimentally validated the model. in terms of esp-r’s prediction capability when it comes to effects such as hysteresis and subcooling, it must be mentioned that a dedicated subroutine (spmcmp56) was programmed by gelissier (2008) and included the possibility to model hysteresis, using the modelling approach developed by hoffmann (2006). later, an experimental validation of the esp-r pcm model was carried out by fallahi, shukla, & kosny (2012), using a base case wall assembly experimentally tested in the oak ridge national lab. testing facility located in charleston, south carolina, and by heim and wieprzkowicz (2016), which instead followed the methodology from the international energy agency (iea) annex 23 by johannes et al. (2011). there is no evidence in the literature about the use of esp-r to simulate transparent envelope applications of pcm system. as for energyplus, possible paths to enable simulation of pcm transparent system with esp-r include the development of a dedicated subroutine, as well as the adoption of strategies (e.g. the conditioned cavity method) to work around the limitation of the current state of the software tool. 141 journal of facade design & engineering volume 6 / number 3 / 2018 3.5 ies-ve ies-ve (which stands for integrated environmental solutions) does not incorporate, for the time being, any direct modelling possibility for pcm-based components (either in opaque or in transparent building envelope components). however, a successful work-around, using the apachesim model, was developed in the past few years, and the official support of the code (ies-ve 2018) recommends it – though highlighting some limitations. this work-around is based on the so-called ‘conditioned cavity method’ (kendrick & walliman, 2007), and is based on the modelling of the pcm layer as a virtual cavity, which is maintained at a set-point temperature corresponding to the nominal melting temperature of the pcm by means of an ideal heating and cooling system. such a cavity has an infinitesimal volume, whose surface have (almost) no thermal resistance and no heat capacity, while the cavity boundaries themselves have a thermal resistance equal to that of the pcm layer. the equivalent heat capacity method is embedded in this approach. the complexity of this work-around lies in the fact that the control of the energy to be delivered or removed (through an airflow) in the virtual cavity requires an iterative process to determine the schedule to control the fictitious heating and cooling system. this means that, in practice, due to the need to establish a very detailed control schedule, simulations are often limited to short periods (in the range of one or few weeks). favoino (2015) employed the “conditioned cavity method” in ies-ve by choosing to control the air temperature of the virtual cavity and assuming an infinite latent heat storage capacity for the pcm layer. if, on the one hand, such an approach enables longer simulations to be carried out, on the other hand it requires further verification to be carried out (i.e., the energy balance on the fictitious pcm layer needs to be carried out to ensure that the sum of the virtual cavity’s heating and cooling loads, at least on a daily basis, needs to be equal or lower than the actual latent heat storage capacity of the pcm layer. other examples of implementation of this method in ies-ve are reported in padovani jensen, and hes (2010) and ahmed, mateo-garcia, mcgough, caratella, and ure (2018). however, it must be stated that there is no evidence in the literature about the validation of the simulation results obtained though this work-around in ies-ve. it is also not possible to find an application of this approach for transparent pcm-based envelope components. 3.6 ida ice until recently, ida-ice had not supported an open, direct modelling of pcm layers. the new explicit module to model pcm has been now (early 2018) embedded in the latest version 4.8 of this tool. documentation on this new approach is not yet publicly and fully available, and this prevents a comprehensive understanding of the features of the modelling strategy implemented in the code, which, anyway, seems to be based on the enthalpy method equation. this new integrated solution is based on a custom model, developed by the software house of idaice, but until version 4.7 was made available only for research purposes. in 2017 and 2018, cornaro, pierro, puggioni, and roncarati, (2017) and cornaro, pierro, roncati, and puggioni (2018) tested and compared the ‘pcm wall’ module, written in neutral model format language (nmf, the programming 142 journal of facade design & engineering volume 6 / number 3 / 2018 language of ida-ice), with experimental data. this custom mode simulates the behaviour of a pcm layer based on the enthalpy method with a finite difference method, where one node represents the pcm layers and two nodes are placed a the two interfaces of the pcm, one at each surface of the layer. this model and implements two enthalpy-temperature curves, which may better replicate the behaviour of pcms with strong hysteresis effects. on the contrary, no evidence is found to assess whether or not subcooling can be addressed. the validation of the model was done using paraffin wax as pcm, a type of pcm that shows little hysteresis and almost no subcooling. a custom model for transparent pcm layers was developed and validated by (plüss et al. 2014), and used to estimate the effect of pcm transparent glazed system by (bionda, kräuchi, plüss, & schröcker 2015). this model is the only model known to replicate pcm in transparent envelope components that is integrated (though through a custom version) in software for bes. the model, written in nmf, uses a 1-d formulation, and is aimed at accurately representing subcooling effects (as it was developed to reproduce the behaviour of a salt hydrate-based system), based on the equivalent capacity method. the optical part of the model is based on the work of weinläder (2003) and weinläder et al. (2005). since only recently, and primarily through custom releases, ida-ice is presently the only tool capable of addressing both opaque and transparent building envelope components that integrate pcms. however, the potentials and limitations of the implemented simulation codes have not been extensively communicated and a comprehensive validation effort might be necessary to fully demonstrate the reliability of these codes. 4 validation of pcm modules or approaches for bes tools validation of numerical models is crucial to ensure accuracy, precision, and reliability of simulation results. validation is usually referred to direct comparison between experimental data and simulation results by means of concepts as average errors or relative maximum errors. however, other processes can also be used, such as analytical validation and model verification. analytical validation consists of comparing the simulation results of a simple case with its analytical solution, while model verification consists of comparing the simulation results with those of a validated model. in a validation process, experimental errors must be considered, as well as errors in input data. when simulating pcm in building façades, errors in ambient conditions and in pcm thermo-physical properties are of great importance and must be carefully evaluated (dolado, lázaro, marín, & zalba, 2011). moreover, special attention must be paid to the physical phenomena modelled, since some physical phenomena of the pcm may not be captured by the model, such as hysteresis or subcooling, and thereby affecting the accuracy of the results. for validation processes, the variable to be analysed must be determined with care. when modelling pcm, this variable can be a pcm variable (energy stored/released, pcm temperature evolution, etc.) or a variable of the system (internal temperature of the building, energy demand of the building, etc.). when the validation process includes the system (latter case), errors from the pcm model can be hidden by the system. finally, when using pcm as a passive system (intrinsic control), one must remember that errors in the system model may result in incorrect temperature predictions of the pcm, thus predicting a complete different behaviour. 143 journal of facade design & engineering volume 6 / number 3 / 2018 in the iea annex 23 (johannes et al. 2011; haghighat et al. 2013), a standardised procedure for validation of pcm-enhanced (opaque) walls was proposed. this procedure, based on nine different cases, allows a numerical benchmark of the simulation results to be established. on the contrary, and probably due to the limited amount of research activities in this field, as well as the limited marketready solutions, no standardised procedures have been internationally agreed upon for the validation of pcm-enhanced transparent/translucent elements, where the role of the impinging solar radiation becomes very relevant (grynning et al. 2013). thus, a standardised procedure for pcm model validation in general is still required, based on detailed and reliable experimental data. bes programs usually attempt to experimentally validate their models (kuznik & virgone, 2009; kuznik et al. 2010; tabares-velasco, 2012). however, these validations can be limited to certain situations and thus, attention must be paid to the validity range and conditions. from all of the presented models, 10 out of 18 (56%) were validated (fig. 1). on the other hand, from the most recent models developed, three models in trnsys have been validated (kuznik et al., 2010; al-saadi & zhai, 2015a; al-saadi & zhai 2015b; delcroix et al., 2017). on the other hand, the model in energyplus is also validated (tabares-velasco et al., 2012). a model in esp-r (fallahi et al., 2012; heim & wieprzkowicz, 2016) and a model in ida-ice (cornaro et al., 2017; cornaro et al., 2018) were also validated against experimental data. the model developed by kuznik et al. (2010) used experimental results from a test cell to compare both the internal ambient temperature and the internal surface temperatures for two external temperature evolutions (step and sinusoidal). for the internal air temperature, maximum differences between numerical and experimental results were 1.1ºc and 0.8 ºc for step and sinusoidal external temperature evolution, respectively. on the other hand, mean differences were 0.2ºc and 0.3ºc, respectively. for the internal surface temperature, the model presents good agreement for the step case (maximum difference of 1.1ºc and mean difference of 0.2ºc). for the sinusoidal case (maximum difference of 1.3ºc and mean difference of 0.6ºc), the model predicts the same behaviour for all walls, while experimental results demonstrate some differences that are not captured by the model. although the model shows good agreement with the experimental results, there exist some differences, which can be caused by the aeraulic effects inside the test cells, which cannot be predicted by the model. this limitation could be overcome if the convective heat transfer coefficient is known. regarding the energyplus model, the validation procedure performed by tabares-velasco et al. (2012) consisted of the analytical verification, comparative verification, and empirical validation of three pcm applications: pcm distributed in drywall, pcm distributed in fibrous insulation, and thin concentrated pcm layers. the analytical verification consisted of solving the stefan problem. the three cases analysed showed similar results and were in good agreement with the analytical. however, results determined that the node spacing must be smaller (3 times smaller) than the default one in energyplus. the verification process consisted of a comparative testing relative to the ideal pcm model in heating 7.3, thus representing a more realistic case. results were in good agreement when time steps where shorter than 4 minutes. moreover, the pcm model also determined peak load reduction and shift accurately. finally, experimental validation was performed based on data from the literature (haavi gustavsen, cao, uvsløkk, & jelle 2011; cao et al., 2010). results were in good agreement in terms of temperature for the heating process, but significant differences were observed for cooling. this is due to the incapability of energyplus to simulate pcm hysteresis, at least until the implementation of this feature in version 9, which was recently tested against dedicated experimental data (goia et al. 2018). in this latter activity, it was shown that even if the capability to simulate the hysteresis has been embedded in the code, the reliability of the 144 journal of facade design & engineering volume 6 / number 3 / 2018 simulation to catch this phenomenon is still pretty low, especially in the case when the melting or solidification process is not completed. al-saadi & zhai (2015a) also used data from (cao, 2010) to experimentally validate their models. they used the root mean squared error (rmse) to analyse the accuracy of the models. two different points in the pcm layer were analysed. all models except the non-iterative correction scheme showed an error close to or below 0.1ºc, which was within the uncertainty range of the experimental equipment. they also performed a comparative analysis with energyplus, analysing the interior and exterior surface temperatures. all models show good agreement with energyplus, showing errors lower than 0.1ºc for a duration of 3 minutes. finally, the same authors (2015b) also validated the model with the experimental data from kuznik et al. (2010), showing good agreement. the model developed by delcroix et al. (2017) was validated following the approach proposed by the international energy agency annex 23 (johannes et al., 2011; haghighat et al., 2013). results were in accordance with the ones from iea annex 23, both for the internal and external surface temperatures and for the heat fluxes. fallahi et al. (2012) validated the pcm model in esp-r, against experimental field data obtained from the oak ridge national lab. testing facility located in charleston, south carolina (kośny, 2008; kośny, kossecka, & yarbrough, 2009; kośny, kossecka, brzezinski, tleoubaev, & yarbrough., 2011). to validate the model, the heat flux across the walls was compared with the measured one, showing a total heat gain difference of about 0.6%. cornaro et al. (2017) compared the simulation results obtained with ida-ice with temperature data collected by means of solar test boxes. these are boxes with a linear scale factor of 1:5 and a surface scale factor of 1:25 with respect to a real room, with five opaque walls and one glazed wall, where the opaque walls are equipped with pcm layers. the results of the validation show that the rmse, calculated for the indoor air temperature over a period of 3 days, was in the range 1.6 to 1.8 °c, corresponding to an error of ca. 5%. 5 limitations and desirable further improvements the use of pcm in building envelopes has focused in passive systems (intrinsic control systems), where the pcm is passively charged and discharged by either solar energy/external temperature or an internal heat source. the main goal of such systems is to reduce the energy demand of the building and/or improve the thermal comfort. however, these systems present difficulties in their design process and pcm selection, since they require very specific designs to achieve a suitable performance. for operating conditions (weather conditions, use and occupation of the building, etc.) different from the design ones, the behaviour of the pcm will change, and its phase change temperature may no longer be suitable, thus reducing or even eliminating its benefits. moreover, errors in the building model affect the pcm behaviour, and may result in inaccurate pcm system design (such as phase change temperature) that may reduce its benefits. additionally, the recharging of the pcm is sometimes limited in such applications, compromising the potential benefits of the system. therefore, active systems (extrinsic control systems) are advisable in order to solve some of these problems. 145 journal of facade design & engineering volume 6 / number 3 / 2018 finally, other important issues in the simulation of the pcm remain to be solved. the accurate inclusion of hysteresis, subcooling, natural convection, and ageing in pcm simulation must be solved in future bes tools. although some advances have been done in the newest versions of software tools, there is still room for improvement. 6 conclusions and future works this paper provides an overview of the successful implementation of the modelling and simulation of pcm-based building envelope systems in software tools for building energy simulation (bes). five of the most common bes tools have been analysed and evidence from the scientific literature about their use for simulating pcm-enhanced envelope has been given. in a time when the integration of detailed aspects typical of pcms’ behaviour (i.e. hysteresis and subcooling) is becoming more and more common in bes tools, it is necessary to highlight how validation of the models implemented in bes is probably an underestimated activity. although there have been proposals on standardised procedures, these have not been extensively used, and the comparison of the simulation performance of bes has not been comprehensively carried out – at least as far as the latest developments are concerned. an overall comparison of the performance of bes tools in modelling and simulation of pcm-enhanced envelope would definitely be an important research task to fill a present-day knowledge gap. coupling such a numerical benchmarking with reliable experimental activities would further increase the relevance of the effort. apart from the standardised procedures proposed by the iea annex 23 for model verification (johannes et al., 2011; haghighat et al., 2013), two different sets of experimental data have been used in the literature for experimental validation of different models (kuznik et al., 2010; cao, 2010). while almost all the bes tools analysed in this paper allow the simulation of opaque envelope systems incorporating pcms to be carried out (and, with the help of a work-around, such a simulation is possible with all of them), it is almost impossible at present to simulate the transparent envelope, which includes pcms exposed to solar radiation – only one custom model is available for one software tool. modelling and simulation of transparent solutions incorporating pcms is therefore way behind the simulation of opaque pcm envelope, and future development of tools by the bes community should also focus on enabling this simulation domain. acknowledgements the authors would like to thank eu cost action tu1403 ‘adaptive facades network’ for providing excellent research networking. the authors albert castell and marc medrano would like to thank the catalan government for the project grant (2017 sgr 659) given to their research group. the author francesco goia would like to thank the research council of norway and several partners through the research centre on zero emission buildings (zeb) (2009 – 2017, grant 193830) at the norwegian university of science and technology. 146 journal of facade design & engineering volume 6 / number 3 / 2018 references ahmad, m., bontemps, a., sallée, h., & quenard, d. (2006). thermal testing and numerical simulation of a prototype cell using light wallboards coupling vacuum isolation panels and phase change material. energy and buildings 38, pp.673–681. ahmed, a., mateo-garcia, m., mcgough, d., caratella, k., & ure, z. (2018). experimental evaluation of passive cooling using phase change materials (pcm) for reducing overheating in public building. e3s web of conferences 32, 01001, 1-7. doi: 10.1051/ e3sconf/20183201001 alexides, v., & solomon, a.d. (1993). mathematical modeling of melting and freezing processes. washington: hemisphere publishing corporation, p. 47. al-saadi, s.n., & zhai, z. (2015). systematic evaluation of mathematical methods and numerical schemes for modeling pcm-enhanced building enclosure. energy and buildings 92, pp.374–388. al-saadi, s.n., & zhai, z. (2015). a new validated trnsys module for simulating latent heat storage walls. energy and buildings 109, pp.274–290. andersen, m., roecker, c., & scartezzini, j. l. (2005). design of a time-efficient video-goniophotometer combining bidirectional functions assessment for transmission and reflection. solar energy materials and solar cells 88(1), 97-118. doi: 10.1016/j. solmat.2004.10.009 barbour, j.p., & hittle, d.c. (2006). modeling phase change materials with conduction transfer functions for passive solar applications. transactions of the asme vol. 128, february 2006. bianco, l., cascone, y., goia, f., perino, m., & serra, v. (2017a). responsive glazing systems: characterisation methods and winter performance. solar energy, 155, pp.372–387. bianco, l., cascone, y., goia, f., perino, m., & serra, v. (2017b). responsive glazing systems: characterisation methods, summer performance and implications on thermal comfort. solar energy, 158, pp.819–836. bionda, d., kräuchi, p., plüss, i., & schröcker, m. (2015). simulation of the thermal performance of translucent phase change materials and whole-building energy implications. proceedings of 10th conference on advanced building skins. doi: 10.13140/ rg.2.1.1729.4806 bony, j., & citherlet, s. (2007). numerical model and experimental validation of heat storage with phase change materials. energy and buildings, 39(10), pp.1065-1072. cabeza, l.f., castell, a., barreneche, c., de gracia, a., & fernández, a.i. (2011). materials used as pcm in thermal energy storage in buildings: a review. renewable and sustainable energy reviews 15, pp.1675–1695. doi:10.1016/j.rser.2010.11.018. cabeza, l.f. (ed.) (2015) advances in thermal energy storage systems. methods and applications. woodhead publishing. united kingdom. isbn: 978-1-78242-088-0. cao, s., gustavsen, a., uvsløkk, s., jelle, b.p., gilbert, j., & maunuksela, j. (2010). the effect of wall-integrated phase change material panels on the indoor air and wall temperature hot box experiments, in: zero emission buildings proceedings of renewable energy conference 2010. trondheim, norway. p. 15-26. cao, s. (2010) state of the art thermal energy storage solutions for high performance building. department of physics, university of jyväskylä, finland,2010. cornaro, c., pierro, m., puggioni, v.a., & roncarati, d. (2017). outdoor characterization of phase change materials and assessment of their energy saving potential to reach nzeb. buildings 7(3), p.55. doi: 10.3390/buildings7030055 cornaro, c., pierro, m., roncati, d., & puggioni, v. (2018). validation of a pcm simulation tool in ida ice dynamic building simulation software using experimental data from solar test boxes. proceedings of building simulation application (bsa) 2017. bolzano university press, bolzano, pp.159-166. de gracia, a., navarro, l., castell, a., ruiz-pardo, a., álvarez, s., & cabeza, l.f. (2013). thermal analysis of a ventilated facade with pcm for cooling applications. energy and buildings 65, pp.508–515. doi.org/10.1016/j.enbuild.2013.06.032. de gracia, a., navarro, l., castell, a., & cabeza, l.f. (2015a). energy performance of a ventilated double skin facade with pcm under different climates. energy and buildings 91, pp.37–42. doi.org/10.1016/j.enbuild.2015.01.011. de gracia, a., fernández, c., castell, a., mateu, c., & cabeza, l.f. (2015b). control of a pcm ventilated facade using reinforcement learning techniques. energy and buildings 106, pp.234–242. doi.org/10.1016/j.enbuild.2015.06.045. delcroix, b., kummert, m., & daoud, a. (2017). development and numerical validation of a new model for walls with phase change materials implemented in trnsys. journal of building performance simulation 10 (4), pp.422-437. dentel, a., & stephan, w. (2010, december). thermal comfort in rooms with active pcm constructions. 8th international conference on system simulation buildings, liege pp.13-15. dolado, p., lázaro, a., marín, j.m., & zalba, b. (2011). characterisation of melting and solidification in a real scale pcm-air heat exchanger: numerical model and experimental validation, energy conversion and management 52 (4), pp.1890-1907. doi. org/10.1016/j.enconman.2010.11.017. elarga, h., goia, f., zarrella, a., dal monte, a., & benini, e. (2016). thermal and electrical performance of an integrated pv-pcm system in double skin façades: a numerical study. solar energy 136, pp.112-124. elarga, h., dal monte, a., andersen, r.k., & benini, e. (2017). pv-pcm integration in glazed building. co-simulation and genetic optimization study. building and environment 126, pp.161-175 energyplus (2015). external interface(s) application guide. energyplus (2018). engineering reference (v. 8.9). fallahi, a., shukla, n., & kosny, j. (2012) numerical thermal performance analysis of pcms integrated with residential attics. fifth national conference of ibpsa-usa, wisconsin. fantucci, s., goia, f., perino, m., & serra, v. (2018). sinusoidal response measurement procedure for thermal performance assessment of pcm by means of dynamic heat flow meter apparatus. submitted for publication in energy and buildings. favoino, f. (2015). assessing the performance of an advanced integrated facade by means of simulation: the actress facade case study. journal of facade design and engineering 3, pp.105–127. doi: 10.3233/fde-150038 147 journal of facade design & engineering volume 6 / number 3 / 2018 geissler, a. (2008). spmcmp56 subroutine in esp-r source standard code (software code). ghoneim, a.a., klein, s.a., & duffie, j.a. (1991). analysis of collector-storage building walls using phase-change-materials. solar energy vol. 47, no. 3, pp. 237-242. giovannini, l., goia, f., lo verso, v.r.m., & serra, v. (2017). phase change materials in glazing: implications on light distribution and visual comfort. energy procedia 111, pp.357–366 goia, f., perino, m., & haase, m. (2012). a numerical model to evaluate the thermal behaviour of pcm glazing system configurations. energy and buildings 54, 141–153 goia, f., perino, m., & serra, v. (2014). experimental analysis of the energy performance of a full-scale pcm glazing prototype. solar energy 100, pp.217–233. goia, f., zinzi, m., carnielo, e., & serra, v. (2015). spectral and angular solar properties of a pcm-filled double glazing unit. energy and buildings 87, pp.302–312. goia, f., chaudhary, g., & fantucci, s. (2018). modeling and experimental validation of an algorithm for simulation of hysteresis effects in phase change materials for building components. energy and buildings 174, pp.54–67. gowreesunker, b.l., stankovic, s.b., tassou, s.a., & kyriacou, p.a. (2013). experimental and numerical investigations of the optical and thermal aspects of a pcm-glazed unit. energy and buildings 61, pp.239–249. grynning, s., goia, f., rognvik, e., & time, b. (2013). possibilities for characterization of a pcm window system using large scale measurements. international journal of sustainable built environment 2, pp.56–64. günther, e., mehling, h., & hiebler, s. (2007). modeling of subcooling and solidification of phase change materials. modelling simulation in material science and engineering, 15(8), pp.879-892. haavi, t., gustavsen, a., cao, s., uvsløkk, s. & jelle, b.p. (2010). numerical simulations of a well-insulated wall assembly with integrated phase change material panels comparison with hot box experiments, in: the international conference on sustainable systems and the environment; 2011. sharjah, sharjah, united arab emirates. haghighat, f., yu, z., inard, c., michaux, g., kuznik, f., johannes, k., virgone, j., barzin, r., farid, m., bastani, a., stathopoulos, n., mankibi, m. e., nkwetta, d. n., moreau, a., vouillamoz, p-e., castell, a., adl-zarrabi, b. (2013). annex 23: energy storage in buildings of the future applying energy storage in ultra-low energy buildings. paris, france: international energy agency. heim, d., & clarke, j.a. (2004). numerical modelling and thermal simulation of pcm–gypsum composites with esp-r. energy and buildings 36, pp.795–805. heim, d., & wieprzkowicz, a. (2016). positioning of an isothermal heat storage layer in a building wall exposed to the external environment. journal of building performance simulation 9 (5), pp. 542–554. hensen, j.l.m. (1999). a comparison of coupled and de-coupled solutions for temperature and air flow in a building. ashrae transactions 105 (2), pp.962–969. hoffmann, s. (2006). numerische und experimentelle untersuchung von phasenübergangsmaterialien zur reduktion hoher sommerlicher raumtemperaturen [numerical and experimental investigation on phase change materials to reduce high indoor temperatures during summer]. (doctoral thesis) bauhaus-universität, weimar, germany. hu, h., & argyropoulos, s.a. (1996). mathematical modelling of solidification and melting: a review. modelling simulation material science and engineering 4, pp.371–396. ibáñez, m., lázaro, a., zalba, b., & cabeza, l.f. (2005). an approach to the simulation of pcms in building applications using trnsys. applied thermal engineering 25, pp.1796–1807. ishimaru, a. (1978). wave propagation and scattering in random media. in: single scattering and transport theory, 1. california, usa: academic press. johannes, k., virgone, j., kuznik, f., wang, x., haavi, t., & fraisse, g. (2011). annex 23: applying energy storage in buildings of the future development of sustainable energy storage designs for a variety of ultra-low energy building thermal, phase change materials and electrical storage options. paris, france: international energy agency. jokisalo, j., lamberg, p., & sirén, k. (2000). thermal simulation of pcm structures with trnsys. stuttgart, germany: terrastock 2000. jones, r.w., balcomb, j.d., kosiewicz, c.e., lazarus, g.s., mcfarland, r.d., & wray, w.o. (1982). passive solar design handbook. volume 3: passive solar design analysis. boulder, co: u.s. department of energy ases, kendrick, c., & walliman, n. (2007). removing unwanted heat in lightweight buildings using phase change materials in building components: simulation modelling for pcm plasterboard. architectural science review 50(3), pp.265-273. koschenz, m., lehmann, b. (2004). development of a thermally activated ceiling panel with pcm for application in lightweight and retrofitted buildings. energy and buildings 36, pp.567–578. kośny, j. (2008). field testing of cellulose fiber insulation enhanced with phase change material‖. oak ridge national laboratory reportornl/tm-2007/186, september 2008. kośny, j., kossecka, e., & yarbrough, d. w. (2009). use of a heat flow meter to determine active pcm content in an insulation. proceedings of the 2009 international thermal conductivity conference (itcc) and the international thermal expansion symposium (ites) – august 29 september 2, 2009 pittsburgh, pa, usa. kośny, j., kossecka, e., brzezinski, a., tleoubaev, a., & yarbrough, d. (2011). numerical and experimental thermal analysis of pcm-enhanced insulations. international thermal conductivity conference (itcc) june 26 30, 2011 saguenay, qc, canada. kośny, j. (2015). pcm-enhanced building components. an application of phase change materials in building envelopes and internal structures. springer international publishing, doi:10.1007/978-3-319-14286-9. kuznik, f., virgone, j., & johannes, k. (2010). development and validation of a new trnsys type for the simulation of external building walls containing pcm. energy and buildings 42(7), 1004-1009. doi.org/10.1016/j.enbuild.2010.01.012. lamberg, p., jokisalo, j., & sirén, k. (2000). the effects on indoor comfort when using phase change materials with building concrete products. proceedings of healthy buildings 2000, vol. 2, pp. 751–756, siy indoor air information oy. 148 journal of facade design & engineering volume 6 / number 3 / 2018 li, d., ma, t., liu, c., zheng, y., wang, z., & liu, x. (2016). thermal performance of a pcm-filled double glazing unit with different optical properties of phase change material. energy and buildings 119, pp.143–152. liu, c., zhou, y., li, d., meng, f., zheng, y., & liu, x. (2016). numerical analysis on thermal performance of a pcm-filled double glazing roof. energy and buildings 125, pp.267-275. lu, s., liu, s., huang, j., & kong, x. (2014). establishment and experimental verification of pcm room’s trnsys heat transfer model based on latent heat utilization ratio. energy and buildings 84, pp.287–298. manz, h., egolf, p., suter, p., & goetzberger, a. (1997). tim–pcm external wall system for solar space heating and daylighting. solar energy 61, pp.369–379. mckellar, b.h.j., & box, a.m. (1981). the scaling group of radiative transfer equation. journal of atmospherical science 38, pp.1063–1068. mehling, h., & cabeza, l. f. (2008). heat and cold storage with pcm, an up to date introduction into basics and applications, berlin heidelberg: springer-verlag. navarro, l., de gracia, a., colclough, s., browne, m., mccormack, s.j., griffiths, p., & cabeza, l.f. (2016a). thermal energy storage in building integrated thermal systems: a review. part 1. active storage systems. renewable energy 88, pp.526-547. doi. org/10.1016/j.renene.2015.11.040. navarro, l., de gracia, a., niall, d., castell, a., browne, m., mccormack, s.j., griffiths, p., & cabeza, l.f. (2016b). thermal energy storage in building integrated thermal systems: a review. part 2. integration as passive system. renewable energy 85, pp.13341356. doi.org/10.1016/j.renene.2015.06.064. padovani, r., jensen, c., & hes, d. (2010). approach to thermal modelling innovative green building elements: green roof and phase change plasterboard. aubea 2010 proceedings of the 2010 conference of the australasian universities building education association, 2010, 1 (1), pp. a080, 1 17 pedersen, c.o. (2007). advanced zone simulation in energyplus: incorporation of variable properties and phase change material (pcm) capability. proceedings of building simulation 2007. plüss, i., kräuchi, p., bionda, d., schröcker, m., felsenstein, s., zweifel, g. (2014). modellbildung eines phasenwechsel-fassadenelements in ida-ice [modelling of a facade element with phase change in ida-ice]. proceedings of bausim: fifth german-austrian ibpsa conference rwth aachen university, 1166, p.1-5. schossig, p., henning, h.m., gschwander, s., & haussmann, t. (2005). micro-encapsulated phase-change materials integrated into construction materials. solar energy materials & solar cells 89, pp.297–306. schranzhofer, h., puschnig, p., heinz, a., & streicher, w. (2006). validation of a trnsys simulation model for pcm energy storages and pcm wall construction elements. ecostock 2006 10th international conference on thermal energy storage. pomona, nj, usa. silva, t., vicente, r., & rodrigues, f. (2016) literature review on the use of phase change materials in glazing and shading solutions. renew. sustain. energy rev. 53, 515–535. stritih, u., & novak, p. (1996). solar heat storage wall for building ventilation. renewable energy 8 (1-4), pp.268-271. tabares-velasco, p.c., christensen, c., bianchi, m. (2012). verification and validation of energyplus phase change material model for opaque wall assemblies. building and environment 54, pp.186-196. doi.org/10.1016/j.buildenv.2012.02.019. vigna, i., bianco, l., goia, f., & serra, v. (2018). phase change materials in transparent building envelopes: a strengths, weakness, opportunities and threats (swot) analysis. energies 11, p.111. doi:10.3390/en11010111 voller, v.r. (1997). an overview of numerical methods for solving phase change problems, in minkowycz, w.j. and sparrow, e.m. (eds), advances in numerical heat transfer, vol. 1, basingstoke: taylor & francis, weinläder, h. (2003). optische charakterisierung von latentwärmespeichermaterialien zur tageslichtnutzung [optical characterization of phase change materials for daylighting]. (dissertation) julius-maximilians-universität, würzburg, germany. weinläder, h., beck, a., & fricke, j. (2005). pcm-facade-panel for daylighting and room heating. solar energy 78, pp.177–186. journal of facade design and engineering 3 (2015) 103 doi 10.3233/fde-150041 ios press 103 editorial dear scientists, dear engineers and designers, the papers in this issue of jfde discuss the potential of adaptive building envelopes, component development as well as implementation strategies. the applied practice paper demonstrates decision strategies behind the adaptive sun shading system of the al-bahr towers. additivity in building envelopes is not only a strategy to fulfil the growing demands for energy efficient buildings and comfort but has great architectural implications as well. in general it asks for more complex components as well as control strategies. but complexity also means costs and risks, and we need to discuss the means and effects. this discussion in particular is very interesting because here science and practice meet. the journal of facade design and engineering jfde will actively follow and stimulate by providing high quality contributions. four of the paper contributions have their origins in the conference ‘facades 2014’, held in november 2014 in lucerne. the contributions have been carefully selected and have been subjected to the regular double blind review process of the journal. we want to thank prof. dr. andres luible for the help in making this issue happen. we are proud that jfde is the scientific partner for a number of conferences such as ‘the future envelope’ conference on building envelopes held yearly in delft (nl) or bath (uk), the icae international congress on architectural envelopes in san sebastian (es) and the above mentioned conference ‘facades’ in lucerne (ch) and detmold (d). our latest partner is the icbest 2017 international conference on building envelope systems and technologies in istanbul. the growing number of partners indicates the relevance of jfde for our growing discipline and will assure the continuity of the journal. facade design and engineering is a peer reviewed, open access journal, funded by the netherlands organisation for scientific research nwo (www.nwo.nl). we see ‘open access’ as the future publishing model. but it certainly requires new financial models which we will have to explore in the coming years. the editors in chief, ulrich knaack tillmann klein issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. journal of facade design and engineering 3 (2015) 223 doi 10.3233/fde-150050 ios press 223 editorial dear jfde readers and authors, we are proud to announce that we have officially completed the nwo funding period by successfully establishing the journal. for three years jfde has been supported by the open access fund of the netherlands organisation for scientific research nwo (www.nwo.nl). we have been able to publish 40 papers in the first three volumes. that is slightly less than we have hoped but the numbers have steadily increased to 19 papers in volume 3 alone. we would like to thank nwo for the support. the funding allowed us to cover a large part of the publishing costs. still, the tu delft and our publisher ios press have invested a lot of time and money in the journal. but without nwo’s contribution it would not have been possible to establish jfde. especially that jfde became indexed by the directory of open access journals doaj during the funding period, is a great success for jfde. doaj is a membership organization with the aim to increase the visibility and ease of use of open access journals and is co-author to the principles of transparency and best practice in scholarly publishing. with the indexation, doaj acknowledges our commitment to quality, peer-reviewed open access publishing. based hereupon we can now prepare for other indexation possibilities and we are also working on securing the funding for the coming years. there are two challenges we face: our discipline of facade design and engineering is scientifically relatively young, and the possibilities of new publishing concepts, which include author-fee principles only slowly get acknowledged. therefore, one of the tasks of jfde is to lobby for scientific oa publishing in a field that is located right between science and applied practice. the editors in chief, tillmann klein ulrich knaack issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. www.nwo.nl from city’s station to station city 077 journal of facade design & engineering volume 6 / number 3 / 2018 a redesign procedure to manufacture adaptive façades with standard products bahar basarir1*, m. cem altun2 * corresponding author 1 graduate school of science engineering and technology, istanbul technical university, turkey, baharbasarir@gmail.com 2 department of architecture, istanbul technical university, turkey abstract although their potential for high environmental performance is largely accepted, adaptive façades have not yet become widespread in practice. most of the current examples are developed by engineer-to-order design processes, as project-oriented, custom, and complex solutions. more simple and reliable solutions are needed to support the reuse of technical solutions between projects and increase the feasibility of adaptive façades. therefore, this research aims to develop a procedure to design adaptive façades whose parts are based on engineered standard products with the least number of parts and layers. the research is initiated through the generation of concepts for designing adaptive façades to be manufactured using standard products. from several concepts, ‘redesigning dynamic adaptive façades’ has been selected for further investigation, as it pursues the goals for a solution determined for this research. a preliminary case study is conducted to redesign an adaptive façade to be manufactured with standard products. its process steps are captured and analysed, and the steps that need improvement are revealed. to systematise and improve the captured redesign process, façade design and product design methodologies are analysed in the context of adaptive façade design. redesign and reverse engineering processes used in product design are adapted and merged with façade and adaptive façade design processes, and a 5-phase adaptive façade redesign procedure is outlined. each phase is developed based on mature tools and methods used in product and façade design. an iterative loop of development, application test, and review process is carried out for development of the process steps. thus, a redesign procedure is generated by the combined application of dfma and triz in the synthesis of reverse engineering and redesign processes. consequently, the application of the redesign procedure is demonstrated through a case study. the case study revealed that the procedure has the ability to generate a façade redesign that has a higher constructability index than the reference façade. keywords adaptive façade, constructability, redesign, standard product, reverse engineering, dfma doi 10.7480/jfde.2018.3.2530 078 journal of facade design & engineering volume 6 / number 3 / 2018 1 introduction adaptive façades are considered to be an important step in the development of façade technology. they are receiving increasing attention from researchers and professionals in the building sector, as they provide comfortable interior conditions with low energy consumption. currently, there are more than five hundred building examples with adaptive shells, according to the climate adaptive building shells database (loonen, 2013; attia & bashandy, 2016). however, these examples are mainly ‘experimental, small-scale’ or ‘high-profile, high-budget’ projects (loonen, trcka, cóstola, & hensen, 2013). despite their accepted potential for high environmental performance and wide range of technology options from high-tech to low-tech, the practical application of adaptive façades is very limited. a comprehensive literature review is conducted to determine the problems causing this situation, and the findings are listed below: – adaptive façades are not clearly defined and resolved in the field of architectural research (schnädelbach, 2010; gosztonyi, 2015; attia, favoino, loonen, petrovski, & monge-barrio, 2015). kolarevic (2015) states that change events are not adequately addressed or explored. – designers need to acquire experience and knowledge about designing adaptive façades (meagher, 2015; loonen, favoino, hensen, & overend, 2017). however, detailed information about design and construction processes, performance, and post occupancy evaluations of existing cases are lacking in the literature (attia & bashandy, 2016; attia, 2017). decisions on how adaptive façades are designed, operated, maintained, and assessed remain a challenge (attia, 2017). questions such as: what sort of adaptation is needed, what type of behaviour results in the best performance, and what is the maximum acceptable rate of change are still being researched. – design and performance evaluation of adaptive façades is a complex task, and existing performance assessment tools are insufficient to evaluate the adaptive façade systems (loonen et al., 2017; boer et al., 2011; struck et al., 2015). – standardised procedures, design support tools, and methods are needed for adaptive façade design (bolbroe, 2014; loonen et al., 2015) – majority of the current examples are project-oriented custom solutions that develop complex one-ofa-kind products and involve innovative technologies, resulting in challenging projects with relatively high risks (loonen et al., 2013). – there are social and psychological challenges and barriers related to user interaction (loonen, 2010; ogwezi, bonser, cook, & sakula, 2011). considering the problems listed above, simple, flexible, and easily accessible solutions are needed with well-described procedures to achieve these solutions to increase the practical application of adaptive façades. thus, a basis would be provided for adaptive façades to become customised industrial products like the majority of the regular façade systems on the market. in the context of this need, several approaches could be developed to achieve such solutions. one of these solutions is to simplify the design of adaptive façades using products that are based on engineered standard products with the least number of parts and layers. within the scope of this approach, the term ‘product’ is used to describe all product levels of façades (klein, 2013), between different levels of completeness, from material to component, within the building product hierarchy developed by eekhout (2008). likewise, the term ‘standard product’ covers all levels of products with unalterable characteristics and manufacturing processes, ranging from standard material to component (eekhout, 2008). 079 journal of facade design & engineering volume 6 / number 3 / 2018 in addition to enhancing the feasibility and constructability of adaptive façades, there are several other reasons for proposing the design of adaptive façades using standard products. anderson (2014) states that standard products are less expensive to design and provide time savings, when design, documentation, prototyping, and testing processes are considered. the overhead cost of purchasing all the constituent parts and the cost of non-core-competency manufacturing can be reduced by using standard products. suppliers are more efficient within their own specialty, more experienced in using their own products, continuously improve quality, have proven track records on reliability, have dedicated production facilities, produce parts at lower cost, offer standardised parts, and sometimes pick up warranty and service costs (anderson, 2014). all these features of standard products support the maintenance, repair, and operation processes as well as the manufacturing process. the aim of this research is to develop a design procedure to support designing adaptive façades with standard products that are available on market, to improve constructability through simplification. at first, a solution is sought for how to design adaptive façades to be manufactured with standard products. possible solution paths, namely concepts, are identified and one of them is selected for elaboration. following this, the selected concept is developed with the focus on identification of a design procedure. various research methods are used within this research. a comprehensive literature review of both façade and product design is performed for concept generation and development. a research through design methodology is adopted, and an iterative loop of development, application test, and review process is carried out for development of process steps, checklists, and templates of the design procedure. applicability of the design procedure is tested through a case study and evaluated by interviews with experts such as architects and manufacturers. within this framework, section 2 presents concept generation, selection, and development processes. section 3 describes the phases and steps of the redesign procedure, developed for the selected redesign concept. section 4 presents the application of the redesign procedure through a case study. section 5 concludes the research with revealing characteristics, benefits, and limitations of the redesign procedure. 2 concept generation, selection and development designing adaptive façades to be manufactured with standard products is an open-ended problem with multiple acceptable solutions. indeed, a characteristic of architectural design problems is that there are numerous alternatives and many potentially acceptable solutions (lawson, 1970). the challenge is to find the best solution in relation to the design objectives of the project. when dealing with an open-ended problem, rather than concentrating initially on a specific solution, it is better to look for as many different solutions as possible (dandy, daniell, foley, & warner 2018). in this context, some researchers suggest subdividing and structuring the problem-solving process into three different levels: concept level, system level, and material level (perino & serra, 2015). from this point of view, this research starts from the concept level and continues down to the system level. the material level is outside the scope of this research, since material development is not intended. the concept level aims to explore new ideas and visions, and analyses them from a theoretical point of view to obtain information on the working principles (perino & serra, 2015). an answer is sought for what would be done to solve the problem, without worrying about how to do it. concept level studies respectively include collecting ideas and existing concepts, concept generation, and concept selection. 080 journal of facade design & engineering volume 6 / number 3 / 2018 to reveal existing concepts and collect ideas, the mature principles from manufacturing industry are reviewed in the context of the aim of this research. at this stage, the need for customisation of façade design in each project depending on building specifications comes into prominence. in this context, strategies of designing customised products by combining standard products are reviewed from product development literature, to determine possible design approaches. ulrich (1992) demonstrated that product variety/customisation can be economically realised with product architecture strategies that provide flexibility in the final assembly process without changing the manufacturing process. in the context of product architecture, customisation by standard products is achieved by modular systems (ulrich & eppinger, 2012) and open systems (koren, hu, peihua, & shpitalni, 2013), and by the production approaches, mass customisation, and mass individualisation, which arise from these product architecture systems. open systems and modular systems are embraced in architecture in a similar manner (staib, dörrhöfer, & rosenthal, 2008). according to that information, it has been determined that concept studies should focus on the development of the product architecture. concept generation study begins after re-stating the research problem in clear, general, and unambiguous terms, and collecting ideas and existing concepts. within the set of possible solutions, concept alternatives are defined depending on certain variables that are mainly extracted from collected ideas and existing concepts. the number of these variables varies depending on the defined part of the solution set. in this context, nine variables stand out for concept generation to solve this research problem: design types, adaptive façade types, constructability improvement strategies, standard product ratio, functional requirements, performance requirements, demand for customisation, production volume and project budget (emmitt, olie, & schmid, 2004; charles, crane, & furness, 2001; eekhout, 2008; dieter & schmidt, 2012; jensen, 2014; firesmith, 2015; cantamessa & montagna, 2016; chen, peng, & gu, 2017; başarır & altun, 2017). concepts are generated depending on the value of the choice spectrum for these variables. with respect to this, several concepts are generated, such as open system design, modular system design, and redesign of existing adaptive façades. after a series of different concept solutions are created for the research problem, the next step is to evaluate, compare, and rank them to define the most reasonable concept for development at system level (dandy et al., 2018). in evaluation, the ‘value’, ‘benefit’, or ‘strength’ of a concept is measured according to solution objectives of the research problem. in this research, the aim is to select a solution that leads to the fulfilment of following objectives: low development risk, high development capacity, high façade performance, technical availability, and high standardisation. with respect to these objectives, concept selection criteria are determined as development cost, development time, development capacity, performance, technological availability, and complexity level. generated concepts are evaluated by a weighted decision matrix, and the concept of redesigning dynamic adaptive façades to be manufactured with standard products is chosen for further development. the advantage of redesign is that the product architecture and a part of the new product is known in advance. there are most likely specific areas or problems to focus on, rather than a completely blank slate. redesign solutions are generally more feasible and reliable, since they have already been used successfully in existing systems (han & lee, 2006). it generally focuses on resolving conflicts between current design objectives and reference design capabilities. most techniques start by choosing a reference design that reduces conflicts as much as possible. remaining conflicts, depending upon their degree, are resolved by changing component attributes, replacing components, or changing the structure of the original design (li, kou, cheng, & wang, 2006). 081 journal of facade design & engineering volume 6 / number 3 / 2018 concept level of the research is completed by selecting the concept. at the following system level, the selected concept is further investigated and developed with the focus on identification of the redesign procedure. for development of the redesign concept into a redesign procedure, a researchthrough-design methodology is used. a preliminary case study is conducted to redesign a dynamic adaptive façade to be manufactured with standard products. a systematic design method is not used in this case study. design diary approach (pedgley, 2007) is utilised to capture its process steps. then, these process steps are analysed and grouped, with regard to their intended use and interrelationship. according to this preliminary case study, three fields that need to be improved in the captured redesign process are identified. these are (i) identifying existing parts to be redesigned, (ii) selecting new parts to be used in the redesign, and (iii) solving the contradictions or problems that arise from the reconfiguration process. input captured redesign process is compared with all other processes. missing process steps and actions are identified; depending on their use and applicability, they are either adopted or eliminated. compiled process steps are rearranged according to their functions and separated into phases. development process output adaptive façade redesign procedure outline phase i: planning phase ii: definition of the reference façade phase iii: analysis of the reference façade phase iv: redesign of the reference façade phase v: evaluation of the redesigned façade captured redesign process of preliminary case study adaptive façade design process (attia & bashandy, 2016; attia, 2017) product design process (pahl, beitz & wallace, 1996; jones, 1992; dieter & schmidt, 2012; ulrich & eppinger, 2012) product redesign process (otto & wood, 1998; smith, smith & shen, 2012) reverse engineering process (otto & wood, 1998; abe & starr, 2003) façade design process (oliveria & melhado, 2011; klein, 2013) fig. 1 adaptive façade redesign procedure outline development to systematise and improve the captured redesign process of the preliminary case study, façade design, adaptive façade design, and product design methodologies are reviewed first. captured process steps of the preliminary case study are compared with the reviewed façade design, product design, and redesign process steps, and missing steps and actions are identified. these are subsequently either adopted or eliminated, depending on their use and applicability in the case of adaptive façade design, since not all process steps of product design/redesign are applicable to adaptive façades depending on different characteristics of development processes (jones, 1992; ichida & voigt, 1996; eekhout, 2008). reverse engineering processes, which are used in product redesign to reveal the properties and working principles of the existing products, are adopted in the same manner. compiled process steps are rearranged according to their functions and separated into phases. thus, a 5-phase adaptive façade redesign procedure is outlined (fig.1). then each process phase is developed separately, according to the projected outputs of the phases. 082 journal of facade design & engineering volume 6 / number 3 / 2018 after the redesign procedure has been outlined, studies are initiated on fields that need improvement according to the preliminary case study. approximately sixty design methodologies have been reviewed in the context of this research problem (tomiyama et al., 2009; dieter & schmidt, 2012; tooley & knovel, 2010; eekhout, 2008; ong, nee, & xu, 2008; natee, low, & teo, 2016). since the first field to be improved is the identification of the existing parts to be redesigned through elimination or replacement, research is initially focused on product simplification methods. systematic problemsolving and design improvement methods related to manufacture and assembly are analysed to determine which of them could be utilised to improve constructability through simplification. based on this, the design for manufacture and assembly (dfma) method, which focuses on the same goals as the constructability concept, developed by o’connor, rusch, and schulz (1987), and intended to adapt into architectural design in various researches to increase the constructability (fox, marsh, & cockerham, 2001; gerth, boqvist, bjelkemyr, & lindberg, 2013), is selected to be adapted into the redesign process. dfma is a design-review method with two components: design for manufacture (dfm) and design for assembly (dfa). dfma has three beneficial impacts on design: (i) reducing the number of parts, (ii) reducing the costs, and (iii) increasing reliability and quality of design through the simplified production process. in order to simplify a product’s structure, the dfa method recommends a functional analysis of each part in the assembly to identify and eliminate parts that do not exist for fundamental reasons. furthermore, dfma manuals comprise comparison metrics for generic material, process, and component types and design evaluation metrics. (otto & wood, 1998) elimination or replacement of parts and reconfiguration of the system during the redesign process can lead to contradictions/problems which require design revisions. to support that process, systematic problem-solving methods are analysed. theory of inventive problem solving (triz), which is claimed as a powerful support in tackling technical problems and increasing creativity (chechurin & borgianni, 2016), is selected for adaptation to the redesign process. the method works by restating the specific design task in a more general way and then selecting generic solutions from identified principles, previously-identified evolutionary patterns, and databases of designs and patents collected and abstracted from a wide range of technologies. trlz provides several problemsolving tools, such as inventive principles for overcoming technical contradictions, separation principles for overcoming physical contradictions, inventive standards or scientific effects for coping with a missing function, and trends of technological evolution for solving technical and physical contradictions (lucchetta bariani, & knight, 2005). to develop the fields that were determined through the preliminary case study, the above-mentioned modules and tools of the dfma and triz methods, which are expedient for research purposes, are integrated into the redesign procedure outline. furthermore, to support the selection of parts for replacement in redesign, part selection factors are compiled from literature. by adding checklists and templates to the design steps, improvements are made to facilitate the implementation of the redesign procedure. for a detailed examination, each phase of the procedure is subjected to application testing. an iterative loop of development, application test, and review process is carried out for development of the process steps. the steps that are taken in the development of the redesign procedure, depending on the phase development are shown in detail in the following figures (fig. 2, fig. 3, fig. 4, fig. 5, and fig. 6). 083 journal of facade design & engineering volume 6 / number 3 / 2018 façade design, predesign/brief process (oliveria & melhado, 2011; klein, 2013) product design, planning and clarifying the task process (pahl, beitz & wallace, 1996; dieter & schmidt, 2012) product design objectives checklist (pugh, 1990; roozenburg & eekels,1995) adaptive façade classification checklist (başarır & altun, 2017) façade design decisions (gowri, 1990; brock, 2005; smith, 2010) input process steps are adapted to redesign purposes. a comprehensive design objectives checklist is generated by merging and refining criteria. phase i: planning development process output fig. 2 phase i: planning, development of process steps product redesign, reverse engineering process (otto & wood, 1998; abe & starr, 2003; smith, smith & shen, 2012) din 8593 manufacturing processes joining (schwede & störl, 2016) design objectives checklist (from phase i) adaptive façade classification checklist (başarır & altun, 2017) bill of materials (bom) (otto & wood, 1998; liu & fisher, 1994) input process steps are refined and adapted to adaptive façade redesign. bom template is generated by compiling and adding the analysis criteria to support analysis and redesign phases. phase ii: definition of the reference façade development process output utilised to identify connections to form the assembly diagram. fig. 3 phase ii: definition of the reference façade, development of process steps 084 journal of facade design & engineering volume 6 / number 3 / 2018 product redesign, modeling & analysis process (otto & wood, 1998; smith, smith & shen, 2012) constructability criteria (cii, 1986; ciria, 1983; tatum, 1987; adams, 1989; allen, 1993; asce, 1991; o’connor et al., 1987) evaluation methods (dieter & schmidt, 2012; gerth et al., 2013; natee, low & teo, 2016) dfa method (leaney & wittenberg, 1992; lucchetta et al., 2005; lefever & wood, 1996) material availability assessment questions (juvinall & marshek, 2012) input process steps are refined and adapted to adaptive façade analysis to identify redesign focus. phase iii: analysis of the reference façade development process output constructability criteria that support the façade design phase are compiled and 22 criteria under 10 main topics are defined for design evaluation. weighted decision matrix method is chosen to evaluate constructability of the façade. experts who should evaluate the constructability are identified. functional analysis according to theoretical minimum number of parts criteria is adopted. availability questions are adapted for equipment and part availability. fig. 4 phase iii: analysis of the reference façade, development of process steps façade design, execution and detailing process (oliveria & melhado, 2011; klein, 2013) product redesign (otto & wood, 1998; smith, smith & shen, 2012) triz (ong, nee & xu, 2008; dieter & schmidt, 2012; lucchetta et al., 2005; mann & cathain, 2005) input process steps are compiled and refined according to redesign objectives. phase iv: redesign of the reference façade development process output triz contradiction matrix and triz inventive principles is adapted into process. material selection factors are adapted for façade part and material selection. joining analysis tool is adapted. material selection factors (juvinall & marshek, 2012; jahan, edwards & bahraminasab, 2016) dfma method (molloy, warman & tilley, 2012) fig. 5 phase iv: redesign of the reference façade, development of process steps 085 journal of facade design & engineering volume 6 / number 3 / 2018 product design evaluation methods (embodiment, detail, testing and refinement processes) (jones, 1992; dieter & schmidt, 2012; ulrich & eppinger, 2012) constructability evaluation method (from phase iii) input step-wise evaluation approach is selected; first weighted decision matrix constructability comparison, then prototyping and testing process is adopted. phase v: evaluation of the redesigned façade development process output fig. 6 phase v: evaluation of the redesigned façade, development of process steps 3 a redesign procedure to manufacture adaptive façades with standard products a redesign procedure with a structured approach towards manufacturing adaptive façades with standard products is developed as presented in section 2. it is based on the organisation of form, elimination, replacement or addition of parts, and reconfiguration, depending on the design objectives. it consists of five phases and their application steps. even though the process is linear theoretically, there is a back coupling between and within the phases in practice. application steps and outputs of each phase are explained in the following sections. 3.1 phase i: planning the aim of this phase is to determine the design objectives and constraints of the façade required for the developing architectural project, and in this context selecting the most proper existing adaptive façade for redesign. first, factors, namely the design objectives, affecting the decisions of façade design and defining the characteristics of the façade, are revealed. a checklist approach is adopted for that purpose. the checklist consists of a comprehensive list of design objectives with 22 factors, such as built environment conditions, performance requirements, material properties, regulations, standards, building and façade characteristics, aesthetics, and cost per unit. based on the data obtained from the checklist, an existing adaptive façade that most closely meets the design objectives is selected as the reference façade for redesign. 3.2 phase ii: definition of the reference façade an extensive understanding of the reference façade is needed to lead the redesign process. this phase intends to provide an understanding of the design rationale that motivated the existing design and physical system of the reference façade. it leads to a comprehension of the “whys” that motivated the “hows” of the reference façade. definition of the reference façade is achieved through the concept of reverse engineering. reverse engineering, wherein a product is observed, disassembled, analysed, and documented in terms of its form, components, physical principles, functionality, manufacturability, and assemblability, initiates the redesign process. definition studies are based on the design, production, and installation details obtained from the designers, contractors, and manufacturers. 086 journal of facade design & engineering volume 6 / number 3 / 2018 identify reference façade design objectives (fill in adaptive façades design objectives checklist) identify characteristics of the adaptive façade (fill in adaptive façade classification checklist) start definition of the reference façade determine details of the reference façade review manufacturing and construction processes of the reference façade determine necessary inputs for manufacturing and construction processes of the reference façade create bill of materials (bom) of the reference façade (fill in bom template) create assembly diagrams (use din 8593 to identify joints) finish definition of the reference façade evaluate the reference façade according to constructability criteria (fill in constructability evaluation template) start analysis of the reference façade determine the constructability criteria which need to be improved determine essential and non-essential parts (use functional analysis tool of dfa method) identify parts for elimination that relate to the nonrequired functions in the redesign determine availability of the parts (use availability assessment questions) determine availability of the manufacturing and construction processes inputs determine which parts should subjected to redesign according to availability and constructability criteria finish analysis of the reference façade fig. 7 phase ii: definition of reference façade, process flowchart fig. 8 phase iii: analysis of reference façade, process flowchart a comprehensive collection of information on the reference façade is undertaken at this phase. the adaptive façade design objectives checklist structured in the planning phase is utilised to establish the factors that motivated the reference façade design. the adaptive façade classification checklist is used to identify adaptive façade characteristics. details of the façade system are identified and examined. manufacturing and construction processes of the façade system are reviewed and the necessary inputs, such as equipment, labour, and funds for these processes are determined. one of the most important steps in this phase is generating a bill of materials (bom) for the reference façade. bom is used for displaying data inputs and outputs, defining key characteristics of parts and structuring part relationships in the manufacturing industry. the bom of the reference façade is generated according to bom template to support redesign decisions. the bom template contains information about sub-assemblies, parts, part numbers, functions, quantity, unit of measure, materials, manufacturing process, production, and procurement type, which describes if a particular part has been purchased or manufactured. 087 journal of facade design & engineering volume 6 / number 3 / 2018 as well as identifying the parts that form the façade system, connections of the parts with each other and with other building components should be identified. type of joints between façade parts are identified by assigning manufacturing processes according to din 8593, and assembly diagrams are created. the flowchart showing all process steps of the phase is given in fig. 7. upon completion of this phase, all the information necessary for the analysis of the reference façade is defined. 3.3 phase iii: analysis of the reference façade as a characteristic of redesign, the product architecture and a fraction of the redesigned façade system is known in advance, and conversely, the parts that need to be eliminated or replaced by standard products must be determined. identifying which parts are the focus of the redesign is important, as well as recognising the redesign objectives. analysis of the reference façade starts with the constructability evaluation, which is made according to 22 constructability criteria used in the detailing process in architectural design, such as the use of minimum number of parts and the use of readily available products in common sizes and configurations. a constructability evaluation template is generated according to a weighted decision matrix method to support this step. a constructability index is calculated by the constructability evaluation; as the index value converges from zero to one, the level of constructability increases. an important issue to be considered is that the nature of the constructability evaluation mostly depends on the level of expertise of the evaluator (cf. dorst, 2004), therefore choice of the evaluator should be done very carefully. at this point, level of expertise of the designer who is responsible for the redesign should be identified according to the knowledge required about the design, manufacturing and construction processes of the reference façade. if necessary, experts should be identified on subjects that require deeper knowledge. consequently, the evaluation should be carried out by the designer together with an expert team. the purpose of the evaluation is to clarify to what extent the reference design can achieve the constructability criteria and set a course of redesign. based on this evaluation, the constructability criteria, to which the reference façade design should be improved, are determined. generally, simplification, standardisation, use of easy-to-find products, and use of enhanced details are the most prominent constructability criteria for reducing the complexity of the reference façade and supporting production with standard products. the following step of this phase is to determine which parts of the façade will be subject to redesign. dfa function analysis is performed to determine essential and non-essential parts. in this phase of the analysis, technical or economic limitations are largely ignored to encourage breakthrough thinking by removing the mental constraints of existing solutions. then, the parts that provide the functions that are not required in the redesign are defined by comparing the design objectives of redesign and reference design. with the data obtained from the bom, availability of the parts that form the façade is assessed according to the availability questions. availability of manufacturing and construction process inputs is evaluated to determine redesign constraints. the steps of this phase, which analyse the reference façade according to the constructability, functionality, and availability criteria, are shown in fig. 8. 088 journal of facade design & engineering volume 6 / number 3 / 2018 start redesign is there any part related with the nonrequired functions? is there any design objective that the reference façade can not meet? is it necessary to simplify the form? remove the unavailable or non-essential part evaluate the façade about functionality is it acceptable? remove the part related with the nonrequired functions redesign the façade to fulfill design objectives does it cause any technical contradiction? is there any part that needs to be added? use triz innovative principles for overcoming technical contradictions re-arrange the product architecture without the part redesign connections conduct joining analysis (use dfma joining analysis requirements) is it acceptable? finish redesign can one of the existing parts undertake the function? change design to provide simple solutions for the function does it cause any technical contradiction? use triz innovative principles for overcoming technical contradictions search for a new part evaluate the part candidates according to part selection factors select part revise the existing part to perform neccessary functions is it neccessary to redesign connections? finish redesign yes yes no no no yes yes yes no no yes no yes no no yes no yes yes no fig. 9 phase iv: redesign of the reference façade, process flowchart 089 journal of facade design & engineering volume 6 / number 3 / 2018 3.4 phase iv: redesign of the reference façade redesign of a system is a special case of design activity, which includes not only choosing the parts, but also managing their connections, assigning functions, and reconfiguring the system. parametric, adaptive, or original redesign solutions can be achieved according to the changes made in the reference façade. the redesign approach adopted in this research is based on the organisation of form, elimination, replacement or addition of parts, and reconfiguration, depending on the design objectives. first, the parts that provide the functions that are not required depending on the function analysis are removed from the system. if there are functions that the reference façade does not provide, means of meeting these through use of existing parts are sought. the form is arranged to simplify the design. contradictions encountered in the redesign are eliminated with triz tools. new parts are identified as substitutes for those that cannot be supplied feasibly by current sources. part selection factors, such as material properties, cost, and joinability, are used to evaluate candidates. parts are checked for compatibility; their connections are designed and subjected to joining analysis according to dfma joining analysis requirements, such as load bearing capacity, corrosion resistance, and maintainability. the flowchart showing the process steps is given in fig. 9. 3.5 phase v: evaluation of the redesigned façade in this phase, the façade system obtained as a result of the redesign activities is evaluated in relation to the design objectives. a stepwise evaluation approach is performed. first, constructability evaluation and constructability index comparison are conducted. the constructability evaluation of the redesigned system is repeated with the same method used in phase 3. the purpose is to clarify to what extent the constructability of the redesigned façade has changed in relation to specific constructability criteria. if the evaluation results do not meet the design objectives and a significant constructability improvement has not been achieved, redesign iterations are needed. if the constructability improvement is in the acceptable range and the scope of the changes requires the performance of the façade to be tested, then prototyping and performance testing processes are performed according to the test plan. the test plan gives a description of the test types to be performed and outlines when the test will be done. if the performance test results are acceptable, the detailed design is finalised, and documents related to production, assembly, transportation, and operation are fully prepared. 090 journal of facade design & engineering volume 6 / number 3 / 2018 4 a case study application of the redesign procedure is demonstrated through a case study. the actions performed in the process steps depending on the phases of the procedure are described in the following sections. 4.1 application of phase i: planning the aim of this phase is to determine the design objectives of the required façade system and, in this context, to select the most proper existing adaptive façade for redesign. for this purpose, it is recommended that the design objectives checklist be used for a comprehensive identification of the required façade. since, in this case, the selection of the existing adaptive façade to be redesigned is not dependent on any particular project, the design objectives checklist is not needed in this phase. instead, the existing adaptive façade selection is made on the basis of having access to design and production details of the façade that enables the redesign. in this context, the adaptive façade of the training academy, designed by ackermann und partner and located in unterschleißheim, germany, is selected as the reference façade for the case study (fig. 10). it is assumed that the reference façade is to be redesigned for a project in turkey, with consideration given to intellectual property rights. it is known that not all the design parameters of the reference façade are compatible with a project in turkey. even so, to simplify the redesign process, it is assumed that the environmental parameters and the design objectives remain the same for this case study. the focus of the redesign is using standard products and simplifying the system to improve the constructability of the reference façade in market conditions of turkey. a b fig. 10 a) front view and b) corridor view of the adaptive façade of the training academy in unterschleißheim (schulungsgebäude in unterschleißheim, 2018) 091 journal of facade design & engineering volume 6 / number 3 / 2018 4.2 application of phase ii: definition of the reference façade in this phase, the reference façade is defined by application of the process steps shown in fig. 7, in terms of the data and details obtained from the literature (schumacher, schaeffer, & vogt, 2010; schittich, 2005) and the assumptions made based on them. as a first step, design objectives and constraints that are effective in the design of the reference façade are described. here, the design objectives checklist is used to systematically present the data obtained from the literature and to provide a comprehensive description. in the checklist of 22 criteria, the reference façade is defined in the context of 9 criteria; those most relevant for redesign purposes are shown in table 1. following this, the characteristic features that define the change event performed by the adaptive façade are revealed based on the classification checklist. the simplified adaptive façade classification checklist, based on the characteristics of the reference façade, is given in table 2. the details of the adaptive façade are compiled from the literature (fig. 11 and 12). criteria explanatory questions training academy environment to which environmental influences is the façade subjected during the operation, manufacturing, storage, and transportation? wind, temperature, vehicle vibration performance/ functions which function(s) does the façade have to fulfil? be wide enough to allow the passage of vehicles, prevent solar gains, provide panel load support, and automatic movement according to position of sun by what parameters will the functional characteristics be assessed? dimensions, load capacity, movement capacity, solar shading size and weight what are the dimensions of the proposed façade panel? h: 6.67m; w: 2.50m; d: 0.25m what is the weight of the proposed façade panel? 1000kg does production, transport, or use process define limits in relation to the maximum dimensions or weight? explain the potential constraints. be wide enough to allow the passage of vehicles, be within the dimensions of road transfer, and must be lightweight. aesthetics, appearance, and finish what are the aesthetic preferences? should the façade fit in with an architectural style or concept? sail-like sunscreen panels social and political implications is there a social idea that the design should reflect? symbolic value: sail-like sunscreens symbolize the technical mobility of the training academy and symbolize the dynamic mobility of the bmw group. quantity what is the size of the production? 43 units of sunscreen panel table 1 design objectives related with the redesign of the reference façade 092 journal of facade design & engineering volume 6 / number 3 / 2018 classification criteria training academy adaptive facade characteristics elements of adaptation sunscreen (building component) spatial morphology not integrated to the façade; outside of the façade plane agent of adaptation individual inhabitants, exterior environment, solar radiation response to adaptation agent dynamic type of movement rotation size of spatial adaptation metres limit of motion inclusive (180 degrees on the vertical shaft) structural system for dynamic adaptation plate structure swivel around a vertical shaft type of actuator motor-based type of control/operation direct and indirect control system response time seconds to minutes system degree of adaptability hybrid level of architectural visibility (rush classification) visible, with location or orientation change effect of adaptation prevent solar gains degree of performance alteration medium* system complexity level 2* * these assessments are hypothetical; level 2 describes relatively simple systems in the ordinal scale of 1-4 table 2 presentation of the reference façade characteristics, which define the change event according to adaptive façade classification criteria. a b fig. 11 a) section drawing (schittich, 2005) and b) partial view from the bottom of the reference façade sunscreen panel (schittich,2005; schulungsgebäude in unterschleißheim, 2018) 093 journal of facade design & engineering volume 6 / number 3 / 2018 fig. 12 reference façade sunscreen panel cross section detail (adapted from schittich, 2005) *part numbers are linked with the bom and ‘ref’ indicates the parts of the reference design. 1264 width length height 1 rivet 50x16 join parts aluminium standard allow damage free movement 2 aluminium sheet cladding 16 provide sun shading t:3 2400 83 anodized aluminium standard b edge profile assembly 1 rivet 32x2 join parts aluminium standard allow damage free movement 3 edge profile a 1 create stiffness perpendicular to surface 120 125 6670 aluminium custom prevent material deterioration 4 edge profile b 1 create stiffness perpendicular to surface 65 155 6670 aluminium custom prevent material deterioration 5 circular plate a 1 allow joining of parts 10 aluminium custom 6 circular plate b 1 bear structural loads 10 aluminium custom 7 triangular plate 8 transfer load t:9 45 100 aluminium custom 8 tube profile 1 bear structural loads 7320 aluminium standard allow movement d 1 rivet 38x2 join parts aluminium standard allow damage free movement 9 l profile a 2 allow joining of parts 60 (t:5) 2130 100 aluminium custom transfer load 10 l profile b 2 allow joining of parts 60 (t:5) 2130 100 aluminium custom transfer load 11 l profile c 2 allow joining of parts 40 (t:5) 100 100 aluminium standard transfer load 12 l profile d 2 allow joining of parts 40 (t:5) 50 100 aluminium standard transfer load 13 solid rib 2 bear structural loads 235 2215 t:5 aluminium custom create stiffness perpendicular to surface e 1 rivet 38x7 join parts aluminium standard allow damage free movement 14 t profile a 2x7 allow joining of parts 60 (t:5) 2130 100 aluminium custom transfer load 15 t profile b 1x7 allow joining of parts 40 (t:5) 100 100 aluminium standard transfer load 16 t profile c 1x7 allow joining of parts 40 (t:5) 50 100 aluminium standard transfer load 17 hollow rib 1x7 bear structural loads 235 2215 t:5 aluminium custom create stiffness perpendicular to surface project name: training academy in unterschleißheim total part count: part no part name quantity function c vertical shaft assembly dimensions (mm) w ei gh t s up pl ie r u ni t c os t a cladding assembly material manufacturing process ø240 outer; ø140 inner ø250 outer; ø140 inner ø140 outer; ø120 inner top and bottom rib assembly mid ribs assembly fig. 13 the bom of one sunscreen panel of the reference façade 094 journal of facade design & engineering volume 6 / number 3 / 2018 after this point, the processes that the reference façade has passed, in reverse order from the installation at the construction site, are examined and the system is theoretically taken apart. manufacturing and construction processes of the sunscreen panels are investigated with the experts and the necessary inputs, such as equipment and skilled labour, are determined. accordingly, relatively simple equipment is needed in these processes, such as an aluminium welding machine, a rivet machine, and a low-capacity crane. the bom of one sunscreen panel is created according to the bom template and in the order of theoretical take-apart process (fig. 13). with the information obtained from the previous process, the assembly diagram is created by defining the joints of the parts according to din 8593. 4.3 application of phase iii: analysis of the reference façade based on the data compiled at the previous phase, constructability, availability, and function analysis of the reference façade is performed during this phase, to determine the redesign strategy and the parts to be focused on during redesign. the process flow is carried out according to the steps shown in fig. 8. first, the experts to evaluate the constructability of the reference façade, using the approach explained in section 3.3, are chosen. since the sunscreen panels are completely made from aluminium material, constructability evaluation is carried out by aluminium profile and façade manufacturers operating in turkey who are engaged with aluminium processing and have sufficient knowledge about manufacturing and construction processes. as a result of the evaluation, it is stated that due to the sail-like form of sunscreen panels, materials need custom shaping, which complicates the production process. furthermore, the assembly process gets complicated due to the excessive number of assembly parts. in this context, the constructability criteria on which to focus the redesign are chosen to be simplification and standardisation, in order to manufacture the system with readily available products in common sizes and configurations, and with the minimum number of parts for assembly. thereafter, essential and non-essential parts are identified using the dfa functional analysis tool (table 3). essential parts bom part number non-essential parts bom part number aluminium sheet cladding 2 rivets 1 tube profile (base part) 8 edge profiles (a, b) 3, 4 solid ribs 13 circular plates (a, b) 5, 6 hollow ribs 17 triangular plates 7 l profiles (a, b, c, d) 9, 10, 11, 12 t profiles (a, b, c) 14,15,16 table 3 essential and non-essential parts of the reference façade according to dfa functional analysis since the redesign aims to have the same functional characteristics as the reference façade, there are no unrequired functions, nor parts related to them. the availability assessment of the parts is done on an ordinal scale of 1-5, in the context of the answers given to the seven availability questions. the scale defines the cases in which 5 represents the highest, and 1 represents the lowest availability. according to the assessment made with the experts, this value is set at 3 (medium availability), since each part except the aluminium tube requires geometric configuration and custom shaping, and the complexity level of these processes are considered. required equipment in the production, assembly, and installation processes are also available in turkey’s market conditions, but their cost should be considered. 095 journal of facade design & engineering volume 6 / number 3 / 2018 as a result of the analysis carried out in this phase, the following redesign strategies are identified: (i) removal of non-essential parts from the system, (ii) replacement of parts, which cannot be removed from the system and require special shaping, with standard products, and (iii) simplification of the panel form. 4.4 application of phase iv: redesign of the reference façade in the redesign phase, the process given in fig. 9 is repeatedly used and various alternatives are developed within the strategies determined during the analysis phase. the form of the sunscreen is rationalised in such a way that it would not cause a fundamental change at its functions. the form change also removes the necessity of custom shaping of the adjoining parts: t profile a, l profile a and b, which are identified in fig. 13. the next step after the form change is to remove unavailable or non-essential parts from the system. in this context, custom edge profiles are evaluated first. without their functions, the system is not considered acceptable, and the functions could not be transferred to any of the existing parts. therefore, standard products are sought to undertake the functions of these parts. since they provide integrity of the frame and increase its strength by creating stiffness perpendicular to the surface, as well as protecting the edges of the aluminium sheet cladding from deterioration, proper products that could undertake both functions could not be found in the product catalogue survey. so, it is decided that the functions should be met by separate products. with this new point of view, another product catalogue survey is conducted, and this time suitable products are found. since only one profile pair is considered feasible for replacement, product selection assessment is not needed. the function of preventing material deterioration is provided by a standard profile produced for use in another industry, and the function of creating stiffness perpendicular to the surface is provided by a standard u profile. joining of these two profiles is provided by riveting. a joining analysis is performed according to the dfma joining analysis criteria that are highlighted in the context of this detail, such as load bearing capacity, and the joining is found feasible. this constitutes the first redesign alternative and is detailed as shown in fig. 14. fig. 14 redesigned sunscreen panel cross section detail, alternative 1 *part numbers are linked with the reference bom, and “ref” indicates the unmodified parts of the reference façade and ‘rd’ indicates the replaced or modified parts of the redesign. 096 journal of facade design & engineering volume 6 / number 3 / 2018 furthermore, solutions are investigated to reduce the number of parts by transferring the assembly function of the t and l profiles to the ribs. thus, all t and l-section aluminium profiles and the rivets which join them to the ribs could be eliminated from the system. in this context, three solution alternatives are developed: (i) welding aluminium plates to the rib, (ii) bending the edges of the rib to give a shape of l, and (iii) to obtain the t shape at the edges, replacing the original 5mm rib with two 2.5mm ribs which are bent in l form from their edges and riveted to each other. consequently, the whole redesign process resulted in four redesign alternatives. 4.5 application of phase v: evaluation of the redesigned façade the four redesign alternatives that resulted from the redesign process are introduced into the evaluation process during this phase. it is assumed that there is no significant change in the adaptive performance of each alternative, since the movement mechanism, type of movement control, overall dimensions, and the aluminium sheet surface cladding of the sunscreen panels remain unchanged. with regard to the evaluations of the experts, it is revealed that modifying the ribs to undertake the assembly function is a promising idea in terms of reducing the number of parts and assembly steps; however, aluminium welding is not preferred over riveting in terms of application difficulty and cost. furthermore, it is stated that the bending alternatives should be subject to some evaluations to determine their applicability, such as the complexity that the bending process will bring on the rib shaping and calculation of the changing load bearing capacities. as a result of these evaluations, only the first alternative, with form change and part replacement, is subjected to constructability evaluation. the capability of using products in common sizes and configurations, brought by the form change, and replacement of custom profiles with standard profiles, improved the simplification and standardisation scores of the system. on the other hand, number of parts and assembly steps of the system have increased, since the function of the custom profile is fulfilled by two standard profiles and they are joined by riveting. in this respect, the points taken from the use of a minimum number of parts criterion have been reduced. nevertheless, the redesigned sunscreen panel has a higher constructability index than the reference design. it is also expected that the manufacturing costs are reduced by the redesign. consequently, this redesign alternative does not require further evaluation such as performance testing. however, it is considered useful to develop alternatives to reduce the number of the parts. 5 conclusion despite their high environmental performance, practical application of adaptive façades is very limited. the majority of the current examples are developed by engineer-to-order design processes, as project-oriented, custom, and complex solutions. even though its translation into a readyfor-market product is very challenging, this is still considered to be a very promising idea. as a starting point, simple, flexible, and easily accessible solutions are needed to increase the feasibility of adaptive façades. one of these solutions is to simplify the design of adaptive façades using engineered standard products with the least number of parts and layers. in this context, this paper aimed to develop a design procedure to support designing adaptive façades with standard products to improve constructability through simplification. 097 journal of facade design & engineering volume 6 / number 3 / 2018 the research starts by generating concepts for designing adaptive façades to be manufactured using standard products. among several concepts, ‘redesigning dynamic adaptive façades’ is selected for further investigation, in terms of solution goals determined for this research. a preliminary case study is conducted without a systematic method to redesign an adaptive façade to be manufactured with standard products. the steps of the redesign process are captured and analysed, and the aspects that need improvement are revealed. to systematise and improve the captured design process, façade design, product design, product redesign, systematic problem-solving, and design improvement methods are analysed and adapted to the adaptive façade redesign process. thus, a redesign procedure is generated by the combined application of dfma and triz in the synthesis of reverse engineering and redesign processes. subsequent to the procedure development, its application is tested through a case study. each phase is evaluated separately in terms of functionality and ease of application. determining the factors, namely the design objectives, affecting the decisions of façade design of the developing architectural project in phase i, enables a comparison with the design objectives of the existing façades. this makes it possible to recognise the possible contradictions in the first stage of redesign and to take precautions against them. it is also useful for selecting the most proper existing adaptive façade as a reference façade for redesign. furthermore, redesign can be misleading without an extensive understanding of the reference façade. phase ii and iii provide an extensive analysis of the reference façade and become vital in making the right redesign decisions. the checklists, templates, and evaluation criteria given in the procedure ease its application. in general, the process steps are well described and can be easily followed except for some cases described below. among them, the application of phase iv, the redesign, is relatively complicated as it requires multiple iterations to achieve a reasonable solution. nevertheless, the several redesign alternatives that followed as an outcome of the case study have demonstrated that it is applicable and useful from this point of view. phase v provides a framework for evaluation of the redesign. its stepwise evaluation approach avoids unnecessary workload. the case study has resulted in a redesign which has a higher constructability index and a higher potential for feasible manufacturing in turkey’s construction market compared to the reference façade. in this context, the use of the procedure has yielded positive results. the redesign procedure is both product and process focused, representing a structured approach to manufacturing adaptive façades with standard products. it supports the improvement of constructability through system simplification. it is proposed that it be used by the designer responsible for the adaptive façade design, with experts who have a comprehensive knowledge on required subjects, such as materials, production techniques, and local market conditions. it is sequential in theory; each phase produces input for the next. however, multiple iterations within and between the phases may be needed to achieve the best solution. although it is assumed that such systematic methods could restrict creativity and innovation, it is a case-based approach, and use of the procedure may also provoke thought by imposing actions that the designers had not previously conceived. furthermore, the procedure is suitable for expansion. it can accommodate additional tools for design analysis to support unforeseen design objectives. it can also be utilised for original adaptive façade design after determining the product architecture, to analyse and improve the design for manufacturing. besides all the promising features, the procedure has some limitations. the quality of the redesigned adaptive façade cannot be isolated from the reference façade, nor from the level of expertise of the designers using the procedure. therefore, the right choice of experts and reference façade has a great impact on the quality of the redesign. although redesign is a widely used method in product design, its practical application in adaptive façade design is currently limited due to the lack of 098 journal of facade design & engineering volume 6 / number 3 / 2018 detailed information about existing adaptive façades. in addition, the intellectual property rights of the reference façade must be considered in the redesign. moreover, the absence of product databases makes it difficult to select products in a controlled way, which in turn affects the connection design and can give rise to extra design iterations. references abe, t., & starr, p. (2003). teaching the writing and role of specifications via a structured teardown process. design studies, 24(common ground), 475-489. doi:10.1016/s0142-694x(03)00037-1 adams, s. (1989). practical buildability (ciria building design report). london, uk: butterworths -heinemann ltd allen, e. (1993). architectural detailing: function, constructability, aesthetics. hoboken, new jersey: wiley. anderson, d. m. (2014). design for manufacturability: how to use concurrent engineering to rapidly develop low-cost, high-quality products for lean production. [n.p.]: productivity press. asce. the construction management committee, construction division (1991). constructability and constructability programs: white paper. journal of construction engineering and management, 117(1), 67-89. attia s. (2017). evaluation of adaptive facades: the case study of al bahr towers in the uae. qscience connect, shaping qatar’s sustainable built environment, 2 (6), 1-13 retrieved from http://dx.doi.org/10.5339/connect.2017.qgbc.6 attia, s., & bashandy, h. (2016). evaluation of adaptive facades: the case study of agc headquarter in belgium. in eds. belis, j. & louter, c., challenging glass 5 -– conference on architectural and structural applications of glass. ghent, belgium: ghent university. attia, s., favoino, f., loonen, r.c.g.m., petrovski, a., & monge-barrio, a. (2015). adaptive facades system assessment: an initial review. 10th conference on advanced building skins, 3-4 november, 1265-1273, bern, switzerland. başarır, b., & altun, m.c. (2017). a classification approach for adaptive façades. in tavil, a., & celik, o.c. (eds.), icbest istanbul: interdisciplinary perspectives for future building envelopes, istanbul, turkey: istanbul technical university. boer, b. d., ruijg, g., loonen, r. r., trcka, m. m., hensen, j. j., & kornaat, w. (2011). climate adaptive building shells for the future – optimization with an inverse modelling approach. in proceedings eceee summer study 2011, belambra presqu’île de giens, france, june 2011, 1413-1422 bolbroe, c. (2014). adaptive architecture. non-refereed proceedings of the 2nd media architecture biennale conference: world cities. pp:13-16. aarhus, denmark brock, l. (2005). designing the exterior wall: an architectural guide to the vertical envelope. hoboken, n.j: john wiley. cantamessa m., & montagna f. (2016). design and redesign of product architecture. in: management of innovation and product development. london: springer. doi:10.1007/978-1-4471-6723-5_16 charles, j. a., crane, f. a. a., & furness j. a. g. (2001). selection and use of engineering materials. oxford: butterworth-heinemann chechurin, l., & borgianni, y. (2016). understanding triz through the review of top cited publications. computers in industry, 82119-134. doi:10.1016/j.compind.2016.06.002 chen, y., peng, q., & gu, p. (2017). methods and tools for the optimal adaptable design of open-architecture products. the international journal advanced manufacturing technology, (1-4), 991. doi:10.1007/s00170-017-0925-6 cii (1986). constructability: a primer. austin, tx: construction industry institute, university of texas ciria (1983). buildability: an assessment, special publication 26, london: construction industry research and information association. dandy, g., daniell, t., foley, b., & warner, r. (2018). planning and design of engineering systems. boca raton, fl: crc press dieter, g. e., & schmidt, l. c. (2012). engineering design (5th ed.). new york: mcgraw-hill higher education. din 8593 manufacturing processes joining, standard by deutsches institut fur normung e.v. (german national standard), 09/01/2003 dorst, k. (2004). on the problem of design problems problem solving and design expertise. journal of design research, 4(2) eekhout, m. (2008). methodology for product development in architecture. nl: ios press emmitt, s., olie, j., & schmid, p. (2004). principles of architectural detailing. blackwell publishing ltd firesmith, d. (2015). open system architectures: when and where to be closed. retrieved from https://insights.sei.cmu.edu/sei_ blog/2015/10/open-system-architecture-when-and-where-to-be-closed.html fox, s., marsh, l., & cockerham, g. (2001). design for manufacture: a strategy for successful application to buildings. construction management and economics, 19(5), 493-502. doi:10.1080/01446190110044861 gerth, r., boqvist, a., bjelkemyr, m., & lindberg, b. (2013). design for construction: utilizing production experiences in development. construction management and economics, 31:2, 135-150. doi:10.1080/01446193.2012.756142 gosztonyi, s. (2015). adaptive façade – which criteria are needed? in pottgiesser, u., hemmerling, m. & böke, j. (eds.), proceedings of façade 2015 computational optimisation. sweden, europe: hs owl, detmolder schule für architektur und innenarchitektur. gowri, k. (1990). knowledge-based system approach to building envelope design (doctoral dissertation). concordia university. available from: base, ipswich, ma. han, y. h., & lee, k. (2006). a case-based framework for reuse of previous design concepts in conceptual synthesis of mechanisms. computers in industry, 57(4),305-318. doi:10.1016/j.compind.2005.09.005 ichida, t., & voigt, e. c. (1996). product design review: a method for error-free product development. portland, or: productivity press. jahan, a., edwards k.l., & bahraminasab, m. (2016). multi-criteria decision analysis for supporting the selection of engineering materials in product design. oxford, uk; cambridge, ma: butterworth-heinemann, an imprint of elsevier 099 journal of facade design & engineering volume 6 / number 3 / 2018 jensen, p. (2014). configuration of platform architectures in construction (doctoral dissertation). sweden, europe: luleå tekniska universitet, byggkonstruktion och -produktion jones, j. c. (1992). design methods (2. bs. ed.). new york: van nostrand reinhold. juvinall, r. c., & marshek k.m. (2012). fundamentals of machine component design. hoboken, nj: john wiley & sons. klein, t. (2013) integral facade construction-towards a new product architecture for curtain walls (doctoral dissertation). delft technical university, delft 2013. kolarevic, b. (2015). towards architecture of change. in kolarevic, b. & parlac v. (eds.), building dynamics: exploring architecture of change,1-17, new york: routledge koren, y., hu, s. j., peihua g., & shpitalni, m. (2013). open-architecture products. cirp annals, 62(2), 719-729, issn 0007-8506 lawson, b. r. (1970). open and closed ended problem solving in architectural design. in honikman (eds.), 1971. a.p. 1970 conference, london: riba leaney p. g., & wittenberg, g. (1992). design for assembling: the evaluation methods of hitachi, boothroyd and lucas. assembly automation, 12(2), 8-17 lefever, d. d., & wood, k. l. (1996). design for assembly techniques in reverse engineering and redesign. asme design theory and methodology conference. retrieved from https://www.sutd.edu.sg/cmsresource/idc/papers/1996-_design_for_assembly_techniques_in_reverse_engineering_and_redesign-dfa-sop_force_flow.pdf li, z. s., kou, f. h., cheng, x. c., & wang, t. (2006). model-based product redesign. international journal of computer science and network security, 6(1). liu, t.h., & fischer, g.w. (1994). assembly evaluation method for pdes/step-based mechanical systems. journal of design and manufacture, 4, 1-19. loonen, r.c.g.m. (2013). climate adaptive building shells. retrieved from http://pinterest.com/cabsoverview/ loonen, r.c.g.m., favoino, f., hensen, j.l.m., & overend, m. (2017). review of current status, requirements and opportunities for building performance simulation of adaptive facades. journal of building performance simulation, 10:2, 205-223 loonen, r.c.g.m., trcka, m., cóstola, d., & hensen j.l.m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483–493, doi:https://doi.org/10.1016/j.rser.2013.04.016 loonen, r.c.g.m., rico-martinez j.m., favoino f., brzezicki, m., menezo, c., la ferla g., & aelenei, l. (2015). design for façade adaptability – towards a unified and systematic characterization. proc. 10th energy forum advanced building skins, bern, switzerland, 1274–84. lucchetta, g., bariani, p. f., & knight, w. a. (2005). integrated design analysis for product simplification. cirp annals manufacturing technology, 54(1), 147-150. mann, d., & cathain, c. (2005). using triz in architecture: first steps. the triz journal, retrieved from http://triz-journal.com/ using-triz-architecture-first-steps/ meagher, m. (2015). designing for change: the poetic potential of responsive architecture. frontiers of architectural research, 4(2), 159-165. molloy, o., warman, e. a., & tilley, s. (2012). design for manufacturing and assembly: concepts, architectures and implementation. springer science & business media. natee, s., low, s. p., & teo, e. a. (2016). quality function deployment for buildable and sustainable construction. singapore: springer. o’connor, j. t., rusch, s. e., & schulz, m. j. (1987), constructability concepts for engineering and procurement. journal of construction engineering and management, 113(2), 235-248. ogwezi, b., bonser, r., cook, g, & sakula, j. (2011). multifunctional, adaptable facades. tsbe engd conference, tsbe centre, university of reading, whiteknights, rg6 6af, 5th july 2011. oliveira, l. a., & melhado, s. b. (2011). conceptual model for the integrated design of building façades. architectural engineering & design management, 7(3), 190-204. ong, s. k., nee, a. c., & xu, q. l. (2008). design reuse in product development modeling, analysis and optimization. hackensack, nj: world scientific. otto, k.n. & wood, k. l. (1998) product evolution: a reverse engineering and redesign methodology. research in engineering design 10(4), 226-243. pahl, g., beitz, w., & wallace, k. (1996). engineering design: a systematic approach. london: springer. pedgley, o. (2007). capturing and analysing own design activity. design studies, (5), 463. perino, m. & serra, v. (2015). switching from static to adaptable and dynamic building envelopes: a paradigm shift for the energy efficiency in buildings. journal of facade design and engineering, 3 (2), 143-163, doi:10.3233/fde-150039 roozenburg, n. f. m., & eekels, j. (1995). product design: fundamentals and methods. chichester: wiley. schittich, c. (ed.), (2005). schulungsgebäude in unterschleißheim. detail zeitschrift für architektursteel construction, german/ english edition 2005(4), 325-330. schnädelbach, h. (2010). adaptive architecture – a conceptual framework, in proceedings of geelhaar, j., eckardt, f., rudolf, b., zierold, s, & markert, m. (eds.), mediacity: interaction of architecture, media and social phenomena, weimar, germany, 523-555 schulungsgebäude in unterschleißheim (2018, january). retrieved 20 january 2018 from https://inspiration.detail.de/schulungsgebaeude-in-unterschleissheim-107778.html schumacher, m., schaeffer, o., & vogt, m. (2010). move: architecure in motion-dynamic components and elements. basel; london: birkhäuser. schwede, d., & störl, e. (2016). system for the analysis and design for disassembly and recycling in the construction industry. central europe towards sustainable building prague 2016 (cesb16) smith, r. e. (2010). prefab architecture: a guide to modular design and construction. hoboken, n.j.: john wiley & sons. smith, s., smith, g., & shen y.t. (2012) redesign for product innovation. design studies, 33 (2), 160-184, issn 0142-694x 100 journal of facade design & engineering volume 6 / number 3 / 2018 staib, g., dörrhöfer, a., & rosenthal, m. j. (2008). components and systems: modular construction: design, structure, new technologies. münchen: edition detail, institut für internationale architektur-dokumentation; basel/boston: birkhäuser. struck c., almeida m. g., monteiro da silva s., mateus r., lemarchand p., petrovski a., … de wit j. (2015) adaptive facade systems – review of performance requirements, design approaches, use cases and market needs. 10th conference on advanced building skins, 3-4 november, 1254-1264, bern, switzerland. tatum, c. b. (1987). improving constructability during conceptual planning, journal of construction engineering and management asce, 113(2), 191–207 tomiyama, t., gu, p., jin, y., lutters, d., kind, c., & kimura, f. (2009). design methodologies: industrial and educational applications. cirp annals manufacturing technology, 58(2), 543-565. tooley m.h., & knovel (2010). design engineering manual. (1st ed). amsterdam; london; boston: butterworth-heinemann. ulrich, k. t. (1992). the role of product architecture in manufacturing firm. massachusetts institute of technology, sloan school of management. ulrich, k. t., & eppinger, s. d. (2012). product design and development (5th ed.). new york: mcgraw-hill/irwin. vermaas, p. e. (2014). design theories, models and their testing: on the scientific status of design research. in chakrabarti a., & blessing l. t. m. (eds.), an anthology of theories and models of design. london: springer untitled-2 journal of facade design and engineering 3 (2015) 1 doi 10.3233/fde-150030 ios press 1 editorial jfde special icae 2015 dear scientists, engineers and designers, we are proud to announce that the journal of facade design and engineering is becoming a firm partner for the distribution of scientific knowledge of the icae international congress on architectural envelopes, organised by tecnalia san sebastian. tecnalia is one of the founding members of the european facade network efn, and this partnership supports the development of jfde with regards to the discipline of facade design and engineering. this issue of jfde is dedicated to icae 2015, the viith edition of the congress. the contributions have been carefully selected from 32 abstracts, submitted to the scientific section of the conference. subsequently the finished papers have been subjected to the regular blind review process of the journal. at this point, we want to thank our special editors julen astudillo and jose antonio chica for their effort to make this partnership happen. the paper contributions show an interesting selection of approaches to innovative materials, form finding, simulation and climatic concepts. this demonstrates the special character of the discipline we are working in, bridging research, design and practice. facade design and engineering is a peer reviewed, open access journal, funded by the netherlands organisation for scientific research nwo (www.nwo.nl). we see ‘open access’ as the future publishing model. but it certainly requires new financial models which we will have to explore over the coming years. the editors in chief, ulrich knaack tillmann klein issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. i i from city’s station to station city 039 journal of facade design & engineering volume 8 / number 2 / 2020 development of an offsite prefabricated rainscreen façade system for building energy retrofitting stefano avesani1*, annalisa andaloro1, silvia ilardi2, matteo orlandi2, stefano terletti3, roberto fedrizzi1 * corresponding author 1 institute for renewable energy, eurac research, bolzano, italy, stefano.avesani@eurac.edu 2 arup italia, milano, italy 3 halfen italia, bergamo, italy abstract as the european building stock is in evident need of deep energy retrofitting to meet current european decarbonisation targets, the construction market calls for industrialised systems to boost massive renovations and activate economies of scale. the article outlines the development of an offsite fabricated system for building energy refurbishment through rainscreen façade elements. a focus is placed on such elements as they offer excellent system integration possibilities and the opportunity to boost the level of offsite fabrication, compared to other already industrialised façade systems, such as unitised façades. this research was carried out within the framework of buildheat research project, funded by the european union horizon 2020 framework programme. the system concept is based on a systemic approach that combines energy efficiency, multifunctionality, integration of renewable energies, and ease of installation as design drivers. system development has rolled out through different phases, with an increased level of detail. during the schematic design phase, a set of different prefabricated façade panel dimensions were analysed. afterwards, the component and system integration were assessed according to their impacts in terms of energy performance and fulfilment of mandatory technical requirements. as a last step, the most promising technical combinations underwent detailed design to verify construction feasibility and eliminate any bottlenecks during the fabrication phase. results show that the proposed prefabricated solutions allowed: (i) simplified active system integration (photovoltaics, solar thermal, and building services), (ii) ease of installation on site, minimising the impact of renovation actions on occupants without compromising on final quality and reducing installation costs. current limitations to extensive market diffusion of the system are related to two main aspects: (i) the need for on-site adjustments; and (ii) increased manufacturing costs compared to traditional external insulation interventions (e.g. etics). the current cost of the system (2020) is in the range of 3 1.5x the cost of, respectively, an etics or a vented rainscreen façade. however, as a next step, including the life-cycle perspective in the calculation, as well as accounting for economies of scale, the system will be evaluated, expecting a cost figure comparable to the rainscreen façade. keywords renewable energy integration, re-cladding, prefabricated construction, system integration doi 10.7480/jfde.2020.2.4830 040 journal of facade design & engineering volume 8 / number 2 / 2020 1 introduction when tackling deep energy retrofitting interventions of buildings, higher complexities and costs are incurred with respect to lower impact energy retrofitting solutions, due to the number of components to be considered, as well as their interconnection. moreover, building to meet high energy performance standards have to include the res generation, in particular photovoltaic (pv) and solar thermal (st) systems that are widely used. such solutions incur extra costs needed to adapt the design onto traditional building components and to ensure a reliable installation. for this reason, a better economic case for deep renovation has to be found adopting a more systemic approach to renovation. as such, the retrofit action works has the chance to improve energy efficiency, as well as occupants´ comfort and safety, together with increasing the overall building value. such solutions tend to endow the envelope with multiple functions that complement thermal insulation, it being the primary objective of the intervention. specifically, it is possible to include building services – both energy generation and distribution systems – in the package, as well as punctual system terminals or shading systems. on the other hand, the inclusion of system components in the envelope is deemed to make renovation works more complex and impactful on building occupants. this is mainly due to need to access the building from the inside in order to complete ducts and cabling connections. currently used technologies for opaque façade retrofits rely on the use of external thermal insulation composite systems (etics), which do not offer any dedicated technical solutions to integrate pv panels and st collectors and building services components (electric cabling, water piping and air ducting). in addition to the above, etics also require the use of fixed scaffolding infrastructure during the installation phase and strongly rely on the workmanship handcrafting experience to guarantee results that match expected performance and aesthetics. ad hoc products and systems for pv and st building integration bipv-bist state technologies are available (epfl, 2016), having been developed for many years, but are still mainly a niche market. the swot matrix reported in bonato, fedrizzi, d’antoni, and meir (2019) as well as the barriers highlighted in maurer et al. (2018) show how the limits of implementation of innovative solar façade systems can be found in many interdisciplinary aspects, from the economic to the social fields. besides these developments, which focus only on solar integration, research and innovation has been recently developing envelope solution sets, which may be installed with minimal impact and disruption to occupants, despite the inclusion of active systems for energy production and distribution, as described, for example, in andaloro, avesani, belleri, machado, and lovati (2018) and ochs, siegele, dermentzis, and feist, 2015). a list of further research projects developing multifunctional envelope solutions for the retrofit of buildings is reported in d’oca et al., (2018). in fact, the impact of renovation works on building occupants can be significantly reduced when taking advantage of offsite fabrication techniques, which allow the construction site phase to be speeded up and minimise construction works on the indoor space (colinart, bendouma, and glouannec (2019). this is possibly thanks to the anticipated design and engineering effort that has to take place before the component production phase (arashpour, abbasi, arashpour, reza hosseini, & yang, 2017; lu, chen, xue, & pan, 2018). pre-assembled components can be then installed on-site in a shorter time and with less need for supporting structures and works. it must not be neglected that the adoption of such systems requires an accurate energy and geometrical audit, as it is not possible to perform component modifications on site (lattke & cronhjort, 2014; silva, almeida, bragança, & mesquita, 2013). it is easily inferred that the offsite fabricated solutions allow for a shorter construction site duration and support the achievement of higher quality results, due to the 041 journal of facade design & engineering volume 8 / number 2 / 2020 elimination of several potential installation errors that can possibly occur in traditional construction sites (gasparri & aitchison, 2019; ochs et al., 2015; op‘t veld, 2015). finally, offsite production has a potential 20% cost saving for owners, as estimated in bertram et al. (2019). the offsite prefabricated rainscreen façade (orf) concept presented in this paper takes the need for systemic façade retrofit solutions specifically to ease the integration of pv and st components to decrease the building non-renewable final energy consumption, and the benefits offered by the adoption of an offsite fabrication approach, as development drivers. the orf aim is to bridge the technological gap between the traditional etics-based passive façade and the aforementioned r&d experiences of the multifunctional prefabricated façade, through the development of a systemic façade solution based on components mainly available on the market. hence, the use of a customisable standardised frame system has been addressed as a core aspect of the façade concept development. in this sense, the choice of starting the development from a rainscreen façade is justified by the presence of a substructure, meeting the need to host several kinds of cladding elements (passive and active) and offering the potential for offsite production and plugand-play installation. the objectives of the paper are to give an overview of the development process, to present the façade’s main features as well as to discuss the achieved façade technological solution. the paper is structured in different sections, zooming into details of the design and development process. a complete overview of the development process is provided as follows: (i) first, design drivers on which the façade system is developed are presented, together with the schematic design method applied to preliminary technology development; (ii) then, both mandatory and non-mandatory technical requirements for the new façade system are introduced. once the methodology is thoroughly illustrated, façade development results and its main features are showcased: detailed designs are presented, together with the testing of technical requirements and the achievement of performance targets. a deep dive into main components is also made, focusing on the façade interface with existing wall, active systems, anchoring and fixing systems, and cladding. the paper closes with a list of current application possibilities and limitations, as well as providing preliminary insights into cost issues arising during the development phase. cost issues are presented at an aggregated level of detail and will be further investigated at a later stage. however, authors identify the life-cycle approach as an option to overcome current cost limitations. 042 journal of facade design & engineering volume 8 / number 2 / 2020 2 materials and methods 2.1 façade development drivers and design process the offsite prefabricated rainscreen façade (orf) system has been developed based on the ventilated façade concept, exploiting compatibility with market-available construction materials and components, but proposes a project specific panel concept, which has to satisfy the technical requirements of the european building product regulations and the following peculiar development drivers, derived from the research project framework under which this system was developed. the new façade must (i) ease the installation of passive and solar active cladding, in order to assure the implementation of systemic deep energy retrofit, reducing the building’s heating demand while preventing overheating and assuring indoor comfort; (ii) lower the installation effort and the impact on inhabitants for a deep retrofit action, simplifying and standardising the installation of active elements, piping, and ductwork on the outside of the existing façade, with consideration also given to the maintenance operations. this means that both active and passive cladding, as well as the building services components, have to be accessible and replaceable. the façade system developed within the framework of this research must respond to the technical requirements set by the european commission within the h2020 framework programme call topic, under which this work has been developed. the above requirements determine that the novel façade system has to be: (iii) cost-effective, being able to compete on the market; (iv) replicable, able to be easily adapted to a broad portfolio of residential building typologies; (v) flexible, compatible with the commercial passive and active cladding systems (such as photovoltaic panel – pv, and solar thermal collector st) needed by the energy concept of the renovation. consequently, the orf has been developed through a multi-stage design process, grounded on existing literature review from past research projects on prefabricated façade systems for energy retrofit. research projects in the review include, but are not limited to, the following: h2020 more connect, 4rineu, drive0, energy matching. the adopted approach was “research by design”, refining the technical solution according to design drivers set by the h2020 programme and specific project requirements until a shared solution among research partners was found. this complex design process spanned from the preliminary system concept to the definition of the final solution to be applied on-site for two demo case buildings, using an increasing level of detail. based on the above-mentioned background and guided by the design drivers reported here above, the design process started with a schematic design phase. its objective was to identify possible façade technological concepts, with technical features able to fulfil the product, and to address the expected impacts. the three pre-assembled façade concepts are: (i) micro-panels, (ii) sandwich panels, (iii) macro-panel. 2.2 orf system components the main components that constitute the façade system are described in table 1, including their main functional requirements. 043 journal of facade design & engineering volume 8 / number 2 / 2020 table 1 list of opvf system main components and related function details component function description sub structure the system sub-structure is the façade main frame that hosts the functional components (passive and active claddings), connecting the new façade to the existing external wall, as well as retaining the external cladding. main sub-structure requirements are mechanical resistance, to bear dead and wind loads, ease of installation, construction tolerances absorption, accessibility, and removability of the outer layer for maintenance and flexibility purposes. thermal insulation the thermal insulation is needed to increase overall façade thermal resistance and should also have appropriate fire reaction, in order to minimize risk of fire spread along the façade plan. its hygrothermal behaviour is very relevant to avoid condensation risks and therefore to increase durability. passive cladding the façade finishing determines the aesthetic appearance of the building and its durability. the façade system is conceived to host different façade market-available passive claddings using the same type of substructure. active cladding façade integration of st and pv modules is more and more an option to increase the solar energy self-consumption towards a net zero energy building. a number of products and documentation can be found in literature. nevertheless, active cladding integration increases the degree of complexity of the whole facade and requires dedicated engineering effort to optimize system connection to the inside of the building, as well as the integration of distribution systems within the façade. this is one of the main barriers that prevents their diffusion on the market and that the orf want to tackle through the development of a pre-assembled and flexible façade sub-structure. energy and service distributions façade integrated piping, ducting, and cabling distributions can provide each flat the needed thermal energy, fresh air and electric power minimizing the indoor construction works and therefore the impact on inhabitants. the advantage of such kind of integration might be counterweighted by a more complex façade installation phase, as well as requiring accessibility for maintenance. windows the window node solution is a critical step to achieve overall indoor comfort, energy demand reduction as well as durability of the components. two different approaches for the window integration in the prefab façade has been studied, addressing a fully integrated and a non-integrated window. in the first case, the window is hosted in an insulated frame, loaded on the façade prefabricated substructure. in the second, the new window is not integrated in the prefab façade, which is hence a macro-panel, wider than the base one – constituted of cladding elements, insulation layer and substructure shaped around the window hole heat recovery mechanical ventilation the use of a mechanical ventilation system is considered as necessary when pursuing a deep energy retrofit action, to reduce heat losses and increase indoor air quality. decentralized mechanical ventilation unit can also be considered, as they allow easier integration within the façade system, avoiding space and cost consuming indoor ducting installations. 2.3 façade technical requirements after the finalisation of the façade concept, a further list of requirements was defined based on the essential requirements established in the 305/2011 eu directive for construction products (european parliament, 2011) and complemented with the technical standard etag034 for ventilated façades (eota, 2012b). the regulatory assessment skipped the air-tightness requirement verification due to the assumption that the façade as retrofit kit is a second layer on an already existing façade, for this reason such a requirement has less priority than in the case of new construction. this is not true in the case of window integration, which has not been considered in this paper. the macro-panel structure was developed by adopting an integrated design logic, focused on both construction process optimisation and performance achievement. in fact, on the one hand, it was designed in detail, providing information on materials to be used, macro-panel dimension ranges, anchoring to the existing structure and customisation opportunities in terms of cladding options, as well as active component integration. on the other hand, the designed solution has then been verified in terms of technical performance requirements, such as: mechanical, thermal, hygro-thermal and condensation risk, water tightness, impact and wind resistance, and fire reaction. as the orf is still 044 journal of facade design & engineering volume 8 / number 2 / 2020 a non-standard market product, all of the above have been assessed based on calculations and tests undertaken on a series of 1:1 scale mock-ups. the summary of the performance requirements and their related assessment method within the project is provided in table 2. table 2 list of opvf system technical requirements and related assessment method used in the project requirement assessment method mechanical resistance the mechanical resistance of the orf system is measured on the main frame (see “substructure” in tab. 1). the maximum expected deformation has been assessed on the frame elements as representative of failure risks for the whole system. calculation has been made according to the italian decree d.m 14/01/2008 (italian government, 2008)given that one of the demo-site buildings is in italy, which is derived from the current european regulatory framework for mechanical resistance calculation in the construction sector. kpi: maximum deformation in operation [mm], calculated thermal resistance the thermal resistance calculations have been performed following the procedures defined in the en iso 10077:2017 (en iso, 2017). linear thermal loss coefficient has been calculated based on a bi-dimensional parametric analysis, which has informed the final shaping of panel geometry, as seen in the results section. after that, the incidence of thermal bridges generated at panel edges has been evaluated. the orf has been compared against a reference building standard energy renovation case. the façade has been considered as adjacent to the existing one, in contact through a 100 mm-thick continuous soft rockwool insulation layer. the incidence of thermal bridges has been evaluated as difference between the orf façade and a traditional external insulation (reference). kpi: thermal resistance [m² k/w], thermal transmittance [w/m²/k, thermal linear loss coefficient [w/m/k], calculated hygro thermal and condensation risk the hygro-thermal behaviour of the solution has been assessed coupling a steady state glaser diagram and a dynamic state calculation performed with delphin software. the latter allows to consider the hygrothermal dynamic behaviour on a 2d domain, as needed for the presence of the substructure in the insulation panels. calculations are based on some assumptions: short-wave solar radiation is not considered, as well as rainwater flow on the external side of the surface. the simulation time has been set to two full years, considering the first year as a stabilization phase. kpi: cumulated mass of condensation water [g/m³], calculated water tightness water tightness does not need to be certified in the case of a ventilated façade kit, as stated in the etag034 (eota, 2012a). however, a test campaign has been performed, directing a continuous water jet against the surface for 10 minutes with no interruption. this phase has been followed by a visual inspection at all layers. kpi: presence of water drops, visual inspection impact and wind resistance impact resistance has been verified using a hard body impact procedure on the external cladding system, using three different bodies: 0,5 kg hard body plus 3 joules, 1 kg hard body plus 10 joules, 3 kg soft body plus 10 joules. the test has focused especially on the polymer concrete cladding panel, which has been developed as innovative material within the orf project for the passive cladding. on the other hand, this test was not performed on pv and st modules that can also be used as external cladding, being them commercial products equipped with own product declaration and performance certificates. wind resistance has been assessed using a support test bench with a steel frame, where façade modules have been positioned and joints sealed to create an airtight chamber that allows to apply wind pressure or suction. pressure levels up to 3000 pa have been applied. effects of wind pressure on façade system have been assessed through deformation meters located at fixed points and a visual inspection of components after test completion. kpi: presence of breaks, tested fire reaction fire reaction of the pc cladding panel has been assessed at façade system level based on single burning item test (sbi) according to en 13823:2020 (en, 2020). the sbi test is performed directing a flame source with determined firing power (30,7 ± 2,0 kw) generated through propane burning towards the sample to be tested from an interior corner point. the test lasts for 20 minutes, and the assessment process is based on the following set of parameters, aiming at determining the material fire reaction class. kpis: total heat release during the first 600 seconds (thr 600) [mj], fire growth rate index (figra) [kw/s], lateral flame spread (lfs) [m], smoke growth rate index (smogra) [m2/s2]. 045 journal of facade design & engineering volume 8 / number 2 / 2020 2.4 façade cost calculation façade design, manufacturing, and installation costs have been calculated based on expenses incurred during the research project demonstration phase within which the solution was developed. the façade manufacturer noted in a bill of expenses all costs, allowing the system cost per m2 installed to be calculated (table 3). it must be noted that transportation costs have been accounted for in the bill of expenses. however, in larger deployments of the same system, transportation costs should be less impactful, provided that the designers choose manufacturers located within a certain geographical range of operation. table 3 cost analysis breakdown component description 1 anchoring elements commercial products (as per curtain wall) to anchor the macro panel frame to the building structure 2 soft insulation layer insulation material (e.g. rockwool) and related fixings 3 macro panel frame alu profiles needed for the macro panel assembling of the macro panel assembling of the macro panel with all its components 4 rigid insulation layer insulation material and related fixing 5 waterproof layer gaskets to be applied between macro-panels 6 external finishing layer anchoring system that allows the single finishing panel dismantling external finishing material 7 packaging wooden frame and plastic to allow the safe handling of the assembled macro-panels 8 transport truck from the factory to the construction-site 9 site work installation phase, as well required construction site equipment, additional materials, general expenses 3 results the output of the development and design phases are presented in this section, in terms of façade macro-panel features and performance assessment results. 3.1 façade final design 3.1.1 overall façade system features the schematic design phase resulted in three main concepts being investigated, as illustrated in fig. 1. the micro-panel concept (fig. 1 -1) is built on a metal frame structure carrying both the cladding and insulation layer, equipped with a connection element to favour easy anchoring to the substructure. its functioning mimics a vented rainscreen façade, including the use of a mullion substructure to support the panels. the sandwich panel concept (fig. 1 – 2) is based on a pre-assembled multi-layer element including cladding and insulation, directly screwed to the substructure, with no need for an additional frame, conversely to previous case. no air cavity is 046 journal of facade design & engineering volume 8 / number 2 / 2020 present in this configuration. the macro-panel concept (fig. 1 -3) mimics a unitised system and is equipped with a half frame on each edge of the panel. these halves are then coupled with their twins as two panels are positioned adjacent to one another, in both the horizontal and vertical direction. the frame is conceived with multiple slots to allow both flexibility in terms of cladding anchoring and to allocate further layers as per the rainwater protection. in this case, the existing-new façade fixing is realised exclusively through multi-directional brackets. anchoring system substructure supporting structure legend panel insulation layer fixing system external cladding layer 1. micro-panel 100 100 100 300 100 100 100 2. sandwich panel 300 300 300 3. macro-panel 300 300 300 fig. 1 first preliminary concepts of the offsite prefabricated façade. in red pre-assembled panel units, in blue substructures, in green anchoring system, zigzag line for insulation layer, filled black for building slab with respect to the three façade concepts of fig. 1, the refining phase in concept design led to the exclusion of the micro-panel approach, as the latter brought about similar advantages to the other two proposals, but with increased costs generated by the large number of fixing points and increased total length of the framing structure (needed to cover the same façade surface as in the macro-panel scenario). further on, the sandwich panel was also discarded for two main reasons: it did not provide a suitable solution for the simple integration of active systems and it required a complex geometrical solution to panel joining, in order to avoid manual work being performed on site. the above are deemed to generate extra costs in case system integration is a mandatory project requirement and an extra degree of complexity to be managed during both the production and transportation phases. in fact, the rebated shape of the modules is easily subjected to breakage and needs to be handled with special care. hence, as the first result, the preliminary design identified the macro-panel approach as the most suitable to be developed in further detail. specifically, the macro-panel choice was based on three main features: (i) full exploitation of the industrialisation potential, through the replacement of the traditional metal substructure applied in vented rainscreens with the more comprehensive macropanel metal frame, (ii) flexibility both in cladding and active system integration, as well as in panel sizing, as the metal frame can be adapted to different structural needs, (iii) commercial availability of orf system components, to improve replicability and to allow for a partially optimised value chain of the macro-panel, at present limited to the offsite fabrication phase. 047 journal of facade design & engineering volume 8 / number 2 / 2020 in general, the orf macro-panels are conceived to cover the inter-floor distance in height, to allow for pre-determined fixing points at slab level and so they can have variable widths based on the type of cladding element (both passive and active) to be applied and therefore on the desired façade appearance pattern (fig. 2). cladding panel aluminum macro-panel frame continuous soft insulation layer punctual brackets for macro-panel anchoring air ducts, water pipes, electric cables existing wall fig. 2 rendered image of the buildheat façade final concept and its constituent layers fig. 3 concept horizontal cross section of the different layers for the orf system in the passive cladding configuration (moderately ventilated air cavity and cladding) in terms of system thickness, the system requires a minimum of 200 mm front translation of the façade plan, considering at least 80 mm compressible soft insulation to absorb eventual construction tolerances, related to non-verticality or non-horizontality of the existing wall. this insulation layer sizing is also driven by the final façade thermal transmittance to be reached. from this perspective, the possibility to host a rigid second insulation layer in the macro-panel has to be considered. consequently, this information on system thickness has been calibrated on a mediterranean climate as the first demo installation and is subject to verification and variation in case more severe climate conditions apply. the final schematic design of the orf horizontal or vertical section is illustrated in fig.3. 048 journal of facade design & engineering volume 8 / number 2 / 2020 the preassembled façade module is composed of an extruded aluminium frame, running all along the macro-panel edges, which is able to host a rigid second insulation in its thickness and allows for different type of cladding fixings thanks to its peculiar innovative shape. this frame shape (fig. 4) allows for an easy adaptation to several cladding types, spanning from opaque passive to active systems, such as pv or st, mimicking the vented rainscreen physical functioning. a set of horizontal and vertical gaskets can be additionally fixed to the frame dedicated groove, drastically reducing the risk of driven rainwater penetration to the soft insulation layer. fig. 4 detail of the macro-panel aluminium frame in its main parts and functionalities (drawing not to scale) 3.1.2 macro-panel anchoring and cladding fixing following the driver of flexibility in integration, a further key element of the new façade is the standardised cladding fixing to the sub-structure (fig. 5), which allows for easy removability of passive cladding as well as the pv panel. fig. 5 standardised fixing system for removable passive and active (pv) claddings fig. 6 macro-panel frame anchoring system to the concrete slab, as for curtain wall the st fixing has been developed ad hoc using z-shaped steel plates connecting the macro-panel frame to the st. the collector dead-load is however carried by the macro-panel frame transom (or an additional suspended one). the macro-panel anchoring to the existing façade is realised at slab front level through metal brackets (fig. 6), the same kind of commercially available system used for a curtain walling system. 049 journal of facade design & engineering volume 8 / number 2 / 2020 3.1.3 interface managing the interface between the existing façade and the retrofit module is crucial for a wellexecuted retrofit action. as said before, in the orf concept, as has also emerged from literature, this is done by including a compressible insulation layer located between the new and the existing façade. this first insulation layer aims to guarantee continuity of insulation and provide eventual space for active system distribution components, such as cabling, ducting, or piping. at present, this layer is not engineered to be assembled offsite and its installation requires manual work to be performed on site. however, there is room for the industrialisation of this element. 3.1.4 pv and st integration as active cladding the integration of active components was investigated in depth during the concept definition phase, as a relevant parameter to develop a flexible and replicable façade solution for the systemic deep energy retrofitting of buildings. as mentioned, the integration of commercially available pv and st systems were assessed through a technical market analysis. based on these (pv and st main relevant features such as sizes, fixing requirements, cabling and piping, wet connections) were the macro-panel substructure design, the fixing system selection (in order to allow the removability for maintenance purposes), in close contact with pv and st manufacturers, and the use of a dressing cap (as for a curtain wall) to cover the exposed frame view and improve the overall macro-panel aesthetic. all system connections (electrical, wet connections) have been solved using commercial products for plug-in junctions available on the market. the final design of the macro-panel frames is reported in fig. 7, fig. 8 and fig. 9. fig. 7 base macro-panel frame front view. top, mid, and bottom passive cladding fixings’ types are indicated. fig. 8 front view of the st integration in a dedicated macro panel with bottom passive cladding. in this case, an additional removable transom is foreseen as well as dedicated fixing points for the st collector retention against horizontal loads fig. 9 front view of the pv macro-panel hosting a top passive cladding. the pv panel frame is mechanically fixed to the removable façade fixing system (as per the passive claddings) thanks to rivets. 050 journal of facade design & engineering volume 8 / number 2 / 2020 3.1.5 decentralised mechanical ventilation unit on the other hand, the decentralised mechanical ventilation unit façade integration was investigated in more detail, with the aim of designing a brand-new ventilation machine unit, optimising both the machine performance and the impact on building occupants. the maintenance-related accessibility requirement is easily achieved using a vented rainscreen with removable cladding, which was already a core technical requirement related to the flexibility and cladding customisation of the system itself. hence, four different options for decentralised ventilation unit integration have been qualitatively analysed according to machine positioning with respect to the façade and related window holes. they can be described as (in italics, the short name adopted in the table below): (i) in the existing façade, below the window unit under window, (ii) in the existing façade, hanging from the upper ceiling – ceiling hung, (iii) in the existing façade, above the window unit and exploiting the shutter box space, when possible shutter box, (iv) in the new façade, adjacent to the existing one and located below the window unit – new façade. the four scenarios have been compared in terms of technical requirements and functionalities related to both the façade system in general and the ventilation machine integration. table 4 summary of comparative analysis among the four different scenarios identified for decentralized ventilation machine integration under window ceiling hanged shutter box new facade impact * ** ** *** social acceptance ** * ** *** noise protection * * * *** replicability potential * ** ** *** façade thickness *** *** *** * machine thermal losses *** *** *** * ease of maintenance *** *** *** ** construction cost * ** * *** component cost * ** ** ** duct connection *** ** * *** * worst case / ** average case / *** preferred option results are summarised in table 4, ranked from worst to preferred according to configuration issues. as seen in the synthetic table, there is no optimal solution, each scenario carrying both advantages and disadvantages. as a consequence, within the frame of this project the integration of the decentralised ventilation unit as a stand-alone system has been discarded. the integration of a decentralised ventilation unit as a separate system still remains in the range of customisation opportunities to be further investigated in the future. 051 journal of facade design & engineering volume 8 / number 2 / 2020 3.1.6 window integration as noted in table 1, the prefabricated window macro-panel was conceived in two ways. in the first one, the new window is fully integrated into the window macro-panel thanks to the use of an insulating frame made of high density xps, loaded on the macro panel aluminium substructure. the second scenario foresees that the new window installation is done separately from the façade insulation, cladding, and window jamb finishing, which are integrated in the prefabricated macropanel. as a result, this latter approach was finally chosen because of the overall minor complexity of the prefabrication, transport, and installation phases of a prefab macro-panel without the presence of the window onboard. moreover, the macro-panel development explained up to now has led to the design choice of a distance of at least 80 mm between the macro-panel substructure and the existing wall. as a result, it would have been too challenging for the macro-panel to bear the window weight with such a cantilever. finally, a fully integrated window scenario would have added the requirement for airtightness on to the window macro panel. such a feature would have needed a dedicated development that deviated from the original idea of a unique macro-panel sub-structure able to host different kind of components. the final design of a prefab macro-panel that matches the window opening is represented in fig. 10. fig. 10 design of the window macro-panel without the integration of the window 052 journal of facade design & engineering volume 8 / number 2 / 2020 3.2 technical performance assessment results the orf system performances have been assessed based on the requirements identified in the methodology section. the results are briefly summarised in table. 5. table 5 summary of technical performance assessment results requirement assessment results mechanical resistance the extruded aluminium frame shape has been dimensioned in order to obtain a maximum 1/200 of frame length deflection at centre point. this resulted in a total frame length equal to 100 mm, which ensures a sufficiently rigid system to comply with the constraint of maximum deformation within 15 mm (calculated as 1/200 over the 3000 mm slab-to-slab distance). thermal resistance thermal resistance has been verified against a baseline retrofitted wall made of the following layers: internal plaster, 15 mm; hollow brick layer, 80 mm; air cavity, 80 mm; hollow brick layer, 120 mm. the reference building is retrofitted using 160 mm rockwool insulation, while the orf is characterized by the same amount of insulation plus the external extruded aluminium frame. the calculated u value for the reference retrofit and orf are respectively equal to 0.189 w/m²/k and 0.217 w/m²/k. the psi-value (thermal linear loss coefficient) for the orf is equal to 0.0167 w/m/k. hence, the effect of thermal bridge and consequently reduced average u-value needs to be accounted for when designing the retrofit action. as a further development option, thermal break within the frame or a different framing material, such as timber, could be considered to mitigate the impact of the thermal discontinuity on the overall energy performance. overall, a traditional etics system results more convenient in terms of cost/thermal performance ratio. however, the design driver was to push the industrialized approach towards a plug-and-play façade system. in this case, energy performance falls within regulatory limits but is not pushed to the maximum achievable performance levels. hygro-thermal and condensation risk based on both the steady-state and dynamic calculations, the eventual risk of surface condensation is not expected. however, when wall materials with very low or zero vapor resistance are used in the existing wall, a high moisture transfer towards the outside could occur. in this configuration, in a limited number of peak conditions over the two-year simulation timeframe, the 95 % threshold in relative humidity could be passed at the contact between insulation and the metal frame. however, the spatial integral calculated shows that the surface condensation phase is rapidly followed by a drying phase, with no accumulation foreseen in the insulation material. as a result, the risk of interstitial condensation is also is unlikely to happen in the case of existing buildings. in addition, this risk is analogous to what can take place in the case of a vented rainscreen at the fixing locations, wall to substructure interface. water tightness the visual inspection performed after the water jetting test presented in the methodology section has shown water drops have entered the air cavity behind the external cladding in a significant quantity, entering both vertical and horizontal joints of the system. in addition, the test highlighted inappropriate positioning of the anchoring hook of the panel, which was causing water infiltration from the air cavity towards the back of the panel through the frame. this issue has been however easily solved fixing it on the main-frame back without any break of the watertight layer. the test proved that the system composed by the extruded aluminium frame and inserted gaskets was overall working properly in terms of water tightness. impact and wind resistance impact resistance of the system integrating a peculiar innovative polymer concrete cladding has been assessed and determined as class iii, meaning that the orf façade can be installed in building, but not at ground level in areas with public access. impact tests have been performed on three different sized passive cladding panels, namely 813 x 402 mm, 813 x 410 mm, 1210 x 443 mm, and breaks have been registered for the combination of 1 kg + 10 joules on a 1210 x 443 mm panel. wind resistance has been tested up to 3000 pa pressure/suction for all the three cladding options: passive, pv and st. no breaks have been registered. fire reaction fire reaction has been tested on a façade corner specimen, equipped with pc passive cladding, with two façade elements, respectively 1000 x 1500 mm, and 500 x 1500 mm sized. according to test procedure, two joints need to be placed in the larger module (1000 x 1500 mm). one of the prototypes was coated with a cool surface reflective paint, while the other was left uncoated. the coated prototype performed better than the other in terms of both heat release and smoke production. in terms of lateral flame spread, the distance was below module width. the materials did not produce any hot melted drops within the first 600 seconds. as the system was not intended for certification, the procedure was executed on a single occurrence, instead of the three-time repetition prescribed by the regulation for certification purposes. test results were in the range to obtain a b,s1,d0 classification, over the minimum threshold of b,s3,d0 that is required to operate in both the italian and spanish construction markets, which were involved in the project through demo case building. 053 journal of facade design & engineering volume 8 / number 2 / 2020 3.3 façade cost at the end of the development and after the first demonstration in a real building, the costs of system design, manufacturing, and installation are in the range of 1.5 to 3 times the cost for a traditional façade retrofit solution. more specifically, the authors calculated the orf system costs at around 3 times the cost of a baseline etics insulation and 1.5 times the cost of a vented rainscreen insulated system. a detailed cost breakdown of this system is not shown for confidentiality reasons. however, more details on potential optimisation margins are provided in section 4.2.1. 4 orf system strengths and limitations 4.1 overview the presented façade system in its final configuration results in a flexible façade retrofit kit for the energy deep renovation of buildings. the chosen final solution, based on the macro-panel concept, allows for the integration of different cladding solutions, from passive to active (as per mock-up fig.11 and demo building fig. 12 installations). fig. 11 performance mock-up of the façade integration, passive (left), pv (centre), and st (right) claddings fig. 12 first demo building installation in zaragoza (spain), 2019 the system ensures adequate performance, verified in terms of mechanical resistance, thermal resistance, hygrothermal behaviour, water tightness, impact and wind resistance, and fire reaction. easy access to all components across the system section is guaranteed, thanks to the offsite assembly design approach and to the use of a removable cladding fixing system. 054 journal of facade design & engineering volume 8 / number 2 / 2020 the current sizing of the first and second insulation layer, and the frame features, determines a reduction of the whole façade’s thermal resistance compared to traditional etics, which might be not fully appreciated in cold climates. nevertheless, the macro-panel frame has to be considered under a custom-made vision, for which the use of thermal breaks, grooves, and wooden parts can reduce the thermal performance gap with a continuous insulation system, satisfying the customer specifications. the anticipated engineering effort spent during the project phase also allows for a reduction in construction time, as the substructure and the anchoring systems are both working as plug-and-play. the installation on site is performed without scaffoldings, using just cranes and/ or moving platforms. this solution is flexible enough to accommodate intervention on façades up to 30 m in height (i.e. buildings of approximately 10 storey). the first figure relating to timing performance of on-site installation is about 160 m²/day. a reduction in the installation time is therefore one of the most readily quantifiable notable advantages, saving installation costs in terms of manpower, equipment, and construction-site related expenses (e.g. public land occupancy). however, a more detailed analysis based on a series of complete experiences is needed. 4.2 discussion 4.2.1 costs and business models based on the cost categories in table 3, as applied to the demonstration building mentioned above, average façade-specific costs have been calculated to be in the range of 1.5 – 3 times the cost of a traditional façade external retrofit intervention. this is certainly a current limitation to broad market penetration, and the challenges of cost optimisation and value engineering are already being addressed within the frame of the project. in more detail, the orf system cost is in the range of 3 times the cost of an etics system with no external cladding, and 1.5 times the cost of a vented rainscreen with insulated air gap. if the orf system could be deployed to a larger extent in the construction market, economies of scale can be triggered, lowering down the components’ prices. however, comparing the orf, and multifunctional façade systems in general, to etics should be avoided in the future, given the substantial difference in physical behaviour and the number of functionalities that can be integrated exclusively in the orf system, and not in the passive etics system. to sum up, the main justifications to extra costs incurred in implementing the orf are found in the following: (i) the orf system has been implemented within the frame of a research project and only one demo-case building, with a few square meters of façade, implying that economies of scale have not yet been activated; (ii) the orf value chain is still at an early stage of development, currently initiated but still limited to the offsite fabrication phase, resulting in scattered design and assembly process with very low cost optimisation; (iii) added value to the solution, deemed to lower down costs over the life-cycle lie in novel business models based on the circularity principles. such models have been already analysed in orlandi, ilardi, catgiu, and carra (2019). moreover, experiences of “façade leasing”, like the one discussed in prieto, klein, knaack, and auer (2017), are setting the basis for a paradigm shift in façade from product to service. in this light, ease of dismantling and re-installing is a key feature. in this framework, the orf could be a mature technology to be exploited thanks to its features of being easy to dismantle and to re-install. 055 journal of facade design & engineering volume 8 / number 2 / 2020 for the above, a reduction in cost can be reasonably foreseen once the production line is stabilised, in terms of physical location component supply network, experienced manpower for manufacturing and installation, increased manufactured quantities. a more comprehensive value engineering analysis, currently under development, can also demonstrate that the difference in cost at time of intervention is consistently reduced when considering the entire façade lifecycle. the delta can be further mitigated by the inclusion of the commercial value parameter in this analysis. in fact, the offsite fabricated system allows for technical risk reduction, as it minimises construction works to be performed on-site. this impacts the final quality of the result and can be accounted for in the life-cycle analysis when using the multiple-benefit approach. at present, there is no standardised methodology for a coherent cost comparison between the façade energy renovation standard technologies and the offsite fabricated approach. nevertheless, the inclusion of all relevant technical and commercial parameters in the cost comparison analysis is needed and can support the orf system business case. further effort will be dedicated to this specific task in the future. 4.2.2 technical review diving deeper into the technological limitations of the system, which also bear an impact on the total cost of ownership for the system, the authors have identified the use of multiple framing systems according to cladding type as another issue to be tackled in the next developments. the frame section itself is also a main cost consideration, as the profiles have been purposely extruded to couple and create multiple decompression chambers, to allow for the use of a gasket system in a unitised façade fashion. finally, the use of metal in the orf frame could be coupled with a timberbased frame and be used exclusively to provide increased stiffness to a timber-based system. eventually, it could also be totally discarded and just be applied in the system fixing (link to existing wall and panel substructure). however, these options will be investigated as a potential outlook for the medium-term future, also relying on a number of successful renovation actions developed on a timber-based system. in addition, the integration of a decentralised ventilation unit within the macro-panel (conversely to the level of detail reached in the case of both pv and st modules) has only been developed at a preliminary stage. it would be worth further investigation in the future to check whether the added functionality can support the orf system value determination and decrease the cost difference between the non-prefabricated and the prefabricated solution. 4.2.3 processes in terms of the construction process, the orf presents margins for optimisation, as the system at the current level of development still requires a quota of the assembly work to be performed on site prior to macro-panel installation, namely: anchoring of building services distributions, of the brackets at the concrete slabs and of the first insulation layer. 056 journal of facade design & engineering volume 8 / number 2 / 2020 in addition, a construction process issue arises from the presence of the on-site installed insulation at the interface between the existing wall and the new façade system, as it generates manual work that can reduce the impact of benefits brought about by macro-panel offsite assembly. although the soft insulation layer has to be manually placed over the existing façade, movable auxiliary equipment can be used for this task (no need of fix scaffolds). 5 conclusions this article has presented findings from the research and development activities focused on the development of an offsite fabricated system for ventilated façade energy refurbishment that can provide the construction market with a systemic approach to renovation, combining energy efficiency, multifunctionality, integration of renewable energies and ease of installation as design drivers. building heating demand is reduced thanks to a traditional continuous insulation layer and the additional prefab panel, while overheating is prevented thanks to the natural micro-ventilation behind the cladding. the orf system development drivers has allowed the following strength points to be achieved. the designed orf macro-panel frame (i) allows for the installation of many kinds of different passive and active (pv and st) cladding elements, (ii) the orf system showed lowered installation efforts thanks to the high degree of prefabrication of the solution, based on a design-for-assembly and maintenance approach, as well as simplified integration of extra technical equipment besides insulation, such as wires, ducts, and other distribution elements. in addition, the system is also compatible with a variety of cladding types, spanning from passive opaque materials, traditionally applied in vented rainscreens, to active systems for the renewable energy sources exploitation, such as pv or st modules. the proposed façade system solution can be broken down into three main functional layers (from inside to outside): (i) adaptation layer between the existing wall and the new façade system, composed of soft compressible insulation where eventual piping, ducting, and cabling can be hosted in case an energy system renovation is addressed; (ii) offsite fabricated extruded aluminium macro-panel frame, anchored to the existing wall through brackets and adaptable with both passive or active cladding. this layer also includes an additional insulation element to complement thermal resistance provided by the compressible insulation layer; (iii) external cladding, installed offsite with a removable plug-and-play anchoring system based on a combined gravity and mechanical retention system. the functioning principle of the orf is based on the plug-and-play installation approach, so to allow quick and reliable installation as well as permitting easy access to components during building service life. in the current state, this is achieved through the combination of a metal mullion substructure and a metal frame running all-around the panel edges. this is deemed to increase costs and the system could be further optimised in terms of frame typology to be applied according to panel dimension and weight. in addition, the frame structure at current state still requires minor manual operations to be performed on site. 057 journal of facade design & engineering volume 8 / number 2 / 2020 the cost analysis carried out within the project proved that current cost ratio of a traditional façade retrofit solution versus the orf is in the range of 1:2 – 1:3. however, the attempt to quantify specific costs at a coherent level between orf façade and a traditional approach showed that there is no current methodology available to compare costs. this is due to the difference in system intrinsic value, as well as the current difficulty encountered in determining added value generated by the increased quality output the pre-assembled solution can guarantee, together with the ease of assigning multiple functions to the façade. however, a substantial improvement in the economic case for adopting prefabricated systems in façade energy retrofitting could be supported by analysing cost variations when adopting different load bearing structure, insulation, and external cladding system, as well as adopting a life-cycle costing perspective and including the added-value of system integration in the analysis. in terms of future research development, the authors are investigating cost optimisation opportunities in the system. the current manufacturing cost is deemed excessive to allow a broad market penetration of the system. specifically, cost optimisation is being pursued evaluating both alternative configurations of the same concept (frame dimension variations and number of fixings), as well as the use of alternative framing technologies (e.g. combining a timber frame with a metal mechanical fixing). acknowledgements the authors kindly acknowledge ignacio gonzalez perez and cristina criado camargo (acciona), for the deep involvement in the façade system development, especially focusing on the passive cladding and the verification of the technical requirements. this research was undertaken within the buildheat project, which has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no. 680658. the european commission has no liability for any use that may be made of the information it contains. references andaloro, a., avesani, s., belleri, a., machado, m., & lovati, m. (2018). adaptive window block for residential use: optimization of energy matching and user´s comfort. proceedings of the cost action tu1403 adaptive façades network final conference, 9. lucerne: tu delft open. arashpour, m., abbasi, b., arashpour, m., reza hosseini, m., & yang, r. (2017). integrated management of on-site, coordination and off-site uncertainty: theorizing risk analysis within a hybrid project setting. international journal of project management, 35(4), 647–655. https://doi.org/10.1016/j.ijproman.2017.02.016 bertram, n., fuchs, s., mischke, j., palter, r., strube, g., & woetzel, j. (2019). modular construction: from projects to products (p. 34). mckinsey & company. bonato, p., fedrizzi, r., d’antoni, m., & meir, m. (2019). state-of-the-art and swot analysis of building integrated solar envelope systems. iea shc task 56. https://doi.org/10.18777/ieashc-task56-2019-0001 colinart, t., bendouma, m., & glouannec, p. (2019). building renovation with prefabricated ventilated façade element: a case study. energy and buildings, 186, 221–229. https://doi.org/10.1016/j.enbuild.2019.01.033 d’oca, s., ferrante, a., ferrer, c., pernetti, r., gralka, a., sebastian, r., & op ‘t veld, p. (2018). technical, financial, and social barriers and challenges in deep building renovation: integration of lessons learned from the h2020 cluster projects. buildings, 8(12), 174. https://doi.org/10.3390/buildings8120174 en. (2020, may 20). en 13823:2020 reaction to fire tests for building products—building products excluding floorings exposed to the thermal attack by a single burning item. en. en iso. (2017). en iso 10077 thermal performance of windows, doors and shutters—calculation of thermal transmittance—part 2: numerical method for frames. en iso. eota. (2012a). etag 034-1: guideline for european technical approval of kits for external wall claddings—part i: ventilated cladding kits comprising cladding components and associated fixings. (april). eota. (2012b, april). etag 034, guideline for european technical approval of kits for external wall claddings. eota. retrieved from www.eota.eu/en-gb/content/etags/26/ epfl. (2016). iea shc task 41 innovative solar products for building integration. retrieved 3 august 2020, from https://leso2.epfl. ch/solar/index.php?page=home european parliament. (2011, september 3). regulation (eu) no 305/2011 of the european parliament and of the council. official journal of the european union. retrieved from https://eur-lex.europa.eu/eli/reg/2011/305/oj 058 journal of facade design & engineering volume 8 / number 2 / 2020 gasparri, e., & aitchison, m. (2019). unitised timber envelopes. a novel approach to the design of prefabricated mass timber envelopes for multi-storey buildings. journal of building engineering, 26, 100898. https://doi.org/10.1016/j.jobe.2019.100898 italian government. (2008, april 2). dm 14 gennaio 2008. gazzetta ufficiale della repubblica italiana [official journal of the italian republic]. retrieved from https://www.gazzettaufficiale.it/eli/gu/2008/02/04/29/so/30/sg/pdf lattke, f., & cronhjort, y. (2014). smarttes: introduction to a new retrofit method. münchen: technische universität münchen fakultät für architektur. retrieved from http://www.holz.ar.tum.de/fileadmin/w00bne/www/04_forschung/02_abgeschlossen/bmbf033r057_978-3-941370-44-9_introduction_smarttes.pdf lu, w., chen, k., xue, f., & pan, w. (2018). searching for an optimal level of prefabrication in construction: an analytical framework. journal of cleaner production, 201, 236–245. https://doi.org/10.1016/j.jclepro.2018.07.319 maurer, c., hubschneider, c., maurer, c., taveres-cachat, e., hollick, j., lemarchand, p., aagesen, v. (2018). report on barriers for new solar envelope systems. iea shc task 56. https://doi.org/10.18777/ieashc-task56-2018-0001 ochs, f., siegele, d., dermentzis, g., & feist, w. (2015). prefabricated timber frame façade with integrated active components for minimal invasive renovations. energy procedia, 78, 61–66. https://doi.org/10.1016/j.egypro.2015.11.115 op‘t veld, p. (2015). more-connect: development and advanced prefabrication of innovative, multifunctional building envelope elements for modular retrofitting and smart connections. energy procedia, 78, 1057–1062. https://doi.org/10.1016/j. egypro.2015.11.026 orlandi, m., ilardi, s., catgiu, c., & carra, g. (2019). circular economy principles for active façade systems (deliverable no. d3.10b). retrieved from www.buildheat.eu/wp-content/uploads/2020/05/20200302_wp3_d3.10b_pp01_report-on-active-facekit_circula-economy-addendum.pdf prieto, a., klein, t., knaack, u., & auer, t. (2017). main perceived barriers for the development of building service integrated façades: results from an exploratory expert survey. journal of building engineering, 13, 96–106. https://doi.org/10.1016/j. jobe.2017.07.008 silva, p. c. p., almeida, m., bragança, l., & mesquita, v. (2013). development of prefabricated retrofit module towards nearly zero energy buildings. energy and buildings, 56, 115–125. https://doi.org/10.1016/j.enbuild.2012.09.034 journal of facade design and engineering 4 (2016) 77 doi 10.3233/fde-170055 ios press 77 editorial dear jfde readers and authors, after four volumes of jfde we can conclude that jfde is still being seen as a ‘young’ journal just as the scientific area it is aiming to serve. in the past, building envelope related papers have been published in structure or building physics related journals. but with the development of the professional field the scientific community grows as well, and with it jfde is enjoying more and more acknowledgement. one result is that we have been able to become the scientific partner of two conferences: ‘powerskin’ in january 2017 at the bau building trade fair in munich and the icbest international conference on building envelopes systems and technologies, may 2017 in istanbul. both events will be covered in two special issues. this development stimulates us to continuously evaluate the future scope of jfde. in this sense, we are proud to present a new issue of jfde with innovative contributions that target both the design and engineering of building envelopes. two papers focus on a better understanding of facades in a technical sense. the first one predicts light transmission trough complex fenestration systems including high incident directions, the other aims at understanding the increased thermal load, the effect on structural safety of insulated glass units and the implications for norms and regulations. two other contributions focus on novel applications: the first one looks at photocatalytic selfcleaning coatings for building façade maintenance. the application of nano technologies is rather new and it is important to explore the potential it might provide to the discipline. the second one researches biomimetic inspired natural ventilation facades with integrated green. such combination is prototypic of the scope we envision for jfde. a good mixture of technical and innovative contributions, always trying to relate science and practice, design and engineering. the editors in chief, tillmann klein ulrich knaack issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). journal of facade design and engineering 3 (2015) 81–89 doi 10.3233/fde-150032 ios press 81 experiences when employing different alternatives for envelope upgrading p. elguezabal∗ and r. garay tecnalia, sustainable construction division abstract. the challenges of achieving the 2020 goals in terms of energy savings and improving efficiency are guiding numerous research initiatives looking for more insulated envelopes, dealing with thermal performance of insulation materials and envelope systems. nevertheless, the envelope integrates within the building and this improvement on the insulation performance has to be properly adopted, taking into account the interrelation of main elements composing the overall system (facade, frame, slabs, openings, partitions etc.), as well as side effects originated not only for new erected buildings, but specifically in renovation and retrofitting works. this paper describes real experiences when considering various options for upgrading the facade through the increase of the insulation capacity, starting from external overcladding prefabricated panels and ventilated facades, advancing to more sustainable low carbon systems and ending with even more highly insulated solutions employing aerogels. lessons from these cases, where energy and hygrothermal assessments have being carried out, demonstrate the influence of the design and construction phases and the relevance of disregarded effects such as minor thermal bridges, uncontrolled craftsmanship on site, and moisture transfer for the different technologies considered. finally, possible alternatives are provided to overcome some of the detected difficulties, such as combination with non-metallic structural components and building membranes, and being prepared for future challenges and new developments when these isolative elements are combined with other technologies, as for example, renewable energy harvesting devices. keywords: thermal insulation, building envelope renovation, thermal bridges, performance under real conditions 1. introduction according to unep–sbci (united nations environment programme–sustainable buildings and climate initiative, 2015), collectively the building sector is responsible for about 40% of global energy consumption, including 12% of all fresh-water use and it produces up to 40% of our solid waste. residential and commercial buildings consume 60% of the world’s electricity, and the saving potential is quantified between 30 to 80% by the use of commercially available technologies. in such context of high and unsustainable energy consumption, the ‘europe 2020’ growth strategy became the main continental initiative describing the main path for improving this situation. there are three targets defined; reducing greenhouse gas emissions, increasing the share of renewables and improve the energy efficiency. specifically for buildings, the eu directives (energy performance of buildings directives, 2002), (epdb recast, 2010), and more specifically for each of the systems and products composing the building, the eu construction products regulation (construction product regulation [cpr], 2011) appoint the basic requirements for ‘energy economy and heat retention’. ∗corresponding author: p. elguezabal, tecnalia, sustainable construction division. tel.: +34 946 430 850; fax: +34 946 460 900; e-mail: peru.elguezabal@tecnalia.com. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:peru.elguezabal@tecnalia.com 82 p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading considering the dimension of the european building stock, 25 billion m2 of useful floor space in the eu27 (economidou atanasiu, despret, maio, nolte, & rapf, 2011), with an average consumption of 280 kwh/m2 for the non-residential sector, is at least a 40% greater in the residential case. no significant variations of the stock are expected in various generations as in the case of the united kingdom, where up to 75% of the dwellings of the year 2050 already exist now (ravetz, 2008). renovation of buildings has being adopted as one of the main actions to improve the inefficient situation towards the 2020 goals, with four main actuations: • improvement of the envelope’s insulation capacity against external conditions. • improvement of efficiency of the equipment and services, mainly those related with heating, cooling and lighting. • a higher harness of renewable energies by incorporating them into buildings. • improvement of the control and management of the energy demand. depending on the specific case and the initial conditions of the building to be renovated, the cost effectiveness of these actuations varies, but individually or collectively the four measures described are intended to significantly reduce the overall impact of buildings in terms of energy consumption. 2. energy improvement by increasing the insulation capacity the envelope as main heat exchanger element between the building and the external environment is easily responsible for more than 60% of energy losses in a conventional building considering the facade, roof and windows. due to this strategic position this element has a high potential to improve the overall performance of the building by incorporating additional insulation. once the decision of renovating the building is taken, this measure supposes a basic concept with proven results aimed to reduce energy losses. it consists in providing a new layer, or extending the existing one within the section of the envelope, offering higher resistance to the heat flux in order to obtain a more internally insulated environment. usually the first option to adopt, uniquely or combined with any other, is a concept that will in most cases give a return of the investment in a short-medium term. for that purpose, various systems can be used in the external, internal or central section of the facade, such as external thermal insulation systems (etics), ventilated facades, insulation insufflation and internal insulation, but in the end the improvement is guided by the nature of the employed insulation. roughly insulation materials (table 1) can be classified in three main groups: • conventional materials: mostly developed by the chemical industry, the ones that are currently mostly used, due to the availability and well known performance. eps, xps, polyurethane, mineral and glass fibres are considered in this group. • organic insulations: taking into consideration the current existing awareness of using more ecologically friendly solutions, several traditionally employed organic insulations have become popular and some others have been developed based on those ancient concepts. among these, wooden fibers, sheep wool, hemp, cellulose or natural cork are commonly used, using animal or plant source materials, to provide insulation. their usual weakness is the difficulty to fulfil the fire resistance requirements. p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading 83 • super-insulations: based on advanced thermally performing materials, several developments have recently been carried out to provide more highly insulating materials, under the so-called super-insulation term aiming to obtain overall wall u-values below 0,2w/m2k. two solutions are currently leading this concept: aerogels made of low-density silica or carbon and vacuum insulation panels where vacuum is encapsulated in a rigid external frame. however, ideal conditions of insulation cannot be achieved as this material has to be integrated into a specific building part, the facade, interacting with all the building. this leads to new configurations, where traditional construction systems may substantially reduce the thermal performance of such materials when the relevance of thermal bridging in auxiliary elements is increased. 3. kubik as a demonstration platform for efficiency improvements on facade solutions kubik by tecnalia is a an external building test facility oriented for r&d activities aimed at the development of new concepts, products and services to improve the energy efficiency of buildings. the possibility of configuring different realistic scenarios to analyze the energy efficiency of isolated or coupled constructive elements covering the envelope, floors and partitions and their interrelation with building’s hvac and lightning systems, gives to kubik a singularity to better understand the performance at room or at building level. additionally to the thermal performance, assembly and erection procedures are also evaluated and developed as well, especially for industrialized demountable solutions where joints and connections have to be designed to avoid local effects such as thermal bridges. this flexibility for building realistic scenarios with different building configurations using various components and systems, gives to kubik an internal and external variable look as the building changes with the specific testing case (fig. 1). during the technical development of a new or adapted process that starts simulating in virtual scenarios, continues with laboratory testing under standards and controlled conditions and finishes with the market deployment of the product, kubik offers an intermediate step between the lab and real working conditions, allowing to evaluate the product’s performance in a more realistic way prior table 1 main characteristics of insulating materials for building applications thickness range of panels (mm) thermal conductivity λ (w/m◦k) density range kg/m3 eps 5–55 0.046–0.030 10–50 xps 30–180 0.029–0.038 20–50 mineral wool 30–75 0.032–0.04 30–70 pu 25–100 0.023–0.029 30–100 sheep wool 15–100 0.04 13.5/15/20 hemp wool 20–200 0.04 40–60 wood fibres 20–200 0.037–0.044 45–200 cellulose fibre 40–100 0.04 30–65 natural cork 20–200 0.04 120–205 aerogel panels 20–60 0.017 – vips 10–60 0.012 230 84 p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading fig. 1. left: south view of kubik in 2010; right: in 2014, with several facade changes in between. to market deployment. this speeds up the product development and reduces risks of malfunction of highly innovative products or cases without previous experiences on such applications. in the following chapters, three real cases are presented for different research projects that have being carried out in kubik, as part of the development or demonstration of alternatives for energy improvement of currently existing buildings, as well as for the ones to be constructed in the future. 4. ventilated facade and etic as overcladding solutions 4.1. description of the case the solution adopted was to attach directly the insulation to an existing facade in order to improve the overall u-value. the difference between both systems consists of the external layer and the generated air chamber in the case of the ventilated facade that does not exist in the etic solution. the initial facade consists of a brick cavity facade whose heat transfer was experimentally obtained in the kubik research facility. two test-rooms in a vertical arrangement were conditioned in the west-oriented facade, including three beam elements to generate horizontal thermal bridges originated by slabs between storeys. this typology is present on spanish buildings erected between 1960 and 1990 and was designed using equivalent materials and thicknesses to those solutions. the experimentation carried out in the eraikal project (basque government, 2012) assessed the thermal bridge quantification in real conditions determining the influence of the slab thermal bridges in a 20% by linear meter. the variation of the u-value from an initial situation of 1.89w/m2k was improved up to 0.6w/m2k, with a renovation system consisting of 50mm of insulation and 50mm of air chamber (fig. 2). p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading 85 fig. 2. left: brick facade test configuration in kubik (2012); right: configuration of each experimental phase. fig. 3. etic solution’s thermal assessment for different cases of existing facade. as an alternative, an overcladding solution was calculated for the same facade but also for some other situations as well (fig. 3). in this case the system was composed by 80mm of insulation, but with the particularity of being an industrialized etic system. this design combining insulation and low conductivity plastic profiles was developed in the sirein+ project. in this case simulations demonstrated the relevance of the initial situation and the influence of joining systems to obtain thermal break-free solutions. from the initial situation the transmittance was lowered to 0.48w/m2k and the linear thermal bridge coefficient � was demonstrated to be negligible for initial u-values below 1.5w/m2k. 4.2. relevance of the continuity of the insulation the main issue in systems for retrofitting as the ones presented is the combination between various materials and solutions for solving the thermal performance. the relevance of the geometries in two and three dimensions, different densities, conductivities and other effects have a significant influence in terms of the final solution behaviour, as they may interrupt the continuity of the insulation layer. 86 p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading fig. 4. profile arrangements in an internal insulation system. left: vertical; right: crossed. the dynamic effects due to the presence or absence of massive solutions represent a phenomenon that can also have a significant effect in the final behaviour (martin, flores, escudero, apaolaza & sala, 2011). additionally, the use of industrialized solutions highlights the importance of the joints for fulfilling other requirements such as water and air tightness that have to be properly designed to comply with the expected performance (capozzoli, gorrino & corrado, 2013) as well as to avoid the origin of new pathologies such as condensations. 5. highly insulating material in an internal insulation system 5.1. description of the case high performing materials due to their excellent relation between thickness and conductivity provide a very promising development path for internal insulation systems, as they solve in less space the indoor actuation when available surface is usually a valuable asset. in this case, the relevance of the support profile system was studied in the aerocoins project, in order to solve the internal insulation system without losing the benefits provided by the aerogel material. cold formed steel profiles and wood studs as common solutions were studied, and as alternative, composite profiles, which are recently becoming a new constructive element as thermal effective solutions with mechanical properties (facomp, 2013). the position of the profile was also assessed considering vertical and crossed orientations fig. 4. 5.2. the structural frame to support the system can ruin the super insulation effect the study about the influence of the profiling options has demonstrated that depending on the configuration chosen, the increase of the thermal conductivity changes (garay, 2013). for the case p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading 87 fig. 5. left: scheme of the main elements composing the system; right: installed panel in kubik. of vertical steel profiles, this increase varies between 40% to 70%, for wooden studs conductivity is higher by at least 40% as well. some crossed configurations for metal or wooden stud systems increase the overall thermal conductivity in 15%, but require more space for the framing, not aligned with the thin wall concept of such solution. the best solution is the one employing plastic composite profiles, as the decrease of the thermal resistance of the wall results significantly lower (<10%). 6. wooden panels insulated with hemp-lime 6.1. description of the case as a final contribution, a hemp-lime biocomposite, framed in a structural factory-made panel is presented, combined with hemp fibre as main insulator. the benefits of the solution, offering thermal inertia and regulating internal humidity, combined with the breathable insulation, gives to this solution a competitive performance for a low embedded energy solution. the 0.04w/mk conductivity of the fibres combined with 0.074w/mk of the biocomposite provides a very competitive solution with final u-values close to 0.15w/m2k in a 300mm section. this system was installed in kubik at the end of 2014 as part of the hempsec project and is still being monitored (fig. 5). although final results are still not available, similar experiences have demonstrated the overall effectiveness in terms of energy efficiency as well as overall performance for an eco-friendly facade like this, with special fire resistance performance (up to 60min) for such a solution. 6.2. eco-efficient solutions do perform similar to conventional systems for this case the use of materials from the same natural origin integrated into an eco-friendly balanced solution, is likely to perform similar to conventional systems (latif, ciupala & wijeyesekera, 2014), in a panel with more thickness, but offering an interesting alternative for energy efficient envelopes as well as for overcoming the difficulties of fire resistance performance. the performance of 88 p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading the facades and its interrelation with the humidity are being currently assessed as well, highlighting the necessity of providing additional information to the steady state calculated u-values (shea, lawrence & walker, 2012) to get a more detailed performance of the solution. 7. conclusions the increase of insulation is a simple actuation for energy renovation of buildings with a lower cost than most of the possible alternatives for energy improvement of buildings, with a good return of the investment, thanks to the feasibility of the results. different alternatives have been presented in the paper to renovate facades under this scope. firstly overcladding solutions are described highlighting the relevance of thermal bridge treatment in order to obtain a balanced solution, decreasing initial u-values from 1.89w/m2k to a range between 0.48–0.6w/m2k. secondly, the influence of the framing distribution to support an aerogel blanket has been studied, resulting that less conductive composite profiles are those that less affects the final insulation performance, with a decrease below the 10%. finally hemp-lime-based insulated panels are presented as competitive and sustainable solutions with additional features, thanks to the higrothermal performance of such solutions. although there is not a standard solution that better performs in all cases, it is important to understand how these solutions are integrated in order to get as much efficiency as possible. research in the development of new materials with new or improved performances is needed, but it is necessary as well to solve how these are integrated in the building’s system, considering the adjacent elements as well as different stages in the building’s life cycle. a non-coordinated operation can waste all the effort and the investment and what is even worse, create new problems that did not exist initially. finally a better understanding of all these concepts is required, in order to be prepared for future developments when combining isolative solutions with renewable energy harvesting devices, as well as with efficient control and management systems, in line with more complete and energy efficient upgrade measures. acknowledgments the research leading to the results reported in this work has received funding from the european union seventh framework programme fp7/2007–2013, the eco-innovation initiative of the european union, the spanish government and the government of the basque country, under the following projects: hybrid nanomaterials for cost-effective building super-insulation systems (aerocoins, grant agreement no260141), pre-fabricated, pre-dried panelized system of hemp-lime construction (hempsec, eco/12/332972), sistema integral de rehabilitación energética (sirein+), estudio de investigación sobre eficiencia energética y viabilidad de la aplicación de fachadas ventiladas en soluciones de rehabilitación (eraikal-12). references basque government (2012). eraikal 12 project research study in energy efficiency and feasibility of ventilated façades as renovation solution. retrieved april 17, 2015 from http://www.garraioak.ejgv.euskadi.eus/contenidos/informacion/estudiokubik/ eu kubik/adjuntos/kubik3.pdf http://www.garraioak.ejgv.euskadi.eus/contenidos/informacion/estudiokubik/eu_kubik/adjuntos/kubik3.pdf http://www.garraioak.ejgv.euskadi.eus/contenidos/informacion/estudiokubik/eu_kubik/adjuntos/kubik3.pdf p. elguezabal and r. garay / experiences when employing different alternatives for envelope upgrading 89 economidou, m., atanasiu, b., despret, c., maio, j., nolte, i., & rapf, o. (2011). europe’s buildings under the microscope. a country-by-country review of the energy performance of buildings. buildings performance institute europe (bpie). capozzoli, a., gorrino, a., & corrado, v. (2013). a building thermal bridges sensitivity analysis. applied energy, 107, 229-243. council directive 2002/91/ec on the energy performance of buildings. epdb. council directive 2010/31/eu, on the energy performance of buildings. epdb (recast). council regulation (e.u.) no. 305/2011 laying down harmonised conditions for the marketing of construction products. cpr. facomp, polymeric nanocomposite profiles for curtain walls, eu fp7 grant agreement no. 221989. retrieved april 17, 2015 from http://cordis.europa.eu/project/rcn/94166 en.html garay, r. (2013) new challenges on façade insulation systems with superinsulation materials. first workshop. high performance thermal insulation (hpi) – 2013 towards near zero energy buildings. 27th & 28th november. würzburg, germany. retrieved april 17, 2015 from http://www.buildup.eu/sites/default/files/content/workshop first announcement.pdf latif, e., ciupala, m. a., & wijeyesekera, d. c. (2014). the comparative in situ hygrothermal performance of hemp and stone wool insulations in vapour open timber frame wall panels. construction and building materials, 73, 205-213. martin, k., flores, i., escudero, c., apaolaza, a., & sala, j. m. (2011). problems in the calculation of thermal bridges in dynamic conditions, energy and buildings, 43, 529-535. ravetz, j. (2008), state of the stock – what do we know about existing buildings and their future prospects? energy policy, 36(12), 4462-4470. shea, a., lawrence, m. & walker, p. (2012). hygrothermal performance of an experimental hemp-lime building. construction and building materials, 36, 270-275. united nations environment programme – sustainable buildings and climate initiative, retrieved march 30, 2015 from http://www.unep. org/sbci/aboutsbci/background.asp http://cordis.europa.eu/project/rcn/94166_en.html http://www.buildup.eu/sites/default/files/content/workshop_first_announcement.pdf http://www.unep.org/sbci/aboutsbci/background.asp http://www.unep.org/sbci/aboutsbci/background.asp from city’s station to station city 073 journal of facade design & engineering volume 9 / number 2 / 2021 wind forces in overgrown rope façades drag coefficient suggestion for climbing plants based on study review kilian arnold*, susanne gosztonyi, andreas luible * corresponding author lucerne university of applied sciences and arts engineering and architecture, kilian.arnold@hslu.ch abstract modern cities face a climatic problem due to the high proportion of sealed surfaces that increase the urban heat island (uhi) effect. green surfaces offer a way to mitigate the uhi effect, as they positively influence the thermal energy storage and air temperature. to support an increase of green surfaces in the limited resources of cities, vertical spaces, e.g. façades, must be exploited. a possible realisation of a vertical green system are overgrown rope façades. overgrown rope façades have pre-fitted ropes in front of façades on which climbing plants can grow. however, such systems have to deal with dynamic wind forces, which pose static challenges to the climbing system. in order to design such systems for the effective wind forces, so-called drag coefficients of the climbing plants must be known. unfortunately, there are no guidelines or known values that provide such specific drag coefficients for climbing plants. in this study, based on a study review of relevant data for drag coefficients on deciduous and coniferous trees and leaves, findings are made comparable by applying the power function. six critical factors to be considered are identified and a drag coefficient for climbing plants is derived from the investigations on deciduous trees. their transferability to overgrown rope façades is analysed and discussed. keywords green façades, wind forces, overgrown rope façades, drag coefficient, climbing plants, urban greenery, uhi, vertical green systems 10.7480/jfde.2021.2.4831 074 journal of facade design & engineering volume 9 / number 2 / 2021 1 introduction in the year 2050, 66% of the global population will live in cities, thereby inhabiting 3% of the earth’s surface and consuming 60% to 80% of worldwide energy (leal filho et al., 2017). due to the large share of sealed surfaces in cities and the related increase in the heat storage capacity of the surfaces, cities are heating up at a faster pace than rural areas (mohajerani et al., 2018). this phenomenon is known as urban heat island (uhi) effect (gartland, 2012). available data show that cities are heating up due to the uhi by up to 10°c over the average temperature compared to rural zones (santamouris et al., 2001). a promising approach for mitigating the uhi problem involves increased usage of horizontal and vertical green spaces in cities (kolokotsa et al., 2013). vertical green systems can contribute to the decrease of urban temperatures more effectively than roof greening because they influence the thermal absorption of ground level directly, which affects the urban heat island effect, thermal comfort, and building cooling demand substantially more than rooftops facing to the sky (djedjig et al., 2015; alexandri & jones, 2008). for vertical green systems, three further greening approaches exist besides ground-based (direct) vegetation (e.g. ivy) (1); “overgrown rope façades” (2); “balcony boxes greening systems” (3); and “living wall” wall-based surface greening (4), (pfoser, 2016)(see figure 1). fig. 1 different vertical green systems: 1) ground-based; 2) overgrown rope façades; 3) balcony boxes system; 4) living wall. (pfoser, 2016, p. 126/127, fig. 137) & (pfoser, 2018, p. 166-167) this paper focusses on the type “overgrown rope façades” since this vertical greening system belongs to the category of wind flow-through systems. compared to typical vertical green systems their spans of climbing systems with ropes are larger and consequently the forces acting on the building structure are higher. ropes have no inherent rigidity, which means that the wind forces are transferred via tensile forces in the ropes, resulting in very high anchorage loads (meskouris et al., 2012). a climbing aid based on rope constructions is provided in front of façades that allows growth heights of the climbing plants, rooted in the ground, of up to 30m (pfoser, 2016). these climbing systems must be able to bear forces including the wind forces acting upon the plants. in order to determine these wind forces for moving in flow bodies and thus correctly design these systems according to the respective building standards, such as e.g. the swiss standard (sia 261, 2014) or european standard (en 1991-1-4, 2010), so-called drag coefficients c d are required, which have to be calculated as: fw = cd ◊ a◊ r ◊v2 2 [n] (1.1) 075 journal of facade design & engineering volume 9 / number 2 / 2021 where f w is the wind force, c d the drag coefficient, a the projection area, ρ the density, and v the flow speed of the medium. for the calculation of wind forces associated with simple geometries of rigid bodies, suitable drag coefficients can be found in the respective standards for flow force determination. the wind speeds on a building are dependent on the building site as well as the building shape. thus, different speeds arise for different buildings. the research questions discussed in this article are as follows: what is the interaction between the speed of the wind and the resulting force? can the wind force of the climbing plants of overgrown rope façades in the wind flow be calculated by formula (1.1)? if so, what are the drag coefficients of climbers and how do they behave with increasing speed? it is important to determine the correct loads for the climbing systems of overgrown rope façades. the resulting higher loads lead to uneconomic anchorage systems and load bearing structures as well as higher material consumption, thereby entailing aesthetic compromises as well as higher building costs. these factors contribute to lower acceptance of the use of overgrown rope façades in practical implementations. in section 2, the results of a comprehensive literature search are presented, which involves the identification and examination of study data related particularly to drag coefficients of plants. in connection to the data identification, basic rules are presented that contain statements about wind load reductions for overgrown rope façades. in section 3, the data findings are analysed based on a comparative analysis methodology. tables with comparable drag coefficients of coniferous and deciduous trees as well as individual leaves and clusters are established. section 4 focuses on the discussion of the data and approaches in order to identify and apply the critical parameters for an appropriate calculation model. this model envisages six influencing factors and is suggested to enable calculation of dynamic wind forces on overgrown rope façades. the model is developed based on the analysed data. finally, a drag coefficient is suggested as a hypothesis for climbing plants on overgrown rope façades in section 5. 2 data review a literature review has been conducted to get an overview of peer-reviewed articles and guidelines, initially linked to the keywords “wind” and “climbing plants” or “vertical green systems.” further keyword combinations have been added, such as “wind loads” and “plants,” “trees,” as well as “drag coefficient” and “plants,” “leaves,” “trees,” and finally also “cd value” and “plants” to refine the results towards wind dimensioning issues. this approach has led to 58 publications, of which only nine deal specifically with wind forces on plants. most of the publications identified in this search deal with the interrelation between wind and plants without providing insights into specific dimensioning parameters, such as the drag coefficient. the search was done in electronic databases of sciencedirect and googlescholar. the most relevant articles and publications, which are listed in table 1, are examined in more detail in order to understand and compare the approaches for assessing wind forces on plants. the findings of this review are presented in brief in the following subsections, 2.1 to 2.4, and analysed in section 3. 076 journal of facade design & engineering volume 9 / number 2 / 2021 table 1 relevant publications # author(s) main focus and challenges publication type ref. to subsec. 1 mayhead, 1973 drag coefficients for forest trees (fir), size of samples, wind tunnel testing scientific article 2.1 2 rudnicki et al., 2004 static and dynamic drag coefficients for forest trees (fir) over the projection area, wind tunnel testing scientific article 2.1 3 vollsinger et al., 2005 static and dynamic drag coefficients for forest deciduous trees over the projection area, wind tunnel testing scientific article 2.2 4 kane & smiley, 2006 drag coefficients for red maple, size of samples, pick-up truck testing scientific article 2.2 5 koizumi et al., 2009 drag coefficients of poplar crowns under natural condition scientific article 2.2 6 vogel, 1984 drag and flexibility in sessile organisms, e-values scientific article 2.3 7 vogel, 1989 drag coefficients for leaves and cluster of deciduous trees, e-values scientific article 2.3 8 kane et al., 2008 drag coefficients for deciduous trees, e-values, pick up testing scientific article 2.3 9 fll*, 2018 planning of façade greening systems planning guidelines 2.4 * forschungsgesellschaft landschaftsentwicklung landschaftsbau e.v. 2.1 drag coefficient of fir mayhead (1973) investigated the drag coefficients of various fir species in a wind tunnel. the samples had a height of 5.8m to 8.5m. for mayhead (1973), it was important for the fresh samples to be as large as possible since they have a different morphology compared to small, young firs. the effective drag resistance in the airflow of the wind tunnel was measured in the speed range from 9.1m/s to 26.5m/s. the crown area of the samples was determined in still air based on photographs. mayhead (1973) calculates the resistance forces according to formula (1.1) and characterised the course of the drag coefficient c d using a polynomial function according to formula. cd = a + b ◊v + c ◊v 2 [-] (2.1) where v is the flow speed, and a,b and c are plant-specific constants. the drag coefficients calculated from the resistance measurements were determined at various measured speeds. based on the experimental data, mayhead (1973) subsequently determined critical drag coefficients using extrapolations that can be applied in the calculation of critical tree heights (table 2). table 2 values of drag coefficient for use in critical height determinations. extract from (mayhead, 1973, p. 129, table iii) species drag coefficient species drag coefficient grand fir 0.36 scots pine 0.29 sitka spruce 0.35 douglas fir 0.22 norway spruce 0.35 lodgepole pine 0.20 corsican pine 0.32 western hemlock 0.14 077 journal of facade design & engineering volume 9 / number 2 / 2021 in contrast, rudnicki et al. (2004) investigated three different coniferous species in terms of how the different projection area in the flow impacts the drag coefficient. the investigated coniferous species had heights ranging from 2.5m to 5m, but had to be trimmed to the wind tunnel height of 1.9m. they measured the drag resistances in the flow at speeds of 4, 8, 12, 16, and 20m/s at intervals of 30 seconds. the classic formula in this study for wind drag is again the formula (1.1). the measured data were evaluated based on two different models: the static model is based on determination of the projection area of the samples in the stationary state, whereby the drag coefficient is always referenced to the stationary projection area. the dynamic model is based on the variable projection area at different flow speeds, whereby the drag coefficient is referenced to the variable projection area at each measurement speed. the drag coefficients of the static model at 20m/s are shown in table 3. table 3 values of drag coefficient. extract from (rudnicki et al., 2004, p. 670, fig.2) species speed [m/s] drag coefficient red cedar 20 0.2 hemlock 20 0.53 lodgepole pine 20 0.47 2.2 drag coefficients of deciduous trees as in the study by rudnicki et al. (2004), vollsinger et al. (2005) performed tests on deciduous trees in a comparable wind tunnel setting. the classic formula for calculating wind drag is again formula (1.1). they also compared the static model for the drag coefficient with the dynamic model. the samples were 3m to 5m in size and were trimmed to a height of 1.9m for the analysis due to the limited wind tunnel size. they measured the drag resistances in the flow at the same speeds of 4, 8, 12, 16, and 20m/s at 30 second intervals, like in the above presented study. again, the drag coefficients of the static model at 20m/s are shown in table 4. table 4 values of drag coefficient. extract from vollsinger et al (2005, p. 1243). species speed [m/s] drag coefficient paper birch 20 0.15 black cottonwood 20 0.17 red alder 20 0.22 big leaf maple 20 0.26 quaking aspen 20 0.28 in order to allow testing of larger samples, kane and smiley (2006), in another study on deciduous trees, mounted 80 red maple trees with a height of 2.7 – 5.1m on a pickup truck for the purpose of driving over a straight course with the samples. the test course was driven in both directions in order to take the average value of the measurements in both directions. they only performed tests on calm days with wind speeds of less than 2m/s. the projection area of the samples was determined in the stationary state using graphics software (photoshop). the drag coefficient was calculated based 078 journal of facade design & engineering volume 9 / number 2 / 2021 on the force measurements in the ropes, the speed measurement above the pickup truck and the determined projection area. the pickup truck accelerated from 0 to 20m/s, whereby the force values were determined at speeds of 11, 16. and 20m/s. the force f in [n] (average value) for the 5m tall red maple samples was calculated according to formula (2.2), in which the force is proportional to the speed to a power of 1.4 (f∝v1.4) (kane & smiley, 2006, p. 1953). f = 7.81◊v1.4 [n] (2.2) in the third study involving deciduous trees, koizumi et al. (2010) examined the drag coefficient on three free-standing poplar trees. by means of pull pre-tests, the deflection of the trees was determined at the effective pulling force. based on the wind speed measurement and the deflection of the tree during the measurement duration, the effective force was extrapolated under real wind loads. they determined the drag coefficients from these force values and the photograph of the crown projection areas. in their measurements, koizumi et al. (2010) realised wind speeds from 2 to 15m/s. the wind drag is determined by formula (1.1). the determined drag coefficients were interpolated with power functions for which the values are documented in table 5. table 5 drag coefficients of poplar trees. extract from (koizumi, motoyama, sawata, sasaki, & hirai, 2010, p. 192, fig. 6) poplar tree high [m] projected area a [m2] function of drag coefficient c d poplar tree no. 1 13.1 32.5 y = 1.77 ◊ x -0.911 poplar tree no. 2 12.3 22.0 y = 1.14◊ x -0.824 poplar tree no. 3 12.9 29.0 y = 1.79◊ x -0.714 2.3 e-values for the determination of drag coefficients the e-value is referred to as follows: “e represents the exponent to which the speed must be raised to be directly proportional to either the drag coefficient or the drag divided by the square of speed” (vogel, 1989, p. 943). if the e-value in the proportion in formula (2.3) assumes a value of -2, the speed v is cancelled out of the equation such that the force f is no longer proportional to the speed. f v2 µve (2.3) the derivation of the e-value according to (vogel, 1984) is stated as following: “we’re left with formula (2.3) as our baseline, plotting f / v2 (ordinate) versus v (abscissa) and looking for deviations from horizontality. f / v2 might be termed the “speed specific drag”. for regions on a graph without inflection points, the exponent can be derived by a linear regression as the slope of the plot of the logarithms of f / v2 an v. this slope, then, can be taken as a “figure of merit” – the lower (more negative) the value, the more noteworthy the relative reduction of drag as speed is increased; we will denote the slop or exponent as “e”.” (vogel, 1984, p.39). 079 journal of facade design & engineering volume 9 / number 2 / 2021 vogel (1984) calculated the e-value based on various studies of resistance measurements on bodies and plants. the plant species were subjected to a flow comprising the medium of water or air. selected e-values calculated by vogel, which are relevant in connection with plants for overgrown rope façades, are documented in table 6. most of the tests of these selected e-values were carried out at a speed of 8 to 20 m/s. table 6 selected e-values of different species. extract from (vogel, 1984, p. 40, table 1) species e-value species e-value pinus sylvestris -0.72 ilex opaca, branch -0.10 pinus taeda, 1 m high -1.13 pinus taeda, control -1.11 ilex opaca, 1 m high -1.30 pinus taeda, shaken -1.12 pinus taeda, branch -1.16 a few years later, vogel (1989) determined further drag coefficients on individual leaves and clusters of trees. the samples were mounted directly at the outlet of a chaotically turbulent airflow from a pipe. the justification for usage of a turbulent stream of air is that the leaves of the trees are exposed to a highly turbulent airflow, meaning that the drag resistances must be measured in a comparable test environment, i.e. in a turbulent airflow. the tests were performed in a speed range from 10 to 20m/s at individual speed levels of 10, 12.5, 15, 17.5, and 20m/s. in the study by vogel (1989), the resulting drag coefficients were documented in the form of diagrams as well as calculated e-values (table 7). table 7 selected e-values of different species. extract from (vogel, 1989, p. 945, table 2) species e-value species e-value black locust (leaf) -0.52 tuliptree (leaf) -1.18 black walnut (leaf) -0.76 tuliptree (cluster) -0.91 pignut hickory (leaflet) -0.2 white oak (leaf) +0.97 pignut hickory (leaf) -0.78 white oak (cluster) -0.44 red maple (leaf) -0.79 white poplar (cluster) -0.60 red maple (cluster) -0.64 willow oak (cluster) -1.06 kane et al. (2008) repeated the test course of kane & smiley (2006) on three further tree species (freeman maple, shingle oak, and swamp white oak) and calculated the corresponding e-values which they took over from vogel (1984). in the acceleration of the vehicle, attention was paid starting at 11m/s to ensure that the additional effective force caused by the acceleration was linear, so that it can later be subtracted from the drag resistance of the samples. this additional force due to the acceleration is less than 2% of the drag resistance of the samples. the drag resistance f of the samples is proportional to the speed v with an approximate exponent of 1.3 (f∝v1.3). the drag coefficient for the studied tree species is inversely proportional to the speed and varies from species to species. the e-values determined by kane et al. (2008) are documented in table 8. 080 journal of facade design & engineering volume 9 / number 2 / 2021 table 8 e-values of three tree species. extract from (kane, pavlis, harris, & seiler, 2008) tree species e-value freeman maple -0.77 shingle oak -0.70 swamp white oak -0.80 2.4 guidelines for green façades in contrast to the previously presented studies, the guideline set entitled “fassadenbegrünungsrichtlinien” (“green façade guidelines”) (fll, 2018) by the forschungsgesellschaft landschaftsentwicklung landschaftsbau e.v., in brief fll, provides data without any information about their development. the document contains information for wind load dimensioning for climbing plants, including reduction factors (drag coefficients). the reduction factors are based on assumptions about the throughput: “climbing plants are flown through by the wind in a similar way to deciduous trees, even if their foliage is arranged in shingle-like overlapping patterns and the growth is very dense. the reduction factors mentioned above take into account that stronger shoots are fixed to climbing aids or wall surfaces and that plants react therefore less elastically to wind pressure (and suction) than trees” (fll, 2018 p. 92). accordingly, this guideline divides climbing plants into load classes. for each of these load classes, the reduction factors (only drag coefficients) are documented in table 9. table 9 reduction factors (drag coefficients) for climbing plants due to throughput. selected value extract from: (fll, 2018 p. 92, table 7) load class 1 very light 2 light 3 medium 4 heavy 5 very heavy wind loads possible reduction due to throughput (drag coefficients) 0.55 0.6 0.6 0.65 0.7 3 analysis of existing drag coefficients examination of the factors applied in the analysed studies are compared in subsection 3.1. in section 3.2, the mathematical approaches in the studies are reflected, and in section 3.3, a comparison basis is developed. finally, in section 3.4 the study data is shown in comparable values and tables. 3.1 test variables of the examined studies in the examined studies, the test parameters vary. table 10 gives an overview of the various test parameters applied in the studies. the guideline set is omitted in this comparative analysis due to its different approach and lack of comparable factors relating to the test environment. 081 journal of facade design & engineering volume 9 / number 2 / 2021 table 10 different test parameters of the relevant studies author(s) species size of species size of sample high of sample [m] test speed [m/s] airflow test method mayhead, 1973 fir 7 1 4 5.8 – 8.5 9.1 – 26.5* laminar wind tunnel rudnicki et al., 2004 fir 3 8 1.9 4 20 laminar wind tunnel vollsinger et al., 2005 d. trees 5 8 1.9 4 20 laminar wind tunnel kane & smiley, 2006 red maple 1 80 2.7 – 5.1 0 20 laminar pickup truck kane et al., 2008 d. trees 3 13 18 4.4 – 4.8 (mean) 0 – 24.5 laminar pickup truck koizumi et al., 2009 poplar tree 1 3 12.3 – 13.1 2 14 turbulent field test vogel, 1989 leaves 8 5 8 10 20 turbulent pipe * only one sample was tested at a speed of up to 38.3m/s despite the somewhat different test environments and factors, some essential conclusions can be drawn. 3.2 methodical derivation of the mathematical model in almost all reviewed publications of section 2, the flow forces on plants are determined according to formula (1.1), the classical formula for wind drag determination. the drag coefficient characterises the aerodynamic properties of the drag body in the flow. the more aerodynamically the flow surrounding a body behaves, the smaller the drag coefficient gets, resulting in a smaller airflow resistance force according to formula (1.1). the proportional relationship between flow speed and force with the e-value that was examined by vogel (1989, p. 943) is characterised in formula . fw v2 µve æ fw µv 2 ◊ve æ fw µv 2+e (3.1) this proportionality formula (3.1) contains the quadratic speed term that is known from fluid dynamics according to formula (1.1). the e-value represents the deformation capacity of the plants (reduction in projection area at increasing flow speeds) according to vogel (1984). the deformation capacity of the plants at increasing speeds can be characterised by a potential course of the drag coefficient c d , whereby the proportion from formula (3.1) can be applied to the drag coefficient according to formula (3.2). cd µv e (3.2) by inserting the proportion constant b (b-value) of the power function, the proportionality symbol in formula (3.2) is replaced by an equals sign, thereby yielding formula (3.3). alongside the e-value, the b-value is a second constant of the power function that characterises the drag coefficient versus speed. 082 journal of facade design & engineering volume 9 / number 2 / 2021 cd = b ◊v e [-] (3.3) by combining formula (3.3) with formula (1.1), formula (3.4) is obtained. this formula can also be derived by adding the proportion constant 𝐵𝐵 ∙ 𝐴𝐴 ∙ 𝜌𝜌 2 into formula (3.1). fw = b ◊v e ◊ a◊ r ◊v2 2 = b ◊ a◊ r ◊v2+e 2 [n] (3.4) formula (3.4) shows that the wind force does not increase proportionally to the wind speed on a quadratic basis (as is usual) and that the increasing porosity of the plants is taken into account at increasing speeds by the e-values. 3.3 determination of comparison values for the analysis due to the difficulty of comparing the different study results and values documented in section 2, it is necessary to prepare the data for a comparative analysis. thus, the function values for the drag coefficients are determined by interpolating the various study data on the drag coefficients with the least squares method and specification of the function course according to formula (3.3). in cases where no numerical values are documented for the drag coefficients in the studies, individual data points are deduced from the curve progressions of the drag coefficients at different speeds. the quality of the interpolated curve is calculated based on the coefficient of determination r2. the coefficient of determination r2 indicates the extent to which the interpolated function follows the extracted data points. a function with a determination close to 1 perfectly follows the data points, whereas a function with a determination close to 0 does not follow the data pints. figure. 2 shows an example for an interpolation of the data points of drag coefficients for scots pine. fig. 2 interpolation of data points for scots pine, source of the data by (mayhead, 1973) the obtained functions can thus be compared based on the b-value and e-value parameters. to ensure that the functions remain comparable in terms of the magnitude, the function value is specifcoefficients 083 journal of facade design & engineering volume 9 / number 2 / 2021 3.4 analysis of the drag coefficients based on the preparation for the comparison of the various data, as described in subsection 3.3, the results of the examined studies are presented in an adapted comparative form, arranged by the static and dynamic models and by coniferous and deciduous tree species, as well as individual leaves and clusters of deciduous trees. 3.4.1 drag coefficients for coniferous trees the analysis results for the drag coefficients for a static projection area (applying the static model) in different coniferous tree species are documented in table 11. table 11 drag coefficient of the static projection area of fir fir b-value e-value c d c d [v=20m/s] r2 source corsican pine 1 3.88 -0.68 3.88 • v-0.68 0.51 0.995 mayhead, 1973 corsican pine 2 3.23 -0.68 3.23 • v-0.68 0.43 0.998 mayhead, 1973 corsican pine 3 2.45 -0.60 2.45 • v-0.60 0.41 0.998 mayhead, 1973 corsican pine 4 1.42 -0.49 1.42 • v-0.49 0.32 0.991 mayhead, 1973 douglas fir 1 2.34 -0.73 2.34 • v-0.73 0.26 0.989 mayhead, 1973 douglas fir 2 2.92 -0.71 2.92 • v-0.71 0.35 0.989 mayhead, 1973 douglas fir 3 2.02 -0.66 2.02 • v-0.66 0.28 0.989 mayhead, 1973 grand fir 4.50 -0.74 4.50 • v-0.74 0.49 0.999 mayhead, 1973 lodgepole pine 1.94 -0.55 1.94 • v-0.55 0.37 0.918 mayhead, 1973 sitka spruce 1 1.95 -0.48 1.95 • v-0.48 0.46 0.978 mayhead, 1973 sitka spruce 2 2.24 -0.55 2.24 • v-0.55 0.47 0.984 mayhead, 1973 sitka spruce 3 2.61 -0.49 2.61 • v-0.49 0.61 0.966 mayhead, 1973 scots pine 1 2.06 -0.53 2.06 • v-0.53 0.42 0.999 mayhead, 1973 scots pine 2 1.61 -0.48 1.61 • v-0.48 0.38 0.991 mayhead, 1973 scots pine 3 1.55 -0.50 1.55 • v-0.50 0.34 0.972 mayhead, 1973 scots pine 4* 2.29 -0.72 2.29 • v-0.72 0.26 0.972 mayhead, 1973 western hemlock 1 1.59 -0.72 1.59 • v-0.72 0.18 0.988 mayhead, 1973 western hemlock 2 1.43 -0.64 1.43 • v-0.64 0.21 0.974 mayhead, 1973 western hemlock 2.00 -0.87 2.00 • v-0.87 0.55 0.949 rudnicki et al., 2004 western red cedar 2.86 -0.87 2.86 • v-0.87 0.21 0.982 rudnicki et al., 2004 lodgepole pine 2.07 -0.45 2.07 • v-0.45 0.53 0.916 rudnicki et al., 2004 * for this sample, (mayhead, 1973) determined the drag coefficients in a speed range from 9 to 38m/s. 084 journal of facade design & engineering volume 9 / number 2 / 2021 the drag coefficients for the coniferous tree species with the dynamic model, taking into account the variable projection area, are shown in table 12. table 12 drag coefficient of the dynamic projection area of fir fir b-value e-value c d c d [v=20m/s] r2 source western red cedar 1.39 -0.29 1.39 • v-0.29 0.58 0.976 rudnicki et al., 2004 western hemlock 1.39 -0.14 1.39 • v-0.14 0.91 0.949 rudnicki et al., 2004 lodgepole pine 1.36 -0.18 1.36 • v-0.18 0.79 0.916 rudnicki et al., 2004 3.4.2 drag coefficients for deciduous trees as above, the results for the analysed drag coefficients for deciduous trees are divided into results for the static model (table 13) and the dynamic model (table 14). table 13 drag coefficient of the static projection area of deciduous trees tree b-value e-value c d c d [20m/s] r2 source bigleaf maple 2.62 -0.76 2.62 • v-0.76 0.27 0.974 vollsinger et al., 2005 black cottonwood 2.28 -0.85 2.28 • v-0.85 0.18 0.989 vollsinger et al., 2005 paper birch 1.90 -0.82 1.90 • v-0.82 0.16 0.997 vollsinger et al., 2005 red alder 2.41 -0.83 2.41 • v-0.83 0.20 0.999 vollsinger et al., 2005 trembling aspen 1.80 -0.60 1.80 • v-0.60 0.30 0.996 vollsinger et al., 2005 red maple 3.92 -0.63 3.92 • v-0.63 0.59 0.998 kane & smiley, 2006 white oak 4.18 -0.76 4.18 • v-0.76 0.43 0.998 kane et al., 2008 freeman maple 3.91 -0.68 3.91 • v-0.68 0.51 0.999 kane et al., 2008 shingle oak 3.01 -0.62 3.01 • v-0.62 0.51 0.996 kane et al., 2008 poplar tree no. 1 1.77 -0.91 1.77 • v-0.91 0.12 0.997 koizumi et al., 2009 poplar tree no. 2 1.14 -0.82 1.14 • v-0.82 0.10 0.983 koizumi et al., 2009 poplar tree no. 3 1.79 -0.71 1.79 • v-0.71 0.21 0.999 koizumi et al., 2009 table 14 drag coefficient of the dynamic projection area of deciduous trees tree b-value e-value c d c d [20m/s] r2 source big leaf maple 1.21 -0.25 1.21 • v-0.25 0.57 0.827 vollsinger et al., 2005 black cottonwood 1.16 -0.26 1.16 • v-0.26 0.53 0.979 vollsinger et al., 2005 paper birch 0.92 -0.13 0.92 • v-0.13 0.62 0.817 vollsinger et al., 2005 red alder 1.18 -0.29 1.18 • v-0.29 0.50 0.968 vollsinger et al., 2005 trembling aspen 1.14 -0.21 1.14 • v-0.21 0.61 0.949 vollsinger et al., 2005 085 journal of facade design & engineering volume 9 / number 2 / 2021 3.4.3 studies on the flow force on individual leaves and clusters of deciduous trees the analysed drag coefficients for clusters are documented in table 15, while the results for leaves are in table 16. table 15 drag coefficients of clusters of deciduous trees tree b-value e-value c d c d [20m/s] r2 source black locust 0.16 -0.58 0.16 • v-0.58 0.03 0.931 vogel, 1989 black walnut 0.49 -0.89 0.49 • v-0.89 0.03 0.959 vogel, 1989 pignut hickory 1.84 -1.25 1.84 • v-1.25 0.04 0.933 vogel, 1989 red maple 0.74 -0.89 0.74 • v-0.89 0.05 0.979 vogel, 1989 tuliptree 1.09 -1.08 1.09 • v-1.08 0.04 0.965 vogel, 1989 white oak 0.15 -0.10 0.15 • v-0.10 0.11 0.965 vogel, 1989 white poplar 0.46 -0.76 0.46 • v-0.76 0.05 0.981 vogel, 1989 willow oak 0.96 -1.07 0.96 • v-1.07 0.04 0.994 vogel, 1989 table 16 drag coefficients of leaves of deciduous trees tree b-value e-value c d c d [20m/s] r2 source tuliptree 3.19 -1.21 3.19 • v-1.21 0.08 0.986 vogel, 1989 pignut hickory 2.68 -1.27 2.68 • v-1.27 0.06 0.955 vogel, 1989 red maple 1.42 -0.90 1.42 • v-0.90 0.10 0.964 vogel, 1989 white oak 0.02 1.17 0.02 • v-1.17 (0.39) 0.996 vogel, 1989 4 discussion the literature search revealed that no detailed studies on wind force dimensioning for overgrown rope façades could be found. for deciduous and coniferous trees, drag coefficients are generated by various experimental studies that have been reflected and compared. the guideline document, “green façade guidelines” (fll, 2018), however, contains assumption-based reduction factors (drag coefficients) for wind load determination on overgrown rope façades whose physical basis is not clarified. the studies on deciduous and fir trees often take an approach involving the critical wind speed at which the trees are at risk of toppling. in their calculations, the authors set the drag coefficient at the critical wind speed to a constant value. depending on the actual location, environment, building shape, and building height, however, different wind speeds arise for the calculation of wind forces on buildings, as shown, for example, in the swiss standard sia 261 (sia 261, 2014) or in the european standard en 1991-1 to 4 (en 1991-1-4, 2010). in a corresponding calculation of the resultant forces, as suggested in subsection 3.4, these forces would therefore be underestimated or overestimated at different speeds when using a constant drag coefficient. 086 journal of facade design & engineering volume 9 / number 2 / 2021 the dynamic course of the drag coefficient plays a major role in the calculation of the wind forces of plants on buildings. if the drag coefficient is applied as a function in calculating the resultant forces on overgrown rope façades, the exact drag resistance is calculated at each resultant wind speed. 4.1 evaluation of the different methodologies the methodology applied in the examined studies varies, although all studies provide established drag coefficients on plants, which makes them comparable if the relevant parameters in table 10 are taken into account. since the type of plant (fir tree, deciduous tree, or individual leaves) has an influence on the test results, these parameters are divided and shown separately in section 3 and its subsections. with regard to the methodology, it should be noted that when a pickup truck is driven through a stationary mass of air, a laminar flow around the body is established. this means that the tests by kane and smiley (2006) and kane et al. (2008) are laminar flow profiles, just as it applies for the wind tunnel tests by mayhead (1973), rudnicki et al. (2004), and vollsinger et al. (2005). if these studies are broken down to the plant species, they can be directly compared with each other if the sample-specific parameters according to table 10 are taken into account. a turbulent flow profile was used by koizumi et al. (2010) under real conditions and vogel (1989) in the pipe flow of a fan. vogel (1989) provides, however, the only study that examined individual blades. thus, the influence of turbulent pipe flow cannot be identified. 4.2 evaluation of the different factors based on the analysis of the studies in section 3, the following points are discussed, which are assumed to be relevant for determination of the drag coefficients of overgrown rope façades, as well as for the course of the drag coefficient: 1 size of the samples 2 flow profile in the environment 3 projection area 4 course of the drag coefficient 5 e-value 6 b-value 4.2.1 size of the samples using a wind tunnel with a larger cross-section, mayhead (1973) was able to investigate fir samples with heights from 5.8m to 8.5m, whereas rudnicki et al. (2004) only had access to a smaller wind tunnel cross-section and thus needed to trim the fir samples to a height of 1.9m. in the study by rudnicki et al. (2004), drag coefficients were obtained at a flow speed of 20m/s for fir species with values of 0.55 (western hemlock) and 0.53 (lodgepole pine). whereas in the study by mayhead (1973) for the same fir species and the same flow speed, drag coefficients were obtained with values of 0.22 (western hemlock) and 0.36 (lodgepole pine). unlike vollsinger et al. (2005), kane and smiley (2006) and kane et al. (2008) were not subject to any limitations on the size of the samples in their field studies on drag coefficients for deciduous trees. by performing a field test on a moving pickup truck, they were able to test significantly larger 087 journal of facade design & engineering volume 9 / number 2 / 2021 samples. although identical species of deciduous trees were not investigated in these studies. kane and smiley (2006) and kane et al. (2008) obtained a drag coefficient between 0.43 and 0.59 at 20m/s for red maple, white oak, freeman maple, and shingle oak. whereas vollsinger et al. (2005) obtained significantly smaller drag coefficients for a comparable typology of deciduous trees in the range from 0.16 to 0.30 at 20m/s for big leaf maple, black cottonwood, paper birch, and red alder. however, since vollsinger et al. (2005) investigated significantly smaller and younger specimens of deciduous trees, the hypothesis is supported by this comparison that older specimens of deciduous trees exhibit greater stiffness and thus less deformation capacity, thereby resulting in higher drag resistances. older plants therefore absorb considerably more wind energy. it can be assumed that experiments on young plants are not representative, as older plants show significantly higher drag coefficients. koizumi et al. (2010) investigated the drag coefficients for standing poplars under natural conditions. in this study, drag coefficients from 0.10 to 0.21 were obtained at 20m/s. these values for the drag coefficients are significantly smaller than what was obtained in the studies of kane and smiley (2006) and kane et al. (2008). in the study of koizumi et al. (2010), the flow profile is real and not laminar like in the wind channel and pick-up truck studies. this means that a dynamic effect due to the gusty wind flow may be responsible for the lower measured drag coefficients of koizumi et al. (2010). the dynamic interplay between the plant’s deflections in gusts of wind is thus assumed to significantly reduce its resistance. the influence of the flow profile is discussed in section 4.2.2. 4.2.2 flow profile in the environment the study by koizumi et al. (2010) is performed under real wind flow conditions. the natural wind spectrum exhibits gusts, for example, which have a significant influence on the results compared to studies in a laminar constant airflow. koizumi et al. (2010) compared the obtained test results with the test results from the wind tunnel measurements of mayhead (1973). however, the extent to which the parameters of the different airflow had an influence was not discussed by koizumi et al. (2010). it is evident that significantly lower flow speeds are obtained due to the real conditions during their measurement compared to laboratory tests. the lower flow speeds in the study by koizumi et al. (2010) and the resultant uncertainty in the extrapolations could definitely have a further influence on the test results at 20m/s. the analysis shows that for full-grown tree specimens in the natural turbulent airflow, significantly smaller drag coefficients are obtained compared to the wind tunnel studies with laminar airflows. the results of the laminar wind tunnel studies should therefore be on the safe side, as higher drag coefficients result. it is not known whether further influences have an impact on the research results in the tests of koizumi et al. (2010) in addition to the turbulent wind profile. therefore, it is not possible to discuss these results in further detail. however, it can be assumed that for the determination of the drag coefficients on plants of overgrown rope façades, a turbulent flow profile must be applied; otherwise, the drag coefficients are overestimated. 4.2.3 projection area the studies of rudnicki et al. (2004) and vollsinger et al. (2005) compared the models of the static and dynamic projection areas. based on the results, it can be concluded that the dynamic model of the drag coefficients exhibits smaller e-values compared to the static model. in the force determination, the assumption of a dynamic projection area a yields, in addition to the dynamic drag 088 journal of facade design & engineering volume 9 / number 2 / 2021 coefficient, is a further dynamic constant in formula (3.4) that makes the calculation more difficult. in contrast, the assumption of a static (fixed) projection area with dynamic drag coefficients has the advantage of only a single dynamic constant reflecting all of the dynamic influences. therefore, the model approach based on static projection areas appears to be the most suitable one for wind force determination on overgrown rope façades. with exception of table 12 and table 14, the studies analysed in section 3 applied static projection areas in the calculation of the drag coefficients, which means that the study results regarding the projection area are comparable. 4.2.4 course of the drag coefficient according to the reviewed studies, the hypothesis of a potentially decreasing drag coefficient can be confirmed in the wind speed range from 4m/s to 25m/s based on the quality of the functions (r2) with coefficient of determinations close to 1. the power function can be well fitted to the data points in this speed range. in the lowlands of switzerland, for example, peak wind speeds occur for gusts in the wind profile of up to 45m/s (sia 261, 2014). in the speed range from 25 to 45 m/s, no tests were carried out in the studies except for the sample “scots pine 4”. this means that no conclusion can be drawn in this speed range (with the exception of the singular sample) on the agreement of the power function due to the missing data points. in fig. 2, the sample “scots pine 4” is interpolated. this power function can be well fitted to the data points. accordingly, it may be concluded that the analysed drag coefficients can be extrapolated by the power function up to a speed of 45m/s. mayhead (1973) interpolated the drag coefficients with a 2nd degree polynomial function (cp. formula 2.1). the coefficient of determination is better with a 2nd degree polynomial function. the 2nd degree polynomial function has one degree of freedom more than the power function and can thus be better fitted to the data points. polynomial functions are applicable only in a certain range and can oscillate outside of this range. furthermore, the 2nd degree polynomial function does not express any specific e-values, meaning this function does not agree with the model in section 3.2. due to the constant and the linear term of the polynomial function, this function cannot be directly converted to a power function and can thus not be directly compared to the quadratic speed term of the airflow resistance force formula according to formula (3.4). in order to characterise the drag coefficient of different plants for overgrown rope façades as a function, power functions are thus suitable according to formula (3.3) with b-values and e-values. 4.2.5 e-value in the study results, the e-values range from -0.43 to -0.87 for firs, -0.60 to -0.85 for deciduous trees, and -0.10 to -1.27 for individual leaves and clusters. for individual leaves and clusters, they exhibit a larger range and differ the most from fir and deciduous trees. the fir and deciduous trees differ only in the lower range and are nearly identical in the upper range with values of -0.87 (fir tree) and -0.85 (deciduous tree). the e-values determined in section 2.3 are compared in table 17 with the analysed e-values from the interpolated drag functions. except for four significant deviations (δe-value > 0.2), the comparison confirms the proposed model in section 3.2 with a potential course of the drag coefficients according to formula (3.3). the four deviations of the δe-value are identified in the data 089 journal of facade design & engineering volume 9 / number 2 / 2021 of vogel (1989) for the drag coefficients of leaves. the e-values for deciduous and fir trees exhibit a maximum deviation of 0.09. table 17 comparison of e-values from research findings (source) and analysis species e-value source e-value analysis δe-value data source red maple -0.60 -0.63 0.03 kane & smiley, 2006 white oak -0.77 -0.76 0.01 kane et al., 2008 freeman maple -0.77 -0.68 0.09 kane et al., 2008 shingle oak -0.70 -0.62 0.08 kane et al., 2008 scots pine 4 (pinus sylvestris) -0.72 -0.72 0.00 mayhead, 1973; vogel, 1984 black locust leaf -0.52 -0.58 0.06 vogel, 1989 black walnut leaf -0.76 -0.89 0.13 vogel, 1989 pignut hickory leaflet -0.2 -1.25 1.05 vogel, 1989 pignut hickory leaf -0.78 -1.27 0.49 vogel, 1989 red maple leaf -0.79 -0.90 0.11 vogel, 1989 red maple cluster -0.64 -0.89 0.25 vogel, 1989 tuliptree leaf -1.18 -1.21 0.03 vogel, 1989 tuliptree cluster -0.91 -1.08 0.17 vogel, 1989 white oak leaf +0.97 +1.17 0.2 vogel, 1989 white oak cluster -0.44 -0.10 0.34 vogel, 1989 white poplar cluster -0.60 -0.76 0.16 vogel, 1989 willow oak cluster -1.06 -1.07 0.01 vogel, 1989 the influence of the e-value on the function is apparent in figure 3: the two curves are nearly identical for the same b-value and different e-values in the wind speed range from 0 to 2m/s. the curves of the e-values then diverge from one another at the speed range between 2 and 8m/s and, subsequently, continue with a slightly tapering offset with respect to one another. a large e-value thus produces a downward shift of the curve. the influence of the e-value on the drag coefficient is very large, so that when deriving an e-value for climbing plants of overgrown rope façades from analysed data, the correct e-value must be chosen. fig. 3 shifting of the function for different e-values while maintaining the same b-value 090 journal of facade design & engineering volume 9 / number 2 / 2021 4.2.6 b-value the b-value is only the constant of the power function before the velocity term (cp. formula (3.3)). along with the e-value, the b-value has a significant influence on the course of the curve of the drag coefficient (cp. figure 4). a high b-value causes the function of the drag coefficient to be shifted upward. however, this shift is not constant and it decreases at higher speeds. when deriving a b-value from the analysed data for a drag coefficient for climbing plants of overgrown rope façades, the correct b-value must be chosen. fig. 4 shifting of the function for different b-values while maintaining the same e-value 5 hypothesis for new drag coefficient determination for climbing plants on overgrown rope façades since climbing plants do not have needles, they are not analogous to coniferous trees and the studies on the wind forces for coniferous species cannot be applied for wind force determination on overgrown rope façades. due to the analogy between the leaf shape of climbing plants and the leaf shape of deciduous trees, the study results for deciduous trees for wind force determination could be applied. however, the morphology of climbing plants does differ from the morphology of deciduous trees. the fact that the morphology has a significant influence on the drag coefficient is demonstrated by the divergent results in studies by kane and smiley (2006) and kane et al. (2008) on larger (and thus older) deciduous tree samples in comparison with the results obtained by vollsinger et al. (2005) on smaller deciduous tree samples. in the studies on the drag coefficient for individual leaves of deciduous trees, vogel (1989) shows that the drag coefficients are mostly less than 0.10 at 20m/s. contrary to the study results of kane and smiley (2006) and kane et al. (2008) with resultant drag coefficients from 0.43 to 0.59 at 20m/s, individual leaves absorb a small share of the wind force. according to this comparison, the morphology of the tree absorbs a large share of the wind force. an essential role can be attributed to the stiffness of the tree branches. 091 journal of facade design & engineering volume 9 / number 2 / 2021 comparing the left image in figure 5 with the right image, an analogy between deciduous trees and climbing plants is apparent: the difference lies in the morphology. deciduous trees develop a considerably larger crown diameter and bear their own weight as well as the applied loads through their stiff branches, whereas climbing plants are reliant on a host that allows them to climb upwards. they transfer their own weight as well as the applied loads to the host and do not bear these forces on their own. fig. 5 morphology of climbing plants for overgrown rope façades (left) and morphology of deciduous trees (right) an analogy between the leaves of deciduous trees and those of climbing plants can also be interpreted from figure 5. the main difference between these two species lies in the morphology of the wood content. compared to deciduous trees, climbing plants exhibit a significantly lower wood content. the stiff wooden mass of a deciduous tree is relatively inflexible under flow loading by the wind and is able to align itself in the direction of flow only to a small extent. due to these inflexible resistance elements, a larger drag coefficient is expected in deciduous trees in comparison with climbing plants unlike what is, for example, described in the “green façade guidelines” (fll, 2018). the guidelines base their assumption of higher loads on climbing plants compared to deciduous trees on the denser leaf value associated with the climbing plants. climbing plants have main shoots on which the leaves grow in a ring pattern, whereas trees have different branches in the tree crown and thus multiple leaves in succession. according to the study by vogel (1989), the individual leaf generates drag coefficients < 0.1. denser foliage cannot significantly impact the resultant wind load, because significantly fewer leaves are located in succession in the twodimensional structures of overgrown rope façades compared to deciduous trees. multiple leaves in succession in tree crowns add up the individual resistance of the foliage, which means that significantly greater drag resistances can be expected in the airflow for deciduous trees compared to climbing plants. the results of kane and smiley (2006) and kane et al. (2008) with resultant drag coefficients between 0.43 and 0.59 at 20m/s thus provide a reference value for force determination on climbing plants. in order to stay on the safe side (to calculate higher forces in the calculation than actually occur), the highest drag coefficient at 20m/s for the red maple sample in table 13 is used for wind force determination on overgrown rope façades. the largest b-value (4.18) and smallest e-value (-0.60) in table 13 are close to the b-value (3.92) and e-value (-0.63) of red maple. this drag coefficient therefore meets the requirements of safe e-values and b-values, making it acceptable for determining the wind forces of climbing plants of overgrown rope façades. 092 journal of facade design & engineering volume 9 / number 2 / 2021 by specifying the drag coefficient as a function and not as a constant at a critical wind speed, the appropriate drag coefficient can be determined for each investigated wind speed on a building. for load determination on overgrown rope façades, the drag coefficient can thus be established based on the curve in figure 6 at any flow speed. fig. 6 reference value of the drag coefficient for overgrown rope façades the curve in figure 6 is based on an average value of all drag resistances for the samples. in civil engineering, limit values are derived from the test data by forming a characteristic value from the data points of the sample size for the influence (loads) on the materials as well as for the characteristic resistance values of the materials. however, since the exact test data are not available, it is not possible to determine a characteristic value. the function in figure 6 thus serves as a rough estimation of the forces and must be considered subject to uncertainties. fig. 7 comparison of the different curves a (top), b (middle) and c (bottom) figure 7 shows the potential that lies in the determination of the exact drag coefficients for calculation of the wind forces for overgrown rope façades. the top curve (a) is based on the full wind forces with a drag coefficient of 1. at 45m/s, an airflow resistance of 1235n/m2 is obtained. the middle curve (b) is based on diminished wind forces with a constant drag coefficient according to (fll, 2018) of 0.6 for the “medium” load class. at 45m/s, an airflow resistance of 740n/m2 is obtained. the bottom curve (c) is based on the dynamic drag coefficient (cp. figure 6) that is defined versus speed and expressed as a function. at 45m/s, an airflow resistance of 400n/m2 is obtained, 093 journal of facade design & engineering volume 9 / number 2 / 2021 representing 1/3 of curve (a). in order not to overestimate the wind loads on the climbing plants of overgrown rope façades, it is essential to determine the drag coefficients by means of a power function. the assumption of full wind loads (a) or the estimation of the drag coefficients by constants (b) (not as functions over the speed range) leads to high loads at the wind speeds occurring on the building and therefore to higher material wear and higher costs. 6 conclusion in the review of the identified studies, different methods are analysed for determination of the flow forces on coniferous and deciduous trees, as well as on individual leaf types. through the subsequent analysis and evaluation of the data, it can be assumed that the study results of kane and smiley (2006) can be applied to obtain a rough estimation of the wind forces on overgrown rope façades. the analogy between deciduous trees and climbing plants is drawn due to the similar leaf shape. however, deciduous trees differ from climbing plants in terms of the morphology. due to the rigid branches of deciduous trees, a greater wind force is presumed on deciduous trees, whereby the drag coefficients of deciduous trees form an upper limit. the red maple plant species from the results of the study by kane and smiley (2006) achieved the highest drag coefficient at 20m/s. this result is applied as the upper limit for a drag coefficient of climbing plants. the analysis and verification of the course of the drag coefficient based on a power function represent an important result. by specifying the drag coefficient as a function, the drag coefficient and thus the wind forces may be determined at any given wind speed in the design of overgrown rope façades. the drag coefficient suggestion determined in this paper is based on various assumptions and serves as an initial rough approximation for determination of the wind forces. the hypothesis that the different morphology of climbing plants compared to deciduous trees leads to smaller drag coefficients must be confirmed in a further study. since no studies are found concerning the wind force on climbing plants, tests are necessary to determine plant-specific parameters for climbing plants for use in wind force determination. the tests must be carried out in an airflow that simulates the natural airflow resulting from wind. in order to further investigate the calculation parameters for the wind force on overgrown rope façades, a further study is planned that will focus on the measurement of the drag resistances on five selected climbing plants in a turbulent wind tunnel, followed by determination of the drag coefficients to the statistical characteristic value. acknowledgement this work was carried out in the course of an in-depth study for a master’s programme. the main author would like to thank lucerne university of applied sciences and arts (lucerne school of engineering and architecture) for its support in elaborating the results of this paper. 094 journal of facade design & engineering volume 9 / number 2 / 2021 references alexandri, e. & jones, p. (2008). temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. building and environment, 43(4), 480–493. doi: 10.1016/j.buildenv.2006.10.055 djedjig, r., bozonnet, e., & belarbi, r. (2015). experimental study of the urban microclimate mitigation potential of green roofs and green walls in street canyons. international journal of low-carbon technologies, 10(1), 34–44. doi: 10.1093/ijlct/ctt019 en 1991-1-4. (2010). eurocode 1: actions on structures part 1-4: general actions wind actions. [standard]. fll. (2018). fassadenbegrünungsrichtlinien-richtlinie für die planung, ausführung und pflege von wandund fassadenbegrünungen [façade greening guidelines guideline for the planning, execution and maintenance of wall and façade greenings]. [guideline]. bonn. gartland, l. m. (2012). heat islands: understanding and mitigating heat in urban areas. routledge. doi: 10.4324/9781849771559 kane, b., pavlis, m., harris, j. r., & seiler, j. (2008). crown reconfiguration and trunk stress in deciduous trees. canadian journal of forest research, 38, 1275–1289. doi: 10.1139/x07-225 kane, b. & smiley, e. t. (2006). drag coefficients and crown area estimation of red maple. canadian journal of forest research, 36(8), 1951–1958. doi: 10.1139/x06-086 koizumi, a., motoyama, j., sawata, k., sasaki, y., & hirai, t. (2010). evaluation of drag coefficients of poplar-tree crowns by a field test method. journal of wood science, 56(3), 189–193. doi: 10.1007/s10086-009-1091-8 kolokotsa, d., santamouris, m., & zerefos, s. c. (2013). green and cool roofs’ urban heat island mitigation potential in european climates for office buildings under free floating conditions. solar energy, 95, 118–130. doi: 10.1016/j.solener.2013.06.001 leal filho, w., echevarria icaza, l., emanche, v. o., & quasem al-amin, a. (2017). an evidence-based review of impacts, strategies and tools to mitigate urban heat islands. international journal of environmental research and public health, 14(12). doi: 10.3390/ijerph14121600 mayhead, g. j. (1973). some drag coefficients for british forest trees derived from wind tunnel studies. agricultural meteorology, 12, 123–130. doi: 10.1016/0002-1571(73)90013-7 meskouris, k., butenweg, c., hake, e., & holler, s. (2012). baustatik in beispielen (2nd ed.) [structural analysis in examples]. berlin heidelberg: springer-verlag. doi: 10.1007/978-3-642-23530-6 mohajerani, a., bakaric, j., jeffrey-bailey, t., & mohajerani, a. (2018). the urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete. journal of environmental management, 197, 522–538. doi:10.1016/j. jenvman.2017.03.095 pfoser, n. (2016). fassade und pflanze. potenziale einer neuen fassadengestaltung [façade and plant. potentials of a new façade design]. [dissertation]. technische universität darmstadt, https://tuprints.ulb. tu-darmstadt.de/id/eprint/5587 pfoser, n. (2018). vertikale begrünung [vertical greening]. stuttgart: verlag eugen ulmer. isbn 978-3-8186-0088-4 rudnicki, m., mitchell, s. j., & novak, m. d. (2004). wind tunnel measurements of crown streamlining and drag relationships for three conifer species. canadian journal of forest research, 34(3), 666–676. doi: 10.1139/x03-233 santamouris, m., papanikolaou, n., livada, i., koronakis, i., georgakis, c., argiriou, a., & assimakopoulos, d. n. (2001). on the impact of urban climate on the energy consumption of buildings. solar energy, 70(3), 201–216. doi: 10.1016/s0038-092x(00)00095-5 sia 261. (2014). einwirkungen auf tragwerke [actions on supporting structures]. [standard]. switzerland vogel, s. (1984). drag and flexibility in sessile organisms. american zoologist, 24(1), 37–44. jstor. doi: 10.1093/icb/24.1.37 vogel, s. (1989). drag and reconfiguration of broad leaves in high winds. journal of experimental botany, 40(8), 941–948. doi: 10.1093/jxb/40.8.941 vollsinger, s., mitchell, s., byrne, k., novak, m., & rudnicki, m. (2005). wind tunnel measurements of crown streamlining and drag relationships for several hardwood species. canadian journal of forest research, 35, 1238–1249. doi: 10.1139/x05-051 from city’s station to station city 021 journal of facade design & engineering volume 7 / number 2 / 2019 rethinking adaptive building skins from a life cycle assessment perspective manuela crespi1, sandra g. l. persiani*2 * corresponding author 1 university la sapienza, department of planning, design, and technology of architecture, italy 2 technical university of munich, department of architecture, germany, sandra.persiani@tum.de abstract adaptive building technologies have opened up a growing field of research aimed at ensuring indoor comfort while reducing energy consumption in buildings. by focusing on flexibility over short timeframes, these new technologies are, however, rarely designed for sustainability over their entire lifecycle. this paper aims to address an information gap between the research field of architectural life cycle assessment (lca) and the state of the art of adaptive façades, by presenting an analysis of the main aspects in traditional and adaptive façades that are relevant to understanding whether parallels can be drawn between available lca databases. the literature is reviewed following an inductive method based on a qualitative data collection aimed at answering a list of research questions, and a deductive method starting from the descriptions of adaptive building envelopes. the findings highlight four main points: i) where and how adaptivity is integrated, ii) the design targets that are able to reduce the environmental impact, iii) the importance of a qualitative as well as a quantitative lca of the technology, and iv) lists a number of knowledge gaps currently limiting the diffusion of lca as a design and verification tool in adaptive building skins. keywords life cycle assessment, building skin, adaptive, systematic mapping, design parameters 022 journal of facade design & engineering volume 7 / number 2 / 2019 1 introduction the building sector is the largest consumer of energy, accounting for over one-third of final energy consumption and carbon dioxide (co2) emissions globally. according to the european commission, the energy use during the active life of the buildings in europe is responsible for approximately 40% of energy consumption and 36% of co2 emissions. in order to address these issues, research in the building sector has mainly focused on maximising the supply of energy from renewable sources and reducing the operational energy consumption in buildings’ life cycle by massively integrating low-energy building technologies and systems (iea, 2013). the concept of ‘energy’ in buildings has often been used in referring to ‘operational energy’ (oe), while largely disregarding embodied energy (ee) or embodied carbon (ec). this encompasses initial, recurring, and demolition embodied energies (azari & abbasabadi, 2018). although it is true that in many conventional buildings oe represents a relatively larger proportion of the life cycle energy (oe 80-90% compared to ee 10-20%) the rates vary depending on the building type (in an adobe/clay residential building the rate is closer to oe 66% ee 33%) (dixit, culp, & fernández-solís, 2013; ramesh, prakash, & shukla, 2010). the need to consider the complete life cycle of the building is therefore significant, especially since the amount of embodied energy is expected to grow with the rising number of low energy buildings that reduce their oe at the expense of an increase in their ee by integrating active and passive technologies and building systems (thicker envelopes, shading devices, etc.) (azari & abbasabadi 2018; dixit, culp, & fernández-solís 2013). it is mainly in answer to the demands for optimisation of operational energy in buildings that the field of architectural façades has developed a great variety of technological solutions that advocate for higher comfort conditions while reducing energy use. much of the technological research on adaptive building envelopes or skins (abs) is centred on developing flexibility of the building surfaces within the timeframes of the human activity cycle, ranging from interactive systems reacting within seconds to seasonal adaptations changing the building skin over a range of months. as most building technologies, abss rarely take into consideration other aspects than the energy efficiency or the user comfort, reflecting only a very partial view of the system’s real sustainability. therefore, if the aim of adaptive building technologies truly is to improve on the sustainability of the built environment, abss need to be designed and contextualised within the broader framework of a complete life cycle assessment (lca), evaluating the technologies throughout all building lca stages, as defined by the european standard en 15804:2012 (table 1). this paper takes a further step towards the integration of lca principles in the design of abss by reviewing the differences between adaptive and traditional façades, highlighting information gaps and focusing on aspects regarding architectural life cycle assessment which are mostly not considered in the abs research field. the study is based on an analysis of the state of the art of adaptive façades and integrates definitions and classifications with insights on the possible environmental impacts involved, setting the bases for a life cycle inventory. the aim is to give a more comprehensive understanding of the function and the assembly of materials and technological parts of the building skin, but also of the effects each design choice has throughout the phases in the life cycle, and by extension, its impact on the environment. the outcomes integrate the previously mapped framework by crespi, persiani, and battisti (2017), preparing for a complete lca system for abss. 023 journal of facade design & engineering volume 7 / number 2 / 2019 production stage (a1-a3) (a1) raw material supply, including processing of secondary material input (a2) transport of raw material and secondary material to the manufacturer (a3) manufacture of the product, and all upstream processes from cradle to gate construction stage (a4-a5) (a4) transport of the products to the building site (a5) installation/construction (of the product) use stage (related to the product) (b1-b5) (b1) use of the product (b2) maintenance of the product (b3) repair of the product (b4) replacement of the product (b5) refurbishment of the product use stage (related to operation) (b6-b7) (b6) operational energy use (b7) operational water use (not relevant for abs) end of life (c1-c4) (c1) demolition (/disassembly) of the product (c2) transport of the waste to waste processing facility (c3) waste processing operations for reuse, recovery, recycling (c4) final disposal of end-of-life product benefits and loads beyond the product’s boundary (d) reuse/ recovery/ recycling potential evaluated as net impacts and benefits table 1 building lca stages according to (en 15804:2012) 2 literature review existing classifications of adaptive building envelopes are broadly recognised to be partial and few (loonen,trčka, cóstola, & hensen, 2013; loonen et al., 2015; luible et al., 2015; sachin, 2016). in order to provide an inclusive review and directly address the aspects relating to lca, the research is structured according to the method of data analysis of the 5 ws (creswell, 1998), aimed to identify basic questions that are relevant to the topic for information gathering and problem solving (who, what, where, when, why, how). with the overview of the abs classification systems taken as a base, the study proceeds to redefine abss from an lca perspective by answering the research questions in table 2. questions who and why are answered by the body of the paper and are therefore not further developed. 024 journal of facade design & engineering volume 7 / number 2 / 2019 abs abs in terms of lca lca stages involved1 what? 1. what is commonly defined as an abs? 2. what are abss in terms of lca? which parts compose an abs and how are these assembled? (fig. 2) a1-a3 production stage b6 operational energy use c3 waste processing c4 final disposal, end of life d reuse/ recovery/ recycling potential -. how are distinctions adaptive/static, active/passive relevant in lca? where? 3. at which component level, and where in the façade are adaptive proprieties integrated? which are the most common abs technologies and materials? (fig. 3) a3 manufacturing a4-a5 construction stage b2-b4 use stage c1 de-construction demolition d reuse/ recovery/ recycling potential at which scale of the building skin is adaptivity integrated? are users involved in the operation of the technology? a3 manufacturing a4-a5 construction stage b2-b4 use stage b6 operational energy use c1 de-construction demolition how? 4. how does the adaptation work? (fig. 4, fig.5) a1-a3 production stage b2-b4 use stage b6 operational energy use when? 5. within which timeframe do adaptive processes occur? what impact does the timing of adaptation have on lca? (fig. 6) b2-b5 use stage b6 operational energy use c1-c4 end of life how can adaptive processes be assessed for an lca? b6 operational energy use 1 life cycle stages according to the european standards en 15804 (2012) (refer to table 1). table 2 ‘ws’ research questions mapping layout of lca parameters for the design of sustainable abs sustainable categories and functions related to cabs life cycle a3 manufacturing production stage a1-a3 a2 transport use stage b1-b7 end of life c1-c4 construction a4-a5 th em a ti c a re a s c a te g o ri es su pp o rt in g fu n c ti o n s mapping of lca parameters connection scheme sustainable requirements / cabs supporting functions building part building segment building b6.1 heating b6.2 cooling b6.3 ventilation b6.5 lighting b6.6 automation control b1 use: installed product b2 maintenance b3 repair b4 replacement b5 refurbishment c1 de-constrution/demolition c2 transport c3 reuse / recycling c4 disposal use stage b1-b7 end of life c1-c4 a5 construction/ installation on site a4 transport pre-products product development material sub-element element super element sub-component component super-component commercial material pre-products a1 raw materials supply a3 manufacturing production stage a1-a3 construction a4-a5 a2 transport + ...+... a2 transport ...+... lc a in pu ts q u a li ta ti v e pa ra m et er s pr o c es se s b6 operational energy use connection scheme processes stage b6 / cabs supporting functions fig. 6 within which timeframe do adaptive processes occur? fig. 5 why is it important to investigate and produce abs? fig. 4 how does the adaptation work? fig. 3 at which component level, and where in the facade are adaptive proprieties integrated? fig. 2 abs? what is an abs in terms of lca? a2 transport a3 manufacturing raw material composite material commercial material pre-products product developmentq u a n ti ta ti v e pa ra m et er s su b -c a te g o ri es fu n c ti o n s fig. 3 fig. 2 fig. 4 fig. 5 fig. 6 cs fig. 1 general mapping of the lca process and parameters for abss (from crespi et al., 2017), with the layout of how figs. 2-6 in this paper can be included in the mapping. 025 journal of facade design & engineering volume 7 / number 2 / 2019 the main aspects characterising abss in an lca perspective are summed up in five infographic representations (figs. 2 6), that can be further included in the mapping framework (fig. 1). 2.1 data collection data collection was conducted by reviewing databases such as sciencedirect, scopus, and researchgate. among the keywords searched were: adaptive, innovative, dynamic, responsive, climate, building envelope, façade components, building shells, building skins, lca, materials, and photovoltaic. the academic literature was reviewed following two main paths: – an inductive method based on a qualitative data collection aimed at answering the research questions; – a deductive method starting from the aforementioned descriptions of adaptive building envelopes. in a first step, a broad range of academic publications were selected based on the innovative technologies introduced in the building envelope. although not directly mentioning ‘adaptive building skins’, these allowed for the incorporation of a great number of technological solutions that are effectively employed in abss, such as photovoltaic systems, which are among the most widespread technologies in active façades. the importance of identifying a method of classification used for existing envelopes’ products lies in the possibility of highlighting shortcuts to available information on substances’ emission data to be further employed in future life cycle inventories for abss (such as the ecoinvent database, 2007), without needing to reconstruct the emissions path due to the individual production processes of the materials making up the product. in a second step, the research focused on the more recent findings on adaptive façades, examining only literature published after 2012. the literature was classified by topic, terminology, and methodological approach used (technological, life cycle, systematic, biomimetic). the outcomes are summarised in the annexes. this approach helped to identify the many nuances the concept of abs spans, not necessarily related to specific technological solutions. 2.2 state of the art review the study of the existing literature on adaptive façades reveals a very broad understanding of these technologies, although, in many cases, ’adaptiveness’ is not directly mentioned. definitions and classifications reveal the recurring features and characters typical of abss that are important to take into consideration within the lca. existing and emerging building skin technologies have been classified, of which two main aspects were identified: – a classification of the physical features (tucci, 2012), with innovative materials to building parts categorised according to behaviour (active/passive) and appearance (opaque, semi-transparent, translucent, transparent). – a classification of the functional behaviour (loonen et al., 2015 & 2013) listing eight basic criteria for façade adaptivity: goal, responsive function, operation, technologies (materials & systems), response time, spatial scale, visibility, and degree of adaptability. the annexes give a further overview of how the collected literature has addressed the evolution of building envelopes through a technological, biomimetic, or systematic approach. the multiplicity 026 journal of facade design & engineering volume 7 / number 2 / 2019 of approaches is indicative of the interdisciplinary nature of the topic and the broad category of technologies employed in abs. emerging technologies identified by the literature review (detailed list in annexes) require further integration in abs inventories to enable a further mapping in terms of lca. these can be subdivided into three macro-families: 1 façades that integrate renewables, from solar façades (quesada, rousse, dutil, badache, & hallé, 2012a &b), solar cooling (prieto, knaack, auer, & klein, 2017), building integrated solar thermal (bist) technologies (zhang et al., 2015), and dynamic building integrated photovoltaic systems (bipv) (jayathissa, jansen, heeren, nagy, & schlueter, 2016; curpek & hraska, 2017). 2 active building envelopes, integrating smart glasses and motor-based shading devices (sachin, 2016), robotic materials that combine sensing and controlling features (mcevoy & correll, 2015), iot sensor network systems and the several devices associated with them (e.g. sensors, actuators, controllers) (konis & selkowitz, 2017). 3 passive stimuli responsive materials and components. although being mostly at an experimental stage, these elements are considered to be of strategic importance for the coming generation of abs. examples are hygromorphic materials, phase change material (pcm)-based mortars (curpek & hraska, 2017; koláček, charvátová, & sehnálek, 2017), self-shading building tiles with shape memory polymers, etc. (among others aresta, 2017; bridgens, holstov, & farmer, 2017; clifford et al., 2017; mao et al., 2016; persiani, molter, aresta, & klein, 2016b; ribeiro silveira, louter, eigenraam, & klein, 2017). with such a broad variety of technologies and functions characterising adaptive building envelopes, it is understandable that many sibling concepts are used to describe adaptive systems. adaptive building skins are described from a systematic point of view as sets of interacting parts with specific multiple functions, behaviours, and goals. the most diffused way to distinguish between types and categories of adaptive envelopes, however, is to identify their purpose and the dynamic behaviour of the components. climate adaptive building shells (cabs), for instance, address more specifically the energy efficiency and performance of the building envelope (loonen et al., 2013). the review also highlighted further directions for developing abss in terms of sustainability. – a number of studies were reviewed where the generation of design concepts is tackled through a biomimetic problem-solving methodology (wang, beltrán, & kim, 2012; persiani, battisti, & wolf, 2016; badarnah, 2012, 2016, 2017). from an lca point of view, investigating the relation environmental agents – means of adaptation – building functions – operation of the technology – lca can create a systematic design-oriented framework open to innovative and creative concepts. these concepts have been introduced in the early design phases in previous research through a preliminary (simplified) systematic lca mapping (crespi, persiani, & battisti, 2017). the framework, built on a method for the design and construction of integral façades, aims to enable decisionmaking in the early design phases of adaptive envelopes and introduces lca optimisation through an evolutionary design method with a multi-objective solution finding. – a new methodology which is widely recognised as a reliable means of data acquisition, information feedback, and a solid base for decision making in the context of sustainable design and lca is building information modelling (bim). the model enables cross referencing of graphic and numerical information of the building and its parts, allowing not only the system to be controlled during its design and construction phase, but also allows it to be managed throughout its complete lifecycle (soust-verdaguer, llatas, & garcía-martínez, 2017; volk, stengel, & schultmann, 2014). 027 journal of facade design & engineering volume 7 / number 2 / 2019 – research reveals that no single mitigation strategy alone can tackle the problem of transiting to a low-carbon built environment. a pluralistic approach is absolutely necessary, combining better design, the use of low-embedded carbon (ec), and reuse of high-ec materials together with stronger policy drivers (pomponi & moncaster, 2016). the state-of-the-art review underlines four topics of importance for abs in terms of lca: – different classifications of abss and abs technologies, highlighting possible shortcuts to available information on substances’ emission data to be further employed in future life cycle inventories for abs; – a list of emerging technologies to be further integrated in abs inventories and mapping of abs in terms of lca; – commonly shared definitions of abs; – directions for further development: the biomimetic approach, integration of information through bim, and a pluralistic approach. what appears to be missing in the state of the art is the implementation, comparison, and alignment of the terminology of building products with those in bim libraries and standards. this would allow a shared base of understanding through the different design and simulation software, from design to facility management, and greatly facilitates the lca process. 3 adaptive building skins from an lca perspective in order to describe which aspects are relevant for abs in terms of lca in a straightforward way, the study is structured through thirteen research questions listed in table 2. 3.1 what is commonly defined as an adaptive building skin? adaptive façades, or adaptive building envelopes, is a general term used to refer to a new generation of multifunctional façade systems that are able to change their function, features, or behaviour over time in response to transient performance requirements and boundary conditions with the aim of improving the overall building performance (cost action tu1403, 2018; persiani et al., 2016a). this emerging research area can be found at the crossroads between environmental architecture, building technologies, and artificial intelligence. as in all emerging fields, the first stages are characterised by an non-uniform variety of terms and definitions with analogous meanings. adaptive building skins (abs), climate adaptive building shells (cabs), adaptive façades, autoreactive façades, and acclimated kinetic building envelope (ake) are just a few of the many sibling concepts that can be found in the current state of art. these terms describe variations of entities within the same family of technologies with a common ‘blueprint concept’, highlighting and focusing on some aspects more than others. there are four definitions of abs indicated in the reviewed studies (wang et al. 2012; badarnah, 2012; loonen et al., 2013; persiani et al., 2016a). while the wording has evolved over time, the core of the concept is mostly shared. the definition focuses on goals and performances to be achieved in a responsive way by the building envelope, which is described of as a system of parts. physical characteristics or technological solutions are not mentioned, although built examples are given in 028 journal of facade design & engineering volume 7 / number 2 / 2019 some cases. the aesthetics of the movement are not considered central to the definition, its potential to involve the users and raise awareness with a positive impact on behaviour is however widely recognised. this approach is shared for the purpose of this research, as it gives the opportunity for façade designers to have unlimited creative boundaries inside a systematic framework driven by specific performance goals and dynamic behaviours. 3.2 what are abss in terms of lca? as mentioned previously, abss enable dynamic responses to changing environmental conditions, boosting indoor comfort and energy performances in the operation stage (b6) but should also contain environmental impacts in the other life cycle phases, such as production, use of the product (b1), maintenance (b2-b4), refurbishment (b5), and end of life (c1-c4), in order to fully justify their use. on the one hand, lca is a means to measure the real impact of abss on the environment, and on the other hand it is a tool to optimise its design, initiating a cycle of experimentation and verification (table 3). among many objectives, an lca identifies opportunities to improve the environmental performance of products at various points in their life cycle (iso 14040 & 14044 2006). adaptive building skins can therefore be redefined in the broader perspective of the entire life cycle where ‘adaptivity’ assumes a broader meaning, involving the conservation of natural resources and the reduction of pollution. design target reduces impact on lca phase use low-ec materials a1 a2 a3 a4 a5 b1 b2 b3 b4 b5 b6 c1 c2 c3 c4 use of local materials x x x x use renewable materials x x x use of materials with low processing energy x x include waste, by-products, and used materials x x x x design for disassembly a1 a2 a3 a4 a5 b1 b2 b3 b4 b5 b6 c1 c2 c3 c4 enable re-use and recovery of materials (especially of ee/ec materials) x x x x x x enable refurbishment of existing structures extending the product’s life x x x x x x x x develop more efficient construction processes / techniques a1 a2 a3 a4 a5 b1 b2 b3 b4 b5 b6 c1 c2 c3 c4 increased use of prefabricated elements/off-site manufacturing x x x x x x x x prefabricate bigger parts of the façade x x x design for autoreactivity a1 a2 a3 a4 a5 b1 b2 b3 b4 b5 b6 c1 c2 c3 c4 enable operation at zero energy x x dynamics are embedded in the material, reducing the number of parts x x x x x x x x x x table 3 design targets to reduce the impact on different phases of the lca 029 journal of facade design & engineering volume 7 / number 2 / 2019 adaptive building envelopes are multifunctional façade systems able to change their features or behaviour over time in response to transient performance requirements and boundary conditions, with the aim of improving the overall performance of the building, while contributing to the reduction of the environmental impacts in all the phases of the building’s life. as previously pointed out, abss are strongly focused on energy efficiency in the operational energy use phase (b6). for a full lca approach, it is necessary to identify and evaluate which among the commonly adopted technologies, components, and materials can have a significant impact on the other phases in the life cycle. high-tech components for instance typically have a shorter lifecycle than that of the building and become obsolete increasingly quickly as newer products are developed, with the common side effect of a higher impact on the production (a1-a3) and maintenance phases (b2-b4) of the system. when designing new abs technologies, the main variations on lca impacts can be expected in the following phases (see also table 3): – production phase (a1-a3), due to use of resources to produce specific components, elements and materials, rising complexity and use of high-tech materials to achieve kinetic façade components, etc. – construction phase (a4-a5), depending on the effectiveness of the assembly (and disassembly) of the product, construction times, and resources can be reduced. – maintenance, repair, and replacement phases (b2, b3, b4) and the end of life phase (c1-c4) can be strongly impacted through designing for disassembly (especially of interest for the replacement of kinetic parts in abss). – benefits and loads in the phase of reuse/ recovery/ recycling (d) are mainly considered beyond the product’s boundaries, as it enters another system’s life cycle when integrated under any of the three forms. 030 journal of facade design & engineering volume 7 / number 2 / 2019 3.2.1 which parts compose an abs and how are these assembled? imullions hinges brackets shading actuator i-mullions unitized curtain wall super-component adaptive facade building part glazing system component shading system component glazing system component sub-component windows i-mullions i-mullions windows ( commercial materials + elements) sub-component (+ standard materials) rubber gaskets chain actuator igu brackets insulated glass unit (igu) element aluminium spacers silicone plate glass plate glass (standard materials + commercial materials) shading system component sub-component shading panels (+ standard materials) shading panels shading panel (commercial materials) element from building part to components from component to elements from component to elements fig. 2 study of a hierarchical disassembly of a basic façade unit composed of glazing and dynamic shading (based on klein, 2013). lca stages involved: (a3-5), (b2-4), (c1), (d). 3.2.2 how are distinctions adaptive/static, active/passive relevant in lca? there are fundamental differences between active and kinetic, adaptive and static systems. ‘active’ and ‘passive’ refer to the energy requirements of the technology: while an active system is powered 031 journal of facade design & engineering volume 7 / number 2 / 2019 though an input of energy (mainly electrical), a passive system uses the latent energy from its surroundings (as for thermal phase change materials) (persiani et al., 2016a). ‘adaptive’ and ‘static’ refer to the physical capacity of the material or the technology to change in determinate conditions. because of the tendency to design increasingly complex façade systems, the boundaries between active and passive systems slowly disappear: adaptive properties are no longer characteristic of active systems, as latent energy reaction can now also be enabled in passive systems (persiani et al., 2016b). in an lca, these characters need to be considered, including stratigraphic façade solutions (like shaded double-glazing systems) and spatial structures with climate-regulating purposes (like greenhouses), which may reduce the impacts in the production phase (a1-a3). 3.3 at which component level, and where in the façade are adaptive properties integrated? a great variety of aspects in an lca depend on the hierarchy of the parts in the abs, on the assembly methods and above all, the wear of elements or components. a designer aware of these processes can effectively have an impact on: – controlling at which stage in the production chain the manufacture and assembly takes place (in factory / on site), with the related impacts; – design disassembly to reduce impacts in the use stage (b2-b4), simplify maintenance and repair, avoiding the replacement of a whole when only part is damaged; – design disassembly for deconstruction (c1), maximising the possibility of reuse, recycling, and separate materials that need special disposal. static envelopes are also included in this framework (traditional passive spatial solutions in fig. 3), being the technical base for many technologies. these can be implemented with adaptive elements, components, or materials, and can be used as reference for future solutions. the main purpose with identifying these solutions is to highlight the presence of elements with a substantial impact on the production and maintenance phase. 3.3.1 which are the most common abs technologies and materials? technologies. the most commonly used technologies are different types of glazed components with shading systems (c1 c3) that may also include elements with controlled solar light and heat transmittance (such as chromogenic e1 e3). mechanical ventilation systems can be found in some static and dynamic building façade technologies (building part bp2) as well as energy generating components (bp3, bp4, bp7, bp8, bp9). a new trend is represented by building integrated solar cooling technologies (bp10), where the cooling system, integrated in the façade, also generates energy through solar electrical or solar thermal processes. the cooling generation principles are several (thermoelectric cooling, absorption cycle, indirect evaporation, vapour compression) and the transfer medium can be either solid-based, water-based, or air-based. the delivery systems, depending on the medium, are radiative walls, mounted pipes, induction units, diffusers, or may be absent. in this case, abss include hvac systems and the life cycle impact might be consistent. 032 journal of facade design & engineering volume 7 / number 2 / 2019 pv (x) pv (x) (x) (x) ra w m at er ia ls m at er ia ls (m ) ex p er im en ta l m at er ia ls (e m ) co m m er ci al m at er ia ls (c m ) su b -e le m en ts el em en ts (e ) su p er el em en ts su b -c om p on en ts (c ) co m p on en ts (c ) su p er co m p on en ts b u ild in g p ar t ( b p ) building parts (bp) related to static envelopes bp 1. naturally ventilated transparent facade (nvtf) bp 2. mechanically ventilated transparent facade (mvtf) bp 3. semi-transparent building integrated pv system (stbipv) bp 4. semi-transparent buildingintegr. pv thermal system (stbipv/t) bp 5. thermal storage wall bp 6. solar chimney bp 7. building-integrated photovoltaic system (bipv) bp 8. building-integrated solar thermal system (bist) bp 9. bipv thermal system (bipv/t) bp 10. thermo-electric facades components (c), elements (e), experimental/commercial materials (em/cm) related to dynamic systems cm 1. phase change materials em 1*. vpm under development: shape change materials (scp) thermal expansion materials (tem) thermobicomposite materials (tbm) shape memory foam (smf) shape memory ceramics (smc) shape memory biosystems (sm-bs) bicomposite materials (bm) absorbent polymers (aps) superabsorbent polymers (saps) wood based hygromorphic materials pcm based mortars thin glass em 2. shape memory polymers + shape memory alloys em 3. shape memory polymers + hydrogel em 4. thermobimetal sc 1. dynamic pv c 1. ventilated double skin facade (dsf) with shading system c 2. prismatic louver c 3. translucent ventilated with chromogenic glass & shading system ec 1*. autoreactive components ec 2*. ventilation units with pcm for vdsf e 1. chromogenic: photochromic, electrochromic, thermochromic, gasochromic chromogenic / angular selective glass e 2. spd smart windows e 3*. vtg under development: micro electromechanical systems other most used materials (m)° m 1. 3. wood derived timber products m 2. 4. metals steel products, aluminium, zinc , lead m3. 6. plastics sealing materials m4. 7. components for windows and curtain walls * materials, elements and components under development variable property materials (vpm) variable transmission glazing (vtg) switchable glazing automated shading systems dynamic pv building integrated solar facade building integrated solar cooling transpare nt semi-tran sparent transluce nt opaque high visibility low visibility no visibility ° classi�cation from the ökobau database, built on gabi database fig. 3 systematic illustration of the typologies of abss, with classified the most widespread technologies and materials (classification from the ökobaudat database). lca stages involved (a1-3), (b6), (c3-4), (d). material innovation in construction depends, to a large degree, on technological improvements in other manufacturing sectors (such as medical or communications). a number of reviewed publications list new materials used in the context of adaptive façades (refer to the literature review in the annexes). during the production phase of the envelope, the most used materials are glass, aluminium, and inorganic polymers for films and textiles, of which the energy embodied in the manufacturing process is hardly ever taken into consideration. however, in 2017, an environmental product declaration (epd) on an etfe-based cladding system was published, showing growing concern and interest of stakeholders for environmental issues (maywald, 2017). 033 journal of facade design & engineering volume 7 / number 2 / 2019 at the current rate of technological development grow rapidly obsolete, the long-term sustainability of specific high-tech solutions becomes challenging with respect to both the production phase (a1-a3) and to the end of life scenarios (c1-c4). adaptive materials (em1 em4) for instance, are able to change their physical features in reaction to the action of external agents (humidity, heat, radiance, etc.). these are mostly under development for the field of building technologies, with few exceptions (as pcm, that are already available on the market). the category is expected to grow increasingly wider, adding on new technologies making use of them. in order to fully evaluate the sustainability of these materials and technologies more specific lca studies are needed. 3.3.2 at which scale of the building skin is adaptivity integrated? adaptivity can be manifested either at material or at component scale. designing for disassembly allows the adaptive parts to be easily removed and replaced, benefitting the life cycle of the whole façade as: – adaptive parts tend to become worn out more quickly when compared to static solutions, because of their changing characters (as for kinetic adaptivity). moreover, the duration and resistance of these new materials has not been tested over many years of use; – technologies grow obsolete increasingly quickly, and disassembly allows adaptive materials or parts to be replaced with more advanced solutions without changing the whole façade system. so far, major innovations on adaptivity have been developing at material scale, followed by a few categories of elements and components such as chromogenic glasses and shading devices that have existed for many years on the market. 3.3.3 are users involved in the operation of the technology? the possibility of users directly interacting with the functioning and the dynamics of abss introduces the question of whether the lca should address the operational energy use (b6) from a qualitative or a quantitative point of view. as comfort is a very subjective matter, it is difficult to achieve optimal conditions that satisfy all users. from a qualitative point of view, users are therefore often enabled to intervene and bypass the system (e.g. opening the windows for ventilation). on the other hand, when users are allowed to override the set conditions, the quantification of energy consumption (lighting and hvac) becomes difficult to control and is likely to rise. building automated domotic monitoring systems have been suggested as high-tech solutions, that are however difficult to evaluate from an lca point of view, as the system is tailored to the users and the potential variations are infinite. distinctions between transparent and opaque elements can give additional information on the performance, as a common low-tech way to introduce adaptivity is through visual and thermal permeability. the increased daylighting and thermal performance have a varying range of energy efficiency, which very much depends on use. 034 journal of facade design & engineering volume 7 / number 2 / 2019 3.4 how does the adaptation work? abss are programmed to adapt to surrounding environmental conditions and transfer energy in different forms (radiant, kinetic, potential) to achieve human comfort requirements. the great majority of abss are actuated through systems of sensors that analyse the surrounding conditions, communicating with a control unit that takes simple decisions and orders counter-actions. to these systems belong hvac technologies and active building systems. user’s controller sensor hvac controllable lighting central control unit actuator processor learning solar cool facades high visibility low visibility no visibility autoreacti ve reactive responsive interactive disassemb ly of the faca de automate d control virtual spa ce dynamic f acadeliving space m at er ia l (m ) ex pe rim en ta l m at er ia l (e m ) co m po ne nt su bco m po ne nt (c , s c) bu ild in g pa rt (b p) in tr in si c in tr in si c ex tr in si c responsive autoreacti ve reactive robotic m aterials smart mat erials fig. 4 systematic illustration of the typologies of abs through the possible variations in adaptivity (adapted from konis & selkowitz, 2017; loonen et al., 2013; loonen et al., 2015; persiani et al., 2016a). lca stages involved (a1-3), (b2-4), (b6).. 035 journal of facade design & engineering volume 7 / number 2 / 2019 research goals are generally aimed at improving abs effectiveness by reducing uncontrolled user behaviour and energy (hvac, lighting, and plug loads) through the integration of smart materials and systems. in continuous dynamic skins, users’ interaction is often enabled through an energy management and control system (emcs), which on one hand aims to optimise but on the other adds up to the energy consumption during the usage phase being an active system. developing trends are energy-generating kinetic devices (as dynamic pv sub-components) and unpowered kinetic features that are however still in a prototyping phase (persiani et al., 2016b). these latter technologies, referred to as “autoreactive”, lack the control unit and wiring as reactions to specific stimuli are predetermined and embedded in the material itself. these systems react to latent energy conditions and can therefore be considered as high-tech passive systems requiring zero-energy in the operational energy use phase (b6). moreover, the reduction of wiring and information technology devices noticeably reduces the impact on the production stage (a1-a3) and the use stage (b2-b4). methods of actuation: motor based hydraulic pneumatic material based motion parameters: system type, geometry, energy, motion how to reach the goals h u m a n co m fo rt humidity potential rain snow air water vapour b6.1 heating b6.4 hot water b6.5 lighting user b6.6 automation control b6.3 ventilation light heat b6.2 cooling prevent energy losses monitor performance ensure low running costs guarantee energetic performance control air exchange rate collect solar thermal energy collect solar pv energy include thermal mass arti�cial thermal mass prevent surface void void kg kg void kg o th er fu n ctio n s transfer functionsnatural processes end goalagents energy gain retain dissipate prevent radiation metabolic rate conduction convection absorption retroscattering emission evaporation increase reduction control daylight radiation comfortable light levels glare protection allow visual contact void kg lca transmission absorption move exchange sun wind void mass radiant energy kinetic evaporation control irradiation reduction gravity capillary action evaporation gain conserve transport lose adaptive motion generate energy store energy lighting shading generate energy natural convection emcs interaction include components for temperature di�erences condensation di�usion pressure di�erence velocity gradient unidirectional /countercurrent �ow di�usion lca stages involved [b6]energy transferenergy typeenvironmental agents end function fig. 5 summary of the connections between environmental agents and abss final goals highlighting the means of energy transfer and the lca processes involved (b6) (summarised from badarnah 2012, 2016, 2017; persiani et al., 2016a). 036 journal of facade design & engineering volume 7 / number 2 / 2019 3.5 within which framework do adaptive processes occur? as adaptive building technologies adapt to both indoor and outdoor changing contexts, the translation of situational information in real time is among its main advantages and purposes. in this framework, lca should be carried out considering more aspects than those pertaining only to static building skins. lcas are mostly based on the collection of a great amount of hard data describing the system through analysis (en 15804 2012, ecoinvent database 2007) and includes information on single materials (embodied energy, recyclability potential), material quantities, usage patterns, and stage processes (as extraction, production, maintenance and recycling processes). this quantitative (calculated) data is largely based on assumptions and estimations, wherever more precise information is not available. every lca, however, is affected by a varying degree of uncertainty derived from the cumulative effect of imprecisions either due to lack of knowledge in the available data or to variability in the data. this is why qualitative considerations (transient or subject to interpretation) can play an important role in determining the overall environmental impact of a given object. soft data refers to human intelligence and behaviour, and is bound to interpretation, contradictions, and uncertainty but is also very useful to understand environmental occurrences and situational nuances. this is why sensitivity analyses, estimating the effects of the choices made regarding methods and data on the outcome are recommended as part of an lca (iso 14040 2006, budavari et al., 2011). moreover, as the current technologies quickly evolve towards increased connectivity and internet of things (iot), the relationships between hard and soft data become ever more intertwined. the integration of variated typologies of information – such as user behaviour – into the analysis is therefore all the more interesting in abs than in more traditional façade technologies. 3.5.1 what impact does the timing of adaptation have on lca? to achieve environmental comfort, the technology will ideally perform better if it can be adjusted more continuously, calling for a very reactive technology that will adapt within short timeframes. from an lca point of view, however, constant reactivity in active abss also means constant use of energy resources, as well as rising maintenance issues due to the frequency of usage. energy use in abss is hypothesised in fig. 6, referring exclusively to active systems, as passive systems are intended to operate at zero energy. timeframe parameters (from loonen et al., 2015) as seconds, minutes, hours, day-night, and seasons refer to climate adaptivity, while years and decades refer to the capacity of extending the life of building parts through maintenance, repair, replacement, or refurbishment. abss are expected to have a higher energy cost the faster and the more frequent the adaptations, as reacting within seconds requires the system to be constantly ready for change. moreover, fast movements typically require active and more complex energy-intensive brain elements (persiani, 2018). 037 journal of facade design & engineering volume 7 / number 2 / 2019 1yr 50yr hrsminsec seconds minutes hours day/night seasons years decades timeframe frequency of adaptations energy use per adaptation cycle velocity of adaptation energy use of the abs system fig. 6 temporary and metabolic framework of adaptations in abss (timeframe, number of adaptations and hypothesised energy intensity). lca stages involved (b2-6), (c1-4). in view of optimising the relationship energetic expenditure/adaptive output, the metabolic cost (energy use per adaptation cycle) of the reactions is hypothesised in relation to the adaptation timeframe. by observing the energy expenditure in animal gaits, where each mechanism reaches its optimal relationship between energy expenditure and kinetic output at specific speeds (persiani, 2018), the energetic cost per adaptation in abss is suggested as higher at slow and very fast speeds. what is of interest is to highlight these aspects in the context of an lca, where the balance between product’s lifespan and operational energy phase must be reached. 3.5.2 how can adaptive processes be assessed for an lca? the definition of abss being characterised by their specific functioning – and not as many other systems, a set of parts – is in this context of great relevance. it is not only the embodied energy of the system that is of interest, but also its potential to reduce the environmental impacts on the usage phase. for this, other methods of calculation are needed. adaptive processes can be considered as peculiar characteristics in the façade system and can be assessed separately in the operational energy use phase (b6). the methods of assessment and calculation of the adaptive features play a decisive role in the evaluation of an lca, when compared with traditional façades, and hence also in the design of the technology. assessment of the energy-intensity of abss in the operational energy use stage (b6) is achieved through dynamic simulations during the design phase and is confirmed through monitoring during usage. post occupancy reports also help to evaluate the optimal response time in relation to the user’s ability to intervene in the regulation of abss, and whether it interferes negatively with the targeted energy efficiency. for all other life cycle phases (a1-a5, b1-b5, c1-c4) the methods of calculations are essentially the same for abs as for traditional façades, which, however, does not mean that the results are the same, as the inputs can vary substantially. 038 journal of facade design & engineering volume 7 / number 2 / 2019 4 conclusions the research has suggested an understanding of current and emerging abss and their functioning, focusing on aspects regarding lca which have been mostly unconsidered up to now. the following points have been highlighted: – abss are described as systems characterised by sets of interacting parts with specific multiple functions, behaviours, and goals. an integration to the definition is suggested to include “containing the environmental impacts in all the phases of the building’s life” in the scope of the technology. illustrations of the typologies of abss and a summary of the connections between environmental agents, energy transfer, lca processes, and abss’ final goals are provided. – adaptivity is either integrated by designing completely new technologies and uses or optimising traditional passive building systems with adaptive features. however, as increasingly sophisticated adaptive technologies are developed, the boundaries between active-dynamic and passive-static systems blur. – the integration of variated typologies of information, as situational and real time information is among the main advantages and purpose of abss. both quantitative and qualitative assessment, such as dynamic simulations and information on user behaviour, play a decisive role in lca the evaluation of the technology. – energy use in abss is hypothesised in terms of metabolic costs (energy use per adaptation cycle) through the relationship energetic expenditure/adaptive output. – lca is suggested as a tool to optimise the design of abss by identifying opportunities to improve the environmental performance of products at various points in their life cycle. to effectively enable lca as a design and verification tool in abss, a number of knowledge gaps need to be filled: – the terminology and ontology of a building’s products need to be implemented for an effective comparison with bim libraries and standards in order to allow for a shared base of understanding from design to facility management, through the different design and simulation software. – future developments of smart materials need to be further investigated in terms of lca to provide good databases of knowledge to support the integration of new adaptive features in façade technology. – designers need to be more aware of the hierarchy of parts, the processes of production, assembly, and the end of use of these technologies in order to be enabled to effectively design better and support industry to develop sustainable solutions. specifically, designers can contribute by carrying forward specific design targets able to reduce the impact on different phases of the lca. a study of a hierarchical disassembly of a basic façade unit is provided. this system mapping is not intended to be exhaustive, but as a base for further implementation on the basis of stakeholders’ needs. it is a first step to facilitate the process of life cycle inventory during lca and life cycle design. adaptive building skins’ energy-saving behaviour need to balance out its environmental impacts during the production, the usage, and the end of life phases to be considered fully sustainable. as adaptive envelopes can be expected to extensively grow in use and address an increasingly wider range of building technologies and construction scales, from building parts to components, the need for lca to support abs research and development greatly increases. indeed, with the purpose of broadening the approach to abss and consider the full range of their environmental impact, this study will be the basis on which to carry out a comparative analysis between traditional and adaptive façades. 039 journal of facade design & engineering volume 7 / number 2 / 2019 references aresta, c. (2017). auto-reactive strategies. a catalogue of materials for innovative façade components. in molter, p.l., mungenast, m., banozic, m., englhardt, o., & klein, t. (eds.), proceedings of the international mid-term conference of the european cost action tu1403 adaptive façade network, munich, germany, tum, 16-17. azari, r., & abbasabadi, n. (2018). embodied energy of buildings: a review of data, methods, challenges, and research trends. energy and buildings, 168, 225-235. doi:10.1016/j.enbuild.2018.03.003 badarnah, l. (2017). form follows environment: biomimetic approaches to building envelope design for environmental adaptation. buildings, 7, 40. doi:10.3390/buildings7020040 badarnah, l. (2016). light management lessons from nature for building applications. procedia engineering, 145, 595-602. badarnah, l. (2012). towards the living envelope. biomimetics for building envelope adaptation (doctoral dissertation). doi:10.4233/uuid:4128b611-9b48-4c8d-b52f-38a59ad5de65 bridgens, b., holstov, a., & farmer, g. (2017). architectural application of wood based responsive building skins. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 179-189. budavari, z., szalay, z., brown, n., malmqvist, t., peuportier, b., zabalza, i., krigsvoll, g., wetzel, c., cai, x., staller, h. & tritthart, w. (2011). methods and guidelines for sensitivity analysis, including results for analysis on case studies, lorelca project. retrieved from http://sintef.no/projectweb/lore-lca/ clifford, d., zupan, r., brigham, j., beblo, r., whittock, m. & davis, n. (2017). application of the dynamic characteristics of shape-memory polymers to climate adaptive building façades. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 171-178. cost action tu1403 (2018). retrieved from the cost tu1403 adaptive façades network. retrieved from http://tu1403.eu/?page_id=32 crespi, m., persiani, s.g.l. & battisti, a. (2017). mapping of lca parameters as a tool for the design of sustainable cycle-based adaptive building skins. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 179-189. creswell, j.w. (1998). qualitative inquiry and research design: choosing among five traditions. thousand oaks, ca: sage publications. curpek, j., hraska, j. (2017). ventilation units with pcm for double-skin bipv façades. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 538-547. dixit, m. k., culp, c. h., & fernández-solís, j. l. (2013). system boundary for embodied energy in buildings: a conceptual model for definition. renewable and sustainable energy reviews, 21, 153-164. doi:10.1016/j.rser.2012.12.037 european standards en 15804. (2012). environmental product declarations core rules for the product category of construction products. brussels, belgium: european committee for standardization. ecoinvent database. (2007). life cycle inventories of building products data v2.0 (2007), ecoinvent report, 7. dübendorf, switzerland: swiss centre for life cycle inventories. international energy agency (iea) (2017). world energy outlook report 2017, 14 november 2017. for more information see: https://www.iea.org/weo2017/ international energy agency (iea) (2013). transition to sustainable buildings, strategies and opportunities to 2050. executive summary. retrieved from https://www.iea.org/textbase/npsum/building2013sum.pdf iso standards. (2006). iso 14040: 2006 environmental management life cycle assessment principles and framework. geneva, switzerland: international organization for standardization. iso standards. (2006). iso 14044:2006 environmental management life cycle assessment requirements and guidelines. geneva, switzerland: international organization for standardization. jayathissa, p., jansen, m., heeren, n., nagy, z., & schlueter, a. (2016). life cycle assessment of dynamic building integrated photovoltaics. solar energy materials and solar cells, 156, 75-82. doi:10.1016/j.solmat.2016.04.017. klein, t. (2013). integral façade construction. towards a new product architecture for curtain walls. a+be | architecture and the built environment, 3, 1-298. doi:10.7480/abe.2013.3. koláček, m., charvátová, h., & sehnálek, s. (2017). experimental and numerical research of the thermal properties of a pcm window panel. sustainability, 9, 1222. doi:10.3390/su9071222. konis, k., & selkowitz, s. (2017). effective daylighting with high performance façades, emerging design practices. cham, switzerland: springer international publishing ag. loonen, r.c.g.m., rico-martinez, j.m., favoino, f., brzezicki, m., menezo, c., la ferla, g. & aelenei, l. (2015). design for façade adaptability – towards a unified and systematic characterization. in proceedings of the 10th energy forum advanced building skins, bern, switzerland, 1274-1284. loonen, r.c.g.m., trčka, m., cóstola, d., & hensen, j.l.m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483-493. doi: 10.1016/j.rser.2013.04.016. luible, a., overend, m., aelenei, l., knaack, u., perino, m., & wellershoff, f. (2015). adaptive façade network – europe. cost tu 1403. retrieved from http://tu1403.eu/?page_id=209. mao, y., zhen, d., yuan, c., ai, s., isakov, m., wu, j., wang, t., dunn, m.l. & qi, h.j. (2016). 3d printed reversible shape changing components with stimuli responsive materials. scientific reports, 6, 24761. doi: 10.1038/srep24761. maywald, c. (2017). texlon etfe green building factsheets – product data base for leed, breeam and dgnb. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 644-652. mcevoy, m.a., & correll, n. (2015). materials that couple sensing, actuation, computation, and communication. science, 347(6228), 12616891-12616898. doi:10.1126/science.1261689. molter p.l., bonnet, c., wagner, t., reifer m. & klein, t. (2017). autoreactive components in double skin façades. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 133-141. 040 journal of facade design & engineering volume 7 / number 2 / 2019 olivieri, l., tenorio, j.a., revuelta, d., bartolomé, c., sanchez ramos, j., alvarez dominguez, s., navarro, l., cabeza, l.f., & cano aguarón, j.l. (2017). development of pcm-enhanced mortars for thermally activated building components. in advanced building skins gmbh (eds.), proceedings of the 12th international conference on advanced building skins, bern, switzerland, 561-571. persiani, s.g.l. (2018). biomimetics of motion, nature-inspired parameters and schemes for kinetic design. springer international publishing, cham, switzerland. doi10.1007/978-3-319-93079-4 persiani, s.g.l., battisti, a., & wolf, t. (2016a). autoreactive architectural façades – discussing unpowered kinetic building skins and the method of evolutionary optimization. in advanced building skins gmbh (eds.), proceedings of the 11th international conference on adaptive building skins. bern, switzerland. persiani, s.g.l., molter, p.l., aresta, c., & klein, t. (2016b). mapping of environmental interaction and adaptive materials for the autoreactive potential of building skins. in proceedings of the 41st iahs world congress sustainability and innovation for the future. algarve, portugal. pomponi, f., & moncaster, a. (2016). embodied carbon mitigation and reduction in the built environment – what does the evidence say?. journal of environmental management, 181, 1-14 prieto, a., knaack, u., auer, t. & klein, t. (2017). solar coolfaçades: framework for the integration of solar cooling technologies in the building envelope. energy, 137, 1-16. quesada, g., rousse, d., dutil, y., badache, m. & hallé, s. (2012a). a comprehensive review of solar façades. opaque solar façades. renewable and sustainable energy reviews, 16, 2820-2832. quesada, g., rousse, d., dutil, y., badac,he, m., & hallé, s. (2012b). a comprehensive review of solar façades. transparent and translucent solar façades. renewable and sustainable energy reviews, 16, 2643-2651. ramesh, t., prakash, r., & shukla, k k. (2010). life cycle energy analysis of buildings: an overview, energy and buildings, 42 (10), 1592-1600. doi:10.1016/j.enbuild.2010.05.007 ribeiro silveira, r., louter, c., eigenraam, p. & klein, t. (2017). flexible transparency a study on adaptive thin glass façade panels. in molter, p.l., mungenast, m., banozic, m., englhardt, o., & klein, t. (eds.), proceedings of the international mid-term conference of the european cost action tu1403 “adaptive façade network”, munich, germany: tum, 44-45. sachin, h. (2016). dynamic adaptive building envelopes – an innovative and state-of-the-art technology. creative space 3(2), 167183. doi:10.15415/cs.2016.32011. soust-verdaguer, b., llatas, c., & garcía-martínez, a. (2017). critical review of bim-based lca method to buildings. energy and buildings, 136, 110–120. doi:10.1016/j.enbuild.2016.12.009 tucci, f. (2012). ecoefficienza dell’involucro architettonico. la pelle dell’edificio da barriera protettiva a complesso sistema-filtro selettivo e polivalente. (2nd ed.). roma, italia: librerie dedalo. vlachokostas, a., & madamopoulos, n. (2015). liquid filled prismatic louver façade for enhanced daylighting in high-rise commercial buildings, in proceedings of the conference on optical nanostructures and advanced materials for photovoltaics, 23(15). doi:10.1364/oe.23.00a805. volk, r., stengel, j., & schultmann, f. (2014). building information modeling (bim) for existing buildings − literature review and future needs, automation in construction, 38, 109–127. doi:10.1016/j.autcon.2013.10.023 wang, j., beltrán, l.o. & kim, j. (2012). from static to kinetic: a review of acclimated kinetic building envelopes. in proceedings of the solar conference, 5, 4022-4029. ökobaudat database. (2016). retrieved from http://www.oekobaudat.de/en.html. 041 journal of facade design & engineering volume 7 / number 2 / 2019 annexes innovative technologies for the building envelope author year topic approach answers to ws terminology quesada et al. 2012a review of solar façades technological what building-integrated solar thermal system (bist); building-integrated photovoltaic system (bipv); building-integrated photovoltaic thermal system (bipv/t); thermal storage wall; solar chimney quesada et al. 2012b review of solar façades technological what mechanically ventilated transparent façade (mvf); semi-transparent building-integrated photovoltaic system (stbipv); semi-transparent building-integrated photovoltaic thermal system (stbipv/t); naturally ventilated transparent façade (nvtf) tucci 2012 innovative materials and components technological systematic what where innovative technologies; variable property materials vpm: tim, pcm, dynamic gel; variable conduttance insulation vci, aereogel, dielectric glass; variable transmittance glass vtg, variable convection diodes vcd, chromogenic glass, prismatic panes and films; dynamic trombe walls; shading systems. klein 2013 integral façade construction technological systematic what where why integral façade; systematic design; product levels; supporting functions zhang et al. 2015 bist and applications technological what where how building integrated solar thermal (bist): air based, water based, refrigerant based, pcm based adaptive façades badarnah 2012 biomimetics for building envelope adaptation biomimetic why how multi-functional interface: key functions, morphological means, multi-regulation; environmental challenges; processes wang et al. 2012 review of acclimated kinetic building envelopes (ake) biomimetic technological what how acclimated kinetic building envelope (ake); static vs kinetic; (climate) responsive, active, intelligent, (climatic) adaptive, smart, interactive, (high) performative, kinetic, dynamic; architectural aesthetics; solar responsive, air-flow responsive; loonen et al. 2013 state of the art climate adaptive building shells (cabs) systematic what where how when relevant physics; time scale; scale of adaptation; control type; typology loonen et al. 2015 classification approaches for adaptive façades systematic what where why how when unified and systematic characterization; façade classification; responsive function; operation: intrinsic, extrinsic; response time; spatial scale; visibility; adaptability; dynamic exterior shading and louver façades; pcm glazing; bipv double-skin luible et al. 2015 common cabs research topics mixed what pv; advanced materials; façade glazing; façade shading; control systems; façade functions mcevoy & correll 2015 materials that couple sensing, actuation, computation, and communication technological what how sensing; actuation; multifunctional materials; robotic materials; shape-changing materials >>> 042 journal of facade design & engineering volume 7 / number 2 / 2019 innovative technologies for the building envelope author year topic approach answers to ws terminology vlachokostas & madamopoulos 2015 daylighting technology in highrise commercial buildings technological what where how liquid filled prismatic louver (lfpl); badarnah 2016 light management: lessons from nature systematic biomimetic why how biomimetic design process; morphological means jayathissa et al. 2016 lca of dynamic bipv technological life cycle what how when building-integrated photovoltaic system (bipv); adaptive solar façade (asf); actuator mao et al. 2016 3d printed reversible shape changing components technological what where how stimuli responsive materials; reversibly actuating components; shape changing components; shape memory polymers; hydrogels; 3d printed components; persiani et al. 2016a autoreactive architectural façades systematic biomimetic how unpowered kinetic building skins; adaptive systems: responsive, reactive, interactive, autoreactive; motion parameters: system type, geometry, energy persiani et al. 2016b adaptive materials and autoreactive building skins (abs) biomimetic technological what where how type of energy in the environment: radiant, potential, kinetic; adaptivity in materials: smp, scp, tem, tb, tbm, scp, smp, sma, smf, smc, sm-bs, bm, aps, saps sachin 2016 dynamic adaptive building envelopes (dabe): state of the art technology technological what how methods of actuation: motor based, hydraulic actuators, pneumatic actuators, material based; robotic materials; smart glass aresta 2017 auto-reactive strategies. materials for innovative façade components technological what where how innovative; adaptive; passive; auto-reactive systems; input-energy and output-stategy badarnah 2017 environmental adaptation in buidling envelope design systematic biomimetic why how environmental adaptation; adaptation means; bridgens et al. 2017 wood based responsive building skins technological life cycle what where when wood based responsive; hygromorphic materials; responsiveness; reactivity; actuation capacity; durability; sustainability, aesthetics; weathering clifford et al. 2017 application of shape-memory polymers to climate adaptive building façades technological what where how shape-memory polymers; climate adaptive building façades; dynamic materials; smart materials; smart tiles curpek & hraska 2017 ventilation units with pcm for double-skin bipv façades technological what where how pcm; double-skin bipv façades >>> 043 journal of facade design & engineering volume 7 / number 2 / 2019 innovative technologies for the building envelope author year topic approach answers to ws terminology koláček et al. 2017 thermal properties of a pcm window panel technological what where how pcm konis & selkowitz 2017 advancing façade performance technological what where how iot-based sensor network: dynamic façade, sensor, controllable lighting, user input maywald 2017 texlon etfe green building factsheets – product data, leed, breeam and dgnb technological life cycle what where when etfe foils; etfe cladding system; epd; building certification systems molter et. al. 2017 autoreactive components in double skin façades technological what where how autoreactive components; double skin façades; adaptive building envelope; closed cavity olivieri et al. 2017 development of pcm-enhanced mortars for thermally activated building components technological what where how pcm; thermal energy storage (tes); thermally activated building systems (tabs); radiant wall prieto et al. 2017 solar cool façades, review of solar cooling integrated façade concepts technological what where how solar cooling technologies; integration; high-performance, intelligent, adaptive façades ribeiro silveira et al. 2017 adaptive thin glass façade panels technological what where chemically strengthened thin glass; adaptive panels; lightweight façade; kinetic façade table 4 overview of the academic literature p:\ios_press\finals\fde\fde 4(1-2)\web\fde0053.dvi journal of facade design and engineering 4 (2016) 1 doi 10.3233/fde-160053 ios press 1 editorial advanced materials and nanotechnology cluster buildings are the largest consumer of energy and the largest emitter of greenhouse gases. more than 50% of the residential buildings in europe were built before 1970; thus renovation and upgrading of the existing building stock is a priority in the eu energy policies and for all the eu member states. the increasing focus on energy conservation necessitates the adoption of innovative materials. government regulations and support for zero energy buildings provide an impetus to the adoption of innovative materials (directive 2010/31/eu). amanac is a collaboration and coordination platform across all of the eu funded advanced materials and nanotechnology projects within the framework of the energy efficiency in buildings public private partnership (eeb-ppp); its activities address development of advanced materials, components and systems for the improvement of the energy efficiency in the built environment. amanac is a cluster of 29 projects, representing 255 project partners, 63% of which are large enterprises or smes. amanac aims to maximize the impact of the amanac projects on the european industry and society. more information about amanac can be found at http://amanac.eu/ the adoption of innovative materials in building envelopes offers great potential for future design and construction. this is why the journal of facade design and engineering dedicates this special issue to this topic. jfde is a firm partner for the distribution of scientific knowledge of the icae2015 international congress on architectural envelopes (www.icae2015.com/en), organised by tecnalia in san sebastian, spain. the papers presented in this current issue originate from five amanac research projects that researched new advanced materials and systems for energy efficient buildings. the works have been selected among the amanac project research results, initially presented in a special session at icae2015. the extended papers have been subjected to the regular double blind review process of the journal. guest editors maria founti school of mechanical engineering national technical university of athens, greece e-mail: mfou@central.ntua.gr. julen astudillo larraz sustainable construction division tecnalia, spain e-mail: julen.astudillo@tecnalia.com. editors in chief ulrich knaack and tillmann klein issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). http://amanac.eu/ www.icae2015.com/en mailto:mfou@central.ntua.gr mailto:julen.astudillo@tecnalia.com journal of facade design and engineering 3 (2015) 71–80 doi 10.3233/fde-150027 ios press 71 an experimental and numerical simulation study of an active solar wall enhanced with phase change materials dionysios i. kolaitisa,∗, roberto garay martinezb and maria a. fountia alaboratory of heterogeneous mixtures and combustion systems, thermal engineering section, school of mechanical engineering, national technical university athens, athens, greece bsustainable construction division, tecnalia, parque cientı́fico y tecnológico de bizkaia, c/geldo, edificio, derio (bizkaia), spain abstract. solar walls can be used to increase the overall energy efficiency of a building. phase change materials (pcm) are capable of increasing the effective thermal mass of building elements, thus decreasing the overall energy consumption. recently, the incorporation of pcm in a solar wall has been proposed, aiming to increase the total energy efficiency of the system. the main scope of this work is to investigate the thermal behaviour of a pcm-enhanced solar wall (pcmesw), using experimental and numerical simulation techniques. a prototype pcmesw is installed in a large-scale test facility and is exposed to dynamically changing climate conditions. a broad range of sensors, used to monitor the time-evolution of several important physical parameters, is employed to assess the dynamic response of the pcmesw. in addition, a computational fluid dynamics tool is used to numerically investigate the thermal behaviour of the pcmesw prototype. predictions of the developing flowand thermal-field in the pcmesw’s air cavity are validated by means of comparison with the obtained measurements; in general, good levels of agreement are observed. results of the numerical simulations may support the design optimization process of innovative pcmesw systems. keywords: solar wall (sw), trombe wall, phase change materials (pcm), experimental data, cfd, simulation, building energy performance, energy savings 1. introduction primary energy use in buildings accounts for approximately 40% of the total annual energy consumption and co2 emissions in the european union (balaras et al., 2007). therefore, energy efficiency of buildings is increasingly becoming a major worldwide energy policy objective. there is a large variety of construction techniques and materials available that can be used to improve the energy efficiency of buildings. solar walls (sws) can be used as passive building elements aiming to decrease energy consumption for heating and cooling purposes. in addition, incorporation of phase change materials (pcm) in building elements can be used to increase their thermal inertia. recently, the use of pcm to increase the effectiveness of a conventional sw has been proposed (amundarain suarez et al., 2014); the pcm-enhanced sw can be installed both in new and existing buildings, as a means to improve the total energy efficiency. the main scope of the current work is to investigate the dynamic ∗corresponding author: dionysios i. kolaitis, laboratory of heterogeneous mixtures and combustion systems, thermal engineering section, school of mechanical engineering, national technical university athens, heroon polytechniou 9, polytechnioupoli zografou, 15780 athens, greece. tel.: +30 210 7724002; fax: +30 210 7723527; e-mail: dkol@central.ntua.gr. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:dkol@central.ntua.gr 72 d.i. kolaitis et al. / pcm-enhanced solar wall operational behaviour of an innovative active pcm-enhanced sw, using detailed experimental and numerical simulation techniques. 1.1. solar wall a solar wall (sw) is essentially a thermal system comprising a glazing panel and a high thermalmass wall, separated by an air cavity. the orientation of the sw is selected to maximise the incident solar radiation (e.g. southern orientation in the northern hemisphere). the temperature of the high thermal mass wall is increased, thus heating the air in the cavity; the heated air flows upwards, due to thermal buoyancy (passive operation, natural convection) or mechanical devices, e.g. fans (active operation, forced convection). air is allowed to enter and exit the cavity through ventilation openings located in its bottom and top side, respectively. the air ventilation openings can be controlled in order to allow air to enter/exit the cavity either from/to the external (ambient) or internal (indoors) boundary of the system. sw are installed aiming to reduce the overall energy demand for heating and cooling. 1.2. phase change materials latent heat storage systems can be used to decrease the overall energy use in buildings; phase change materials (pcm), exhibiting high values of latent heat of melting (or solidification), are extensively used in latent heat storage systems. use of pcm in building elements has been recently proposed, aiming to increase the indoor thermal comfort (reduction of air temperature peaks, decrease of diurnal indoor temperature fluctuations), decrease the required energy consumption for heating/ cooling (reduction of the heating and cooling peak loads) and take advantage of off-peak energy savings (soares et al., 2013). latent heat storage by incorporating pcm in building elements is an attractive way to compensate for the limited thermal mass of modern lightweight buildings. aiming to improve the overall building energy efficiency, pcm can be used in a variety of building elements such as walls (construction units, insulation materials), glazing and shading devices (glazing systems, solar walls), thermally activated constructions or even hvac systems (pomianowski et al., 2013). 1.3. pcm-enhanced solar wall when a conventional sw is enhanced with a pcm the total thermal mass of the system is increased. in this case, it is possible to reduce the overall volume of the wall, thus resulting in significant advantages in terms of lower cost and dead load. a number of experimental and theoretical analyses has been conducted to investigate the applicability of using pcm in sw systems (manz et al., 1997; weinlaeder et al., 2005; khalifa & abbas, 2009; zalewski et al., 2012; soares et al, 2013). in a pcmenhanced solar wall (pcmesw), the large sensible thermal mass of a conventional sw is essentially substituted by the large latent heat exchange, associated with the pcm phase change process. an innovative technological concept based on this idea has been proposed in a recent european patent (amundarain suarez et al., 2014); the proposed system can be installed either to new or existing (energy retrofitting) buildings. the pcmesw system (fig. 1) comprises a double glazing layer, a highly reflective solar protection ‘roller blind’ system, a high thermal mass wall which incorporates pcm and an insulation layer located between the pcm-enhanced wall and the indoor environment; an air cavity is formed between the glazing and the wall. in the pcmesw system examined in this work, d.i. kolaitis et al. / pcm-enhanced solar wall 73 fig. 1. schematic overall layout of the pcmesw system (left) and detailed cross section near the bottom air inlet (right). fig. 2. schematic layout of the three operational modes and six sub-modes. a commercial macro-encapsulated pcm was used. the temperature range for melting/solidification of the employed pcm was 28–30◦c; its latent heat storage capacity was 190kj/kg and its mean density was 1510kg/m3. on the bottom and top sides of the cavity are located air ventilation openings to allow air to enter/exit the cavity either from/to the external (ambient) or internal (indoors) boundary of the system. air ventilation is assisted by the use of four small electrically driven fans, installed at the bottom of the air cavity; therefore, the examined system is characterized as an ‘active’ device. this innovative pcmesw system is able to operate in three main operational modes; each mode is further split in two sub-modes, one for ‘daylight’ operation and the other for ‘night-time’ conditions. the general configuration of the pcmesw system in each operational mode is shown in fig. 2. in operational mode 1, used mainly during the heating season (winter), the pcmesw system operates as a conventional trombe wall; air is allowed to enter and exit the system only from/to the ‘indoors’ side. during daytime (sub-mode 1-d), the solar protection roller blinds are inactive, thus allowing the incident solar radiation to heat both the air and the pcm-enhanced wall. during night-time (sub-mode 1-n), the solar protection is activated to mitigate heat losses to the ambient. operational mode 2, also used during the heating season (winter), corresponds to a parietodynamic wall. in this case, ambient air is allowed to enter the system through the bottom inlet vents and, after being heated by the high thermal mass wall, is directed indoors, through the top outlet vents. similar to operational 74 d.i. kolaitis et al. / pcm-enhanced solar wall table 1 system configuration for the three operational modes and six sub-modes operation trombe wall parietodynamic wall ventilated façade mode 1 2 3 sub-mode 1-d 1-n 2-d 2-n 3-d 3-n bottom vent in in out out out out top vent in in in in out out roller blind up down up down down down ventilation fan on on on on off on mode 1, the roller blinds are inactive during daytime (sub-mode 2-d) and are activated during the night (sub-mode 2-n). in operational mode 3, commonly used during the cooling season (summer), the pcmesw system operates as a ventilated facade; air is allowed to enter and exit the system only from/to the ‘ambient’ side. in this case, the solar protection ‘blind’ system is always active, thus preventing the incident solar radiation to reach the high thermal mass wall. in daytime (sub-mode 3-d), the ambient air that enters the system is moving upwards, thus cooling the wall by means of natural ventilation. in night-time (sub-mode 3-n), the wall releases the stored thermal energy to the ambient air moving in the cavity, thus reducing the energy demand for cooling. operation of the pcmesw system is optimized by adapting to the varying meteorological conditions. selection of the appropriate operational mode depends on the prevailing ambient conditions and the desired indoor thermal comfort conditions; changes between operational modes can be made either on predefined time periods (diurnal and annual) or dynamically, using real-time information provided by sensors. operational modes 1 (trombe wall) and 2 (parietodynamic wall) are used during the heating season, whereas operational mode 3 (ventilated facade) is used during the cooling season. selection of the actual operational mode of the pcmesw system can be made either automatically, using appropriate control algorithms, or manually. the different operational modes are set by modifying the position of the air ventilation openings (air can enter or exit either from/to the external (ambient) or the internal (indoors) environment), activation of the solar protection roller blind system (when activated it prevents solar heat flux to reach the high thermal mass wall) and activation of the ventilation fans installed at the bottom of the system (active or passive operation). the configuration of each controllable device for every operational mode and sub-mode available is presented in table 1. 2. experimental study as part of the development of the pcmesw system, a scientific and technical assessment of the pcmesw concept was carried out, comprising a full-scale test under real atmospheric conditions. the testing campaign was aimed at providing important information of the energy performance of the system and gaining confidence in the level of integration between different elements in the solar collector system, a significant step before industrialization of the design. for this purpose, a south-oriented part of the building envelope in the experimental research facility kubik by tecnalia was modified to house the pcmesw prototype. although this paper refers mainly to the thermal d.i. kolaitis et al. / pcm-enhanced solar wall 75 fig. 3. view of the pcmesw system installed in the southern facade of the kubik building (left) and schematic layout of the installed sensors (right). assessment of the pcmesw development, manufacturability, installation, wear, accuracy of actuators, suitability of fans, wiring and other aspects were also tested in the process. 2.1. layout of the test facility kubik by tecnalia is a full-scale multi-rise building aimed at realistic testing of energy efficient building concepts and architectural systems, providing a fully adaptable environment (internal boundary conditions, hvac system layout, adaptation of building envelopes, fully customizable building automation and control). it is located in derio, on the atlantic coast of spain, which is characterized by a cfb climate based on the koeppen climate classification system (kottek et al., 2006). the cfb climate is typical for the majority of central and western europe, including the british islands and some locations on the mediterranean coast. the kubik building is designed and operated as a test facility to bridge the gap between laboratory testing and full scale deployment and is customized on a case-by-case basis to meet the specific needs of each project. in this particular case, a part of the south-facing curtain wall system was removed to accommodate the pcmesw system (fig. 3, left). the internal space (room) to which the pcmesw system was attached has an area of 45 m2; thermal comfort measurements for the indoor environment were obtained using a full boundary condition measurement system, comprising temperature measurements at three height levels and a radiant temperature sensor. heat transfer to other rooms of the building, through internal partitions or floor and roof slabs, was negligible, since the hvac system of the building provided steady indoor conditions. the bottom inlet and top outlet vents to the indoor space were located 0.3m above the floor level and 0.3m below the ceiling level, respectively. the pcmesw system provided additional heat at the upper area of the indoor space, increasing thermal stratification, however, as this system was conceptualized as a forced (active) ventilation system in which fans are operated whenever the system introduces heat to the building, this eased up the recirculation process through the lower air inlet. in more advanced implementations, a more complex ducting system that may separate the inlet and outlet openings could help in alleviating such issues. the examined system was heavily instrumented to verify the behaviour of the pcmesw as a passive system (solar thermal collector) and as an adaptive active system (control algorithm). temperature sensors were located at different heights within the air cavity and the pcm surface of the pcmesw 76 d.i. kolaitis et al. / pcm-enhanced solar wall fig. 4. recorded measurements of temperatures in the pcmesw system (left) and incident solar radiation (right) over a typical 8-day period. system. these measurements were complemented with the weather station at the kubik test facility, additional ambient air temperature and radiant temperature sensors installed in the facade, hot wire anemometers in the air cavity and indoor measurements (fig. 3, right). temperatures were measured with pt100 class-a sensors, with an estimated accuracy of 0.1◦c. solar radiation measurements were obtained by kipp & zonen cmp 6 pyranometers. air velocity measurements were obtained using schmidt technology ss-20.500 thermal flow sensors, exhibiting a measurement accuracy of 3%. 2.2. experimental results the pcmesw system was monitored during the autumn-winter period of 2013–2014. within this period a complete dataset covering approximately eight weeks was collected. in fig. 4, representative temperature and solar radiation data are shown. recorded temperatures at the exposed surface of the pcm-enhanced wall are found to reach peak values of 50–60 ◦ c, when the ambient air temperature is far lower (10–20 ◦ c), thus demonstrating the potential of the pcmesw system to assist space heating. the high pcm surface temperature values of 50 ◦ c clearly suggest that a full phase-change occurred during the tests. the optimal phase change temperature and thickness of pcm was found to be greatly dependent on latitude and local shading; therefore, pcm selection should be based on a local and regional optimization process. 2.3. net output energy for space heating the net output energy of the pcmesw system, available for space heating, was estimated using the obtained measurements. empirical correlations were developed, based on a detailed analysis of the measured data, to allow estimation of the maximum daily energy output of the system during a period of one year. as expected, the monthly energy output of the system was found to be highly dependent on the available solar energy for that particular period. in fig. 5, the estimated monthly energy output for a typical building located in bilbao (spain) is presented; a multi-rise residential building with typical construction characteristics of the building stock built in the late 1970s in the area, i.e. uninsulated cavity walls and single glazed fenestration, was considered. during the winter period (november-march), an average monthly energy output of 4 kwh per unit area (m2) of the d.i. kolaitis et al. / pcm-enhanced solar wall 77 fig. 5. estimated monthly energy output for space heating, per unit surface area of the pcmesw. pcmesw system can be achieved. as expected, larger output energy values for the summer period are obtained; however, this output should be filtered out, according to the actual heating needs of the building during the summer (cooling) period. 3. numerical simulations an in-depth view of the thermal behaviour of sw in various operational conditions can be obtained by means of numerical simulations; the obtained results may be used to assist the design process of such complex thermal systems (kolaitis & founti, 2014). in this work, a detailed numerical simulation study is performed, using the ansys-cfx 15.0 computational fluid dynamic (cfd) tool. cfx is a general-purpose cfd tool, capable of simulating complex turbulent, multi-phase, multi-component and reacting flows. the performed time-transient simulations allow investigation of the developing flowand thermal-fields, aiming to determine the dynamic thermal response of the pcmesw system. 3.1. simulation setup a simplified geometry of the actual force-ventilated pcmesw prototype system, investigated in the experimental campaign, is simulated. the system consists (from outdoors to indoors) of a double glazing (6mm and 4mm glazing panels, separated by a 6mm thick enclosed air cavity), a 73mm thick ventilated air cavity and a 17mm thick pcm layer (c.f. fig. 1). the dimensions of the system have been determined in order to maintain design compatibility with the structural elements where the pcmesw system will be integrated; the thickness of the pcm layer has been dictated by commercially available macro-encapsulated pcm solutions. the total height of the simulation domain is 1830mm. a 2d non-uniform cartesian grid, comprising 51380 elements, is used for the simulations; the gasphase grid is refined close to the solid boundaries in order to improve the flow resolution in the region of the developing boundary layers (fig. 6, left). the total simulation time corresponds to a full 24h period; an adaptive time-step is used, ranging from 15s to 300s. the reynolds-averaged navier-stokes formulation is used to describe turbulent flow phenomena; the shear stress transport turbulence model is employed. 78 d.i. kolaitis et al. / pcm-enhanced solar wall fig. 6. indicative view of the mesh (left) and time-varying thermal boundary conditions (right). measurements of the time-varying ambient temperature and solar incident heat flux during a typical 24-hr period (12 october 2013) are used as thermal boundary conditions (fig. 6, right); these values are imposed on the outer surface of the external glazing that is directly exposed to the environment. the pcm layer on the indoor side of the system was attached to the well-insulated external wall of the kubik building; adiabatic boundary conditions are used in this case. at the bottom entrance of the air cavity, a constant volumetric air flow (26.3 l/s) is employed, corresponding to the forced ventilation conditions induced by the four fans installed at the air inlet region. a single operational mode (submode 1-d, trombe wall) is used for the entire duration of the simulation, following experimental practice. therefore, ambient air temperature measurements are used to estimate the air temperature at the bottom inlet of the system. the thermal behaviour of the commercial pcm, used in the actual pcmesw system, is simulated using the effective specific heat methodology (kolaitis & founti, 2013); in this case, the specific heat of the material is artificially modified in the phase change temperature region, in order to account for the pcm’s latent heat of melting/solidification. a range of important pcm thermal properties required for the simulations was unavailable to the authors; plausible assumptions (e.g. thermal radiation emissivity equal to 0.9 and thermal conductivity equal to 0.6w/mk) have been made for the simulations. 3.2. numerical results in fig. 7 (left), cfd predictions for the air cavity and pcm wall surface temperatures are compared to the respective experimental values, obtained at a height of 1600mm. numerical results for the pcm surface temperature are found to be slightly higher than the measured values; these discrepancies are attributed mainly to the assumptions used to estimate the (unavailable) thermal properties of the commercial pcm system used in the prototype (e.g. thermal conductivity, emissivity) and the adiabatic boundary conditions used on the ‘indoor’ side. however, a very good quantitative agreement is achieved for the air cavity temperature, suggesting that the cfd code may be effectively used to analyse the characteristics of the flow-field developing in the cavity. a comparison of the developing velocity and temperature profiles in the air cavity is presented in fig. 7 (right). the depicted curves correspond to three characteristic positions along the height of the cavity, i.e. 5%, 50% and 95% of the total height, as measured from the bottom inlet; the time instant corresponds to 14:20. predictions of air velocity and air temperature in the cavity suggest that d.i. kolaitis et al. / pcm-enhanced solar wall 79 fig. 7. temporal evolution of air cavity and pcm wall surface temperatures (left) and spatial distribution of air cavity velocities and temperatures at various heights (right). the mean temperature is increased with increasing height; due to the increased thermal mass of the pcm, higher air velocity and temperature values are observed at the boundary layer developing near the pcm wall. the almost homogeneous velocity profile at the inlet (5%) is gradually transformed to a highly distorted profile, where high velocities are observed near the wall. a similar behaviour is observed in predictions of the temperature profiles. near the top outlet of the cavity, the air temperature near the pcm wall is almost 31 ◦ c higher than the respective temperature close to the inner glazing. 4. conclusions an innovative solar wall, enhanced with phase change materials, has been investigated, using both experimental and numerical simulation techniques. a prototype pcmesw has been installed in a real-scale testing building and was monitored for eight weeks during the autumn-winter season. the obtained measurements indicated that the potential monthly net output thermal energy of the system, during the winter period in a typical cfb climate region, may be as high as 4 kwh/m2. numerical simulations of the prototype device have been also performed, using a cfd tool. predictions have been found to agree reasonably well with the measured values. cfd results allowed an in-depth view of the developing thermaland flow-field in the prototype pcmesw. acknowledgments this study has been financially supported by the e.c. in the frame of the fp7 project ‘meefs: multifunctional energy efficient façade system for building retrofitting’ (eeb.nmp.2011-3, grant no. 285411). references amundarain suarez, a., campos dominguez, j. m., chica paez, j. a., meno iglesias, s., uriarte arrien, a., garay martinez, r., et al. (2014). passive solar collector module for building envelope. european patent ep 2520870 b1, 5 march 2014. balaras, c. a., gaglia, a. g., georgopoulou, e., mirasgedis, s., sarafidis, y., & lalas, d. p. (2007). european residential buildings and empirical assessment of the hellenic building stock, energy consumption, emissions and potential energy savings. building and environment, 42, 1298-1314. 80 d.i. kolaitis et al. / pcm-enhanced solar wall khalifa, a. j. n., & abbas, e.f. (2009). a comparative performance study of some thermal storage materials used for solar space heating. energy and buildings, 41, 407-415. kolaitis, d. i., & founti, m. a. (2013). development of a solid reaction kinetics gypsum dehydration model appropriate for cfd simulation of gypsum plasterboard wall assemblies exposed to fire. fire safety journal, 58, 151-159. kolaitis, d. i., & founti, m. a. (2014). solar wall enhanced with phase change materials: a detailed numerical simulation study. proceedings of the 9th energy forum on advanced building skins, 28-29 october 2014, bressanone, italy, 183-195. kottek, m., grieser, j., beck, c., rudolf, b., & rubel, f. (2006). world map of the köppen-geiger climate classification updated. meteorological zeitschrift, 15(3), 259-263. manz, h., egolf, p. w., suter, p., & goetzberger, a. (1997). tim-pcm external wall system for solar space heating and daylighting. solar energy, 61, 369-379. pomianowski, m., heiselberg, p., & zhang, y. (2013). review of thermal energy storage technologies based on pcm application in buildings. energy and buildings, 67, 56-69. soares, n., costa, j. j., gaspar, a. r., & santos, p. (2013). review of passive pcm latent heat thermal energy storage systems towards buildings’ energy efficiency. energy and buildings, 59, 82-103. weinlaeder, h., beck, a., & fricke, j. (2005). pcm-façade-panel for daylighting and room heating. solar energy, 78, 177-186. zalewski, l., joulin, a., lassue, s., dutil, y., & rousse, d. (2012). experimental study of small-scale solar wall integrating phase change material. solar energy, 86, 208-219. from city’s station to station city 021 journal of facade design & engineering volume 9 / number 2 / 2021 what makes a façade beautiful? architects’ perspectives on the main aspects that inform aesthetic preferences in façade design alejandro prieto*, mimi oldenhave * corresponding author delft university of technology, faculty of architecture and the built environment, department of architectural engineering + technology, architectural facades & products research group, a.i.prietohoces@tudelft.nl abstract the aesthetic of our built environment is perceived as an important aspect to consider for the design of human-centred cities, but a problem quickly arises in the presence of clashing conceptions of what we understand to be aesthetically pleasing. this paper adds to this discussion, by exploring architects’ aesthetic preferences in façade design, aiming to include design practitioners in a debate that so far has remained largely academic. thus, the goal of the study was to identify relevant aspects, based on a series of semi-structured interviews with practitioners representing 34 architectural firms from the netherlands, carried out from january to april 2020. it was possible to identify two major types of aspects, and subsequent sub-groups. intrinsic aspects (compositional, plastic, detail design, and character) comprise aspects that are characteristic of a façade as an object, while extrinsic aspects (human, intellectual, and contextual connection) consist of relational features, determining the perceived beauty of a façade in terms of its connections with an outside agent. the main identified aspects in each one of the groups, the potential relations among them, and their relative relevance within the surveyed sample were part of the assessment, comparing the outcomes against previous results from the literature. keywords façade, design, aesthetics, architecture, interviews, content analysis 10.7480/jfde.2021.2.5540 022 journal of facade design & engineering volume 9 / number 2 / 2021 1 introduction the aesthetic of our built environment is perceived as an important aspect to consider in our quest for the design of human-centred cities. our built environment should not only respond to physical measurements of comfort, but should also aim to strengthen the psychological well-being of our communities, by means of advancing people’s happiness and boosting a sense of pride in their surroundings. to live surrounded by beauty is something we all aspire to, but a problem quickly arises in the presence of clashing conceptions of what we understand as aesthetically pleasing, beautiful, or harmonious. thus, it becomes necessary to thoroughly explore and discuss different preferences and points of view when it comes to the design of beautiful buildings, and particularly their façades, as the main architectural element carrying the symbolic expression of the building and the architect’s intent, while serving as a backdrop for public space and urban life. the meaning of beauty in architecture and the discussion of aesthetic preferences when it comes to the design of our buildings, have been explored since classical times, with vitruvius’ treatise on architecture as the earliest example. in it, the author proposed an understanding of beauty based on symmetry and proportion as the main traits found in nature’s designs, using the proportions of the human body as a model of natural perfection (vitruvius & morris, 1914). since then, this understanding of beauty in architecture has been the leading theory, exemplified over time by the widespread use of the ‘golden ratio’ in renaissance architecture (alberti, 1986), or the use of an anthropometric scale of proportions in the work of le corbusier during the mid-twentieth century (le corbusier, 1953). in recent decades, the aesthetic perception of the built environment has been addressed by several scholars, mostly on two fronts: either from a theoretical perspective, anchored in a historically rooted philosophical discourse; or through the assessment of the responses of different groups of participants, following surveys and questionnaires from psychology studies. it should be mentioned that there are also relevant efforts from the field of cognitive neuroscience, aiming to understand how our brain perceives visual environmental stimuli (kirsch et al., 2016; pearce et al., 2016; rolls, 2014). however, these are broader in scope, not directly tackling particular aspects associated with architectural and urban design. first, several theoretical studies in recent decades have discussed aesthetics in architectural design, aiming to identify underlying aspects and resources for the design of beautiful or harmonious buildings and façades. krier (1988) wrote in his book about architectural composition that the fulfilment of aesthetic requirements depends on the artistic interpretation of proportion, structure, material, and colour. moreover, he stated that geometry is the basis for all forms of architectural expression, and architecture, as organised geometry, draws its strength from opposing rather than adopting the laws of nature. salingaros (1995; 1999; 2000) on the other hand, declared that structural orders present on historical buildings are the main reason we find them pleasing; they follow the ordered internal complexity found in nature-based and organic forms. this direct link to nature was expanded by alexander (2004) in his four-volume set on the art of building and the nature of the universe. in it, he wholeheartedly advocates for liveliness and elements derived from nature as defining traits used to understand beauty in architecture, proposing fifteen properties to guide the design of lively geometries, inspired by patterns and visual cues found in nature. other theoretical works worth mentioning have focused on the impact that certain visual cues have on the observer. nasar (1994) stated that architecture is the cause of diverse feelings, such as pleasantness, excitement, or calmness, which may be promoted through the use of certain key 023 journal of facade design & engineering volume 9 / number 2 / 2021 design aspects (complexity, order, style, and atypicality). smith (2003) stated that the recognition of patterns and the incorporation of opposites into a balanced whole, are the two sides of the coin of aesthetic experience. moreover, he also acknowledged the influence of nature in human perception, describing the essence of beauty—as we find it in nature—as the clash between complexity and order. lastly, de botton (2006), in his seminal book the architecture of happiness, explored the links between aesthetics, architecture, and their influence on our happiness, emotions, and behaviour. he focused on the communicative aspects of architecture, stating that buildings embody social values, which we perceive through the lens of our own experiences. hence, what we perceive as beautiful serves as evocation of the values that we hold dear, directly connecting our buildings and their façades to the human scale and experience. discussing the outcomes from experimental research and surveys, coburn et al. (2019) studied psychological responses to natural patterns in architecture, following the biophilic hypothesis previously discussed. in the study, 167 participants were asked to arrange images based on aesthetic preference, obtaining a close relation between aesthetic preference ratings and presence of natural patterns in architecture. tinio & leder (2009) studied the role of symmetry and complexity in the aesthetic judgement of buildings, through individual and group assessments of sets of images with different scenarios. symmetry was found to be more important than complexity to explain aesthetic preferences of the participants. another study by keshtkaran et al. (2017) studied aesthetic preferences in high-rise buildings based on the definition of two sets of contrasting factors: (a) primary factors (balance, symmetry, regularity, simplicity, unity, economy, understanding, predictability, subtlety, neutrality, opacity, consistency, scale, flatness, sequentiality, proportion, colour, materials, and style); and (b) distinctive factors (asymmetry, complexity, spontaneity, activity, stasis, boldness, emphasis, transparency, variety, scale, depth, randomness, colour, materials, solids/voids). the study showed that people preferred designs that tend to follow distinctive factors. finally, other explorations have focused on the role of specific aspects in the aesthetic perception of buildings, such as the use of colour and its communication potential (meerwein et al., 2007; mikellides, 2012; o’connor, 2008), or the role of patterns and variations in architectural composition (breen, 2019; chamilothori et al., 2019; el-darwish, 2019). as the presented overview shows, the matter of aesthetic preferences in architectural design has been a clear topic of interest for scholars; however, the discussion has remained largely academic, failing to acknowledge the perspective of architects and urban designers, who are the ones shaping our buildings and cities. their insights are undoubtedly relevant to understanding the logic behind façade design—the face of our buildings—where their preferences have a clear impact on not just the look, but also the performance, of our built environment. on a more practical note, acceptance from architects and façade designers is often cited as a common barrier for the integration of new technologies in the built environment, technologies that could help mitigate the environmental issues we currently face (farkas & horvat, 2012; prieto et al., 2017; tablada et al., 2020). further discussion and understanding of aesthetic preferences in architectural design could potentially lead to the development of a wide array of multifunctional building products for façade applications, tailored to different aesthetic sensibilities from both architects and the general public. this paper adds to the knowledge in the field, by showing the results of a study conducted to assess the perception of architects regarding aesthetic preferences in façade design. the aim of the study was to identify parameters and relevant aspects involved in the appearance of buildings, based on a series of interviews with practitioners from different architectural firms in the netherlands. 024 journal of facade design & engineering volume 9 / number 2 / 2021 the choice to use interviews as the source of the study followed the intention to understand conscious preferences when it comes to façade design, as part of a larger discussion about these topics involving experienced practitioners. the results presented in the paper are framed within a larger research project, where, at a later stage, they will be used to design an image-based survey for mass-distribution, assessing the underlying preferences of a much wider sample based in the identified aspects from the interviews, testing their validity in a larger context. in that sense, it is important to state that while the results from the interviews are regarded as valuable information, they do not claim to be exhaustive nor universally valid; they aim to provide insights on aesthetic preferences in façade design, through the perspective of dutch architects, so the cultural background of the sample must be kept in mind when assessing the results. 2 materials and methods 2.1 data-gathering methodology the study follows the qualitative evaluation of a series of interviews with architects, by means of content analysis techniques. an initial list of architectural firms in the netherlands was composed through internet queries in specialised architectural portals and professional networks, considering established companies with a dedicated website, with a base of operations in amsterdam, rotterdam, delft, or the hague. it was also an initial condition that the work shown on their websites comprised more than just single houses, showing a certain degree of experience with complex projects. based on the initial list, 82 firms were contacted via e-mail inviting them to participate in the study, using the contact information listed on their websites. following that, 43 firms replied (52% of the total firms contacted), which finally resulted in 34 successfully conducted interviews (41% of the original firms invited). in all cases, the e-mails were directed to one or more of the partners, inviting them to take part in the study personally, or alternatively to appoint someone who could represent the work of the firm. the interviews were conducted from january to april 2020. most of them took place on location, but the last 7, originally scheduled in march, had to be cancelled due to covid-19 restrictions, and later rescheduled online. the interviews followed a semi-structured questionnaire comprising open-ended questions, and took 45 minutes on average. the questionnaire was developed with the larger aim to identify and explore certain aspects to be considered in façade design. hence, both general and specific information about the façade design process of each firm was considered in the data gathering. consequently, the questionnaire was structured around five main themes: (i) general information; (ii) general design approach; (iii) façade design elements and intentions; (iv) aesthetic perception of façades; and (v) sustainability in façade design. the results presented in this paper refer to the fourth theme, which aims to explore what the interviewees perceive as a beautiful façade, focusing on the definition of specific traits, elements, or aspects involved in their aesthetic perception (what are in your opinion the main aspects or elements involved in the aesthetic perception of façades? what makes a façade beautiful?). 025 journal of facade design & engineering volume 9 / number 2 / 2021 2.2 description of the sample the first set of questions from the questionnaire (general information) aimed to characterise both the firm and the interviewee, to better describe the sample. basic information about the firm (location and size) was registered, and the role/position of the interviewee within the firm, their gender, and their years of experience as an architect/designer were gathered from the interviewee. as mentioned in the previous section, 34 interviews were successfully conducted. the interviews were recorded and transcribed, and then coded for the assessment following conventional content analysis techniques using the software atlas.ti, resulting in a database in excel to allow for the qualitative and quantitative analysis of the gathered information. the database consists of architects’ responses from 34 different architectural firms in the netherlands, located mostly in amsterdam (12 firms / 35%) and rotterdam (16 firms / 47%), and the remaining ones in delft and the hague (6 firms / 18%), as shown in figure 1. regarding the size of the firms (figure 2), most are small sized companies, having between 10 and 49 employees (47%), followed by medium ones (41%). within the latter group, a sub-distinction is made in the graph, between medium sized companies with fewer than 100 employees (10 firms / 29%) and mediumlarge sized companies employing 100-250 people (4 firms / 12%). lastly, 4 micro sized companies (fewer than 10 employees) also took part in the study (12%). 0 2 4 6 8 10 12 14 16 18 amsterdam rotterdam delft the hague in te rv ie w ed a rc hi te ct s 0 2 4 6 8 10 12 14 16 18 <10 (micro) 10-49 (small) 50-99 (medium) 100-249 (medium) in te rv ie w ed a rc hi te ct s fig. 1 location of the interviewed firms fig. 2 size of the firms (n° employees) about the interviewees, 79% of the sample are male, and 21% are female; the vast majority holds a partner position in the firm (85%, comprising 9 partners and 20 founding partners). the remaining 5 interviewees are either architects or associate/senior architects in each company (figure 3). the participants were also asked to state their years of experience in architectural design (figure 4). roughly a third of the group has between 10 to 19 years of experience (32%), and another third has been designing for 30-39 years (32%). following these groups, 6 interviewees declared that they had 20-29 years of experience (18%), and another 5 had 40 or more years (15%). lastly, only one participant claimed to have less than 10 years of professional experience as an architect. 026 journal of facade design & engineering volume 9 / number 2 / 2021 0 5 10 15 20 25 architect associate architect partner founding partner in te rv ie w ed a rc hi te ct s 0 2 4 6 8 10 12 less than 10 10-19 20-29 30-39 40 or more in te rv ie w ed a rc hi te ct s fig. 3 position of the interviewee in the firm fig. 4 years of experience in architectural design 3 results: aesthetic preferences in façade design – what makes a façade beautiful? the interviewees were asked to state what are, in their opinion, the main aspects or façade elements involved in how we aesthetically perceive façades, or more broadly, what makes a façade beautiful according to their own perception. the responses were then coded as part of the assessment following conventional content analysis techniques, to group, explore, and conceptualise the outcomes. thus, the codes were obtained directly and inductively from the responses, without using predefined categories. fig. 5 word cloud of the identified aspects involved in the aesthetic perception of façade 027 journal of facade design & engineering volume 9 / number 2 / 2021 the word cloud shown in figure 5 depicts the codes/keywords obtained from the responses, with word sizes illustrating the amount of total mentions. the assessment of the results mostly focused on the identification and discussion of certain themes throughout the sample, from a qualitative perspective. thus, the amount of mentions per keyword or general theme will only be regarded as referential. nevertheless, it is worth noting that after coding the responses, material expression was the aspect that received most mentions, alongside others such as detail quality, and tectonics. this seems to point out that the interviewed architects have a marked susceptibility for aspects related to material and construction when it comes to façade preferences, which was also previously evidenced by the presence of material as the most mentioned basic resource for façade design. the assessment does not aim to be comprehensive, so it is important to keep the particularities of the sample in mind when reviewing the outcomes. another issue worth mentioning, before delving into the responses in more detail, is that most of the interviewees stated several themes or aspects that contribute to the perceived beauty of building façades. hence, it was found to be nearly impossible to find one defining trait from their own experience that would explain what makes a façade beautiful. even though specific aspects did stand out in their responses, this serves as further evidence of the complexity of the subject of study. based on the initial coding of the responses, it is possible to first identify two major types of aspects that seem to have an impact on the aesthetic perception of façades: intrinsic and extrinsic. the former type comprehends aspects that are characteristic of a façade as an object, while the latter consists of relational features, determining the perceived beauty of a façade in terms of its connections with an outside agent. within these major groups, some sub-groups were identified by exploring the coded responses, which resulted in the generation of a categorisation tree for the organisation and assessment of the gathered data. the identified groups and sub-groups are depicted in figure 6. all groups were identified after the initial exploration of the responses instead of being predefined, so the categorisation tree is regarded as the first outcome of the assessment. intrinsic extrinsic compositional plastic detail design character & expression intellectual connection human connection contextual connection identified aspects related to the aesthetical perception of facades fig. 6 categorisation tree proposed for the identified aspects from the responses, based on the distinction of intrinsic and extrinsic groups of aspects the intrinsic groups comprise aspects that describe a façade in terms of its own inherent qualities, isolated from any information regarding the context. examples of elements within these groups that inform our perception are composition, colour, proportion, or texture, among others. it feels important to point out that even though these focus on the object, there is always a subject experiencing it, so an inherent level of subjectivity will always be part of the appraisal. within this major group, it was possible to identify four sub-groups of intrinsic aspects: (i) compositional, (ii) plastic, (iii) detail design, and (iv) character & expression. 028 journal of facade design & engineering volume 9 / number 2 / 2021 on the other hand, extrinsic aspects qualify a façade, considering how it establishes a relation to an external agent. hence, in these cases, the mental connections built by the observer are what govern the overall perception of a building façade, instead of its isolated attributes. within this group, it was possible to identify three sub-groups: (a) human connection, (b) contextual connection, and (c) intellectual connection. both intrinsic and extrinsic sub-groups will be examined in more detail in the following section, providing a number of selected quotations as examples from each group, for a deeper understanding of the gathered responses. 3.1 intrinsic aspects for the aesthetic perception of façades the first group (compositional) comprises aspects that are based on a compositional approach to façade design, emphasising the relative arrangement of different visual elements, as a defining trait for an aesthetically pleasing façade. this seems to correspond to a more classical view of façade design, bringing it closer to other pictorial arts, similar in a way to organising visual elements on a blank canvas. further evidence of this ‘classical’ viewpoint, beyond its ties to the visual arts, is seen in the wording used by some of the interviewees (table 1). statements such as “it is of course the composition,” “it’s certainly the composition of elements,” or “there is of course something which lots of people before me have tried to describe about proportion and rhythm”; show a categorical stance on the matter, backed by consolidated knowledge and tradition. within this sub-group, the main identified codes from the responses were related to proportions (n=10) and the composition of solids and voids (n=7), the latter being what most interviewees referred to when further explaining their preference for composition (“it’s certainly the composition of elements, mostly open and closed”). the importance of this compositional resource (solids vs. voids) was particularly stressed by one of the interviewees, placing it in a contemporary context by declaring that due to the lack of ornamentation, craftsmanship and relief of modern façades, the main design resource currently left is the relation between the open and closed parts of the façade, alongside proportion. proportion on the other hand, was clearly identified as an aspect to consider by close to a third of the sample, even being regarded as a central one by some of the interviewees (“it all starts with proportion”). its relevance was further stressed in some of the responses by appealing to proportions and “harmonic measures” as overarching rules that dictate how we assess beauty, transcending personal subjectivity. other identified aspects that share the same group were rhythm (n=3), and stratification (n=4), the latter referring to the acknowledgment of different strata within the building (base, middle, and top), and its clear reflection on the design of the façade. the second group of intrinsic aspects (plastic) refers to sculptural qualities of the building façade, thus exceeding its appreciation as a mere plane, in favour of a volumetric approach to it. this group then gathers concepts such as “sculpturality” or “plasticity” as general notions that apply to beautiful façades, according to some interviewees (table 2). here, the most mentioned aspect by the interviewees was material expression (n=16), further explained as getting a sense of the material, following a tactile approach to façades; letting the material and its inherent qualities show through the design. this expression was also mentioned to carry over time, so the durability and ageing of materials were explicitly mentioned as relevant aspects that define how this expression will change over time, without losing its beauty. 029 journal of facade design & engineering volume 9 / number 2 / 2021 table 1 selected quotations from the interviewees categorised under “compositional aspects” identified aspects / keywords selected quotations position (years of experience) composition solids/ voids “it is of course the composition. how the solid and the open part of a façade form a certain structure, composition”. partner (30) composition solids/ voids “it’s a lot about indeed composition and being open or closed, or being approachable and non-approachable. that those aspects are in the right balance”. founding partner (17) composition solids/ voids “it’s certainly the composition of elements, mostly open and closed”. partner (36) composition solids/ voids; rhythm “the way that the windows are put in the facade. it could be a very nice rhythm or a play of different kind of windows”. partner (15) proportion; scale; composition solids/ voids “(nowadays) you have no ornament, you have no craftsmanship and you have a flat facade. and then the only thing that remains are the proportions, they stay. the proportion stays, scale stays and i think the most important aspect is the relation between the parts that are open and the parts that are closed. that is the main thing”. founding partner (24) proportion; rhythm “there is of course something which lots of people before me have tried to describe about proportion and rhythm”. founding partner (18) proportion “i think it all starts with proportion” founding partner (30) proportion “the proportions is of course something that is coming back always, also in smaller elements, but also in the facade as a whole, is an important parameter” architect (10) proportion “it’s about harmonic measures in a way” partner (37) proportion “i think that if you design a façade... and i might not like your style or you are not a fan of mine, but if it’s well-proportioned, i think there is somehow a generic rule for answering something in a proper way” associate architect (15) stratification “(i don’t like it) if the building neglects that the base has a different function than let’s say a middle part and the upper part”. founding partner (17) stratification “i like it when… i would almost say that i’m in that way a kind of classical thinker. i mean, everything has a plinth, a middle and a top”. partner (37) accordingly, several interviewees declared a preference for certain materials, deemed more expressive, particularly considering how they look over time. therefore, natural stone, brick, or concrete were preferred by a section of the sample for their “plastic expression” and response to ageing, in contrast to the use of aluminium or steel, which were deemed to result in “too smooth” façades, which do not naturally age (“in fact, you should clean them every year, but no one does it, so then it gets dirty and you see it getting older (…) but’s it’s just getting more ugly”). two concrete aspects explicitly mentioned in relation to this material expression in façades, were texture or relief (n=7); and depth (n=5). the use of these resources in façades directly reflects their sculptural qualities, either by playing with the finishing of the material, promoting roughness over smoothness (“very smooth façades have no appeal to me at all, because, what can you read from that?”; “most of the time we don’t strive for super smooth façades”), or by purposely misaligning façade components to configure a volumetric perception (“for me it’s about depth, in the literal sense of the word, how you play with the position of the window in the brickwork and position of the lintels. it is about these 40 centimetres or something, 24-20 in the brickwork”). moreover, it was stated that the use of texture and depth bring liveliness into the façade, besides serving as a means to reflect human scale on to the building. 030 journal of facade design & engineering volume 9 / number 2 / 2021 it is noteworthy that two respondents expressed a concern for the current loss of sculptural qualities in façades, identifying its source on the impact that the requirement for continuous insulation has had in façade construction. thus, the insulation layer acts as a barrier between the structure of the façade and its outermost layer, the cladding, which is in turn what we see from the outside. cladding, then, and not a load-bearing massive element is what ultimately defines the expression of the façade, which was an issue that was shared by other interviewees and will be expanded later in the text. table 2 selected quotations from the interviewees categorised under “plastic aspects” identified aspects / keywords selected quotations position (years of experience) material expression; roughness; sculptural materials “what i personally like very much is to stress a certain solidity and sculptural quality of a façade. so you know, very smooth façades have no appeal to me at all, because what can you read from that? (…) i prefer materials than can really, have this kind of sculpturality or plastic expression, so i am more into brick, concrete, even with plasterwork you could do it”. partner (30) texture; material expression; roughness “i think that a façade, for me, it has to have texture, so it has to express some material… so, that can be concrete, i like brutalism very much, for example;. but there has to be a very specific balance between… not too smooth, and not too much designed, but there have to be some, maybe rough things in it”. partner (35) material expression. “the plasticity, i love plastic façades. that you have a kind of... you have a sense of the material, the tactility of a façade”. partner (15) material expression. “also the expression of material, the durability of this. how does the building look in ten years? how does it get old? does it get old in a beautiful way?”. founding partner (30) texture; depth “i think the material is also very important. the plasticity, tectonics, texture of the rhythm. i like it when you have more.. sometimes you play with the depth, i think that makes a facade beautiful”. partner (15) texture; depth “i think texture. that is really something to start with if you want to, you know, something with a lot of relief, structure elements and depth, shade and all that”. founding partner (33) texture; depth “if you talk about depth and relief in the façade, that will bring the facade to life. so, most of the time we don’t strive for super smooth facades”. architect (10) material expression. “we started to understand the material better, and then it’s just very nice to figure out how we can really create items that have certain qualities that you really can’t reach with any other technique”. founding partner (14) depth “i really like to have depth in my facade. that is also something that we are really happy to use”. partner (34) depth “i think that for me it is about depth in a way. depth in the literal sense of the word, how you play with the position of the window in the brickwork and position of the lintels. it is about this 40 centimetres or something, 24-20 in the brickwork”. founding partner (27) the third identified group (detail design) refers to constructional aspects with emphasis on the resolution of details in an aesthetically pleasing manner. specifically, the quality of the detail, in terms of how the different components are assembled, was regarded as a relevant aspect by a third of the sample (n=11), which adds up to almost half of it (n=16) if we also consider the interviewees who mentioned tectonics (n=9) in their responses. when discussing this aspect, some interviewees expressed awareness of the fact that while it is widely perceived as relevant for architects and other 031 journal of facade design & engineering volume 9 / number 2 / 2021 professionals in the field (statement that echoes the frequency of mentions received in the study), it is probably perceived as a minor or non-existent issue for the general public. nonetheless, they were inclined to surmise that a well-designed detail would be reflected on other aspects that would make people appreciate the façade anyway. this fact serves as a good reminder of the scope of the study and the clear differences in aesthetic perception that would arise from a comparison between architects’ and non-architects’, as other studies have suggested. table 3 selected quotations from the interviewees categorised under “detail design” identified aspects / keywords selected quotations position (years of experience) detail quality; tectonics; calm details “the way things are mounted and constructed on the site is a very important aspect. i think that for the audience is a minor detail; but for the… let’s say the people who are part of the profession, these are huge differences; and i think for the audience if you would compare it somehow they would say that the other one looks calmer and better, more pure”. founding partner (17) detail quality “i think it’s nice if there’s some sort of aspect where you think “oh this looks really nice”, or well detailed, interesting choice”. partner (21) calm details; simplicity “i always get a bit nervous when we have a lot of joints in our facades, a lot of things have to meet each other, like... could we think of some reduction there? i mean, life is complicated enough already, with all these complicated materials. so, let’s avoid unintended, too much fuss”. founding partner (30) calm details; tectonics; simplicity “the absence of superfluous-ness, superfluidity, how you’d call it. the fact that you can return to a building as it was meant to be, how it was created”. partner (36) simplicity “to make something look simple is extremely difficult”. architect (6) simplicity “if you see a building properly done by good architects, then i would appreciate about its beauty, when there is as little design as possible. when it is too much… i don’t like big things on the table. keep it simple”. founding partner (40) refinement “i think it’s super important that it is refined, but it’s difficult to grasp as well; but it has something to do with what i said earlier, i can pretty much say that i always try thinning the profiles that we are using, and make it elegant; and because of that you can make it stronger, or robust in other elements”. founding partner (18) refinement “refinement can also come from designed elements that are obviously designed, so, i don’t know, the little hands that neutelings riedijk put on the mas, in antwerp, for instance. that’s a blatant example, so if it didn’t have that, probably it wouldn’t be as refined because it would just be this big natural stone block”. founding partner (18) refinement; ornament “you can make kind of ornaments. my colleague here makes beautiful concrete ornaments for what are called functional elements, such as a gutter downspout at the top of the facade, these things. so also on that level you can put in more refinement”. founding partner (27) within the responses related to detail design, two aspects explicitly appeared in some of the interviews: a preference for calm details (n=4) or simplicity (n=2) and the mention of refinement (n=5) as another condition for a beautiful façade. calm details, and simplicity (n=5, considering both mentions) were two codes identified for the clear preference for keeping the appearance of the façade simple, avoiding “unintended, too much fuss,” “too much noise,” superfluous elements, or over-designed solutions (table 3). when expanding on this issue, the interviewees stated that a more simple, calmer look does not necessarily imply having less detail on the façade, but instead depends 032 journal of facade design & engineering volume 9 / number 2 / 2021 on the way façade components are connected, which circles back to the aforementioned quality of the detail. after all, as one of the respondents put it, in façade design, “to make something look simple is extremely difficult.” on the other hand, refinement was less clearly defined, but the word itself popped up in five separate interviews, invoking a general elegance or grace in the design. based on the responses, it was possible to identify two ways to achieve this. one way was the inclusion of carefully designed ornamental elements in certain places, such as gutter downspouts and door handles, or the mention of the museum aan de stroom, in antwerp, and the cast aluminium hands that adorn its stone façade panels. the other identified way to add refinement to a façade was by controlling the dimensions and proportions of certain elements, particularly window frames and metal profiles, where thinner profiles were declared to be more elegant. on the other hand, another interviewee stated that even when they have to use thicker window profiles, by using large-scale, robust elements such as façade cladding, they manage to maintain a certain refinement by contrast. on a similar note, several interviewees declared an aversion to pvc window frames, due to both their width and their material expression (or lack thereof). finally, the last group of intrinsic façade qualities comprises aspects related to its “character and expression.” within this group, part of the sample generally stated that a façade needs to have “character” to be perceived as beautiful (n=4). what constitutes character was not clearly identified, due to the fact that it could come from several features. in this regard, a resemblance to a person was posed by some interviewees, as when we encounter certain people that have a presence, an aura of self-confidence, or seem to be in balance (table 4). therefore, it could be something about the whole façade, or about certain features that capture our attention and give identity to the façade. the perceived attractiveness of a façade arose in the responses, in the sense that it needs to attract our attention to be perceived as beautiful. mentions of a “wow-effect,” amazement, or surprise were stated to be conditions pertaining to finding a façade beautiful (n=5), related to originality and innovation on its design. certainly, this surprise effect could be generated in a myriad of ways; however, three specific façade features that boost their potential for attraction were identified among the responses: having a dynamic or changing expression; having a layered design; and the use of colour. first, the choice of façades that do not always look the same, and thus change their expression, was declared to be an explicit preference by some interviewees (n=6), arguing that it increases the liveliness of the façade. moreover, this dynamic expression comes from the reaction of the façade, or its components, to variable environmental stimuli, mostly light inputs from local weather conditions. hence, through the use of particular materials, special surface treatments, or texture; these façades look differently throughout the day, or differ between sunny and overcast days, for example. the second identified feature refers to another way to generate different expressions in the façade, through a layered design approach (n=4). here, the façade is designed with consideration given to different layers of detail and information (and sometimes literal constructive layers), responding to different scales of perception. thus, instead of making the façade itself change or react to the environment, a layered façade uses the relative position of the observer to seemingly change its expression when perceived from different points of view. in other words, the building and its façade will be recognised from a distance, but distinct elements and details will appear as we approach it, enriching the visual experience and thus increasing the attractiveness of the façade. 033 journal of facade design & engineering volume 9 / number 2 / 2021 table 4 selected quotations from the interviewees categorised under “character & expression” identified aspects / keywords selected quotations position (years of experience) character “it’s beautiful when it has this radicality. when you see that it is self-evident and it gives the building.. it’s like with people, some people are self-assured and they have a presence when they walk somewhere”. founding partner (33) character “i think for me it’s character which makes something beautiful. it could be anything. it can be something that is enlarged. it can also be something that is very much in proportion”. founding partner (20) character “if a building has a good character and it approaches me on the same adult level as i am. so if the approach to the building is equal to mine, i think there is beauty. i recognise the building as my partner and i can love it as such”. founding partner (40) amazement “i think buildings should never be boring. so architecture needs to be discovered (…) that you really have this wow-effect and that you kind of have the feeling ‘how did they do this’ for instance”. architect (6) amazement “i think it’s beautiful when it sort of fits the building itself, fits its environment, but it also surprises you a bit”. partner (21) changing expression “i think it’s nearly always important that there is a liveliness to the materials that you use, so that they are different on a rainy day or on a light day, so they’re in some way responsive to the light and the conditions around them”. founding partner (18) changing expression “i think it’s very beautiful if the building can transform a bit over the day and the night, that it plays with light, i guess, as an effect on the façade”. founding partner (30) changing expression “sometime i prefer facades that have a change in themselves. if you think about anodised aluminium as an example, it changes with the lights. so in the morning, it has a different reflection than in the twilight. so it really changes with the weather and the sun, that can make a facade very alive”. founding partner (15) colour; character “i do like to use colour, because it works fantastic with light. i’ve made a lot of buildings in colour. and colour is important though you must not over react. colour might express a certain character, but it’s not so ‘i have a green shirt so i’m now all of a sudden durable’, not at all. it is actually the tone of the light versus the dark, that works. so it’s the tone colour and it’s tone value rather than colour itself”. founding partner (40) colour “i spend a lot of time for colour, in the interior is quite difficult. i really think about the outside facade for instance, i often give another colour than the inside because of the sun, because of the contrast, because... i think of a lot of complimentary materials and colours because i like when an interior speaks”. partner (37) colour “generally i’m wary of colour, lots of colour, unless it’s done well. it has to be regarded with a bit of distrust until you’re absolutely sure”. founding partner (18) colour “a mere colour is very cheap and shallow. it has no deeper meaning or it’s not very… intellectually, it’s not very pleasing”. partner (23) third, the use of colour was found to be somehow controversial, and different viewpoints were gathered on the matter. while some interviewees considered it as a feature that may make a façade beautiful (n=4); some others were wary of it, advising its use only when absolutely necessary, and one interviewee even declared the use of colour as “cheap and shallow,” and not pleasing from an intellectual point of view. these claims, being gathered from open questions, show that this is clearly 034 journal of facade design & engineering volume 9 / number 2 / 2021 a relevant issue on the visual perception of façades, acknowledging it even if it did not fall within their own preferences. an interesting point regarding the use of colour was brought by one of its advocates, stating that colour is important, but it is not really about the colour itself, but instead about its tone value, in contrast with other visual elements. this sentiment somehow grounds its application in façades, adding a deeper understanding of colour and its role within a composition, as opposed to the mere “shallow” application of paint coating over a building surface. 3.2 extrinsic aspects for the aesthetic perception of façades as mentioned earlier, the extrinsic aspects that may explain aesthetic preferences when it comes to façades, are defined by the relations between these façades (or their components) and external agents. these agents are the foundations for the sub-groups presented in this section: (a) contextual connection, (b) human connection, and (c) intellectual connection. out of all the identified sub-groups (both intrinsic and extrinsic), the relationship of a façade to its context was the one that received the least number of mentions (n=8). within this sub-group, most mentions referred to the explicit reaction of the façade to its local urban context (n=6), followed by its interpretation or acknowledgement of local culture (n=3). the low number of relative mentions may seem surprising, especially considering that virtually all interviewees stressed in a different section of the questionnaire that awareness of the context is one of the key issues in façade design. even though further information is needed to pose a comprehensive explanation for this seeming mismatch, this may be regarded as a basic competence when it comes to architectural design. as one of the interviewees articulated it, the design of the façade in an urban setting always entails a “balance between blending in and standing out.” therefore, it could be the case that this dichotomy is one more set of aspects to successfully integrate in a design, without being largely understood as an explicit condition to find a façade beautiful (although probably the opposite would hold true, with façades that do not fit their environment being perceived as unappealing or shallow). table 5 selected quotations from the interviewees categorised under “contextual connection” identified aspects / keywords selected quotations position (years of experience) urban context “i think when a building is beautiful it’s when it reacts to its urban context”. founding partner (17) local culture “it is several things, it’s culture and it’s history”. founding partner (41) urban context “the question is, does it work in the public space? so, as an object, i could really like it. in relation to the public space? hmm… is it something i want to relate to if i’m just walking by? partner (34) urban context “last century the opinion, the common opinion was that the facade is not a free thing. it is directly connected with the interior, with the structure. i believe very much that the facade can be separated (…); in that opinion the facade belongs to the public space and not to the structure” founding partner (45) urban context “it depends really on all aspects; how it is connected to the context (…), how you can see what the program is inside”. founding partner (40) 035 journal of facade design & engineering volume 9 / number 2 / 2021 table 6 selected quotations from the interviewees categorised under “human connection” identified aspects / keywords selected quotations position (years of experience) human touch “facades i like, somehow they should not be perfect. perfect façades are normally not very human; so, they look perfect but they’re very anonymous (…) i like it when there’s some kind of personal touch in the façade, you see a certain struggle in the façade, you know? the beauty of imperfectness”. founding partner (13) human touch “so we like to strive also for little imperfection now and then. so adding the human hand in the process and in the design. that makes it even more special and it works on this (small) scale, but it also works from a big distance as well”. architect (10) readability “for me, for us, for the office, it is this important than you can read and understand, in a way, the facade. that means that it has to be regular, on the edge of getting boring”. founding partner (27) readability “i think that the readability of buildings is very important for people. that they can understand, read the language of the architecture, because architecture is also a form of communication. too much abstraction makes buildings dead and public spaces dead”. founding partner (45) readability “understandableness. you have to understand the façade, what is it about, that it is still understandable how it works. and that you know if it is closed or that it can be opened”. founding partner (40) human scale “that it also reacts to.. or at least.. knows how it behaves itself to the human proportion and you can deliberately say ‘i want to be bold’ by making a huge building. but at least that it reflects to the human scale”. founding partner (17) human scale “i think relief in general in a facade is an architectural means of relating to human scale and people”. founding partner (33) human scale “something on the human scale that gives something to the people”. architect (10) how it feels “it’s about haptics. how does a façade feel for instance. is it very shiny? or is it a combination of reflections?”. founding partner (13) how it feels; user interaction “i like a facade to be not.. let’s say hard, in the sense of getting a facade that you just walk by and it doesn’t interact with you”. partner (34) how it feels; user interaction “it’s always that it’s not like this cold thing which is inside-outside, but it has like a bench going on through the inside or the outside”. partner (15) unsurprisingly, several mentions of extrinsic aspects referred to different relations we can establish between the building façade and ourselves, the observers, which were then grouped under the label “human connection.” the first identified aspect that is worth mentioning is the preference for façades that acknowledge the human scale (n=5), as a way for the building to relate to us. it was stated in one of the answers that this may not particularly imply a direct translation of human dimensions, but rather it is about a clear approach that guides how the building behaves in relation to human proportions, reflecting the human scale even in large scale buildings (table 6). related to this human reflection, another aspect mentioned by a group of interviewees was their preference for façades that are not perfectly finished, but rather have imperfections caused by “having a human hand in the process and in the design,” alluding to a “beauty of imperfectness” (n=3). “perfect” façades, on the contrary, were described as anonymous and not very human. thus, the result of a human touch in façade construction was stated to convey “a certain struggle” that makes them relatable. 036 journal of facade design & engineering volume 9 / number 2 / 2021 two other identified aspects associated with this human connection seem to be related to distinct ways of how we perceive and react to façades, either from a more intuitive standpoint or a more rational approach to the object. the first aspect refers to statements about how a façade feels, grouping different sensorial inputs that shape our interactions with it (n=5). besides general notions of the feeling that a façade gives and references to our experience of it, different themes expressed by the interviewees were its reaction to touch (haptics), the perceived temperature of façade surfaces, visual cues related to its specularity, and a certain softness on the façade that allows us to interact with it, through specific functional elements (like a bench), or other unidentified features that makes it approachable as a whole. on the other hand, the readability of façades was considered to be an important issue by some interviewees, as a means by which they can connect to a façade in a rational manner (n=4). hence, it was declared that people need to understand what the façade is about and how it works, providing clear visual cues that help us interact with it (making clear which elements can be operated, or the location of the entrance for instance). this was also argued in one of the responses, which stressed that architecture is also a form of communication, a role that, according to the interviewee, does not relate to the abstraction that seems to be increasingly used in architectural design, which may lead to disconnected buildings and “dead public spaces.” another respondent stated that the need for readable façades shapes their designs through the use of regularity, “on the edge of getting boring,” a sentiment that may clash with other recorded preferences for originality and amazement when it comes to façade design. the last sub-group identified within the boundaries of the study expands on this rational connection to façades, comprising aspects that reflect the intellectual process behind façade design (intellectual connection). therefore, while this set of aspects also speaks of a connection between the façade (the object) and humans (us, the subjects experiencing it), the focus here is on the appreciation of the reasoning behind a certain façade design, rather than a clear reading of its visual elements and functions. consequently, responses that fall into this category refer to the logic behind façades (n=8), the story that the designer tries to tell with it (n=8), and in a more general way, how all the different requirements are integrated into a successful design (n=5). these three themes were, in one way or another, regarded by almost half the interviewees (n=15) as aspects by which to judge the beauty of a façade, which shows their relevance within the sample. nonetheless, it must be stressed that this is true for designers, which clearly makes sense from the point of view of professional curiosity, but it is not expected to be necessarily relevant for people without a background in architecture, design, or building engineering. the responses coded by “logic behind it,” address beauty “in the thinking behind,” as declared by one interviewee. moreover, in these cases, the way a façade looks should come from logical design decisions that could be reasonably traced back by looking at the façade and its details; simply put, it has to make sense. similarly, mentions of façades telling the “right story” imply that we should recognise the idea behind a façade, and this in turn needs to be an appropriate answer to the goals it sought to achieve. the difficulty of handling multiple, distinct (and, most of the time, clashing) requirements throughout the design process was expressed by generally mentioning how all aspects are integrated in the final façade design, as a guiding aspect for its aesthetic perception. this sentiment was explicitly declared by one interviewee who declined to pinpoint specific aspects that shape his aesthetic preferences, due to the influence of all aspects on each other (“i think i cannot really answer the question (…) because it’s actually all. how everything is coming together”). 037 journal of facade design & engineering volume 9 / number 2 / 2021 table 7 selected quotations from the interviewees categorised under “ intellectual connection” identified aspects / keywords selected quotations position (years of experience) logic; right story “the best feeling is when aesthetics and reasons to be, are so closely linked that it’s undisputedly the best answer to the question”. founding partner (18) logic “it will only be beautiful if it’s well-designed. if it does what it does, if you understand why we did it; even if you sometimes make a facade that’s very special, then you should be able to see on it, or to recognize what’s the idea behind a façade”. founding partner (42) logic “that the building, thereby also the facade, shows really well the ideas you had. if it is a showcase of the ideas you had, then the building will be good and beautiful in that way. if there is a disruption between the ideas and the things, or you don’t built it in a logical way, you changed your mind during the way, there are a lot of possible disruptions, then it is not a nice and aesthetic valuable building”. founding partner (34) logic; ties to architectural history “it works for me if it’s beautiful, but in such a way that i can understand; in such a way that beautiful has this aesthetic aspect to it, but also this intelligent aspect. so, if we manage to make something that from an engineer point of view, turns out to be the right answer to the most important questions; and it’s also then, culturally something that is embedded in architectural history; and it is also very aesthetically pleasing, that is, i think, what it’s all about”. partner (23) logic “the beauty is for me in the thinking behind. if i see something that is extremely excessive in shape and form and you know, an explosion of things, this is not beautiful for me. beautiful is for me is if somebody really put a lot of thought in there. i mean that is probably my perception of beauty, logic. i find a lot of beauty in logic”. associate architect (14) right story “i would say the best facade is the facade who tells the right story”. partner (37) logic; right story “if i turn the question around. what i dislike in a facade is when it looks very not-logic or when you think ‘what are they doing here?’, ‘what is the story.’ expressive architecture is interesting if it comes with a reason or with a story”. founding partner (30) honesty “i think a beautiful facade is somehow making sense, and it’s also honest. and it utilises its materiality in a very clever way. i think that’s what i mean by making sense (…) so, i think it’s when something is not trying to be something it’s not”. founding partner (14) right story; relation to building function “that’s because the story of the façade… the façade is trying to tell you something, and to explain you something which is happening behind the façade. if it’s only just an empty façade, then it’s, i think, after 5 years it’s boring (…) what’s really beautiful, that’s coming from the inside, and you were looking to a translation of the inside”. founding partner (30) honesty; right story “also what i think, the honesty. so it’s not only about making it beautiful; but also if it’s correct, if it’s fitting the program and the location, if it’s an honest façade, that’s also beautiful. it’s not only in the bricks, but also in the story that the façade tells”. associate architect (17) honesty “honesty is of course, a very problematic thing in architecture (…) if you bring it up, you have to choose sides. do you think that a building needs to be honest or not? i don’t know. i’m not sure, but it’s interesting to think about that, i think. and it comes up in facades all the time”. partner (23) 038 journal of facade design & engineering volume 9 / number 2 / 2021 due to the nature and diversity of ways to explain aesthetic preferences, it is not possible to directly relate façade characteristics to potential preferences; the perception of the resulting façade will depend on the successful balance of various different requirements, following a narrative that shapes the building through logical design decisions. nonetheless, when expanding on their answers, a set of interviewees did declare their preference for “honest façades” (n=5)—an aspect that is worth mentioning due to some conflicting views on the topic. honesty was explicitly declared as a desired attribute for a beautiful façade by five respondents, referring to both the need for it to reflect the programme behind it, and a coherent use of the material (“it utilises materiality in a very clever way (…) i think it’s when something is not trying to be something it’s not”), seeing it as a way to strengthen the consistency between story and resolution. on the other hand, some interviewees declared their apprehension for façades that directly reflect the indoor programme, alluding to the fact that it may decrease the flexibility of the building to cope with new functions in the future, making it less resilient. similarly, when discussing material choices, two interviewees stated their annoyance with the whole discussion about “the honesty of materials” that seems to come up every now and then in the field. according to one of them, the discussion is a fake one, because “there is no honesty in façade detailing anymore,” considering the irruption of polymers and multi-material components, besides the aforementioned use of cladding as finishing layer. in the view of the interviewee, this argument renders the whole discussion moot, because if we were to completely follow material honesty in façades nowadays, we would greatly limit our architectural repertoire. instead, his advice was to avoid choosing sides and embrace the artistic potential from the “contradiction of honesty and fake building technology.” 4 discussion figure 7 shows a bar-graph summarising the main identified aspects and the frequency of their mentions, categorised in the themes discussed in the previous section (the graph only considers aspects with more than two mentions). as previously discussed, material expression, the quality of the detail, and proportion were the aspects most mentioned within the sample, which shows certain proclivities of the surveyed group of designers. when comparing the results from the interviews with the existing literature, certain aspects appear in both. this is explicitly true for aspects such as proportion, material, and colour, declared in previous studies as relevant factors (keshtkaran et al., 2017; krier, 1988). likewise, the mention in the literature of underlying structural orders in façades, to explain their perceived beauty (bell, 1993; salingaros, 1999, 2000; smith, 2003); considers the use of a series of compositional resources, such as proportion, stratification, and rhythm, which were separately mentioned by the interviewees. in that sense, the aspects identified from the interviews tend to be more concrete when compared to the more general theoretical treaties, which does not come as a surprise given the grounded experience of the practitioners. another mentioned aspect that echoes the literature is the expression of the character of a building, through its façade, to explain our perception of it. this view circles back to de botton’s ideas (de botton, 2006), when declaring that architecture embodies social values, so we find appealing certain features that remind us of values we hold dear, at conscious and subconscious levels. this becomes evident in the answers that explicitly compare buildings to people, assigning them a certain presence and self-assurance. lastly, some aspects from both primary and distinctive groups of aspects defined by keshtkaran et al. (2017) appeared in the responses. proportion, materials, simplicity, and the importance of being able to understand or read 039 journal of facade design & engineering volume 9 / number 2 / 2021 façades from the former, and depth, the play of solids and voids, and a certain spontaneity related to the changing expression of the façade or its capacity to amaze the viewer, as distinctive factors for design. the categories presented by keshtkaran et al. (2017) reflect a dichotomy between blending-in and standing-out when it comes to façade design, which was stated as a relevant practical issue in some responses. consequently, it is not really about choosing one or the other approach, but rather about using diverse design resources from both approaches to find a balance between them, one that suits the brief and context of the building. 0 2 4 6 8 10 12 14 16 18 20 p ro po rti on s ol id s/ vo id s s tr at ifi ca tio n r hy th m m at er ia l e xp re ss io n t ex tu re d ep th s ur fa ce ro ug hn es s q ua lit y of th e de ta il t ec to ni cs r ef in em en t s im pl ic ity / ca lm d et ai ls c ha ng in g ex pr es si on a m az em en t / o rig in al ity c ha ra ct er la ye re d de si gn c ol ou r lo ca l u rb an c on te xt lo ca l c ul tu re h um an s ca le h ow it fe el s r ea da bi lit y h um an to uc h lo gi c be hi nd it r ig ht s to ry in te gr at io n of a ll as pe ct s h on es ty compositional n um be r of m en tio ns p er id en tif ie d as pe ct ( an d su bgr ou p) plastic detail design in tr in s ic a s p e c ts e x tr in s ic a s p e c ts character & expression context connection human connection intellectual connection fig. 7 number of mentions per identified aspect (and sub-group) having discussed common aspects between the responses and the literature, it is also important to shed light on some of their differences. first, regarding aspects that commonly appear in the specialised literature, there was no direct mention of nature-based or organic forms and patterns as an aspect behind aesthetic preferences. similarly, there was no mention of symmetry—a classical feature related to beauty—nor complexity, an aspect that appeared extensively in the reviewed literature (akalin et al., 2009; jennath & nidhish, 2016; keshtkaran et al., 2017; megahed & gabr, 2010; nasar, 1994; salingaros, 1999, 2000; smith, 2003; tinio & leder, 2009). in this regard, it is important to reiterate the fact that the interviews helped to identify conscious preferences from the sample; this means that non-disclosed aspects previously studied by scholars may very well be highly relevant at a subconscious level, but they are not consciously perceived as such by the interviewed practitioners when asked to discuss their aesthetic preferences. moreover, while complexity was not explicitly mentioned, simplicity on the other hand, was declared to be something to strive for when it comes to detailing. 040 journal of facade design & engineering volume 9 / number 2 / 2021 nevertheless, the strong sentiment expressed in favour of plasticity (texture, depth, and material expression) over flat façades, arguably speaks of a higher surface complexity. furthermore, the appeal of façades with changing expressions, and an overall amazement or originality as conditions to find façades beautiful, also advocate for visually complex experiences as opposed to dull surfaces. in that sense, for the interviewed sample, there seems to be a conscious preference for simplicity, but a subconscious desire for complexity, mixed together to explain their aesthetic preferences. hence, simplicity and complexity do not seem to stand in direct contradiction, but rather it appears to be a matter of how these concepts are being considered in the design, where simpler and refined details are preferred, but without compromising complex visual experiences that capture our attention, through the use of expressive materials, plasticity or a layered design, among other resources. on the other hand, aspects identified in the responses which have not been particularly explored in the literature mostly refer to the specialised background of the sample. this clearly applies to the focus on the quality of the detail and the tectonics of the façade, which understandably sparks attraction out of professional curiosity (how did they do that?). moreover, the same applies to the intellectual connection that experts can build with the subject of the study, which makes them appreciate a façade by understanding the logic that shaped it through the design process, or by acknowledging the appropriateness of the physical response to a set of requirements. it is confidently expected that these aspects would not be present, or at least their frequency would greatly decrease, had the questionnaire been aimed at a general audience instead. also, besides certain exceptions, such as the work of de botton (2006), previous studies in the field have focused almost exclusively on the aspects dubbed as intrinsic in this study. these aspects have been easier to explore through experimental research, assessing the response of people to a set of pictures, previously categorised according to said aspects. however, the task grows in difficulty if we need to consider the context of where a particular picture was taken from, or the scale of it compared to the observer, to name a couple of other aspects that will definitely have an impact on our perception of a given façade. on-site questionnaires and/or interviews would help on this regard, for instance, while understanding the intellectual connection to façades will require an open debate on these topics alongside practitioners. up to this point, the assessment of the results has focused on the different identified aspects, discussing their mentions independently. however, aesthetic perception is a multi-variable phenomenon, where all the discussed aspects are present at once, having an impact on each other in an interlinked map which informs the observer’s experience. this was clearly evidenced in the study by the fact that the majority of interviewees declared more than one aspect when asked to describe what makes a façade beautiful in their opinion, showing the complexity of succinctly describing their aesthetic preferences. therefore, as a second layer in the study, the mentions were assessed considering the full responses of the interviewees, mapping what informs their aesthetic experience in terms of the relations between the aspects they declared, or in other words, which themes/subgroups were mentioned in the responses from each interviewee. this was conducted in an effort to further understand and qualify the gathered responses, potentially leading to the identification of distinct profiles or types within the interviewed sample. these maps are shown as venn-diagrams in figures 8 and 9. the former shows the relations between the mentions of intrinsic aspects, categorised in the four discussed sub-groups, while the latter does the same for the three sub-groups of extrinsic aspects. each dot represents one interviewee, so its position shows the themes that were mentioned within her/his response. 32 interviewees mentioned aspects categorised in one of the four intrinsic sub-groups (figure 041 journal of facade design & engineering volume 9 / number 2 / 2021 8), while 27 declared aspects categorised as extrinsic (figure 9), out of the total 34 interviewed designers. the relations within each set of sub-groups will be discussed separately to simplify the assessment and will then be finalised with a comprehensive view of the crossovers between them. first, looking at the intrinsic sub-groups (figure 8), it is possible to see that the mentions are fairly distributed, and most interviewees mentioned aspects belonging to more than one sub-group (24 out of 32). moreover, the number of responses that mentioned aspects from two different subgroups is quite similar for virtually all the pair combinations, ranging from 11 to 13 matches, with the sole exemption of the intersection between character & expression aspects and compositional aspects, which gathered just 7 mentions. this shows that compositional aspects were less present for the interviewees who mentioned those relating to character & expression (and vice versa), which could lead to the possible conclusion that compositional aspects have a lesser role in defining the character of buildings, as expressed by the interviewees. nevertheless, further research on this relation would be needed in order to fully assert this as a valid statement. plastic de tatt il d esign compositional ch ar ar ct er & ex pr es si o n human connect ion contextual co n n e c tio nin te ll ec tu al c on nec tion fig. 8 intrinsic aspects mentioned per interviewee fig. 9 extrinsic aspects mentioned per interviewee the wide distribution of the responses across all the intrinsic sub-groups shows how interlinked all these themes are for the interviewees, when asked to explain their aesthetic preferences in façade design. therefore, it was not possible to relate isolated themes to their declared preferences. on the other hand, looking at the extrinsic groups, the situation drastically changes. figure 9 shows that about a third of the sample only mentioned extrinsic aspects later categorised under “intellectual connection” (n=11), while the same holds true for mentions under “human connection” (n=8). no interviewees declared only aspects categorised under “contextual connections,” hence, the interviewees who mentioned aspects within that group also mentioned aspects belonging to the other two. the fact that the contextual connection theme had no standalone mentions, on top of the minor number of overall mentions for that theme, seems to show that it is perceived as less relevant compared to the other themes. nonetheless, it is the authors’ opinion that the relation of the building with its context is largely perceived as a given when it comes to basic architectural design. thus, this could explain that while it appeared within the responses, it did not do so manifestly. the most noteworthy takeaway from assessing the mentions gathered under extrinsic themes, are the marked preferences encountered between either the “human connection” or “intellectual connection” themes. further research will be undoubtedly needed in order to fully explore the validity of this assertion, but the separate recognition of these distinct preferences seems to point 042 journal of facade design & engineering volume 9 / number 2 / 2021 towards two clear profiles among the sample: designers who explain their façade preferences in terms of the relations they establish with us, human observers; and others who explicitly favour façades where it is possible to understand the logic behind them. in fairness, these two groups speak of a relatable connection between object and subject, however the former focuses on the relations that we can experience directly from the façade as a physical object, while the latter seeks to establish a connection to the design process that led to that object, aiming to follow the rationale behind such an object. the relation between intrinsic and extrinsic aspects was also explored throughout the interviewees’ responses. figure 10 depicts a sankey-graph that shows the links between mentioned aspects belonging to either the intellectual connection or the human connection group, and mentions of other aspects by the same interviewee, which belong to any of the identified intrinsic groups. mentions within the contextual connection theme were not depicted separately, given that those responses were already contained in one of the other extrinsic groups. at first glance, it is possible to see that not only are there several links between both columns, but these links are also fairly distributed among the intrinsic sub-groups. this means that these aspects are definitely interlinked in the interviewees’ minds, but no direct correlation between specific groups is distinguished. compositional plastic detail design character & expression 18% (n=12) 29% (n=20) 26% (n=18) 26% (n=18) intellect 39 57% in te ll ec tu al co n n ec ti o n h u m an co n n ec ti o n 57% (n=39) 43% (n=29) 7 7 5 9 11 12 6 11 7 9 12 11 5 11 6 7 extrinsic aspects intrinsic aspects fig. 10 mentions of extrinsic and intrinsic aspects by the interviewees the mention of both intrinsic and extrinsic aspects by the majority of the interviewees (25 out of 34) shows that both types of aspects are relevant to define their aesthetic preferences when it comes to façades. moreover, just two interviewees declared only extrinsic aspects in their responses, while seven only declared aspects later categorised as intrinsic. by looking at these responses, the two who only mentioned extrinsic aspects provided a more general answer to the question, stating that a façade is beautiful when everything fits in a logical way. on the other hand, when interviewees provided a more detailed response, intrinsic aspects would appear, as a way to pinpoint specific façade design elements or resources, and thus grounding a more general design intent. in that sense, intrinsic aspects seem to have a twofold condition when it comes to the aesthetic perception of façades. these aspects could be perceived as beautiful features on themselves (a nice composition, sculptural quality, pattern, among others), or they could be used as design resources to enhance certain relations between the façade and its context, or between the façade and the observer, being 043 journal of facade design & engineering volume 9 / number 2 / 2021 regarded as the physical expression of these underlying relations. the exploration of the relation between the design intent and the physical result, and its role in the aesthetic perception of façades, although highly interesting, escapes the scope of the present article. 5 conclusions this paper tackled aesthetic preferences in façade design from the point of view of designers, aiming to identify certain parameters and relevant aspects involved in the aesthetic perception of the building façade. the study was based on a series of interviews with practitioners from 34 architectural firms in the netherlands. in this regard, the research project was devised as an exploratory study, providing insights on aesthetic preferences through the perspective of a sample of dutch architects, so the outcomes, while valuable for the broader discussion around these aspects, do not claim to be exhaustive or universally valid. after gathering and coding the responses from the interviews, two main groups were identified to categorise the aspects that inform the aesthetic perception of façades for the sample: intrinsic and extrinsic; the former dealt with characteristics of the façade as an object, and the latter focused on the relational features between façades and an outside agent. moreover, sub-groups were identified in an effort to further explain the differences and similarities between the mentioned aspects. both intrinsic and extrinsic aspects were mentioned in the responses from the majority of the interviewees, which shows that both types of aspects play a role in defining their aesthetic preferences when it comes to façades, considering potential relations between them to nurture their aesthetic experience. nonetheless, while these mentions were fairly distributed among the different sub-groups in the case of intrinsic aspects, it was possible to see a clear distinction within the sample between extrinsic aspects, where some interviewees declared aspects that refer to a human connection (human scale, human touch) while others clearly mentioned aspects that refer to an intellectual connection to the façade design process (logic behind it, story) to explain what aspects inform their aesthetic preferences. when compared to the existing literature on the topic, the results showed both similarities and divergences. on the one hand, mentions of proportion, material, and colour have been largely documented in the literature, along with the notion of structural orders in façades, and the expression of the character of a building through its façade, to explain our attraction or preference towards it. on the other hand, regarding aspects commonly discussed in the literature, there was no direct mention in the responses of nature-based or organic forms, as an influence on aesthetic preferences. similarly, there was no mention of symmetry, a classical feature related to beauty, nor complexity, an aspect that appeared extensively in the reviewed literature. simplicity, in contrast, was declared to be something to strive for when it comes to detailing. nonetheless, in this regard, the authors speculate that the strong sentiment expressed in favour of plasticity over flat façades, or the pursue of changing building expressions throughout the day, arguably speak of a desire for visually complex experiences in opposition to dull surfaces. thus, the responses from the sample may be exhibiting a conscious preference for simplicity and refinement when it comes to the design of details, but a subconscious desire for complexity in the overall character of the façade, through the use of expressive materials, plasticity, or a layered design, among other resources. 044 journal of facade design & engineering volume 9 / number 2 / 2021 finally, it feels important to reiterate the specialised profile of the interviewed sample, composed of architectural design practitioners, which explains the focus on the quality of the detail and the tectonics of the façade, or the intellectual connection that architects can establish with the thinking behind a façade, and not just the building as an object. this undoubtedly shaped the outcomes discussed in the paper, following the initial goals of the study; however, as next steps, it will be interesting to compare and test these findings with a larger sample, and against the aesthetic preferences of the general public, to include societal perspectives into an open discussion striving for the design of beautiful façades, buildings, and cities. acknowledgements this paper is part of the project prettyface – exploration of aesthetics in façade design, funded by the dutch research council (nwo) through their programme creative industry knowledge innovation mapping (kiem), under the dossier number ki.18.037. the consortium behind the project comprises delft university of technology, kaan architecten, barcode architects, thijs asselbergs architectuurcentrale and archdaily. the authors wish to acknowledge the architectural firms that took part in the study (in alphabetical order): architecten van mourik, arconiko, thijs asselbergs architectuurcentrale, atelier kempe thill, barcode architects, benthem crouwel architects, braaksma & roos architecten, cepezed architectenbureau, de nijl architecten, dok architecten, dus architects, ector hoogstad architecten, gaaga, gortemaker algra feenstra, heren5 architects, hulshof architects, kaan architecten, kcap architects&planners, kraaijvanger architects, mecanoo, mei architects and planners, moederscheim moonen architects, mollink soeters pphp, mvrdv, mvsa architects, next architects, orange architects, oz architects, paul de ruiter architects, team v architectuur, un studio, v8 architects, vanschagen architecten, and wdj architecten. references akalin, a., yildirim, k., wilson, c., & kilicoglu, o. (2009). architecture and engineering students’ evaluations of house façades: preference, complexity and impressiveness. journal of environmental psychology, 29(1), 124-132. doi:10.1016/j.jenvp.2008.05.005 alberti, l. b. (1986). the ten books of architecture the 1755 leoni edition. (g. leoni, trans.). dover publications. alexander, c. (2004). the nature of order four volume set: an essay on the art of building and the nature of the universe. routledge. bell, s. (1993). elements of visual design in the landscape. spon press. breen, j. (2019). patterns & variations: designerly explorations in architectural composition and perception. (doctoral dissertation). delft university of technology, delft, the netherlands. chamilothori, k., chinazzo, g., rodrigues, j., dan-glauser, e. s., wienold, j., & andersen, m. (2019). subjective and physiological responses to façade and sunlight pattern geometry in virtual reality. building and environment, 150, 144-155. doi:10.1016/j. buildenv.2019.01.009 coburn, a., kardan, o., kotabe, h., steinberg, j., hout, m. c., robbins, a., . . . berman, m. g. (2019). psychological responses to natural patterns in architecture. journal of environmental psychology, 62, 133-145. doi:10.1016/j.jenvp.2019.02.007 corbusier, l. (1953). le modulor [the modulor] (r. vera, trans.). editorial poseidon. de botton, a. (2006). the architecture of happiness. hamish hamilton ltd., penguin books. el-darwish, i. i. (2019). fractal design in streetscape: rethinking the visual aesthetics of building elevation composition. alexandria engineering journal, 58(3), 957-966. doi:10.1016/j.aej.2019.08.010 farkas, k., & horvat, m. (2012). t.41.a.1: building integration of solar thermal and photovoltaics barriers, needs and strategies (t.41.a.1 ). report t.41.a.1. iea shc task 41, subtask a jennath, k. a., & nidhish, p. j. (2016). aesthetic judgement and visual impact of architectural forms: a study of library buildings. procedia technology, 24, 1808-1818. doi:10.1016/j.protcy.2016.05.226 keshtkaran, r., habibi, a., & sharif, h. (2017). aesthetic preferences for visual quality of urban landscape in derak high-rise buildings (shiraz). journal of sustainable development, 10(5). doi:10.5539/jsd.v10n5p94 kirsch, l. p., urgesi, c., & cross, e. s. (2016). shaping and reshaping the aesthetic brain: emerging perspectives on the neurobiology of embodied aesthetics. neurosci biobehav rev, 62, 56-68. doi:10.1016/j.neubiorev.2015.12.005 krier, r. (1988). architectural composition. rizzoli. meerwein, g., rodeck, b., mahnke, f. h., bruce, l., gaskins, m. d., & cohen, p. (2007). color: communication in architectural space. springer verlag ny. megahed, y. & gabr, h. (2010). quantitative architectural aesthetic assessment applying birkhoff’s aesthetic measure in architecture. mikellides, b. (2012). colour psychology: the emotional effects of colour perception. 105-128. doi:10.1533/9780857095534.1.105 nasar, j. (1994). urban design aesthetics the evaluative qualities of building exteriors. environment and behaviour, 26(3), 377-401. o’connor, z. (2008). façade colour and aesthetic response: examining patterns of response within the context of urban design and planning policy in sydney (doctoral dissertation). the university of sydney, sydney, australia. 045 journal of facade design & engineering volume 9 / number 2 / 2021 pearce, m., zaidel, d., vartanian, o., skov, m., leder, h., chatterjee, a., & nadal, m. (2016). the cognitive neuroscience of aesthetic experience. perspectives on psychological science, 11(2), 265-279. prieto, a., klein, t., knaack, u., & auer, t. (2017). main perceived barriers for the development of building service integrated façades: results from an exploratory expert survey. journal of building engineering, 13, 96-106. doi:10.1016/j.jobe.2017.07.008 rolls, e. t. (2014). neuroculture: art, aesthetics, and the brain. rendiconti lincei, 25(3), 291-307. doi:10.1007/s12210-013-0276-7 salingaros, n. a. (1995). the laws of architecture from a physicist’s perspective. doi:10.4006/1.3029208 salingaros, n. a. (1999). architecture, patterns, and mathematics. nexus network journal, 1(1-2), 75-86. doi:https://doi.org/10.1007/ s00004-998-0006-0 salingaros, n. a. (2000). the structure of pattern languages. architectural research quarterly, 4, 149-161. doi:10.1017/ s1359135500002591 smith, p. (2003). the dynamics of delight architecture and aesthetics. tablada, a., kosorić, v., huang, h., lau, s. s. y., & shabunko, v. (2020). architectural quality of the productive façades integrating photovoltaic and vertical farming systems: survey among experts in singapore. frontiers of architectural research, 9(2), 301-318. doi:10.1016/j.foar.2019.12.005 tinio, p. p., & leder, h. (2009). just how stable are stable aesthetic features? symmetry, complexity, and the jaws of massive familiarization. acta psychologica, 130(3), 241-250. doi:10.1016/j.actpsy.2009.01.001 vitruvius, m., & morris, m. t. (1914). vitruvius: the ten books on architecture (english edition) (m. h. morgan, trans.). harvard university press. 046 journal of facade design & engineering volume 9 / number 2 / 2021 journal of facade design and engineering 3 (2015) 91–102 doi 10.3233/fde-150029 ios press 91 composite uhpc-aac/clc facade elements with modified interior plaster for new buildings and refurbishment. materials and production technology lorenzo miccolia,∗, patrick fontanaa, nelson silvab, andrea klingec, christer cederqvistd, oliver krefte, dirk qvaeschningf and christer sjöströmg abam federal institute for materials research and testing, division 7.1 building materials, unter den eichen, berlin, germany bcbi swedish cement and concrete research institute, c/o sp, brinellgatan, borås, sweden croswag architekten gva mbh, berlin, germany daercrete technology ab, tallvagen bankeryd, sweden exella technology and research, hohes steinfeld, kloster lehnin, germany fdyckerhoff gmbh, biebricher straße, wiesbaden, germany gsvenska aerogel ab, strömmavägen, gävle, sweden received 2 april 2015 revised 17 april 2015 accepted 20 april 2015 abstract. the awareness of the environmental impact of the building sector is increasing. steel reinforced concrete is the most commonly used construction material, though with a high-embodied energy and carbon footprint. large environmental gains may arise if an alternative to steel reinforced concrete is developed. in this context, ultra-high performance concrete (uhpc) materials are shown to be promising alternatives with advantages such as lower embodied energy and reduced environmental impact. predictions suggest that uhpc composite elements for building envelopes could have other benefits such as an increased service life, optimised use of building area due to thinner elements and minimised maintenance due to the absence of reinforcement or use of non-corrosive reinforcing materials such as carbon fibres. in the framework of the h-house project funded by the european commission, composite elements are developed. the aim is to create facade panels combining an autoclaved aerated concrete or cellular lightweight concrete insulation layer with an external uhpc supporting layer. to enhance occupant comfort and health, hygroscopic materials that are capable to buffer indoor air humidity shall be applied to the inside of such elements. indoor air humidity levels are expected to be more stable, which shall subsequently improve the indoor climate and minimise potential decay to the construction. keywords: composite panels, ultra-high performance concrete (uhpc), autoclaved aerated concrete (aac), cellular lightweight concrete (clc), aerogel, modified earth plaster ∗corresponding author: lorenzo miccoli, bam federal institute for materials research and testing, division 7.1 building materials, unter den eichen 87, 12205 berlin, germany. tel.: +49 30 8104 3371; fax: +49 30 8104 1717; e-mail: lorenzo.miccoli@bam.de. issn 2214-302x/15/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:lorenzo.miccoli@bam.de 92 l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster 1. introduction the purpose of an adequate building envelope is protection against moisture ingress, heat loss in winter, excessive heating in summer and noise. components for the interior should be able to buffer heat and humidity peaks and prevent pollutants and noise. solutions for both components for building envelope and components for the interior have to be durable, energy-efficient and affordable. in this framework the development of prototype facade elements comprising hygrothermally treated ultra-high performance concrete (uhpc) in combination with autoclaved aerated concrete (aac) or cellular lightweight concrete (clc) are presented. to improve the indoor environment quality with regards to balanced indoor air humidity levels, an earth plaster modified with aerogel, demonstrating an increased moisture buffer, was developed. uhpc exhibits extreme high strength and excellent chemical durability. the exceptional properties of uhpc are the result of a high packing density based on an optimised particle size distribution and significant reduction of water in the cement paste compared to ordinary concrete (larrard & sedran, 1994). the workability of uhpc is adjusted by adding highly efficient plasticisers, obtaining mixes capable to flow or even with self-compacting properties. the very high density of the material is of course beneficial to its durability. numerous studies showed that due to the limited adsorption of moisture and negligible moisture transport the resistance of uhpc against any kind of deterioration mechanism is drastically increased compared to normal concrete. in the case of building envelopes the excellent resistance against freeze-thaw attack and penetration of chloride ions in marine environments is a particular advantage (ahlborn et al., 2008; thomas et al., 2012; piérard et al., 2012). uhpc was already applied successfully to building constructions, such as lightweight roof constructions, facade elements (acker & behloul, 2004; behloul & batoz, 2008; rebentrost & wight, 2008a; szolyd, 2014) and protection panels (rebentrost & wight, 2008b). in this study, lightweight aac with a dry density between 100 to 115kg/m3 was employed. this material provides a low thermal conductivity in combination with mechanical properties adequate for the use as insulation layer in composite elements (eta, 2011). the use of clc in residential building applications has been limited, so far, to social housing projects where a large number of units needs to be constructed in a short period; at densities of around 600kg/m3 clc constitutes an affordable and sustainable alternative providing both structural and insulation characteristics. in this study the typology of a half-panel element was developed. the typology is non-load bearing and it was conceived to be used for new buildings and for renovation of existing buildings. small-scale half elements were developed to assess the feasibility of the production technology process. 2. facade element components the general idea is to realise the external uhpc shell as a box-shaped element (fig. 1). due to the support from the edges of the box no shear forces are generated in the uhpc-aac/clc interface during transport and service life. thus, no additional connectors are necessary, provided that the bond between uhpc and aac/clc is sufficiently high to prevent from detachment of the layers when the composite element is tilted after demoulding and during transport. moreover, the edges are forming a frame and improve the stiffness of the box-shaped element, allowing the decrease of the thickness of the exterior uhpc layer. in the corners, the cross section of the frame is broadened to include the assemblies for anchoring and transport/mounting. figure 2 and table 1 give an overview l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster 93 fig. 1. examples of the uhpc fibre-reinforced cladding in a curtain wall (szolyd, 2014). fig. 2. half panel, non-load bearing: 5m × 3m. (a) axonometric view; (b) vertical section (mid of the panel); (c) lateral view. of the geometry of the panels. the design was based on load assumptions required by eurocode 2 (en 1992-1-1, 2004). in particular a wind speed of 44m/s equivalent to a wind load of 1.66 kn/m2 was considered. 2.1. uhpc due to the extraordinary high strength and the high density of uhpc, it is possible to produce very thin and durable facade elements. the use of uhpc for light-weight elements would reduce the environmental impact in relation to manufacturing, transport and installation processes. 94 l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster table 1 geometrical parameters of half panels insulation l × h uhpc ext. layer insulation total total weight (m2) thickness (mm) thickness (mm) thickness (mm) weight (kg) (kg/m2) aac 5 × 3 30 350 380 1880 125 clc 5 × 3 30 350 380 2340 156 table 2 composition of uhpc mixtures and obtained density material nanodur® compound 5941 sand superplasticiser water dry density (kg/m3) 1050 1150 17.9 178.5 2440 the uhpc adopted is based on dyckerhoff nanodur® technology. nanodur compound contains ultrafine components (portland cement, blast furnace slag, quartz, synthetic silica) smaller than 250 �m that are dry mixed intensively. in this way the homogeneity and dense packing of the particles is reliably achievedandthewetmixingprocessoftheuhpcwithastandardconcretemixerissimplifiedsignificantly (table 2). nanodur cement is a cem ii b-s 52.5r according to the standards (en 197–1, 2011). further reduction of embodied energy was achieved by replacement of portland cement with less energy intensive types of cement or supplementary cementitious materials (scm) originating also from industrial residuals. in order to increase the performance of uhpc, hydrothermal curing (autoclaving) is applied, a technique used for the industrial production of aac elements. solutions are referred to minimum compressive strength of 100n/mm2 for non-load bearing applications and high quality of the formed uhpc surface. with screening tests three superplasticizers were identified for optimum workability of the fresh uhpc. shrinkage of the uhpc was identified as potential problem with regard to bond behaviour and large sizes of composite elements. with the use of a shrinkage-reducing admixture promising results were obtained. 2.2. insulation materials 2.2.1. aac the material structure of aac is characterised by a solid skeleton and aeration pores being formed during the aluminum-driven expansion of the slurry. the solid skeleton consists of hydrothermally synthesized crystalline calcium-silicate-hydrates (thereof mainly tobermorite) and, moreover, minor contributions of unreacted sand. the foam-like structure of aac, with its solid skeleton acting as partitioning walls between the aeration pores (alexanderson, 1979), leads to an optimum correlation between weight and compressive strength. millions of aeration pores lead to a low thermal conductivity making aac a highly thermal insulating building material. thermal conductivity depends on temperature, density, structure and chemical nature of the material. in aac, it is largely a function of density and moisture content (narayanan & ramamurthy, 2000; oel, 1980; lippe, 1986). l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster 95 fig. 3. correlation between aac dry density and: thermal conductivity (a); compressive strength (b). table 3 aac. range of mix proportions tested and obtained densities material cement sand quick lime anhydrite/gypsum mineral aggregate aluminium∗ dry density (kg/m3) 250–500 250–400 50–250 30–70 100–200 5–8 85–115 ∗used as porosing agent/blowing agent. for this reason, improvements of the thermal performance of aac had been mainly achieved by reducing the dry density (fig. 3a). although the strength of the remaining solid skeleton could be steadily improved in the last decades, decreasing the dry density by trend leads to losses in the compressive strength (fig. 3b). in other words, the material properties of aac always represent a compromise of mechanical and thermal properties. in case of a certain minimum mechanical requirement, options for reducing the thermal conductivity are limited. for aac, the lowest range of lambda-values (declared thermal conductivities=42 to 47 mw/(m·k) (eta, 2011; en iso 10456, 2010) was accomplished at dry densities between 85 and 115kg/m3. due to its extremely low mass, such light-weight aac is a pure insulation material without any load bearing capacity (see table 3). the difference is only the dry density, being achieved by altering the amount of aluminum (the more aluminum the lower the dry density). 2.2.2. clc in order to be used as a high performance insulation material, very low density clc must be developed; the goal is to achieve a thermal conductivity of 30–35 mw/(m·k) at a density around 150kg/m3. given the high volume of foam, the main challenge is to guarantee that the cementitious matrix sets fast enough to sustain the porous structure without collapse of the foam. for this purpose, calcium aluminate cement was chosen as binder, which sets much faster when compared to portland cement. table 4 lists the range of mix compositions tested and respective target and obtained densities. initial tests had as objective to evaluate the compressive strength and thermal conductivity of a series of samples to be used as a benchmark for further development. the results in fig. 4a show that, at low densities, very low values of compressive strength are obtained; in addition, the scatter is also large. this is typical for clc in which the mechanical properties are very much dependent on the homogeneity of the air void distribution. 96 l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster fig. 4. correlation between clc dry density and: thermal conductivity (a); compressive strength (b). table 4 clc. range of mix proportions tested and obtained densities material cement sand sp w/c vf (l) density target wet dry (kg/m3) 72–303 76–258 0.4–1.8 0.25–0.5 737–952 155–637 165–958 175–734 sp, superplasticizer; vf, volume of foam. the results from thermal conductivity measurements (fig. 4b) are quite promising; at a density about 300kg/m3, the λ-value is around 70 mw/(m·k). given the good linear correlation with density, a λ-value below 45 mw/(m·k) can be expected for the target density of the research. 2.3. modified earth plaster to increase the moisture buffer capacity of clay minerals, earth plasters will be modified with aerogels. due to the highly porous structure of the aerogels, it is important to optimise the water addition to maintain workability of the material mixture and to reduce drying shrinkage. aerogels were therefore customised with regards to their size, porous structure as well as their densities. furthermore, fibres were added to certain material mixtures, which increase the allowance for drying shrinkage of the base material. different mixing methodologies have also been applied and evaluated. two different types of quartzene® were used in the form of granules (gi) or powder (pi), denoted cms and nd. cms is composed of hydrated calcium magnesium silicate, whereas the other one consists of pure hydrated silicon dioxide (nd). five different earth plasters were used: earth plaster base coat (epb), earth plaster mineral 16 (m16), earth plaster rough final coat (eprf), earth plaster rough final coat fine (eprf fine) and earth plaster fine final coat (epff). results demonstrate a very large scatter with regards to drying shrinkage (table 5). although it was aimed to keep the water addition low, some material mixtures demonstrated an inacceptable level of shrinkage. through the incorporation of fibres, shrinkage values could be significantly improved. in addition to drying shrinkage, strength properties of the developed materials have been conducted to verify an acceptable material performance with regards to fitness for purpose (table 5). l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster 97 table 5 strength properties and results from drying shrinkage tests for different material mixtures material shrinkage (%) water fibres density flexural compressive mixture addition (%) (kg/m3) strength (n/mm2) strength (n/mm2) din 18947 min. 2a/3b/4c according to – min 0.9 min 0.7 min 1.5 manufacturer max 2.2 nd gi<0.5/epb 2.3a 25 straw 1.60 0.3 1 nd gi 0.5/m16 5.6 23 – 1.45 n/a n/a nd pi /epb 1.7b 27 straw 1.75 0.9 2.5 nd pi /m16 6.6 n/a – nd pi/eprf 2.9b 24 straw 1.79 0.9 2.4 nd pi/eprffine 1.9b 26 straw 1.72 0.6 1.6 nd pi/epff 3.3c 26 cellulose 1.79 1.3 3.7 nd pi/epfffine 3.6c 28 cellulose 1.75 1.1 3.1 cms gi/eprf 0.8c 23 straw 1.47 0.1 1.4 cms gi/m16 2.4b 25 hemp 1.37 0.3 1.2 cms pi/eprf 1.9b 25 straw 1.71 0.4 1.2 aupper limit for mineral earth coat. bupper limit for earth coat, stabilised with fibres. cupper limit for thin earth coat, stabilised with fibres. inacceptable results marked in grey. n/a=not available. although a number of material mixtures have passed all tests, the results demonstrated that the use of aerogel in combination with earth plasters is sensitive to failures. replicability of tests has proved to be difficult and results from test series show a relatively large scatter, even though sample production and testing were conducted in exactly the same way. in addition, certain tests with different material mixtures provided unexpected and in certain cases contradicting results. 3. energetic and hygrothermal performances following the targets of the european commission related to the primary energy demand for buildings by 31st december 2020 all new constructions shall be nearly zero-energy buildings (nzeb). in this framework the goal of facade elements here proposed is therefore to achieve or undercut a u-value of 0.15w/(m2·k). a first assessment of the thermal behaviour of the composites elements was carried out considering the physical and thermal properties reported in table 6. these properties are expected values that are assumed to be reached with a high certainty. further improvements are expected with the incorporation of aerogels, in particular for the thermal conductivity. in the current configuration the half panel present a u-value of 0.140w/(m2·k) for aac and 0.142w/(m2·k) for clc. to minimise the energy consumption of a building further, the uhpc concrete composite elements are designed to be highly airtight. it is therefore suggested to apply modified earth plasters to the inside of these panels. the developed earthen plaster materials are expected to demonstrate an increased water vapour adsorption capacity, and are hence able to balance indoor air humidity levels and to provide healthy and comfortable spaces for occupants. modified earth plasters as well as base materials have been tested with regards to their hygrothermal performance. the moisture buffer capacity of the developed materials was assessed via water vapour adsorption tests according to 98 l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster table 6 physical and thermal properties of the materials used component function dry density (kg/m3) thermal conductivity (mw/(m·k)) uhpc structural 2500 1500 aac insulation 85–95 42∗ clc insulation 180 45 ∗here, the design-value λd of 42 mw/(m·k) corresponds to a λ10,dry-value ≤39.2 mw/(m·k) 0. fig. 5. results water vapour adsorption test for modified and base earth plasters. lines ws i, ii and iii represent water vapour adsorption classes according to the standards (din 18947, 2013). the standards (din 18947, 2013). water vapour adsorption classes for earth plasters, set out in din 18947, are incorporated in the results. findings demonstrate the positive material performance of the developed materials (fig. 5). the modified base coat plaster adsorbs nearly 100% more water vapour than the pure material, whereas the modified final earth plaster, applied on top of the modified base coat plaster adsorbs approximately 50% more than the pure earth plaster (after 12 hours). however, it is surprising that the latter material mixture adsorbs 2/3 less than the modified base coat plaster, although both original materials adsorb almost the same amount of water vapour. during the mixing process it has been observed that the aerogel material influenced the formation of the surface. fine material particles became visible at the sample surface, which seemed to make the surface denser. 4. production technology of composite uhpc-aac/clc 4.1. manufacturing of uhpc boxes the purpose of this section is to present the product technology used for producing uhpc-aac composite elements. first trials were dedicated to the one-step production of the box-shaped uhpc elements, i.e. the exterior uhpc layer and the upturning edges are cast with a single concrete batch. for this purpose a ‘floating body’ was adopted. the protection of the floating body against buoying l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster 99 fig. 6. procedure for two-step production of box-shaped uhpc elements. (a) cast of exterior layer. (b) placement of a rigid frame or a block as internal formwork on hardened exterior layer. (c) cast of upturning edges. fig. 7. uhpc box manufactured with the two-step procedure: joint between exterior uhpc layer and upturning edges. upwards requires accurate measures when full hydrostatic pressure is considered. in the case of fullscale elements, where the buoyancy may reach high values, it might be too complex to accurately fix the floating bodies. therefore, in a second approach, further trials were dedicated to a two-step production procedure of the uhpc box with the upturning edges of the box being cast on top of the exterior layer after initial hardening (fig. 6). in the tests a multipor® block was used as internal formwork. one day after the cast of the exterior layer the multipor® block was placed on its top without fixation and the upturning edges were cast. the uhpc was poured into the gap between formwork and multipor® block at one corner of the formwork. the uhpc was easily flowing around the multipor® block filling the gap completely without generating any buoyancy; i.e. during the cast the multipor® block was simply held in place manually and the uhpc was not penetrating under the multipor® block, even though the backside of the exterior layer was not perfectly smooth. due to the two-step manufacturing procedure the uhpc boxes cannot be regarded as monolithic, like in the case of the one-step manufacturing. in fact, a distinct layering was observed, visible as a joint between exterior uhpc layer and upturning edges (fig. 7). in order to evaluate the bond strength between the two uhpc layers, preliminary shear and pull-off tests were performed. 100 l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster fig. 8. manufacturing of uhpc-aac small-scale samples. (a) empty uhpc box with highly porous internal surface. (b) cast of aac slurry. (c) swelling process of the aac slurry driven by the aluminum reaction. it is supposed that ‘gluing’ of multipor® blocks on the back side of the exterior uhpc layer with a rapidly hardening mineral-based adhesive with low shrinkage will ease the cast of the upturning edges, and thus the production of full-scale elements. on the other hand, when uhpc boxes are needed as ‘moulds’ for the cast of fresh aac and subsequent joint autoclaving, a rigid frame as internal formwork on top of the exterior uhpc layer is supposed to be more efficient for the cast of the upturning edges than a block. a frame consisting of several parts will be more flexible and easier to install as well as to remove when the element is demoulded after hardening of the uhpc. 4.2. manufacturing of insulation small-scale box-shaped uhpc elements as shown in fig. 8a were prefabricated at dyckerhoff laboratories and were shipped to xella and cbi for the manufacturing of half panels using aac and clc respectively. after sufficient hardening of the uhpc, the developed aac/clc are cast directly on these panels to realise the insulation layer of the small-scale element. in the case of aac the uhpc boxes were filled with fresh slurries so that the swelling process induced by the reaction of the aluminum and the set of the aac occurred inside the uhpc boxes (fig. 8b, c). after 24 hours the elements were autoclaved. after autoclaving two aac composites samples revealed severe crack formation, presumably as a consequence of differences in thermal strain between the aac insulation layer and the encasing uhpc box. the observed results suggest that the pursued strategy of manufacturing uhpc/aac half panels is not suitable for aac with dry densities ≥175kg/m3. it is assumed that the observed cracks both in the aac and in the uhpc box are a consequence of a restrained thermal dilation of the material in particular during the cooling phase of the autoclaving process, resulting in tensile stresses. concerning the manufacturing of clc there are, however, two main aspects that need to be considered: thorough wetting of the internal surfaces of the uhpc before casting to avoid clc collapse and decrease shrinkage; and after casting, enough time should be given in order to allow the clc to dry and thus avoid excessive moisture to be entrapped in the insulation. so far, small-scale samples (fig. 9) were prepared using clc with a density about 300kg/m3. after hardening, the clc generally maintained its original dimensions. mainly around the edges, cracking and detachment of the clc from the uhpc was observed but without compromising the integrity of the panel. l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster 101 fig. 9. manufacturing of uhpc-clc small-scale samples. (a) cast of clc. (b), (c) homogenous distribution of clc in the uhpc box. 5. conclusions the box-shaped concept is a simple and robust solution for the facade elements; besides the good structural performance, the concept enables efficient protection of the insulation material during transport, installation and use. additionally, due to the absence of reinforcement and connectors through the insulation, the production technology does not involve major labour-intensive tasks, which is desirable for scale-up. the preliminary studies showed more advantages of a two-step manufacturing procedure of the uhpc boxes than a one-step procedure. in this framework the bond between uhpc layers plays a key role with the production of non-monolithic uhpc elements. the bond strength between the uhpc substrate and the uhpc top layer was evaluated by shear tests and pull-off tests with promising results. however, future activities need to include more systematic investigations, in particular with regard to the surface properties of the uhpc substrate. in the manufacturing of small-scale uhpc-aac composite elements a satisfying bond between the uhpc and the aac was observed. however, the investigations of the thermal deformation behaviour of the composite elements will be part of future activities. a possible optimisation of production technologies will cover both two-step manufacturing of uhpc boxes and ‘gluing’ of aac blocks on hardened uhpc. the one-step manufacturing of full-scale uhpc boxes appears too complex and will not be investigated further. before casting the insulation layer, thorough wetting of the uhpc substrate is recommended to avoid collapse and harmful shrinkage of the clc. special attention has to be paid to the drying of the clc to avoid excessive moisture entrapped in the insulation layer after mounting of the facade elements. the hardening process of clc seems compatible with panel configuration. the clc generally maintained its original dimensions. only around the edges, minor cracking and detachment were observed without compromising the integrity of the panel. future activities will include quantification of the bond strength of the uhpc-clc interface. future work will focus on the incorporation of fibres and aerogels. the first is expected to contribute to an increase in the mechanical stability of the clc whilst the second will considerably lower the thermal conductivity to values in the range of 30–35 mw/(m·k). the use of an earth plaster modified with aerogel seems a promising way to improve indoor hygrothermal conditions. the material development to date indicates that aerogels increase the 102 l. miccoli et al. / composite uhpc-aac/clc facade elements with modified interior plaster water vapour adsorption capacity of an earth plaster by approximately 70–90%. the next steps shall investigate, if the water vapour adsorption process is restricted through the modified surface of the earth plaster. in addition, adhesive strength and abrasion tests will be conducted. furthermore, the optimisation of the material mixtures shall be progressed to enable the development of a marketable product. acknowledgments this research study was made possible with the support of the european union’s seventh framework programme for research, technological development and demonstration under grant agreement no. 608893 (h-house, www.h-house-project.eu). references acker, p. & behloul, m. (2004). ductal® technology: a large spectrum of properties, a wide range of applications. in: proc. int. symp. on ultra high performance concrete, september 13-15, 2004, kassel, germany, 11-23. ahlborn, t. m., misson, d. l., peuse, e. j., & gilbertson, c. g. (2008). durability and strength characterization of ultra-high performance concrete under variable curing regimes. in: proc. 2nd int. symp. on ultra high performance concrete, fehling, e., schmidt, m., & stürwald, s. (eds.) kassel, germany, march 5-7, 2008, schriftenreihe baustoffe und massivbau (10), kassel university press, 197-204. alexanderson, j. (1979). relations between structure and mechanical properties of autoclaved aerated concrete. cem concr res, 9(4), 507-514. behloul, m., & batoz, j. -f. (2008). ductal® applications over the last olympiad. in: proc. 2nd int. symp. on ultra high performance concrete, kassel, germany, march 5-7, 2008, schriftenreihe baustoffe und massivbau (10), kassel university press, 855-862. bus center ratp in thiais, france (www.szolyd.com). de larrard, f., & sedran, t. (1994). optimization of ultra-high-performance concrete by the use of a packing model. cement and concrete research, 24, 997-1009. din 18947 (2013). earth plasters – terms and definitions, requirements, test methods. en iso 10456 (2010), building materials and products – procedures for determining declared and design thermal values. en 1992-1-1 (2004). eurocode 2: design of concrete structures – part 1-1 – part 3. en 197-1 (2011). cement. composition, specifications and conformity criteria for common cements. european technical approval, eta-05/0093 (2011). multipor thermal insulation panel, valid to june 1, 2019. lippe, k. l. (1986). entwicklung hochporöser c-s-hwerkstoffe mit minimaler wärmeleitfähigkeit. bmft forschung band 86, fachinformationszentrum energie/physik/ mathematik. narayanan, n., & ramamurthy k. (2000). structure and properties of aerated concrete: a review. cement & concrete composites, 22, 321-329. oel, h. j. (1980). wärmeleitfähigkeit und festigkeit von calzium-hydrosilicat-produkten. abschlußbericht dfg forschungsvorhaben mo 256/6. piérard, j., dooms, b., & cauberg, n. (2012). evaluation of durability parameters of uhpc using accelerated lab tests. in: schmidt, m. et al. (eds.): proc. of hipermat 2012, 3rd int. symp. on uhpc and nanotechnology for high performance construction materials, march 7-9, 2012, kassel, germany, 371-376. rebentrost, m., & wight, g. (2008a). experiences and applications on ultra-high performance concrete in asia. in: proc. 2nd int. symp. on ultra high performance concrete, fehling, e., schmidt, m. and stürwald, s. (eds.), kassel, germany, march 5-7, 2008, schriftenreihe baustoffe und massivbau (10), kassel university press, 19-30. rebentrost, m. & wight, g. (2008b). behaviour and resistance of ultra high performance concrete to blast effects. in: proc. 2nd int. symp. on ultra high performance concrete, kassel, germany, march 5-7, 2008, 735-742. thomas, m., green, b., o’neal, e., perry, v., hayman, s., & hossack, a. (2012). marine performance of uhpc at treat island. in: schmidt, m. et al. (eds.): proc. of hipermat 2012, 3rd int. symp. on uhpc and nanotechnology for high performance construction materials, march 7-9, 2012, kassel, germany, 365-370. www.h-house-project.eu (www.szolyd.com) from city’s station to station city 127 journal of facade design & engineering volume 10 / powerskin / 2022 retrofitting potential of building envelopes based on semantic surface models derived from point clouds edina selimovic1, florian noichl1, kasimir forth*1, andré borrmann1 * corresponding author, kasimir.forth@tum.de 1 technical university munich, germany abstract to meet the climate goals of the paris agreement, the focus on energy efficiency needs to be shifted to increase the retrofitting rate of the existing building stock. due to the lack of usable information on the existing building stock, reasoning about the retrofitting potential in early design stages is difficult. therefore, deconstructing and building new is often regarded as the more reliable and economical option. digital methods are missing or not robust enough to capture and reconstruct digital models of existing buildings efficiently and automatically derive reliable decision-support about whether demolition and new construction or retrofitting of existing buildings is more suitable. this paper proposes a robust, automated method for calculating existing buildings' life cycle assessments (lca) using point clouds as input data. the main focus lies in bridging the gap between point clouds and importing semantic 3d models for lca calculation. therefore, the automation steps include a geometric transformation from point cloud to 3d surface model, followed by a semantic classification of the surfaces to thermal layers and their materials by assuming the surface elements by building age class. keywords retrofitting potential, lca, point cloud, semantic enrichment doi https://doi.org/10.47982/jfde.2022.powerskin.8 128 journal of facade design & engineering volume 10 / powerskin / 2022 1 introduction the construction sector is responsible for a large share of the overall situation of increasing emissions. in 2019, the amount of waste produced by the construction industry in germany alone amounted to 230.9 million tons in 2019 (umweltbundesamt, 2021). most used materials are from non-renewable resources, mainly found in the existing building stock. at the same time, landfill capacities are decreasing and pose an additional environmental challenge to this industry sector (hillebrandt, 2018; rosen, 2018). furthermore, the manufacturing, transportation, construction, and disposal of newly built assets, including buildings and infrastructure, contribute to 11% of the total carbon emissions worldwide (worldgbc, 2019). for this reason, a particular emphasis is placed on the consideration of retrofitting instead of demolishing and building new. consequently, retrofitting is regarded the most sustainable and ecologically most significant solution for the previously described problems in industrialized countries (lottner, 2014; worldgbc, 2019). due to the lack of usable information on the existing building stock, reasoning about the retrofitting potential in early design stages is difficult. therefore, deconstructing and building new is often regarded as the more reliable and economical option (matthias hüttmann, 2018). robust methods are missing to efficiently capture and reconstruct digital models of existing buildings (deutscher abbruchverband. 2007). the automatic derivation of reliable decision-support about whether demolition and new construction or retrofitting of existing buildings is more suitable is currently not supported (matthias hüttmann, 2018). with the introduction of digital methods in the construction sector, such as building information modeling (bim), many conventional planning processes were facilitated (borrmann, 2015). synergies from many different disciplines have been identified, such as in the field of sustainability (bbsr & bbr, 2019). since then, model-based sustainability analysis, for example, life cycle assessment (lca), can be realized in early design stages. however, previous research focused on pre-existing bim models and neglected the existing building stock (akbarieh et al., 2020). the current way of creating a quantity take-off of an existing building is mostly a manual process, representing only an estimation and focused mainly on costs (deutscher abbruchverband, 2007). remodeling would be highly time-consuming. to circumvent this, techniques have been developed in recent years that use digital methods to automatically generate 3d building models of a building through point cloud capture and processing. automated processes for 3d reconstruction and object recognition are therefore needed. these are yet not fully mature or applicable for the case of lca calculation of existing buildings (chen, 2021). addressing the mentioned problems, this paper proposes a method for bridging the gap between point clouds and importing semantic 3d models into tools for holistic calculation of the environmental impact of different scenarios. this research proves the opportunity to compare the environmental impacts and emissions between an existing building and a retrofitting variant and supports future decision-making without extensive manual preparation. 2 background and related work this section discusses the background, related work, and the research gap for our proposed methodology. firstly, we discuss the limitations of bim in the end-of-life phases of building design. secondly, we introduce the topic of geometric reconstruction of point clouds. in the end, we show approaches of object classification with a focus on windows as an exemplary addition and essential contribution to lca calculations. 129 journal of facade design & engineering volume 10 / powerskin / 2022 2.1 bim in the end of life (eol) design phase akbarieh et al. investigated the application of bim in the eol phase of buildings and to what extent bim is used for planning in the eol phase (akbarieh et al., 2020). however, their research showed that most studies on bim-based eol topics were based on existing bim models and did not investigate a simplified automated remodeling for different use cases. the applications developed mostly represent application programming interfaces (apis) for specific proprietary bim tools in the form of plug-ins connected to lca software. a direct data transfer between the 3d model and the lca tool is often missing and not applicable without a workaround that includes specialized third-party software. studies conducted on existing buildings to investigate an eol-related topic and therefore create a 3d model were carried out by ge et al. (2017). they manually reconstructed the model with the help of plans, point clouds, and pictures, which is time-consuming. volk et al. (2018) investigated an automatic approach that enabled the 3d reconstruction of the interior of a building based on a point cloud collected by a depth sensor. the building components were detected automatically, and the quantity take-off was calculated based on standard values and experience data to eventually perform a deconstruction analysis. nevertheless, at some points, manual intervention was still needed. 2.2 3d reconstruction employing digital surveying methods, existing buildings can be captured as point clouds. reconstructing a 3d model from these point clouds has been a challenge in computer vision and computer graphics for years and is still an active field of research (chen, 2021). the poisson reconstruction by kazhdan & hoppe (2013) creates watertight surfaces from oriented point clouds. however, this approach requires complete data free of outliers, which is rarely the case for realworld measurements. other papers address the problem of manhattan-world scene reconstruction (coughlan & yuille 2000; li et al., 2016), which represents the 3d scene with axis-aligned nonuniform boxes. li et al.’s (2016) method successfully creates faithful reconstructions from various data sources. still, this approach dramatically simplifies the geometric complexity of the real world. nan & wonka (2017) developed polyfit, an approach that allows robust results for simple geometries with a slicing method. for this purpose, in a first step, the ransac algorithm (schnabel et al., 2007) detects planes in the dataset. the next step clips the model with the detected planes in polyhedral cells, representing the candidates’ faces. described as an integer programming problem, the results of this approach are watertight and manifold. the only disadvantage lies in the scalability due to the extensive computation time needed for complex geometries (nan & wonka, 2017). 2.3 object classification: window classification more semantic information can be added to create a model representation beyond the pure geometry of the building envelope. the window-to-wall ratio provides important information for energy simulation (schneider & coors, 2018). therefore window classification is a suitable extension for a pure 3d reconstruction process. there are many studies concerning window classification. volk et al. use an image rendering of the point cloud walls to extract the windows (volk et al., 2018). yang et al. (2016). create a binary image to detect ”holes“ in the image taraben & kraemer (2021) use a deep learning approach to map the detected windows from 2d images on a previously generated surface model. schneider & coors (2018) recognize windows directly in the point cloud using a contouring algorithm. 130 journal of facade design & engineering volume 10 / powerskin / 2022 nevertheless, most studies treat windows as “holes” in the building hull and do not consider further features as radiometric properties. one advantage of point cloud acquisition by laser scanning is the ability to record intensity values. intensity represents the amount of light reflected from a surface and is usually stored per point with a value from 0 to 255. it is assumed that each material can be approximated through a range of intensity values (macher et al., 2021). the evaluation of intensity features is robust to light changes or shading but depends on scanner position, distance to object, and object colour (macher et al., 2021; park & cho, 2021). park & cho developed a deep learning method for material classification on 3d point clouds that consider intensity values in addition to geometric information and colour values (park & cho 2021). macher et al. (2021) used intensity values to segment the windows from the facade based on a histogram analysis. for this study, a laser scan point cloud should be used to investigate window classification based on intensity values. 3 methodology this section proposes a methodology for geometrically reconstructing surface models using point clouds as input, semantically enriching these models and importing them into an lca tool. the acquisition of the point cloud derived from terrestrial laser scanning (tls) is out of scope for this paper, as it is only used as the input for the proposed methodology. the geometric transformation from point cloud to 3d surface model is conducted using the polyfit approach (nan & wonka, 2017). transparent elements such as windows are identified through laser intensity values of the points. in the next step, the faces of the model can be assigned to thermal surface classes and finally imported into lca software. the materials and material layers of the model faces are approximated by assuming the building age class. figure 1 shows an overview of the workflow. the following subchapters introduce the steps of this workflow in more detail. fig. 1 the general workflow 3.1 3d reconstruction the process of creating the 3d shape and appearance of real objects is referred to as 3d reconstruction. in the following, the steps for reconstructing a 3d model out of a point cloud are explained in more detail. the entire methodology is shown in figure 2. 131 journal of facade design & engineering volume 10 / powerskin / 2022 fig. 2 methodology for the 3d reconstruction to perform 3d reconstruction, the polyfit approach is used. this approach is based on the ransac algorithm (schnabel et al., 2007), which can identify planes from the point cloud. thus, building surfaces, such as walls or roofs, can be captured individually. in polyfit, the previously detected planes intersect with the point cloud’s bounding box. this results in several faces, referred to as face candidates (figure 3 left). to determine the faces from the face candidates that form a common surface, polyfit converts the reconstruction problem into a binary linear programming problem, resulting in the right picture of figure 3. fig. 3 polyfit result: face candidates (left) and selected face candidates (right) the result can be exported as an obj-file (fileformat.info, 2022). the obj file contains information about all exterior surfaces and their corner points. the surfaces in the file represent the selected face candidates pictured on the right in figure 3. however, for further applications, the combined building surfaces are more valuable. hence, step 3 determines the combined surfaces formed by the selected face candidates. to identify them, this study uses the face normals. face orientation can be determined using the dot product, starting from an xy-plane. in the last step the faces with the same normal orientation and at least one same point can be merged. after the related faces were detected, inner surfaces, in this case floors, are estimated using the building height. for this purpose, a floor height was assigned manually, which allowed the number of floors and their respective height coordinates to be estimated. the area of the floors is assumed to be identical to the ground floor. consequently, the corner points of the resulting surfaces are identified, as they are needed for further processing in lca tools. because the surface edges represent an intersection of straight lines, the condition can be set that corner points whose count is odd in the total set of points will represent a corner point of the total surface. this condition will take convex as well as concave polygonal surfaces into account. in this step, the order in the clockwise (cw) or counterclockwise (ccw) direction gets lost, and the corner points are listed in a random sequence. however, it is impossible to define a polygonal surface only by the point coordinates, so the order has to be rebuilt. to accomplish this, the point-edge relationships are considered beforehand. consequently, the identified corner points can be ordered in a cw or ccw order. 132 journal of facade design & engineering volume 10 / powerskin / 2022 3.2 object classification the surface model provides purely geometric information. to further use the model for lca calculation, semantic information has to be added for a retrofitting potential analysis. in a first step, each surface is assigned to a thermal surface class, which will be later enriched with more material assumptions. in a second step, windows are classified to derive the window-to-wall ratios for the model. 3.2.1 thermal surface classification each surface is assigned to a thermal class. the thermal surface classes describe the boundary conditions of each surface, e.g., wall to exterior or wall to the ground. there are 13 different classes existing (hollberg et al., 2018). in this study, the following assumptions are made: 1 there is no basement floor 2 the flat and the pitched roof are referred to as the roof class 3 the walls considered only represent the walls to exterior 4 there is no distinction between ceilings 5 there is no distinction between windows in walls and roof windows the thermal classes considered in this study can be classified based on simple geometric conditions, collected along with the thermal classes in table 1. the basis for this classification is normalized surface normal vectors, rounded to one decimal. the classes are determined through the surfaces’ orientation to the xy-plane, where rounding to one decimal leads to a tolerance of 2.8° for single-axis rotation and 4.0° rotation around two axes. table 1 thermal surface classification thermal surface class condition 01 condition 02 floor to ground orientation parallel to xy-plane lowest z-coordinates flat roof orientation parallel to xy-plane not the floor surface pitched roof orientation neither parallel nor perpendicular to xy-plane external-wall orientation perpendicular to xy-plane ceiling assumed floor height and floor number, dependent on total building height orientation parallel to xy-plane and equally shaped as the floor to ground 3.2.2 window classification the window classification is carried out using intensity values. these are obtained from the laser scan and stored with the points in the point cloud. therefore, the window classification is performed directly on the point cloud in contrast to the thermal surface classification. for the lca calculation, the window-to-wall ratio was estimated. an accurate geometric representation was not needed. figure 4 shows a summary of the classification process. 133 journal of facade design & engineering volume 10 / powerskin / 2022 fig. 4 process for the window classification in this study, the window points are extracted by their intensity values. therefore, the values of the whole point cloud are analyzed in a histogram. hence giving an initial range for the façade values, from which the range of values for the windows can be approximated. the range must be adjusted specifically to the project and is refined within the case study (chapter 4.3). the range with the most window points and the fewest false points is iteratively filtered out. false points refer to points whose intensity values overlap with the window points. to define a clear assignment of points to each window, the points defining one window are clustered into a group, which is necessary to extract the windows individually. the window points are clustered with the density-based dbscan-clustering by ester et al., (1996). the algorithm depends on the parameter eps, which describes the minimal radius that combines all points within this radius to a cluster. a weakness of the intensity-based approach is that the window points are not fully captured in the point cloud processing due to the transparent material property of glass, which leads to an inaccuracy in the detected clusters and thus only represents an approximation. consequently, the bounding boxes around the clusters, determined in the last step, were not perfectly rectangular. 3.3 information requirement and processing for the lca certain information is required to perform an lca from a semantic enriched 3d model. the information can be represented in alphanumerical numbers, coordinates, etc., and is needed for different reasons within the lca calculation. table 2 gives an overview of the required types of information. table 2 lca required input information information in the form of usage for lca calculation surfaces xyz-coordinates of the corner points essential for whole lca area of the surface alphanumerical number essential for whole lca surface material by assumption of the surface elements by building age class essential for whole lca thermal layer of the surface text label calculation of the use phase outward-facing surface normal vector calculation of the use phase window openings in surface number of the surface area and id of the window essential for whole lca window-to-wall ratio the surface area of the polygonal surfaces and windows is calculated based on the corner point coordinates. therefore, the surface is split into triangles, which allows taking convex as well as concave surfaces into account. the area can be calculated using the cross product of each triangle. the sum of all triangle areas determines the total area. afterwards, the window-to-wall ratio is calculated by dividing the previously classified window area by the total area. in the last step, the outward-facing normal of every surface is defined, needed to calculate the use phase in the lca. 134 journal of facade design & engineering volume 10 / powerskin / 2022 this computation represents a common problem, which is solved with the trimesh library (dawsonhaggerty et al. 2019) and is not applicable for objects with high geometric complexity. as a final step, in the lca calculation tool, the material layers and material thicknesses of the model faces are approximated by assuming the building age class. therefore, the information from the tabula database is combined with the lca database using oekobaudat (bbsr 2022). 3.4 calculation of the retrofitting potential the calculated retrofitting potential in this study aims to compare the environmental impacts of the existing building with a retrofitting variant using lca. to simplify and enable the comparability of the variants, the following assumptions as well as system boundaries and boundary conditions were made: – no consideration of the lifecycle phase c for the demolition of reconstructed old substances according to the rating system sustainable building (bmub, 2017) – considered useful lifespan of the building: 50 years – chosen environmental indicators: primary energy non-renewable and global warming potential (gwp) – manual specification of an identical energy system in summary, the retrofitting potential will compare only newly installed materials in all lifecycle phases. a detailed analysis for recycling and demolitions efforts of existing building components is out of scope, as in this early design phase, no reliable assumptions can be made. 4 case study 4.1 dataset and preprocessing to test and validate the proposed method, this study conducted a case study on a point cloud (see figure 5.a) obtained by a tls of an existing 5-storey multi-apartment building from the 1960s (merko 2022). the point cloud of the abandoned building, situated in adazi, latvia, was acquired due to the building’s need for retrofitting. first of all, the point cloud with 10.315.268 points was subsampled within the open source software cloudcompare (cloudcompare 2022) through the specification of space between the points to 0.1 m. in the used point cloud, the points of the interior of the building were also contained. to simplify computation, these points were removed manually. to reconstruct the 3d model, the planes were detected with the ransac algorithm (see figure 5.b) and the surface model was generated using the polyfit software. here the complexity was reduced with the provided polyfit parameters, allowing simplified further processing. the obtained surface model consisted of selected face candidates from polyfit (see figure 3 right). in the next step, related faces were combined with building surfaces (see figure 5.c), allowing the extraction of the corner points’ coordinates. 135 journal of facade design & engineering volume 10 / powerskin / 2022 fig. 5 a) the point cloud with b) the detected planes from ransac and c) the reconstructed surface model from polyfit 4.2 semantic enrichment and window classification subsequent steps deal with semantic enrichment. thermal surface classification is conducted on the geometric surface model. due to simplified assumptions, the implemented classification system classified all walls as exterior walls and the floor as floor-to-ground. a building with a cellar is not considered here. furthermore, the ceiling structure is always the same, and there is no ceiling against unheated space. in addition, recesses in the ceilings cannot be taken into account. extensions or garages with flat roofs are excluded since only one roof is assumed. the windows were detected separately in the point cloud file, providing the laser intensity information. after the performed histogram analysis of intensity values, it was observed that the intensity values of window elements were heavily spreading. nevertheless, almost all window intensity points lay in a range from 0 to 40, with some outliers at value 255. in between, there were virtually no window points detected. however, choosing all points in a range [0;40] would lead to many false points, mainly from the roof and the building edges (see figure 6.b). this is most likely the case due to standard sensitivity factors of laser scanners, such as the scanner position, the incidence angle, and the distance to the acquired object. the determination of the window range is thus project-specific, and a range of [0;25] was iteratively set as the most accurate for this point cloud. on this basis, the window points were extracted, and the db-scan clustering was performed to further extract the windows individually (see figure 6.c). here the parameter describing the minimal distance of a point to a cluster was set to 0.4 m. this value is set to prevent nearby window points from being merged into one cluster. the clustering results still showed that some windows were merged, but on the downside, some were neglected totally. nevertheless, this was largely compensated in the overall results. due to the intensity-based approach, some false points that were not describing windows, but overlapping in intensity values, were extracted too. this led to falsely detected clusters from these points. some were removed based on their orientation parallel to the xy-plane, as they were representing balconies. others were removed based on their very low height, which is not common for windows, thus representing the lower building edge. in the last step, the bounding boxes were defined around each cluster (see figure 6.d). due to some missing window points, the boxes were not perfectly rectangular but also rhomboid-shaped. this was acceptable since only the ratio is to be examined, and the geometrical form of the recognized windows is of secondary importance. consequently, the corner points were determined and the area calculated. the results showed a detection rate of approximately 94 %, with a total window area of 1129.62 m² compared to the correct area of around 1200 m². 136 journal of facade design & engineering volume 10 / powerskin / 2022 fig. 6 a) point cloud with b) filtered window points and c) clustered windows, and d) cluster-fitted bounding boxes 4.3 lca calculation of retrofitting potential subsequently, the gained information was processed as input in an lca tool. due to its lca-based direct variant comparison function, the lca software used in this study was caala (caala 2020). moreover, caala enables the calculation of the total emissions, embodied and operational energy, using a single-zone calculation approach. for this purpose, the data was converted into a caalajson file. therefore, the area, the surface outward normal, and the window-to-wall ratio was calculated, as described in chapter 2.3. after the geometry was read in the software, the material layers were assumed by building age class. the investigated building complex was assumed to be building type nbl_mfh_e (1958-1968) according to the tabula building type defined by the institute for housing and environment in the course of an eu project (loga et al., 2015). in the next step, the retrofitting variant was developed considering the building age, including the following measures, which caala suggests for this construction: 1 wood fibre insulations in an external thermal insulation composite system (etics) 2 triple glazing windows with a wooden frame, u=0.8, g=0.6 3 the energy system of a condensing gas boiler the refurbished variant leads to much better results regarding energy performance as well as greenhouse gas emissions. this is represented in table 3 with the total non-renewable primary energy consumption (penrt) and the greenhouse gas emissions (gwp) of the existing building in comparison to the emissions of the refurbished variant. all lifecycle phases were considered for the calculation as described in section 3.4. furthermore, figure 7 depicts the cumulative emissions of the gwp over the whole lifecycle. here the existing building starts with no emissions and finishes the first year with 78.53 kg co2-eq./ (m²ngf*a) from the building’s operational phase b6. the retrofitted variant starts with -24.0 kg co2eq./(m²ngf*a) resulting from phases a1-3 due to sustainable materials used and increases annually by 26.56 kg co2-eq./(m²ngf*a) on account of the operational phase b6. in contrast to the retrofitted variant, the emissions from the end of life phase c of the existing building are not considered as described by the considered systematic of the sustainable rating system (bmub, 2017). this can be seen in figure 7 from the different incline at the end of the two graphs.only simple retrofitting measures were chosen to validate the workflow, as the focus is on the automated generation of geometric and semantic information rather than on different and complex retrofitting scenarios. table 3 lca results of the variant comparison lca indicator unit existing building retrofitted variant primary energy non-renewable (penrt) [kwh/(m²ngf*a)] 358.74 129.14 global warming potential (gwp) [kg co2-eq./(m²ngf*a)] 78.53 28.43 137 journal of facade design & engineering volume 10 / powerskin / 2022 fig. 7 cumulative emissions of the gwp of the compared variants 5 discussion the proposed method enables lca calculation of retrofitting scenarios on the basis of semantic 3d models generated and enriched from point clouds. the method was tested on a 5-storey multiapartment building. the study verified that a point cloud of the building’s envelope could be processed in such a way that the minimum geometric and semantic requirements needed for an lca calculation and the import into an lca tool can be obtained. the proposed methodology leads to widening the scope of action for construction decisions on retrofitting the existing building stock already in early design stages. moreover, the ability of semantic enrichment of 3d models with intensity values for window classification was demonstrated. limitations of the developed method can be faced if the geometric complexity is too high, resulting in an inaccurate surface model or a wrong surface outward normal computation. the latter would lead to an incorrect lca calculation of the use phase and, therefore, only enable the calculation of the embodied energy, which is insufficient for the making of a meaningful decision. furthermore, the classification of the windows requires manual adjustment for filtering the intensity values of the window points. the investigation of laser intensity values restricts the digital surveying technology to laser scanning and the results to the quality of the acquired window points. it has to be mentioned that the results of the window classification represent a window-to-wall ratio and not an accurate geometric shape, which is, however, sufficient for a simplified lca calculation in an early design phase. the lca results strongly depend on the selected materials and thicknesses and on the assumptions of the tabula building typology. furthermore, the material allocation is not performed independently but requires this function in the software. 138 journal of facade design & engineering volume 10 / powerskin / 2022 6 conclusion and outlook regarding the building sector’s climate goals and environmental impacts, the retrofitting rate needs to be increased instead of the number of new constructions. hence, this paper proposes an automated method to calculate the lca of existing buildings by using point clouds as input data to close the gap between point clouds and lca software. to validate the method, a point cloud acquired with a laser scanner of an existing building was used, containing the intensity values of each point. the geometrical reconstruction was conducted using the polyfit approach. the semantic enrichment of the building components was based on geometrical information, while the windows were classified using intensity values. for this study, the window-to-wall ratio was sufficient. thus, the geometric representation of the windows was not needed. the developed method detected around 94 % of the total window area and mapped the windows to the appropriate walls. in future works, the importance of 3d reconstruction, and similarly, scan-to-bim processes, will increase, opening up a wide synergy potential for many research fields, such as sustainability. furthermore, radiometric features such as intensity should be investigated in future studies, especially in the promising field of deep learning. the consideration of intensity values could enable a material classification from the point cloud and therefore help to identify more information about surface materials. this could lead to a direct assumption of building classes. besides, the window classification can be more sensitized to determine the geometric shape. thus, a combination of the usage of intensity values with a deep learning technology could lead to promising results. references akbarieh, a., jayasinghe, l. b., waldmann, d., & teferle, f. n. (2020). bim-based end-of-lifecycle decision making and digital deconstruction: literature review. sustainability, 12(7), 2670. https://doi.org/10.3390/su12072670 bbsr, & bbr (2019). ökobilanzierung und bim im nachhaltigen bauen [life cycle assessment and bim in sustainable construction]. retrieved from https://www.bbsr.bund.de/bbsr/de/forschung/programme/zb/auftragsforschung/2nachhaltigesbauenbauqualitaet/2019/oekobilanz-bim/01-start.html bbsr, ö. (2022, may 20). ökobaudat. retrieved from https://www.oekobaudat.de/en.html bmub (2017). bewertungssystem nachhaltiges bauen (bnb). büround verwatungsgebäude. modul komplettmodernisierung. bnb_bk 1.1.1: ökologische qualität. wirkungen auf die globale und lokale umwelt. treibhausgaspotential (gwp) [sustainable building rating system (bnb). office and administration buildings. module whole-building modernization. bnb_bk 1.1.1: environmental quality. effects on the global and local environment. greenhouse gas potential (gwp)]. retrieved from https://www.bnb-nachhaltigesbauen.de/bewertungssystem/buerogebaeude/steckbriefe-bnb-bk-2017/ borrmann, a. (2015). building information modeling: technologische grundlagen und industrielle praxis [building information modeling: technological basics and industrial practice]. vdi-buch ser. wiesbaden: springer fachmedien wiesbaden gmbh. retrieved from https://ebookcentral.proquest.com/lib/kxp/detail.action?docid=3567925 caala (2020, august 3). ihr digitaler assistent für ganzheitliches entwerfen. – caala [your digital assistant for holistic design. – caala]. retrieved from https://caala.de/ chen, z. (2021). learning to reconstruct compact building models from point clouds (master thesis). delft university of technology. retrieved from https://repository.tudelft.nl/islandora/object/uuid:e33e7fa1-118e-41d8-904f-5f03eb36e887?collection=education cloudcompare (2022). cloudcomapre (version 2.12.1) [computer software]. cloudcompare: cloudcompare. retrieved from http:// www.cloudcompare.org/ coughlan, j. m., & yuille, a. l. (2000). the manhattan world assumption: regularities in scene statistics which enable bayesian inference. dawson-haggerty et al. (2019). trimesh (version 3.2.0) [computer software]. retrieved from https://trimsh.org/ abbrucharbeiten: grundlagen, vorbereitung, durchführung [demolition work: fundamentals, preparation, execution]. (2., aktualisierte und erw. aufl.) (2007). köln: r. müller. fileformat.info (2022, may 11). wavefront obj: summary from the encyclopedia of graphics file formats. retrieved from https:// www.fileformat.info/format/wavefrontobj/egff.htm ge, x. j., livesey, p., wang, j., huang, s., he, x., & zhang, c. (2017). deconstruction waste management through 3d reconstruction and bim: a case study. visualization in engineering, 5(1), 1–15. https://doi.org/10.1186/s40327-017-0050-5 139 journal of facade design & engineering volume 10 / powerskin / 2022 hollberg, a., lichtenheld, t., klüber, n., & ruth, j. (2018). parametric real-time energy analysis in early design stages: a method for residential buildings in germany. energy, ecology and environment, 3(1), 13–23. https://doi.org/10.1007/s40974-017-0056-9 kazhdan, m., & hoppe, h. (2013). screened poisson surface reconstruction. acm transactions on graphics, 32(3), 1–13. https://doi. org/10.1145/2487228.2487237 li, m., wonka, p., & nan, l. (2016). manhattan-world urban reconstruction from point clouds. in (pp. 54–69). springer, cham. https://doi.org/10.1007/978-3-319-46493-0_4 loga, t., diefenbach, n., & born, r. (2015). deutsche gebäudetypologie: beispielhafte maßnahmen zur verbesserung der energieeffizienz von typischen wohngebäuden [german building typology: exemplary measures for improving the energy efficiency of typical residential buildings]. darmstadt. macher, h., roy, l., & landes, t. (2021). automation of windows detection from geometric and radiometric information of point clouds in a scan-to-bim process. the international archives of the photogrammetry, remote sensing and spatial information sciences, xliii-b2-2021, 193–200. https://doi.org/10.5194/isprs-archives-xliii-b2-2021-193-2021 matthias hüttmann (2018). graue energie abreißen oder sanieren? [demolishing or retrofitting embodied energy?] bund-jahrbuch 2018 ökologisch bauen und renovieren: nachhaltige orientierung für bauherren, 16–19. retrieved from chrome-extension:// efaidnbmnnnibpcajpcglclefindmkaj/viewer.html?pdfurl=https%3a%2f%2fwww.bund-bawue.de%2ffileadmin%2fbawue%2fdokumente%2fthemen%2fklima_und_energie%2foekologisch_bauen_und_renovieren_2018_graue_energie._abreissen_oder_sanieren.pdf&clen=913329&chunk=true merko (2022, may 11). demo data | drupal. retrieved from http://www.merko.lv/en/demo-data nan, l., & wonka, p. (2017). polyfit: polygonal surface reconstruction from point clouds. https://doi.org/10.1109/iccv.2017.258 park, j., & cho, y. k. (eds.) (2021). laser intensity-assisted construction material classification in point cloud data using deep learning. orlando, fl. schnabel, r., wahl, r., & klein, r. (2007). efficient ransac for point-cloud shape detection. computer graphics forum, 26(2), 214–226. https://doi.org/10.1111/j.1467-8659.2007.01016.x schneider, s., & coors, v. (2018). automatische extraktion von fenstern in 3d punktwolkenmittels einer hierarchischen methode [automatic extraction of windows in 3d point clouds using a hierarchical method] (38. wissenschaftlich-technische jahrestagung der dgpf und pfgk18 tagung in münchen no. 27). münchen. retrieved from https://www.researchgate.net/ publication/323749844_automatische_extraktion_von_fenstern_in_3d_punktwolken_mittels_einer_hierarchischen_methode taraben, j., & kraemer, k. (2021). automatisierte generierung von stadtmodellen aus uas-befliegungen für die energetische bewertung von quartieren [automated generation of city models from uas flights for the energetic evaluation of neighborhoods] (32. forum bauinformatik 2021). umweltbundesamt (2021, november 29). abfallaufkommen [volume of waste]. retrieved from https://www.umweltbundesamt. de/daten/ressourcen-abfall/abfallaufkommen#bau-abbruch-gewerbe-und-bergbauabfalle volk, r., luu, t. h., mueller-roemer, j. s., sevilmis, n., & schultmann, f. (2018). deconstruction project planning of existing buildings based on automated acquisition and reconstruction of building information. automation in construction, 91, 226–245. https://doi.org/10.1016/j.autcon.2018.03.017 yang, j., shi, z.-k., & wu, z.-y. (2016). towards automatic generation of as-built bim: 3d building facade modeling and material recognition from images. international journal of automation and computing, 13(4), 338–349. https://doi.org/10.1007/ s11633-016-0965-7 140 journal of facade design & engineering volume 10 / powerskin / 2022 journal of facade design and engineering 3 (2015) 49–57 doi 10.3233/fde-150034 ios press 49 solar active envelope module with an adjustable transmittance/absorptance c. villasantea,∗, i. del hoyoa, i. pagolab, m. sánchezb and e. aranzabea amechanical engineering unit ik4-tekniker, c/iñaki goenaga, eibar, gipuzkoa, spain bsolar thermal energy department, national renewable energy center, ciudad de la innovación, sarriguren (navarra), españa abstract. a solar active envelope module with a high flexibility degree is proposed in this paper. the transparent module controls the day-lighting of the room, improving the indoor environment, while absorbing the superfluous solar energy inside. that energy is used to increase the efficiency of heating, ventilation, and the air-conditioning (hvac) system of the building. this is carried out through a fine control of the absorptance of the envelope module. the active envelope module consists of three glazed chambers with advanced coatings and frames to assure a minimum thermal transmittance while allowing transparency. a fluid containing heat-absorbing nanoparticles flows inside the central chamber and is heated up due to the impinging solar energy. unlike other systems proposed in the past, which included transparency control systems based on complex filters and chemical processes, the absorption of the module is controlled by the variation of the thickness of the central chamber with a mechanical device. that is, varying the thickness of the central chamber, it allows controlling the absorptance of the whole system and, as a result, indoor day-lighting and thermal loads. therefore, a new system is proposed that enables to: • dynamically control the thermal loads and solar day-lighting. • improve the thermal performance of building envelope and reduce the energy demand of the building. • increase the solar fraction of the heating and cooling, and reduce the thermal losses improving the efficiency of the building. the solar active device proposed in this article is protected by a spanish patent application (fundación tekniker, fundación cener-ciemat, 2014). keywords: solar collector facade, architectural module, adaptive envelope, absorptance regulation, indoor comfort, dynamic control, thermal loads reduction 1. introduction several facade solutions that control the indoor day lighting for an improved environment can be found in the literature. for instance, traditional blinds and shutters have been widely used and recently, products like isolette® include built-in blinds. some other existing modules, in addition of controlling the day-lighting through the facade, absorb the part of the solar energy that is not needed inside, and use it for different purposes, such as the increase of heating, ventilation and air-conditioning (hvac) system of the building efficiency. ∗corresponding author: c. villasante, mechanical engineering unit ik4-tekniker, c/iñaki goenaga, 5 20600 eibar, gipuzkoa, spain. tel.: +34 943 20 67 44 / 943 25 69 00; fax: +34 943 20 27 57; e-mail: cristobal.villasante@tekniker.es. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:cristobal.villasante@tekniker.es 50 c. villasante et al. / solar active envelope module with an adjustable amongst these facade solutions that control the lighting, smart windows for solar control are available, which allow blocking a controllable amount of impinging solar energy. thus, the window allows changing the solar factor and transmittance properties to adjust to outside and inside conditions, reducing the energy needed for heating and cooling. there are several working principles for this kind of adaptable windows. firstly there are the passive ones, the photo-chromic or thermo-chromic, which change their opacity depending on the incident light or temperature conditions. as their properties change according to ambient conditions, it is not possible to implement a control system (bertsch, oppliger, & menzi, 2013). there are also active technologies, such as the gas-chromic ones, which base their opacity control on the injection of different gases in the gap between the different glass layers that compose the window (wittwer, graf, & georg, 2002). another active technology is the electro-chromic window (ecw) (lee et al., 2004), which reversibly changes its optical and energy transmittance properties when an external potential is applied. the ecw is the most suitable technology for architectural applications. nevertheless, all these solutions control the day-lighting of the rooms for an improved indoor environment by rejecting most of the impinging solar energy. as already mentioned, some facade solutions that take advantage of the incident solar energy are available on the market. water-filled glasses are the most common solution in this group. glass facades have very low thermal inertia, and solar radiation causes the greenhouse effect inside the buildings, causing indoor overheating and comfort decrease, forcing thus to include energy consuming air-conditioning systems. in order to limit these negative effects, some authors suggest filling the gap between the glass panes with water. this way, water blocks some solar radiation in the infrared spectrum as well as increases the thermal inertia of the building’s interior, reducing the internal temperature variation and therefore, reducing the peak demand of energy. although most of the solar energy is still entering the building, this system creates an isothermal envelope of the building to reduce the hvac energy costs without waiving the advantages of the use of glass. double-glazing panes with circulating hot or cold water can also be used in glass facades. some prototypes, which can be integrated into the building’s facade or roof, have been developed (www.intelliglass.es). another solution is the fluidised glass facade element (gstöhl, 1998) with two fluid layers, which uses the outer liquid layer for controlling the energy transmission through the absorption of solar radiation. despite the fact that the system unifies the functions of a shading device, solar collector and heating and cooling element, different concentrations of colorant are needed for the control of the absorptivity of the fluid. the latter requires a complex separator and a dosing feeder to be included in the system. a quite similar system was proposed later (schwarz, 2001). an additional related patent was previously introduced (frantl, 1991). in this patent, a window, facade or wall construction includes at least three panes of glass, which are separated to delimit at least two intermediary spaces. a heat transfer medium, which can be coloured or pigmented, is circulated through one of the intermediary spaces. if a change in coloration or pigmentation is desired, this can be performed by means of a dosed supply, by filtering out dyes or pigments from the heat transfer medium, or by a chemical procedure that changes the ph-value of the medium. all these solutions are difficult to be implemented and integrated in a building. the solar active envelope module presented in this paper controls the lighting and ventilation of the room for an improved indoor environment and absorbs the part of the solar energy that is not needed inside, reducing the energy consumption of hvac systems of the building or other applications, such as swimming pool heating. the use of solar energy is carried out through a fine and simple control of the absorptance of the whole system (fundación tekniker, fundación cener-ciemat, 2014). c. villasante et al. / solar active envelope module with an adjustable 51 2. the solar active envelope module aiming to combine all the advantages of the solar active facade and solar collectors, the proposed system consists of a smart glass envelope with the ability of controlling dynamically both light and room/building temperature, taking advantage of all incident solar energy reaching the system. in the system at hand, lightness control does not imply rejecting solar energy. on the contrary, excess of solar light will be absorbed into a heat transfer fluid (htf) to improve the energy efficiency of the building. this goal will be achieved by implementing a transparent envelope module (both roof and facade solutions are possible) that will include a htf flowing inside. thickness of this htf channel will be adjusted in order to absorb the desired amount of solar light, based on the combination of the indoor user needs and the outdoor environmental conditions, and transform it into thermal energy, converting thus the building envelope into a distributed solar collector. each facade module will be regulated independently, so it can behave as a totally transparent to totally opaque module or with any intermediate transparencies. the proposed solar active envelope module consists of three chambers between glass panes. the fluid containing heat-absorbing nanoparticles flows inside the central chamber and is heated up due to the incident solar energy. by varying the thickness of the central chamber with a mechanical device, the absorption of the module is adjusted, as light has to cross a different fluid thickness (see fig. 1). fig. 1. different module transparency depending on fluid thickness. note: the fluid composition is always the same and so will be the transparency observed in the real cross section. the change in the color considered in this figure is just an indication of the effect of thickness adjustment in the module. 52 c. villasante et al. / solar active envelope module with an adjustable vents fig. 2. details of the concept (3d design). fig. 3. the movable glass (third one from outside, left side) adjusts the thickness of the central chamber (where the fluid flows). the thickness of the central chamber is varied with a mechanical device, which acts on one of the glasses (see figs. 2 and 3). depending on the position of the movable glass, the system transparency will be different, achieving as a result, a completely adjustable and automatic day lighting regulation. although fig. 2 describes a system including linear actuators, a number of possible driving systems can be proposed (fundación tekniker, fundación cener-ciemat, 2014) including screws, pneumatic actuators, cams etc., depending on the requirements (accuracy, force, window size, space available, etc.) in each case. water tightness is also an issue that has been taken into consideration, flexible plates or bellows must be attached to the fixed structure and to the movable glass in order to seal the whole perimeter. moreover, glass resistance and bending due to pressure loads could be an issue that will lead to a multiple supporting points/drives design or that will limit the maximum height of a single module. in addition, the frame of the window includes four ventilation vents, with an integrated smart filter system. two of the ventilations vents are opened to the outside and the other two to the inside, controlling thus the indoor air-quality and comfort (see fig. 2). c. villasante et al. / solar active envelope module with an adjustable 53 outside inside fig. 4. different coatings proposed for the system. besides, each glass layer can be covered with special coatings (see fig. 4). optimized optical coatings for reducing thermal losses and improving optical behaviour, and anti-soiling coatings for reducing cleaning operations, are considered in the proposed module. 3. operational modes of the solar active envelope module the proposed system is suitable to have a smart control system integrated, which will be able to dynamically adapt the system (position of the movable glass pane, flow of the fluid containing heat absorbing nanoparticles and ventilation flows) depending on the indoor needs and considering the outdoor conditions, guaranteeing thus their maximum exploitation. in this way, the system is able to: • act as a smart active curtain in such a way that the solar lighting is controlled by means of the thickness of the central chamber and the transparency given to the fluid. a completely adjustable and automatic daylighting regulation is achieved and indoor air-quality and comfort are guaranteed with the included ventilation vents. • optimize the thermal loads of the building and performance of the building envelope. as a result, the energy consumption from auxiliary heating systems and production of domestic hot water (dhw) will be reduced. in addition, the thermal loads will be substantially reduced during hot days thanks to fluid blind, reducing therefore the cooling necessity and energy consumption. • take advantage of the energy absorbed in the fluid increasing the solar fraction of heating and cooling, reducing the thermal losses and improving the efficiency of the building. in that way, the control system could act on the system following the next foreseen procedure: • the control system decides the lighting needs under the measured conditions and the usage pattern of the room to be controlled. hence, it acts on the mechanical devices to adjust the thickness of the central chamber in order to obtain the required indoor illumination. • the control system commands the mass flow inside the chamber depending on needs, being possible to absorb the excess of energy on the building envelope into the htf as useful thermal energy. 54 c. villasante et al. / solar active envelope module with an adjustable 4. simulations of the proposed module 4.1. methodology in order to characterize the performance of the active module proposed in this paper, a model of the system has been implemented and several simulations have been made. the model has been developed in a flexible, well-structured and object-oriented way, which has been accomplished applying the modelica modeling language. modelica is a multi-purpose physical system modeling language and has been developed in the frame of an international initiative in order to unify already existing similar modeling approaches and to enable developed models and model libraries to be easily exchanged. the concept is based on non-causal models featuring true ordinary differential and algebraic equations, i.e. differentialalgebraic equation (dae) systems (elmqvist & mattsson, 1997). the object-oriented approach, the possibility of multiple inheritance and the re-declaration feature lead to a clear model structure, avoid multiple definitions of frequently used code and offer an incredible model flexibility. the code syntax and application guidelines are defined in the regularly updated modelica language specification (https://www.modelica.org). furthermore, the use of modelica clearly decouples the modeler from the equation system solving. instead of developing a specific solving algorithm for each modeling task, the modelica tool reads the developed modelica code, performs symbolic manipulations of equations and translates the modelica model into a numerical simulation code, using state-of-the-art algorithms developed for general application. thus, developed models and model libraries are exchangeable, i.e. can be read and simulated using different modelica environments. today, commercial, as well as open-source modelica environments are available. the system module model has been implemented, splitting the system in several submodels. by dividing the whole facade module into model subunits, categorized by the three basic modes of heat transfer, conduction, convection and radiation, many already existing freely available standard model structures and components could be used. additionally, modelica’s object-oriented way of modeling led to a clear model structure and maximized the reusability of a newly developed code for future applications. from the outside to the inside of the building, the facade module model has been divided in the following subunits (see fig. 5) in order to compute temperatures (t) and mass flows (m): • natural convection representing the outside atmospheric conditions (tout). • double glazed window (tw) representing the first chamber and the corresponding two glass panes facing the exterior. • an absorption fluid flow (tff) representing the fluid with nanoparticles flowing through the middle chamber and absorbing solar energy. • a glass pane (tlg) with heat conduction. • an air flow (taf) representing the air flowing through the chamber facing the interior and ventilation vents. • a glass pane (trg) with heat conduction. • natural convection representing the indoor conditions (tin). ascanbeseeninfig.5,eachmodule(e.g.awindow)issplitinnnodesinordertoresolvetheequations. asanexample,thetemperature(t)ofeachnodeandtheabsorptionfluidflowmassflow(ṁff)havebeen obtained for two clear-sky days in a specific location assuming the indoor and outdoor conditions. (https://www.modelica.org) c. villasante et al. / solar active envelope module with an adjustable 55 fig. 5. system model scheme and related implemented sub-models corresponding to a single module. the specific location which has been chosen for this analysis is san sebastián (43.32◦n, 1.98◦w) located in the north of spain. an hourly clear sky year has been created and, after that, two representative days have been selected for the simulations. the selected days are the 21st of june and the 21st of december. the incident solar irradiance is needed as an input in the system model and, therefore, the impinging irradiance to a vertical facade module facing south had to be calculated from the clear-sky hourly values. some assumptions have to be made for the simulations. in the first place, the outdoor ambient temperature has been selected from a typical meteorological year for the location of san sebastián. an indoor temperature of 24◦c and an air flow of 0.1kg/s through ventilation vents have been assumed. for the fluid flow absorbing solar energy an inlet temperature of 35◦c, and outlet temperature of 70◦c and an absorptance of 0.8 have been supposed. 4.2. results using the implemented modular system model some simulations have been carried out for the specific location of san sebastián on the 21st of june and the 21st of december. the indoor and outdoor ambient temperatures and the absorbing fluid flow inlet and outlet temperatures have been imposed, so the results of the simulation are the temperature of each node of every subsystem and the mass flow of the absorbing fluid flow. the results of the simulations corresponding to hours with impinging solar radiation are presented in the appended tables. 56 c. villasante et al. / solar active envelope module with an adjustable table 1 simulation results of the proposed facade module for the 21st of june in san sebastián month day hour direct normal irradiance impinging t out ṁff irradiance clear module (w/m2) (◦c) (kg/h m2) sky (w/m2) 6 21 9 746 102 27.3 1.20 6 21 10 803 194 28.6 2.59 6 21 11 837 261 29.8 3.65 6 21 12 854 290 30.7 4.08 6 21 13 857 279 31.5 3.96 6 21 14 844 227 32 3.12 6 21 15 816 142 32.3 1.75 6 21 16 767 37 32.3 0.22 total 20.57kg/day m2 table 2 simulation results of the proposed facade module for the 21st of december in san sebastián month day hour direct normal irradiance impinging t out ṁff irradiance clear module (w/m2) (◦c) (kg/h m2) sky (w/m2) 12 21 9 379 274 9.1 3.60 12 21 10 596 492 11.1 7.20 12 21 11 700 625 12.9 9.53 12 21 12 745 684 14.4 10.56 12 21 13 749 675 15.5 10.32 12 21 14 714 601 16 9.12 12 21 15 625 467 15.8 6.86 12 21 16 441 273 14.8 3.67 total 65.42kg/day m2 as it can be seen in table 1, a total of 20.57kg per day and per m2 of facade module of fluid at 70◦c is expected to be obtained in this location, the 21st of june being a clear sky day. as it can be seen in table 2, a total of 65.42kg per day and per m2 of facade module of fluid at 70◦c is expected to be obtained in this location the 21st of december being a clear sky day. one major advantage of the proposed system in this paper is that those quantities of fluid heated at 70◦c, obtained as a result of controlling the indoor illumination, will be used for the production of dhw and for hvac if the building is equipped with an absorption machine. in addition, the blocking part of the incoming solar radiation will contribute to reduce even more the building thermal loads decreasing the cooling needs and energy consumption. 5. conclusions this paper proposes a solar active envelope module, with a high degree of flexibility. aiming to combine all the advantages of the solar active facade and solar collectors, the proposed system c. villasante et al. / solar active envelope module with an adjustable 57 consists of a smart glass envelope with the ability of dynamically controlling both light and room/building temperature, taking advantage of all incident solar energy reaching the system. the proposed solar active envelope module consists of three chambers between glass panes. the fluid containing heat absorbing nanoparticles flows inside the central chamber and is heated up due to the incident solar energy. by varying the thickness of the central chamber with a controlled mechanical device, the absorption of the module is adjusted, as light has to cross a different fluid thickness. the module transparency is based on the combination of the indoor user needs and the outdoor environmental conditions. the proposed system is suitable to have integrated a smart control system which will be able to dynamically adapt the system. as a result, the module will: • act as a smart active curtain. • optimize the thermal loads of the building and performance of the building envelope. • reduce the thermal losses and improve the efficiency of the building. in order to characterize the performance of the proposed active module a model of the system has been implemented using modelica language. the system module model has been implemented splitting the system in several subunits. using the implemented module system model some simulations have been carried out for the specific location of san sebastián the 21st of june and the 21st of december, obtaining the temperature of each node of every subsystem and the mass flow of the absorbing fluid flow. the results of the simulations show that, in the location of san sebastián, a total of 20.57kg per day and per m2 of facade module of fluid at 70◦c is expected to be obtained on the 21st of june being a clear sky day. in addition, a total of 65.42kg per day and per m2 of facade module of fluid at 70◦c is expected to be obtained in this location on the 21st of december being a clear sky day. those quantities of fluid heated at 70◦c, obtained as a result of controlling the indoor illumination, will be used for the production of dhw and for hvac if the building is equipped with an absorption machine. in addition, the blocking part of the incoming solar radiation will contribute to a even higher reduction of the building’s thermal loads decreasing the cooling needs and energy consumption. references bertsch, s., oppliger, d., & menzi, t. (2013). u.s. patent no. 20130263535al. elmqvist, h., & mattsson, s. e. (1997). modelica the next generation modeling language – an international design effort, proceedings of the 1st world congress on system simulation, singapore. frantl, e. (1991). u.s. patent no. us4993235. fundación tekniker, fundación cener-ciemat (2014). spanish patent no. p201431102. oepm madrid, spain. gstöhl, d. (1998). world organization patent no. wo 98/51973. intelliglass: www.intelliglass.es/ (accessed 28.03.2015) lee, e.s. et al. (2004). the energy saving potential of electrochromic windows in the us commercial building modelica-association, (2012). modelica® a unified object-oriented language for systems modeling language specification – version 3.3, modelica-association, https://www.modelica.org (accessed 28.03.2015) schwarz, d. (2001). u.s. patent no. us6216688. sector building. technologies program, environmental energy division, lawrence berkeley national laboratory, 2004. wittwer, v., graf, w., & georg, a. (2002). gaschromic glazings with a large dynamic range in total solar energy transmittance. nrel 5th international meeting on electrochromism, 2002. www.intelliglass.es/ https://www.modelica.org from city’s station to station city 41 journal of facade design & engineering volume 6 / number 1 / 2018 integrated façades as a product-service system – business process innovation to accelerate integral product implementation juan f. azcárate-aguerre1, alexandra den heijer2, tillmann klein1 1 department of architectural engineering and technology (ae+t), faculty of architecture and the built environment, tu delft, delft, the netherlands 2 department of management in the built environment (mbe), faculty of architecture and the built environment, tu delft, delft, the netherlands abstract the circular economy (ce) attempts to realign business incentives, across all fields of human industry, to support the preservation of raw materials within closed economic loops. within this conceptual frame, product-service systems (pss) combine the use of tangible products such as building technologies, with intangible maintenance and monitoring services, to enhance the delivery of valuable performance while limiting the use of materials and other finite resources. this paper explores the potential for the application of ce and pss organisation principles in the delivery of façades-as-a-service. it explores how the benefits brought about by this way of thinking lower initial capital requirements, material ownership retention by suppliers, and long-term interdisciplinary collaboration could lead to a more efficient façade construction industry, while accelerating the rate and depth of building energy renovations. within the current process for designing, manufacturing, and operating façades there is a gap between supply-side discoveries and demand-side needs, which hinders the implementation of resource-efficient façades. façade-leasing as a form of product-service system keeps suppliers committed, throughout the building’s service-life, to safeguard optimum performance in operation, while actively stimulating clients to adopt innovative technical solutions. the paper elaborates on both supply-side façade innovations and the demand-side conditions necessary to implement such business models, and also explores the costs and benefits of product-service systems as new collaboration models to align supply and demand incentives. it builds upon the research project “façade leasing” (azcarate-aguerre, j.f., 2014) and combines knowledge about façade design and engineering (supply-side approach) with the knowledge about client needs, performance criteria, and willingness to pay (demand-side approach). the research methodology includes a literature review and expert interviews, integrating both theory and practice. this paper argues that a product-service system approach to façade design, construction, operation, and renovation could accelerate the rate and depth of building energy renovations. it could also provide incentives to supplyand demand-side stakeholders, to implement circular economy principles through new models of product ownership, service contracting, and performance delivery. it aims at establishing the general conceptual frame of a product-service system for leasable façades, setting the basic parameters to be taken into account when designing a pss-based business model, and formulating its value proposition. keywords product-service systems (pss), façade-as-a-service, leasing, resource-efficient innovation, integrated façades, circular economy doi 10.7480/jfde.2018.1.1840 42 journal of facade design & engineering volume 6 / number 1 / 2018 1 introduction how does the construction process hinder the implementation of energy saving measures and components? the last few decades have seen an exponential development in the field of energy conservation and generation technologies within the construction sector. goals for the reduction of co2 emissions, established by the eu for the years 2020 and 2050, have set a strong regulatory frame for the implementation of such technologies in all fields of architectural and infrastructural development. as a result, many organisations with large real estate portfolios – such as universities, hospitals, and financial institutions – have signed agreements in the past decade to reduce their ecological footprint and stimulate resource-efficient projects (den heijer & teeuw, 2010; joustra, de jong, & engelaer, 2013; abn amro, 2014; ing, 2015). however, the mass-market implementation of energy-efficient products is being negatively affected by the traditional business and supply processes that dominate the construction industry (vrijhoef, 2011). innovative business models and contracting mechanisms are required to support and accelerate the market absorption of energy-efficient technologies in the industry, share the performance risks (and benefits) of innovative products, and enhance the financial accessibility of performance-based renovations (gondrie, 2015). this would upscale the impact of upcoming technologies on the reduction of co2 emissions, by facilitating their marketwide implementation. in other words, innovative business and management processes are required to act as a catalyst for the accelerated implementation of innovative technological products. despite the current technological capacity to produce energy-neutral and even energy-positive buildings (marszal & heiselberg, 2011) the real-life application of these technologies is still limited to a relatively small segment of high-performance, high-cost, iconic, and experimental constructions (banfi, farsi, filippini, & jakob, 2008). low-energy buildings – though highly significant from a scientific and marketing perspective – are still a small fraction of the european building stock. the main hindrance to the wider utilisation of such systems lies in the incentive structure that dominates the industry, as short-term stakeholders are offered no direct incentives from the longterm operational benefits provided by their products (van nederveen & gielingh, 2009; vrijhoef, 2011). direct operational benefits from energy savings must be complemented by the multi-stakeholder incentive structure proposed by a circular economy systemic approach. in the current, productcentred transaction structure, stakeholders involved in the construction and operation of a building have short-term participation in the project based on the sale of components. a transition towards a service-based structure, founded on the long-term collaboration between project partners with shared performance goals, will more effectively create shared value while improving a building’s performance and ecological impact from an energy and raw material consumption perspective. many of the theoretical assumptions presented in this paper have been extrapolated from betterdocumented examples belonging to other manufacturing industries, as this is a relatively unexplored topic in the construction industry. this study builds upon these examples to establish a value proposition for new models of collaboration, by outlying the theoretical costs and benefits of a longterm, product-service system (pss) for the delivery of façades-as-a-service. this is done from the point of view of both supply and demand side stakeholders. in order to evaluate this proposition, we combine, on the one hand, a building technologies approach to describe upcoming technological products, as well as their potential impact on a building’s energy performance. on the other hand, we assess, from a real estate management perspective, the value these product-service combinations could represent for a (client) organisation. this is based on their contribution to the organization’s functional, financial, strategic and sustainability/energy goals (den heijer, 2011; den heijer, 2013) (fig. 1). 43 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 28 “4-value” performance criteria for determining added value for clients and end-users of real estate. adapted from den heijer, 2013 (based on real estate management theory (heijer, 2011)) this paper proposes that the general focus on short-term financial gains (or losses) that currently dominates decision-making, often results in missed opportunities to collaboratively extract longterm value in the functional, strategic, and energetic fields, while also preventing the implementation of circular resource management and conservation strategies. it explores the ways in which the current supply process hinders the application of new, and more efficient, building products and technologies (section 2). it then explores the supply-side challenges and opportunities (section 3.1), as well as the demand-side requirements and interests that must be taken into account (section 3.2) in order to determine the brief for new business-to-client models that encourage innovation (section 4). the objective is to outline the changing role of stakeholders, the added value for demand and supply sides, and, lastly, to define further necessary research along these lines. 2 problems in the current construction process the current construction industry is characterised, as are many other manufacturing industries, by a strong linear process (vrijhoef, 2011; joustra, de jong, & engelaer, 2013). the flow of materials, services, and knowledge through the supply chain is largely interrupted at every step of the process, as long-term collaboration between supply tiers, contractors, and clients is hardly promoted by current contracting methods (vrijhoef, 2011). the general tendency to look at buildings as “finished products” rather than “ongoing processes” leads to an overall short-sightedness when defining the most efficient operation and end-of-service scenario design for the construction and the materials that compose it. a failure to define long-term goals that can be shared by all stakeholders (on the supply and demand sides), results in a process that assigns a high value to materials as materials and components are the elements being traded between stakeholders while underestimating the value of services (or capabilities) delivered by or through such products. we have identified two primary mechanisms 44 journal of facade design & engineering volume 6 / number 1 / 2018 embedded in the construction process that contribute to a fragmented supply-chain and a slow technological progress curve. these are further described below as: 2.1 business and supply mechanisms, and 2.2 technological innovation mechanisms. 2.1 business and supply mechanism the rate of innovation, development, knowledge transfer, and implementation in other words, the technological life cycle of the construction industry is relatively slow. this section elaborates on those negative circumstances, which lie within the business and supply practices of the construction industry: (a) the industry structure, and (b) the small scale of supply companies. a. the industry structure -when compared to other manufacturing industries (such as automotive and consumer electronics), the construction industry stands out for its general lack of central coordination (van nederveen & gielingh, 2009; vrijhoef, 2011). a central driving force, in this context, is defined as a stakeholder with incentives to optimise the entire production process from design through fabrication, operation, and end-of-service reprocessing who has clear leverage on suppliers and subcontractors, and therefore the power to reshape the entire supply chain towards more efficient or sustainable practices. as a reference, automobile manufacturers act as central driving forces throughout the entire process from the design to the collection/reprocessing of a car, even when they may not be necessarily responsible for individual steps in the process such as designing and manufacturing individual components, or providing aftermarket servicing and maintenance. their crucial role in the production of the car, in terms of design, assembly, branding, and even financing, provides them with an important leverage to re-define their processes and demand suppliers to follow their guidelines. as established by the principles of “lean manufacturing” (womack, jones, & roos, 1990) this long-term relationship between the product assembler/marketer and component suppliers promotes innovation and efficiency by setting common and extended performance-oriented goals throughout the supply-chain. in the construction industry, however, there are two mayor conflicts that prevent the application of such processes. on one hand, the supply-chain, consortium, and contracts differ from project to project (vrijhoef 2011). they are as customised as the individual projects they are related to, which hinders standardisation in collaboration approach or product solution (gjaltema, 2013). therefore, the risks associated with the implementation of innovative solutions are relatively high, as supplier involvement after the realisation of the project is generally limited to a series of operational guarantees. on the other hand, none of the individual stakeholders collaborating in the construction process has enough leverage to demand substantial changes, in terms of practices and methods, from the other parties involved in the project. a possible exception to this would be the client, who could decide to maintain active involvement in the process as a decision-maker, but whose technical knowledge would generally be insufficient to demand significant structural changes. a shift towards a more active participation from clients and investors has been recently recognised. real estate developers and managers such as delta development group (scott, 2014), and banking institutions such as abn amro (abn amro, 2014), are taking steps to improve the long-term health and sustainability of their projects and investments. b. the small scale of supply companiesfaçade suppliers and producers, worldwide, are in general relatively small companies (cleton, 2014). the typical project portfolio of one of these producers could comprise between 10 and 20 projects, of varying sizes, at any given time. this means that a problem with product-delivery or guarantees in any given project can have a substantial negative 45 journal of facade design & engineering volume 6 / number 1 / 2018 effect on the overall yearly performance of the company (and can have devastating consequences when accentuated by times of financial crises). this currently results in a façade industry that is overly cautious when it comes to implementing new technologies with a limited testing history. instead, systems with which suppliers and contractors are familiar are chosen, and which have been proven consistently over time, even if these systems are below the state-of-the-art in terms of energy efficiency or other performance criteria. consequently, the small scale hinders innovation, as sme suppliers cannot often afford to deviate from traditional solutions, or are prevented from doing so by market forces or decision-makers further up the value chain. on the positive side, a small project portfolio means that sme suppliers are likely to be interested in implementing new business models, which extend their involvement in projects and ensure a long-term, steady source of revenue. this is in contrast to a product-delivery-based business structure, which forces them to constantly look for new clients and projects in order to secure a highly volatile cash-flow. 2.2 the technological innovation mechanism next to the structural disablers that the construction industry brings from a business and supply perspective, there are technological obstacles that affect the market-integration of innovative products: a) the rate of effective technology implementation and b) risks and uncertainties for the client. a. the rate of effective technology implementation the rate at which technological innovation can occur within a system is, necessarily, closely tied to the length of its (effective) implementation cycle, also known as its “vital life” (arthur d. little, 1981). by an effective implementation cycle we mean the time it will take between the creation of the first working prototype of a technological product, and the moment in which this product reaches the mature economy of scale, in production, which would facilitate its mass-market application. this rate is also tied to the expected service-life of the previous generation of an equivalent product, as few users will replace a system before it is technically (or in some cases socially) required. for example, smartphone suppliers are able to make modifications to their platforms at a rate of one or even two new releases per year, because the market absorption and expected service-life of these units is, on average, 18 to 24 months. in this specific case, replacement rarely comes as a technical obligation, but is generally due to trends, marketing, and other social behaviour. in the case of façades and façade-integrated components, service-life is generally expected to fall within the range of 20, 30, or even 40 years. if we consider each product generation to be a mass-market testing prototype, it is easy to see why mobile phones have exponentially increased their involvement in our everyday life over the last 10 or 15 years, by radically changing their functionality, while façades today look quite similar to how they did 80 or 100 years ago, even though their performance and functionality have vastly improved. new methods for product development and implementation are required, closer to those of the automotive and electronics industries, if we expect to shorten the rate at which upcoming technologies are launched into the market, tested, upgraded, improved, and replaced. since façades are massive assemblies, hardly comparable to an automobile or a smartphone in terms of material use and replaceability, a possible approach would be to fragment the façade assembly into smaller, more manageable pieces, which can be constantly and individually reassessed with the introduction of new, more effective technologies. 46 journal of facade design & engineering volume 6 / number 1 / 2018 b. risks and uncertainties to the client from the client’s perspective, the decision to invest in energyefficient systems also carries a significant risk. when we consider current occupation trends, especially in the case of residential real estate, we see that the average time a building owner will live in a single property is around 7 years (gondrie, 2015). this is considerably shorter than the average time required for the return on investment of an energy-saving system. for example: photovoltaic (pv) solar panels are calculated to reach socket parity when the return on investment is equal to or higher than 5%, depending on the system’s efficiency and the rate of inflation of energy prices (bazilian et al., 2013). it is also a risk choosing the right moment to make a capital investment on sustainable technologies. going back to the example of pv panels: the cost of a pv installation per watt output has dropped by an average of 21% per year over the last 30 years (mayer, simon, philipps, schlegl, & senkpiel, 2015). this means the capital investment required to buy such systems before they reach maturity or mass-production scale could negatively offset the return on investment of the system from energy savings due to a faster relative depreciation. 3 solutions to transform the construction process innovation in building technologies, and especially energy-saving systems, has been accelerating at an unprecedented rate. residential zero-energy renovations, which until recently represented an expensive, experimental concept, can now be realised for a relatively small additional investment of between 20% and 25% (azcarate-aguerre et al., 2017). however, as we have discussed before, the market-wide implementation of these systems, whether in new construction or in deep renovations, is significantly slowed by organisational and information-exchange bottlenecks in the construction industry. this tendency can be counteracted through innovative business models that consider the accelerating rate of innovation in the supply industry, and reconciles it with the long-term financial commitment these systems represent for real estate demand interests. new products, released at shortening intervals, cannot be integrated into the market through traditional supply mechanisms. innovative products and services demand innovative business practices and a deep industrial reorganisation (van nederveen & gielingh, 2009; vrijhoef, 2011). design, build, finance, maintain, and operate (dbfmo) contracts are a promising step in the direction of re-assigning long-term decision-making powers to a party (in this case a general contractor backed by a multidisciplinary consortium) with sufficient technical understanding of the construction and operation process (straub, prins, & hansen, 2012). in such contracts, the centralised contractor in charge of developing and managing the building over a 40 or 50 year contract period, would have the level of responsibility and control needed to demand deep structural changes from product and service suppliers. however, as we will discuss further, dbfmo contracts are only partly successful as a circular economy implementation mechanism. 3.1 product-service systems as an industry-transforming strategy the business and supply solution product-service systems are a business-to-consumer (b2c) strategic model that fits within the frame of a circular economy structure. a pss business model replaces a traditional purchasing scheme, in which a supplier transfers ownership and responsibility of an asset to a buyer, 47 journal of facade design & engineering volume 6 / number 1 / 2018 maintaining only limited liabilities over it in the form of technical guarantees. from a pss perspective, the product on its own does not hold the final value, but is merely a mechanism through which a service can be delivered to a client (baines & lightfoot, 2013). to put this into an example involving pv cells: a traditional purchasing method would have a client buy the pv cells from a manufacturer, through a one-time cash payment or through a financial lease. the client would then own the panels, in many cases pay an additional fee for any required maintenance, suffer the technology’s capital depreciation, and deal with the product’s end-of-service scenario. in exchange for this he would generally get a return-on-investment from the energy savings in his property’s operating costs. in a pss model, on the other hand, the physical pv panels are not the item being sold, but are instead combined with the continual service delivered by those panels through a longterm contract with the client or end-user and charged relatively to their actual performance. in such a scenario, the client would not pay for (nor ever legally own) the pv panels, but would instead pay a fixed monthly or yearly amount based on the effective operation of the system, or even a variable amount related to the system’s output (eg. euro per watts generated in a given month). the client is therefore paying not for the materials embedded in the pv panels, but for the performance provided by these to produce passive energy through the building’s envelope. product-service systems act, therefore, as a dematerialisation strategy. they remove financial incentives from the sale of physical products, and force manufacturers and service providers to optimise their service-delivery by minimising their use of material and human resources (baines & lightfoot, 2013). from an industry perspective, this offers a number of advantages and a huge field for the development of new business structures to organise and manage a long-term, ongoing relation between suppliers of technologies (original equipment manufacturers oems), contractors in charge of delivering product-service packages, building owners, and end-users. in fact, pss thinking is already being applied for individual components with an external interface with the building (meaning they are not embedded into the construction, nor interconnected with other components, and can therefore be installed/uninstalled with a relatively small effort). an example of this is the combination of products and financial/technical services offered by photovoltaic-leasing companies in the united states and the netherlands (liu, eric, tyner, & pekny , 2014). a more ambitious approach to pss implementation would not only deal with the way in which technological systems and financial/legal contracts are packaged and sold to the end user. an integral pss approach would completely redefine the way in which systems are designed to interact with each other by, for example, increasing standardisation and reducing compatibility issues. it would also restructure the supply chain in terms of contractual obligations. oem suppliers would hold a long-term contract with the general contractor, who is, in turn, contracted in the long-term to deliver an optimal performance to the client. lastly, it would promote a new form of design which facilitates replacement, upgrade, and reprocessing of obsolete components within a larger system, while guaranteeing that removed parts can be easily reused or recycled for new purposes in an expanding second hand market. 3.2 integrated façades the technological solution integrated façades are complex building assemblies in which a large part of the building’s service and climate-control systems are contained within the modular construct of the building’s envelope. integrated façade principles can be found in both curtain wall designs, as well as in self-standing modular window boxes (klein, 2013). in most cases, a wide frame surrounding the glass façade surface will contain diverse technical systems such as: cooling and heating, ventilation, heat-exchange, shading, energy generation and/ or storage, media projection, electric and water supply, and performance-monitoring sensors. 48 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 29 schematic shift from a traditional product-system to a circular product-service system for integrated façades. for certain building typologies, such as cell offices, integrated façade systems can virtually eliminate the need for centralised building services which results, from a pss perspective, in two major advantages: 1) it combines two of the four basic building elements (structure, envelope, building services, and building infill) into one; and 2) it facilitates the distributed functioning of envelopeintegrated services according to room occupation trends, thereby avoiding the negative centralisedsystem effect in which large sections of the building are conditioned even when not in use. combining the envelope and building services elements of a construction is a logical step, when we consider how closely related they are in terms of expected service-lives. while the structure of a building is generally expected to last for 50, 100, or 200 years and the interior finishes and mobiliary can be changed as often as every 5 or 10 years, building services generally provide a technical service-life within the range of 15 to 20 years. envelopes are expected to perform for between 20 and 40 years. combining these systems on the outside of the building can facilitate and coordinate renovation and system-replacement processes in terms of both logistics and use of materials. concentrating climate-control mechanisms in the façade also means the envelope will play a more determinant role than ever before in the efficient climatic and energy performance of the building, which can be an advantage when defining utilities-inclusive contracts. a product-service system approach to the design, installation, and operation of integrated façade modules (fig. 2) would allow a service-provider to estimate, within a reasonable range, the impact of his modular products on a specific building’s indoor climate and energy consumption, therefore allowing him to offer a longterm, performance-based contract, as opposed to a single outright-purchase option. it is important to note that the effect of façade and building services on climatic and energetic performance can vary according to diverse building types. it is, however, beyond the scope of this paper to analyse the extent to which a façade service provider can guarantee a determined indoor comfort level. 49 journal of facade design & engineering volume 6 / number 1 / 2018 3.3 the potential of pss for the implementation of a circular economy the potential of product-service systems for the implementation of a circular economy model lies in the correct distribution of ownership, responsibilities, and interests throughout the supply chain (joustra, de jong, & engelaer, 2013). under the current business structure, producers and installers of technological systems and building components are only tied to their products by a legal mechanism based on guarantees and liabilities. such a system “punishes” the under-performance of a product, instead of “rewarding” its over-performance (fig. 3). a linear supply chain (the one currently dominating the architecture, engineering, and construction industry) will have each step of the supply mechanism surrendering ownership of the physical products to the next, in exchange for a certain degree of technical guarantees. all systems are ultimately transferred to the client (by definition, in most cases, the party with the most limited technical knowledge), who then has to hire a team of facility management experts to extract the best possible performance out of these systems. long-term efficiency, apart from major faults which would have to be covered by guarantees, are not in the interest of suppliers and manufacturers, as they no longer maintain financial ties or incentives to this performance. end-of-service scenarios are also negatively affected by this business structure; the client, and owner of the materials contained in the building, will frequently surrender ownership of these materials to the company in charge of demolition as a form of payment. the materials will then be extracted with varying degrees of effectiveness, and the output sold in the global market. processing and logistic costs are high, as components are not originally designed for disassembly, making their separation process difficult and inefficient. their sale on a global market reduces the chances that these materials will be re-used locally, thus increasing transportation expenses and related co2 emissions. an intermediate business model would have a driving stakeholder on the supplier side being responsible for the construction and operation of the building over a determined period of time. in dbfmo contracts, for example, the general contractor in charge of the project’s 30to 50-year service-life will retain responsibility over the effective performance of the building and its systems, he will then rent the building to the client for a fixed yearly fee. dbfmo contracts do not, however, strictly follow the principles of pss thinking, and are instead a form of extended financial lease. the contractor is effectively the constant manager of the building, and is in charge of the financing and maintenance of all components, but these contracts often do not include utility costs (meaning the contractor cannot draw direct incentives from the energy-efficiency of the building, and instead is only penalised if the performance is below a specified benchmark) and they do not specify a strategy for dealing with the building’s materials at the end of the contracted period. at that point in time the client might simply become a traditional owner/manager of the building, or it might be sold in the market, or rented out in a new lease contract. the end-of-service scenario for the construction materials is therefore equally uncertain, as demolition and recycling are generally not included as part of the original planning and contracting process. a true pss-oriented business model will have all stakeholders tied, materially and financially, to the optimum performance of the building throughout its service life, including end-of-service material extraction and reuse. this is however, not in the form of penalties for below-expected performance, as in the case of an operating lease, but in the form of incentives for above-expected results. a pss method for the installation of integrated façades would include utility costs from climate control. 50 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 30 schematic linear vs. circular supply-chain structure, from “punishment” to “reward”. the product-service system would use the integrated façade modules as a product to deliver a final indoor comfort and energy performance as a service. this means pss façades can become a method of energy performance contracting (epc) in which the cost of a façade renovation, through leased components, can be partially or totally repaid through the savings resultant from their increased energy efficiency. the continuous nature of the service-provider’s role throughout the components’ service lives, and the fact that an improved energy performance will result in direct profit increase, means it will be in his primary interest to maintain an optimal overall building performance through the use of updated technologies. 4 discussion and conclusion in theory, integrated façades as product-service systems have the potential to permanently bridge the technical, financial, and legal knowledge gap between producers of building technologies, builders, managers, and clients. by treating each building project as an ongoing service (which may last decades or even centuries) instead of as a delivered product, a pss can not only integrate a circular economy mindset into the construction industry, but also set up the business mechanisms that will ensure all parties in a project are committed, in the long term, to a single goal: the best possible functional performance of a building with the most effective, minimum use of resources. however, this transition requires changes in the innovation process, which starts with persuading stakeholders to explore the advantages, and weigh these advantages against the uncertainties and risks. conclusions about the required pss characteristics and conditions – the pps brief – are summarised below. the following section presents our arguments, from a multi-stakeholder perspective, on how this transition towards a service-oriented industry can be achieved (section 4.1), and why this transition is in the interest of the principal parties involved (section 4.2). it then discusses the state of research, and proposes a series of future steps necessary to bring this concept closer to its practical realisation (section 4.3). 51 journal of facade design & engineering volume 6 / number 1 / 2018 4.1 changing roles in the innovation process successful radical innovation requires a major crisis or market opportunity. if the construction industry wants to develop into a market with more innovative capabilities, the innovation process has to change as well (bers, dismukes, miller, & dubrovensky, 2009; vrijhoef, 2011; joustra, de jong, & engelaer, 2013). such an industry-wide shift will not be reached without a fair amount of restructuring and collateral damage. the financial demands and long-term stability required by a long-term ongoing project could, ultimately, be unfeasible to many smaller players in the supply chain (such as subcontractors or system providers), who might have to expand and merge their businesses, or sacrifice profit under pressure from larger players further up the supply chain. general contractors will have to plan their future operations based on how many buildings they can afford to manage at any given time, while ensuring they maintain a diverse enough portfolio, instead of the current model based on delivery dates and a constant search for new contracts. this represents a major shift in the traditional business practices of such companies. the cost and risk of this transition could have a negative effect on a number of organisations and stakeholders, but could be rewarded with a greater financial stability and improved solidity to face economic fluctuations or crises. a. the financial sector needs to stimulate and support changes in supply and demand business models by applying new financing mechanisms. recent studies by large banks (ing economics department, 2013; abn amro, 2014) already show that they are exploring more innovative technical/ financial packages to support new business models. the road for this has been set by relatively simpler contracting methods employed by other industries, with mobile phones and cars being among the most common. such industries have certain advantages in this regard such as, to name but a few: a longer service-based contracting track-record, higher product-service standardisation, a clearer demarcation between client and supplier roles, and standard contract termination terms. all of these contribute to lower uncertainty and hence lower risk premiums. the construction industry, meanwhile, is entering an exploratory phase, in which such contracts and multi-stakeholder relations are being tested in customised scenarios, while standard contracts and risk management structures are still to be developed. the 2008 financial crisis will provide a useful background for this development, as large financial institutions have been forced to change their strategy and (further) diversify their investment portfolios, thereby providing leverage against the uncertainties of increasingly fluctuating economic cycles. financial regulation on real estate investments, which is currently based on preserving overall property value through clear ownership structures, must adapt to understand and include models of collaborative service-focused ownership. non-regulated, or improperly regulated, investment models could have a negative effect on the sustainability of the building stock if they lead to the creation of complex, deceptive, high-risk financial products such as those which triggered the 2008 global financial crisis. b. the architectural practice needs to re-assess the value given to unique, irregular forms and highly customized assemblies, and consider decisions based on a modular interaction between high-performance building components. this does not mean architectural design should become a secondary priority, completely restricted by the functional requirements of standardised building technologies. however, a leasable system would rely on a certain degree of modularity and interchangeability as a way of increasing the residual value of components, which would in turn have an effect in the conceptualisation and design development processes of architects and designers. 52 journal of facade design & engineering volume 6 / number 1 / 2018 c. building owners – and other demand-side stakeholders – will need to explore the added value of alternative business models and (re)evaluate the traditional concept of ownership. in fact, this exploration has already been taken place in many organisations, under the influence of agreements to reduce the carbon footprint and ambitions to be frontrunners in innovation. examples can be found at universities, hospitals, and financial institutions with large building portfolios (den heijer & teeuw, 2010; joustra, de jong, & engelaer, 2013; ing economics department, 2013; abn amro, 2014). ongoing research has found that tools to accurately compare total cost of ownership for diverse investment options still need to be further developed. this is particularly evident in the case of accurate methodologies for evaluating direct and indirect operational costs in existing buildings, information which is crucial in determining the economic attractiveness of a traditional or service-based façade renovation project. d. new generations of decision makers have seen great advances in the concept of use and access above the concept of ownership (rifkin, 2000). innumerable modern assets, such as printers, phones, cars, and real estate, are now frequently leased, rented, or shared. this represents a significant cultural deviation from a traditional tendency to own a wide range of physical assets. internet-based applications have facilitated the dissemination of “sharing-economy” models in which people within a certain region can share products or services upon demand without the need for intermediaries (apart from the internet-based application itself). these socio-cultural changes create a positive atmosphere for the growth of more complex systems of performance-based business-to-business (b2b) contracting in which physical components constitute a means and not an end. the broader social and economic consequences of such disruptive models are still, however, not fully understood. a shift towards product-service systems could also entail a concentration of resource ownership in the hands of companies, which could exacerbate economic polarisation trends contributing to growing wealth gaps, both locally and globally. such systemic consequences are difficult to model and predict, and must therefore be taken into account and monitored throughout the development of circular business models such as the one presented in this thesis. 4.2 added value of pss for stakeholders an additional complexity built into the construction industry is the highly significant impact this sector has on a wide range of direct and indirect stakeholders. while a poorly functioning household item will only create a problem for the user and can most likely be returned to the manufacturer for reprocessing without any major effort, a building has a permanent presence within its context over one or more human generations. this means that the stakeholders in a building are not only the supply and demand parties directly involved in its construction and operation, but also its end users, city inhabitants, regulatory bodies, infrastructure providers, and countless others. the adoption of a circular economy process, in the form of pss building components, offers considerable incentives to most of the parties involved (fig. 4), especially in times of economic uncertainty when preconceptions about our economic and industrial activities should be revised. the wider groups affected by processes in the industry are listed below: a. demand-side stakeholders could initially benefit the most, especially now that real estate managers are more likely to focus on total costs of ownership, and not only initial investment. rising energy prices, social trends that value the aesthetics of a “brand-new” and “high-tech” appearance, and accelerating technological innovation create a substantial economic pressure, which causes buildings to depreciate at an ever-faster rate. real estate owners and managers are 53 journal of facade design & engineering volume 6 / number 1 / 2018 more aware than ever of the value of maintaining their building portfolio in optimal conditions. dealing with this depreciation, however, requires deep technical understanding of the systems operating within the building (more so as buildings become more complex and filled with highlyspecific technologies). by outsourcing the entire life cycle of diverse building components to technical experts who have a clear understanding of them, clients can avoid the struggle and financial risks associated with managing these systems themselves. a pss approach would provide the following advantages according to the 4-value performance criteria identified at the start of this paper (fig. 1): sustainable/energy as mentioned thoroughly in this article: sustainable, energy-performative technologies are being released into the market at accelerating rates. the strategic and financial value these technologies offer to a client institution is closely related to the use of state-of-the-art systems. such systems can be made available and replaced (efficiently) within shorter intervals through leasing mechanisms that guarantee operational consistency and material conservation. risks presented by lower than expected actual energy-savings, for example caused by the documented rebound effect (guerra santin, 2013), need to be taken into account, and might lead to additional monitoring and/or financial costs. functional rapidly shifting real estate trends demand increasing levels of flexibility in a building’s architectural programme, occupancy, aesthetic design, and technical services. a service-based supply business model would significantly increase the capacity of real estate managers to respond to these changes by modifying the performance, appearance, and specifications of their building portfolio, without being weighed down by long-term investment cycles. strategic the european market for commercial real estate is currently suffering from a high vacancy rate. the excessive supply of commercial floor space in certain regions forces building owners to think about additional values, which they can offer to potential clients in order to distinguish themselves from their competitors. leasable façade systems would allow more frequent renovations and a wider functional flexibility (as stated above), which would in turn result in more attractive properties with more frequent maintenance schedules and a higher energetic performance and user comfort. financial in many cases, real estate ownership and management is not the primary business activity of the client institution. leasable products provide more equally distributed, constant cash-flows, making real estate investments more predictable and eliminating the current peaks in capital flow over a building’s service life: construction, major maintenance/renovation works, and deconstruction. outsourcing technically-demanding services, while eliminating financial peaks, would allow clients to focus resources on their primary business activities. b. supply-side stakeholders, on the other hand, could exploit entirely new areas of business development. as we see with other manufacturing industries, operation and financial services are among the most profitable activities a company can engage in. combining building components (products) with a combination of technical and financial services would thus expand the range of activities from which construction companies currently derive their revenues. not only would it expand it but, as mentioned earlier, it would spread these revenues over a constant, steady income flow, stabilising their long-term finances and reducing their vulnerability in times of economic turmoil. this is especially relevant to the sector because, as we know, the construction industry is generally among the first and hardest hit by financial crises due to their high dependency on a small number of large, short-term projects (cleton, 2014). 54 journal of facade design & engineering volume 6 / number 1 / 2018 fig. 31 benefit overview of a pss façade concept according to den heijer’s 4-value criteria. the focus on product’s performance could meanwhile incentivise product innovation by shifting the focus to entire life-cycle engineering. design decisions could, for example, justify higher material content or quality in exchange for longer service-lives, or lower maintenance costs. additional investment on disassembly mechanisms could be financially justified if they lead to component or material preservation within closer economic loops of reuse, repair, and remanufacturing. this replaces traditional recycling processes which often entail the down-cycling of valuable and critical materials due to unfeasible separation costs. c. regulatory bodies and society as a whole would benefit from the more efficient use of material and financial resources resulting from keeping complex technical systems in the hands of industry experts. a circular business model, in which all parties involved in the project have a permanent interest in the correct performance of the building, would naturally lead to a more effective use of energy and raw materials (as waste of either one of them would negatively affect their business’ profitability), while guaranteeing the best possible end-of-service management of all systems. constantly involved supplyside partners would have a technical platform, and the direct incentives, to integrate new technologies more quickly into the market, making transition happen at a faster rate throughout the construction industry. demand-side clients with more regular cash flows dedicated to covering real-estate costs would have more stability to focus on their primary processes and business objectives. 4.3 following steps this paper has established the general conceptual frame of a product-service system for the delivery of façades-as-a-service. it has set the basic parameters to be taken into account when designing a pss-based business model, and formulated a value proposition from the diverse perspectives within the supply and demand sides of the construction industry. as mentioned before, many of the concepts 55 journal of facade design & engineering volume 6 / number 1 / 2018 presented in this article have been extrapolated from better documented examples belonging to other manufacturing industries which have undergone a transition towards servitisation. in order to better understand the differences of applying such models in the construction sector, our team is currently in the process of developing a pilot project and testing environment with the active participation of industry representatives from the identified stakeholder groups. barriers and opportunities can already be identified in the transition towards façades-as-aservice, and need to be further explored. one possible drawback of this system would come from the complexities of user behaviour. as seen in numerous studies, energy-based renovations frequently create a “rebound” effect, in which “occupant behaviour has a significant effect on energy consumption, given the higher temperature settings in dwellings with insulation, mechanical ventilation and more efficient temperature control” (guerra santin, 2013). a possible solution for this problem would be to include a maximum-energy-use clause in the contract, specifying the range of energy consumption guaranteed by the service provider, above which the difference will be charged to the user. another solution could be the implementation of complex monitoring systems which differentiate the building’s base consumption from additional losses due to negative user behaviour. more in-depth knowledge is also needed regarding the current process for decision-making, procurement, and contracting, which governs the building practice during the development stages. further research will elaborate upon the value proposition offered by a pss business model according to the project’s target market, and offer alternatives as to which stakeholders within this supply chain could drive the transition to this new form of thinking. a strong focus on the demand and regulatory side will be crucial to determining further conclusions, as we believe the reorganisation of the supply industry would not be effective on its own. instead, clients and governing bodies must clearly recognise the value these ideas hold to support their activities and interests, and be ready to undertake the structural changes necessary for their implementation. a clearer picture of the needs and processes undertaken by owners and operators of real estate could lead to higher definition in the applicability of a pss-based integrated façade, from a business perspective. further research must elaborate on the conditions and incentives (established in section 4 of this paper) that are required for this model to be applicable in practice. a combination of schematic technical and business prototypes must be used to develop and analyse a real-life pilot project. the objectives of this exercise must be to bring into the discussion many of the diverse stakeholders analysed, and discuss the value proposition of our model through tangible examples based on a real building case. this will facilitate a practical evaluation of the pros and cons of façades-as-a-service, assessed through a realistic pilot project. acknowledgements the authors want to thank the branch organisation of the dutch metal façade industry, vmrg (representing the supply side), as well as tu delft’s facilities management and real estate agency, fmvg (representing the demand side) – for their input during the preliminary stages of this research project. their contribution will continue to be highly valuable throughout the following steps of this research. we also want to thank eit climate-kic for their active and ongoing support of this initiative. 56 journal of facade design & engineering volume 6 / number 1 / 2018 references abn amro (2014). circular construction – the foundation under a renewed sector. amsterdam: circle economy for abn amro, december 2014. azcarate aguerre, j., konstantinou, t., klein, t., guerra santin, o., steensma, s., & silvester, s. (2017). investigating the business case for a zero-energy refurbishment of residential buildings by applying a pre-fabricated façade module. eceee 2017 summer study on energy efficiency. pp. 1113-1122[3] baines, t., & lightfoot, h. (2013). made to serve: how manufacturers can compete through servitization and product service systems. chichester, west sussex, united kingdom: wiley. banfi, s., farsi, m., filippini, m., & jakob, m. (2008). willingness to pay for energy-saving measures in residential buildings. energy economics, 30(2), 503-516. bazilian, m., onyeji, i., liebreich, m. et al. (2013). re-considering the economics of photovoltaic power. renewable energy. bers, j. a., dismukes, j. r., miller, l. k., & dubrovensky, a. (2009). accelerated radical innovation: theory and application. technological forecasting and social change, 76(1), 165-177. doi: doi 10.1016/j.techfore.2008.08.013 chao-duivis, m.a.b., koning, a.z.r., & ubink, a.m. (2013). a practical guide to dutch building contracting. ‘s-gravenhage: instituut voor bouwrecht. cleton, i. (2015). creating value with sustainability. pd eng. dissertation, stan ackermans institute, eindhoven 2015. c2c bizz (2013), guided choices towards a circular business model, interreg iv b report of c2c bizz consortium (authors: douwe jan joustra, egbert de jong, frits engelaer) for project samenwerkingsverband regio eindhoven (sre), november 2013. den heijer, alexandra (2013). assessing façade value – how clients make business cases in changing real estate markets. journal of façade design and engineering (1) pp. 3–16, december, 2013. den heijer, alexandra (2011). managing the university campus – information to support real estate decisions. delft: eburon academic publishers, march 2011. den heijer, alexandra and peter teeuw (2011). sustainable visions for the campus of the future. misbe 2011, management and innovation for a sustainable built environment. 20-23 june 2011, amsterdam, the netherlands. gjaltema, g.m., vrijhoef, r. (2013). experimental design strategy as part of an innovative construction industry. 7th nordic conference on construction economics and organization (2013), 227-238. gondrie, n. (2015). renovation, leasing a prefab renovation in the netherlands. msc thesis tu delft, delft: faculty of architecture and the built environment, january 2015. guerra santin, o. (2013). occupant behaviour in energy efficient dwellings: evidence of a rebound effect. journal of housing and the built environment, 28(2), 311-327. doi: 10.1007/s10901-012-9297-2 ing economics department (2015). rethinking finance in a circular economy: financial implications of circular business models. amsterdam: ing bank, may 2015. klein, t. (2013). integral façade constructiontowards a new product architecture for curtain walls. dissertation, delft technical university, delft 2013. little, a. d. (1981). the strategic management of technology. liu, x., eric, g. o., tyner, w. e., & pekny, j. f. (2014). purchasing vs. leasing: a benefit-cost analysis of residential solar pv panel use in california. renewable energy, 66, 770-774. marszal, a. j., & heiselberg, p. (2011). life cycle cost analysis of a multi-storey residential net zero energy building in denmark. energy, 36(9), 5600-5609. mayer, j. n., simon, p., philipps, n. s. h., schlegl, t., and senkpiel, c. (2015). current and future cost of photovoltaics. long-term scenarios for market development, system prices and lcoe of utility-scale pv systems (fraunhofer ise, study on behalf of agora energiewende, freiburg, 2015). rifkin, j. (2000). the age of access: the new culture of hypercapitalism. where all of life is a paid-for experience. new york: tarcher. scott, l. (2014). park 20|20, amsterdam: born to be recycled. urban land (article based on interview with coert zachariasse, delta development group), may 5, 2015. retrieved from http://urbanland.uli.org/sustainability/park-2020-amsterdam-born-recycled/ straub, a., prins, m., & hansen, r. (2012). innovative solutions in dutch dbfmo projects. architecture science van nederveen s. and w. gielingh (2009). modelling the life-cycle of sustainable, living buildings. journal of information technology in construction (itcon special issue building information modeling applications, challenges and future directions). vol. 14, pg. 674-691. vrijhoef, ruben (2011). supply chain integration in the building industry: the emergence of integrated and repetitive strategies in a fragmented and project-driven industry. phd thesis, delft: ios press, november 2011. womack, j. p., jones, d. t., roos, d. (1990). the machine that changed the world: based on the massachusetts institute of technology 5-million dollar 5-year study on the future of the automobile. new york: rawson associates. from city’s station to station city 001 journal of facade design & engineering volume 7 / number 2 / 2019 superposition matrix for the assessment of performancerelevant adaptive façade functions jens boeke*1, ulrich knaack1, marco hemmerling2 * corresponding author 1 tu delft, architecture and the built environment, j.boke@tudelft.nl 2 cologne university of applied sciences, institute for architectural design abstract the environmental boundary conditions and the demand for comfort change constantly during the use of a building. by dynamically balancing changing conditions and requirements, adaptive façades contribute to the energy efficiency of buildings. the façade fulfils a multitude of functions that are interdependent and relate to environmental conditions and requirements. by negotiating mutually supportive and competing adaptive functions, intelligent coordination offers the potential for better performance of façades in building operation. the strategy is already being applied in other application areas, such as the intelligently cooperating machines in industry 4.0. there, individual automated production plants are networked to form intelligent technical systems with regard to a common production goal. the research presented follows the assumption that this strategy can be applied to automated and adaptive functions of the façade to increase the building performance. the study identifies those functions which, due to possible automation and adaptivity, as well as effect on performance, can be considered as possible components of an intelligently cooperating system. in addition, characteristics are determined which can be used to evaluate the extent of automation and adaptivity of an individual façade function. the study shows that the detailed analysis of the automation and adaptivity within identified façade functions is possible. with a superimposition matrix, it also provides a tool that enables this assessment of the degree of automation and adaptability. keywords intelligent building envelope, superposition matrix, adaptive façade, multi-functionality, intelligent technical system 002 journal of facade design & engineering volume 7 / number 2 / 2019 1 introduction 1.1 background the façade determines the overall performance of the building. it has an impact on the indoor comfort and the buildings´ energy efficiency. in view of current objectives related to energy saving and the increased demands on the well-being of users within buildings, there is a demand for more efficient façade systems. researchers see a potential in adaptive façades. (aelenei et al., 2015) the climatic conditions of the outdoor environment and the indoor requirements are constantly changing. the balancing of both presents a continuous optimisation of the construction properties with regard to the performance of the façade. this enables savings in the operation of energy-consuming building services, which can be reduced if the building envelope guarantees a desired indoor climate. the façade fulfils a multitude of additional functions in the role of a mediator between the exterior and interior. the façade functions are derived from the influences of the environment and the requirements of building use. the façade functions are mutually interdependent. they can conflict or positively influence and complement each other. moloney (2011) and loonen, trčka, cóstola, and hensen (2013) formulate the demand for holistic concepts instead of fragmented solutions for adaptive façades. adaptive façades are being researched and realised, but the development is still at an early stage. (aelenei et al., 2015) the implementation of adaptive façades often includes automation technology. over the past decades, research and development in this field provided the technical basis for the realisation of such systems. (schumacher, schaeffer, & vogt, 2009) these include the miniaturisation of electronics and computer technologies, as well as the developments in sensor and actuator technologies. the close interaction of computer-based control and communication with physical technical systems forms the concept of cyber-physical systems. an important aspect is the cooperation of distributed system components. (monostori, 2014; rajkumar, lee, sha, & stankovic, 2010; wang, torngren, & onori, 2015) the internet of things (iot) describes the comprehensive and internet-based networking of physical objects. all devices that have an embedded control system and the ability to communicate can be part of it. (bittencourt et al., 2018) in the current development of an industry 4.0, the flexibility and productivity of manufacturing processes is increased by networking individual production facilities into so-called intelligent technical systems. technological developments in various research fields, such as it and neurobiology, are merged to provide mechatronic systems with intelligence based on embedded sensors, actuators, and cognitive abilities. the individual technical systems within an intelligent technical system work autonomously and are able to communicate and cooperate with regard to a common production goal. (dumitrescu, jürgenhake, & gausemeier, 2012) böke, knaack, and hemmerling (2018) assume that such strategies can be applied to the operation of the building envelope. by networking automated adaptive façade functions within an intelligent system, the efficiency of the façade in building operation is to be increased in the sense of greater flexibility and productivity in industrial production. for the networking, a differentiated understanding of the individual façade functions and the individual possibilities of automation and adaptivity is required. there are different lists of façade functions that do not take adaptivity into account, such as the “façade function tree” by klein (2013). 003 journal of facade design & engineering volume 7 / number 2 / 2019 loonen et al. (2015), for example, provide characteristics of adaptivity for the overall system of the façade without consideration of individual functions. 1.2 problem statement the possibility of an intelligent cooperation of automated adaptive façade functions according to the model of networked production plants in industry has not yet been clarified. the role of an individual adaptive façade function as a component of an intelligently networked façade can only be assessed by comparing it with the project-specific environmental conditions and performance requirements. it is uncertain which façade functions can be considered as a part of an intelligently networked system due to an adaptive feasibility and an effect on the performance of the façade. previous listings of façade functions, like the “façade function tree” by klein (2013) or the “façade as an interface” by hausladen, de saldanha, liedl, and sager (2005) refer to the façade generally, and differ in organisation, scope, and detail. the transfer of the networking strategy from industrial production plants to the façade depends, according to the technical basis of an industry 4.0, on the comprehensive automation and adaptivity of the individual functions. there is a lack of assigned characteristics by which the degree of automation and adaptivity of an individual façade function can be assessed. previous studies on a possible characterisation of adaptive façades, such as the composition by (loonen et al., 2015), refer to the façade as an overall system. they do not provide a complete result, since they refer to partial aspects such as either functionality or the degree of automation. 1.3 objectives the aim of the study is to develop a holistic view of adaptive façades in the interplay of requirements and external boundary conditions. the knowledge about dynamically changing factors of the boundary conditions supports a later decision as to which information must be collected via sensors for the intelligent operation of a networked façade system. for this purpose, the individual factors to which the adaptive façade must react are to be recorded. in addition, the various requirements for interior comfort are to be compiled as target values for the intelligently networked façade system. façade functions that can be automated and adaptive, and that affect the performance of the façade, meet the requirements for a possible cooperation with other façade functions within a networked system. one aim of this study is to identify these façade functions and assign characteristics to assess the degree of an automated adaptivity. as a tool for the subsequent examination of the actual requirements in practice, the identified façade functions are to be superimposed with the identified characteristics of automation and adaptivity in a superposition matrix. 004 journal of facade design & engineering volume 7 / number 2 / 2019 1.4 research question main question: how can the automated adaptivity of façade functions be assessed to systematically identify them as a possible part of an intelligently networked façade? sub questions: – what are the boundary conditions of an intelligently networked façade and from which environmental conditions and comfort requirements derive its adaptive functions? – which façade functions are possible part of an intelligently networked façade due to a possible adaptive implementation and an impact on the building performance? – how can the automated adaptivity of a façade function be assessed? 2 methodology the investigation is based on a literature review. it is organised into two main parts. in the first part, under section 3, the boundary conditions of an intelligently networked façade are recorded in response to the first sub question. these include the environmental conditions with dynamic parameters, as well as the different requirements for interior comfort. both fields are extensively researched and documented in literature and standards. this first section, as shown in fig. 1, provides the context for the subsequent assessment of whether a façade function can be implemented automated adaptive and whether it has an effect on building performance. literature review filtering & consolidation superposition indoor comfortenvironmental conditions façade changing parameters requirements façade functions characteristics of adaptivity superposition matrix of façade functions and characteristics for the assessment of automation and adaptivity context filtering & consolidation fig. 1 graphical abstract 005 journal of facade design & engineering volume 7 / number 2 / 2019 a focus of the study is on the automated adaptive façade functions examined in the second part in section 4. previous lists such as the “façade function tree” by klein (2013) contain an overview of all façade functions without restriction as to whether an adaptive implementation is feasible and whether it affects the performance of the building envelope. the list “façade as an interface” by hausladen et al. (2005) corresponds to the approach of classifying façade functions in a holistic view, taking environmental conditions and comfort requirements into account. it serves as a starting point for the identification of relevant façade functions in this study. in order to ensure the completeness and accuracy of the functions identified by them, the list is overlaid with alternative layouts developed by klein (2013) and in the research of mppf the multifunctional plug&play approach in facade technology 2015). the overlaid data sets of façade functions is consolidated and filtered, as shown in table 1 of section 4.1, according to performance relevance as well as a basically possible adaptivity. the identification of characteristics that can be used to assess the automated adaptivity of a façade function is also based on existing literature. different references are overlaid and consolidated based on the research by loonen et al. (2015) with the goal of a complete list of evaluation characteristics of automation and adaptivity. façade function 1 characteristics of adaptivity fu n ct io n characteristic 1 characteristic 2 characteristic (...) characteristic 1 characteristic 2 characteristic (...)façade function 2 façade function (...) fig. 2 schematic representation of the superposition matrix for the assessment of the automated adaptivity, the determined characteristics are assigned to the previously identified individual façade functions. this step is done regarding the third sub question in a systematically structured superposition matrix according to the representation in fig. 2. the usability of the superposition matrix is tested on the basis of an exemplary application to an existing façade project. the necessary project information for the application example is derived from literature. 006 journal of facade design & engineering volume 7 / number 2 / 2019 3 the context of environmental boundary conditions and comfort requirements 3.1 identification of environmental boundary conditions and related dynamic parameters the climate is composed by the variations of different elements. according to dahl (2010), the detached discussion of individual aspects is difficult since they are in a constant and dynamic relationship to one another. bitan (1988) identifies temperature, humidity, wind, precipitation, solar radiation, and special features as parameters with a high impact on the building. hausladen, liedl, and saldanha (2012) designate the same climate elements as the most important for the construction planning, but without the addition of “special features”. in the consideration of the “façade as an interface” hausladen et al. (2005) designate the sound as an influencing element. van den dobbelsteen, van timmeren, and van dorst (2009) also supplement this aspect. dahl (2010) focusses on, from his point of view, the most important aspects: heat, humidity, wind, and light. in an overarching view, the following climate elements relevant to the building industry are identified. with regard to the adaptivity of the façade, the focus is on the dynamic parameters of an environmental condition. solar radiation solar radiation is electromagnetic radiation emanating from the sun (givoni, 1976). the energetic radiation power determines the intensity of the solar radiation. it changes with respect to the time of day or season as well as to the weather. another important aspect is the duration of irradiation, depending on the geographical location and the weather (ranft & frohn, 2004). global radiation is composed of direct sunlight and indirect, diffuse radiation. the angle of incidence of the direct sunlight is another important aspect. the greatest amount of energy is released at an incidence angle of 90 degrees to a surface. the intensity, the duration of irradiation, and the angle of incidence of the irradiation can be summarised as the important parameters of the solar radiation. temperature solar radiation indirectly affects the outside air temperature by heating the earth’s surface. this provides heat energy to the air layers above. also, the exchange with inflowing air affects the temperature. depending on the geographic location, the season, and the time of day, the temperature is subject to great variations. between 1.5 and 3 m depth, the average temperature of the ground remains constant throughout the year. (givoni, 1976; hausladen et al., 2012) the air temperature measured in degrees celsius is determined as the decisive parameter of this climatic element. air quality the air quality is determined by its oxygen content as well as its pollution. air pollution occurs mainly in dense urban areas as a result of traffic or industrial processes. plants can also be the cause of reduced air quality due to the formation of pollen (hasselaar, 2013). 007 journal of facade design & engineering volume 7 / number 2 / 2019 sound developments in both transport and industry are accompanied by noise emissions. due to the density of urban areas noise pollution can often not be avoided. noise in the environment has an impact on human comfort and depends on the distance between the source and the building. sound differs by type, duration, and transmission. hausladen et al. (2005) distinguish linear and selective sound sources. the duration of a noise load can be continuous, interrupted, or recurring. sound transmission can take place via air or via building components and materials. the sound applied to the building is measured outside in decibels (hausladen et al., 2005). wind wind is an effect of the earth’s air flow and pressure system. it is based on the distribution of air pressure, the rotation of the earth, the alternation of heat and cool over land and over water, and the topography of a respective region. winds vary according to seasons. there are global wind systems like the trade-winds, westerlies, and polar winds. additionally, time-bound winds exist due to high temperature differences. local winds occur between water and land or mountains and valleys (givoni, 1976). the microclimatic conditions such as terrain and building form have a great impact. for example, nozzle effects are possible, depending on the building arrangement (hausladen et al., 2012). the pressure acting on a building depends on the local wind force. force and direction of the wind are identified as crucial parameters. precipitation precipitation is a component of the water cycle. depending on the temperature, the water changes its form from gaseous to liquid. the cooling of the air condenses the moisture stored in it and leads to precipitation. the dew point is the temperature at which this process takes effect. along with rain, mist and dew are also forms of precipitation (givoni, 1976). the precipitation quantity and the possible precipitation direction are identified as parameters of the climate element. humidity humidity is defined as either relative humidity or absolute humidity. absolute humidity refers to the location-related, stored water vapour in the air. it is dependent on precipitation and the distance to the sea. absolute humidity is constant during the day, while relative humidity is subject to temperature fluctuations. cold days have a decreasing effect, warm days have an increasing effect (hausladen et al., 2012). 008 journal of facade design & engineering volume 7 / number 2 / 2019 3.2 identification of requirements for the indoor comfort the comfort in buildings can be in conflict with energetic performance goals. the quality of an interior climate has a significant impact on the well-being, health, and productivity of users. the demands on the interior climate of a building are guided by the needs of the comfort of the human being. they vary according to subjective perceptions and preferences (ranft & frohn, 2004). comfort can be determined by a range of factors, some of which are related to each other in a way that is not scientifically understandable (ranft & frohn, 2004). essential aspects of comfort are, according to knaack et al. (2014), perceived temperature, visual comfort, hygienic comfort, and acoustic comfort. a clear assignment of the aspects relevant to a user’s comfort cannot be determined universally due to varying individual perceptions. in order to determine the climatic quality of an interior environment, specialist planners rely on the level of satisfaction of users (hasselaar, 2013). in many cases, there are legal requirements that a building must meet. in germany, minimum requirements for the indoor climate are defined in the following guidelines and standards: important for the evaluation of interior comfort are ansi/ ashrae 55 thermal environmental conditions for human occupancy and din en 15251 indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics; german version en 15251:2007. for indoor environments in general, the iso 7730 defines requirements with regard to thermal comfort and din 5035 with regard to daylight. requirements for comfort in office buildings are defined by the standards din1946 and din 33403: “climate at the workplace”. the acoustic requirements in office buildings are regulated by din 2569: sound insulation in office buildings. (ranft & frohn, 2004) al horr et al. (2016) identify the thermal comfort, acoustic comfort, and visual comfort as important factors for the well-being and productivity of users. dahl (2010) complements aero-comfort and hydro-comfort in terms of the indoor air quality. thermal comfort the operative temperature, also known as the sensed temperature determines the thermal comfort and is composed of the radiation and air temperature of the room (hasselaar, 2013). it is often understood as the most important aspect in terms of interior comfort. the human body maintains an operating temperature of about 37 degrees celsius (dahl, 2010). hausladen, saldanha, and liedl (2008) determine the operative temperature in combination with the air speed as decisive factors for the thermal comfort. al horr et al. (2016) name air temperature, average radiation temperature, relative humidity, air speed, and individual aspects such as clothing as the six influencing factors that affect thermal comfort. an uneven distribution of the room temperature leads to discomfort. the activity of a person, clothing, age, sex, health and duration of stay in an environment influence, according to hausladen et al. (2005), the sensitivity towards temperature. too high temperatures can weaken the performance of a person while cold temperatures lead to illness. hausladen et al. (2005) formulate the following temperatures as average demand values separated by winter and summer season: in winter, the comfortable operating temperature is 22c° at air speeds of approx. 0.16m/s, while in summer it is 24c° with an air speed of 0.19m/s. 009 journal of facade design & engineering volume 7 / number 2 / 2019 aeroand hydro-comfort according to dahl (2010), comfort also depends on air movement and cooling, both of which occur with convection and evaporation. as the temperature increases, larger air movements are perceived as positive. (ranft & frohn, 2004) the quality of the indoor air is based on the quality of the ventilated external air and possible influences by users, technical installations, materials, or indoor plants. air pollution outdoors can have a negative effect on the air supply. high-quality air is based on a high oxygen concentration and a low dust and pollution load. the perceived contamination of indoor air is measured in decipol (dp). the co2 concentration is also an aspect of air quality. in addition to the activity of the user, hausladen et al. (2008) also name behaviours such as smoking as an influencing factor. the perceived quality of the air decreases with increasing humidity and temperature. the olf measure corresponds to the air pollution of a user doing light office activities. hausladen et al. (2008) give 0.15vol% as the maximum co2 concentration. dahl (2010) describes the hydro-comfort with regard to the humidity. relative humidity can be subject to large fluctuations between about 20% and 70%. it has an impact on health and how people feel within interior climates. large rooms can more easily compensate for humidity due to a larger air capacity, whereas small rooms require more extensive air exchange. (dahl, 2010; ranft & frohn, 2004) visual comfort hausladen et al. (2008) establish that natural light has a positive effect on the visual comfort of users. according to them, the quantity of the light provided, and its distribution, are crucial. the human eye adapts to the prevailing light conditions (dahl, 2010). glare can have a negative impact on the user. it occurs as a result of direct radiation originating from sun or artificial light sources, as well as reflections of light irradiation. large contrasts in the lighting also lead to possible glare. low contrasts and low shadows reduce it and promote spatial perception. visual references to the outside contribute to the well-being of the users. (hausladen et al., 2008) acoustic comfort acoustic comfort is based on the protection from noise and the guarantee of a sound environment which corresponds to the use of the building (al horr et al., 2016). acoustics is associated with well-being and the ability to concentrate within a room. sources of noise pollution may be outside the building or may result from the activities within a room. sound is measured in decibel (db). the volume of sound is a decisive factor. the weighted sound level considers people to be more sensitive to specific frequencies than to others. the addition (a) indicates the correspondingly filtered measured variable. silence corresponds to the value 0db(a) and noise above 140db(a) is perceived to be painful (hasselaar, 2013). the reverberation time describes the duration of a noise and has a great effect. the noise should not collide with communication or concentration in a building. hausladen et al. (2005) formulate a noise load of 30-45db (a) as an acceptable maximum. 010 journal of facade design & engineering volume 7 / number 2 / 2019 3.3 interpretation of the identified environmental conditions and comfort requirements a total of seven categories of environmental conditions have been identified as illustrated in fig. 3. different dynamic parameters are possible within the respective category. the influencing factors are not always subject to a natural origin but can also be the result of human intervention in the environment. examples of such artificial influencing factors are the noise environment within urban areas or traffic-related air pollution. depending on the project’s geographical and temporal context, different patterns of influencing factors are possible. on the other hand, there are a total of four identified categories for interior comfort. different requirements can be assigned to the individual categories. hausladen et al. (2008) distinguish detailed requirements of the interior comfort in the “façade as an interface”. the “room temperature”, the “inside surface temperature”, and the “supply air temperature” named by them can be assigned to the thermal comfort. illuminance, glare protection, and visual relationships affect the visual comfort. the aero& hydro-comfort can be detailed into air changes, air quality, and air speed, while the sound load named by hausladen et al. (2008) can be assigned to the acoustic comfort. it is not claimed that the listed requirements are complete. they are the basic requirements that can be supplemented depending on the conditions of different building uses. fig. 3 illustrates the context of the environmental conditions and comfort requirements from which the adaptive functions of the façade derive: environmental conditions solar radiation temperature air quality sound wind precipitation humidity indoor requirements thermal comfort aero& hydro-comfort visual comfort acoustic comfortfaçade fig. 3 context of environmental conditions and comfort requirements the context provides a holistic view of the façade in terms of its dependence on environmental conditions and comfort requirements. it contributes to the understanding of individual reactions of an adaptive façade function. possible intersections between the individual reactions can be identified with regard to the differentiated environmental conditions and indoor comfort requirements. the context is intended to serve as a decision aid in the selection of façade functions for networking and existing dependencies and interferences in cooperation. 011 journal of facade design & engineering volume 7 / number 2 / 2019 4 identification of façade functions and characteristics of adaptivity 4.1 performance relevant and possibly adaptive functions of the façade the façade has a great impact on the energy and comfort-related quality of a building. as shown in fig. 4, it balances the dynamic climatic conditions of the exterior environment with regard to the requirements of the interior (hausladen et al., 2008). it determines the appearance and contributes to the design assessment of a building (fassaden, 2015; knaack et al., 2014). features of the façade can be distinguished according to whether they have an effect on the building’s performance or solely on the aesthetic design of the façade. in this respect, loonen et al. (2013) exclude, for example, media façades, which exclusively present visual adaptive features without contributing to the performance, from the definition of climate-adaptive building envelope. the expectations and demands on the functional spectrum of façades continuously increased throughout the development history. (mppf the multifunctional plug&play approach in facade technology, 2015) at the same time, technical possibilities available for the façade construction have multiplied. klein (2013) notes an extensive mechanisation of the façade, which, in his view, in the latest development also fulfils additional comprehensive tasks of building services. klein (2013) describes the functions as an elementary aspect for the investigation and development of façade constructions. according to herzog et al. (2004) the building envelope separates and filters between the outdoor environment and interior of the building. from a historical point of view, it is the job of the façade to provide protection against the dangers and the weather of the exterior. herzog et al. (2004) state that additional requirements for the building envelope result from the local external environment and the requirements of the interior. in this context they specify further control and regulatory functions in addition to the protection function of the façade. according to knaack et al. (2014), the façade is a dividing element between the exterior and interior, that satisfies multiple functions with the simplest structure possible. they argue that these functions include the provision of visual openings, balancing of wind loads, and load-bearing properties. herzog, krippner, and lang (2004) see the façade as an interface, which ensures a comfortable interior climate. depending on the different requirements of different seasons, they are also confronted with a target conflict of different façade functions. herzog et al. (2004) also identify conflicts of interest in the different requirements of different seasons. they differentiate between different requirements in summer and in winter. according to herzog et al. (2004), it is not only crucial which functions the building envelope fulfils, but also how the functions are organised with regard to one another. the interplay of individual façade functions can allow for synergy effects. there are different, differentiated representations of the functions of a façade. they differ in scope, detailing, and organisational structure. in an overlapping composition of façade functions “the façade as an interface”, hausladen et al. (2005) contrast the functions with corresponding influencing factors and requirements. in this way, they also manifest superimpositions, for example when façade functions are derived from several external conditions or when they affect various interior requirements. they identify a total of thirteen climaterelated functions. participating researchers of the project “multifunctional plug & play facade” formulate functions of the façade in three categories. the first category refers to the basic, mainly protective and climate-related functions of the façade. solar thermal energy and photovoltaics are listed in a separate section on energy production. in the third category, supporting functions of the façade are named. this category includes tasks such as heating, cooling, or mechanical ventilation (mppf the multifunctional plug&play approach in facade technology, 2015), klein (2013) differentiates 012 journal of facade design & engineering volume 7 / number 2 / 2019 the functions of the façade in an objective tree, based on strategies from product design. it organises the functions stepwise into primary, secondary, and support functions. the “façade function tree” represents a comprehensive and detailed breakdown of the façade functions in five categories. these include the durability of the construction, an appropriate manufacturing process, ensuring sustainability, support for building use, and the shape of the façade. it is assumed that the functions of the categories: “create a durable construction”, “allow reasonable building methods” and “spatial formation of façade” can in principle not be implemented in an adaptive manner. it is also assumed that not all functions affect the performance of the building in operation. against this background and due to the size of the composition, a pre-selection is made regarding the categories “provide comfortable interior climate” and “responsible handling in terms of sustainability”. fig. 4 facade functions by knaack, klein, bilow, and auer (2014) the assembly of façade functions in table 2 is derived from the consolidation of the previously identified lists by hausladen et al. (2005), herzog et al. (2004), klein (2013), and (mppf the multifunctional plug&play approach in facade technology, 2015). the layout by hausladen et al. (2005) is already tailored to the climate related functions of the façade. it serves as the starting point for the merging with the alternative constellations. in the superposition of the compositions it becomes clear that many of the functions, which are designated identically or slightly modified, overlap. duplicates are removed as part of the consolidation. 013 journal of facade design & engineering volume 7 / number 2 / 2019 consolidated list filter action consolidated list filtered general function specification sun 1 glazing fraction irrelevant to an adaptive implementation skipped 1 solar shading 2 shading specified to „solar“ modified 3 solar control glass specification 4 light deflection 2 light deflection 5 glare protection 3 glare protection 6 control daylight radiation 4 control daylight radiation provide a comfortable daylight level 8 allow natural lighting of interior allow to control modified allow natural lighting of interior temperature 9 thermal insulation 5 thermal insulation heat protection 10 heat insulation glass specification (of thermal insulation) modified   maintain indoor temperature 11 thermal storage mass irrelevant to an adaptive implementation skipped 12 decentralised equipment is more of a property than a function skipped 13 maintain air tightness irrelevant to an adaptive implementation skipped air 14 window ventilation specification (change to ventilation) modified 6 ventilation control air exchange rate 15 control air exchange rate specification ventilate excessive heat 16 ventilate excessive heat specification user 17 allow visual contact allow to control modified 7 control visual contact visual contact to outside   visual protection for inside acoustic 18 sound insulation sound insulation = reduction? level of success 8 sound insulation 19 sound reduction 20 insulation of connection to dividing walls irrelevant to an adaptive implementation skipped 21 insulation of floor connection irrelevant to an adaptive implementation skipped energy 22 collect solar thermal energy 9 generate energy collect solar thermal energy 23 collect solar energy   collect solar energy 10 store energy supply (not dependent on outdoor climate but relevant to indoor comfort) 24 heating combination with cooling modified 11 heating and cooling 25 cooling modified 26 humidification combination with dehumidification modified 27 dehumidification modified 12 de/ humidification 28 electricity 13 electricity 29 artificial light 14 artificial light 30 communication 15 communication table 1 development of the consolidated and filtered list of façade functions 014 journal of facade design & engineering volume 7 / number 2 / 2019 in addition, the functions are filtered as shown in table 1 in consideration of the environmental factors and requirements identified in section 3 according to two decisive aspects: first, an adaptive implementation must be possible in principle. this precondition is not given for example in the case of “glazing fraction” listed by hausladen et al. (2005). the function is definitively determined within the planning and manufacturing process and not dynamically changeable in the operating phase of the building. it is therefore not taken into account for the present assembly. on the other hand, the function must have an impact on the performance of the building. within the scope of the consolidation, further decisions are made that have an impact on the result. in some cases, klein (2013) provides the detailing of individual, opposite states of a front function. with regard to an adaptive implementation, these functions can be summarised. an example of such a merge are the functions “block radiation” and “let radiation pass”. correspondingly, the reduction and the insulation of sound can be combined as different degrees of fulfilment of the function. klein formulates several of the identified façade functions with the addition “allow”. with respect to an adaptive implementation of the respective function, an opposite state is assumed to be possible and the term is converted to “control”. in the category “responsible handling in terms of sustainability”, klein (2013) identifies functions which do not directly affect the performance of the building. the collection of solar and solar-thermal energy can contribute to the functioning of the façade depending on the climatic conditions of the exterior. the corresponding functions are also named in the list by mppf the multifunctional plug&play approach in facade technology 2015) and are taken into account in the consolidated summary. in the “supply” category, they also name functions of building technology, which have an effect on the interior climate, regardless of external boundary conditions. they are also added to the list of functions. general function specification general function specification sun acoustic 1 solar shading   8 sound insulation   2 light deflection   energy 3 glare protection   9 generate energy collect solar thermal energy 4 control daylight radiation provide a comfortable daylight level   collect solar energy control natural lighting allow natural lighting of interior 10 store energy   temperature supply 5 thermal insulation heat protection 11 heating and cooling     maintain indoor temperature 12 de/ humidification   air 13 electricity   6 ventilation control air exchange rate 14 artificial light     ventilate excessive heat 15 communication   user 7 control visual contact visual contact to outside   visual protection for inside table 2 consolidated and filtered assembly of performance-related façade functions 015 journal of facade design & engineering volume 7 / number 2 / 2019 4.2 characteristics of adaptivity loonen et al. (2015) require uniform aspects which can be used to determine the adaptiveness of a building envelope. they identify eight characteristics of adaptivity which they also assemble in a matrix. the starting point of the investigation by loonen et al. (2015) is the adaptivity. in the first aspect, they question the goal and purpose which should be achieved with it. loonen et al. (2015) name a total of six objectives, which are derived from the requirements of interior comfort, user control and energy generation. for each objective, they identify appropriate, responsive functions. in contrast to the listing by loonen et al. (2015), this study is organised according to the façade functions themselves. according to the identified façade functions in context of environmental conditions and indoor comfort requirements in chapter 3 of this research, the objectives of a façade function are not questioned again. a relevant characteristic is whether a façade function can be adaptive at all. a distinction is made between the flexibility of the building envelope, i.e. the adaptivity which has to be initiated by the user, and the adaptiveness which requires independent, selfregulated adjustments (ross, rhodes, & hastings, 2008). in addition, the technology, which can be a construction component or a material that carries out the function, is questioned as an aspect by loonen et al. (2015). the accordingly named “spatial scale” is understood to be directly coupled to it. under “degree of adaptivity” they summarise the possible states of an adaptive process. these can, according to them, map the direct change between extreme states (on-off) or smooth transitions (gradients). with regard to the application to a façade function or a component, this characterisation appears to be insufficient. the question arises as to whether an open window is on or off. as a supplement to this characteristic, a generalisation of the state description is proposed in “active” and “inactive”. additionally, one should define when a corresponding state is reached for each function. the response time is adopted as a criterion of adaptivity. it is assumed that it provides information on whether the adaptation processes meet the dynamic requirements of a façade function. loonen et al. (2015) identify visibility as a characteristic of an adaptive façade. as this aspect has no effect on the performance of the façade it is not taken into account in the present study. loonen et al. (2015) distinguish between intrinsic adaptations, the construction or material inherent adaptations in response to ambient stimuli, and extrinsic adaptations, which are based on additional automation technology. from a technical point of view, therefore, the scope of introduced automation technology can also be a criterion for the capabilities of an adaptive façade function. in this context, moloney (2011) and ochoa and capeluto (2008) refer to the components of a mechatronic system, an existing input system, a processing system, and an output system. an existing sensor system, which continuously collects data on the relevant environmental conditions, can be identified as a criterion for adaptive façades. additionally, an existing control, which processes the determined data, as well as actuators, which initiate adjustments within the façade construction, are further criteria. table 3 is a revised list of characteristics for the adaptivity of façades. 016 journal of facade design & engineering volume 7 / number 2 / 2019 general criteria description possible parameters general 1 technology the construction-related element, which ensures the fulfilment of the function. building component / system / material 2 flexible possible flexibility of the construction regarding the function yes / no 3 adaptive self-initiated adaptations (applied automation technologies) yes / no behaviour 4 operation component or material-integrated self-adaptation or on the basis of information processing intrinsic / extrinsic 5 response time time intervals of adaptation processes seconds, minutes, hours, day-night, seasons, years, decades 6 degree of adaptability the number and type of possible states that the adaptive system can map. active / inactive / gradual automation 7 input system existing information gathering (sensors) yes / no 8 processing system existing processing of the gathered information (controller) yes / no 9 output system existing actuators, which implement adaptations of the design with regard to the function yes / no table 3 revised list of characteristics of adaptivity using the overlapping of façade functions and characteristics of an adaptive system, a database is created in which the identified functions are organised vertically, the corresponding characteristics of adaptivity horizontally. the additionally identified specifications support the understanding of general functions and can be used for a detailing in subsequent investigations. in the present database, only the general functions are taken into account. the present breakdown of functions is made based on the assumption that they are confronted with individual dynamic factors and requirements within the façade as a holistic system. 5 superposition of façade functions and characteristics of adaptivity in the superposition matrix shown in table 4, the identified façade functions are overlaid with the determined characteristics of adaptivity. the façade functions are listed vertically in the table. the respective characteristics of adaptivity contrast them in horizontal organisation. 017 journal of facade design & engineering volume 7 / number 2 / 2019 general behaviour automation technology flexible adaptive operation response time degree of adaptivity input system processing system output system sun 1 solar shading 2 light deflection 3 glare protection 4 control daylight radiation temperature 6 thermal insulation air 7 ventilation user 8 control visual contact acoustic 9 sound insulation energy 10 generate energy 11 store energy supply 12 heating and cooling 13 de/ humidification 14 electricity 15 artificial light 16 communication table 4 superposition matrix 6 example for the application of the superposition matrix table 5 shows the application of the superposition matrix to the façade of the kfw westarkade in frankfurt, germany. the project, designed by sauerbruch hutton and completed in 2010, was selected because the building and the façade are extensively documented in the literature (fortmeyer & linn, 2014). the double façade of the building is characterised by vertical coloured blinds. it has automated ventilation flaps. a glare shield is installed in the space between the façade. based on the literature sources, the façade functions can be assigned the characteristics of adaptivity and automation shown in table 5. characteristics which cannot be determined due to missing data are marked with n.a. (gonzález, holl, fuhrhop, & dale, 2010; winterstetter & sobek, 2013) 018 journal of facade design & engineering volume 7 / number 2 / 2019 general behaviour automation technology flexible adaptive operation response time degree of adaptivity input system processing system output system sun 1 solar shading lamella screen yes yes extrinsic n.a gradual yes yes yes 2 light deflection no no no no no no no no no 3 glare protection lamella screen yes yes extrinsic n.a gradual yes yes yes 4 control daylight radiation lamella screen yes yes extrinsic n.a gradual yes yes yes temperature 6 thermal insulation intermediate space yes yes extrinsic n.a gradual yes yes yes air 7 ventilation ventilation flaps yes yes extrinsic n.a activeinactive yes yes yes user 8 control visual contact lamella screen yes yes extrinsic n.a gradual yes yes yes acoustic 9 sound insulation absorber no no no no no no no no energy 10 generate energy no no no no no no no no no 11 store energy no no no no no no no no no supply 12 heating and cooling no no no no no no no no no 13 de/ humidification no no no no no no no no no 14 electricity no no no no no no no no no 15 artificial light no no no no no no no no no 16 communication no no no no no no no no no table 5 superposition matrix applied to the façade of the kfw westerkade the case study shows that the characteristics determined can principally be applied to buildings. an absolute assignment of a function to a particular component of the façade is not always possible, since there are undefinable overlaps between them. in the investigated project, for example, the function of sound insulation is fulfilled by the intermediate space of the double façade. in this context, the opening of the ventilation flaps has an effect on the noise protection. absorbent surfaces also contribute to the fulfilment of acoustic requirements, which, however, are not able to adapt. such complex contexts can only be mapped abstractly, and interpretations are necessary in the assignment. this also applies to the determination of individual characteristics. thus, the opening state of a ventilation flap can be evaluated as an active or inactive state; however, in the case of a plurality of ventilation flaps that are capable of opening, it is also possible to estimate that a gradual adaptation is present. it is necessary to clarify whether the respective evaluation refers to a single element or to the overall system. 019 journal of facade design & engineering volume 7 / number 2 / 2019 7 conclusion the study compiles the external boundary conditions and interior comfort requirements of adaptive façades. based on existing literature, the following seven external influencing factors were identified: “solar radiation, temperature, air quality, sound, wind, precipitation, and humidity”. as stated above in section 2.3, apart from natural factors, human intervention in the environment also has an impact on the functions of the façade. in terms of interior comfort, the four requirement categories “thermal comfort, aero& hydro-comfort, visual comfort, and acoustic comfort” were identified. the listing provides an initial basis and is not understood to be complete. depending on the field of application and in further research, more aspects may be added. taking the identified framework conditions into account, the study provides a detailed breakdown of performance-relevant and possibly automated adaptive façade functions as potential parts of an intelligently networked, adaptive façade system. while some of the identified functions have a direct impact on the façade performance by balancing of environmental conditions and comfort requirements, others are cited because of an existing reference to environmental conditions and an indirect impact on the system, for example, through the provision of energy. the “supply” functions category lists such functions that are not necessarily linked to external influences but do affect the interior comfort. as an extension of previously existing research results, the found characteristics of an adaptive façade as an overall system are summarised, supplemented with automation aspects, and applied to the individual functions. this detailing enables a differentiated consideration of dependencies and shared requirements between individual performance-relevant and automated adaptive façade functions within an intelligently networked façade system. the superposition matrix developed in this study can contribute to the design of intelligent-networked and multifunctional-adaptive building envelopes. as a theoretical assembly based on existing literature, it does not provide information about the actual realised automated adaptive functions in building practice. the superposition matrix can be used as organisational tool for the systematic assessment of automated and adaptive façade functions in realised building envelopes to examine the technical basis for intelligent networking. a corresponding practical investigation is identified as a future research task for the clarification of a possible intelligent cooperation of automated adaptive façade functions according to networked production plants in industry. acknowledgements the study is part of a phd research project and is not subject to any specific funding. references aelenei, l., brzezicki, m., knaack, u., luible, a., perino, m., & wellershoff, f. (2015). cost action tu1403 adaptive facades network. al horr, y., arif, m., katafygiotou, m., mazroei, a., kaushik, a., & elsarrag, e. (2016). impact of indoor environmental quality on occupant well-being and comfort: a review of the literature. international journal of sustainable built environment, 5, 1-11. doi:10.1016/j.ijsbe.2016.03.006 bitan, a. (1988). the methodology of applied climatology in planning and building. energy and buildings, 11, 1-10. doi:10.1016/0378-7788(88)90018-7 bittencourt, l., immich, r., sakellariou, r., fonseca, n., madeira, e., curado, m., . . . rana, o. (2018). the internet of things, fog and cloud continuum: integration and challenges. internet of things, 3-4, 134 155. doi:https://doi.org/10.1016/j.iot.2018.09.005 böke, j., knaack, u., & hemmerling, m. (2018). state-of-the-art of intelligent building envelopes in the context of intelligent technical systems. intelligent buildings international, 1-19. doi:10.1080/17508975.2018.1447437 dahl, t. (2010). climate and architecture (1st ed ed.). milton park, abingdon, oxon ; new york, n.y: routledge. dumitrescu, r., jürgenhake, c., & gausemeier, j. (2012, 2012). intelligent technical systems ostwestfalenlippe. fassaden: best of detail -facades. (2015). (c. schittich, s. lenzen, & i. f. i. architektur-dokumentation eds. 1. aufl ed.). münchen: inst. für internationale architektur-dokumentation. fortmeyer, r. m., & linn, c. d. (2014). kinetic architecture: designs for active envelopes. mulgrave, victoria: images publishing group. 020 journal of facade design & engineering volume 7 / number 2 / 2019 givoni, b. (1976). man, climate and architecture: applied science publ. gonzález, b., holl, c., fuhrhop, d., & dale, m. (2010). kfw westarkade frankfurt am main energy-efficient office building. berlin: stadtwandel-verl. hasselaar, b. (2013). the comfort unit: developed as part of a climate adaptive skin. ‘s-hertogenbosch: uitgeverij boxpress. hausladen, g., de saldanha, m., liedl, p., & sager, c. (2005). climate design: solutions for buildings that can do more with less technology. basel, switzerland: birkhäuser. hausladen, g., liedl, p., & saldanha, m. d. d. (2012). klimagerecht bauen: ein handbuch [building to suit the climate: a handbook]. basel: birkhäuser. hausladen, g., saldanha, m. d., & liedl, p. (2008). climateskin: concepts for building skins that can do more with less energy. basel ; boston: birkhäuser. herzog, t., krippner, r., & lang, w. (2004). facade construction manual. basel; boston: birkhauser-publishers for architecture. klein, t. (2013). integral facade construction: towards a new product architecture for curtain walls. doi:10.4233/uuid:f90c1c7eb6b3-42de-8f17-22cc23981de7 knaack, u., klein, t., bilow, m., & auer, t. (2014). façades: principles of construction (second and revised edition ed.). basel, switzerland ; boston: birkhäuser. loonen, r., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483-493. doi:10.1016/j.rser.2013.04.016 loonen, r. r., rico-martinez, j. m., favoino, f., brzezicki, m., menezo, c., la ferla, g., & aelenei, l. (2015). design for façade adaptability -towards a unified and systematic characterization. bern, switzerland. moloney, j. (2011). designing kinetics for architectural facades: state change: routledge. monostori, l. (2014). cyber-physical production systems: roots, expectations and r&amp;d challenges. procedia cirp, 17, 9-13. doi:10.1016/j.procir.2014.03.115 mppf the multifunctional plug&play approach in facade technology. (2015). (t. mach, m. grobbauer, w. streicher, & m. j. müller eds.). graz: verl. der technischen univ. graz. ochoa, c. e., & capeluto, i. g. (2008). strategic decision-making for intelligent buildings: comparative impact of passive design strategies and active features in a hot climate. building and environment, 43, 1829-1839. doi:10.1016/j.buildenv.2007.10.018 rajkumar, r., lee, i., sha, l., & stankovic, j. (2010, 2010). cyber-physical systems: the next computing revolution. paper presented at the proceedings design automation conference. ranft, f., & frohn, b. (2004). natürliche klimatisierung [natural climatisation]. basel: birkhäuser. ross, a. m., rhodes, d. h., & hastings, d. e. (2008). defining changeability: reconciling flexibility, adaptability, scalability, modifiability, and robustness for maintaining system lifecycle value. systems engineering, 11, 246-262. doi:10.1002/sys.20098 schumacher, m., schaeffer, o., & vogt, m.-m. (2009). move: dynamic components and elements in architecture. basel : london: birkhäuser ; springer [distributor]. van den dobbelsteen, a., van timmeren, a., & van dorst, m. (2009). smart building in a changing climate: techne press. wang, l., torngren, m., & onori, m. (2015). current status and advancement of cyber-physical systems in manufacturing. journal of manufacturing systems, 37, 517-527. doi:10.1016/j.jmsy.2015.04.008 winterstetter, t., & sobek, w. (2013). innovative and energy-efficient façade technology for the kfw westarkade highrise in frankfurt/main. stahlbau, 82, 35-46. doi:10.1002/stab.201390076 journal of facade design and engineering 3 (2015) 15–25 doi 10.3233/fde-150025 ios press 15 development and construction of a thermoelectric active facade module marı́a ibáñez-puy∗, josé antonio sacristán fernández, césar martı́n-gómez and marina vidaurre-arbizu department of building construction, services and structures, school of architecture, university of navarra, pamplona, navarra, spain submitted 30 march 2015 revised 13th-20th april 2015 accepted 29 april 2015 abstract. in order to fulfil the current challenges for the european building sector, building design has diverged into two alternative directions: active technologies and passive design strategies. in the last few years, advanced and responsive building envelope components have represented a promising answer to these challenges. this paper presents the design and construction process of a project that aims to design, build and control the energy performance of an industrial-scale modular active ventilated facade prototype with a new themoelectric peltier system (tps). the tps is a thermoelectric hvac heat pump system designed to be located in the building envelope and providing a high comfort level. trying to optimize the energy performance of the traditional ventilated opaque facade, and make more efficient the energy performance of the tps, the concept of adaptability has been applied to ventilated opaque facades. the essential research theme is to control the natural phenomena that take place inside the ventilated air cavity of the facade: taking advantage when heat dissipation is needed, and avoiding it when heat losses are not welcome. in order to quantify the previous statements, some facade prototypes are being built in pamplona (spain) and their energy performance is going to be analyzed during a year. keywords: ventilated opaque facade, energy efficiency, peltier, prototype, adaptive facade 1. introduction the building envelope is the construction element that has the greatest impact on the overall energy consumption of the building (manioğlu & yilmaz, 2006). on the one hand, active technologies aim at enhancing the level of sustainability in the built environment via the introduction of innovative technical device. these devices are used for supply of energy from renewable sources or for conversion of resources at higher overall efficiencies (fabrizio, corrado, & filippi, 2010). on the other hand, the term ‘passive’ (chwieduk, 2003) refers to buildings where the design of construction and shape of the building itself plays more roles in capturing, storing and distributing wind and solar energy, normally with the aim of displacing fossil fuels for space conditioning and lighting (sadineni, madala, & boehm, 2011). ∗corresponding author: marı́a ibáñez-puy, department of building construction, services and structures, school of architecture, university of navarra 31009, pamplona, navarra, spain. tel.: +34 948 425600/ext. 803310; e-mail: mibanez.3@alumni.unav.es. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:mibanez.3@alumni.unav.es 16 m. ibáñez-puy et al. / development and construction of a thermoelectric active facade the fact that some of the most well-known producers of technological facade modules and windows have started to develop integrated and modular multifunctional systems (favoino, goia, perino, & serra, 2014; hindrichs & behaling, 2008; schuster & mueller, 2007) supports the vision that advanced and responsive building envelope components could represent a promising answer to the challenge posed by the newest energy regulations. these innovative facade concepts (loonen, trčka, cóstola, & hensen, 2013) are almost ‘self-sufficient’ building skins that show a dynamic behaviour and incorporate different technologies (e.g. decentralized heating/cooling units, heat exchangers, energy supply devices, energy storage, lighting equipments, shading devices, ventilated cavities) aimed at reducing the energy demand of the building, on one side, and at converting energy from renewable sources, on the other. 2. objectives in this frame, a project called ‘development, construction and analysis of an active facade module with peltier cells’ (theen project) is presented, which aims to design an industrial-scale modular active ventilated facade prototype with a new thermoelectric peltier system (tps). trying to optimize the energy performance of the traditional ventilated opaque facade and to make more efficient the energy performance of the tps, the concept of adaptability has been applied to ventilated opaque facades. the focus is a high control of the natural heat transfer phenomena that take place inside the ventilated air gap. the aim is to promote heat dissipation during hot season, and increase heat storage during cold season. this paper is focused on describing and analysing the design and construction process of the active ventilated facade system that incorporates the tps. however, no results of the energy monitoring performance are shown because it is still under construction. 3. the thermoelectric envelope (theen) project the objective of the theen project is the characterization of an industrial-scale modular adaptive opaque facade prototype incorporating a peltier cell (thermoelectricity) driven hvac system supplied by energy from pv panels. 3.1. adaptive ventilated opaque facade during last decades the use of a ventilated opaque facade as a solution for residential building envelopes has considerably increased, especially in mediterranean countries (marinosci, strachan, semprini, & morini, 2011). a ventilated opaque facade is usually defined as an opaque external layer composed of lightweight and thin cladding, and an opaque internal skin which acts as thermal and acoustic insulation. between both layers there is an air cavity, drained and always ventilated (no control). no air is transferred from the cavity to the room. although it is a really common solution, the ventilated facade is not always the best option, because its energy performance could change heavily according to the climate characteristics and orientation (mesado, chiva, juliá, & hernández, 2010). that is why its application must be deeply studied before its installation (giancola, sanjuan, blanco, & heras, 2012). m. ibáñez-puy et al. / development and construction of a thermoelectric active facade 17 many studies have been published analyzing the energy performance of a ventilated facade: • during summer (lópez, jensen, heiselberg, & ruiz de adana santiago, 2012) air flows upwards inside the air gap, due to the action of solar radiation, removing part of the heat loads throughout the facade, thus reducing the heat flux through the indoor environment. • during cold seasons, when solar radiation tends to be lower, the temperature of the air that leaves the ventilated cavity is usually lower than the indoor air temperature, and the energy balance is negative (suárez, sanjuan, gutiérrez, pistono, & blanco, 2012). regarding the advantages and disadvantages of the ventilated air cavity, the proposal is to be able to control this ventilation. so as to avoid the heat losses during winter, it is proposed to close the cavity (no ventilation) in order to promote the heat storage in the air cavity, and hence the heat loses are lower. however, during summer the cavity will be opened, thus the heat dissipation is increased. 3.2. thermoelectric peltier system (tps) the facade design incorporates an improved model of the universidad de navarra’s patent (fig. 1.) (patent number 201101142) of a tps (martín-gómez et al., 2010). fig. 1. scheme of the original tps patent. 18 m. ibáñez-puy et al. / development and construction of a thermoelectric active facade there are many studies that evaluate the behaviour and show the different applications of the peltier cells and the materials that make them up (yamashita, 2008, 2011). in our case, thermoelectric technology is used as a cooling or heating system (rodriguez, vián, & astrain, 2008). the peltier effect is produced when electric current flows through two different types of semiconductor metals. the current starts the heat transfer from one union to the other: while one union is getting cooler the other starts to heat up. if the direction of the current is changed, the heat transfer direction changes too, hence peltier cells can be used as heat pumps. from our point of view, in winter it would absorb heat from the exterior air and supply it into the inside room space while in summer, it would absorb heat from the air in the enclosed space and remove it to the outside. there are some peltier’s applications in buildings: (arenas alonso, pagola de las heras, palacios hielscher, rodrı́guez pecharromán, & vázquez arias, 2007; cheng, cheng, huang, & liao, 2011; he, zhou, hou, chen, & ji, 2013; van dessel & foubert, 2010; xu, dessel, & messac, 2007). the vast majority of these applications uses thermoelectric as a way to transmit heat between two environments at different temperatures. most of them associate the peltier cells with the transparent part of the facade. this new hvac system offers: high precision and reliability, easy installation and reduction of the installation volume. one of the initial requirements of the project is to drive the tps by photovoltaic solar panels. however, due to economic reasons it is not possible to incorporate a pv energy supply system at the moment. nevertheless, there is going to be a quantification of the electricity consumption. 4. design and construction the project aims to evaluate the possible energy savings and the energy performance of an innovative thermoelectric hvac system, linked also with ventilation control of the facade air cavity, by means of comparison with a conventional hvac system and facade system. this article shows the different parameters that have been taken into account not only to design the facade but also to build it. 4.1. site parameters in order to quantify the previous statements, two identical prefabricated building modules (pbms) are being built at the school of architecture in pamplona (spain) (fig. 2). the fact of having two different pbms offers the possibility of getting comparative energy and temperature performance data and, therefore, achieves more objective findings. the two pbms are located in an isolated place; no buildings can throw any shade on them and they are highly exposed to wind (mainly north direction). however, not only the orientation but also the exact disposal of the pbms has been studied. the long side of both pbms is perfectly orientated south-west, since a simulation made with the software ecotect® shows that the south-west facade reaches higher temperatures than the south. so, the facades which are going to be studied are the ones in the most hostile orientation, which means, the south-west. despite the fact that the pbms are located in an isolated site, the software sketchup® has been used to simulate the solar movement, and avoid that the two pbms cast shadow one on the other (fig. 3). m. ibáñez-puy et al. / development and construction of a thermoelectric active facade 19 fig. 2. location of the two pbm at the school of architecture, pamplona (spain). fig. 3. screen view of the software sketchup® for september at 12:24 a.m. 4.2. building parameters the differences between these two pbms (fig. 4) are: • pbm 1: it will incorporate a conventional hvac system (inverter type) and the envelope will be composed by a conventional ventilated opaque facade (no control of the cavity ventilation: always opened). • pbm 2: it will incorporate the tps and the air gap ventilation of the opaque facade will be controlled through air dampers on/off (only south-west orientation). 20 m. ibáñez-puy et al. / development and construction of a thermoelectric active facade fig. 4. distribution of the pbms and the type of building walls. f1 (always ventilated facade), f2 (adaptive peltier facade: ventilation control +tps). table 1 composition and properties of the facade (types f1a and f1b) layers from outside to inside layer t (mm) ∧ (w/mk) r (m2k/w) (6) trapezoidal steel sheet (open/close) 1 –/50 –/0.00 (5) air gap (1) (open/close) 210 –/0.18 (4) panel of rock wool 65 0.034 2.03 (3) thick aluminum sandwich panels with pur rigid foam core 35 0.023 1.13 (2) panel of rock wool 46 0.035 1.43 (1) laminated gypsum board 12.5 0.250 0.05 (1)the thickness of the air gap is too wide to calculate its thermal resistance as the coefficient between thickness and thermal conductivity, because of the convective effects that take place inside it. 4.2.1. walls parameters all the sides of the pbms, except the f2 that incorporates the peltier system, are composed by always open ventilated facade systems, and highly isolated (table 1). there is no wall-window that could alter the energy performance data of the opaque facade. the external layer has a particular shape section mainly because of design reasons (fig. 5). nevertheless, this particular shape increases the section of the cavity, which increases the space to heat accumulation during winter. besides, some air turbulences are expected to take place rising up the heat dissipation during summer. a steel external layer has been chosen in order to avoid thermal inertia. the colour of this steel sheet is a medium grey, trying to avoid too much solar radiation absorption during summer and too much solar radiation reflection during winter. m. ibáñez-puy et al. / development and construction of a thermoelectric active facade 21 fig. 5. wall building section. 4.2.2. cavity parameters a double height pbm has been designed with the intention to generate natural ventilation in the facade cavities. besides, the cavity is divided into vertical boxes (shaft-box) (oesterle & lieb, 2001). in this type of division the height of the cavity is not interrupted so it presents a temperature gradient. the air heats up in this space due to the distance between exterior bottom and top openings, which means a better ventilation rate and, consequently, a decrease in the heat gains inside the room during summer. 4.3. active thermoelectric facade parameters 4.3.1. principals it is important to highlight that the hvac system is integrated in the ventilated facade. the principal of this new hvac system is the heat transfer. as a global vision of the process, during hot seasons the tps extracts the overheating from the inside of the room to the air cavity. on the other hand, during cold seasons the inverse process takes place. at the same time, in winter the cavity is heated by solar radiation and captured (air cavity ventilation in close position) while during summer it can be effective in extracting the excessive heat from the cavity (air cavity ventilation in open position). so, in this way, it is expected that the adaptive design of the facade would improve the efficiency of the tps. 4.3.2. active peltier facade composition the composition of the active peltier facade is the same as the rest of the walls, but incorporates the following elements: • adaptive facade. controllable ventilation dampers, installed at the top and at the bottom of the facade. it allows the control of the air cavity ventilation. • tps. consists of a thermo-electric modular equipment with a power of 1000w. the system incorporates 20 peltier cells of 51,4w each. its dimensions are 1200mm x 1800mm x 250mm, which makes its perfect integration on the facade system possible. the inner layer of the system is a refractory stone of 14mm. 22 m. ibáñez-puy et al. / development and construction of a thermoelectric active facade it must be taken into account that the thickness of the air gap is higher than the ones commonly used for the conventional opaque ventilated facade (6 to 8 to 10cm). the thickness of the profitable cavity (210mm) in this case corresponds basically to construction reasons: it is the minimum one that makes the installation of dampers that allow controlling the air gap ventilation possible. one the one hand, during hot season narrower cavities produce an accentuated stack effect and a stronger air movement which leads to a more effective extraction. on the other hand, in larger cavity depths there is a reduction in the stack effect and the heat transfer towards the interior room increases. this increase could be an advantage for cold seasons, which is the critical season in pamplona. 4.3.3. expected energy performance figure 6 shows the expected energy performance of the active ventilated facade with the tps. it must be said that no detail of the active peltier facade can be shown at the moment because of legal reasons. the set point to open the cavity inlet and outlet takes into account the difference of temperature between the outside (to) and the inside (ti) ambient. the cavity will be opened when to>ti and closed when to<ti. 4.4. test building in order to quantify the thermal behaviour, both pbms were equipped with different types of sensors (table 2), placed in different orientations and height (fig. 7) so as to have complete information about all the phenomena that take place throughout the facade. fig. 6. energy performance during cold season (left) vs. energy performance during hot seasons (right). m. ibáñez-puy et al. / development and construction of a thermoelectric active facade 23 table 2 sensor specifications measurement notes sensor description n◦ layout parameter air flow (c) buoyancy driven inside ultrasonic anemometer 8 pbm1: 2 sw +2 ne the cavity pbm2: 2 sw +2 ne (f) external wind direction 1 outside the pbms and speed temperature (d) shielded external ambient platinum resistance 1 outside the pbms thermometer (prt) (i) shielded internal ambient 2 inside the pbms (b) mid cavity 8 pbm1: 3 sw +1 ne pbm2: 3 sw +1 ne (a) surface 24 pbm1: 9 sw +3 ne pbm2: 9 sw +3 ne relative humidity (g) external ambient 1 outside the pbms (h) internal ambient 2 inside the pbms global solar radiation (e) 1 x sw surface pyranometer 1 outside the pbms fig. 7. layout of the measurements system. front view (a) and section view of the south-west (b) and north-east facade (c). 24 m. ibáñez-puy et al. / development and construction of a thermoelectric active facade 5. conclusion and future works the design process of the facade module has been studied deeply. the facade composition and energy performance expectations are already developed. it cannot be denied that the design of this kind of complex building skins is an arduous task, and full of uncertainties. the next step is to build and monitor the prototypes in order to evaluate their energy behaviour. but, it might be said that the building process has already started. nowadays, all the civil and services installation works has been done to adapt the site for the pbms and the frameworks of the modules has been installed (fig. 8). fig. 8. photo of the pbms (under construction). once the building process will be finished, their energy performance is expected to be analysed during an entire year. at the same time, a model is being developed of the two pbms so as to simulate them not only with energyplus® but also with abaqus®. the targets of the project are not only to measure the functioning and response capacity of the control of the air cavity ventilation against real situations, as well as its building viability, but also to evaluate the possible benefits and the costs/profit ratio simulating and comparing the system with traditional hvac installations (validating, as well, the system’s adequacy to different climatological features). besides, the fact that there is going to be a real prototype allows to assess the viability of commercialization of the system and its possible implementation in other fields. acknowledgments we have to thank the support provided by the bia2013-46463-r project. ministerio de ciencia e innovación del gobierno de españa. m. ibáñez-puy et al. / development and construction of a thermoelectric active facade 25 references arenas alonso, a., pagola de las heras, f. l., palacios hielscher, r., rodrı́guez pecharromán, r., & vázquez arias, j. (2007). paramento transparente activo (pta) en aplicaciones de climatización. re revista de edificación, 101-109. cheng, t.-c., cheng, c.-h., huang, z.-z., & liao, g.-c. (2011). development of an energy-saving module via combination of solar cells and thermoelectric coolers for green building applications. energy, 36(1), 133-140. doi:10.1016/j.energy.2010.10.061 chwieduk, d. (2003). towards sustainable-energy buildings. applied energy, 76(1-3), 211-217. doi:10.1016/s0306-2619(03)00059-x fabrizio, e., corrado, v., & filippi, m. (2010). a model to design and optimize multi-energy systems in buildings at the design concept stage. renewable energy, 35(3), 644-655. doi:10.1016/j.renene.2009.08.012 favoino, f., goia, f., perino, m., & serra, v. (2014). experimental assessment of the energy performance of an advanced responsive multifunctional façade module. energy and buildings, 68, 647-659. doi:10.1016/j.enbuild.2013.08.066 giancola, e., sanjuan, c., blanco, e., & heras, m. r. (2012). experimental assessment and modelling of the performance of an open joint ventilated façade during actual operating conditions in mediterranean climate. energy and buildings, 54, 363-375. doi:10.1016/j.enbuild.2012.07.035 he, w., zhou, j., hou, j., chen, c., & ji, j. (2013). theoretical and experimental investigation on a thermoelectric cooling and heating system driven by solar. applied energy, 107, 89-97. doi:10.1016/j.apenergy.2013.01.055 hindrichs, d. u., & behaling, s. (2008). schüco e2 façade. profile, 5, 18-37. loonen, r. c. g. m., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483-493. doi:10.1016/j.rser.2013.04.016 lópez, f. p., jensen, r. l., heiselberg, p., & ruiz de adana santiago, m. (2012). experimental analysis and model validation of an opaque ventilated facade. building and environment, 56, 265-275. doi:10.1016/j.buildenv.2012.03.017 manioğlu, g., & yilmaz, z. (2006). economic evaluation of the building envelope and operation period of heating system in terms of thermal comfort. energy and buildings, 38(3), 266-272. doi:10.1016/j.enbuild.2005.06.009 marinosci, c., strachan, p. a., semprini, g., & morini, g. l. (2011). empirical validation and modelling of a naturally ventilated rainscreen façade building. energy and buildings, 43(4), 853-863. doi:10.1016/j.enbuild.2010.12.005 martı́n-gómez, c., eguaras-martı́nez, m., mambrilla-herrero, n., torres, j., ramos, j. c., & rivas, a. (2010). prototype thermoelectric climate system for its use in residential building. in 37th iahs world congress on housing. santander. mesado, c., chiva, s., juliá, e., & hernández, l. (2010). two dimesnional modelling with cfd of the behavior of a ventilated ceramic façade. in j. c. f. pereira & a. sequeira (eds.), 5th european conference on computational fluid dynamics, eccomas cfd 2010. lisbon, portugal. oesterle, e., & lieb, r.-d. (2001). double-skin facades: integrated planning: building physics, construction, aerophysics, air-conditioning, economic viability. munich, london, new york. rodriguez, a., vián, j. g., & astrain, d. (2008). design and thermal analysis of the components in a thermoelectric finger icemaker incorporated in a domestic refrigerator. in 6th european conference on thermoelectrics. parı́s, francia. retrieved from http://ect2008.icmpe.cnrs.fr/contributions/p1-21-rodriguez.pdf sadineni, s. b., madala, s., & boehm, r. f. (2011). passive building energy savings: a review of building envelope components. renewable and sustainable energy reviews, 15(8), 3617-3631. doi:10.1016/j.rser.2011.07.014 schuster, h. g., & mueller, h. f. o. (2007). interdisciplinary development of a modular façade system with decentralised building services. in 2nd palenc conference and 28th aivc conference on building low energy cooling and advanced ventilation technologies in the 21st century (pp. 908-912). crete island, greece. retrieved from http://www.inive.org/members area/medias/pdf/inive/palencaivc2007/ volume2/palencaivc2007 v2 063.pdf suárez, m. j., sanjuan, c., gutiérrez, a. j., pistono, j., & blanco, e. (2012). energy evaluation of an horizontal open joint ventilated façade. applied thermal engineering, 37, 302-313. doi:10.1016/j.applthermaleng.2011.11.034 van dessel, s., & foubert, b. (2010). active thermal insulators: finite elements modeling and parametric study of thermoelectric modules integrated into a double pane glazing system. energy and buildings, 42(7), 1156-1164. doi:10.1016/j.enbuild.2010.02.007 xu, x., van dessel, s., & messac, a. (2007). study of the performance of thermoelectric modules for use in active building envelopes. building and environment, 42(3), 1489-1502. doi:10.1016/j.buildenv.2005.12.021 yamashita, o. (2008). effect of temperature dependence of electrical resistivity on the cooling performance of a single thermoelectric element. applied energy, 85(10), 1002-1014. doi:10.1016/j.apenergy.2008.02.011 yamashita, o. (2011). effect of interface layer on the cooling performance of a single thermoelement. applied energy, 88(9), 3022-3029. doi:10.1016/j.apenergy.2011.03.017 http://ect2008.icmpe.cnrs.fr/contributions/p1-21-rodriguez.pdf http://www.inive.org/members_area/medias/pdf/inive/palencaivc2007/volume2/palencaivc2007_v2_063.pdf http://www.inive.org/members_area/medias/pdf/inive/palencaivc2007/volume2/palencaivc2007_v2_063.pdf journal of facade design and engineering 3 (2015) 39–47 doi 10.3233/fde-150028 ios press 39 full-scale performance testing and evaluation of unitized curtain walls e. ilter, a. tavil∗ and o.c. celik department of architecture, istanbul technical university, istanbul, turkey abstract. unitized curtain wall systems have been widely seen on high-rise buildings’ facades by bringing benefits with regard to ease of construction, lightness, etc. however, some design and application problems related to structural and infiltration performance of a facade system might arise during its life cycle, which is difficult for the building to compensate. this paper presents a comparative analysis of the structural and infiltration performance of the two identically detailed and produced unitized curtain wall system mock-ups. in order to understand long-term environmental effects on the curtain wall system, a fatigue process was applied on one system in addition to the standard test procedures, while the standard test procedure was applied on the other reference specimen. the tests on the two identical specimens were conducted in accordance with ts en 13830 and aama 501.4 standards. as a result of air infiltration and wind load resistance tests, air infiltration and frontal deflection values on the facade surface were obtained. hence, experimental performance of the systems was compared and the effect of the fatigue procedure on the facade performance was evaluated. keywords: unitized curtain wall, full-scale test, structural and infiltration performance 1. introduction recent curtain wall applications in the building construction sector show that unitized curtain wall system becomes more preferable by architects and system manufacturers especially for high-rise buildings, owing to the advantages with regard to the ease of construction, lightness, weather tightness, quality detailing, thermal performance, etc. unitized systems enable straightforward processing by means of serial production and pre-assembly of the individual facade units. most of the system components are assembled in a plant under controlled working conditions, promoting quality assembly and allowing for fabrication lead-time and rapid closure of the building without being affected by weather conditions (horowitz, 1991). right along with the advantages of the unitized curtain walls, some design and application problems related to the structural and infiltration performances might arise during their life cycle. repeated variable wind loads and climatic conditions are the most significant factors affecting the long-term structural and infiltration performances. experiences verify that the criteria such as structural integrity, provision for movement, and weather tightness are of great importance during their design. certainly, there are a number of other considerations, most of which are of less critical importance and some of which vary in importance depending on the location and type of building (wong, 2007). ∗corresponding author: prof. dr. aslihan tavil, department of architecture, istanbul technical university, taskisla, taksim, 34367 istanbul, turkey. tel.: +90 2931300/ext. 2356; fax: +90 212 2514895; e-mail: tavil@itu.edu.tr. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:tavil@itu.edu.tr 40 e. ilter et al. / full-scale performance testing and evaluation curtain wall construction has stimulated the development of the most significant test programs. programmed testing has become a requirement on curtain wall mock-ups for air-leakage, water leakage, structural strength, thermal performance and further testing for capabilities such as seismic, acoustical, dust or snow performances. full-scale preconstruction curtain wall mock-up testing is today normally considered a standard practice for buildings where the curtain wall is a custom application for high-rise buildings, and for buildings where a high confidence level is desired. curtain wall mockup tests comprise a full size representative portion of the proposed exterior wall system along with the simulated environmental conditions, such as wind, temperature extremes, and seismic exposure. standard test methods, however, may not be sufficient to specify all the requirements for a mock-up test program with regard to some performance assessment. for example, there is no standard test procedure to assess the long-term performance of curtain wall systems (sakhnovsky, 1991; kasket et al., 1998). mock-up tests provide an indication of the performance of the curtain wall only at the time it is brand-new. workmanship quality of the mock-up is usually very good because every element is constructed under a thorough inspection that is not expected during construction of the curtain wall on the building. whether through aging or because of sealant related problems, most curtain wall systems let some water enter, which causes decrease on the structural and infiltration performance (sakhnovsky, 1991). this paper presents the test procedures for evaluating long-term structural and weather (air, water) infiltration performances of unitized curtain wall systems. in order to reach this objective, full-scale mock-up testing of two identical unitized curtain wall panels was carried out. during the testing process, one specimen was subjected to fatigue process before conducting the standard test procedures while only the standard test procedures were applied on the other identical specimen. experimental performances of the two specimens were compared. 2. experimental process as is customary, for approval of the facade for a specific project, a series of various performance tests must be carried out before starting the production and the site erection. all these performance tests, that can help to increase the probability of trouble-free performance of the wall on the completed building, are conducted on a 1:1 scale model of the facade, having exactly the same characteristics of the real final product (galli, 2011). performance testing of the specimen, both prior to construction and during construction, is a well-established and respected element of quality assurance. testing representative segments of curtain wall, or windows, for example, comprises three basic tests such as air infiltration, water penetration, and structural loading. when the systems succeed the performance tests, the system can be accepted to fulfil the requirements of water penetration, strength, air infiltration, seismic performance, etc. that ensure the quality. 2.1. mock-up design the experimental study was carried out on a large test rig located at the facade testing institute, istanbul (fig. 1a and b). the test specimens shown in fig. 1, consist of two identically detailed unitized curtain wall systems with 2,40m width, 4,45m height dimensions, and are comprised of aluminum framing members and insulating glass units. 4mm clear float and 8mm tempered glass double-glazing units with structural silicone spacer were used for the transparent parts. total surface area of each e. ilter et al. / full-scale performance testing and evaluation 41 a c b fig. 1. mock-up design (1:1 scale). a: panel mounted in test rig (interior face). b: exterior face of the panel. c: specimen geometry, applied static pressures and measurement points (m1, m2, m3 and cross-sections a-b). 42 e. ilter et al. / full-scale performance testing and evaluation specimen is 10,68 m2, see fig. 1c. specimen p is the reference specimen and specimen fp is the one that the fatigue procedure was applied on to investigate the long-term environmental effects (ilter et al., 2014). the main difference between the two specimens is that the water penetration under dynamic pressure and fatigue procedure tests were additionally conducted on specimen fp besides the standard test procedure. 2.2. performance tests the commonly used performance tests, which are static air infiltration, water penetration under static and dynamic pressures, wind resistance for design load and safety requirement, seismic resistance and fatigue process are explained below with respect to the related norms. in addition to them, acoustic performance, impact resistance of glass, thermal cycling tests are also conducted on curtain wall systems in turkey but these tests are out of the scope of this study. air infiltration test under static pressure: using the chamber developed for the mock-up testing, the rate of air leakage through the specimens is determined at the project specified pressure differential induced across the assemblies. the air leakage rates determined are compared against the acceptable rates identified for the project. prior to testing, the specimens’ exterior faces are covered with a watertight steel plate. subsequently, the specimens are subjected to the positive and negative pressure of 660pa (more than 10% of normal pressure value) by holding for 3 seconds, and then released. air infiltration is tested up to a test pressure of 600pa by increasing with steps of 50, 100, 150, 200, 250, 300, 450, 600pa (ts en 12153, 2004). water penetration test under static pressure: a pressure differential is applied across the curtain wall assembly, while simultaneously applying water spray onto the exterior surfaces. the testing is performed to show the water penetration resistance of the curtain wall systems and transitions between the system components. static water penetration tests are conducted at a flow rate of 3.4 lt/(m²·min) at up to a test pressure of 600pa according to ts en 12155 standard. specimens are subjected to 660pa positive pressures and held for 3 seconds, then released. afterwards, considered pressures of 50, 100, 150, 200, 250, 300, 450, 600pa are applied to the specimens and held for 300 seconds per each pressure step (aama 501.1, 2005). negative and positive wind pressures are applied on the exterior face of the panel during the testing. water penetration test under dynamic pressure: water penetration testing consists of utilizing a portable wind generator and installing a spray rack system at the exterior of the specimen. a rotax airplane engine equipped with 1850mm diameter propeller producing 1350pa (165km/h) wind velocity was used. the engine and prop assembly are mounted into a steel frame that incorporates a sheet metal shroud and lifting fixture. dynamic wind stream has been generated along with the water spray perpendicular to the outdoor face of the specimen for a period of fifteen minutes, right along with the water applied at the rate of 3.4 lt/(m²·min) on the face completely and continuously, see fig. 2 (ts en 12179, 2000). wind resistance test based on design load and safety requirements: the test is conducted through turkish standards; ts en 13116 describes performance requirements due to design load while ts en 12179 specifies the method to determine the performance of curtain wall components under positive and negative static air pressure regarding the safety issues. positive test pressure is conducted before the negative test pressure. prior to testing, three measurement points were determined. m1 and m3 were at the level of structural supports on the middle axis while m2 was in the middle of m1 and e. ilter et al. / full-scale performance testing and evaluation 43 fig. 2. dynamic water penetration test for the fatigue process. m3 (fig. 1c). the specimens are subjected to 1200pa (50% of the design load) positive and negative pressures, held for three seconds, and then released. afterwards, pressures with increasing steps of 0, 600, 1200, 1800, 2400pa are applied to the specimens and held for 10 seconds for each pressure step. lateral deflections were measured at positive and negative wind pressures up to ± 2400pa. frontal deflections of the specimens were determined according to ts en 13116. at the beginning of the test three pulses of air pressure (50% of design wind load or 500pa whichever is greater) is applied. then, the sample is subjected to 25%, 50%, 75%, and 100% of design wind load. the test specimens are exposed to positive and negative wind loads by applying 150% of the design load (pe= ± 3600 pa) (aama, 2009, ts en 12155, 2005). seismic resistance test: this test provides performance evaluation of curtain wall systems when subjected to the specified horizontal displacements representing an earthquake. furthermore, it is intended to analyze the seismic safety of architectural glass components within a curtain wall. the static test method is conducted through aama 501.4, which is applied on the same test chamber with air infiltration, water-tightness, and wind resistance. testing is done by means of a bottom structural support unit for producing horizontal movements and completed at three full cycles where a cycle is defined as a full displacement in one direction, back to the originating point, full displacement in the opposite direction and back to the originating point. each cycle is completed in almost 2 minutes. the design displacement is taken as 34mm (0.010 × the greater of the adjacent story height), and measured at the movable structural support unit. after the tests, all areas were inspected in detail, 44 e. ilter et al. / full-scale performance testing and evaluation and no disengagement, metal distortion, sealant or glazing failures were observed (ts en 12155, 2005). fatigue procedure: uniformly distributed suction loads were applied on the outer surface of the specimen. etag 17 comprises a static wind suction test, which is applied to specimens for the fatigue procedure. the test was performed in successive steps of 500pa to 3500pa (lower than the extreme load pressure: 3600pa) with a return to zero at each level, and loads were held for 10 seconds per each pressure step. the results of the fatigue test are given in fig. 3. fig. 3. fatigue procedure applied on the specimen fp (etag 17). for the two specimens, test sequence was determined according to ts en 13830 and aama 501.4. the tests were conducted at three phases during nine months with 90 days intervals between each testing sequence. in total, six test sequences were conducted on the reference specimen p and fatigued specimen fp. the categories of the tests for p are air infiltration, water penetration under static pressure, wind resistance, seismic resistance as the standard tests. water penetration under dynamic pressure tests and fatigue procedure were the additional tests for fp along with the standard tests. although the design load value (pd, ±2400 pa) was defined by the manufacturer, normal pressure value (pn; %25 of pd, ±600 pa) and extreme wind (safety) load (pe; %150 of pd, ±3600 pa) were calculated according to ts en 12154 and ts en 12179. the applied test sequence in order is given in table 2. 3. test results and performance evaluation this experimental work revealed that the measured maximum air infiltration values were within the acceptable limits (i.e. 1.50 m³/m²·h) for the reference specimen p on which the fatigue process was not applied during all three testing phases. however, the air infiltration values highly increased and reached 2.31 m³/m²·h for positive pressure and 2.04 m³/m²·h for negative pressure after the fatigue process was conducted in fp. the fatigue process was applied till the last testing phase. water leakage was not observed in p at the end of the three testing phases, while water leakage was observed in fp after the second and third testing phases showing that the fatigue process caused loss of performance on the curtain wall system as expected, see table 1. the water was observed at the intersection of the four panel pieces (a and c) on the vertical axis of the curtain wall panel, indicated as w1 (fig. 1c). e. ilter et al. / full-scale performance testing and evaluation 45 table 1 infiltration and structural test results specimen time load air air water frontal deflection frontal deflection label (days) type infiltration infiltration leakage value at 2400pa (mm) limit value (mm) value at 600pa limit value (m³/m²·h) (m³/m²·h) p1 0 positive 0.63 1.50 no leakage 10.34 15.00 negative 0.52 1.50 no leakage 11.27 15.00 p2 90 positive 0.73 1.50 no leakage 10.97 15.00 negative 0.53 1.50 no leakage 12.00 15.00 p3 180 positive 1.07 1.50 no leakage 10.09 15.00 negative 0.80 1.50 no leakage 16.60 15.00 fp1 0 positive 0.89 1.50 no leakage 10.84 15.00 negative 1.20 1.50 no leakage 8.77 15.00 fp2 90 positive 0.92 1.50 at w1∗ 12.06 15.00 negative 1.24 1.50 at w1∗ 12.62 15.00 fp3 180 positive 2.31 1.50 at w1∗ 13.47 15.00 negative 2.04 1.50 at w1∗ 17.78 15.00 ∗w1 is the intersection point of a and c units on vertical middle axis of the panel (fig. 1c). note: highlighted cells show the results that exceeded the limits. table 2 results and performance classification according to the test categories and sequence test test name test standard peak test result performance classification number pressure (pa) p1 p2 p3 fp1 fp2 fp3 1 1air infiltration ts en 12153 600 pass a4 a4 a4 a4 a4 a2 2 2water penetration (static) ts en 12155 600 pass r7 r7 r7 r7 r5 r5 3 3water penetration (dynamic) aama 501.1 600 see note4 no water penetration at w1∗ 4 wind resistance (design load) ts en 12179 2400 see note5 ∗∗ ∗∗ ∗∗∗ ∗∗ ∗∗ ∗∗∗ 5 1air infiltration ts en 12153 600 pass a4 a4 a4 a4 a3 a1 6 2water penetration (static) ts en 12155 600 pass r7 r7 r7 r7 r5 r5 7 seismic resistance aama 501.4 – pass no deformation 8 1air infiltration ts en 12153 600 pass a4 a4 a4 a4 a3 a1 9 2water penetration (static) ts en 12155 600 pass r7 r7 r7 r7 r5 r5 10 wind resistance (extreme load) ts en 12179 3600 pass no deformation 11 seismic resistance (static) aama 501.4 – pass no deformation 12 3fatigue procedure etag 17 3500 pass – – – no deformation 1classification was performed to ts en 12152 and cwct. 2classification was performed to ts en 12154 and cwct. 3applied only on fp1, fp2, and fp3. 4there is no classification or performance requirement for water penetration (dynamic) testing in aama standard. 5p1, p2, fp1, and fp2 successfully passed the wind resistance (design load) test requirements in the classification standard of wind resistance testing (ts en 13166). but specimen p3 and fp3 exceeded the limit value (15mm) at the peak pressure. ∗w1 is the intersection point of a and c units on vertical middle axis of the panel (see fig. 1c). ∗∗frontal deflection value <15mm. ∗∗∗frontal deflection value ≥15mm. for air infiltration test, if exceeded the limit value: 1.5 m3/m2 h at 150pa ⇒ class: a1; at 300pa ⇒ a2; at 450pa ⇒ a3; at 600 pa ⇒ a4 (ts en 12152). for water penetration test (static), if exceeded the limit value: 15mm at 150pa ⇒ class: r4; at 300pa ⇒ r5; at 450pa ⇒ r6; at 600 pa ⇒ r7 (ts en 12154). 46 e. ilter et al. / full-scale performance testing and evaluation frontal lateral deflection values from wind loading increased at each testing phase, however some values slightly exceeded the limit of 15mm at high negative pressure values while the positive deflections were within the acceptable limits. results show that, as expected, the overall stiffness of the tested panel is higher for positive pressures than negative pressures (table 1). test sequence and infiltration and structural tests results were grouped with regard to the available standards for performance classification (table 2). the experimental results attained are demonstrated by the infiltration and structural performance criteria, which were numerically obtained with the tests for performance evaluation. it is a crucial issue for the facade manufacturer, owner or contractor, and designer to evaluate the long-term performance of a facade system. tables 1 and 2, and fig. 4 show that, at the maximum wind pressure of 2400pa, frontal deflections are exceeded by 6.67% and 18.53% for p and fp, respectively. air infiltration values for fp are measured as 2.31 m³/m²·h and 2.04 m³/m²·h at the third testing phase. these values exceeded the acceptable limits by 54% and 36% in positive and negative loading type respectively while the air infiltration values of p are within the acceptable limits at all testing phases. fig. 4. frontal deflections under wind pressure (2400pa) and air infiltration values of p and fp. 4. conclusions the following results can be drawn from this full-scale experimental work: • although the majority of today’s unitized curtain wall systems is technologically improved and performs higher requirements, the test results indicate that the continuous impact of environmental conditions cause a significant loss in performance (up to 54% due to air infiltration, 18.53% due to wind pressure) during the curtain wall system’s life time. e. ilter et al. / full-scale performance testing and evaluation 47 • the most critical criterion seemed to be the frontal deflections in p while water leakage limit was exceeded first in fp in addition to exceedance limits of air infiltration and frontal deflections. • considering that the recent test procedures provide an indication of the curtain wall only at the time when it is brand-new, test procedures which evaluate real life performance of the curtain wall systems should be developed for more realistic evaluations. • the process of mock-up testing plays an important role towards understanding the performance of the curtain wall system regarding the critical issues. however, whole experiment process requires experience, time, high efforts and cost issues. acknowledgments this experimental research was fully supported by façade testing institute (fti) and metal yapı a.ş., istanbul. this support is gratefully appreciated. however, any opinions, findings, conclusions, and recommendations presented in this paper are those of the authors and do not necessarily reflect the views of the sponsors. references aama 501.1 (2005). standard test method for water penetration of windows, curtain walls and doors using dynamic pressure. american architectural manufacturers association, usa. aama 501.4. (2009). recommended static testing method for evaluating curtain wall and storefront systems subjected to seismic and wind induced interstory drift. american architectural manufacturers association, usa. etag 17 (2006). standard for systemized building envelopes. centre for window and cladding technology cwct, uk. galli, u. (2011). seismic behavior of curtain wall facades. a comparison between experimental mock-up test and finite element method analysis. doctoral dissertation, politechnico di milano. horowitz, j. (1991). the interrelation of exterior wall and structural systems in buildings, exterior wall systems-glass and concrete technology, design and construction. edited by barry donaldson, astm stp. ilter, e., tavil, a., celik, c. o., seyhan, m. (2014). full-scale performance testing of unitized facade systems. icbest 2014, aachen, germany. kaskel, b. s. et al. (1998). critical review of curtain wall mockup testing for water penetration. water leakage through building facades. edited by r. j. kudder and j. l. erdly, astm stp 1314. sakhnovsky, a. a. (1991). full-scale performance testing of curtain walls. exterior wall systems-glass and concrete technology, design and construction. edited by b. donaldson, astm stp 1034. ts en 12179 (2000). curtain walling, resistance to wind load/test method. turkish standards institution, ankara, turkey. (classification standard: ts en 13116, curtain walling, resistance to wind load/performance requirements, turkish standards institution, ankara, 2004). ts en 12153 (2004). curtain walling, air permeability/test method. turkish standards institution. ankara, turkey. (classification standard: ts en 12152, curtain walling, air permeability/performance requirements and classification, turkish standards institution, ankara, 2004). ts en 12155 (2005). curtain walling, watertightness/laboratory test under static pressure. turkish standards institution, ankara, turkey. (classification standard: ts en 12154, curtain walling, water-tightness/performance requirements and classification, turkish standards institution, ankara, 2004). wong, w. (2007). analysis and design of curtain wall systems for high rise buildings. university of southern queensland faculty of engineering and surveying, australia. jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 005 journal of facade design & engineering volume 5 / number 1 / 2017 editorial dear jfde readers and authors, this issue is the result of the facade conference powerskin, held on 19 january 2016 at the building tradefair ‘bau’ in munich. the conference was organised in a cooperation of tu munich, tu darmstadt and tu delft. all three universities maintain a facade research and education unit on building envelopes. the conference featured a mix of practice and education related and scientific contributions. again, jfde was chosen as the platform to publish a selection of scientific papers. a total of 22 abstracts were reviewed of which 17 were pre-selected. from those, we received 13 papers that had been subjected to our strict double blind peer review process. a final selection of 9 papers forms this issue. we thank our guest editors thomas auer and jens schneider who have been key partners in creating this special issue. as discussed in the last editorial, we have successfully completed the nwo funding period (netherlands organisation of scientific research) to start up jfde. open access publishing strongly depends on author publishing fees and the introduction in our scientific ((field?)) needs some more time. in search of a new business model, we proudly announce that we have found two partners wo believe in the relevance of jfde. the society of facade engineering sfe is a joint initiative of cibse, istructe and the riba with the aim to create a forum to advance knowledge and practice in facade engineering. cwct it is an industry funded centre and a leading information provider and trainer in the field of building envelopes and glazing. both organisations acknowledge the need for an independent scientific publishing organ which will contribute to the development of the field of facade design, engineering and construction. this support enables us to continue jfde. we want to thank the publisher ios press and especially mark eligh to have guided jfde to the level where it is now. this issue is the first published by tu delft open, a publishing house established by the tu delft library. we will continue to deliver high quality contributions with the aim of 3 issues per year. thank you for supporting jfde. the editors in chief, tillmann klein ulrich knaack journal of facade design and engineering 3 (2015) 1–13 doi 10.3233/fde-150031 ios press 1 free-form architectural envelopes: digital processes opportunities of industrial production at a reasonable price e. castañeda∗, b. lauret, j.m. lirola and g. ovando department of construction and technology in architecture, school of architecture, technical university of madrid, spain abstract. free-form architecture is one of the major challenges for architects, engineers, and the building industry. this is due to the inherent difficulty of manufacturing double curvature facades at reasonable prices and quality. this paper discusses the possibilities of manufacturing free-form facade panels for architectural envelopes supported by recent advances in cad/cam systems and digital processes. these methods allow for no-mould processes, thus reducing the final price. examples of actual constructions will be presented to prove the viability of computer numerically controlled (cnc) fabrication technologies. scientific literature will be reviewed. promising fabrication methods (additive, subtractive, forming) to accomplish this proposal will be discussed. this research will provide valuable information regarding the feasibility of manufacturing freeform panels for architectural envelopes at lower prices. keywords: free-form architecture, double curvature facade panel, digital manufacturing, no-mould processes, cnc processes 1. introduction free-form buildings play a progressively important position in contemporary architecture. in the digital age computer generated 3d forms are easily achievable and architects benefit from a series of computer software applications to help them get a broader freedom of design (hauschild & karzel, 2011). not only because of the possibility of creating new undulating, sinuous skins, but also because of the development of new tools that enable the production of double-curved panels. some well-known ‘free-form’ buildings, as railway stations in innsbruck (fig. 1), the kunsthaus in graz, or the bmw pavilion in frankfurt (fig. 2), have been manufactured using moulding processes which are, unfortunately, only available for limited projects and budgets. the problem is that every single panel of the envelope is unique, which means that every piece has to be produced using a unique mould (pronk et al., 2010). in this study, the writers review the latest methods that can be applied on the free-form envelopes field and introduce the challenges, which appear when realizing the manufacturing of double-curved panels for curvy buildings and how these challenges can be best settled. ∗corresponding author: estéfana castañeda vergara, phd. student of department of construction and technology in architecture, school of architecture, technical university of madrid, spain. tel.: +34 638823178; e-mail: estefanacastaneda@ gmail.com. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:estefanacastaneda@{penalty -@m }gmail.com 2 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price fig. 1. railway station, innsbruck, made of glass panels (2007), zaha hadid. fig. 2. styrofoam mould for polycarbonate panel of ‘the bubble’, bmw pavilion, frankfurt (1999), bernhard franken. 2. digital fabrication in construction around 98% of the planning, calculation, optimization, tendering and marketing in construction is based on digital data. along with this comes more and more direct interface between the computerized design process and the physical implementation (hauschild & karzel, 2011). for this reason, the digital processes of production mean that the constructability in building design is an undoubtable condition of computability. today, different processes are being explored to find new construction efficient methods of translating the designed architecture to a real building. 3d digital modeling software based on nurbs (non-uniform rational b-splines) has opened a universe of complex forms that were, until the appearance of cad/cam1 technologies, very difficult to conceive, develop and represent, let alone manufacture (kolarevic, 2005). in this context the production of complex panels are possible by means of computer numerically controlled (cnc) manufacturing processes. 1cad, computer-aided-design; cam, computer-aided-manufacturing. e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 3 2.1. subtractive procedures subtractive procedure involves the separation of particles of a raw material using different techniques (electric, chemically or mechanically-reductive). cnc subtractive methods comprise punch, lasering, water-jet, plasma-arc, hot-wire, milling two to six axis, robotic lasering (multi-axial) (hauschild & karzel, 2011). usually subtractive processes are based on two-dimensional fabrication. the production strategies used for two-dimensional fabrication include contouring, triangulation (or polygonal tessellation), use of ruled, developable surfaces, and unfolding (kolarevic, 2003). they all involve the extraction of two-dimensional, planar components from geometrically complex surfaces or solids comprising the building’s form. nevertheless the milling of three-and-more axes has the ability to raise or lower the drill-bit along the z-axis. undercuts cannot be milled with three-axis machines. to achieve manufacturing undercuts, four or six axes machines are used. the cnc milling has been used to produce moulds for free-form panels, and parts of large proportions. in gehry’s zollhoff towers (dusseldorf, germany) (fig. 3), the undulating reinforced concrete walls were made with moulds of styrofoam (polystyrene foam). 335 different moulds were cnc milled that became the forms for the casting of the concrete. ‘the bubble’, is the bmw pavilion, designed by berhard franken, for which free-form acrylic glass panels were formed, using polystyrene moulds, as well as created, using multi-axis milling. nio architecten designed the curvy bus stop in hoofddorp, netherlands (fig. 4). the structure was manufactured from polystyrene foam pieces with hot-wire procedure. the total measures of the construction are 50 × 10 × 5m, and a layer of grp coating protects the surface. the construction costs were d 1 million, and it would have been twice, if the structure had been traditionally built with concrete and steel. using large scale hot-wire cutting is very well suited to concrete formwork and the manufacture of light, voluminous construction parts, as we can see in these constructions. subtractive processes are commonly the procedures more used in construction, due to the manufacturing industry, which has used them to mass-produce elements of conventional orthogonal geometries. these techniques have large relevance within free-form panels fabrication, based on their ability to create personalized architectural forms starting from typical construction materials fig. 3. the reinforced concrete panels for zollhof tower, düsseldorf (2000). 4 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price fig. 4. moulds of polyestirene foam for bus station in hoofddorp, the netherlands (2003). (naboni & paoletti, 2014), overall producing mock-ups and moulds. however, the question of material economy continually arises due to waste of material with these subtractive procedures. it does not appear sensible to mill a specific individual component from a block of solid material if there is no decisive added value in doing so (hauschild & karzel, 2011). 2.2. forming procedures forming processes are particular manufacturing methods, which make use of mechanical forces, restricting forms, heat or steam to cause plastic deformation of a material to produce required shapes. some forming processes include cnc punching, cnc folding, cnc bending, hydroforming, linear flow splitting, flexible roll forming and bending, thermoforming, injection moulding, welding and multi-point forming. sheet metal (aluminum, steel) have been traditionally formed by hand or in stamping moulds. if cnc processes could be used to form a metal sheet their main problem would be solved: cost. this is due to the fact that skilled labour and runtime are reduced. three promising processes for manufacturing free-form panels on metal forming are under development: flexible roll bending, incremental forming and multipoint forming. incremental sheet forming (isf) (fig. 5) is an umbrella term for a range of processes in which a sheet is formed incrementally by a progression of localized deformation (alonso et al., 2014). mass production technologies for making aerodynamic automobiles, airplanes and ships using sheet metal exist, but are not feasible for irregularly shaped buildings because the requirement for panels of different shapes. thus, the high price of mould fabrication cannot be justified. for this reason, existing buildings with curved shapes have rarely employed true double-curved panels, and have instead used tricks (lee & kim, 2012). double-curved panels can be approximated by arrays of adjustable height and numerically controlled pins (fig. 6), which could be used for the production of moulded glass and plastic sheets, and for curved stamped metal (alonso et al., 2013). for this and previous case studies, we can affirm it is technically possible to fabricate panels of complex shapes with sheet metal processing methods. however the costs of the existing methods normally exceeded the project budget due to the high price of the machines that manufacture such panels and the material itself. e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 5 fig. 5. an example application of isf: a 1/8 scale model of the front section of a shinkansen (bullet train) made by amino in japan. fig. 6. a multipoint forming (mpf) prototype unit was built at jilin university in china. 2.3. additive procedures in contrast to the subtractive processes, the additive procedure is characterized by adding layers of material to produce a part, without the need of tools or preforms (tool-less), and it allows great freedom of geometry. the main advantage comes from the possibility to produce unique components, which would not be economically sustainable to produce with traditional manufacturing techniques (hauschild & karzel, 2011). this feature, together with the high freedom enabled in realizing complex forms, makes the additive manufacturing process particularly relevant within the perspective of advanced customization, and explains the increasing involvement of architects in the development of techniques and application in this field (naboni & paoletti, 2014). 6 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price fig. 7. printer baam (big area additive manufacturing). the additive process is frequently cited as being layered manufacturing, solid free-form fabrication, rapid prototyping or rapid manufacturing. the technologies associated to this method are stereolithography (sla), fused deposition modeling (fdm), laser sintering (ls), digital light processing (dlp), 3d printing (3dp), polyjetttm methods, direct metal fabrication (dmf), direct metal deposition (dmd), contour crafting and d-shape. the principle of the additive procedure is the same for all of the different methods: special computer software breaks the cad 3d model down into layers. these layers form horizontal layers/building plans/foot prints of the model. the breaking down process is called slicing. the additive manufacturing output device processes each layer of the model consecutively, whereby the contours and fillings of the part are cured. depending on the method, this is done by exposition, heating, or bonding in a process chamber, which typically is confined to certain dimensions. different technical strategies are used to bond each new layer with the previous one. thus, layer-by-layer, the physical representation of the virtual cad model is generated (strauß, 2013). the additive manufacturing process was considered limited in building production, because of the reduced size of the objects that it could produce. additionally they are used mainly for the fabrication of mock-ups with complex geometries like desktop printers. currently, those assessments are changing, since the 3d printing industry is improving its machines. a start-up in germany has created a fused deposition printer called bigrep one, which has a capacity of 1.15 × 1.00 × 1.19m, and prints material such as pla, abs, pva, hdpe, pp, nylon, tpe, laywood and laybrick, with a dual extrusion head. the company 3dp as well has a large printer (1.00 × 1.00 × 0.5m), which can extrude a variety of materials as bigrep one, and guarantee the possibility of working in series. the oak ridge national laboratory created baam (big area additive manufacturing) (fig. 7), which 3d printed the shelby cobra car in 24 hours (fig. 8). the baam can manufacture strong, lightweight composite (abs reinforced by carbon fiber), the print area is 2.00 × 4.00 × 0.87m and can produce components 500 to 1000 times faster than today’s industrial additive machines. the bigrep one and the baam are demonstrating the potential of large-scale additive manufacturing as an innovative and viable manufacturing technology. e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 7 fig. 8. 3d printed shelby cobra in 24 hours. fig. 9. d-shape machine. the machine uses sand mixed with an inorganic binder. in the last few years large-scale processes for construction applications have been developed based on additive manufacturing principles. pegna considers a layer deposition method suitable for traditional construction materials (pegna, 1997). these processes offer construction automation, the promise of design freedom (buswell et al., 2007). enrico dini developed in 2004 the world’s biggest 3d printer, which is capable of building structurally sandstone constructions automatically with virtually no human intervention. the d-shape processes (fig. 9) uses a powder deposition method, which is selectively hardened, using an inorganic binder. dr. richard buswell at loughborough university has focused on using traditional construction materials (fig. 10). the machine uses a gantry and can produce components in a built volume of up to 2.00 × 2.50 × 5.00m. contour crafting (cc) is a parallel process created by prof. behrokh khoshnevis of the university of southern california, which follows the same principle of the generative build-up of other rapid prototyping methods. khoshnevis’s goal is the construction of entire buildings on the 8 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price fig. 10. the freeform construction project’s proof of concept is a 1 × 1m, one-tone reinforced concrete panel. earth and on the moon. these processes and researches can be extrapolated for building envelopes facade panels with no moulds. in china the printing techniques that have been developed seem to be more advanced than contour crafting (cc), d-shape and buswell’s researches. winsun decoration design engineering co. has been capable to print entire family houses made from recycled rubble, fiberglass, steel, cement and binder, in contrast to the other large 3d printers that have just produced a few building elements. the concept proposed by winsun is similar to cc. both techniques propose hollow printed walls inside, apart from corrugated filler, a design that saves material without sacrificing strength. however, whereas winsun prints the building in parts off-site, cc has fully assembled its technology so that it can build directly on the construction site. this localization will be key to bringing the costs down. khoshnevis observes: “compared with prefabricated construction, 3d printing is not much cheaper, unless it can occur locally, onsite, eliminating costs of transporting materials and labour” (ver bruggen, 2014). an obvious divergence exists between additive manufacturing and current building processes. the size, timelines and production terms are the main discussion topics. nevertheless, in friedberg, voxeljet is manufacturing vx4000, a large-format 3d printing system of 8.00 m3 (4.00 × 2.00 × 1.00m), which printed a small shelter, designed by peter ebner. it is possible to use this technology for printing facade panels, because of the size of the machine that is suitable for the proportions of architectural elements. contrary to the opinion of other 3d printing companies it is not technically impossible to increase the size of the system, even if it is true that some material and technology parameters cannot simply be scaled up (strauß, 2013). questions such as how much the elimination of labour compensates for the technical skills and time required and how well this balances against the cost of equipment, materials and maintenance can be answered aptly by stating that rapid prototyping or manufacturing does not allude only to ‘cost’ but also to ‘added value’ (buswell et al., 2005). there is an extensive range of materials available for rapid prototyping, but limited materials of varying properties and application are available for additive manufacture at large scale, despite that it is possible to print colours and maybe textures using for example the 3d printer. the conservation e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 9 industry, as dr. cooper of the conservation centre, liverpool, states, still prefers to use traditional methods to finish objects that have been re-formed using additive manufacturing or machining, in order to achieve a high level of reality or ‘reproduction fidelity’. if we translate concepts from the automotive industry, for example, (‘concept cars’, ‘just-in-time management’, new collections from standard components) into facade technology, we can expect an increase in precision and implementation quality by using digital and high-tech tools without compromising individuality. let us use the ‘file-to-factory’ concept for the most diverse applications by combining a few standard components with highly specialized details and shaped pieces (knaack, et al., 2008). there are no tooling requirements for additive processes and so customization of every ‘build’ comes at no extra cost. the cost-per-part of components manufactured using rapid manufacturing are constant, opposed to the high initial set-up costs for tooling used in conventional processes (hopkinson & dickens, 2003). the main component for freeform construction is in scaling-up these additive manufacturing processes, from manufacturing ‘desktop’ sized items to something the size of a wall (buswell et al., 2008). 3. free-form panel no-mould processes currently there are several research centres that are using all their efforts to solve manufacturing double curved panels for architecture envelopes. in the product development of the technical university of eindhoven, they have created a machine that allows for the creation of free-form plastic panels using an adjustable mould by vacuum pressure with 1.20 × 1.80m format. the relevant point of the machine is that it uses a unique matrix for creating different panels, which is a versatile and economic solution. the freeform construction project at loughborough university, 3d printed a concrete reinforced panel (fig. 10) without the requirement for formwork. dr. richard buswell choses concrete to apply in additive manufacturing processes, because it is a well-known material in the construction industry. it is a worthy approach to achieve a feasible process to manufacturing free-form architectural envelope panels. the dongdaemun design park (ddp) building, designed by zaha hadid, was developed by a sheet metal forming called multipoint stretch forming, which is a hybrid method of multipoint forming and stretch bending (fig. 11), through trial and error. the cost of fabrication is 90% less than the traditional forming methods. furthermore, the average fabrication time per panel was reduced from several hours to 15 minutes per panel using the new multipoint stretch forming method (lee & kim, 2012). formtexx manufacturing facilities are conceptually a closed-box, digitally controlled devices analogous to desktop printers, but larger. instead of paper, is a sheet metal; instead of a print head with a usb connection is a robotic double-curvature forming machine with a usb connection. significantly, both printer and forming machine are oblivious of the quantity of data spooled to the individual sheet. feed speed is constant regardless of content. the innovative manufacturing process, which avoids impacting the surface of the material, provides opportunities for lightweight construction, employing engineering techniques typical of the automotive industry. produce panels are 1.20 × 1.20m with double curvature every 20 minutes (fig. 12). 10 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price fig. 11. multipoint stretch forming machine (photographs: courtesy of steellife). fig. 12. free-form display 10 × 3m at the building centre, london. formtexx. high tooling costs of the multipoint method leads to exploration to lower cost procedures. desktop 3d printers lower prices and their large format versions (bigrep) are a promising alternative for freee. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 11 fig. 13. kamermaker. large 3d printer in a tuning shipping container. fig. 14. one of the pieces to the 3d print canal house. form architectural panel production. recent developments such as kamermaker (fig. 13), a large size 3d printer by dus architects show the feasibility of plastic 3d printing building facade components, without the need of moulds (fig. 14). 12 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 4. conclusions recent advances in digital processes allow for free-form architectural facade panel production at a reasonable price, by means of avoiding the use of moulds, which reduce significantly the associated costs, as in architecture none or little repetition is expected. some of these digital processes, such as multipoint one, have demonstrated the viability of free-form-no-mould architectural panel production. however, sophisticated tooling prices are still high and far from being widely accepted by the construction industry. nevertheless if the use of moulds is mandatory, digital additive, subtractive, and forming processes will allow obtaining complex free-form moulds at lower prices, without the use of craftsmanship, which reduce the cost of labour and as a result, the final budget. the fast spreading of additive plastic based 3d desktop printers and the raising attempts to scale up these machines for architectural purposes, such as kamermaker by dus architects, sets out new alternatives in no-mould low cost free-form architectural panels. these 3d printing processes allow for free-form manufacturing panels, to show the bare material or to be finished on-site and off-site by adding one or more finishing coatings. drawbacks in energy use and time consuming printing periods must be addressed. material development plays a key role in realizing true functionality in parts produced, using additive procedures. reliability and commercial availability of these bigger printers are yet to be achieved. architects and small enterprises are leading this innovative path for 3d printing architectural components. it is yet to be proven that this will be a widely accepted method by the building industry or just another good idea that fails, due to the complex constraints involving the architectural product as a whole. references alonso, l., bedoya, c., lauret, b., & alonso, f. (2013). f2te3: sistema de cerramiento transparente, ligero, de altas prestaciones energéticas que permite el diseño con formas libres. informes de la construcción, 532(65), 443-456. doi: 0.3989/ic.12.068 alonso, l., lauret, b., castañeda, e., domı́nguez, d., & ovando, g. (2014). free-form architectural facade panels: an overview of available mass-production methods for free-form external envelopes. 2nd international conference on construction and building research, 2, 149-156. doi: 10.1007/978-94-007-7790-3 20 buswell, r. a., soar, r. c., pendlebury, m., gibb, a., edum-fotwe, f., & thorpe, a. (2005). investigation of the potential for applying freeform processes to construction. 3rd international conference on innovation in architecture, engineering and construction (aec), s. sariyildiz & b. tuncer (eds.), delft university of technology, rotterdam, 141-150. buswell, r. a., soar, r. c., gibb, a. g. f., & thorpe, a. (2007). freeform construction: mega-scale rapid manufacturing for construction. automation in construction, 16(2), 224-231. doi: http://dx.doi.org/10.1016/j.autcon.2006.05.002 buswell, r. a., thorpe, a., soar, r. c., & gibb, a. g. f. (2008). design, data and process issues for mega-scale rapid manufacturing machines used for construction. automation in construction, 17(8), 923-929. doi: 10.1016/j.autcon.2008.03.001 hauschild, m., & karzel, r. (2011), digital processes. planning, design, production. detail practice. munich, germany: birkhäuser. hopkinson, n., & dickens, p. (2003). analysis of rapid manufacturing using layer manufacturing processes for production. proceedings of the institution of mechanical engineers. part c: journal of mechanical engineering science, special issue on rapid manufacturing, 217(1), 31-39. knaack, u., bilow, m., & strauss, h. (2008). rapids. layered fabrication technologies for facades and building construction. rotterdam, the netherlands: 010 publishers. kolarevic, b. (2003), architecture in the digital age: design and manufacturing. new york, ny: taylor & francis group. kolarevic, b. (2005). digital praxis: from digital to material. proceedings of the 3rd conference on innovation in architecture, engineering and construction (aec), s. sariyildiz & b. tuncer (eds.), delft university of technology, rotterdam, 1, 5-18. lee, g., & kim, s. (2012). case study of mass customization of double-curved metal facade panels using a new hybrid sheet metal processing technique. journal of construction engineering and management, 138, 1322-1330. doi: http://dx.doi.org/10.1061/(asce)co.19437862.0000551 http://dx.doi.org/10.1061/(asce)co.1943-7862.0000551 e. castañeda et al. / digital processes opportunities of industrial production at a reasonable price 13 naboni, r., & paoletti, i. (2015). advanced customization in architectural design and construction, springerbriefs in applied sciences and technology. milan, italy: springer & politecnico di milano. pegna, j. (1997). exploratory investigation of solid freeform construction. automation in construction, 5(5), 427-437. doi: 10.1016/s09265805(96)00166-5 pronk, a., van rooy, i., & schinkel, p. (2010, january). double-curved surfaces using a membrane mould. in: symposium of the international association for shell and spatial structures (50th. 2009. valencia). evolution and trends in design, analysis and construction of shell and spatial structures: proceedings. editorial universitat politècnica de valència, 618-628. strauß, h. (2013). am envelope. the potential of additive manufacturing for facade construction. delft university of technology: architecture and the built environment. ver bruggen, s. (2014, may 21). chinese company prints 10 homes in one day. retrieved from http://www.forumforthefuture.org/ greenfutures/articles/chinese-company-prints-10-homes-one-day http://www.forumforthefuture.org/greenfutures/articles/chinese-company-prints-10-homes-one-day http://www.forumforthefuture.org/greenfutures/articles/chinese-company-prints-10-homes-one-day journal of facade design and engineering 3 (2015) 59–70 doi 10.3233/fde-150033 ios press 59 freeform surfaces adaptation using developable strips and planar quadrilateral facets francisco gonzalez-quintial∗, javier barrallo and ana artiz-elkarte higher technical school of architecture, university of basque country upv-ehu, donostia, spain abstract. almost all industrialized materials commonly employed at engineering and building construction are approximately unstretchable, metal sheets, plywood or glass. so there is no doubt about the advantages offered, mainly from the economic point of view, by the processes that use flat or developable surfaces in the resolution of doubly curved ones. using two prototypical kinds of developable surfaces, an adaptation method of double curvature surfaces is formulated using either developable strips or planar quadrilateral surfaces. through the geometric concept of apparent contours and by the systematization of a process inspired by traditional projective geometry an algorithm is built, using the newest and outstanding cad processes. they systematize and allow obtaining single-curvature strips or flat facets, in order to be able to address its construction, using materials that can be bent in one direction or rigid material with no possibility of being bent at all by simple and economical procedures. these strips obtained are from the geometrical viewpoint absolutely developable. they are patches extracted from cones or cylinders. the main subject to be developed in this paper is on one hand showing of the system and its geometric basis and on the other the exhibition of the results that are being obtained over physical models and prototypes built in diverse materials. keywords: freeform surfaces, double curved surfaces, developable surfaces, algorithmic approach 1. introduction architecture in recent years has seen an extensive use of double curvature surfaces. simultaneously supported in new design and digital fabrication systems there has been an important development in the construction of this kind of surfaces that have allowed to see examples of buildings with spectacular results, like frank gehry´s designs, supported by the researches and geometric developments of d. shelden (2002). this has led to a revolution within the systems of geometric control of less conventional shapes, free form and double curvature shapes, not only from the point of view of the cad technology, but through the impulse that has occurred through the theoretical geometry knowledge and the interesting theoretical developments, as those published by h. pottmann (2007), intended for specific use in the development of that which is almost a new science, known as architectural geometry. ∗corresponding author: francisco gonzalez-quintial, higher technical school of architecture, university of basque country upv-ehu. donostia, spain. tel.: +34 943 018 407; e-mail: francisco.gonzalez@ehu.eus. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:francisco.gonzalez@ehu.eus 60 f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets in the works mentioned, a way of approach to geometric resolution of the construction of free form shapes is its adaptation through developable surfaces. shelden calls them paper surfaces, but pottmann talks about the generation of planar quad meshes from families of conjugate curves of principal curvature. this type of planar or developable elements is used because of the simplification and economy that it offers in regard to economic and constructive aspects. a developable surface can be constructed from a planar surface, the other way round a developable surface can be deployed on a plane without any crease or distortion. this question is decisive in manufacturing any element within any constructive system, like a front facade panel or the hull of a ship. from the geometric point of view there are many aspects that define a developable surface. one of them is its gaussian curvature. while a developable surface is characterized by having zero gaussian curvatures, on a free surface any value equal to or different from zero gaussian curvature can be found, whether positive or negative. in this way the adaptation of any freeform surface passes through a process of geometric adaptation, where the degree of restrictions a developable surface must have, is superior to a freeform surface. we can say that a developable surface is undoubtedly less free, by being subject to important geometrical impositions. in his research h. pottmann has an amazing new focus and uses mainly a differential geometry basis to elaborate a complete approach to freeform surfaces resolution using developable surfaces, following a refinement process based among others on the use of a principal curve network, which is a special kind of conjugate curve network. this approach allows to obtain both quadrilateral planar facets and developable strips (pottmann, schiftner, bo, schmiedhofer, wang, baldassini, et al., 2008). in the work of axel killian (2006) the use of genetic algorithms allows an interesting approach to the developable surface issue. the process of adaptation and transformation of a set of strips over a surface supposes a new way to focus on geometric questions. using an adaptive genoma, an algorithm is built to give a geometric solution through a numerical way. j. glymph (2004) uses a geometric strategy for generating quadrilateral planar facets. from a constructive point of view this approach is of great interest. this method incorporates basic geometric principles into a parametric framework. so the obtained form is as result of a predefined strategy not an adaptation of any shape. the aforementioned methods raise and solve similar aims that have been proposed. the method developed is inspired by all of them in some way. however what is proposed hereafter is not an approach based on differential geometry or genetic algorithms. furthermore reproducing any freeform shape accurately is one of the principal aims of the method. so it is not possible to use a predefined strategy to obtain the form. this is not a form-finding method. in a similar way as is proposed by glymph (glymph, shelden, ceccato, mussel, & schober, 2004), the method is based on a classical geometrical basis, mainly on projective geometry expositions as shown by izquierdo asensi (1999). although the method uses a specific kind of conjugate curve network, there is not a differential refinement as in the one pottman (2007) proposes. it is shown below that apparent contour curves have a conjugate curve network and they have many properties that can be used. actually these curve networks are quite less regular than principal curve networks but an infinite set of different apparent contour curve networks could be obtained in return. in addition, as will be seen, the use of this specific kind of curves, such as the apparent contour ones, as a basis for the design of developable surfaces, allows to get results that confer a very particular image, and keep up a special link between the perception and the point of view from which the contours are drawn, in other words from which the forms are perceived. f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets 61 furthermore, the use of a generative algorithm, using grasshopper (grasshopper, n.d.), has allowed to transliterate the geometric basis as we used to do previously in a projective way. so it has been possible to check and explore the possibilities of these approaches, which could not have been done otherwise. we have developed the basis of a method of adaptation of double curvature surfaces following the proposed philosophy, trying to respond to both sets of adaptation, as well through the use of developable strips as through planar facets. 2. geometric foundations when a surface is drawn using any system of representation, whether by hand on paper or by rendering a computer model, it is necessary to define a curve that marks the edge of the represented object. this curve separates the part of the object the observer is able to see, from the part that is hidden. otherwise it divides the illuminated surface area from the one that remains in the shadow. this is basically the property of the apparent contour curve, also known as separatrix on that surface. in order to get the apparent contour, an auxiliary projecting surface is used, wrapping the object on the curve that is wanted to be obtained from a defined point of view. this surface is tangent to the object along a series of points that define its apparent contours. it is generated by straight lines or projection rays, starting from the selected point of view they are touching the surface tangentially. these straight lines of projection are the generatrices of a developable surface whether a cone or a cylinder. the theoretical concept is very simple, however obtaining these apparent contours is complex enough, especially when the object shapes move away from simple primitive shapes. even on primitive forms, obtaining boundary lines of shadow or separatrices, which is a common application of the apparent contours related to projective geometry, is laborious when it is carried out manually. using computer-aided design software facilitates and improves this work by providing powerful tools in this regard. on any surface several apparent contour curves are generated through the displacement of any point of view along any spatial trajectory. here, straight paths, either proper (fig. 1) or improper ones, are used (fig. 2). these apparent contours are generated as tangency between projecting tangent surfaces and basic surface. in this way we wrap the primitive surface with a series of tangent developable surfaces, which are supported on the surface along the apparent contour and intersect each other two by two along an outer curve to the surface. as a result projecting tangential surfaces are delimited by two curves being external to the primitive surface. these curves are intersections between two by two projecting adjacent surfaces. besides they contain obviously the proper tangent curve that is the apparent contour. in this way a series of strips are obtained wrapping the original surface externally. these strips are absolutely developable, since they are obtained from the projecting tangent surface that defines the apparent contour. its boundaries are determined by two tangent developable surfaces, and the width of the strip in turn by the distance that separates points of projection from which the tangent surfaces are obtained and with them the apparent contours. the extent of this document does not allow an exhaustive explanation of geometric principles. this process and the details of the geometric basis are profusely detailed in previous works (gonzález quintial, & sánchez parandiet, 2012). 62 f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets fig. 1. contours obtained using proper straight trajectory, projecting conical surfaces. fig. 2. contours obtained from improper straight trajectory. projecting cylindrical surfaces. 3. graphical and physical adaptation of double curvature surfaces using developable strips this paragraph is about obtaining developable strips that fit freeform surfaces. the problem is the adaptation of this kind of surfaces on which the curvature is not uniform and doesn’t have zero f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets 63 value. otherwise, this is the usual situation on complex surfaces or freeform shapes. the success of the strategy focuses on the achievement of an appropriate projection path, which could be satisfactory in terms of setting the apparent contour curves which facilitate ultimately the construction of developable strips. with the chosen path apparent contours are obtained. it does not depend on any quality of the surface, but it depends particularly on the point of view. there could be an infinite number of apparent contours on a surface, since an object can be observed from an infinite number of positions. the observer, the point of view, is what determines the curve over the surface. the method takes advantage of this flexibility allowing to be related to a mechanism that is subjective, like the human visual perception process. geometric resolution of the surface has much to do with appearance or the way one is looking. the process is applied in an empirical way, aided by a system of tracing apparent contour curves, which is built using a generative algorithm in grasshopper. in this way it is possible to probe multiple options using the same primitive freeform shape; if appropriate, the best set will be chosen, meaning the path that enables to obtain the optimal contour set and consequently allows to ensure a more favorable set of curves which could be used to fit the developable strips. the adaptation of a freeform surface, where the variation of the curvature is noticeable, is shown in the image (fig. 3). in this case an improper straight trajectory of projection is used, resulting in apparent contours from tangent surfaces of cylindrical projection. a set of apparent contour outlines should be selected from points of view at the infinite. here a regular division of the path is defined, along an improper path. from these set of points of view the corresponding set of tangent projecting cylindrical surfaces is generated. the extraction of the patch bounded by the intersections between cylinders allows to obtain a cylindrical developable strip that fits the surface. once the graphical way of the surface of double curvature adaption is solved, the checking of the validity of the method must be done by transferring the geometric resolution results to physical models. several cases have been studied from both graphical and physical points of view through the construction of prototypes using different types of light materials. we are talking about prototypes, not models. on the prototypes and during their implementation process, the behaviour of materials could be anticipated in relation to the way in which they are arranged, and their physical interaction. the prototype has an added value in relation to a purely formal model. the results in terms of tolerances can be checked. furthermore the deformation and the adaptability in lightweight materials from which the prototypes are made up, anticipate somehow the behaviour of materials that will be used to build the final model. consequently, from the constructive point of view, a key feature of the developable surfaces from which the most useful application is obtained, is that theoretically they can be deployed on a plane. as said initially they can be developed without creases or folds, on a flat surface. conversely, a developable surface can be constructed from a plane. it means that by using any kind of flat or planar constructive element a developable surface can be built. because the strips obtained are developable surfaces, it is possible to construct these prototypes using light laminar materials. in other words, it is possible to transfer flat strips unfolding directly, using a cutting machine. as can be seen in the images (fig. 3), a high fidelity degree in the physical reproduction of the model have been achieved. 64 f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets fig. 3. development of freeform surface. application of the method on a freeform surface by a straight improper trajectory. physical model made applying the process. 4. graphical and physical adaptation of double curvature surfaces using planar quadrilateral facets the adaptation method of double curvature freeform surfaces through the use of flat panels or quadrilateral facets allows us to generate a mesh, wrapping the surface with flat elements, in order to use this one for the primitive shape construction. this process could be carried out not only using materials that can be curved along one direction, but also using flat rigid materials which do not allow to be curved or bent; these processes imply important extra charges, like glass, otherwise profusely employed, whose process of folding or bending raises its price considerably. up to now the apparent contours used to obtain developable strips have been obtained by using points of view located along straight proper or improper lines. the reason is not only a simplification. of course, using points along straight lines constitutes a better way to order, control and systematize f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets 65 fig. 4. generation of apparent contours and conjugated curves using a proper straight path. the sets of apparent contour curves. it is mainly due to a geometric strategy. this is caused by the fact that if any straight trajectory is used, already being a proper or an improper line, the intersections of the sheaf of planes which are cut along the trajectory and the basic surface (fig. 4) make up into a group of conjugated curves to the apparent contours set, which were obtained on the original surface using the aforementioned path. they become two families of conjugated curves, thus acquiring all of their properties. in this way, whatever the conjugated curves networks are, if any curve from the network is taken and the tangent is calculated to the other family curves at each one of the cutting points, these tangents form a developable surface along each of the network curves. reciprocally this is true as well. therefore, in our case, by using an apparent contour curve it is possible to calculate their tangents at each of the cutting points with the planar curves of the conjugated curves family. furthermore, due to their conjugacy property, these ones form a developable surface. in this specific case the tangents are the generatrices of the projection surface. the projecting surfaces, cone and cylinder, are obviously developable surfaces and both are tangents along the apparent contour curve. in the case of the planar sections, defined by cutting the sheaf of planes and the original surfaces, the result is not as evident, but it can be proved easily. in this way, on one hand it is possible to obtain in an easy way the families of conjugated curves when the paths through which they are traced are straight, and on the other hand it is possible to take advantage of several properties that are very useful in practice. when the path is a proper straight line, the sheaf of planes is easy and directly determinated, using the path as intersection of this sheaf. in the other 66 f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets fig. 5. generation of apparent contours and conjugated curves using an improper straight path. case, if an improper line is used, the sheaf of planes cuts at the infinite and a set of parallel planes could be used (fig. 5). these properties are profusely used along the method development, due to the fact that from the constructive point of view it is particularly important to have a quadrangular network which could be used on one hand to trace two sets of interwoven strips on the surface and on the other hand, using the same network, it is also possible to get a reticle or mesh whose inner spaces constitute fig. 6. conjugated curves families over a freeform surface. in red, apparent contour curves obtained by using several paths; in blue, planar sections under the original surface and the sheaf of planes that contains each trajectory. f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets 67 fig. 7. planar quadrilateral envelopment derived from the mesh under the conjugated curves network. planar quadrilateral facets. the less coarse the mesh is, the better approximation to the surface. in this way the general application of the method should be refined on special zones of the surfaces that are adapted (figs. 6, 7). in general terms, it is possible to affirm that using a conjugated curves network, obtained from a set of apparent contours, the adaptation of a freeform surface could be carried out through the use of planar rigid materials, like for example laminated glass. fig. 8. graphic resolution of a surface that is not continuous. contour curves arrangement, planar conjugated sections supporting principal structure and approaching to the envelopments using planar quadrilateral facets. 68 f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets finally, in addition, having a set of planar curves supposes an appropriate starting point of the construction development process of the surface structure and of the approaching to the structural framework. so, the use of planar frames allows for the possibility to utilize structural linear profiles or those that are curved only in their own geometric plane. as a result, from the economic point of view it supposes a much easier fabrication process than methods by which curving is needed of the profiles forming the framework. fig. 9. adaptation of a variable double curvature surface using complementary strategies articulated through contour lines and their planar conjugated curves. f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets 69 in this way as well, both the waterproof envelope as the structural one adapt the geometric shape faithfully. besides they establish a symbiotic link. so the geometrical approach to the structure and the one used in the construction of the envelop are the same (fig. 8). 5. conclusion in conclusion, adaptation is a procedure that implies an alteration of the original shape, so it is necessary to find a balance between constructive economy and fidelity in the reproduction of the form. based on these premises the theoretical geometric method of adaptation of double curvature surfaces has been formulated, using developable strip surfaces as well as quadrilateral planar facets. the method pretends to be suitable for use over any kind of free form surfaces construction. the formulation of a geometrical method is shown; as we have seen it offers satisfactory results, being suitable for the objectives indicated at the beginning of the paper: graphic adaptation of form and its consequent development and construction, prototypes as well as finished architectural elements. there are important add-on aspects about the development. the extent of this paper does not allow to show them entirely. first, it is noteworthy to mention the strategy developed to give continuity to the contours on surfaces which are not continuous, i.e. cutting surfaces and tangents, where the contours, due to their own geometric principles, lose their continuity. the flexibility of the layout of contours allows to combine contours, splicing separatrix curves obtained from different points of view, by making a path that chains the contour curves, using their intersections along the secant curve to obtain the tangent planes over the adjoining surface. each one of the complete contours set obtained over the total surface are not produced using the same point of view, but all points of view are located in the same straight trajectory. the result is a family of contours with a remarkable order and continuity on the set of all the sub-surfaces, as can be seen in figure 8. besides, the resolution of the surface in figure 9 has required some additional strategies consisting of the mix of several theoretical aspects about developable surfaces. in this way it is possible to improve some areas of weakness where the exclusive use of contours do not allow an easy way to build the surface. the next step in the course of the research is to extrapolate a full-scale construction of these theoretical issues. the method is versatile enough. for example, the construction of the structure and the envelopment of the facade could be considered at the same time, using in that way the same geometric concept. thus a natural interrelation between both elements is possible. in a certain way these results could have been achieved through the consequences extracted from the prototypes, although it is necessary to check the results in large-scale or one to one scale. references glymph, j., shelden, d., ceccato, c., mussel, j., & schober, h. (2004). a parametric strategy for free-form glass structures using quadrilateral planar facets. automation in construction, 13(2), 187-202. gonzález-quintial, f. (2012). método de adaptación de formas de doble curvatura mediante superficies desarrollables. doctoral dissertation, upv-ehu. gonzález-quintial, f., & sánchez parandiet, a. (2013). freeform surfaces adaptation through developable surfaces using apparent contours. global design and local materialization. proceedings caad futures (2013), shanghai, china, 358-367. grasshopper (n.d.). algorithmic modelling for rhino. retrieved april 26, 2015, from http://www.grasshopper3d.com/ izquierdo asensi, f. (1980). geometrı́a descriptiva superior y aplicada. madrid: dossat. http://www.grasshopper3d.com/ 70 f. gonzalez-quintial et al. / freeform surfaces adaptation using developable strips and planar quadrilateral facets izquierdo asensi, f. (1995). geometrı́a descriptiva. madrid: paraninfo. kilian, a. embodied intelligence. final project. using genetic algorithms to generate developable strips from free formed surfaces. overview of projects. (n.d.). retrieved april 26, 2015, from http://www.designexplorer.net/ pottmann, h., & bentley, d. (2007). architectural geometry. exton, pa: bentley institute press. pottmann, h., schiftner, a., bo, p., schmiedhofer, h., wang, w., baldassini, n., et al. (2008). freeform surfaces from single curved panels. acm trans graph, 27(3), 1. shelden, d. (2002). digital surface representation and the constructability of gehry’s architecture. doctoral dissertation, massachusetts institute of technology, department of architecture. http://www.designexplorer.net/ from city’s station to station city 105 journal of facade design & engineering volume 10 / number 1 / 2022 process automation to improve the building engineering design analysis of nonrepetitive façade geometries jacopo montali * 1, thomas henriksen * 1 * corresponding authors, jacopo@henriksenstudio.com, thomas@henriksenstudio.com 1 henriksen studio ltd abstract this paper evaluates how parts of the building engineering design processes can be automated using software automation, with a focus on the analysis of thermal bridges in façades. reduced repetition in façade design requires the automation of routine tasks that would otherwise be performed manually. because full software automation is seldom achievable, a preliminary decision-making process that considers both the effort to create automation and the benefit to exploit it is required. a methodology is presented to achieve beneficial trade-offs between effort and benefits, by using heuristic knowledge. the knowledge was gathered by interviews with façade professionals. the methodology was tested on two case studies based on the analysis of the expected thermal bridge heat loss of two large-scale and low-repetition buildings. the results of the automated process described in the methodology were compared against information obtained from traditional approach, where the engineer/consultant performs each individual task manually. the results shows that the introduction of automation leads to time savings of 44%, if compared to the manual approach. keywords façade design, process automation, thermal bridges, building physics, heuristic knowledge, software development doi http://doi.org/10.47982/jfde.2022.1.05 106 journal of facade design & engineering volume 10 / number 1 / 2022 107 journal of facade design & engineering volume 10 / number 1 / 2022 1 introduction façade engineering requires many routine tasks to be performed on façade types or interfaces, especially when repetition in the project is minimal. with architectural expression growing in ambition, and with increasing performance requirements that need to be coordinated with many different engineering disciplines, this aspect becomes a source of risk due to the lack of precision in the expected performance, requiring analysis of the most critical parts of the project and leading to product over-engineering and, therefore, less sustainable solutions. façade design automation is a current topic of interest in the sector as it provides ways of streamlining the design of complex systems, while capturing the underlying mechanisms that govern the design of the product (henriksen et al., 2016; montali et al., 2017). the paradigm is fostered by recent advances in software programming languages, propelled in turn by the growth of data and advances in web technologies in the last decades. an opportunity therefore emerges in combining software automation and façade engineering to achieve a better understanding of the expected performance when designing non-repetitive façades. the traditional use of software via graphical user interface (gui), requiring manual mouse and keyboard interaction on every single task to be performed, is currently being replaced by more sophisticated software applications based on programming interfaces (api). commercially available, off-the-shelf software applications like rhinoceros (robert mcneel & associates, 2021) or revit (“revit,” 2021) nowadays provide users with access to the same gui objects via api that can be controlled via commonly-available programming languages (such as python or c#). such api normally extend the functionality the software’s gui. excluding human labour from repetitive tasks, while leaving a supervising role to people who would otherwise perform the tasks in question, simplifies processes and provides opportunities for increasing productivity and creativity. to this end, research has shown that there is no clear solution on how to manage repetitive tasks manually as they prove intrinsically challenging. for instance, it has been shown that interruptions in repetitive tasks provide benefits (back et al., 2010). but, if exceedingly short and unwanted, they can reduce productivity and lead to more human errors (altmann et al., 2014). it has also been shown that a key role is played by how the gui is built and its level of user-friendliness (tsai & byrne, 2007)computer-based routine procedural tasks involving execution of multiple steps. differences were found in error frequencies at particular steps between the three tasks, a result that is consistent with predictions derived from altmann and trafton’s (2002. as israelian nobel prize winner daniel kahneman would say: “humans are noisy”, hinting that the same person will produce different outputs (some of which incorrect) by performing the same task repetitively. computer automation thus allows for more time to be spent on value-adding and less error-prone activities, and partly addresses the construction sector “productivity imperative” (mckinsey & company, 2015). in this paper, a method is presented to show how to implement software solutions for task automation. the method builds upon existing literature in the field of knowledge management (milton, 2007), as well as optimization techniques (ashby, 2011) that prioritise which tasks to automate first in order to minimise the initial cost of software implementation. a case study of the total heat loss calculation through the envelope for two uk-based, large-scale passivehaus projects will be used to demonstrate the validity of the proposed method. 108 journal of facade design & engineering volume 10 / number 1 / 2022 the choice of the heat loss through the building envelope for energy-efficient buildings as a case study is driven by recent international ambitions to tackle the energy crisis. it is estimated that the incorrect incorporation of thermal bridges can lead to an underestimation of the heating peak power by 29% and of the annual energy demand by 37% in passive houses (berggren & wall, 2011). furthermore, the adoption of a fixed percentage increasing the centre-of-pane building envelope thermal loss to account for thermal bridges was proven not suitable in cold climates (berggren & wall, 2013). therefore, a more in-depth analysis of the actual thermal bridging is required, and automation can be a valuable instrument in this regard. the present study is structured as follows. first, the methods will be shown, along with a description of the two case-study projects. the paper will follow with the application of the method to the above-mentioned projects, while it will end with discussions of the results and conclusions and final remarks. 2 methods the method is based on the technique described in milton (2007), and adapted by selecting specific steps (in particular step numbers 7, 28, 32, 33) to reduce the time required to complete and to streamline the automation process. an overview of the method is shown in figure 1 in a bpmn (“bpmn,” 2017) view. fig. 1 bpmn view of the method adopted. the method consists in the following tasks: 1 map the process to be automated this task requires to create a detailed process map showing each individual task required. each task is a detailed and individual operation that is normally performed manually and whose automation will be evaluated. see figure 3 in the results section for an example. bpmn (“bpmn,” 2017) can be used as it is a widely used formal process mapping language, although any process mapping style is valid. 109 journal of facade design & engineering volume 10 / number 1 / 2022 2 create a scoping diagram the automation of each task from the process map created at the step above is then evaluated in terms of benefits and cost. for this purpose, two tables are produced (example in table 1): one addressing the benefits and another one the cost of automating each task (table rows). each task is evaluated against a series of topics (table columns), which depend on the project and may differ between the two tables. the evaluation score could be a number ranging from 1 to 5, where 1=lowest score and 5=highest score. the topics are weighted in case the relative importance of topics differs. an example of a topic for evaluating costs can be “time required”, while an example for evaluating benefits could be “increased quality”. each task is then given a weighted average, calculated as a linear combination of the scores and the weightings. the weighted average represents the overall cost or benefit in automating a specific task. see table 3 and table 4 in the results section for a practical example. table 1 example of table for capturing cost or benefits in automating a specific task topic 1 topic 2 weighted average weighting 2 1 task 1 1 1 3.0 task 2 2 2 6.0 task 3 3 3 9.0 the evaluation scores in each diagram are normally based on heuristics. the matrices can be completed by one or more expert people. if more than one person is required to complete the assessment, either individual tasks (rows) or aspects (columns) are assigned to each person. in the case of a multitude of people contributing to a single task/topic entry, a simple average can be used. once the two tables are prepared, each task is prioritised via a scoping diagram (figure 2). given that each task presents two weighted scores, a diagram (“scoping diagram”, named after the “scoping matrix” in milton (2007) can be drawn to prioritise the tasks to be automated. as the diagram presents benefits and costs on the axes, a point in the diagram identifies a task. the optimal tasks, i.e., tasks laying on the pareto front (sawaragi et al., 1985), are those that have the priority in terms of implementation as they improve in both cost and benefit at least another task. for this reason, optimal tasks are also referred to as “non-dominated”, while the remaining are defined “dominated” (sawaragi et al., 1985). fig. 2 scoping diagram showing which tasks shall be automated first 110 journal of facade design & engineering volume 10 / number 1 / 2022 3 implement software solutions to automate tasks the third step consists of an iterative process where the most urgent task from the scoping diagram is automatised first via software implementation. given that all tasks on the pareto front have the same level of priority, an expert judgment may be required. depending on the inclination of the front, it is possible to start with the task with the lowest cost (nearly vertical front) or largest benefit (nearly horizontal front). alternatively, it is possible to transform the problem from multi-objective to single-objective via a penalty function approach, by determining the appropriate exchange coefficient, if possible (ashby, 2011). in this case, the most urgent task is the task with the minimum penalty function value. once the task is chosen, both the time required to implement the software solution and an assessment of the time required to manually perform the task are produced. if the assessment proves that the automation of the task increases productivity (e.g., by showing that the time required to implement the software and run it is lower than what measured in a manual approach) or quality (e.g., by counting the number of typos/errors in the final output), then the team marks the step as completed and proceeds with a second iteration. the second iteration will automate the second most urgent item between those on the pareto front in the scoping diagram. 3 data the data used in the results section are taken from two real-world projects currently under development in london, uk. both projects present miscellaneous end uses, such as student accommodation, office, and retail and are pursuing the passivhaus certification. fig. 3 render images of the two projects used to test the automated process. courtesy of alford hall monaghan morris, apt and urbanest. the two projects are characterized by a series of tower blocks and a podium area at ground level. the tower blocks are mainly student accommodations, while the podiums present a mixed use. both projects present two additional levels below ground. the total gfa area of the projects is approximately 25000 m2 and 60000 m2. 111 journal of facade design & engineering volume 10 / number 1 / 2022 the passivhaus certification required in-depth evaluation of the total heat loss through the building, including thermal bridging. all thermal bridges related to the curtain wall façade were excluded in the present analysis as analysed separately in the u cw , as per bs en 12631. therefore, only thermal bridges at roof and basement levels were considered in this study, including those at the interface between roofs/slabs and curtain walls (a.k.a. non-façade thermal bridge types). the number of nonfaçade thermal bridge types identified in the two projects and analysed in this paper was 77 and 109. 4 results 4.1 map the process to be automated the first step comprised mapping the process by defining all individual tasks. in this case, the process consisted of automating the calculation of the lengths and number of thermal bridges for the two projects investigated. this process was deemed to contain repetitive and error-prone activities as it required the determination of the length and count of a large number of different types of thermal bridges. for this reason, the process was first performed manually for a few representative examples and then represented in a process map as shown in figure 4. fig. 4 bpmn process map for determining all thermal bridges lengths and number. the individual tasks of the process were: import general arrangements in rhino: in this step, the architectural plans in cad format were retrieved and imported in rhinoceros. the process consisted in using the “import…” function for each general arrangement imported. identify thermal bridge types: all possible thermal bridges were identified by inspecting the above general arrangements. when a new thermal bridge type was identified, a new layer in rhino was generated with the following naming convention: xxx-yyy-l00-tb123-0_0_0_0_0-description-h123-v123 where the hyphen-separated fields are: xxx is the project code yyy is an additional descriptor field l00 identifies the project level at which the thermal bridge refers 112 journal of facade design & engineering volume 10 / number 1 / 2022 tb123 is the thermal bridge code where: – tb identifies the thermal bridge type and can be pt (point), lh (linear horizontal), lv (linear vertical) – 123 is a progressive number for numbering the thermal bridge 0_0_0_0_0 is the ratio of the thermal bridge heat loss to a specific project zone/area. for instance, a 0.1_0_0.9 code assigns 10% of the heat loss to the first project zone, zero to the second, and 90% to the third. this field is useful in case of projects requiring the total heat loss calculation for different zones/areas of the same building, such as in passivhaus certifications. description is a natural language description of the thermal bridge in question h123 is a mandatory field for lv thermal bridge types, representing only their lengths, as this is not captured in plan views, where 123 is the thermal bridge length in metres. v123 is the value of the thermal bridge loss, where 123 is either in w/k for point thermal bridges (χ) or w/m2k otherwise (φ) – draw thermal bridges for each type: each of the identified thermal bridges were drawn in rhinoceros as either points (for “pt” or “lv” types) or lines (for “lh” for linear horizontal types) – compute the quantity: per each thermal bridge type, the total quantity is calculated by determining the number of point thermal bridges or the total length of linear thermal bridges. – generate output: the output consists of determining the total heat loss 𝑄𝑄![ " # ] f for each thermal bridge type i: 𝑄𝑄! = 𝜑𝜑! ∙ %𝑙𝑙",! 𝑄𝑄! = 𝜒𝜒! ∙ %𝑗𝑗 where ∑l j,i is the sum of all individual length of a specific linear thermal bridge type i and ∑j is the count of all point thermal bridges of type i, as calculated from the step above. the values of φ i and χ i are determined via separate fem modelling or tabular data. 4.2 create a scoping diagram the purpose of this step is to assess the cost/benefit of automating the above tasks by creating a scoping diagram. for each task from the process map, two tables were created to independently assess benefits and cost. table 2 defines the benefits of automation and it was evaluated against two possible aspects: time savings and increased quality. in this instance, the former was given a weight of 2, while increased quality was assigned a weight of 1, indicating that the benefit of automation was expected to have more of an impact on productivity than quality, if compared to a manual approach. 113 journal of facade design & engineering volume 10 / number 1 / 2022 table 2 assessment of the benefits in automating each step time saving increased quality weighted average weighting 2 1 import general arrangements in rhino 1 1 1.0 identify thermal bridges types 2 2 2.0 draw thermal bridges for each type 3 3 3.0 compute the quantity 3 1 2.3 generate output 3 1 2.3 similarly, table 3 was used to assess the cost of automating each task from the process map. two cost aspects were analysed: time required to automate i.e., time to produce code, and cost in terms of additional specialist people i.e., the task requires advanced techniques to be used, such as machine learning. table 3 assessment of the cost of automating each step time required to produce code specialist people required weighted average weighting 2 1 import general arrangements in rhino 1 1 1.0 identify thermal bridges types 5 3 4.3 draw thermal bridges for each type 5 2 4.0 compute the quantity 1 1 1.0 generate output 2 1 1.7 in this case, both tables were completed by experts in the field. all tasks are then given a weighted average both in terms of cost and benefit, which were represented in the scoping diagram show in figure 5. the scoping diagram evaluates all individual tasks (triangles) against their cost/benefit, with the best tasks to automate being the ones that have lower costs and greater benefits. fig. 5 scoping diagram (milton, 2007) for the investigated process. the scoping diagram evaluates all individual tasks (triangles) against their cost/benefit, with the best tasks to automate being the ones which have lower costs and larger benefits. 114 journal of facade design & engineering volume 10 / number 1 / 2022 4.3 implement software solutions to automate tasks this step is an iterative as it requires the identification of the most urgent tasks from the process map at step 1). the iteration is terminated when there is no cost/benefit opportunity or if, after having implemented a specific task, measured benefits don’t meet the expected results. the most urgent item on the scoping diagram is chosen from the subset of optimal tasks, i.e., those lying on the pareto front. in this case, there are three out of five tasks that are optimal and they are, in ascending order of cost of automation: “compute the quantity”, “generate output”, and “identify thermal bridge types”. in this case, the task with the lowest cost of automation was chosen, as all three pareto front tasks seemed to have similar values in terms of benefit (vertical pareto front). iteration 1 the “compute the quantity” task was chosen to be the most urgent as it was the one with the lowest cost of those in the pareto front. the task was implemented via custom c# scripting by creating dedicated classes of objects that represented the thermal bridges (figure 6). note that the naming convention of the rhinoceros layers containing thermal bridges of the same type can be interpreted as a serialised version of the objects in figure 6, in that it includes all object properties except for the read-only properties “totalquantity” and “heatloss”. the former is calculated as the sum of all thermal bridge quantities (either number or length), while the latter is equal to the product between the two properties value and totalquantity. there is also an array of thermal bridges quantities “quantities” which is excluded from the layer-naming convention as it is a property that gets generated at runtime and serves to determine the “totalquantity”. fig. 6 uml class diagram representing the taxonomy of the thermal bridges in the software application. the benefits were measured by comparing the time to perform the task manually with the time to implement the software solution and run it. for this task, an average of 15s per element to be measured (if a rhino point, adding it to the count, whereas if a rhinoceros line, by reading the length and reporting it onto an excel spreadsheet, including a 10 minute break every hour of work) was determined. the time required to automate the task was measured to be 4 hours, while the time to run it required approximately 1 ms per task. for the two projects, the total number of rhinoceros elements was 1756. figure 7 represents the obtained results, showing that the time required to complete the task manually is 440 minutes. as task automation required overall 240 minutes (time to run the automation was negligible in this case), the increase in productivity was positive and it was thus possible to move to the next task automation. 115 journal of facade design & engineering volume 10 / number 1 / 2022 fig. 7 measured benefits from the automation of the task “compute the quantity”. iteration 2 the second iteration involved the automation of the “generate output” task. due to project requirements, it was necessary to both report the total heat loss per each individual thermal bridge and their location in plan view. as the number of identified thermal bridge types was larger than 100, a plan view of each thermal bridge and their location was required. an example of the expected output is shown in figure 8. fig. 8 expected output for the two projects investigated. left: summary of total heat loss per each thermal bridge. right: example of the location of a thermal bridge in plan view (courtesy of allford hall monaghan morris). 116 journal of facade design & engineering volume 10 / number 1 / 2022 building upon the code from iteration 1 and the classes shown in figure 5, first an extension of the c# code was made to export results into an excel table automatically. this code was used to generate the clustered bar chart on the left of figure 9. then, additional code was produced to automatically take screenshots of each thermal bridge and the rhinoceros entities representing coloured in red, along with the project general arrangements shown in the background in a lighter colour, as shown on the right of figure 9. once the code was implemented, results were measured and compared. it was measured an average of 5 minutes per thermal bridge, if the task were to be performed manually. this value considers the act of turning all rhinoceros layers off except those identifying the thermal bridge and the background general arrangements, setting the right layer colours and including a 10-minute break every hour of work. the time to create and use the automation was measured to be 240min and 1ms/task, respectively. fig. 9 measured benefits from the automation of the task “generate output”. the measured benefits arising from automating this task are shown in figure 6. while the manual approach would require approximately 880 minutes of work, the automated task would slightly exceed 240 minutes. iteration 3 the third iteration would involve the automation of the “draw thermal bridges for each type” task. this would mean automating a task with a software solution capable of recognizing complex concepts such as “building”, “thermal bridge” and even more abstract ones, such as representing a thermal bridge with a line on a building’s general arrangement plan view. while these concepts would be quite elementary to be performed manually by an experienced professional, it is harder to embed them into a software application such as an artificial intelligent agent via, for instance, machine learning. for this reason, the iteration was stopped and the rest of the tasks were performed manually. 117 journal of facade design & engineering volume 10 / number 1 / 2022 5 discussion the proposed method was used to automate part of the calculation of the total heat loss through the building envelope on two large-scale uk projects. the method was proven to produce a series of results that increased productivity. first, it demonstrated how to identify bottlenecks in a manual engineering task. for the two projects analysed, these were the tasks associated with counting the number and length of thermal bridges, generating the output, and identifying thermal bridge types on architectural drawings. second, the time required to perform a large number of repetitive operations was significantly reduced. by comparing the time required to perform the two tasks requiring automation, a 63% reduction was measured (figure 10). overall, the time required to perform all tasks from figure 3 was calculated to be 1893 minutes for the fully manual approach and 1054 minutes for the semi-automated route, thus leading to a global 44% reduction in time (figure 11). fig. 10 time (in minutes) to perform “compute the quantity” and “generate output” tasks. fig. 11 time (in minutes) to perform all tasks. lastly, given the repetitiveness of the tasks being automated, there is an expectation that quality would be increased. it is envisaged that human repetition may lead to random errors, such as incorrect reporting of thermal bridges lengths/counts onto an excel spreadsheet or when generating graphical output. while quality may prove challenging to measure and/or assess, the authors believe that automation of repetitive and easy-to-automate tasks produces high-quality results. producing experiments where people are required to complete a repetitive task, to be then reviewed in terms of number of errors against what is produced by a machine, may be a possible route. as these types of experiments fall outside of the expertise of the authors, further research in this field by design engineers and psychologists is required. the assessment of the quality in automation is deemed to be both a point in favour and a limitation of this study. other limitations consist of the inability to automate tasks requiring complex reasoning, such as the above-mentioned “draw thermal bridges for each type” task. novel computer science techniques, appropriately implemented in off-the-shelf software libraries, are required by construction professionals. 118 journal of facade design & engineering volume 10 / number 1 / 2022 6 conclusion the present paper has proposed a method to analyse and prioritise manual tasks in a typical façade engineering process for successive software process automation. the method consisted of three major tasks, requiring process mapping, the subsequent evaluation of the tasks to be automated with the largest priority, and a final part where automation is implemented iteratively on each task. the method was applied concurrently to two large-scale uk projects for the assessment of the total heat loss through the building envelope via a large number of thermal bridges. results have shown that the time to complete the engineering tasks can be reduced up to 44%, if appropriate automation on the most urgent tasks is implemented. the tasks that were automated in this study suggested that the more repetitive the task, the higher the likelihood of running into errors when performing it manually. this aspect is as crucial as it is complex to demonstrate and assess in advance. further research requires methods to assess the expected increase in quality, as this aspect plays a fundamental role in increasing productivity in the engineering design of façades. references altmann, e. m., trafton, j. g., & hambrick, d. z. (2014). momentary interruptions can derail the train of thought. journal of experimental psychology: general, 143(1), 215–226. https://doi.org/10.1037/a0030986 ashby, m. f. (2011). materials selection in mechanical design. back, j., brumby, d. p., & cox, a. l. (2010). locked-out: investigating the effectiveness of system lockouts to reduce errors in routine tasks. conference on human factors in computing systems proceedings, 3775–3780. https://doi. org/10.1145/1753846.1754054 berggren, b., & wall, m. (2011). thermal bridges in passive houses and nearly zero-energy buildings. in 4th nordic passive house conference, 2011. berggren, b., & wall, m. (2013). calculation of thermal bridges in (nordic) building envelopes risk of performance failure due to inconsistent use of methodology. energy and buildings, 65, 331–339. https://doi.org/10.1016/j.enbuild.2013.06.021 bpmn. (2017). retrieved from http://www.bpmn.org/ henriksen, t., lo, s., & knaack, u. (2016). a new method to advance complex geometry thin-walled glass fi bre reinforced concrete elements. journal of building engineering, 6, 243–251. https://doi.org/10.1016/j.jobe.2016.04.002 mckinsey & company. (2015). the construction productivity imperative. retrieved from www.mckinsey.com/industries/ capital-projects-and-infrastructure/our-insights/the-construction-productivity-imperative milton, n. r. (2007). knowledge acquisition in practice: a step-by-step guide (1st ed.). springer-verlag london. https://doi. org/10.1007/978-1-84628-861-6 montali, j., overend, m., pelken, p. m., & sauchelli, m. (2017). towards facades as make-to-order products the role of knowledge-based-engineering to support design. journal of facade design and engineering, 5(2). https://doi.org/10.7480/ jfde.2017.2.1744 revit. (2021). retrieved from https://www.autodesk.com/products/revit/overview robert mcneel & associates. (2021). rhinoceros webpage. retrieved from https://www.rhino3d.com/ sawaragi, y., nakayama, h., & tanino, t. (1985). theory of multi-objective optimization. academic press. tsai, j., & byrne, m. d. (2007). evaluating systematic error predictions in a routine procedural task. proceedings of the human factors and ergonomics society, 2, 817–821. https://doi.org/10.1177/154193120705101209 from city’s station to station city 155 journal of facade design & engineering volume 10 / number 1 / 2022 the acoustic and daylighting effects of external façade sun shading systems simone secchi * 1, patrizio fausti 2, gianfranco cellai 1, martina parente 1, andrea santoni 2, nicolò zuccherini martello 3 * corresponding author, simone.secchi@unifi.it 1 university of florence, department of architecture, italy 2 university of ferrara, department of engineering, italy 3 formerly university of ferrara, department of engineering, italy abstract external sun shading devices are increasingly used in sustainable buildings to reduce the greenhouse effect in the summer and the glare effect due to direct solar irradiation through transparent surfaces. the acoustic effects of these devices have been investigated in recent studies that suggest the possibility of optimising these elements to improve acoustic comfort in indoor environments, even with open windows. nevertheless, there are few studies that analyse the combined effect of these devices on acoustic attenuation and improved daylighting. in this paper, the results of acoustics and daylighting simulations are reported, considering different dimensions, distances of the louvres and orientations of the façade. the main results of previous works concerning the effect of lining the bottom side of each louvre with sound-absorbing material are also briefly summarised. the acoustic effects of different configurations of the louvres are evaluated in terms of insertion loss in the façade plane. for the lighting simulations (daylight factor, daylighting uniformity and daylight glare probability), the variation of the shielding effect is studied considering the spacing between the louvres and the orientation of the façade for different times and seasons for latitude in the south of europe. keywords façade, indoor comfort, light shelves, insertion loss, acoustic absorption, daylighting doi http://doi.org/10.47982/jfde.2022.1.07 156 journal of facade design & engineering volume 10 / number 1 / 2022 1 introduction solar shading systems installed on building façades are necessary to reduce the summer energy consumption of buildings, based on the technical standards en 13363 (en 13363-1, 2003) and en 14501 (en 14501, 2006), received within the energy performance buildings directive (epbd) released in 2002 and revised in 2010 (directive 2010/31/eu, 2010). these standards and the eu directive concern building construction techniques to achieve very high energy-efficient buildings, also known as nearly zero energy buildings (nzeb); buildings that significantly reduce their environmental footprint. solar shading devices can influence visual comfort (cellai, carletti, sciurpi, secchi, nannipieri & pierangioli, 2014) (ecbcs annex 29/shc task 21, 2010) negatively by diminishing daylight amount in inner spaces, but also positively by reducing glare effects and improving daylighting uniformity, especially with clear sky conditions. this paper evaluates the effect of variation in daylight availability in a specific period of the year in south europe. the threshold parameters that define the quality and quantity of natural lighting are regulated by national laws and calculated by international standards such as en 12464 (en 12464-1, 2021), en 14501 (en 14501, 2005) and en 17037 (en 17037, 2018). the assessment and management of environmental noise are also very important for the wellbeing of people (directive 2002/49/ec, 2002). poorly designed façade shading systems can lower the acoustic comfort and undermine the overall acoustic performance of the façade if the sound coming from the traffic is directed toward the windowpane. this negative effect can be suppressed and converted into a positive effect by proper louvre design that reflects the traffic noise away from the façade surface. the acoustic performances of building façades have been considered in several studies, which investigated multiple aspects such as their shape (busa, secchi & baldini, 2010), the presence of balconies (li, lui, lau & chan, 2003), the effects of a green envelope (van renterghem, hornikx, forssen & botteldooren, 2013), the influence of the windows (granzotto et al., 2017) and the flanking transmission (secchi, cellai, fausti, santoni & zuccherini, 2015). a comprehensive review related to the acoustic performances of building façades was recently published (hu, zayed & cheng, 2021), including studies on sound insulation, the effect of façade on street noise, and noise reduction techniques. on the other hand, the acoustic influence of façade shading systems has not been widely investigated. sakamoto and aoki (sakamoto & aoki, 2015) presented a numerical study to examine the sound reduction effects due to the presence of louvres mounted on the building façade, validating the results with experimental measurements on a 1:20 scale model. numerical analysis and a 1:1 scale model were also used by zuccherini et al. (zuccherini, fausti & secchi, 2016) and fausti et al. (fausti, secchi & zuccherini, 2019) to analyse variations in the acoustic sound pressure field on the building façade, considering different sound source positions and changing the configuration of the louvres and their tilt angle. further investigations of the acoustic effects of building façade shading systems, involving in-situ experimental measurements (sakamoto, lee, ishii, katayama, iwase & takahashi, 2017) (zuccherini, fausti, santoni & secchi, 2015) and evaluation of the related psychoacoustic effects (zuccherini, aletta, fausti, kang & secchi, 2019), highlighted that the louvres can reduce the sound pressure levels across the building façade and the magnitude perception of the noise. the most common indoor shading devices were experimentally analysed by catalina et al. (catalina, ene & biro, 2019), assessing their influence on both the improvement of the sound insulation performance and the reverberation time of the room. however, in both cases, results showed negligible differences between different shading device systems. 157 journal of facade design & engineering volume 10 / number 1 / 2022 at present, most studies deal with the effect of façade shape and shielding devices, with analyses referring only to acoustics or daylight, whereas the human reaction to sound and light stimuli are strongly correlated, as some studies show (buratti, belloni, merli & ricciardi, 2018) (huang, zhu, ouyang & cao, 2012) (hangzi, xiaoying, & yue, 2020). the main aim and the novelty of this work are, therefore, the use of a multi-domain approach to evaluate the acoustic and daylight effects given by the presence of an external shading device made of shading louvres. this study was carried out by means of both simulations and measurements in a real building and in a scale model realised in an acoustic laboratory. in the follow-up to this work, questionnaire-based investigations will make it possible to analyse the reciprocal influence between the daylighting and acoustic parameters currently described by independent parameters and limit values. a virtual mock-up building was designed as a reference for various evaluations. the building could be assimilated into an 8-storey office building with a flat façade. the shading system was simulated by evaluating various sizes of louvres and the spacing between them. acoustic and daylight simulations were carried out on the same geometries. the louvres’ tilt angle was not considered in this work for daylight simulations because it was assumed that shading devices with horizontally oriented louvres constitute the best compromise between acoustic and daylight comfort: previous works have shown that louvres tilted towards sound sources can increase the sound pressure level on building façade, offering, on the other hand, the best shading to the building itself (fausti et al., 2019) (zuccherini et al., 2019). acoustic performances have been evaluated as sound pressure level differences (insertion loss – il) on building façades, between a reference scenario, without shading devices on the façade, and with various shading systems. in previous works, a standard shading system was compared to an acoustically optimised one, having louvres with sound-absorbing properties on the bottom side (zuccherini et al., 2016) (fausti et al., 2019) (zuccherini et al., 2015) (zuccherini et al., 2019). the conclusions of these studies are assumed in the present work. the daylighting effect of external louvres is evaluated in many studies for their importance in the reduction of thermal loads but also for the improvement of visual well-being (technical report of iea shc task 50.c2, 2016) (technical report of iea shc task 61.c1, 2019) (ahmad, kumar, prakash & amana 2020) (carletti, cellai, pierangioli, sciurpi & secchi, 2017). several parameters have been introduced to assess the quantity, quality and glare of light (carlucci, causone, de rosa & pagliano, 2015). in this study, the daylighting effect is analysed with reference to three parameters, and the relevant results are compared with the corresponding recommended values. in particular, the amount of daylight is evaluated by means of the daylight factor (d), described in many national and international standards; the distribution of daylight is analysed with the daylighting uniformity (u), as defined in the standard en 12665 (en 12665, 2013); the daylighting glare effect is analysed by means of the daylight glare probability (dgp), described in the standard en 17037 (en 17037:2018) (galatioto & beccali, 2016). also, the useful daylight illuminance (udi) is an important parameter to analyse the probability of glare occurrence due to daylight. it is defined as the annual occurrence of illuminances across the work plane where all the illuminances are within the range 100 to 3000 lx; the degree to which udi is not achieved because illuminances exceed the upper limit is indicative of the potential for occupant discomfort due to glare (nabil & mardaljevic, 2005). in any case, as has been shown (mardaljevic, andersen, roy & christoffersen, 2012), there is a strong correlation between the useful daylight illuminance and the daylight glare probability used in this study. 158 journal of facade design & engineering volume 10 / number 1 / 2022 2 methodology 2.1 description of the case study the virtual mock-up considered in the present study is a 28 m tall building with 8 floors (ground floor + floors 1 to 7). each floor is 3.1 m high, having 0.4 m slabs dividing two consecutive floors. a road was set in front of the building façade, made up of two lanes (3.5 m each), lateral pavements/ sidewalks and cycling paths, a total of 6.5 m wide (figure 1). the sound source was set as a traffic lane, 8.25 m away from the building façade. fig. 1 left: section of the building façade. dimensions are in meters (m). each floor is 3.50 m high, including the thickness of the floor slab. the sound source s is placed at 8.25 meters from the building façade and 0.50 m from the ground, simulating the sound emission of a traffic lane. f1-7 indicates the floor level. fig. 2 right: axonometry of the inner space. dimensions in meters. the third dimension of the test room was only considered with daylight simulations. the inner space was simulated as a room 5 m wide, 4 m long and 3.1 m high (figure 2), with windows 4.6 m wide and 1.5 m high (34.5% of the floor surface). both the acoustic and the daylight simulations consider louvres 3 cm thick and variable between 20 / 30 / 40 cm in spacing. 2.2 acoustic simulations methodology the acoustic effects of external louvres were investigated with a commercial finite element (fe) solver (comsol® multiphysics), whose detailed description is reported in (fausti et al., 2019). this model was previously validated with reference to a 1:1 scale model analysed in a semi-anechoic chamber (zuccherini et al., 2016) (figure 3). the laboratory mock-up used in the cited work was characterised by tiltable louvres, sized 0.2 m x 2 m, 1.8 cm thick: the tilting possibility of the louvres helped study the variability of acoustic insertion loss (il) associated with the tilt angle of sun shading louvres. the il was calculated as the differences between the sound pressure level (spl) 159 journal of facade design & engineering volume 10 / number 1 / 2022 measured without the shading system (considered as the reference value) and the spl measured with the shading devices. the il was calculated first considering a traditional façade shading system (figure 3 (b)) and then adding sound-absorbing material on the bottom faces of each louvre in the shading system (figure 3 (c)). fig. 3 (a) empty floor of the semi-anechoic room simulating a flat building façade; (b) non-absorbing shading louvres; (c) sound absorbing louvres; (d) microphones used for measuring spl without the shading device; (e) microphones used for measuring spl with the shading devices. figure 4 shows measured values (en iso 10534-2, 2001) of the normal incidence sound absorption coefficient of the melamine foam used in the experiment. fig. 4 measured normal incidence sound absorption coefficient (α) of the material (expanded melamine foam) used in the mock-up. a 2d fe model, representing the experimental mock-up in a semi-infinite acoustic domain, was first validated with laboratory measurements and then expanded to further investigations, simulating urban situations (fausti et al. 2019). to exactly reproduce the laboratory condition, the louvre system was modelled with the same geometry as the mock-up, backed by perfectly reflecting boundary conditions representing the 160 journal of facade design & engineering volume 10 / number 1 / 2022 floor of the semi-anechoic chamber (as well as the glazing surface in real conditions). the acoustic fluid domain was modelled with a radius of 4.8 m and truncated by using perfectly matched layer (pml) elements to approximate the semi-infinite domain condition (figure 5 a). the louvres in the standard condition (bare wood material with no sound-absorbing layer) were approximated as rigid boundaries of the fluid domain. the condition representing louvres lined with a layer of sound-absorbing material was simulated with the poroacoustic built-in feature in comsol®, applied to sound-absorbing domains: a delany-bazley-miki (dbm) (delany & bazley, 1970) (miki, 1990) equivalent fluid model was used to replicate measured absorption properties. after the validation of the model, further fe analyses were carried out using an extended model, as shown in figure 5 b. the sound source, which reproduced the experimental configuration, was implemented as a monopole (omnidirectional) point source (5 pa) placed in the middle of the left lane, 0.5 m from the ground, 8.25 m from the building façade. although the section of the simulated road includes two traffic lanes, only one was simulated. the acoustic effects of the shading louvres were evaluated in terms of insertion loss (evaluation of the effects on a building façade facing the one with the shading louvres). further details on this modelling approach can be found in the reference (fausti et al., 2019). a b fig. 5 (a) bi-dimensional fem set-up reproducing the experimental set-up. (b) extended fem model of an entire 7-storey building façade facing a traffic lane at ground level; red dots represent virtual pressure sensors to measure the sound pressure level. 2.3 daylighting simulations methodology the daylighting parameters described in section 1 (d, u and dgp) were calculated with relux® (www.relux.com), a software to simulate artificial light and daylight according to standards en 12464-1:2021 and en 17037:2018, one of the most used and advanced lighting simulation tools for daylight and lighting calculation (maamari, fontoynont, adra, 2006) (bhavani & khan, 2011) (kaempf et al., 2016). relux® is validated (bouroussis, nikolaou & topalis, 2019) regarding the methodology described by the technical report cie 171:2010 (cie 171, 2010) that defines and proposes a set of several test cases to validate the calculations accuracy of a lighting simulation software. an indepth study of the validity of nine different software tools, including relux®, showed that they can 161 journal of facade design & engineering volume 10 / number 1 / 2022 simulate a standard room, also with louvres, with minimal percentage differences between them (iversenet al., 2013). in this work, simulations were performed with reference to the cie sky types 1 (overcast sky, for daylight factor) and 12 (clear sky, low turbidity, for daylighting uniformity and daylight glare probability). the investigated solutions assessed the ability of the software to simulate the influence of an obstruction, like an external horizontal louvre, on the diffuse reflection and on the internal direct illuminance. the analysed model was placed in a geographical location in the south of europe with the correct north orientation. the calculation method uses radiosity algorithms to evaluate the lighting characteristics in discrete points of the environment after dividing each surface into meshes with homogeneous photometric properties. radiosity algorithms require that all surfaces be ideal diffusers that follow lambert’s law. the following summarises the main lighting properties of the surfaces and the calculation parameters. – reflection factor of walls and ceiling in gypsum plaster, white matt: 80%. – reflection factor of floor: 40%. – reflection factor of external louvres: 80%. – transmission factor of window glasses: 80%. – position of the calculation surface: 0.75 m from the floor and 0.5 m from the walls. – position of the case study: florence, central italy (43° 46’ n 011°15’ e). – orientation of the case study: south and east (west façade in the morning is equal to east façade in the afternoon about the altitude of the sun; therefore, only the east façade has been evaluated). – sky conditions: standard cie overcast sky for daylight factor simulations; standard cie clear sky with sun and overcast sky for uniformity factor and discomfort glare probability simulations. – percentage of indirect light used in simulations: average. reflection factors of internal surfaces were taken from the acceptable ranges described by both en 12645-1:2021 and en 17037:2018 (0.7 to 0.9 for the ceiling, 0.5 to 0.8 for walls and 0.2 to 0.4 for the floor). annex b of en 17037 recommends using lower values of reflection factors in the abovereported range to take into account the presence of furniture. however, we considered the case of a room furnished with light-coloured furniture. simulation refers to the following parameters described in the introduction: – daylight factor d. – daylight uniformity u. – daylight glare probability dgp. the daylight factor, d, is believed to have been developed by alexander pelham trotter towards the end of the nineteenth century (mardaljevic, j., 2013) as the ratio of the internal horizontal illuminance to the unobstructed external horizontal illuminance, usually expressed as a percentage. nowadays, the required levels of daylight factor range between 1% and 5%, depending on building types and activities, and it can be evaluated in standard overcast conditions as under various unobstructed skies (danny et al., 2018) (xu, yuehong, xin, 2014). daylighting uniformity, u, (en 12665, 2013) is the ratio of minimum illuminance to average illuminance. 162 journal of facade design & engineering volume 10 / number 1 / 2022 the daylight glare probability, dgp, (en 17037, 2018) is the parameter used to analyse the daylight glare effect that considers both the illuminance at eye level and individual glare sources of high luminance to estimate the fraction of dissatisfied people. both u and dgp are useful parameters to evaluate the effect of louvres in clear sky conditions (kose & kazanasmaz, 2020). for daylighting purposes, the international commission on illumination, cie, defines 15 different standard sky conditions (cie s 011/e, 2003) (darula & kittler, 2014) that are described by functions, depending on the solar altitude, even when the sun is obscured. in this study, the analysis of the parameters u and dgp was performed with clear sky conditions (less than 30 % cloud cover or none) combined with sunny sky conditions; in this case, the sky distribution corresponds to the standard cie clear sky condition with additional direct illumination from the sun. this model of the sky is useful when visual glare and thermal discomfort studies are performed (suk & kensek, 2011). standard en 17037 (en 17037, 2018) gives recommendations for the daylight factor d with reference to different locations. table 1 shows the values of d to obtain a given target illuminance e t (lx) from 100 (minimum target illuminance) to 750 lx (high target illuminance), referred to the median external diffuse illuminance e v,d,med of florence and rome , for a fraction of daylight hours f time,% = 50%. as an example, in the case of central italy (florence), the value d equal to 2% allows exceeding 300 lx for 50% of the time daylight hours. table 1 values of d for daylight vertical openings to exceed an illuminance level from 100 to 750 lx for a fraction of daylight hours f time,% =50% place latitude (°) median external diffuse illuminance e v,d,med l x d to exceed 100 lx d to exceed 300 lx d to exceed 500 lx d to exceed 750 lx florence 43.46 15017 0.67% 2.00% 3.33% 5.00 % rome 41.80 19200 0.50% 1.60% 2.60% 3.90% table 2 shows the correspondence between values of daylight glare probability and the statistical perception of glare according to the standard en 17037. table 2 recommended values of daylight glare probability according to annex e of en 17037 criterion dgp value glare is mostly not perceived dgp ≤ 0.35 glare is perceived but mostly not disturbing 0.35 < dgp ≤ 0.40 glare is perceived and mostly disturbing 0.40 < dgp ≤ 0.45 glare is perceived and mostly intolerable dgp ≥ 0.45 to assess daylight glare probability, the luminance distribution within the field of view and the size, intensity and location of the glare sources in regard to the line of sight have to be taken into account. consequently, dgp values have been calculated for the three positions shown in figure 6: dgp1 and dgp2 are symmetrical but facing the opposite walls, located 2.0 m from the façade and with a view direction parallel to the window, while dgp 3 is perpendicular to the façade and located 3 m away; all points are 1.2 m high from the floor. 163 journal of facade design & engineering volume 10 / number 1 / 2022 fig. 6 position of the points views for the calculation of dgp. the analysis of dgp was carried out for different configurations of louvres for the same orientations of the façade (south and east), periods of the year (two solstices and equinox) and at the same hours of calculation (09:00, 12:00 and 15:00) as for the daylighting uniformity. 3 results and discussion 3.1 acoustic results simulations were performed with the above-described fe software and method: they produced a detailed description of the sound distribution across the façade at different frequencies. figure 7 shows a sample result obtained by fe simulations: the sound pressure level at 1 khz across the building façade portion is affected by the presence of the shading louvres; the sound pressure level also appears to be highly reduced by the presence of sound absorbing louvres, with respect to the traditional ones. fig. 7 sound pressure level simulated in correspondence with the 4th floor of the simulated building façade. the solid thick black vertical line represents the façade surface. left: without louvres. center: non-absorbing louvres. right: sound-absorbing louvres. 164 journal of facade design & engineering volume 10 / number 1 / 2022 as shown in figure 7, the sound pressure level with louvres present is far from uniform across the façade. the results of the simulations have therefore been further evaluated in terms of arithmetic averages of the insertion loss, with each average referring to a single floor. simulations have been conducted considering five virtual measurement positions on each floor. each average considers five measurement points across the virtual building façade. in the fe acoustic model, the effect of the variation of the following parameters was studied: – louvres’ tilt angle (0, -30°, -45°), considering: 0° for horizontal louvre; -45° for louvre tilted towards the soil and the sound source. – louvres’ width (20, 30, 40 cm). – louvres’ spacing (20, 30, 40 cm). the following summarises the main results. 3.1.1 tilt angle of louvres the louvres’ tilt angle with respect to the sound source has an impact on the acoustic insertion loss: it is shown that tilted louvres offer less acoustic protection than horizontal ones. therefore, only horizontal louvres have been considered in further daylight analyses. it is very clear how il increases with respect to the building height: this consideration can be made while evaluating other geometrical aspects of the shading system. fig. 8 left: average of il in db(a) considering sound-absorbing louvres’ tilt angles (0°, 30°, 45°) with respect to building height. right: overall evaluation of the effect of louvre tilt angle on il: standard versus sound-absorbing louvres results are shown. 3.1.2 louvre width in this case, the width of the louvres varied between 20, 30 and 40 cm, with respect to the floor height and an overall average. 165 journal of facade design & engineering volume 10 / number 1 / 2022 results show that the louvres’ section width is not relevant when acoustic il in dba is observed: it is important to mention that simulations have been executed considering the spacing between louvres being no shorter than their width. in other words, the ratio between the spacing and width of louvres has been kept equal to 1. this fact can partially explain the minor effects of louvre widths on the acoustic il. fig. 9 left: average of the il in db(a) considering sound-absorbing louvres section width (20, 30 and 40 cm) with respect to building height. right: overall evaluation of the effect of louvre section width on il: standard versus sound-absorbing louvres results are shown. 3.1.3 louvre spacing results showed that values of il in db(a) are affected by the louvres’ spacing: wider distances between louvres give poorer sound protection to the building façade. fig. 10 left: average of il in db(a) considering the spacing between sound-absorbing louvres (20, 30 and 40 cm) with respect to building height. right: overall evaluation of the effect of louvre section width on il: standard versus sound-absorbing louvres results are shown. 166 journal of facade design & engineering volume 10 / number 1 / 2022 3.1.4 preliminary considerations on acoustic results figures 8, 9 and 10 indicate that the acoustic effect of the louvres is mainly influenced by the floor level and by their tilt angle and spacing. since these results are the starting point for daylighting analysis, it must be considered that the floor level does not influence the indoor daylighting for a building not obstructed by other buildings, as in the case study analysed. on the other hand, louvres tilted downward (30° or 45°) negatively affect both the reduction of the insertion loss and the daylighting factor. for this reason, daylighting simulations have been performed with reference to a typical floor level and horizontal louvres (0° of tilt angle) 20 cm wide, spaced 20, 30 and 40 cm from each other. 3.2 daylighting results figure 11 shows the results with reference to the values of maximum, average and minimum daylight factor, d, calculated with standard cie overcast sky, in the conditions expressed in paragraph 2. the second vertical axis in the same graph shows the fraction of the reference plane (f plane ) where the target daylighting level (500 lx) or the minimum target daylighting level (300 lx) are achieved, according to annex a of en 17037:2018. the reduction of the daylight factor is a consequence of the presence of the louvres and their spacing. with horizontal louvres 20 cm wide and spaced 20 cm from each other, the average daylight factor is 3.1%, and 47% of the reference plane (f plane ) reaches the target illuminance level of 500 lx, while 100% of the reference plane reaches the minimum target level of 300 lx for 50% of the daylight hours. according to en 17037:2018, minimum values of f plane should be 50% for the target level and 95% for the minimum target level. therefore, it can be deduced that, for the latitude of southern europe, even with external louvres spaced 20 cm from each other, the minimum values of daylighting level are achieved with good approximation under overcast conditions (47% instead of 50% can be considered within the uncertainty margin of the method). fig. 11 variation of the average daylight factor as a function of the louvres’ spacing with overcast skies. 167 journal of facade design & engineering volume 10 / number 1 / 2022 from figure 12 it can be observed that the presence of the louvres spaced 20 cm improves the uniformity of daylighting, u, (e min /e avg ) with clear sky, at the latitude of central italy and with reference to a different orientation of the façade, the two solstices and the equinox, both in the morning and in the afternoon. an important exception to this is the south façade, without louvres, at 12:00 of the winter solstice, when the direct sun radiation involves all the evaluation area of the office. here, the minimum and the average values of daylighting are very similar (very high uniformity factor). it can also be noted that at the equinox (21 march), the illuminance uniformity with louvres spaced 20 cm is much higher than with louvres more broadly spaced. this is due to the fact that with louvres spaced 20 cm, the direct solar radiation is completely intercepted by the louvres, as can be seen in figure 13, which compares the luminances of the room surfaces with louvres spaced 20, 30 and 40 cm for 9.00 am on 21 march. we must consider, for this purpose, that the eastern orientation of the sun’s altitude at 9:00 corresponds to the western orientation at 15:00, while the eastern orientation at 15:00 corresponds to the western orientation at 09:00. for this reason, the western orientation is not considered in these evaluations. fig. 12 variation of the uniformity factor as a function of the louvres’ spacing with clear sky. a b c fig. 13 luminance values (cd/m2) as a function of the louvres’ spacing with clear sky. from left to right, louvres spaced 20, 30 and 40 cm. also, with overcast skies, when the orientation and the date and hour of the evaluation are irrelevant, a better uniformity factor is achieved with louvres’ spacing of 20 cm (figure 14). 168 journal of facade design & engineering volume 10 / number 1 / 2022 fig. 14 variation of the uniformity factor as a function of the louvres’ spacing with overcast and clear sky (average values over the year). tables 3 and 4 show the dgp values calculated for the three positions and for east and south exposure. table 3 daylight glare probability for south orientation of the façade. the cell colours refer to dgp range of values in table ii date 21-december 21-march 21-june louvres' spacing 0.20m 0.30m 0.40m no louvers 0.20m 0.30m 0.40m no louvers 0.20m 0.30m 0.40m no louvres dgp1 h 9.00 0.28 0.29 0.29 0.3 0.36 0.4 0.42 0.46 0.31 0.37 0.4 0.45 h 12.00 0.22 0.23 0.23 0.23 0.23 0.24 0.24 0.26 0.23 0.24 0.24 0.24 h 15.00 0.19 0.19 0.19 0.2 0.19 0.2 0.2 0.21 0.19 0.2 0.2 0.21 dgp2 h 9.00 0.28 0.29 0.3 0.31 0.37 0.41 0.42 0.46 0.32 0.37 0.4 0.45 h 12.00 0.22 0.22 0.23 0.23 0.24 0.24 0.25 0.25 0.23 0.24 0.24 0.24 h 15.00 0.19 0.19 0.19 0.2 0.2 0.2 0.2 0.21 0.2 0.2 0.2 0.21 dgp3 h 9.00 0.37 0.39 0.39 0.41 0.49 0.55 0.57 0.62 0.41 0.5 0.55 0.62 h 12.00 0.28 0.29 0.29 0.29 0.31 0.31 0.31 0.34 0.31 0.31 0.32 0.32 h 15.00 0.21 0.21 0.22 0.23 0.22 0.23 0.23 0.25 0.22 0.23 0.23 0.25 table 4 daylight glare probability for east orientation of the façade. the colours of cells refer to dgp range of values in table ii date 21-december 21-march 21-june louvres' spacing 0.20m 0.30m 0.40m no louvers 0.20m 0.30m 0.40m no louvers 0.20m 0.30m 0.40m no louvres dgp1 h 9.00 0.86 0.87 0.88 0.89 0.28 0.31 0.33 0.35 0.24 0.25 0.25 0.26 h 12.00 0.49 0.53 0.54 0.57 0.32 0.37 0.4 0.45 0.25 0.26 0.27 0.34 h 15.00 0.33 0.35 0.35 0.37 0.28 0.32 0.35 0.39 0.23 0.24 0.24 0.29 dgp2 h 9.00 0.28 0.29 0.3 0.31 0.26 0.3 0.32 0.35 0.22 0.23 0.23 0.27 h 12.00 0.4 0.44 0.44 0.48 0.31 0.36 0.4 0.45 0.25 0.26 0.27 0.35 h 15.00 0.33 0.34 0.35 0.4 0.3 0.34 0.36 0.4 0.24 0.25 0.26 0.29 dgp3 h 9.00 0.37 0.39 0.39 0.41 0.34 0.4 0.44 0.49 0.29 0.3 0.3 0.36 h 12.00 0.54 0.58 0.59 0.63 0.41 0.5 0.55 0.61 0.31 0.34 0.35 0.47 h 15.00 0.44 0.46 0.47 0.49 0.37 0.44 0.48 0.54 0.29 0.3 0.31 0.4 169 journal of facade design & engineering volume 10 / number 1 / 2022 results of dgp show that the presence of the louvres produces a generalised reduction of glare effects. in particular, horizontal louvres spaced 20 cm from each other produce better results. for observers in positions dgp1 and dgp2, which are the more usual in office buildings, and louvres spacing 0.2 m, only in the morning of the winter solstice, when the sun altitude is very low for the east orientation of the façade, glare problems occur. during the summer season, the presence of louvres spaced 20 cm from each other reduces the dgp below the value of 0.35 (glare mostly not perceived according to en 17037) at each of the hours examined; with the only exception of the observer position dgp3 at 9.00 in the morning with south orientation (dgp = 0.41). 4 conclusions in this work, the effect of external louvers on a case study building 28 meters high, unobstructed by other buildings on the opposite side of the road, is evaluated regarding both acoustic and daylighting parameters. the results of previous works by the authors concerning only the acoustic effect are summarised and recalled here. to better understand the hypothesis and the details of these results, it is necessary to refer to the cited papers ((zuccherini et al., 2016) (fausti et al., 2019) (zuccherini et al., 2015) (zucherini et al., 2019). in general, the acoustic effect of an external louvre could be negative, as it can cause an increase in the sound pressure level on the façade. however, if properly designed and with the bottom side of the louvres lined with sound-absorbing material, this effect can become positive and contribute to reducing the façade sound pressure level by some decibels. on the other hand, the presence of these façade devices significantly reduces the amount of daylighting in the interior, especially with overcast sky conditions, and modifies the distribution of daylight, especially with clear sky conditions. in this study, only fixed horizontal louvres 20 cm wide, with different spacing and with different façade orientations, are considered for both acoustic and daylighting simulations since acoustic simulations showed that this is the better configuration to reduce the façade sound pressure level. regarding the latitude of central italy (florence) and an office building with large windows (34.5 % of the floor surface), unobstructed by other buildings on the opposite side of the road, in this configuration, the reduction of daylight factor with overcast skies is acceptable also with louvres spaced 20 cm from each other. moreover, results show that the uniformity factor (ratio between minimum and average daylighting level) is significantly improved both with clear and overcast sky conditions with white (80% reflection factor) external louvres 20 cm wide and spaced 20 cm from each other. moreover, the presence of the louvres produces a generalised reduction of glare effects in sunny sky conditions. the results presented in this paper demonstrate that a proper design of external louvres, specifically referring to the characteristics of the building and the latitude of the location, can improve both acoustic insulation properties of the façade and the distribution of daylight in the indoor space, reducing the probability of daylight glare. further development of this work will involve evaluating the daylighting effect at other latitudes and studying the effect of the reduction of thermal loads in summer conditions. 170 journal of facade design & engineering volume 10 / number 1 / 2022 acknowledgements this work is the extended version of the paper presented at the international conference on construction, energy, environment & sustainability (cees 2021), held in coimbra, portugal in october 2021 (secchi, fausti, cellai, parente, santoni & zuccherini, 2021). references en 13363-1: 2003. solar protection devices combined with glazing calculation of solar and light transmittance simplified method. https://standards.iteh.ai/catalog/standards/cen/25977ecf-12cd-4a04-9038-f9734c657740/en-13363-1-2003a1-2007 en 14501: 2006. blinds and shutters thermal and visual comfort -performance characteristics and classification. https:// standards.iteh.ai/catalog/standards/sist/f7041878-9426-466c-a3cc-ab9102bf8454/sist-en-14501-2006 directive 2010/31/eu of the european parliament and of the council of may 19, 2010, on the energy performance of buildings (recast). https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj:l:2010:153:0013:0035:en:pdf cellai, g., carletti, c., sciurpi, f., secchi, s., nannipieri, e., pierangioli, l. (2014). transparent building envelope: windows and shading devices typologies for energy efficiency refurbishments, cap.2 “building refurbishment for energy performance, green energy and technology”, springer international publishing, switzerland. https://www.springerprofessional.de/en/ transparent-building-envelope-windows-and-shading-devices-typolo/2056852 ecbcs annex 29/shc task 21, 2010. project summary report daylight in buildings. https://www.iea-ebc.org/data/publications/ ebc_annex_29_psr.pdf en 12464-1:2021. light and lighting lighting of workplaces part 1: indoor workplaces. https://standards.iteh.ai/catalog/ standards/cen/53fc4ff7-e7df-4ebd-a730-0d5f0ea888e0/en-12464-1-2021 en 14501:2005. blinds and shutters thermal and visual comfort performance characteristics and classification. https:// standards.iteh.ai/catalog/standards/cen/08a6e37a-8784-484f-88e9-05794adfbd27/en-14501-2005 en 17037:2018, daylight in buildings. https://standards.iteh.ai/catalog/standards/cen/836e5b91-1eb0-4643-a2ba-7ca5a5988e64/ en-17037-2018 directive 2002/49/ec of the european parliament and of the council of june 25, 2002, relating to the assessment and management of environmental noise. https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj:l:2002:189:0012:0025:en:pdf busa, l., secchi, s., & baldini, s. (2010). effect of façade shape for the acoustic protection of buildings. building acoustics, 17(4), 317-338. https://doi.org/10.1260%2f1351-010x.17.4.317 li, k. m., lui, w. k., lau, k. k., & chan, k. s. (2003). a simple formula for evaluating the acoustic effect of balconies in protecting dwellings against road traffic noise. applied acoustics, 64(7), 633-653. https://doi.org/10.1016/s0003-682x(03)00020-3 van renterghem, t., hornikx, m., forssen, j., & botteldooren, d. (2013). the potential of building envelope greening to achieve quietness. building and environment, 61, 34-44. http://dx.doi.org/10.1016/j.buildenv.2012.12.001 granzotto, n., bettarello, f., ferluga, a., marsich, l., schmid, c., fausti, p., & caniato, m. (2017). energy and acoustic performances of windows and their correlation. energy and buildings, 136, 189-198. https://doi.org/10.1016/j.enbuild.2016.12.024 secchi, s., cellai, g., fausti, p., santoni, a., & martello, n. z. (2015). sound transmission between rooms with curtain wall façades: a case study. building acoustics, 22(3-4), 193-207. https://doi.org/10.1260/1351-010x.22.3-4.193 hu, z., zayed, t., & cheng, l. (2022). a critical review of acoustic modelling and research on building façade. building acoustics, 29(1) https://doi.org/10.1177%2f1351010x211022736 sakamoto, s., & aoki, a. (2015). numerical and experimental study on noise shielding effect of eaves/louvers attached on building façade. building and environment, 94, 773-784. http://dx.doi.org/10.1016/j.buildenv.2015.05.015 zuccherini martello, n., fausti, p., secchi, s. (2016). acoustic measurements on a 1:1 scale model of a shading system for building façade in a semi-anechoic chamber, proceedings of internoise 2016, august 21-24, hamburg, germany. https://www. researchgate.net/publication/306505595_acoustic_measurements_on_a_11_scale_model_of_a_shading_system_for_building_facade_in_a_semi-anechoic_chamber fausti, p., secchi, s., zuccherini martello, n. (2019). the use of façade sun shading systems for the reduction of indoor and outdoor sound pressure levels, building acoustics, 26, 3, 181-206. https://doi.org/10.1177%2f1351010x19863577 sakamoto, s., lee, h., ishii, h., katayama, t., iwase, s., & takahashi, k. (2017). in-situ experiment and numerical analysis on an effect of noise shielding louvers attached on a building façade. in inter-noise and noise-con congress and conference proceedings, vol. 255, no. 6, pp. 1484-1491. institute of noise control engineering. https://www.ingentaconnect.com/ contentone/ince/incecp/2017/00000255/00000006/art00057 zuccherini martello, n., fausti, p., santoni, a., secchi, s. (2015). the use of sound absorbing shading systems for the attenuation of noise on building façades. an experimental investigation, buildings 5(4), 1346-1360, https://doi.org/10.3390/ buildings5041346 zuccherini martello, n., aletta, f., fausti, p., kang, j., secchi, s. (2019). a psychoacoustic investigation on the effect of external shading devices on building façades, applied sciences, 6(12), 429. https://doi.org/10.3390/app6120429 catalina, t., ene, a., & biro, a. (2019). visual and acoustic performance of shading devices–real scale laboratory measurements. in e3s web of conferences. vol. 111, p. 06072. edp sciences. https://www.researchgate.net/publication/335141827_visual_and_ acoustic_performance_of_shading_devices_-_real_scale_laboratory_measurements 171 journal of facade design & engineering volume 10 / number 1 / 2022 buratti, c., belloni, e., merli, f. & ricciardi, p. (2018). a new index combining thermal, acoustic, and visual comfort of moderate environments in temperate climates, building and environment, 139, july 2018, 27-37, https://doi.org/10.1016/j. buildenv.2018.04.038 huang, l., zhu, y., ouyang, q. & cao, b. (2012). a study on the effects of thermal, luminous, and acoustic environments on indoor environmental comfort in offices, building and environment, 49 (2012) 304-309, http://dx.doi.org/10.1016/j. buildenv.2011.07.022 hangzi, w., xiaoying, s. & yue, w. (2020). investigation of the relationships between thermal, acoustic, illuminous environments and human perceptions, journal of building engineering, 32, november 2020, 101839, https://doi.org/10.1016/j. jobe.2020.101839 technical report of iea shc task 50.c2/ methods and tools for lighting retrofits-state of the art review. april 2016. http://task50. iea-shc.org/data/sites/1/publications/technical_report_t50_c2_final.pdf technical report of iea shc task 61.c1 /workflows and software for the design of integrated lighting solutions. november 2019. https://task61.iea-shc.org/data/sites/1/publications/iea-shc-task61-workflows-and-software-for-the-design-of-integratedlighting-solutions.pdf ahmad, a., kumar, a., prakash, o., amana, a.; daylight availability assessment and the application of energy simulation software – a literature review. materials science for energy technologies 3 (2020) 679–689. https://doi.org/10.1016/j.mset.2020.07.002 carletti, c., cellai, g., pierangioli, l., sciurpi, f. & secchi, s. (2017). the influence of daylighting in buildings with parameters nzeb: application to the case study for an office in tuscany mediterranean area, energy procedia, 140, december 2017, 339-350, https://doi.org/10.1016/j.egypro.2017.11.147 carlucci, s., causone, f., de rosa f., and pagliano l. (2015) a review of indices for assessing visual comfort with a view to their use in optimisation processes to support building integrated design. renewable & sustainable energy reviews. 2015, 47:10161033, https://doi.org/10.1016/j.rser.2015.03.062 en 12665: 2013. light and lighting basic terms and criteria for specifying lighting requirements. https://standards.iteh.ai/ catalog/standards/cen/d7c62c9a-95ac-4ed8-9a40-862805aa5afc/en-12665-2011 galatioto, a. and beccali, m. (2016). aspects and issues of daylighting assessment: a review study. renewable and sustainable energy reviews. 66, 852–860.2016. https://doi.org/10.1016/j.rser.2016.08.018 nabil, a. & mardaljevic, j. (2005). useful daylight illuminance: a new paradigm for assessing daylight in buildings, lighting research and technology 37(1), march 2005, 41-59, http://dx.doi.org/10.1191/1365782805li128oa mardaljevic, j., andersen, m., roy, n., & christoffersen, j. (2012). daylighting metrics: is there a relation between useful daylight illuminance and daylight glare probability? proceedings of bso conference 2012: 1st conference of ibpsa-england, loughborough, uk, 10-11 september 2012, 189-196. https://publications.ibpsa.org/conference/paper/?id=bso2012_3b1 en iso 10534-2:2001. acoustics – determination of sound absorption coefficient and impedance in impedances tubes – part 2: transfer-function method. https://standards.iteh.ai/catalog/standards/cen/8f9ca271-960d-4042-8937-ccb8cbedcd20/ en-iso-10534-2-2001 delany, m. e., & bazley, e. n. (1970). acoustical properties of fibrous absorbent materials. applied acoustics, 3(2), 105-116. https:// doi.org/10.1016/0003-682x(70)90031-9 miki, y. (1990). acoustical properties of porous materials-modifications of delany-bazley models. journal of the acoustical society of japan (e), 11(1), 19-24. https://doi.org/10.1250/ast.11.19 www.relux.com maamari, f., fontoynont, m., adra, n. (2006). application of the cie test cases to assess the accuracy of lighting computer programs. energy and buildings, 38(7), july 2006, 869-877. https://doi.org/10.1016/j.enbuild.2006.03.016 bhavani, r.g., and khan, m.a. (2011). advanced lighting simulation tools for daylighting purpose: powerful features and related issues. trends in applied sciences research 6(4), 345-363. https://scialert.net/abstract/?doi=tasr.2011.345.363 kaempf, j., paule, b., basurto, c., bodart, m., boer, j., bueno, b., dubois, m.c., geisler-moroder, d., fusco, m., hegi, m., jorgensen, m., roy, n., wienold, j. (2016). methods and tools for lighting retrofits: state of the art review, a technical report of iea shc task 50, online: http://task50.iea-shc.org/data/sites/1/publications/technical_report_t50_c2_final.pdf bouroussis c.a., nikolaou d.t., topalis f.v. (2019). test report on the validation of relux desktop 2019 against cie 171:2006 – may 2019, https://relux.com/assets/static/global/documents/reluxdesktop_validation_report_final.pdf cie 171:2010. test cases to assess the accuracy of lighting computer programs. cie, vienna, 2006. https://cie.co.at/publications/ test-cases-assess-accuracy-lighting-computer-programs iversen, a., roy, n., hvass, m., jørgensen, m., christoffersen, j., osterhaus, w., johnsen, k. (2013) daylight calculations in practice: an investigation of the ability of nine daylight simulation programs to calculate the daylight factor in five typical rooms. sbi 2013:26 danish building research institute, aalborg university. 2013. https://vbn.aau.dk/en/publications/daylight-calculations-in-practice-an-investigation-of-the-ability mardaljevic, j. (2013). rethinking daylighting and compliance, journal of sustainable design & applied research. 1, 3, article 1. available at: https://arrow.tudublin.ie/sdar/vol1/iss3/1/, https://doi.org/10.21427/d7hj0c danny, h.w.li, shuyang, li, wenqiang, c. and siwei l. (2018). analysis of point daylight factor (pdf) average daylight factor (adf) and vertical daylight factor (vdf) under various unobstructed cie standard skies. iop conf. ser.: mater. sci. eng. 556 012044. https://iopscience.iop.org/article/10.1088/1757-899x/556/1/012044 xu, y., yuehong, s., xin, c. (2014). application of relux simulation to investigate energy saving potential from daylighting in a new educational building in uk, energy and buildings, volume 74, 2014. https://doi.org/10.1016/j.enbuild.2014.01.024 kose, b., & kazanasmaz, t., (2020). applicability of a prismatic panel to optimise window size and depth of a south-facing room for a better daylight performance. light & engineering. 63-67. http://dx.doi.org/10.33383/2019-038 cie s 011/e:2003 spatial distribution of daylight cie standard general sky. https://cie.co.at/publications/spatial-distribution-daylight-cie-standard-general-sky 172 journal of facade design & engineering volume 10 / number 1 / 2022 darula, s. and kittler, r. (2002). cie general sky standard defining luminance distributions. proc. conf. esim 2002.the canadian conference on building energy simulation. montreal, canada. http://www.ustarch.sav.sk/wp-content/uploads/ darula_kittler_proc_conf_esim_2002.pdf kensek, k., & suk, j.y. (2011). daylight factor (overcast sky) versus daylight availability (clear sky) in computer-based daylighting simulations. https://www.semanticscholar.org/paper/daylight-factor-(overcast-sky)-versus-daylight-sky)-kensek-suk/ 155fe1ef6424944a9056249809ae9d941e140539 secchi, s., fausti, p., cellai, g., parente, m., santoni, a. & zuccherini, n.m. (2021). the acoustic and daylighting effects of external façade sun shading systems. proceedings of international conference on construction, energy, environment and sustainability, coimbra, portugal, 12-15 october 2021 journal of facade design and engineering 2 (2014) 223–233 doi 10.3233/fde-150023 ios press 223 structural glass envelopes – implementation of environmental studies into viscoelastic analysis lisa rammiga,b, peter lenka,∗ and vladimir marinova aeckersley o’callaghan, kingsgate, london, uk bfacade research group, faculty of architecture and the built environment, delft technical university abstract. the focus of this paper is to promote close collaborations of facade and structural engineers through integration of design services in order to deliver more optimal glass envelopes. to achieve the best performance, this process involves a careful selection of glass and coatings, radiation analysis to maximise daylight while controlling solar gain, and preventing overheating of spaces as well as careful detailing. site-specific environmental data are used to accurately determine the performance requirements of the glass. once the environmental impact is assessed the temperature in the interlayer is calculated over any chosen time domain. as part of the proposed process, a technique for determining the time-dependent structural behaviour of sgp laminated glass under varying temperature loads is also considered. in conclusion the authors examine the possibility of integration of both environmental and structural design approaches through the use of parametric software. keywords: building envelopes, structural glass, parametric design, energy efficiency 1. introduction the increased need for more energy efficient envelopes is becoming the cornerstone for the design of many building structures and facades involving glass. building envelope design becomes more complex due to increasingly stringent energy requirements as well as technological developments. furthermore building envelope physics play an increasing role in the design and are today considered in great detail to ensure occupant comfort. as thermal comfort is dependent on many dynamic parameters these have to be analysed and balanced carefully to achieve a comfortable interior climate throughout the year. in addition, structural glass is increasingly becoming a starting point in the architectural exploration of envelopes. almost always structural glass is employed as a laminate of two or more layers of glass, bonded with a plastic interlayer. furthermore in certain design situations both energy and structural requirements need to be considered simultaneously. the scope of this study is to evaluate the potential for an increase in performance of structural glass through a combined site-specific environmental and structural analysis. ∗corresponding author: peter lenk, eckersley o’callaghan, kingsgate, london, n19ae, uk. e-mail: peter@eckersleyo callaghan.com. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:peter@eckersleyopenalty -@m callaghan.com 224 l. rammig et al. / structural glass envelopes – implementation of environmental studies 2. environmental parameters due to the inherent characteristics of glazing, such as high thermal conductivity and transparency, the thermal impact on the interior space and as such on the thermal comfort, is much greater compared to other more solid materials like concrete or stone. as relates to the transparency, high performance coatings and external shading systems are commonly used to limit the amount of solar gain into the building and prevent overheating. the selection of these coatings and the type of glass determine the absorption coefficient of the overall build-up; the higher the absorption, the higher the temperature within the glass build-up. while glass temperature is a comfort parameter in itself, in this study it is used to inform the structural behaviour of glass elements. this is particularly important for laminated glass, as typically interlayers are made of viscoelastic materials, which properties also vary with temperature. for horizontal or inclined glazing this becomes an important consideration as the load transfer through the viscoelastic material decreases rapidly when exposed to different temperatures. this can lead to a large increase in glass deflections and stresses. 2.1. methodology with the thermal and radiation analyses of the building envelope based on location, orientation and exposure the temperature gradient through the glass build-up can be determined more precisely (fig. 1, left). rather than determining a set of peak temperature loads to design to, the proposed method uses site-specific weather data that allow the build-up temperature to be determined as a function of time. tools such as ies, ecotect or diva for rhino are used to carry out a radiation analysis on the building envelope. the volume of the envelope is exposed to the location specific solar radiation for a typical year, outputting radiation levels for certain intervals during this time. to ensure occupant comfort within an interior space several parameters have to be taken into account and set against each other to find the most optimal solution. here these shall be defined as u-value, g-value, visible light transmission, surface temperature of the build-up and glare potential. for the case study shown in figure 1, right, several glass configurations have been specified and analysed under site-specific environmental conditions. for the analysis an energy plus weather file provided by the us department of energy has been utilised (energyplus, 2014). surface temperatures as well as solar incident radiation in specific locations are calculated and then used in a further calculation to assess the temperature distribution throughout the glass build-up. here peak loads fig. 1. total solar radiation in ies (left), roof geometry (right). l. rammig et al. / structural glass envelopes – implementation of environmental studies 225 fig. 2. temperature division through a clear glass build-up (top left), with high performance coating on pos. 2 (top right) and with low e coating on pos. 3 (bottom left) in winslt, distribution of temperatures above and below 50 ◦ c for the high performance coating option. for highly exposed locations are chosen and analysed in greater detail. figure 2 shows three of the analysed build-ups with temperature distribution due to the lower absorption compared to coated glass; the clear build-up on the top left shows the lowest temperature distribution and has been used as a benchmark as it does not meet the performance requirements. the low e coating (fig. 2, bottom left) leads to significantly larger temperatures on the internal panes of the igu due to the location of the coating and the increased absorption due to a thicker glass build-up. the high performance coating indicated on the top right leads to a temperature increase in the outer laminate, however is less critical than the previously mentioned option. the data in figure 2, bottom right, show a distribution of temperatures above and below 50 ◦ c occurring for the build-up using the high performance coating. this threshold has been chosen due to the significant step change in shear capacity of sgp interlayer at this temperature. the trend clearly shows that temperatures above 50 ◦ c occur simultaneously with greater exterior temperatures, however, high radiation in colder months can also lead to high temperatures within the interlayer of the laminate. for the particular location and glass configuration described in this study, the annual time with temperatures above 50 degrees is 62 hours. for pvb and sgp interlayers, which are most commonly used today, the material stiffness changes with temperature and load duration. through the use of viscoelastic structural analysis the designer can optimise the glass thickness in laminates if temperature variation over time can be determined 226 l. rammig et al. / structural glass envelopes – implementation of environmental studies precisely. this is crucial to a more optimised design. as the composite action with a viscoelastic interlayer increases the structural efficiency of glass laminates and reduces their weight and thickness. furthermore, in particular for horizontal applications it is crucial that the design temperatures are known to great detail as the failure of the interlayer could lead to either strength or serviceability failure of the panel. 3. structural behaviour of sgp laminated glass under transient temperature loads having established a set of site-specific temperature data for the part of the glass envelope of interest, a procedure for structural investigation is used herein. the transient state of the body temperature of the glass laminate, could lead to a number of challenges in the structural design. often during the design process, the impact of this transient nature of the laminate behaviour is overlooked. this part of the study investigates sentryglass plus (sgp) laminated glass elements under varying temperature conditions. due to the low transition temperature, such materials could change from solid state into semi-viscous state during the structure’s lifetime or under elevated thermal conditions. induced shear forces acting over longer time periods will cause viscous flow and therefore shear deformations. a few examples of structural challenges related to the thermal and viscoelastic behaviour of laminated glass are presented below: – increased temperature gradient within a glass element may introduce significant thermal stresses. when thermal tension stresses exceed the glass edge strength, micro flaws will propagate and cause failure. in vanderbroek, belis, & louter, (2013) the authors presented a study where the thermal glass stress was calculated based on real climatic data. yearly temperature maxima over twenty years were considered. the effect of the number of stress cycles on the fatigue strength of glass components and especially their connections should be studied further. analysing day/night temperature gradients predominantly in climates susceptible to higher temperature variations needs extra attention. – a decrease in the stiffness of the interlayer at an elevated temperature could shift the behaviour of the laminated glass component from monolithic to layered behaviour. hence, a significant increase in glass bending stresses will be expected. similarly due to the reduction in lateral stiffness, the elements could be susceptible to euler buckling or lateral torsional instability. in galuppi, royer-carfagni (2014) the authors presented a study of a laminated beam in bending and buckling where the interlayer was considered to be viscoelastic. the authors concluded noteworthy differences between the actual laminated glass beam response and the one evaluated through the quasi-elastic approximation. – due to the significant decrease in the bending stiffness at elevated temperatures, a laminated curved panel could be susceptible to snap through instability. preventing abrupt global stability failure and establishing element capacity is critical as higher safety factors are usually associated with such sudden failure modes. the snap through effect of monolithic glass sheets was extensively researched in galuppi, massimiani, & royer-carfagni (2014), where the authors investigated whether the global stability of the cold-bent panel could be enhanced by stiffening the glass edges with a metal frame bonded with soft adhesive. the authors proved by calculations that the soft adhesive distributes forces across the length rather uniformly. in fildhuth et al. (2014) the authors presented a detailed numerical and experimental study of the recovery behaviour of laminated l. rammig et al. / structural glass envelopes – implementation of environmental studies 227 cold-bent glass. after initial short-term spring back (shape loss up to 20%), only marginal shape loss was measured in long term for sgp laminates. however, further studies are required to investigate whether such elements, subjected to temperature cycles, especially in applications without mechanical clamping, could preserve required shape. – increased deformations of glass components may relate to undesirable reflections and caustic. ponding of rainwater could increase maintenance cost as well as it might create significant aesthetic issues. although this is primarily a serviceability and maintenance issue a possible structural problem is the increased risk of glass stress corrosion due to the water and air pollution. 3.1. theoretical background in order to determine the structural behaviour of laminated glass for a set of material properties which are time and temperature dependent, a material model is required that can accurately represent the mechanical properties of the interlayer. the investigated material is sentryglass plus, which falls into the category of ionomeric polymers. as such the material exhibits viscoelastic mechanical properties. a material is said to be viscoelastic if the material has an elastic (recoverable) part as well as a viscous (non-recoverable) part. upon application of a load, the elastic deformation is instantaneous while the viscous part occurs over time (ansys, 2010). the viscoelastic model usually depicts the deformation behaviour of glass or glass-like materials and may simulate cooling and heating sequences of such material. these materials at high temperatures turn into viscous fluids and at low temperatures behave as solids. further, the material is restricted to be thermo-rheologically simple (trs), which assumes the material response to a load at a high temperature over a short duration is identical to that at a lower temperature but over a longer duration. methods to define viscoelastic material model: – prony series – curve fitting in this paper we have investigated the possibility of deriving prony coefficients from experimental data by the curve fitting method. experimental data were provided by the interlayer manufacturer dupont for the sentryglass plus. a dynamic mechanical analysis (dma) was carried out by dupont following the recommendations of astm 4065. this test consists of applying small oscillating strain (<10%) displacement to a tensile specimen at specified frequencies and temperatures. while the displacements are applied and loads recorded the sample compliance is determined in situ and the modulus extracted directly during the test. the tensile dma data were analysed using the time-temperature-superposition method to extract the young’s modulus (e) as a function of time for a specified reference temperature following the method described in ferry (1980). curve fitting involves comparing experimental data to certain predefined nonlinear material models. viscoelastic material curve fitting determines material constants by relating experimental data to the prony series expansion for both the shear and bulk modulus of the hyper-viscoelastic material. after inputting the experimental data, the order of prony series expansion is defined and a nonlinear regression is performed. the curve fitting results together with the experimental data are shown graphically in figure 5, left. the fitted coefficients are then written to the database to define the new viscoelastic material. 228 l. rammig et al. / structural glass envelopes – implementation of environmental studies fig. 3. shear modulus-time /temperature dependency for sentry glass interlayer as published by dupont. fig. 4. curve fitting, prony series vs. master curve of sentry glass (left) and wlf shift function (right). for viscoelastic curve fitting with multiple temperatures, coefficients can be evaluated at each discrete temperature point and written as a temperature-dependent prony data table, or the williamslandau-ferry (wlf) or tool-narayanaswamy (tn) shift functions can be used to account for the temperature dependency. shear modulus – time/temperature dependency for sentryglass plus is presented in figure 3, while the wlf shift function is graphically displayed in figure 4, right. l. rammig et al. / structural glass envelopes – implementation of environmental studies 229 3.2. numerical verification if the viscoelastic material is loaded with a constant force the polymer relaxes in time and strain increases with time. the corresponding shear stress decreases with time. shear stress depends on the current strain as well as the past strain history. due to the above effect, in general, models utilising viscoelastic materials should be stiffer than those utilising linear elastic materials which are commonly adopted in design practice. a simple numerical experiment to test the behaviour of the viscoelastic material model was carried out. in figure 5 the basis of the numerical model is presented. plane stress state analysis of the interlayer region measuring 10mm by 1.5mm and subjected to a uniform shear load was carried out. the specimen was loaded as shown in figure 5, left, with constant uniformly distributed load for 5e8 seconds (∼15 years). after that, a short rectangular impulse with constant load, 1.5 times higher than the base load, was introduced for 1e6 seconds (12 days). then the constant base load was reapplied for another 1e9 seconds (∼30 years). the domain was meshed with eight node quadrilateral elements with initial elastic shear modulus of 200 mpa and poisson ratio of 0.48. simplified viscoelastic material modelled with prony series for time-dependent shear modulus was used as presented in figure 4, right. the results from this numerical experiment are presented in figure 7. three points are evaluated through the thickness (1.0mm, 0.5mm and 0.0mm from top) at the centre of the sample. in figure 7, left, the normal stress in xx direction over time is plotted. stress relaxation, strain flow and elastic phase followed by viscoelastic recovery are showing good correlation with theoretical viscoelastic fig. 5. numerical experiment, definition, viscoelastic element subjected to time dependent shear. fig. 6. loading diagram (left) and simplified viscoelastic material model (right). 230 l. rammig et al. / structural glass envelopes – implementation of environmental studies fig. 7. numerical experiment, stress xx (left) and numerical experiment, strains xx (right). behaviour. the modelled impulse load could be replaced with a short-term temperature increase acting on the glass envelope as discussed earlier in this paper. similar structural behaviour would be expected as materials obey trs theorem. to summarise, a number of techniques can be used to accurately predict the viscoelastic behaviour of the interlayer in long-term conditions. having obtained a set of temperature data from the environmental analysis, the structural designer can use prony series or wlf shift function to predict deflection and stress in laminated glass for a chosen set of temperature and load durations. due to the large amount of data that are typically handled for even the simplest exercises of this sort, the authors propose the implementation of parametric tools which help to automate the processes. 4. parametric implementation having established a methodology for carrying out environmental and structural analysis as described in this study, the designer is faced with the task of manipulating the vast amount of data that become available at each iteration. as a typical example (fig. 8) a glazed roof structure may have thousands of potential solutions depending on the pitch angle and number of bays. the proposed temperature analysis used weather data to determine the actual temperature of the interlayers which is then used as an input for the structural analysis. by reducing the design parameters such as roof geometry, it is possible to manually design the worst-case glass panel for this example. while this is useful in itself and qualitative selection of design options remains a fundamental part of the process, there is a potential for added value to be introduced if a multi-parameter study is carried out. a viable way to tackle this is to use a common collaboration platform where the effect of each design change can be readily interrogated and its effects understood at early stages of the process (imbert, et al., 2012). 4.1. parametric environment building envelope design can be described as a multi-parameter problem with a large number of competing and often contradicting requirements. the solution of such a problem can be approached through the use of parametric tools, which are already well established in the industry. tools such l. rammig et al. / structural glass envelopes – implementation of environmental studies 231 fig. 8. roof example (left) and incremental geometry change during optimisation (right). fig. 9. integrated parametric envelope design workflow. as grasshopper, generative components and designscript allow parametric design to be successfully employed in building and envelope design. in addition a large number of third-party applications for environmental and structural analysis can be integrated into the parametric environment through api scripting in one of the common languages such as c++, python etc. within this parametric environment the user can dynamically explore a large number of solutions and at every step a feasibility calculation can be carried out. for instance for the roof study in figure 8, left, we employ the process described in figure 9. at the first step the geometry is loosely set up by the designer, with some flexibility built into the logic (harding, et al., 2012). based on the available variables, e.g. pitch, ridge height (fig. 8, right) an environmental assessment is carried out. to arrive at this, a number of tools are employed for 232 l. rammig et al. / structural glass envelopes – implementation of environmental studies environmental design, e.g. ecotect, diva for rhino, which are combined with bespoke scripting to determine the amount of solar radiation and temperature within the glass build-up. the result of the assessment is a set of temperature versus time data for each glass panel centroid. in addition purpose built tools are then employed to select the thermal characteristics of the glass, i.e. whether the panels are monolithic or insulated and also have a solar coating and/or frit options. as a result the user can be informed on the current u, g-values and light transmission. that is then quantitatively verified against the overall building performance requirements. following this, a structural analysis can be executed through interfacing gh with finite element software, such as strand 7, api or ansys workbench. the fore mentioned viscoelastic laminate analysis technique is then included in the parametric study. by knowing the temperature distribution in the glass build-up an accurate prediction of the glass relaxation and stresses can be calculated. all the current parameters are then fed into a budget estimate tool, which can give an approximate material cost to further inform the designer on the feasibility of the solution. the next step in the process employs an optimisation routine, which can deliver a more structured approach to arriving at a more optimal solution. natural selection type techniques are available in the forms of evolutionary and genetic algorithms, which can be tailored to optimise a multi-parameter task if an appropriate set of fitness functions are chosen (rutten, 2010). 5. conclusion and further research building envelope physics play an increasing role in the design of structural glass envelopes and are analysed in greater detail to ensure occupant comfort. this analysis today is mainly performed by building physics experts or mechanical engineers. as such there is little or no information being passed between these teams and the structural designer. the authors see this as a vital link which, if utilised, can increase the performance and reduce the cost of envelopes. the study presented here proposes a methodology for tackling this challenge. the information gained by the environmental analysis, more precisely the temperature variation of the interlayer over time, is used as an input to nonlinear structural analysis. in this paper only a single short term peak temperature load (12 hour) was analysed with a base temperature over a total duration of 45 years. the aforementioned numerical experiment was carried out to test the stability of the numerical analysis as well as the behaviour of the proposed viscoelastic material model. while this proves the feasibility of the approach the authors acknowledge that carrying out visco-elastic analysis coupled with time dependent load curves is computationally highly expensive. it is acknowledged that analysis of such dimension requires an automated procedure for handling the data. as such a methodology has been presented for the implementation of a parametric system which is capable of iterating through a large number of solutions without the need for manual user input. further research into this topic will be beneficial as relates to the analysis and material models available for the simulation of ionomer interlayers as well as other commercially available laminating substrates. the authors acknowledge the limitations of the wlf shift function as an accurate representation of the sgp behaviour but have included it for completeness. in line with the proposed numerical verifications, small scale physical testing is required which extends into a more realistic time span than the results currently available in the field, e.g. one to five years. to achieve this, a temperature and humidity controlled environment would be beneficial in the further study of permanent and variable strain applied to the specimens. the data would prove invaluable for calibration with existing numerical models. l. rammig et al. / structural glass envelopes – implementation of environmental studies 233 references ansys inc. & sas ip inc. 2010, ansys 13.0 theoretical manual. de bleecker, h., scholz, c., mocibob, d., standaert, p. (2014). a novel glass facade with high environmental targets. in: c. louter, f. bos, j. belis & j.-p. lebet (eds.), challenging glass 4 & cost action tu0905 final conference (pp. 161-170). london, united kingdom: taylor & francis group. energyplus energy simulation software, weather data, wmo region 4 mountain view-moffett field nas 745090 (tmy). retrieved may 1, 2014 from http://apps1.eere.energy.gov/buildings/energyplus/weatherdata about.cfm ferry, j.d. (1980). viscoelastic properties of polymers, 3rd edition, john wiley & sons. fildhuth, t., knippers, j., bindji-odzili, f., baldassini, n., pennetier, s. (2014). recovery behaviour of laminated cold bent glass. in: c. louter, f. bos, j. belis & j.-p. lebet (eds.), challenging glass 4 & cost action tu0905 final conference (pp. 113-120). london, united kingdom: taylor & francis group. galuppi, l., massimiani, s., royer-carfagni, g. (2014). snap-through instability in double curved cold-bent glass. international journal of non-linear mechanics, elsevier 2014, in press. galuppi, l., royer-carfagni, g. (2014). the design of laminated glass under time-dependent bending and buckling. in: c. louter, f. bos, j. belis & j.-p. lebet (eds.), challenging glass 4 & cost action tu0905 final conference (pp. 431-438). london, united kingdom: taylor & francis group. harding, j. (2012). thinking topologically at early stage parametric design, advances in architectural geometry, 77-90. hindrichs, d., daniels, k. (eds.) (2007). plusminus 20◦/40◦ latitude. edition axel menges. stuttgart/london. imbert, f. (2012). concurrent geometric, structural and environmental design: louvre abu dhabi, advances in architectural geometry, 67-76. rutten, d. (2010). evolutionary principles applied to problem solving. retrieved may 17, 2014 from http://www.grasshopper3d.com/ profiles/blogs/evolutionary-principles vanderbroek, m., belis, j., louter, ch. (2013). thermal breakage of glass. in: j. belis, ch. louter, & d. mocibob (eds.), cost action tu0905 mid-term conference on structural glass (pp. 563-569). london, united kingdom: taylor & francis group. http://apps1.eere.energy.gov/buildings/energyplus/weatherdata_about.cfm http://www.grasshopper3d.com/profiles/blogs/evolutionary-principles http://www.grasshopper3d.com/profiles/blogs/evolutionary-principles journal of facade design and engineering 3 (2015) 27–38 doi 10.3233/fde-150026 ios press 27 active materials for adaptive architectural envelopes based on plant adaptation principles marlén lópeza,∗, ramón rubioa, santiago martı́na, ben croxfordb and richard jacksonc adepartment of construction and manufacturing engineering. university of oviedo, ideascad research group. c/ pedro puig adam. campus de viesques, gijon, spain bthe bartlett, faculty of the built environment. ucl cinstitute of biomedical engineering. ucl abstract. in this paper, the authors present research into adaptive architectural envelopes that adapt to environmental changes using active materials, as a result of application of biomimetic principles from plants to architecture. buildings use large amounts of energy in order to maintain their internal comfort, because conventional buildings are designed to provide a static design solution. most of the current solutions for facades are not designed for optimum adaptation to contextual issues and needs, while biological solutions to adaptation are often complex, multi-functional and highly responsive. we focus on plant adaptations to the environment, as, due to their immobility, they have developed special means of protection against weather changing conditions. furthermore, recent developments in new technologies are allowing the possibility to transfer these plant adaptation strategies to technical implementation. these technologies include: multi-material 3d printing, advances in materials science and new capabilities in simulation software. unlike traditional mechanical activation used for dynamic systems in kinetic facades, adaptive architectural envelopes require no complex electronics, sensors, or actuators. the paper proposes a research of the relationship that can be developed between active materials and environmental issues in order to propose innovative and low-tech design strategies to achieve living envelopes according to plant adaptation principles. keywords: active materials, adaptive architecture, biomimetics, energy efficiency, plants 1. introduction “cities are part of the climate change problem, but they are also a key part of the solution.” (kamalchaoui & robert, 2009). nowadays cities consume the larger part of global energy and are therefore major contributors of greenhouse gas emissions. moreover, cities have key competencies to act on climate change through their responsibilities over urban sectors such as buildings. so much so that latterly the european union has been developing a large number of funding building efficiency programs for research and innovation trying to problem-solve these issues, such as the horizon 2020 framework. some of these programs focus on building retrofitting, or the installation of energy-efficient ∗corresponding author: marlén lópez, department of construction and manufacturing engineering. university of oviedo, ideascad research group. c/ pedro puig adam. room 6.1.19. campus de viesques, 33203 gijon, spain. tel.: +34 656 181913; e-mail: arqmarlenlopez@gmail.com. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:arqmarlenlopez@gmail.com 28 m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles technologies, especially on facades. facades have an important role in the regulation and control of energy waste, since they act as intermediary filters between external environmental conditions and inside users and functional requirements. due to this decisive role, in recent years facades or architectural envelopes have been the subject of numerous studies and research, always trying to achieve greater efficiency and performance in terms of energy, comfort or structure (almusaed, 2011; fortmeyer & linn, 2014; park & dave, 2014). as opposed to our buildings, which remain inert, living objects respond to the environment and they are able to adapt to the changing weather conditions (armstrong, 2012). innovation in technological functions of architecture is key to meeting these challenges. it should be noted that the emergence of new construction techniques, such as additive manufacturing, allows to break the geometric constraints of traditional processes (spiller & armstrong, 2011). this research is about an architecture that continuously changes in response to external stimuli. through technological innovation we investigate the application of biomimetics to the development of adaptive architectural envelopes according to plant adaptation strategies. advances in technology have opened new avenues for design; we can more effectively mimic nature’s language (oxman, 2012). this paper is organized in two main sections. in the first one, motivations for application to biomimetic principles from plants to architecture are explained, as well as concepts such as adaptation and why plant adaptation principles are the basis of this research. the second section is related to how to convert these inspirational mechanisms of plants into technical implementations for adaptive architectural envelopes. a selection of possible active materials is proposed, according to the environmental issues defined: humidity, temperature, carbon dioxide and light. finally some ideas about possible manufacturing tools are described. 2. from plant adaptations to architecture 2.1. adaptation adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats (dobzhansky et al., 1968). the multiple environmental and climatic characteristics of the area are variable parameters, while those concerning internal comfort in buildings are largely static; so, we use large amounts of energy to pump heating, or cool, ventilate and light our buildings between quite well defined limits, while external environmental factors can change considerably. the existing solutions to these problems tend to have a static building envelope and dynamic building services. therefore conventional solutions for facades and roofs are not designed for optimum adaptation to contextual issues and needs. however, biological solutions to adaptation are often complex, multi-functional and highly responsive. the building envelope, without distinction between walls and roof, is the interface between exterior environmental factors and the interior demands of the occupants (del grosso & basso, 2010). an adaptive architectural envelope is one that responds to changing environmental conditions both interior and exterior while managing the indoor environment. adaptive architectural envelopes should have adaptation strategies to anticipate exterior environmental variations as well as interior activities and their interactions with inhabitants. the building envelope could be constructed, not with the traditional inert surfaces, but with a ‘living’ cladding, which could house a wide range of technologies based on the behaviour of the envelopes found in nature (beesley, 2006). m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles 29 nowadays biology is a new inspiration for technological thinking. some of these studies have looked at nature as a source of inspiration for subsequent application to architecture, a trend also known as biomimicry or biomimetics. these terms come from the greek words bios, meaning life, and mimesis, meaning to imitate. biomimetics is defined as the ‘abstraction of good design from nature’ (vincent, 2006) or ‘mimicking the functional basis of biological forms, processes and systems to produce sustainable solutions’ (pawlyn, 2011). systems found in nature offer a large database of strategies and mechanisms that can be implemented in biomimetic designs. usually it is a discipline that has been developing for some time in other fields, such as engineering or medicine, however in recent years we begin to see how several research works have been developed around biomimicry to look into new solutions in architecture. some of them try different methodologies for developing new building envelopes based on biomimetic principles, such as “towards the living envelope” (badarnah, 2012) or “architecture follows nature” (mazzoleni, 2013). unlike these studies, this research is based only on plants and their strategies of adaptation to different climates. we only focus on those adaptations to environment shown by plants. 2.2. plant adaptation principles plants, like buildings, lack movement and remain subject to a specific location, so they have to resist weather conditions that affect them at all times and they have developed special means of protection against excessive wind, drought, cold, heat and light. plants, unlike buildings, have adapted to the environment, through processes of evolution over millions of years. these adaptations develop over time and generations as a response to the ever-changing environment. the study of concepts such as bioclimatology or phytosociology (azcón-bieto & talón, 2000) is part of the research process in which plant communities, and their relations with the environment, and the temporal processes that modify them, are analysed. the basis of the success of plants is the ability to compete in their environment and it depends mainly on their physiological evolution and adaptation to the environment. one of the most important applications of physiological principles refers to the study of the relationships between plants and climate, with physical and physiological environmental factors and the relationships between them. climatic factors include intensity and periodicity of heat and light, precipitation and relative humidity, as well as wind or the periodicity and duration of the seasons. this leads us to the analysis of the distribution of plants in different geographical territories as climatic factors, or, in other words, to bioclimatology. this information is particularly relevant as an introductory element to the biological analysis as well as the architectural proposition. taking as a reference the worldwide bioclimatic classification system (rivas-martı́nez, 2004), we focus on europe, where we can see four of the five broad climate types defined: mediterranean, temperate, boreal and polar. in a first biological approach we can validate that evolution and adaptation of plants, like all the organisms, occur in three main ways: morphological, physiological and behavioural. in addition to this first general approach, we define a second more useful one to order the wide range of plant adaptation examples that can be found in nature, as well as make easier the application or transfer solutions from nature to architectural solutions. we establish a classification of plant adaptations and interactions with their environment, so the core of this proposal is movement, resulting in two categories: dynamic mechanisms and static strategies. at the same time, these two main approaches could be further divided into a macro and micro level. within dynamic mechanisms we consider those plants that respond to external stimuli through movement. we focus on those plants that 30 m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles fig. 1. the image on the left shows a dynamic valve mechanism in seeds of many mesembryanthemums as a behavioural adaptation, using rainwater as a trigger to open and to launch their seeds. the image on the right shows a static strategy of hairy leaves of gynandriris setifolia as a morphological adaptation to protect themselves against excessive sunlight and temperature, by strategies of three-dimensional waxes or dense coverage with air-filled hairs. are responsive, that is, exhibit rapid and reactive movements, in a timescale that we can perceive. in this way, we study how plants react in specific ways to light, temperature, fire, darkness, water or drought through reactive mechanisms in the macroscopic and microscopic scales. on the other hand, within static strategies we focus on the highly multifunctional properties of plant leaves. plant surfaces provide more than one solution for environmental conditions and can include, for example, light reflection, superhydrophobic or superhydrophilic surfaces (koch et al., 2009; bhushan, 2009; gibson, 2012). in fig. 1 we can see how plants from the mediterranean climate (dry and hot areas) present different adaptations, with dynamic mechanisms or static strategy depending on the case. this study is part of a larger research and we have found stomata of leaves of particular interest once several plant adaptations examples have been studied for their possible application to adaptive architectural envelopes. we will see in the following section how stomata are an example of dynamic mechanisms and, at the same time, static strategies, and they demonstrate that the classification proposed is not exclusive and therefore stomata are specimens with an exceptional value in the process of biomimetic inspiration. 2.3. stoma definition of stoma, or stomata in plural, related to botany, is that it is a pore, found in the epidermis of leaves used to control gas exchange. the pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening. guard cells perceive and process environmental and endogenous stimuli such as light, humidity, carbon dioxide, temperature, drought, and plant hormones to trigger cellular responses resulting in stomatal opening or closure (azcón-bieto, j. & talón, m., 2000; vogel, 2012) asexplainedabove,stomataarechosenbetweenallpossibleadaptationsinplantsbecausetheyarean example of dynamic mechanisms and static strategies (fig. 2). stomatal valve movements in response to waterandcarbondioxideinterchangesareanexampleofdynamicmechanisms.inaddition,thereisagreat variabilityinmorphologiesonsurfacestructuresaroundthesevalvecells,duetofunctionaladaptationsto environmentalconditions.also,thesemodificationsofsurfacesareexamplesofstaticmechanisms.stomataareresponsiblefortheinterchangeofgasesforrespirationandphotosynthesis.furthermore,stomata allow for the loss of excess water in the form of water vapour, which also allows for cooling. so, functions m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles 31 fig. 2. from left to right: stomata on crucifera leaves; stomatal valve movements on a lavandula dentata leaf; different morphologies of sunken stomata, waxes, hair structure or chimneys to reduce the evaporation of water. fig. 3. diagram showing the mechanism of stomatal movement. ofstomatainclude:interchangingofgases,avoidinglackofwater,transpirationandinterchangingoftemperature. a leaf must continuously adjust the apertures of its stomata as conditions (light intensity and water and dioxide carbon availability) change. stomata open in response to a decrease in concentration of dioxide carbon, as well as respond directly to light. temperature provides another stimulus: at higher temperatures, stomata commonly open, responding to increased carbon dioxide consumption and they close responding to the higher level of carbon dioxide. finally stomata respond to water or high humidity throughguardcellsthatincreasetheirturgiditybyopening(vogel,2012;martinetal.,1983).thus,control ofstomatalmovementsdependsonthecontrolledvariablewithintheleafandtheexternalinputs(fig.3). controlledvariablesaredioxideconcentrationandthewaterlevel(turgidity).externalinputsarehumidity or water availability, temperature, atmospheric carbon dioxide concentration and light intensity. besides this mechanism of stomatal movement, different surface structures have been developed in leaves around stomata to adapt to environmental conditions. we can find different wax morphologies and hair structures, on plants from the temperate climate, whose role is to reduce the evaporation of water. sunken stomata, wax chimneys around the stomata or wax-covered stomata, are found out on plants from the mediterranean climate, in dry and hot habitats. in the same way dense coverage with air-filled hairs, is used to reflect visible light and temperature control, as well as to absorb 32 m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles uv-radiation. we also consider other features in their anatomy such as size and frequency or density (number of stomata) (i.e: in arid zones plants show a reduction per leaf area to reduce water loss) (koch et al., 2009). therefore, we have the key of their environmental adaptations and interactions by studying the micro-structures of the stomata of plants at different climate zones. it is important to understand the principles of adaptation solutions and transferring them into artificial systems for adaptive architectural envelopes rather than simply copying them. as part of a large research, transfer from stomata challenges into architectural design objectives will be carried out through a design concept generation methodology. this methodology has been created to guide the design process from biological challenge, its function and main features, to some innovative ideas according design challenges and benefits, as well as to technical implementation with its features. we are in a preliminary design phase from stomata to technical implementation, and some results of this transfer have not been achieved yet. what follows is a short definition of functions for the architectural envelope, as an adaptive interface between environmental issues and internal comfort. identification of these functions will lead us to define innovation challenges in adaptive architectural envelopes compared to a standard or traditional building system in terms of energy efficiency. 3. environmental issues and active materials 3.1. environmental issues the environment is constantly changing and producing new challenges to cope with. light (solar radiation), temperature, relative humidity, rainwater, wind (air movement), noises and carbon dioxide (air quality) are the basic environmental issues affecting a building. these issues significantly affect occupant comfort demands as well as building performance. we have seen how stomatal movements and different microstructures have a great influence as interfaces between plant and environmental issues. humidity, temperature, carbon dioxide and light are the external inputs that have an effect on stomatal movements. based on this lesson from nature, the objective of this research is to transfer this biological mechanism into a new system for architectural envelopes capable of adapting to the same issues in order to be more efficient and less energy demanding. therefore humidity, temperature, carbon dioxide and light, are the environmental issues selected in this research and others such as rainwater, air movement or noises are not taken into account. external inputs depend directly on the specific climatic zone – data given by the worldwide bioclimatic classification system – as well as the season and the daytime or night-time period. for user demands inside the building some standard regulations for buildings are available, such as ashrae (american society of heating, refrigerating, and air-conditioning engineers) and cte (código técnico edificación), according to the level and type of activity. based on those demands, the following functions are defined for the architectural envelope, as an adaptive interface between environmental issues and internal comfort: • absorbing, collecting or evaporating to regulate humidity • dissipating, gaining or conserving to regulate temperature • filtering or exchanging to regulate carbon dioxide (air quality) • reflecting, absorbing, redirecting or diffusing to regulate light m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles 33 3.2. active materials after defining biological inspiration how can we materialize this mechanism into a technical implementation? this research tries to work out a system capable of growing or shrinking in size or changing in shape to adapt to different environmental issues, through the ingrained properties of the material they are made of, without the need for external energy or complex mechanical parts. we study the possibilities of fabrication of responsive systems, where multiple materials can react to the environment and deform over time, such as mechanisms that transform into a predetermined shape, changing property and function after fabrication through deformation of active materials capable to stretching, folding or bending, depending on environmental stimulus. however, traditional materials such as ceramics, metals or glasses are industrially produced to satisfy the demands of the building sector, so they are homogeneous and uniform in composition, and isotropic, having identical or very similar properties in all directions (menges, 2012). thus, conventional materials and manufacturing processes only provide inert solutions, static results or complex high-tech equipment to achieve kinematic systems. usually, kinetic architecture is developed through mechanical and electronic sensing, actuating and regulating devices, resulting in a non energy-efficient architecture. it seems clear that it would be very difficult to achieve our purpose by means of inert traditional materials. on the other hand, a wide range of smart materials has emerged in recent years. smart materials have properties that react to changes in their environment. the main research directions are based on shape memory alloys (sma), shape memory polymers (smp), piezoelectric materials, magnetostrictive materials, electrostrictive materials and electroactive polymers. there are several ways to classify different types of smart materials. we can mention the one carried out by addington (addington & schodek, 2004) where two kinds of smart materials are described: property change materials, that changes one of their properties (chemical, mechanical, optical, electrical, magnetic or thermal) in response to a change in the conditions of their environment, and do so without the need of external control and energy exchange materials that transform energy from one form to another to effect a desired final state. in addition, in some material centres, information related to smart materials is organized directly according properties and features, such as photoluminescence, electrochromism, shape memory, phase change or magnetorheology among others. in summary, most common active materials are those relying on electrical stimulus to activate movement, such as piezoelectric ceramics. however, this kind of smart materials has been dismissed for our selection, because we focus on low-tech and low-energy adaptive material systems, rather than highly automated and mechanical systems. we are interested in those materials that have structural and physical properties to generate movement or kinetically adapt in real time to environmental changes; active materials, with kinematic behaviours for a better performance that shrink, fold or expand responding to changes and, at the same time, remain stable in their different configurations. what follows is a description of a first approach to active materials with high potential to reduce energy consumption according to the environmental issues explained in previous sections. we look into self-actuating responsive materials with innate characteristics, behaviour and performative capacity to react to humidity, temperature, carbon dioxide and light. these atmospheric conditions act as ‘green’ triggers on active materials with reversible changes. 3.2.1. humidity reactive materials hydrogel: smart gel based on an insoluble network of polymer chains that swell up when water is added, making an expanded mass. hydrogels are highly absorbent, they can store large amounts of 34 m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles fig. 4. experiments of opening and closing movements in conifer cones due to humidity level changes. water, they also possess a degree of flexibility very similar to natural tissue, due to their significant water content. wood: wood is a traditional material, however we include it in this list due to its properties of hygroscopicity and anisotropy, which convert wood into a humidity reactive material. wood is a cellular structure and always seeks to reach equilibrium moisture, thus continuously responding to changes in the relative humidity by adjusting the bound water content, resulting in constant dimensional movement (menges, 2012). hydroskin pavilion, by the institute for computational design (icd), is a research work into autonomously responsive architectural systems through the use of wood as meteorosensitive material using its hygroscopic properties (reichert et al., 2014). conifer cones (fig. 4) have repetitive opening and closing cycles as responsiveness to humidity changes, and thus, they are biological examples of humidity reactive systems with responsive capacity in the structure of the material itself. 3.2.2. temperature reactive materials thermo-bimetal: sheets of differing metal alloys laminated together. when two metals which, when heated, expand at different rates and they are joined together, the structure that they form will bend as the metals fight each other into contortions and these could provide a useful embedded structural response (howes & laughlin, 2012). bloom installation is a research project, by doris sung about the use of these materials as cladding for buildings, with automatical ventilation based on the difference in temperature between the interior and exterior (kim sung, 2013). shape memory alloys: smart metals capable of recalling their original shapes. once deformed, heating them above their transformation temperature will trigger them to return to their original shape. there are a few different varieties, from metals that only remember one shape to metals capable of remembering two shapes, triggered by different temperatures (howes & laughlin, 2012). shape memory polymers: smart polymers capable to return from a deformed state, temporary shape, to their original, or permanent shape, induced by an external stimulus, in this case a temperature change. thermochromic polymers, inks or glasses: thermochromism is the property of substances to change colour due to a change in temperature. these materials alter their colour in reaction to temperature changes. heat sensitive plastics: plastics capable to expand induced by thermal changes (fig. 5). combinations of two plastics of differing coefficients of thermal expansion involve heat sensitive actuation. other m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles 35 fig. 5. experiments of heat sensitive plastic in the ucl healthcare biomagnetics laboratories (the royal institution of great britain) where an infrared camera is used to capture the heat distribution triggered through the temperature changes. polymers can also be blended to enhance this process such as ultra high molecular weight polyethylene (uhmwpe), which expands a lot under heating but is hard to process on its own. microfluidic channels and high surface area structures could also be added to enhance or decrease actuation effects in a smart design. 3.2.3. carbon dioxide reactive materials co2 responsive polymers: responsive polymers use carbon dioxide as a green or eco-trigger as well as to absorb co2 directly from air. two main types of materials are distinguished: carbon dioxide responsive polymers and carbon dioxide polymers for co2 capture (lin & theato, 2013). 3.2.4. light reactive materials photochromic dyes: photochromism is a reversible change of colour upon exposure to uv light. under the influence of uv light, a photochromic molecule will change shape, opening up into a very effective absorber of visible light form. this colour change is a reversible equilibrium, when the source of radiation is removed the molecule will revert back to its inactivated or ‘resting’ state. light responsive polymers: functional polymers that are able to undergo light-induced shape changes. polymer systems that convert photo-induced effects at the molecular level to macroscopic movement of working pieces, such as contraction and bending of azobenzene-containing liquid-crystal elastomers and volume changes in gels. some advanced materials researches focus on light-induced shape-memory polymers that can be deformed and temporarily fixed in a new shape (jiang et al., 2006). after this first approach according to requirements of the environmental issues, a second requirement to select materials would be manufacturing tools. in last years, the huge potential of additive manufacturing and three-dimension printing technologies to generate active systems and structures with complex geometry, have been tested. printing technologies have developed at an exceptional accuracy, speed, material property and manufacturing cost (raviv et al., 2014). recently, some research groups around the world, have gone one step further and have started to work around four-dimension printing concept, which allows materials to self-assemble into 3d structures, adding the extra dimension of time. some teams, such as those made up by skylar tibbits or h. jerry qi and martin l. dunn, develop and test methods for 4d printing, by means of smart materials to create 36 m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles fig. 6. experiments on 3d printing technologies, from left to right: 3d printed flexible chain mail, it has no motion for itself; possible joining combinations of chain mail with other sensitive materials; theoretical opening or closure of chain mail triggered through the temperature changes of sensitive plastic, it is a reactive structure with motion for itself. structures that can assemble themselves. active fibers can be incorporated into composite materials so their behaviour can be predictably controlled when the object is subjected to thermal and mechanical forces (ge et al., 2013; ryu et al., 2012; raviv et al., 2014). this 4d technology provides a new approach to creating reversible 3d surfaces and promises exciting new possibilities, among others the technical implementation for adaptive architectural envelopes we are looking into. consequently, materials with special properties to be manipulated and used in additive manufacturing will be selected (fig. 6). fig. 7. scheme idea of the testing scenario, a homogeneous climate chamber with controlled laboratory conditions to verify different motions. m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles 37 several experiments, with single active materials in simple geometries by 3d printing, will be carried out in order to elaborate laboratory tests in a climate-box (fig. 7). using the same shape element as a starting point, we will control the values of the environmental parameters to observe the response range and behaviour of different materials. these tests should show dimensional changes on element surface (i.e. opening, closing, expansion, fold and other transformations) caused by varying environmental issues, and thus, demonstrate the material capacity to rapidly respond to changes in temperature, humidity, carbon dioxide or light. through these experiments we try to explore the interrelationships between active materials research and its response to external stimuli (humidity, temperature, carbon dioxide and light) seeking meaningful ways to bridge between the natural inspiration, stoma, and the technical implementation, adaptive architectural envelope. once these individual experiments have been made and tested, the next step towards the final prototypes is the union of different materials with different features to manage hybrid materials, that is materials or active composites which can embody multiple functionalities. the materials needed for fabrication of humidity reactive systems may be different from the materials used in temperature reactive mechanisms. therefore, techniques of multi-material printing are needed to be embedded into a single 3d system or structure to control several environmental issues simultaneously, and thus implemented in the multi-adaptive architectural envelope. 4. conclusions the research focuses on the relationship that can be developed between active materials and environmental issues in order to propose innovative and low-tech design strategies to achieve living envelopes according to stomatal movements in plants, as a result of the application of biomimetics in architecture. environmental issues selected are humidity, temperature, carbon dioxide and light and we try to select active materials that respond to changes in these elements. some of these active materials include: heat sensitive plastics, wood or carbon dioxide responsive polymers, with capabilities such as programmable actuation, sensing, self-transformation and workable in 3d printer technologies. the next step in the research is the development of several experiments with single active materials in simple geometries by 3d printing to elaborate laboratory tests in a climate-box in order to demonstrate dimensional changes on surface elements caused by varying environmental issues; to carry out experiments in active material elements with behaviours as actuators for innovative and low-tech design strategies, and subsequently to achieve hybrid materials or active composites which can embody multiple functionalities. acknowledgments the authors are grateful for financial support of their work by the nominative subsidy to the university of oviedo for r&d projects of the ‘conserjeria de economia y empleo del principado de asturias’ (ref. fc-15-grupin14-004). references addington, m., schodek, d. (2004). smart materials and technologies. for the architecture and design professions. oxford, united kingdom: architectural press. 38 m. lópez et al. / active materials for adaptive architectural envelopes based on plant adaptation principles almusaed, a., (2011). biophilic and bioclimatic architecture. analytical therapy for the next generation of passive sustainable architecture. london: united kingdom: springer-verlag. armstrong, r. (2012). living architecture. how synthetic biology can remake our cities and reshape our lives. kindle single, ted books. [online]. available at: http://www.amazon.co.uk/living-architecture-synthetic-biology-ebook/dp/b0076qqjmy azcón-bieto, j. & talón, m. (2000). fundamentos de fisiología vegetal. barcelona, spain: mcgraw-hill interamericana. badarnah, l. (2012). towards the living envelope: biomimetics for building envelope adaptation. delft university of technology. doi:10.4233/uuid:4128b611-9b48-4c8d-b52f-38a59ad5de65 beesley, p. (2006). responsive architectures: subtle technologies. canada: riverside architectural press. bhushan, b. (2009). biomimetics: lessons from nature – an overview. philosophical transactions of the royal society, 367, 1445-1486. doi: 10.1098/rsta.2009.0011 del grosso a. e. & basso p. (2010). adaptive building skin structures. smart materials and structures, 19, 12. doi:10.1088/0964-1726/ 19/12/124011 dobzhansky, t., hecht, m. k., steere, w. c. (1968). on some fundamental concepts of evolutionary biology. evolutionary biology, 2, 1-34. new york, ny: appleton-century-crofts. fortmeyer, r. & linn, c. (2014). kinetic architecture: design for active envelopes. mulgrave, australia: images publishing. ge, q., qi, h. j., dunn, m. l. (2013). active materials by four-dimension printing. applied physics letters, 103(13): 131901. doi: 10.1063/ 1.4819837 gibson, l. (2012). the hierarchical structure and mechanics of plant materials. journal of the royal society interface, 12, 106. doi: 10.1098/ rsif.2012.0341 howes, p., laughlin, z. (2012). material matters: new materials in design. london, united kingdom: black dog publishing. jiang, h. y., kelch, s., lendlein, a. (2006). polymers move in response to light. advanced materials, 18(11), 1471-1475. doi: 10.1002/ adma.200502266 kamal-chaoui, l. & robert a. (2009). competitive cities and climate change. oecd regional development working paper. paris, france: oecd. kim sung, d. (2013). metal that breathes, on-line: tedxusc. available at: http://www.ted.com/speakers/doris kim sung.html koch, k., bhushan, b., and barthlott, w. (2009). multifunctional surface structures of plants. an inspiration for biomimetics. philosophical transactions of the royal society, 367, 1487-1509. doi: 10.1098/rsta.2009.0022 lin, s., theato, p. (2013). co2-responsive polymers, macromolecular rapid communications, 34(14), 1118-1133. doi: 10.1002/marc. 201300288 martin, e. s., donkin, m. e. and stevens, r. a. (1983). stomata, london: edward arnold. mazzoleni, i. (2013). architecture follows nature-biomimetic principles for innovative design, new york, ny: crc press. menges, a. (2012). material computation. higher integration in morphogenetic design. architectural design, 82, london, united kingdom: wiley academy. oxman, n. (2012). programming matter. architectural design, 8, 88-95. doi: 10.1002/ad.1384 pawlyn, m. (2011). biomimicry in architecture. london, united kingdom: riba publishing. park, j. j., dave, b. (2014). bio-inspired parametric design of adaptive stadium facades. australasian journal of construction economics and building conference series, 2, 27-35. doi: http://dx.doi.org/10.5130/ajceb-cs.v2i2.3886 raviv, d., wei, z., mcknelly, c., papadopoulou, a., kadambi, a., shi, b., et al. (2014). active printed materials for complex self-evolving deformations. scientific reports 4, 7422. doi:10.1038/srep07422 reichert, s., menges, a., correa, d. (2014). meteorosensitive architecture: biomimetic building skins based on materially embedded and hygroscopically enabled responsiveness. computer-aided design, 60, 50-69. doi: 10.1016/j.cad.2014.02.010 rivas-martı́nez, s. (2011). worldwide bioclimatic classification systemglobal geobotany, 1(1). doi: 10.5616/gg110001 ryu, j., d’amato, m., cui, x., long, k. n., qi, h. j., dunn, m. l. (2012). photo-origami. bending and folding polymers with light, applied physics letters, 100(16), 161908. spiller, n., armstrong, r. (2011). it’s a brand new morning, protocell architecture. architectural design, 81. london, united kingdom: wiley academy. vincent, j., bogatyreva o., bogatyrev n., bowyer, n., and pahl, k. (2006). biomimetics: its practice and theory. journal of the royal society interface, 3(9), 471-482. doi:10.1098/rsif.2006.0127 vogel, s. (2012). the life of a leaf, university of chicago press. http://www.amazon.co.uk/living-architecture-synthetic-biology-ebook/dp/b0076qqjmy http://www.ted.com/speakers/doris_kim_sung.html http://dx.doi.org/10.5130/ajceb-cs.v2i2.3886 journal of facade design and engineering 2 (2014) 135–136 doi 10.3233/fde-150035 ios press 135 editorial facade design and engineering finding the scope within a growing community facade design and engineering in many cases is related to other fields such as structural design, material technology, design and construction processes, building physics and climate design. in the past, research efforts in the field of building envelopes were directed towards those fields, more or less already established in the scientific world. we started jfde because we felt a specialised journal was needed to react to a constantly growing interest in the discipline and with it, naturally a growing community of readers and authors. there is evidence of this development in different areas: because of rising requirements in terms of energy savings of buildings and more complex constructions, facade design and engineering is becoming a recognised and strong field in building practice. internationally, there are no longer large projects that do not involve facade specialists. most large building engineering offices have specialised facade teams, which was not the case a couple of years ago. but the number of specialist offices grows steadily as well. in parallel, universities worldwide start to create chairs or working groups that focus on the building envelope. the building technology master track at tu delft started a program for facade design in 2007. the program is closely linked to the work of the facade research group. we can proudly say that, despite the ongoing recession on the architectural market, our graduates find jobs easily, within the netherlands and many of them in offices and companies all over the world. we are actively creating our own reader group with young professionals that leave our educational institutions towards practice and research. still, there is the question how facade design and engineering will win recognition in its scientific environment. the ‘if’ is not a question any more! our last special issue focussed on glass, and again this issue features two papers about the topic. one is about structural silicone glazing and the other is an applied practice paper about the novel use of glass bricks for a facade in amsterdam. in our point of view we will see a specialist journal on architectural glass emerging soon. the other papers discuss design tools, constructive strategies for ‘massive’ envelopes and the assessment of building integrated photovoltaic. the scope of jfde is still very broad, and for now we will keep it that way. research efforts always focus on gaining new knowledge and ultimately aim at improving the built environment. the design of building envelopes is a multifaceted and integrated task. our target is to bring together the scientific facade community. we will keep observing new trends in research and subsequently determine whether jfde will need a narrower scope. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. 136 t. klein and u. knaack / editorial facade design and engineering is a peer reviewed, open access journal, funded by the netherlands organisation for scientific research nwo (www.nwo.nl). we see ‘open access’ as the future publishing model. but it certainly requires new financial models which we will have to explore over the coming years. the editors in chief, tillmann klein and ulrich knaack from city’s station to station city 029 journal of facade design & engineering volume 10 / number 1 / 2022 towards a human centred approach for adaptive façades an overview of user experiences in work environments mine koyaz1-2*, alejandro prieto3-5, aslıhan ünlü4, ulrich knaack3 * corresponding author 1 istanbul technical university, construction sciences doctoral program. turkey, koyaz@itu.edu.tr 2 istanbul bilgi university, faculty of architecture, department of architecture. turkey 3 delft university of technology, faculty of architecture and the built environment, department of architectural engineering + technology, architectural facades & products research group. the netherlands 4 özyeğin university, faculty of architecture and design. i̇stanbul, turkey 5 universidad diego portales, school of architecture; faculty of architecture, art and design. santiago, chile abstract adaptive façades are multifunctional systems that are able to change their functions, features, or behaviour over time in response to changing boundary conditions or performance requirements. as one of the significant developments in the façade industry over the last decade, the adaptive façade offers an intelligent solution that can decrease energy consumption and potentially increase users’ comfort in a building. from an engineering perspective, these advanced technologies aim to improve the overall performance of the building while generating a better indoor environment for the users, but unfortunately, investigations show that this goal is not always achieved. this is why, to bridge this performance gap, we embark on a change of perspective in façade design, from a technology-centred to a human-centred one. this research emphasizes that, with their changeability aspects, adaptive façade technologies offer unique potential, although the design of such façades requires a deeper understanding of users. with this as its focus, this paper aims to identify the factors affecting the user experience in a working environment, considering the interactions of the user with building services and façade systems from a holistic point of view, in which façade-user relationships are to be distinguished, towards the larger aim of developing a human-centred approach for adaptive façade design. keywords human centred design, user experience, adaptive behaviour, adaptive façade, façade design, changeability, interaction doi http://doi.org/10.47982/jfde.2022.1.02 030 journal of facade design & engineering volume 10 / number 1 / 2022 1 introduction the complex multifunctional aspects of façade design hold an inherent duality between optimizing the energy performance of a building and providing comfort for its users (klein, 2013). although these two sides are inseparable and highly intertwined, as a result of their conflicting nature one could be easily overlooked in favour of the other, which unfortunately would result in poor design on both sides. worldwide, there have been several research projects that have searched for new ways and developed new technologies to design more energy efficient buildings for a more sustainable future (aelenei et al., 2018). the belief that “technology will save us” has been a clear trend in building engineering, but there has been a general disregard for the people who will use it. even with the use of high technology solutions, in practice we may end up with the following scenarios: a building may be highly energy efficient but its users could be dissatisfied or because of users’ interference the building may not achieve the predicted levels of energy performance and we may end up with dissatisfied users anyway (lazarova-molnar & mohamed, 2017). since the façade is the mediator between the exterior and interior environments, it could be said that its design is the key to shaping the whole performance of the building (knaack et al., 2014). however, implementing high technology solutions to reduce energy consumption should not turn the façade into a barrier between the indoor and outdoor, which would create hermetically sealed spaces in which every physical aspect is regulated through hvac systems (addington, 2009). instead, the façade needs to be carefully designed and integrated with the service systems, where smart technologies would be used as the tools to provide desired indoor environmental conditions in harmony with the users and to find an optimal energy efficient balance. in order to achieve that goal, a human-centred approach in façade design, rather than a technology centred focus, needs to be embraced. the current applications of high performance buildings with smart building automation systems stress even further the need for such human-centred approach (capeluto & ochoa, 2017). these socalled intelligent systems are creating new pathways for user interactions, where user expectations and user experiences are changing along with it (kaasinen et al., 2013). wigginton and harris used the analogy of the human body’s neural system to explain their user-centred perspective on the topic, in which the somatic system that is responsible for the voluntary actions of the muscles that create movement is likened to the actions of the users in the building. conversely, the autonomic system refers to the building’s intrinsic responsive envelope (wigginton & harris, 2002). this approach highlights the co-existent nature of users’ adaptive behaviours and the automated dynamic responses of the building systems. a paradigm shift from occupants being passive subjects in a building to being active participants has begun by acknowledging the effects of occupant behaviour on the energy performance of the building (yan & hong, 2018). although significant efforts are being made to better understand and simulate the adaptive behaviours of the users, the influencing factors behind these interactions and how these affect the user experience in the first place, are still quite theoretical (heydarian et al., 2020). there needs to be further interdisciplinary studies investigating multi-domain exposure situations on users’ satisfaction and behaviour, humans’ perception of the built environment, and user-oriented design of the control interfaces, in order to achieve an occupant-centric building design approach (o’brien et al., 2020). 031 journal of facade design & engineering volume 10 / number 1 / 2022 fig. 1 façade – user – building relationships there are many factors affecting and shaping users’ experience in a building (figure 1). the feeling of comfort (or discomfort) brought about by the effects of the factors related to the indoor and/or outdoor environment, spatial conditions, as well as the individual parameters related to the user itself are the main elements that could be listed. it is not just the comfort-related satisfaction, but also the overall user sensation and perception that results in interactions with the building façade and/or service systems, that are important parts of the user experience in the indoor environment (law et al., 2009). considering the amount of time we spend inside, especially in work environments, several studies highlight the importance of users’ comfort and wellbeing on their health and productivity (bluyssen, 2019; fisk, 2000; humphreys, 2005). even though the façade is highly significant in shaping the indoor environment, there seems to be a lack of understanding about its effects on the overall user experience (alavi et al., 2017). even in low technology buildings, the façade is a system that users commonly engage with, by opening the windows, closing the window blinds, or simply by looking outside. it does not seem far-fetched to state that these interactions have a crucial effect on users’ experience, while they also create a gap between the predicted and achieved performance of the building (masoso & grobler, 2010). such adaptive behaviours of users are one of the key factors that influences a building’s design, performance optimization, and energy simulation (yan & hong, 2018). in the case of adaptive façades, with their smart and dynamic automation systems, it could be said that interactions between the façade and the user become even more significant. from an engineering perspective, being able to change façade properties would enhance their performance –by providing dynamic responses to ever-changing external stimuli–, but this dynamic character may also enhance the effect of the façade on the user experience, hence, it may also potentially deepen the aforementioned performance gap. there is a limited number of studies focusing on adaptive façades, investigating the comfort and well-being of the users as they relate to their interactions. in a recent study, luna-navarro presents a classification scheme for different pathways in occupant façade interaction, which offers a deeper understanding of the conflicts and relationships between the user and intelligent control strategies in adaptive façades (luna-navarro et al., 2020). another study on occupant-centric control strategies for adaptive façades discusses the implementations of adaptive comfort models and personalized control strategies to improve user satisfaction (tabadkani et al., 2021). in addition, there are a few other research studies on post-occupancy evaluations and real-time occupant feedback in buildings with adaptive façades, demonstrating the influences of different physical factors on the feeling of comfort in users (alavi et al., 2017; attia et al., 2018; bakker et al., 2014; lassen et al., 2021; lunanavarro et al., 2021). 032 journal of facade design & engineering volume 10 / number 1 / 2022 acknowledging the contributions of such research towards reaching a human-centred adaptive façade, the influence of human factors on user behaviours and the indirect effects of interactions on the user experience still requires further investigation. therefore, this paper (1) investigates how different factors affect the user experience in a working environment, (2) presents an overview of adaptive user behaviours with a human-centred approach, (3) distinguishes the façade user relationship within the larger context, and (4) identifies the missing links towards reaching the full potential of adaptive façades. a holistic perspective is embraced in this study and the presented overview is built over the user experience concept, which covers all human senses, emotions, behaviours, and interactions with the façade and/or environment. with this focus, this paper aims to bring the scattered information in the literature from different fields of expertise like engineering, architecture and social sciences together, ensuring that not just the effects of the environmental factors but also the human factors on user experience are investigated. as a broader aim, the information provided in this paper offers a starting point towards developing a human-centred approach for adaptive façades. 2 scope and method of research research presented in this paper starts with an initial exploration of literature, which draws the context for further investigation. exploratory research is conducted using science direct, google scholar, itu and tu delft libraries, with the keywords: human-centred design, adaptive façade, user experience. because of the lack of literature specifically focused on the façade and user relationship, the scope of the overview later widens to involve: human-centred design in various fields, façade technologies and applications (traditional or advanced), façade properties and performances in general (physical and energy related), user comfort, satisfaction, well-being, and related studies to improve indoor environment quality. after reading the titles, abstracts, keywords, and highlights of the papers collected in the initial exploration, the context of the detailed investigation is formulated. figure 2 below portrays the research boundaries around respective keywords and concepts, presents an outline of this paper, and draws the relationship of its sections. the users or the occupants are the main subjects of this overview. the scope of the study evolves around the user experience concept, which embraces a holistic approach to cover the users’ comfort, satisfaction, and well-being, as well as users’ emotions, behaviours, and interactions. the relationship between these concepts and their relevance to user experience are examined through studies of human-centred design, from different fields (social sciences, industrial design, environmental psychology, etc.). although the main focus is on the adaptive façades and distinguishing its dynamic relationship with the users, since there is a scarcity of literature with this specific focus, studies related to user experience in indoor environments are investigated in general. workplaces are chosen as the main domain of the research. in order to define a whole list of influencing factors, several agents of the indoor environment and adaptive façades are searched within the context of user experience. thus, various studies evaluating users’ comfort during the occupancy stage, theoretical studies investigating different parameters affecting user’s satisfaction, studies regarding users’ adaptive behaviours and the motivations behind that, users’ effects on the energy performance of the building, studies investigating occupant-centric design and automation of the adaptive façade, and other relevant studies within its broader reach, formed part of the review process. 033 journal of facade design & engineering volume 10 / number 1 / 2022 fig. 2 scope and boundaries of the research as for the structure of the paper, section 3 presents background information on different approaches of human-centred design to establish the point of view of this research, and to describe the relationship between adaptive behaviour and user experience. section 4 introduces the potentials of adaptive façades, presents a brief overview of their relationship with the users, identifying the missing links, demonstrating the perspective of the authors and relevance of the research. section 5 is the main body of the paper, where the findings of the review process are presented in a systematic way and different factors affecting user experience in working environments are identified, through a mechanism of change. lastly, section 6 presents an overall discussion and highlights relevant issues that require further investigation for a human-centred approach for adaptive façades. 3 human-centred design human-centred (or centric) design is a concept which, over the years, has found application in various different fields, mainly in industrial design and software engineering (hoffman et al., 2002). different approaches to the concept in architecture and related behavioural theories that highlight a deeper understanding of the user experience are overviewed in this section. 3.1 human-centred design approaches norman describes human centred design as the process which ensures that the designs match the needs and capabilities of the people for whom they are intended (norman, 1988). in other words, it is a philosophy that starts with understanding humans, to support the design of a useful, usable, pleasurable, and meaningful product/system (ideo.org, 2015; norman, 2013). designing for the users has been a part of design thinking since early ages. the evolution of the concept has started with the consideration of the physical ergonomics of the users while engaging with the product/system (verbeek & slob, 2006). later, this concept evolved into usability, which covers the effectiveness, efficiency, and satisfaction in a specified context of use (iso 9241-210, 2010). then, the consideration of emotions, behaviours, and users’ experiences became part of the approach, often referred to as interaction design in the literature (t. zhang & dong, 2008). recent approaches aim to cover not just the present experiences but also the needs and aspirations of the future (van 034 journal of facade design & engineering volume 10 / number 1 / 2022 der bijl-brouwer & dorst, 2017). designing with the users on the other hand, by means of involving them in the design process (participatory design and generative techniques) or asking for feedback regarding the design (human-computer interaction), could be traced back to the 1970s – 1980s (damodaran, 1996; maguire, 2001; visser et al., 2005; zoltowski et al., 2012). sanders maps the different human-centred approaches in design practice and defines the designer’s mindset as two categories; expert or participatory (sanders, 2008). expert mindset refers to the case where users are seen as subjects and the design aims to achieve the best possible usability for them. participatory mindset, on the other hand, refers to the case when users are seen as partners and are actively involved in the design process from the beginning (abras et al., 2004). involvement of the users in the design phase is a way of ensuring that the design would match their needs, but unfortunately, there are operational barriers (chammas et al., 2015). consequently, in the case of building design, engagement of the users mostly occurs only at the occupancy phase, in the form of post-occupancy evaluations with real-time occupant sensing and/or feedback technologies (wagner et al., 2018). a collective perspective of the different approaches is offered by iso 9241-210, where the human-centred design is defined as an approach to systems design and development that aims to make interactive systems more usable by focusing on the use of the system and applying human factors/ergonomics and usability knowledge and techniques (iso 9241-210, 2010). feedback and communication between the stakeholders (users and designers) are highlighted as the key to this approach. by acknowledging iso’s definition of human-centred design and embracing an expert mindset, we are taking the user as the subject of this research. hence, we start by investigating how users interact with and experience their surroundings, thereby aiming to translate and transfer the information learned from users to the designers. 3.2 adaptive behaviour and user experience relationship the term experience is used to define the encounter of an event or occurrence and the takeaways from it. it could be a piece of knowledge, a skill or an impression, a feeling. in other words, experience defines how people remember their interactions (norman, 2013). from a human centred point of view, any system that interacts in some way with its users should be designed with consideration given to users’ needs and capabilities. therefore, in order to achieve the best user experience, it is crucial to understand the factors that shape the nature of such interactions and how these may differ from one individual to another. according to the stages of action model (norman, 2013), an action (or behaviour) like opening a window is a goal-driven activity that could be triggered by various different physiological, psychological, or sociological factors. the activity starts by making the decision to take action, which is defined as the planning stage, followed by specifying the action and performing it by engaging with the window. the effects of the action will begin to show themselves, like creating changes in the air quality and temperature of the room. then the user will perceive the new condition, interpret the reason behind it, compare it with the previous situation and make a decision to either close the window or keep it open. in this instance a data/event-driven activity occurs, which has surfaced as a result of the user’s emotions in the initial condition (naqvi et al., 2006). in other words, the perception of the environment triggered the interaction with the surroundings, while at the same time the interaction created a change in the perception. 035 journal of facade design & engineering volume 10 / number 1 / 2022 even though norman’s model defines a certain loop between perception and behaviour, it does not explain what exactly drives the behaviour itself. there are several other cognitive behaviour theories used in explaining occupant interactions with different building systems. from these theories, five commonly acknowledged models could be listed as: theory of reasoned action tra, theory of planned behaviour tpb, theory of interpersonal behaviour -tib, norm activation model – nam and value, belief, norm theory – vbnt (ajzen, 1991; heydarian et al., 2020; triandis, 1977), in which affect, attitude, social norms, perceived control, and habits are referred to as the main elements that create the intention of taking an action. affect refers to what the organism feels, and defines the value, in other words, the quality of the experience. interpretation of such feelings could be referred to as emotions that shape preferences for the future or moods for the present situation (ortony et al., 2005). along with the affect, different personal or social norms and individual beliefs are also related to the intention of a behaviour (pee et al., 2008). the perceived control and individual’s attitude towards taking an action are other elements that generate behavioural motivations (heydarian et al., 2020). in addition to these conscious behaviours, habit refers to a sequence of behaviour that has become an automatic response to specific cues in the environment (verplanken & aarts, 1999), which can be defined as an unconscious behaviour or an intentional one. all in all, a behaviour could surface because of a poor affect, with the intention to make it better and as a result, it will create an interaction to change the values of the experience. this phenomenon could also be explained with the concept of adaptive behaviour which is often referred to in the literature as follows: if a change occurs such as to produce discomfort, people react in ways that tend to restore their comfort (nicol & humphreys, 2002). such comfort-driven behaviours are an important part of the user experience and simultaneously affect the overall energy performance of the building by creating unforeseen energy consumptions (jia et al., 2017; stern, 1992) fig. 3 the dynamic cycle of the user experience: the mechanism of change within the context of the built environment, users’ behaviours, in terms of interactions with the building’s service systems or façades, are significant elements of the user experience, which either (1) shape or (2) reflect upon the experience itself (alavi et al., 2017). these behaviours may differ for each individual, due to their different physiological, psychological, or sociological conditions/backgrounds (khalid, 2006). therefore, understanding the mechanism of change is considered to be an important step towards defining how different factors affect the user experience in work environments. 036 journal of facade design & engineering volume 10 / number 1 / 2022 4 potentials of adaptive façades the previous section introduced the dynamic cycle of user experience over a mechanism of change (figure 3). in the case of adaptive behaviours, users are the actors that trigger the mechanism of change. influencers that affect the users’ behaviours may be related to human or environmental factors, which will be overviewed in the following section in relation to the user experience. triggered by those factors in a case of discomfort, to achieve a more desired comfort condition, users can either choose to alter themselves, like changing their clothing level or the occupancy conditions, or they can alter their environmental conditions by interacting with the building’s façade and/or service systems (o’brien & gunay, 2014) (figure 4). adaptive façades with their smart automation systems have a similar mechanism of change, by creating an artificial response to certain impulses, which results in a change in the indoor environmental conditions. fig. 4 relationship of factors indirectly affecting user experience if we take a holistic approach to façade design, it could be said that today’s challenge is to find an energy efficient façade solution that would also create a positive user experience. to deal with that complexity, and with their changeability aspect, adaptive façades offer a unique opportunity. this research explores the hypothesis that there is a missing link (shown in red lines on figure 4) between the human factor and the adaptive façade. the operation of the adaptive façade by means of automation should regard the human factors as agents, along with the environmental factors. moreover, automation systems should be designed to co-exist with and learn from the users’ adaptive behaviours, and to consider the indirect effects on the user experience. if these gaps could be bridged, only then would adaptive façades reach their full potential. adaptive technologies have the potential to significantly reduce the energy use of the buildings (perino & serra, 2015) along with having a profound influence on user satisfaction ( attia, 2017). the performance of the adaptive façade – in terms of comfort and energy – is highly dependent on its dynamic operation strategies (favoino et al., 2018). the automation system is another actor like the user, which is used to manipulate the change in indoor environmental conditions, thus directly and/or indirectly altering the user experience. therefore, design consideration of an adaptive façade system should consider the implementation of occupant-centric automation and integration with the operation of other building services systems, along with the functional requirements of the façade. each adaptive façade technology (such as movable solar shading, switchable glazing, phase change materials, dynamic insulation, multifunctional façades …etc.) requires a unique consideration since they offer different ways of interaction with the user and may have different effects on users satisfaction and well-being ( attia et al., 2019). in terms of a human-centred design approach to 037 journal of facade design & engineering volume 10 / number 1 / 2022 adaptive technologies, existing studies and performance modelling approaches are limited to considering environmental factors as the main agent of change, thus missing the relationship with the human factors and user experience. along this path, one of the key issues that needs to be investigated is the balance between automation and user control. adaptive façades controlled by an intelligent automation system could be designed to predict a possible result of discomfort and act on behalf of the users to alter the indoor environmental conditions (böke et al., 2020). however, there are also studies indicating that users are more satisfied, happier, and more productive when they are given at least a certain degree of control (samani, 2015). this opens up the discussion on how much control should be left to the users considering the building’s energy performance. educating the users and improving the response feedback mechanism would be the highlighted ways to deal with a possible performance gap between the predicted and actual energy consumption of the building (al-obaidi et al., 2017). another issue is the diversity of each user’s needs and preferences. there have been several studies aiming to model the reasoning behind users’ behaviours and simulate them in order to demonstrate the actual energy performance (yan et al., 2015). however, it is quite impossible to predict how each user would react to a certain situation. from another perspective, despite the variety of each human factor and its endless combinations, by observing user’s behaviour under certain conditions, it could be possible to identify some user types (ortiz, 2019), which requires a deeper understanding of the human factor and an interdisciplinary study that incorporates architecture, engineering, and social science. from an expert point of view, by identifying what are the key factors that affect the experience and how they show diversity, the level and type of adaptation that is needed for certain user types could be defined. thus, this would be used as a starting point towards a human-centred adaptive façade design approach. from this perspective, which aims towards a human centred design approach, we are looking from the users’ point of view to understand what experiences that they expect from their façades. identifying the complex and dynamic mechanisms of the user experience in an indoor environment (workspaces in the context of this paper) would be the first step. to that end, this research investigates how each factor affects the mechanism of change and portrays a better understanding of the triggers that make users take action. from there, it would be possible to (1) identify certain patterns in users’ adaptive behaviours that could be mimicked with the adaptive façades, (2) determine the effects of human factors and define certain expectancies of different user groups, (3) distinguish certain aspects of the façades that need to be variable in order to satisfy individual adaptive comfort needs. 5 user experience in work environments user experience is defined as a person’s perceptions and responses that result from the use or anticipated use of a product, system, or service (iso 9241-210, 2010; law et al., 2009). it is a concept that not only covers the past and present experiences but also those anticipated for the future. in the context of this paper, experience is used as a holistic term to cover all human senses and refers to any kind of emotion, perception, sensation over interaction, and any feeling of comfort, satisfaction, or well-being. 038 journal of facade design & engineering volume 10 / number 1 / 2022 although the façade is one of the main systems that shapes the indoor environment, there has been only a limited number of studies exploring how it affects users’ experiences. the ways in which users engage with their façade and experience it could be defined in two ways: active or passive. (1) passive experience refers to the perception, for instance, of feeling the sunlight coming through the glazing or looking outside the window. (2) active experience refers to interaction, for instance, opening the window because of feeling warm, closing the window blind to prevent glare. in other words, passive experience indicates a path for the direct effect of an influencer, while active experience draws an indirect path where certain behaviour is triggered. based on the review of the existing literature, the effects of different factors on the user experience, in a working environment, are identified. figure 5 shows the relationship of the parameters from the findings of the review, which are systematically organized in a way to demonstrate the dynamics between the influencing factor, the mechanism that is triggered by it, the response that results in a change of the initial state, and how this whole cycle affects related user experiences (figure 3). referring to the aforementioned mechanism of change, we classify the influencing factors under two main groups: human and environment. according to occupant behaviour studies, it could be said that it is not only physical factors of the environment, but also the individual factors for each user, that influence the experience in an indoor environment (frontczak & wargocki, 2011). as the elements that could be easily manipulated by the building design, the general tendency in the related research projects is to focus on the environmental factors. there are only a few studies that investigate the diversity of individual factors and their relationship with users’ adaptive behaviours (hong et al., 2017). taking the human as the centre of focus, first of all, personal descriptors and needs that are different for each individual and which affect the experience are outlined. environmental factors, on the other hand, are related to the surroundings of the user. on a building scale, these would be divided into three subcategories: external, internal, and spatial factors. fig. 5 relationship between the parameters within the mechanism of change the main adaptive behaviours that users would take to alter their indoor environment, based on the common mentions within the existing literature as potential causes of unpredicted energy consumptions, could be listed as (1) opening/closing windows, (2) adjusting window blinds (shading elements), (3) adjusting hvac systems (heating, cooling, ventilation units), (4) turning on/off lighting (boerstra, 2016; delzendeh et al., 2017; nicol & humphreys, 2002). in addition to these, in cases of discomfort, users may also choose to (5) adjust their clothing or upon arriving/departing the space (6) change their presence without engaging with their surroundings (haldi & robinson, 2008; zhang et al., 2018). the main focus of this paper is on the experiences of the users that resulted 039 journal of facade design & engineering volume 10 / number 1 / 2022 from their comfort-driven interactions with their façade and integrated service systems, thus the first four behaviours that require engagement with the windows, shading, hvac, or lighting systems are investigated, in the context of a working environment. each of these actions creates a reaction and alters at least one of the properties of the façade and/or the indoor environment. these could be respectively defined as the changes at (1) air exchange rate, level of (2) sunlight transmittance, value of (3) indoor temperature, (4) illumination level (frontczak & wargocki, 2011; wagner et al., 2018). the changed conditions that resulted from the adaptive behaviours indirectly affect the user experience, and thereby become an influencer once again. as mentioned before, within the context of this research, the user experience concept is issued from a holistic point of view that covers all human senses, emotions, behaviours and interactions with the façade and/or environment. it not only refers to a state of comfort or discomfort, but it is about the overall sense of satisfaction that occurs before, during, and after use (iso 9241-210, 2010; law et al., 2009). therefore, from a human-centred point of view, this paper takes the human senses to its centre and classifies user experiences as seeing, feeling, hearing, and controlling. the aim is to broaden the commonly used visual, thermal, air quality, and acoustical comfort titles, and to investigate the indirect effects of comfort-driven adaptive behaviours on the user experience. with that perspective, seeing refers to any experience related to sight, issues regarding visual comfort and aesthetics are covered under this title. feeling is used to cover the experiences related to the haptic and olfactory senses of the human. the factors that are mentioned in relation to the experience of feeling show a similar mechanism of change and are related to the thermal comfort and indoor air quality collectively, and therefore could not be separated. hearing refers to the audial experiences and is related to acoustical comfort. controlling is added since the main focus of this study is adaptive façades and it refers to the anticipation of control during user interaction and its psychological effect on user experience. in addition to these, the overall experience title is also used, referring to the factors that may not have a direct effect on a specific sense, but still found to be in relation to a general positive sensation of the human being. based on the above-mentioned classification, collected and interpreted information from the literature on how each human or environmental factor triggers each comfort-driven adaptive behaviour, which in turn creates a change in the environment that affects user experience, (figure 5), are systematically presented in this section over the defined mechanism of change. 5.1 human factors each person by nature has their own unique traits. these are not just physiological factors, but also psychological, social, cultural, economic, or other contextual factors that show diversity among each individual. these different characteristics alter a person’s needs, habits, aspirations and influence his/her experience (khalid, 2006). these human factors are grouped as personal descriptors and needs. a holistic approach to how each human factor triggers an adaptive behaviour and alters the experience in a working environment is found to be lacking in the literature. therefore, an attempt to gather the existing but scattered information from the literature is made and presented in detail in this section. table 1 below presents the relationship of each factor with each adaptive behaviour and user experience 040 journal of facade design & engineering volume 10 / number 1 / 2022 table 1 the human factors that affect user experience in a mechanism of change impulse (influencer / agent) mechanism (action / behaviour) response (effect / change) related experience human factors o p en in g / c lo si n g w in d o w a d ju st in g w in d o w b li n d a d ju st in g h v a c t u rn in g o n /o ff l ig h ts a ir e xc h a n g e ra te s u n li g h t tr a n sm it ta n ce in d o o r te m p er a tu re il lu m in a ti o n l ev el s ee in g f ee li n g h ea ri n g c o n tr o ll in g o ve ra ll personal descriptors age + + + + + + + o gender + o + + + + o + country of origin o o o o o + + o + educationlevel o o o o o o o + + + type of job + + + + + + + + + + o + + socio-personal traits + + + o + + + + + + o + o personal needs security + o o o o o o o + + privacy + o o + o + o o hygiene o o o o o + view + o + + + o o + (+) direct mention in the literature, (o) interpreted from the literature, (-) not mentioned in the literature 5.1.1 personal descriptors the factors mentioned in this category refer to the characteristic features of an individual, which may be quantifiable tangible conditions or identifiable intangible aspects of the users. age age is mentioned as one of the physiological factors that have an impact on the users’ comfort requirements and mainly on the thermal comfort perceived by the occupants (al horr et al., 2016; d’oca et al., 2016; fanger, 1970). frontczak & wargocki, in their research, portray how individual characteristics influence satisfaction with indoor environmental quality, and age is associated with subjective air quality, visual satisfaction, and adverse perception (frontczak & wargocki, 2011). in another study that identifies the influencing factors on occupants’ energy related behaviours, age is listed as one of the social and personal parameters (delzendeh et al., 2017). in terms of adaptive behaviours, window opening behaviour (fabi et al. 2012; zhang et al. 2018), heating thermostat adjustment (deme belafi et al., 2018), and in general the need for personal control (boerstra, 2016) are affected by the age factor. gender another frequently mentioned physiological factor that influences thermal satisfaction and the overall comfort perception of the users is gender (al horr et al., 2016; boerstra, 2016; d’oca et al., 041 journal of facade design & engineering volume 10 / number 1 / 2022 2016; day & gunderson, 2015). it is an acknowledged fact in the field that in general, due to different body compositions, thermal preference and satisfactory set temperature for a female and a male differ from each other (delzendeh et al., 2017; fanger, 1970; huizenga et al., 2006). in the case of discomfort, altering clothing (holopainen et al., 2014) could be the initial act followed by heating thermostat adjustment (deme belafi et al., 2018), opening or closing the windows (fabi et al., 2012; zhang et al., 2018) or interfering with the hvac system to alter the intensity of airflow (fountain et al., 1996). along with thermal sensation, frontczak & wargocki also mention gender as influencing the satisfaction with air quality and the visual satisfaction along with general preference on indoor environmental conditions (frontczak & wargocki, 2011). country of origin country of origin is listed as another human factor that has both physical and socio-economic aspects. first of all, due to racial features and/or accustomed climatic conditions, users may have different means of thermal comfort (al horr et al., 2016). in addition, the expectation of thermal conditions would also vary from country to country due to cultural differences and defined local standards, which would result in different experiences in terms of feeling (day & gunderson, 2015). as a result of various customs and habits, individuals’ acoustical dissatisfaction and any overall adverse perceptions would also show diversity (frontczak & wargocki, 2011). lastly, in terms of adaptive behaviour, cultural norms are mentioned as an influence on the use of electrical systems (o’brien & gunay, 2014). education level the level of education is mentioned in relation to the user’s knowledge and awareness. as a sociopersonal factor, education affects users’ attitudes, motivations, and behaviours in their working environment (delzendeh et al., 2017). although it is not possible to define a direct correlation, users with different levels of education have a different perception of thermal comfort, air quality, and overall satisfaction (frontczak & wargocki, 2011). day & gunderson states that to minimize energy consumption, especially in high-performance buildings, training and education have a crucial effect (day & gunderson, 2015). informing users on how to control the façade and service systems, and in which way their actions would affect the energy performance of the , greatly influence adaptive behaviours such as opening/closing windows, adjusting hvac units, and turning on/off lighting (yan & hong, 2018). type of job type of job is one of the most comprehensive factors that covers dress codes and activity levels, working hours, time pressure and stress levels, relationship with colleagues, relationship with employees/employers, and overall contentment with the job. according to frontczak & wargocki different factors related to different professions, defining a variety of needs and influencing the perception of thermal comfort, visual comfort, and air quality, as well as overall satisfaction (frontczak & wargocki, 2011). job satisfaction in general is shown to have a significant effect on work performance, productivity, and psychological well-being (samani, 2015). depending on the job description, the required level of activity would define a metabolic rate and clearly have an influence on the thermal comfort parameters (delzendeh et al., 2017; haldi & robinson, 2008; holopainen 042 journal of facade design & engineering volume 10 / number 1 / 2022 et al., 2014; nicol & humphreys, 2002). another influencing factor in terms of thermal satisfaction is clothing (al horr et al., 2016; day & gunderson, 2015). the level of clothing could be defined by a dress code for certain professions or it could be a parameter that is for users to alter and adapt (fanger, 1970; huizenga et al., 2006). in any case, clothing could be mentioned as one of the factors that influences users to take adaptive actions in order to control the indoor temperature (fountain et al., 1996). in addition to these, working routine and arrival and departure patterns are found to be important motivational factors for adaptive behaviours (d’oca et al., 2016). opening the windows on arrival to the office and closing them upon departure is a common habit (o’brien & gunay, 2014). adjusting the window blinds, turning on/off heating and lighting are other actions that are consistent with the working schedule (deme belafi et al., 2018; wagner et al., 2018). moreover, these could be listed as some of the main occupant behaviours that create the gap in the predicted and actual energy performance of the buildings (masoso & grobler, 2010). socio-personal traits the main intangible factors that show diversity among the users and influence behaviours in various different ways are grouped under the title socio-personal traits. there are several behavioural models that try to portray the intentions and motivations of users’ adaptive behaviours based on their personal and social properties (deme belafi et al., 2018). personal factors could be listed as individual traits, beliefs, and attitudes which are the reflections of one’s character (fabi et al., 2012). a study investigating the influence of personality traits on occupant behaviour shows how the big five personality traits influence the behaviours associated with window opening, adjusting blinds, fans, or clothing, in addition to how thermal sensation and preference are affected (schweiker et al., 2016; wagner et al., 2018). briefly, it states that people with high neuroticism traits tend to stick to the actions that they know and are able to control more, like clothing adjustments or operating windows. on the other hand, the extraversion trait could define a person who may very likely act quickly against any discomfort. similarly, people with a high degree of openness to experience would be more open to changes, and hence feel less need to engage in adaptive behaviour. in other words, depending on their level of willingness to take action, a person could be defined as more active or passive (d’oca et al., 2016). apart from the personal profiles, social parameters could also be highly influential as a behavioural motivation (zhang et al., 2018). social constraints, norms, and pressure in a shared office are some of the most effective factors in terms of energy awareness and influence the adaptive behaviours related to energy usage (day & gunderson, 2015; wagner et al., 2018). besides, one’s cultural belonging or certain lifestyle is another factor that affects human cognition and would alter the user behaviour (delzendeh et al., 2017). all in all, perception and preference of comfort are highly interrelated with the socio-personal traits, thus these factors are significant influencers on the user experience. 5.1.2 personal needs the factors mentioned in this category cover the main psychological elements in terms of users’ needs and stressors that affect users in positive or negative ways. 043 journal of facade design & engineering volume 10 / number 1 / 2022 security the need for security is a factor that influences the experience both directly and indirectly. according to vischer, health and safety are part of the physical comfort parameters that are the first step of the habitability pyramid (vischer, 2007). additionally, the feeling of safety and security is a significant psychological factor that affects the indoor environmental satisfaction of the users (russell et al., 2013; zagreus et al., 2004). in terms of adaptive behaviours, in residential or working places, the act of window opening and closing is influenced by the means of providing security (o’brien & gunay, 2014; roulet et al., 2006). to indicate whether someone is outside, adjusting window blinds could also be another adaptive behaviour that is influenced by the feeling of security (deme belafi et al., 2018). privacy the need for privacy is another psychological factor (like security) that both directly and indirectly affects users’ experience in a working environment (vischer, 2007). the need for privacy applies to both visual and acoustical environments (kim & de dear, 2012). especially for open-plan offices, communication privacy is an important parameter that affects the level of satisfaction and productivity in the physical work environment (al horr et al., 2016; samani, 2015). for visual privacy, controlling the shading elements could be listed as the main influenced adaptive user behaviour, which applies to homes as well as offices (deme belafi et al., 2018; lee et al., 2013). hygiene the need for hygiene is one of the basic human needs as a part of physical comfort (vischer, 2007). cleanliness of the working space and maintenance of the building directly influence occupants’ level of satisfaction and productivity in the work environment (huizenga et al., 2006; kim & de dear, 2012; roulet et al., 2006; samani, 2015; zagreus et al., 2004). although there is no mention in the literature about it triggering an adaptive behaviour, by means of protecting the indoor environment it may be interpreted that there is an indirect effect on the window closing action. view the need for a view is mentioned in relation to outside view, visual quality, and aesthetic appearance, which would influence the level of satisfaction with the working environment (samani, 2015). architectural features of the space, colours, textures, materials, and components affect the sense of aesthetics and positively influence the seeing experience (delzendeh et al., 2017). in addition, view out is one of the significant factors that affects visual comfort. lack of view to outside may cause eye tiredness (day & gunderson, 2015), but having a pleasant view like a green landscape positively influences the overall satisfaction (kim & de dear, 2012). after sunlight levels, quality of view is the next main factor that triggers the window blind controls (bakker et al., 2014; kwon et al., 2019; o’brien & gunay, 2014; vischer, 2007; wagner et al., 2018). 044 journal of facade design & engineering volume 10 / number 1 / 2022 5.2 environmental factors apart from the human factors, the conditions of the environment surrounding the users directly affect their comfort, both physically and psychologically (vischer, 2007). as the barrier between the exterior and interior environments, façades undertake multiple functions with the main goal of providing a comfortable indoor environment (klein, 2013). therefore, its design is surfaced with regard to external conditions and it is the main element that shapes the internal conditions. besides, other design characteristics related to the functionality of the building, referred to as spatial conditions, would also be factors that affect users’ comfort in their environment. effects of environmental factors on users’ comfort and behaviours are a more commonly studied aspect, therefore the information presented in this section is a brief mention within the context of the mechanism of change. table 2 below represents the relationship of each factor with each adaptive behaviour and user experience. 5.2.1 external factors the factors mentioned in this category are the main climate-related factors that depend on the geographic location and show variety for each season, time of year, or time of day. outdoor temperature outdoor temperature is one of the main factors that affects thermal comfort. in different seasons, it influences the window opening/closing behaviours of the users, as well as the adjustment of window blinds and heating/cooling units (al horr et al., 2016; bakker et al., 2014; brager et al., 2004; buso et al., 2015; day & gunderson, 2015; delzendeh et al., 2017; fabi et al., 2012; frontczak & wargocki, 2011; haldi & robinson, 2008; wagner et al., 2018; zhang et al., 2018) solar access solar access, depending on the orientation of the building, is one of the main factors that affects visual comfort. along with the time of the day, the intensity of the daylight and sky conditions are mainly effective on the shading elements opening/closing behaviours of the users and turning on/off artificial lighting. besides, the effect of sunlight may cause overheating which is related to thermal comfort. in terms of temperature adjustment, closing window blinds or altering the hvac systems could also be listed as other influenced adaptive actions (day & gunderson, 2015; delzendeh et al., 2017; fabi et al., 2012; huizenga et al., 2006; wagner et al., 2018; zhang et al., 2018). 045 journal of facade design & engineering volume 10 / number 1 / 2022 table 2 the environmental factors that affect user experience in a mechanism of change impulse (influencer / agent) mechanism (action / behaviour) response (effect / change) related experience environmental factors o p en in g / c lo si n g w in d o w a d ju st in g w in d o w b li n d a d ju st in g h v a c t u rn in g o n /o ff l ig h ts a ir e xc h a n g e ra te s u n li g h t tr a n sm it ta n ce in d o o r te m p er a tu re il lu m in a ti o n l ev el s ee in g f ee li n g h ea ri n g c o n tr o ll in g o ve ra ll external factors outdoor temperature + + o + + + o o + + o solar access + + o + + + + + + + + o wind/rain + + + + o o relative humidity + o + + + o o outdoor noise level o o o + o + internal factors indoor temperature + + + + + + o o + + + lighting conditions o + + o + o + + o + + air quality + o + o + o + relative humidity + o + o + o + indoor noise level o o o o o o o + o + spatial factors building function + + + + + + + + o o o o + building layout + + o o + + + + + + + o o façade characteristics + + o o + + + + + + o + o service systems o o + + o o + + + + o + + (+) direct mention in the literature, (o) interpreted from the literature, (-) not mentioned in the literature wind and rain wind and rain conditions are listed together as factors that affect the window opening/closing behaviours of the users. due to their effect, it results in changes in the indoor air quality and it also affects thermal comfort (delzendeh et al., 2017; fabi et al., 2012; haldi & robinson, 2008; zhang et al., 2018). relative humidity the outdoor relative humidity is linked with thermal comfort and indoor air quality, in relation to the window opening behaviour and if there is an active ventilation system, by means of adjusting its air flow (al horr et al., 2016; delzendeh et al., 2017; deme belafi et al., 2018; zhang et al., 2018). outdoor noise levels outdoor noise levels have a direct effect on acoustical comfort, and the resulting hearing experience, and they are also mentioned as one of the factors that influence window closing behaviour (day & gunderson, 2015; haldi & robinson, 2008; roulet et al., 2006; vischer, 2007) 046 journal of facade design & engineering volume 10 / number 1 / 2022 5.2.2 internal factors the factors mentioned in this category are the main quantifiable/measurable conditions of the indoor environment. indoor temperature indoor temperature is one of the main factors that affects thermal comfort. its effects could be investigated under two main sub-categories: mean radiant temperature and temperature variability created by the air movement, in other words, draught (frontczak & wargocki, 2011). due to different needs, it influences the window opening/closing behaviours of the users as well as the adjustment of window blinds and heating/cooling units (al horr et al., 2016; bakker et al., 2014; boerstra et al., 2012; boerstra, 2016; bordass et al., 1993; brager et al., 2004; d’oca et al., 2016; day & gunderson, 2015; delzendeh et al., 2017; deme belafi et al., 2018; fabi et al., 2012; fanger, 1970; frontczak & wargocki, 2011; haldi & robinson, 2008; holopainen et al., 2014; huizenga et al., 2006; kim & de dear, 2012; kwon et al., 2019; nicol & humphreys, 2002; o’brien & gunay, 2014; roulet et al., 2006; samani, 2015; wagner, 2018; zagreus et al., 2004; zhang et al., 2018) lighting conditions lighting conditions are the primary indicator of visual comfort. it covers the level of illumination within the space, homogeneity of the brightness, and formation of glare. it is affected by the external conditions, mainly solar access, and regulated initially by adjusting the window blinds and later by means of artificial lighting. in relation to the outside view and sunlight’s overheating capability, it is linked with the window opening behaviour as well as the adjustment of window blinds (bakker et al., 2014; boerstra et al., 2012; boerstra, 2016; bordass et al., 1993; d’oca et al., 2016; day & gunderson, 2015; delzendeh et al., 2017; deme belafi et al., 2018; fabi et al., 2012; kim & de dear, 2012; kwon et al., 2019; o’brien & gunay, 2014; roulet et al., 2006; samani, 2015; vischer, 2007; wagner, 2018). air quality indoor air quality refers to the co 2 concentration of the air, odours, and air composition as in the content of dust/electrical particles/microorganisms. it is related to the feeling experience and affects the window opening/closing behaviours of the users (bakker et al., 2014; boerstra et al., 2012; boerstra, 2016; bordass et al., 1993; d’oca et al., 2016; deme belafi et al., 2018; fabi et al., 2012; fanger, 1970; haldi & robinson, 2008; huizenga et al., 2006; kim & de dear, 2012; kwon et al., 2019; roulet et al., 2006; samani, 2015; zagreus et al., 2004; zhang et al., 2018). relative humidity the indoor relative humidity, similarly to the external conditions, influences thermal comfort and indoor air quality, in relation to the window opening behaviour and if there is an active ventilation system, by means of adjusting its air flow (al horr et al., 2016; day & gunderson, 2015; deme belafi et al., 2018; holopainen et al., 2014; huizenga et al., 2006; kwon et al., 2019; roulet et al., 2006; zhang et al., 2018). 047 journal of facade design & engineering volume 10 / number 1 / 2022 indoor noise levels indoor noise levels is one of the main factors that creates discomfort in a working environment. it directly affects acoustical comfort. depending on the source of the noise if it is from the outside it impacts window closing behaviour, and if it is an operational sound interfering the window blinds and/or hvac systems, operations would be influenced (boerstra, 2016; day & gunderson, 2015; kim & de dear, 2012; roulet et al., 2006; samani, 2015; vischer, 2007; zagreus et al., 2004). 5.2.3 spatial factors the factors mentioned in this category are the properties related to the design of the space and its function. building function the function of the building is one of the main factors that defines the user requirements. it defines the function of the space and its spatial needs for comfortable usage. therefore, it influences all adaptive behaviours in various ways and could be linked with all the user experiences (al horr et al., 2016; boerstra, 2016; brager et al., 2004; delzendeh et al., 2017; frontczak & wargocki, 2011; huizenga et al., 2006; kwon et al., 2019; nicol & humphreys, 2002; roulet et al., 2006; samani, 2015; vischer, 2007; wagner, 2018). building layout building layout is the other factor that covers a wide range of design aspects like private or openplan working environments, area of workspace per person, and position of each user within the space. it may affect all experiences in terms of seeing, feeling, and hearing. users’ interactions with their façade vary due to their distance from it, hence building layout is listed as one of the influential factors for opening/closing windows and adjusting shading elements behaviours (kwon et al., 2019; luna-navarro et al., 2021). in addition to that, as was also mentioned under the socio-personal traits, being in a private or shared office may alter ones adaptive behaviours by creating social pressure (bakker et al., 2014; boerstra, 2016; brager et al., 2004; buso et al., 2015; d’oca et al., 2016; day & gunderson, 2015; delzendeh et al., 2017; fabi et al., 2012; huizenga et al., 2006; kim & de dear, 2012; o’brien & gunay, 2014; roulet et al., 2006; samani, 2015; vischer, 2007; wagner, 2018; zagreus et al., 2004; zhang et al., 2018). façade characteristics façade characteristics refer to window size and position, wall to window ratio, mass, robust or dynamic character, and other design aspects of the façade. in the case of adaptive façades, it is clearly a factor that influences the controlling experience of the users (tabadkani et al., 2021). the level of manual or automated control of the openings and shading units influence the level of user interactions and along with that effect the visual and thermal comfort of the occupant (bakker et al., 2014; buso et al., 2015; fabi et al., 2012; kwon et al., 2019; vischer, 2007; zhang et al., 2018). 048 journal of facade design & engineering volume 10 / number 1 / 2022 service systems service systems refer to the active, mechanical units of the building, which have a great influence on shaping the indoor environment. their design is outside the context of this paper; however, their control and feedback mechanisms can be mentioned as one of the factors that influences the controlling experience of the users (o’brien et al., 2020). alterations made by engaging with the hvac systems or lighting element can also affect the visual, thermal, and overall comfort of the occupant (brager et al., 2004; buso et al., 2015; delzendeh et al., 2017; fountain et al., 1996; huizenga et al., 2006; kwon et al., 2019; nicol & humphreys, 2002; wagner, 2018; zhang et al., 2018). 6 discussion in the previous section, an overview of the factors that affect user experience is systematically presented over the mechanism of change. according to the findings, it could be said that the most affected experience by the human factors is feeling. there is a direct effect, mainly related to the perception of thermal comfort, and an indirect effect as a result of being triggered by users’ interactions. among these, the most commonly investigated adaptive behaviour is found to be opening/closing windows, followed by adjusting hvac. both of these actions are in relation to the changes in the air exchange rate and/or indoor temperature, hence associated with the feeling experience. other commonly affected experiences are seeing and controlling. related to these experiences, the indirect effect of adjusting window blind behaviour needs to be further investigated, in terms of user-façade interactions. from the listed human factors, type of job and socio personal traits covered a wide range of factors, therefore it was an expected outcome to see that these titles are in relation to all the listed adaptive behaviours and experiences. even though, with a holistic perspective, it was possible to gather information regarding the influences of human factors on user experience, the number of studies with this focus is limited and research targeted on the experience was found to be especially scarce. as for the environmental factors, there was more research in the literature investigating direct or indirect effects on user experience. from the listed external factors, solar access is found to be the most influential one that could trigger all the issued adaptive behaviours. it directly and indirectly affects the seeing and feeling experience, and, related to the mechanism of change, could also be associated with the controlling experience. it is followed by the outdoor temperature, and from the internal factors, indoor temperature. from the listed spatial factors, the function of the building shows a clear relationship with all the adaptive behaviours and is related with the overall experience. however, based on the existing literature, it is difficult to define a direct effect caused by the building’s function on each experience. on the other hand, studies show direct and indirect effects of building layout on seeing, feeling, and hearing experiences. by looking at the results of the review, studies investigating the effects of the environmental factors in relation to hearing experience seems to be lacking. considering the fact that outside or inside sound levels are indicated as some of the main causes for discomfort, the indirect effects of these factors in relation to the mechanism of change need to be explored in a deeper way. besides, controlling experience requires a more targeted study, where indirect effects on the experience would be investigated when the façade systems are fully or partially left to the users’ control or fully automated. in addition to that, façade and service systems characteristics needs to be further explored in terms of their direct effects on each user experience. as a final remark on the review, the collective effect of different parameters is another issue that needs to be further investigated in terms of user experience. 049 journal of facade design & engineering volume 10 / number 1 / 2022 the presented overview shows that the adaptive mechanisms related to the façade, like opening/ closing windows and adjusting window blinds, are highly influenced by human factors. it could be said that the users’ intention behind adaptive behaviours is the reflection of their adaptive comfort needs and adaptive control expectations. a similar outcome could be drawn for the behaviours related to the building service systems. each adaptive behaviour is performed to overcome a certain discomfort and/or to enhance the user experience. therefore, users’ adaptive actions reflect their way of thinking. with a human centred approach, if the adaptive façades are to be designed in harmony with its users, users’ anticipations of their indoor environment, as they relate to their experiences, require further research. transferring this information from the occupancy to the design stage is crucial for developing occupant-centric control and operation systems, predicting the building’s energy consumption, and enhancing users’ positive interactions with their surroundings, with the ultimate aim of reaching the full potentials of the adaptive façades. 7 conclusion an overview of human-centred design and user experience in the context of working environments is presented in this paper. with a holistic perspective, all of the factors affecting users’ experience are investigated through a literature review process. a mechanism of change is defined to explain the dynamic cycle of the user experience with respect to users’ interactions with the building façade and service systems. according to the findings, factors that trigger a change mechanism by influencing an adaptive user behaviour are identified and categorized under two main groups: human and environmental factors. human factors are divided into two sub-groups: (1) age, gender, country of origin, education level, type of job, and socio-personal traits as personal descriptors; (2) security, privacy, hygiene, and view as personal needs. environmental factors are divided into three sub-groups: (1) outdoor temperature, solar access, wind/rain, relative humidity, and outdoor noise level as external factors, (2) indoor temperature, lighting conditions, air quality, relative humidity, and indoor noise level as internal factors, (3) building function, building layout, façade characteristics, and service systems as spatial factors. the main adaptive behaviours are identified as (1) opening/closing windows, (2) adjusting window blinds (shading elements), (3) adjusting hvac systems, (4) turning on/off lighting, and the change effects that these actions would result in are listed as (1) air exchange rate, level of (2) light transmittance, value of (3) indoor temperature, and (4) illumination level. through these impulse, mechanism, and response relationships, the direct or indirect effects of each factor on user experiences are distinguished. where the user experiences are defined as seeing, feeling, hearing, and controlling, they are therefore not only focused on the concept of comfort but also cover all human senses, emotions, behaviours, and interactions. based on the outcomes, in the simplest terms, it could be said that every user has different needs, perceptions, and thus, different behaviours. with a human-centred point of view, adaptive façades with their changeability feature, offer great potential to deal with these diverse needs and to keep the indoor environment in its preferred condition with a reduced level of energy consumption. an understanding of the users’ relationships with the façade is essential in creating an ideal way of interaction, thus improving the user experience. in order to achieve that goal, the relationship between the human factor, as an agent of change, and automation systems of the adaptive façades, as the actor in the mechanism of change, is pointed out as the missing link. therefore, relevant issues and further research areas towards a human-centred adaptive façade design approach 050 journal of facade design & engineering volume 10 / number 1 / 2022 are introduced. the information presented in this paper could serve researchers in the field as a starting point, and show façade designers the adaptive nature of the users, hence transferring the knowledge on the dynamics of occupancy to the design phase. acknowledgments presented study is a part of mine koyaz’s ongoing phd thesis, currently named as "human centred adaptive facades", at construction sciences program, istanbul technical university. research presented in this paper is carried during and after her visit at architectural engineering and technology department, delft university of technology as a guest researcher. the scholarship support from istanbul bilgi university for this visit is also acknowledged. references abras, c., maloney-krichmar, d., & preece, j. (2004). user-centred design. bainbridge, w. encyclopedia of human-computer interaction. https://doi.org/10.1045/january2005-editorial addington, m. (2009). contingent behaviours. architectural design, 79(3), 12–17. https://doi.org/10.1002/ad.882 aelenei, l., aelenei, d., romano, r., mazzucchelli, e. s., brzezicki, m., & rico-martinez, j. m. (eds.). (2018). case studies – adaptive façade network. in cost action tu 1403 adaptive façade network. tu delft open. ajzen, i. (1991). the theory of planned behaviour. organizational behaviour and human decision processes, 50, 179–211. https://doi.org/10.1922/cdh_2120vandenbroucke08 al-obaidi, k. m., azzam ismail, m., hussein, h., & abdul rahman, a. m. (2017). biomimetic building skins: an adaptive approach. renewable and sustainable energy reviews, 79(may), 1472–1491. https://doi.org/10.1016/j.rser.2017.05.028 al horr, y., arif, m., katafygiotou, m., mazroei, a., kaushik, a., & elsarrag, e. (2016). impact of indoor environmental quality on occupant well-being and comfort: a review of the literature. international journal of sustainable built environment, 5(1), 1–11. https://doi.org/10.1016/j.ijsbe.2016.03.006 alavi, h. s., verma, h., papinutto, m., & lalanne, d. (2017). comfort: a coordinate of user experience in interactive built environments. lecture notes in computer science (including subseries lecture notes in artificial intelligence and lecture notes in bioinformatics), 10515 lncs, 247–257. https://doi.org/10.1007/978-3-319-67687-6_16 attia, s., garat, s., & cools, m. (2019). development and validation of a survey for well-being and interaction assessment by occupants in office buildings with adaptive façades. building and environment, 157(april), 268–276. https://doi.org/10.1016/j. buildenv.2019.04.054 attia, s. (2017). evaluation of adaptive façades: the case study of al bahr towers in uae essay uk free essay database. qscience proceedings. http://www.essay.uk.com/essays/architecture/evaluation-adaptive-façades/ attia, s., luna-navarro, a., juaristi, m., monge-barrio, a., gosztonyi, s., & al-doughmi, z. (2018). post-occupancy evaluation for adaptive façades. journal of façade design & engineering volume, 6(3), 1–9. https://doi.org/10.7480/jfde.2018.3.2464 bakker, l. g., hoes-van oeffelen, e. c. m., loonen, r. c. g. m., & hensen, j. l. m. (2014). user satisfaction and interaction with automated dynamic façades: a pilot study. building and environment, 78, 44–52. https://doi.org/10.1016/j.buildenv.2014.04.007 bluyssen, p. m. (2019). towards an integrated analysis of the indoor environmental factors and its effects on occupants. intelligent buildings international, 0(0), 1–9. https://doi.org/10.1080/17508975.2019.1599318 boerstra, a., beuker, t., loomans, m., & hensen, j. (2012). impact of perceived control on comfort and health in european office buildings. 10th international conference on healthy buildings 2012, 1(april), 370–375. boerstra, a. c. (2016). personal control over indoor climate in offices: impact on comfort, health and productivity. böke, j., knaack, u., & hemmerling, m. (2020). automated adaptive façade functions in practice case studies on office buildings. automation in construction, 113(february), 103113. https://doi.org/10.1016/j.autcon.2020.103113 bordass, b., bromley, k., & leaman, a. (1993). user and occupant controls in office buildings. february. brager, g. s., paliaga, g., & de dear, r. (2004). operable windows, personal control, and occupant comfort (rp-1161). ashrae transactions, 110(december 2015), 17–35. buso, t., fabi, v., andersen, r. k., & corgnati, s. p. (2015). occupant behaviour and robustness of building design. building and environment, 94, 694–703. https://doi.org/10.1016/j.buildenv.2015.11.003 capeluto, g., & ochoa, c. e. (2017). intelligent envelopes for high-performance buildings. springer international publishing. https:// doi.org/10.1007/978-3-319-39255-4 chammas, a., quaresma, m., & mont’alvão, c. (2015). a closer look on the user centred design. procedia manufacturing, 3(ahfe), 5397–5404. https://doi.org/10.1016/j.promfg.2015.07.656 d’oca, s., corgnati, s., pisello, a. l., & hong, t. (2016). introduction to an occupant behaviour motivation survey framework. clima 2016, february. http://datos.bancomundial.org/indicador/sl.tlf.cact.fe.zs damodaran, l. (1996). user involvement in the systems design processa practical guide for users. behaviour & information technology, 15(6), 363–377. day, j. k., & gunderson, d. e. (2015). understanding high performance buildings: the link between occupant knowledge of passive design systems, corresponding behaviours, occupant comfort and environmental satisfaction. building and environment, 84, 114–124. https://doi.org/10.1016/j.buildenv.2014.11.003 delzendeh, e., wu, s., lee, a., & zhou, y. (2017). the impact of occupants’ behaviours on building energy analysis: a research review. renewable and sustainable energy reviews, 80(august), 1061–1071. https://doi.org/10.1016/j.rser.2017.05.264 051 journal of facade design & engineering volume 10 / number 1 / 2022 deme belafi, z., hong, t., & reith, a. (2018). a critical review on questionnaire surveys in the field of energy-related occupant behaviour. energy efficiency, 11(8), 2157–2177. https://doi.org/10.1007/s12053-018-9711-z fabi, v., andersen, r. v., corgnati, s., & olesen, b. w. (2012). occupants’ window opening behaviour: a literature review of factors influencing occupant behaviour and models. building and environment, 58, 188–198. https://doi.org/10.1016/j. buildenv.2012.07.009 fanger, p. o. (1970). thermal comfort: analysis and applications in environmental engineering. mcgraw-hill. favoino, f., loonen, r. c. g. m., doya, m., goia, f., bedon, c., & babich, f. (eds.). (2018). building performance simulation and characterisation of adaptive façades – adaptive façade network. in cost action tu 1403 adaptive façade network. tu delft open. fisk, w. j. (2000). health and productivity gains from better indoor environments and their relationship with building energy efficiency. annual review of energy and the environment, 25(1997), 537–566. fountain, m., brager, g., & de dear, r. (1996). expectations of indoor climate control. energy and buildings, 24(3), 179–182. https:// doi.org/10.1016/s0378-7788(96)00988-7 frontczak, m., & wargocki, p. (2011). literature survey on how different factors influence human comfort in indoor environments. building and environment, 46(4), 922–937. https://doi.org/10.1016/j.buildenv.2010.10.021 haldi, f., & robinson, d. (2008). on the behaviour and adaptation of office occupants. building and environment, 43(12), 2163–2177. https://doi.org/10.1016/j.buildenv.2008.01.003 heydarian, a., mcilvennie, c., arpan, l., yousefi, s., syndicus, m., schweiker, m., jazizadeh, f., rissetto, r., pisello, a. l., piselli, c., berger, c., yan, z., & mahdavi, a. (2020). what drives our behaviours in buildings? a review on occupant interactions with building systems from the lens of behavioural theories. building and environment, 179(november 2019), 106928. https://doi. org/10.1016/j.buildenv.2020.106928 hoffman, r. r., ford, k. m., feltovich, a., woods, d. d., feltovich, p. j., & klein, g. (2002). a rose by any other name…would probably be given an acronym. ieee intelligent systems, 17(4), 72–80. https://doi.org/10.1109/mis.2002.1024755 holopainen, r., tuomaala, p., hernandez, p., häkkinen, t., piira, k., & piippo, j. (2014). comfort assessment in the context of sustainable buildings: comparison of simplified and detailed human thermal sensation methods. building and environment, 71, 60–70. https://doi.org/10.1016/j.buildenv.2013.09.009 hong, t., yan, d., d’oca, s., & chen, c. fei. (2017). ten questions concerning occupant behaviour in buildings: the big picture. building and environment, 114, 518–530. https://doi.org/10.1016/j.buildenv.2016.12.006 huizenga, c., abbaszadeh, s., zagreus, l., & arens, e. (2006). air quality and thermal comfort in office buildings: results of a large indoor environmental quality survey. proceeding of healthy buildings 2006, 3, 399–397. https://doi.org/10.12659/pjr.894050 humphreys, m. a. (2005). quantifying occupant comfort: are combined indices of the indoor environment practicable? building research and information, 33(4), 317–325. https://doi.org/10.1080/09613210500161950 ideo.org. (2015). the field guide to human centred design. iso 9241-210. (2010). ergonomics of human–system interaction : human-centred design for interactive systems. jia, m., srinivasan, r. s., & raheem, a. a. (2017). from occupancy to occupant behaviour: an analytical survey of data acquisition technologies, modeling methodologies and simulation coupling mechanisms for building energy efficiency. renewable and sustainable energy reviews, 68(june 2016), 525–540. https://doi.org/10.1016/j.rser.2016.10.011 kaasinen, e., kymäläinen, t., niemelä, m., olsson, t., kanerva, m., & ikonen, v. (2013). a user-centric view of intelligent environments: user expectations, user experience and user role in building intelligent environments. computers, 2(1), 1–33. https://doi.org/10.3390/computers2010001 khalid, h. m. (2006). embracing diversity in user needs for affective design. applied ergonomics, 37(4 spec. iss.), 409–418. https:// doi.org/10.1016/j.apergo.2006.04.005 kim, j., & de dear, r. (2012). nonlinear relationships between individual ieq factors and overall workspace satisfaction. building and environment, 49(1), 33–40. https://doi.org/10.1016/j.buildenv.2011.09.022 klein, t. (2013). integral façade construction : towards a new product architecture for curtain walls [tu delft]. in architecture and the built environment. https://doi.org/10.7480/abe.2013.3 knaack, u., klein, t., bilow, m., & auer, t. (2014). façades: principles of construction (2nd editio). birkhäuser verlag gmbh. kwon, m., remøy, h., van den dobbelsteen, a., & knaack, u. (2019). personal control and environmental user satisfaction in office buildings: results of case studies in the netherlands. building and environment, 179, 428–435. https://doi.org/10.1002/ bdra.20073 lassen, n., josefsen, t., & goia, f. (2021). design and in-field testing of a multi-level system for continuous subjective occupant feedback on indoor climate. building and environment, 189(october 2020), 107535. https://doi.org/10.1016/j. buildenv.2020.107535 law, e. l. c., roto, v., hassenzahl, m., vermeeren, a. p. o. s., & kort, j. (2009). understanding, scoping and defining user experience: a survey approach. conference on human factors in computing systems proceedings, 719–728. https://doi. org/10.1145/1518701.1518813 lazarova-molnar, s., & mohamed, n. (2017). on the complexity of smart buildings occupant behaviour: risks and opportunities. acm international conference proceeding series, part f1309(november). https://doi.org/10.1145/3136273.3136274 lee, e. s., fernandes, l. l., coffey, b., mcneil, a., clear, r., webster, t., bauman, f., dickerhoff, d., heinzerling, d., & hoyt, t. (2013). a post-occupancy monitored evaluation of the dimmable lighting , automated shading , and underfloor air distribution system in the new york times building building technology and. luna-navarro, a., fidler, p., law, a., torres, s., & overend, m. (2021). building impulse toolkit (bit): a novel iot system for capturing the influence of façades on occupant perception and occupant-façade interaction. building and environment, 193(november 2020), 107656. https://doi.org/10.1016/j.buildenv.2021.107656 052 journal of facade design & engineering volume 10 / number 1 / 2022 luna-navarro, a., loonen, r., juaristi, m., monge-barrio, a., attia, s., & overend, m. (2020). occupant-façade interaction: a review and classification scheme. building and environment, 177, 106880. https://doi.org/10.1016/j.buildenv.2020.106880 maguire, m. (2001). methods to support human-centred design. international journal of human computer studies, 55(4), 587–634. https://doi.org/10.1006/ijhc.2001.0503 masoso, o. t., & grobler, l. j. (2010). the dark side of occupants’ behaviour on building energy use. energy and buildings, 42(2), 173–177. https://doi.org/10.1016/j.enbuild.2009.08.009 naqvi, n., shiv, b., & bechara, a. (2006). the role of emotion in decision making: a cognitive neuroscience perspective. current directions in psychological science, 15(5), 260–264. https://doi.org/10.1111/j.1467-8721.2006.00448.x nicol, j. f., & humphreys, m. a. (2002). adaptive thermal comfort and sustainable thermal standards for buildings. energy and buildings, 34(6), 563–572. https://doi.org/10.1016/s0378-7788(02)00006-3 norman, d. a. (1988). the psychology of everyday things. basic books. norman, d. a. (2013). the design of everyday things. basic books, a member of the perseus books group all. https://doi. org/10.5860/choice.51-5559 o’brien, w., & gunay, h. b. (2014). the contextual factors contributing to occupants’ adaptive comfort behaviours in offices a review and proposed modeling framework. building and environment, 77, 77–87. https://doi.org/10.1016/j.buildenv.2014.03.024 o’brien, w., wagner, a., schweiker, m., mahdavi, a., day, j., kjærgaard, m. b., carlucci, s., dong, b., tahmasebi, f., yan, d., hong, t., gunay, h. b., nagy, z., miller, c., & berger, c. (2020). introducing iea ebc annex 79: key challenges and opportunities in the field of occupant-centric building design and operation. building and environment, 178(may), 106738. https://doi.org/10.1016/j. buildenv.2020.106738 ortiz, m. a. (2019). home occupant archetypes: profiling home occupants’ comfortand energy-related behaviours with mixed methods. [tu delft]. https://doi.org/10.7480/abe.2019.5 ortony, a., norman, d. a., & revelle, w. (2005). affect and proto-affect in effective functioning. in who needs emotions (pp. 173–202). https://doi.org/10.1093/acprof pee, l. g., woon, i. m. y., & kankanhalli, a. (2008). explaining non-work-related computing in the workplace: a comparison of alternative models. information and management, 45(2), 120–130. https://doi.org/10.1016/j.im.2008.01.004 perino, m., & serra, v. (2015). switching from static to adaptable and dynamic building envelopes: a paradigm shift for the energy efficiency in buildings. journal of façade design and engineering, 3(2), 143–163. https://doi.org/10.3233/fde-150039 roulet, c. a., flourentzou, f., foradini, f., bluyssen, p., cox, c., & aizlewood, c. (2006). multicriteria analysis of health, comfort and energy efficiency in buildings. building research and information, 34(5), 475–482. https://doi.org/10.1080/09613210600822402 russell, r., guerry, a. d., balvanera, p., gould, r. k., basurto, x., chan, k. m. a., klain, s., levine, j., & tam, j. (2013). humans and nature: how knowing and experiencing nature affect well-being. in ssrn. https://doi.org/10.1146/annurev-environ-012312-110838 samani, s. a. (2015). the impact of personal control over office workspace on environmental satisfaction and performance. journal of social sciences and humanities, 1(3), 163–172. http://www.aiscience.org/journal/jssh sanders, l. (2008). an evolving map of design practice and design research. acm — interactions, xv.6(november + december), 1–7. https://doi.org/10.1093/beheco/arw006 schweiker, m., hawighorst, m., & wagner, a. (2016). the influence of personality traits on occupant behavioural patterns. energy and buildings, 131, 63–75. https://doi.org/10.1016/j.enbuild.2016.09.019 stern, p. c. (1992). what psychology knows about energy conservation. american psychologist. tabadkani, a., roetzel, a., li, h. x., & tsangrassoulis, a. (2021). a review of occupant-centric control strategies for adaptive façades. automation in construction, 122(may 2020), 103464. https://doi.org/10.1016/j.autcon.2020.103464 triandis, h. c. (1977). interpersonal behaviour. brooks/cole pub.co. van der bijl-brouwer, m., & dorst, k. (2017). advancing the strategic impact of human-centred design. design studies, 53, 1–23. https://doi.org/10.1016/j.destud.2017.06.003 verbeek, p.-p., & slob, a. (2006). user behaviour and technology development: shaping sustainable relations between consumers and technologies. springer. verplanken, b., & aarts, h. (1999). habit, attitude, and planned behaviour: is habit an empty construct or an interesting case of goal-directed automaticity? european review of social psychology, 10(1), 101–134. https://doi. org/10.1080/14792779943000035 vischer, j. c. (2007). the effects of the physical environment on job performance: towards a theoretical model of workspace stress. stress and health, 23, 175–184. https://doi.org/10.1002/smi.1134 visser, f. s., stappers, p. j., van der lugt, r., & sanders, e. b.-n. (2005). contextmapping: experiences from practice. codesign, 1(2), 119–149. https://doi.org/10.1080/15710880500135987 wagner, a. (2018). occupant behaviour-centric building design and operation ebc annex 79. in iea ebc (issue october). wagner, a., o’brien, w., & dong, b. (eds.). (2018). exploring occupant behaviour in buildings. springer international publishing ag. https://doi.org/10.1007/978-3-319-61464-9 wigginton, m., & harris, j. (2002). intelligent skins. butterworth-heinemann. yan, d., & hong, t. (eds.). (2018). definition and simulation of occupant behaviour in buildings. in iea, ebc annex 66 final report (issue may). yan, d., o’brien, w., hong, t., feng, x., burak gunay, h., tahmasebi, f., & mahdavi, a. (2015). occupant behaviour modeling for building performance simulation: current state and future challenges. energy and buildings, 107, 264–278. https://doi. org/10.1016/j.enbuild.2015.08.032 zagreus, l., huizenga, c., arens, e., & lehrer, d. (2004). listening to the occupants: a web-based indoor environmental quality survey. indoor air, 14(s8), 65–74. https://doi.org/10.1111/j.1600-0668.2004.00301.x 053 journal of facade design & engineering volume 10 / number 1 / 2022 zhang, t., & dong, h. (2008). human-centred design: an emergent conceptual model. include2009 proceedings, 2008, 7. https://doi. org/10.1.1.426.5107 zhang, y., bai, x., mills, f. p., & pezzey, j. c. v. (2018). rethinking the role of occupant behaviour in building energy performance: a review. energy and buildings, 172, 279–294. https://doi.org/10.1016/j.enbuild.2018.05.017 zoltowski, c. b., oakes, w. c., & cardella, m. e. (2012). students’ ways of experiencing human-centred design. journal of engineering education, 101(1), 28–59. https://doi.org/10.1002/j.2168-9830.2012.tb00040.x 054 journal of facade design & engineering volume 10 / number 1 / 2022 from city’s station to station city 005 journal of facade design & engineering volume 5 / number 2 / 2017 editorial international conference on building envelope systems and technologies – icbest is the premier conference for attendees to benefit from the cutting-edge information on building envelope systems and technologies. icbest is a worldwide forum for building envelope architecture and engineering. it provides information exchange, networking, and discussions of recent developments and their application, thus bridging the gap between architects, designers, engineers, manufacturers, and researchers. the first icbest conference was held in 1994 in singapore, continuing in bath (uk), ottawa (canada), sydney (australia), again in bath (uk), vancouver (canada), and finally in aachen (germany), with generally three year intervals between. icbest istanbul is the eighth conference series that focuses on various aspects of building envelope design and engineering in which the approach is interdisciplinary. therefore, the main theme of the conference is “interdisciplinary perspectives for future building envelopes”. main sub-themes supporting the conference topic are performance & sustainability, structural systems, materials & components, life cycle, envelope technologies, failures & refurbishment, and case studies. to address the conference theme, nine papers were selected for publication in the special issue of journal of facade design and engineering (jfde) from the submitted conference papers, following a blind review process conducted by guest reviewers. the guest editors believe that these papers will contribute to the cutting-edge research and practice in building envelope systems and technologies. the following papers are included in this special issue: – in the context of envelope technologies, an innovative approach to retrofitting as it relates to the use of the adaptive ventilated facade module developed in the eu project e2vent is presented by basso et al. in “e2vent: design and integration of an adaptable module for residential building renovation”. – “used building materials as secondary resources – identification of valuable building material and automized deconstruction”, by zabek et al., deals with a new approach for deconstructing used building elements and re-introducing them in new construction on a regional scale. – basso & del grosso have discussed differences concerning the applicability of adaptive structures to engineering problems of different fields in “directions for the design of energy efficient kinematics in adaptive solar building envelopes”. – bauer et al.’s paper, entitled “climate based daylight simulations with evaldrc : analysis of daylight redirecting components”, discusses the tool that was applied to a daylight optimization case study in order to introduce the new developments in daylight redirection simulation into architectural and engineering study programs. – “modelling of active solar building envelopes for cost-effective evaluation”, by maurer & kuhn, explains the complexities of, and solutions for, solar building envelopes for cost-beneficial designs. – “a literature review of experimental setups monitoring thermal performance of vegetated facade systems” by yuksel & turkeri aims to explain the existing experimental setups for evaluation of the thermal performance of a felt type vegetated facade system, conducted in a temperate humid climate. – models of two daylight redirecting components (drcs) that are generated from measurements are presented in “high-resolution data-driven models of daylight redirecting components” by grobe et al. the paper concludes that different measurement and modelling protocols should be applied to different class systems, rather than aiming at a common low-resolution discretization. 006 journal of facade design & engineering volume 5 / number 2 / 2017 – the paper entitled “towards facades as make-to-order products: the role of knowledge-based engineering to support design”, by montali et al., illustrates how knowledge-based engineering (kbe) can potentially support early-stage design integration through the development of a facade product model for automatic rule checking and knowledge reuse. the relationship between research and practice is an important part the idea of jfde. under the chapter ‘applied sciences’ in this issue, we feature a paper entitled “arup solar: an innovative app for a parametric, holistic and multidisciplinary approach to early design stages” by donato et al. which addresses how an app developed by arup aims to investigate the relationships between envelope features (e.g. window to wall ratio, g values, etc.) and cooling strategies, as well as to identify potential opportunities for renewable solar energy production. the guest editors thank the authors and reviewers for their significant contributions to this special issue. moreover, the guest editors sincerely thank dr. fatih yazicioglu and dr. buket metin for their great assistance during the whole process. last but not least, constructive comments and great help of the jfde editor thaleia konstantinou and editors in chief ulrich knaack and tillmann klein are gratefully acknowledged. aslihan tavil, oguz c. celik guest editors, jfde special issue faculty of architecture istanbul technical university (itu) istanbul, turkey journal of facade design and engineering 3 (2015) 225–235 doi 10.3233/fde-150042 ios press 225 additive manufacturing for future facades: the potential of 3d printed parts for the building envelope holger straußa,b,∗, emmer pfenninger partner aga and ulrich knaackb aspecialist facade engineers and consultants, münchenstein, switzerland bfaculty of architecture, facade research group, delft university of technology, netherlands abstract. the basic principle of ‘3d printing’ is the layer wise production of real parts from virtual data – be it with laser, with power glue, electron beam or uv light processing (hopkinson, hague, & dickens, 2006). the professional application of ‘3d printing’ is ‘additive manufacturing’ (am) and this opens a fascinating new world of engineering. it offers a selection of reliable building construction materials – done in concrete, aluminium, steel, high performance plastics or glass (woodcock, 2011). no matter what applications can be found: to ‘design for function’, rather to ‘design for production’ turns our way of engineering of the last century upside down. as a result, am opens the outlook to applications in our (built) future in combination with the available technologies of today (strauß, 2013). the tool-less production with am allows for new shapes and functional parts in small batch sizes – down to batch size one. the parts performance can be re-interpreted based on the demands within the system, not based on the limitations of conventional manufacturing. am offers new ways of materializing the physical part around its function. advancements can for example be achieved in the semi-finished goods: fittings with less, but higher integrated parts could offer a better performance with lower material consumption. solving the most critical part of a free-form structure and allowing for a smart combination with the approved standards has a great potential, as well. it leads toward customized and enhanced performance. this paper discusses the possible changes and influences those emerging technologies have on the development of building envelopes. to achieve today’s demands of future envelopes, we have to find new solutions. additive manufacturing is one possible way to do so. keywords: architecture, facade engineering, 3d printing, additive manufacturing, bespoke building envelopes, future facades, innovation, performance, file to factory 1. introduction since 1987 ‘rapid prototyping’ (rp) is known as one fast way to materialise concept design ideas in a handy, digital way. by transferring 3d model data into physical parts in various available materials, it is an accepted tool in a broad range of disciplines. limitations in building chamber size, and therefore limited part sizes, firstly brought up applications in fields where larger components were not necessarily needed: scaled models, medical implants, jewellery, or even casting moulds of engine parts as semi finished goods, are a few examples. and it was in those fields of application, where rapid prototyping grew during its first two decades of being. from rp also derived other kinds of applications: ‘rapid tooling’ (rt), which, in industrial mass production has changed the manner of how production tools are made, as well as ‘rapid manufacturing’ (rm) which is specifically ∗corresponding author: holger strauß, emmer pfenninger partner ag, weidenstrasse 13, 4142 münchenstein, switzerland. technical university delft, facade research group. e-mail: strauss.planen@yahoo.de. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:strauss.planen@yahoo.de 226 h. strauß et al. / additive manufacturing for future facades designed for the production of finished products that are immediately usable without the need for subsequent production steps (hopkinson et al., 2006). all available additive processes are based on a few underlying principles of layer wise production, where materials are added onto each other and a part is built up layer by layer (see fig. 1). additive fabrication is clearly separated from the conventional production principles, such as subtractive manufacturing, where material is taken away from the raw material by cutting, grinding, milling, etc., or formative production processes, where raw material is shaped in moulds and forms with injection moulding, deep-drawing and bending. fig. 1. overview ‘additive fabrication’; use and allocation of various terms for the different areas of the am industry. it can be observed that over the last decades all professional participants, that are applying amtechnologies in r&d or business application, have fostered the former niche production processes into a new industrial field of production. changed prerequisites in the am industry led to more awareness and acceptance over the last years. more and more people got aware of the available am technologies and started to use them as an additional tool in industrial design, mechanical engineering and others. also a big part of the newly developing applications and ideas come from the ‘do-it-yourself’ part of the industry. tinkerers across the globe are developing open source desktop 3d printers – so called ‘fabbers’ (burns, 1999–2003). on the one hand this opens decentralised manufacturing of individualised products to the people, on the other hand this makes am technologies affordable for teaching institutions and laymen. this development supported to spread the idea of am further, opening new markets, giving new ideas. parallel to the development of the am technologies, also the so called ‘third industrial revolution’ happened over these last three decades (whadhock, 2012). the first industrial revolution changed the british textile industry with automated weaving looms in the 18th century. it started a fast pace of technical innovation to follow up, changing the society from agricultural to industrial. the second ‘industrial revolution’ was started by the invention of the production line in the automotive industry in the united states of america in the 19th century by henry ford. in his article about the digital revolution whadhock brings up keywords like ‘social manufacturing’, and predicts that “as manufacturing goes digital, a third great change is now gathering pace. it will allow things to be made economically in much smaller numbers, more flexibly and with a much lower input of labor, thanks to new materials, completely new processes such as 3d printing, easy-to-use robots and h. strauß et al. / additive manufacturing for future facades 227 new collaborative manufacturing services available online. the wheel is almost coming full circle, turning away from mass manufacturing and towards much more individualized production. and that in turn could bring some of the jobs back to rich countries that long ago lost them to the emerging world.” this strive for customisation is definitely a growing demand in today’s’ societies, but it also reflects back to all fields of social live (whadhock, 2012). social media, highly individualised live designs, more and more locally detached and independent work spaces, worldwide available cloud computing, highly infiltration of society with digital gadgets, digital tools and digital worlds lead to a higher demand for individualisation in all aspects of life. while the first and second industrial revolution aimed toward more and more repetitive processes in production and assembly, today’s products and everyday items become more and more individualised. starting with customized products – configurators in the automotive industry (volkswagenag, 2015), to partly individualized items ‘your name on your sneakers’ (nike, 2015), to truly customized features – medical implants fitted to your body from ct digital data (christensen, 2014), whereas so called ‘mass customization’ is already reality for a large number of available products (piller, 2012). adding to this aspect of today’s demand for customization, am offers the possibility of highly individualized parts at the same cost of an ordinary standard part, as with am cost per piece is no longer linked to complexity. a straight printed cube of equal volume causes the same amount of print time, consumed material and energy involved as a highly detailed and digitally morphed structure (sandhana, 2013). 2. printed building construction sample facades on this background, it can be expected that facade technology and facade construction will change with the application of additive manufacturing. the building envelope is – due to functionality and industrialisation of the industry – one of the most challenging parts of the building. it has a multitude of expected performances, it separates inside from outside and gives the face to the building. for the one-of-a-kind architectural designs, the facade is the suitable part of building construction to represent the design intent of the customer, architect and user. but in building construction also specific requirements have to be met with the applied products. aspects that are related to every building activity have to be addressed and to be met: legal aspects like building codes and liabilities or economical aspects like market demand and market acceptance. all aspects are regulated by the building authorities; it is therefore a bigger approach to bring am to a certified production process in building construction. but as the market is demanding for bespoke solutions, am is a tool at hands that meets the requirements of low batch production, perfectly matching the one-of-a-kind designs in architecture. already today companies from the facade industry are testing the applicability of am for their products, like in the aviation industry, at first for non-structural components only (cappings, end pieces, etc.), but with further development of material properties in the close future for structural applications. in this context also batch size is one aspect of the decision: investment for tooling and production vs. needed number of parts and fast time-to-market circuits. 3. freeform geometries one example of an innovative application of the new technologies is the demand of freeform geometries by the architect. there are different strategies to deal with freeform: 228 h. strauß et al. / additive manufacturing for future facades – true freeform, that means doubly curved surfaces, that are very difficult to produce in most of the building construction materials; – tessellated surfaces, where an approximation can be made between true freeform and ruled surfaces, ending in an almost freeform surface; – faceted surfaces, where from the beginning the free form is broken down into straight members, that allow for standard production. there are products that allow for ‘true freeform’ (double curved surfaces), like fabrics and membranes. but most of the time they are not meeting the building construction requirements and also follow structural and geometric rules that disturb the pure freeform approach. therefore other solutions are needed. some producers start to open their production toward freeform. e.g. rieder concrete is investigating into freeform facade panels (rieder, 2013). also there have been approaches with free-formed aluminium extrusion profiles for stick system facades. but how to deal with the doubly curved surfaces that will appear for the infill panels? how can the glazing be realized? this leads to bended glass, or to new ways of producing those doubly curved elements with am technologies: for example with 3d printed glass – ‘direct glass fabrication’ (rammig, 2010) (see fig. 2), or with freeform dome structures from clay or concrete – ‘contour crafting’ (khoshnevis, 2006) (see fig. 3). there are ideas at hands, and they can support the further development of additive processes from cooperative research, the specialist’s input and the academic approach. fig. 2. prototype of point fixation made with direct glass fabrication (dgf). 4. 3d system offerings for facades? in the near future solutions for complex geometries with the safety of a system offering may be hybrid systems that combine mass-produced and mass-customized parts in one solution. looking at it, this seems to be a contradiction, because system offerings and bespoke solutions do not go together. but regarding the background of the long-term experienced and – maybe even more important – certified system offerings, it makes sense. to develop a facade for the western market without coming back to the certified and tested system offerings is economically almost impossible. therefore it is logical to implement new technologies into the production line, and achieve more satisfying results for the architect and customer. at the end one-of-a-kind projects will have the individualized look they need, but also have the reliable functioning details regarding building physics and performance. h. strauß et al. / additive manufacturing for future facades 229 fig. 3. scaled prototype of lunar dome structure printed with contour crafting (cc). credits behrokh khoshnevis, university of southern california, usa. 4.1. additive fabrication as a new part of the production chain, additive fabrication will change the way we design and produce as well as how we handle consumer goods and our built environment. in order to make these changes tangible for facade construction, case studies with realistic application concepts from the facade technology were carried out. initially it was difficult to identify such concrete applications for am, due to our habit to think subtractive rather than purely functional. however, it did lead to an intensive examination of the technologies and to defined approaches and results. relevant aspects of the studies include material consumption and assembly as well as component performance in the facade system, amongst others. the development of am is still in the beginning stage; however, am technologies offer the potential to lastingly change design and construction methods. the change in our way of thinking has long begun: file-to-factory, building integrated modelling (bim) and digital materials are the key words in this on-going discussion in the day and age of parametric design and construction called ‘the age of grashopper’ (davidson, 2012). further development of the new technologies is progressing rapidly; it is foreseeable that am will be intuitively and naturally used in the future and, thus, will find an application in many new areas – even in the somewhat conservative building sector (strauß, 2008). the authors were involved in an industry-supported research between the university of applied sciences – hochschule owl, detmold, germany and kawneer-alcoa that focused on the potential of additive manufacturing for facade construction. this paper will not give specific technical background information on the technology, but refers to the previously published papers and books of the writers about the topic (knaack, 2010; strauß, 2013). 4.2. the nematox facade node the nematox facade node is the result of a cooperative research between industry and academics. the project was carried out from 2008 to 2010. it shows that it takes a while to come from initial ideas to a reliable invention. in this case the conducted research was finalized with the presentation 230 h. strauß et al. / additive manufacturing for future facades of the nematox facade node (see figs. 4 and 5). it presents the capability of am technologies to be combined with facade system offerings, and as a result it could offer a new step toward freeform geometry in facades (strauß, 2010). fig. 4. rendering of nematox facade node. fig. 5. aluminium printed mock-up of nematox facade node. h. strauß et al. / additive manufacturing for future facades 231 after the completion of the research project, the company took almost three years to pick the idea back up. but looking at it today, the time passed added to the realistic approach of the initial development. over these last years a lot of progress was made within the am technologies for ‘direct metal fabrication’. production times were reduced, quality assessment was enhanced and prices for the am systems and materials dropped significantly. the reason for kawneer-alcoa to come back to the idea lies between ‘artistic architectural ideas’ and ‘industrial research’: today alcoa clearly envisions a market for individualized facades, digital planning processes and tools. also am offers a product enhancement within each new project; as the parts are virtually planned and designed on the computer, no extra tooling or expensive machinery is needed. the lead times from design to production are drastically reduced by implementing am into this production chain. no tooling costs or tooling times need to be accounted for. all this is in favour for digital production of smart, neuralgic parts in a free-formed facade. klaus-martin hees (research director europe, kawneer-alcoa): “three years ago, we were missing the matching marketing strategy for the nematox node. therefore the topic was only theoretically looked at. in the meantime the market and the requirements hereof have changed constantly. there is a trend toward more and more complex constructions with even shorter lead times for their realisation. on this background we have re-launched the nematox project in 2013 [. . . ]” (hees, 2013). even in 2016, the addendum ‘rapid’ is a relative term in conjunction with am methods, because an actual production process may take up to several days. however, the equipment development has reached a stage where the combination of processing speed and material properties does achieve ‘fast’ effective manufacturing rates when compared to traditional methods. this comparison takes into consideration the time it takes to produce a conventional tool set (e.g. for injection moulding) for traditional methods. but speed is not the most important aspect when evaluating am technologies. the main advantage lies in the great freedom of form compared to traditional methods. when transferring this existing new technology into real facade projects, the innovative potential will face the budget reality of the future projects. still the author strongly believes in the potential of additive manufacturing for facade construction. the nematox facade node is the proof of concept from research cooperation between building industry and academic research and adds extra value to well-known applications (strauß, 2013). there are already prototypes of ‘printed’ houses and building construction components for smaller huts and pavilions existing, that aim directly for the built environment (sandhana, 2013). one example is the ‘kamermaker’ in amsterdam as a direct applied architectural approach – a printed canal house (dusarchitects, 2015) (see fig. 6). life size printed parts and components in materials that suit building construction come to the fore. concrete building structures are the result of research conducted at universities and institutions; here contour crafting by behrokh khoshnevis is a vision with a well known material in building construction: concrete. (khoshnevis, 2006) (fig. 7: cc-concrete part with nozzle). all examples show the potential am has for building construction – all in completely different ways, but all fostering the great advantage of layer wise production. according to a survey conducted by the german research institute dmrc, most of the applications in the research sector aim toward ‘direct manufacturing’ as the major application of am (∼80%). still followed by ‘rapid prototyping’ (∼66%), ‘rapid tooling’ (∼33%), and ‘other applications’ (∼19%). whereas the numbers account over a total of 100%, as numerous activities could have been chosen by the surveys’ participant. nevertheless, it shows exactly the shift in the professional field of am, from pure rapid prototyping to an end-use application of ready-to-use parts (gausemeier, 2013). 232 h. strauß et al. / additive manufacturing for future facades fig. 6. axonometric view of the construction parts being printed by the kamermaker (ba dusarchitects). fig. 7. contourcrafting-concrete part (cc) with printing nozzle. credits behrokh khoshnevis, university of southern california, usa. h. strauß et al. / additive manufacturing for future facades 233 the development of am compliant applications will also bring forth new materials to facade technology that will maybe be more suitable to manufacturing as well as for the required functions than conventional materials. materials do not matter in the first place, but can also lead to new applications – if we think of the massive amount of iron-mongery that is needed to make a functional opening casement: what if the hinges and fittings would be incorporated and ‘digitally’ mounted to the mullions in one corporate material? the healthy, näıve view of the architect helps engineers and constructors to lift, if not take off entirely, blinkers that protect from the unknown. and now material scientists even started to add a new dimension to 3d printing, as the technology itself goes on: introducing now the fourth dimension to 3d printing – called 4d printing. researchers at pittsburgh university now investigate into additional values from smart materials incorporated into am parts via stimuli-responsive components in am materials (materialsviews, 2013). “dare to be naive”, as buckminster fuller said, could be a leading thought for all innovation in the building sector. the intensified normative regulation, the higher and higher requirements as a result of building codes and guidelines limit more and more the optimistic and daring approach that architectural engineering once had. definitely facades face higher and higher demands regarding design intent (geometry) and performance (building physics, technical fit out, services). the building envelope therefore has a high potential for innovation. this starts at a very primary construction of a house and facade, and ends in mike davies’ ‘polyvalent wall’ (davies, 1981). 5. outlook even after three decades since its invention, am is still under development. many different factors add up to the full picture of its eruptive potential for the known production processes. application in building technology, especially in the building envelope has great potential. the ‘rapid’ development over the last ten years gives hope to keep the pace as in the personal computer market. indicators are: – larger building envelopes/chambers: raised every year, the biggest step was undertaken by voxeljet, germany, where the biggest available building chamber now offers a size of 4000 × 2000 × 1000mm (voxeljet, 2015). this offers the possibility to print live size components for building construction – e.g. steel constructions, nodal points for large-scale structures. – faster production times: the speed can be enhanced with various measurements within the am processes; slm solutions, germany, for example came up with a combination of multiple laser sources in one machining centre. this is possible, as cost for industrial lasers dropped significantly over the last years. this leads to faster production times and higher output, and reaches semi-industrial production batch sizes, and was not expected five years ago. – advanced cad-cam interfaces: more and more cooperative software solutions, with more and more intuitive surfaces and functions, alternative file formats and other changes move am toward ‘digital production’. a deeper understanding of complex planning processes and the right tools at hands to produce digital designs with digital tools further incorporates new technologies into existing production processes. one tool is am, other tools are high speed milling, water jet cutting, cnc saws, cnc milling centres, and many more. hybrid technology application is the desirable application of am. architectural and engineering vision for am in the building industry is the smart combination and hybrid solution of well known standards and of the bespoke and tailored individual components. this is what makes the difference between mass produced and mass customized, regarding the need for reliable construction – safe with respect to the lives that are under risk in our built 234 h. strauß et al. / additive manufacturing for future facades environment –, but also regarding the demanding requirements of all ecological aspects that are related to every building activity. am is one eruptive new technology within the digital revolution. after its 25 years of infancy it now faces reality and is entering the architectural market. it has the potential to change the way we design, construct and produce. to start, for example, with a very delicate single piece of a holistic facade approach at first will be an adequate beginning. and it will be the beginning to open new ways in facade building. 6. summary this article shows the potential of additive manufacturing (am) for the future development of building envelopes: am will change the way of designing facades, how we engineer and produce them. new technologies offer one possible way to do so. they open new approaches in designing, producing and processing building construction and facades. am offers the opportunity to manufacture facades ‘just in time’. it is no longer necessary to store or produce large numbers of parts in advance. initial investment for tooling can be avoided, as design improvements can be realized within the dataset of the am part. the parts performance can be re-interpreted based on the demands within the system, and not based on the limitations of conventional manufacturing. this may lead toward customized and enhanced performance. the basic principle of am opens a new world of engineering, no matter what applications can be found: to ‘design for function’ rather to ‘design for production’ turns our way of engineering of the last century upside down. am will never replace established production processes but rather complement them where this seems practical. am is not the proverbial swiss-army knife that can resolve all of today’s facade issues! but it is a tool that might be able to close another link in the ‘file-to-factory chain’. am allows us a better, more precise and safer realization of today’s predominantly free designs that are based on the algorithms of the available software. with such extraordinary building projects, the digital production of neuralgic system components will become reality in the near future – today, an am envelope is close at hand. still, ‘printing’ entire buildings lies in the far future; for a long time human skill and craftsmanship will be needed on the construction site combined with high-tech tools to translate the designers’ visions into reality. references burns, m. (1999-2003). fabbers.com, 2012, from http://www.ennex.com/%7efabbers/ christensen, a. (2014). changing the game in healthcare. tct magazin, 82. davidson, s. (2012). grasshopper generative modeling for rhino, 2012, from http://www.grasshopper3d.com/ davies, m. (1981). a wall for all seasons. riba journal, 2(88). dusarchitects (2015). 3d print canal house. retrieved 31.03.2015, 2015, from http://3dprintcanalhouse.com/kamermaker-1 gausemeier, j. (2013). evaluation of the expert survey on the future relevance of additive manufacturing research. paderborn, germany: dmrc, heinz nixdorf institute, university of paderborn. hees, k.-m. (2013, 20 september 2013). [research director europe. kawneer-alcoa]. hopkinson, n., hague, r. j. m., & dickens, p. m. (2006). rapid manufacturing. an industrial revolution for the digital age. chichester, england: john wiley and sons, ltd. khoshnevis, b. (2006). contour crafting, from http://www.contourcrafting.org/ materialsviews. (2013). entering an new dimension: 4-d printing. materials views newsletter retrieved october, 2013, from http://www.materialsviews.com/entering-a-new-dimension-4-d-printing/ nike (2015). nikeid. retrieved 31.03.2015, 2015, from http://www.nike.com/de/de de/c/nikeid piller, f. t. (2012). mass customization & open innovation a blog by frank t. piller. retrieved april 2012, from http://mass-customization.de/ http://www.ennex.com/%7efabbers/ http://www.grasshopper3d.com/ http://3dprintcanalhouse.com/kamermaker-1 http://www.contourcrafting.org/ http://www.materialsviews.com/entering-a-new-dimension-4-d-printing/ http://www.nike.com/de/de_de/c/nikeid http://mass-customization.de/ h. strauß et al. / additive manufacturing for future facades 235 rammig, l. (2010). direct glass fabrication new applications of glass with additive processes. m.eng. masterthesis, hochschule owl university of applied sciences, detmold. rieder, a. (2013). fibrec 3d fassade. retrieved september 2013, 2013, from http://www.rieder.cc/at/de/main/referenzen/reference/19/ sandhana, l. (2015). the room with 260 million surfaces: 3d printed architecture is here. gizmag online newsletter. retrieved from www.gizmag.com website: http://www.gizmag.com/swiss-architects-3dprint-a-room/29299/ strauß, h. (2008). funktionales konstruieren einfluss additiver verfahren auf baukonstruktion und architektur. dipl.-ing., hochschule owl, detmold. strauß, h. (2010). am facades influence of additive processes on the development of facade constructions. final report of research project. hochschule owl university of applied sciences. detmold. strauß, h. (2013). am envelope the potential of additive manufacturing for facade construction. delft university of technology, delft. (ab+e, 01/2013) knaack, u, bilow, m., strauß, h. (2010). rapids layered fabrication technologies for facades and building construction (imagine 04). rotterdam: 010 publishers. volkswagenag. (2015). konfigurator von volkswagen.de. retrieved 31.03.2015, from http://app.volkswagen.de/ihdcc/de/configurator.html voxeljet. (2015). vx4000: das großformatige 3d-drucksystem. retrieved 31.03.2015, from http://www.voxeljet.de/systems/vx4000/ whadhock, i. (2012). a third industrial revolution. the economist. retrieved from http://www.economist.com/node/21552901 woodcock, j. (2011). living in a (multi) material world object focus on democratizing multi-materials process and new functional materials. tct magazin. http://www.rieder.cc/at/de/main/referenzen/reference/19/ www.gizmag.com http://www.gizmag.com/swiss-architects-3dprint-a-room/29299/ http://app.volkswagen.de/ihdcc/de/configurator.html http://www.voxeljet.de/systems/vx4000/ http://www.economist.com/node/21552901 journal of facade design and engineering 2 (2014) 235–254 doi 10.3233/fde-150022 ios press 235 visual assessment of bipv retrofit design proposals for selected historical buildings using the saliency map method ran xu∗ and stephen wittkopf cc ease, lucerne university of applied sciences and arts, horw, switzerland abstract. with the increasing awareness of energy efficiency, many old buildings have to undergo a massive facade energy retrofit. how to predict the visual impact which solar installations on the aesthetic cultural value of these buildings has been a heated debate in switzerland (and throughout the world). the usual evaluation method to describe the visual impact of bipv is based on semantic and qualitative descriptors, and strongly dependent on personal preferences. the evaluation scale is therefore relative, flexible and imprecise. this paper proposes a new method to accurately measure the visual impact which bipv installations have on a historical building by using the saliency map method. by imitating working principles of the human eye, it is measured how much the bipv design proposals differ from the original building facade in the aspect of attracting human visual attention. the result is directly presented in a quantitative manner, and can be used to compare the fitness of different bipv design proposals. the measuring process is numeric, objective and more precise. keywords: bipv, visual assessment, visual impact, solar retrofit, saliency map 1. introduction 1.1. the swiss urban planning law and solar installations the swiss raumplannungsgesetz rpg (urban planning law) has been published in order to prevent the urban sprawl and to fully utilize the existing urban areas before occupying new, untouched rural lands. generating clean energy with large-scale solar parks by destroying existing rural landscapes is therefore definitely not encouraged. instead, small-scale building-integrated photovoltaic (bipv) installations on the existing building facades are preferred. however, bipv installations need to follow certain restrictions. according to the rpg article 18a that came into force on may 1st 2014, it is stated that “solaranlagen [. . . ] dürfen solche denkmäler (kulturund naturdenkmälern) nicht wesentlich beeinträchtigen” (solar installations [. . . ] should not significantly influence the original appearance of the cultural and natural heritages) (rpg, 2014). ‘cultural heritage’ hereby refers to historical buildings that are selected by the individual swiss cantons (swiss federal constitution, 2014) and (to be) put into the cantonal inventory list of historical buildings. rpg article 18a was proposed with the purpose to ensure both ∗corresponding author: ran xu, cc ease, lucerne university of applied sciences and arts, 6048 horw, switzerland. tel.: +41 41 349 36 34; fax: +41 41 349 39 60; e-mail: ran.xu@hslu.ch. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:ran.xu@hslu.ch 236 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings a careful integration of new clean energy and protection of the heritage of swiss urban and nature landscape. regarding the historical buildings, aside from the rpg article 18a, the government has also published a document on their website, giving suggestions on the sensitive issues that a renovation work on a historical building may face (bundesamt für energie & eidgenössische kommission für denkmalpflege, 2009). the general attitude of this document is to encourage keeping the historical buildings as original as possible, and the change after the solar facade renovation should be as less as possible compared to the buildings’ original state. however, the exact meaning of the expression ‘not significantly’ in the rpg article 18a has been left to each cantonal/local authority to define. in this case, local authorities can refer to the canton government, commune (gemeinde) or the local cultural heritage preservation department (denkmalschutz). some cantons have published guidelines on how to install solar panels on the house roof/facade so as to smoothen the process of solar installation applications (kanton basel, 2013, 2012; kanton luzern, 2014; kanton thurgau, 2009). in the guideline brochures, suggestions are being made so that the solar panels will ‘not significantly’ affect the existing historical building. 1.2. problem statement the solar installation guidelines for historical buildings from the swiss cantons are putting a lot of weight on its appearance rather than on its construction, and guidelines are mostly interpreted with semantic and qualified descriptors. the direct result of rpg article 18a and its derivatives is that people of different backgrounds have different understandings on how solar panels are ‘not significantly’ affecting the historical buildings and whether they co-exist harmoniously or not. the biggest barrier for the solar installation often comes from local authorities with their disagreement on the changed appearance on the original building facade, leading to confusion and conflict between them and the applicants (von arx, 2013; häne, 2012; ludin, 2013; stadt zürich hochbaudepartement, 2008; vitelli, 2013; zurbriggen, 2009). the main reason for the conflict is because the applicants (building owner, architect and solar engineers etc.) and local authorities (e.g. cultural heritage preservation department) often have different interests in this matter. the building owners and the engineers prefer costand energy-efficiency of the solar installation, and attenuate its visual aspect subconsciously. architects may weigh cost and energy-efficiency equally with the visual aspect of the building, while the cultural heritage preservation department focuses entirely on the visual aspect and harmony of the solar installation and the existing building (see fig. 1). fig. 1. stakeholders and their approximate percentage of interests in energy & cost aspect versus in visual aspect. r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 237 here people deal with the co-existence of qualified (visual) and quantified (cost, energy) descriptors. qualified descriptors hereby refer to linguistic descriptions such as ‘good’, ‘ugly’, ‘not harmonious’ etc. to describe how one feels about the visual aspect of the solar installation. quantified descriptors refer to energy conversion, cost etc. that can be presented with numbers. the drawback of a qualified and relative descriptor is that different preferences, different cases and different persons will have different opinions about what is called that the solar installation is ‘not significantly’ affecting the existing historical building. it is relative, subjective and not always reasonable. quantified descriptors, on the opposite, are absolute, objective and the scale do not vary according to the case, situation or person. the co-existence of two entirely different value systems can lead to conflicts and hence the failure of the project. in order to guarantee a reasonable judgment, a coherent value system with a fixed evaluation scale should be developed. one way to do this is to develop a transformation system that can translate the qualified, subjective descriptor into a quantified, objective one, so as to ensure that everything is objective and absolute. in this paper, a method to quantify the term ‘significance’ is introduced, so as to be able to quantify the word ‘not significantly’ subsequently. 1.3. current approaches even though this paper deals with the visual assessment of bipv installation, literature on visual assessments of both photovoltaics and solar thermal installations is being reviewed so as to gain a holistic perspective in this matter. the words solar panel and solar installation therefore refer to both photovoltaics and solar thermal installations. the architectural and visual values of solar panels as a new building element are being discussed widely. mostly people discuss whether the solar panel is well integrated into its environment (the building facade) by starting from evaluating its material, colour, form and detail integration, which is already an indirect investigation of the term ‘not significantly influencing the appearance of cultural heritage’ in an intuitive way. guidelines and recommendations are developed on how to evaluate solar panels in a holistic manner (frontini et al., 2013; munari probst & roecker, 2009, 2011, 2012). bipv is being analysed by dividing their main characteristics into three main categories – functional, constructive and formal, where the functional category includes the basic building envelope function and solar energy collection ability, questioning whether the bipv product fulfil its multifunctional purpose as a facade element. the constructive category cares about whether the bipv are integrated well into the overall facade construction. the formal category is evaluating the solar panels on subcategories such as shape & size flexibility, pattern, colour, joint/frame, dummy part availability. the assessment gives + for positive, ± for mean and – for negative evaluation of the above mentioned aspects. the result for a solar panel product is produced by listing all the categories in a list with + and – symbols. the evaluation process is made manually and intuitively, and offers three grades (positive, mean and negative) for each aspect. another way to discuss the term ‘not significantly’ is by investigating the visibility of the solar panels, namely to assess whether the solar panels are visible from the streets or not. this method was developed by using simple 2d geometry (dessi, 2013). others also focus on the visibility but have used a different approach. by adapting the gaussian probability function, the probability of noticing the solar panel within the human visual scene is being calculated from a pure mathematical point of view (xu, 2014). both approaches, however, are radically simplified methods and only take the visibility element into consideration, where the colour, reflectivity, material properties, details of the solar panel and the overall building environment are omitted. 238 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings in another research, daylight reflection issues of bipv are being singled out to study their impacts on the visual appearance of the building. by using radiance software (wardothers, n.d.) and annual weather data (wittkopf et al., 2014; yang et al., 2013), one is able to quantify whether the bipv installation is ‘significantly’ visible by the amount of daylight reflection it produces. this kind of research only covers one small part of the visual aspect of solar facade and hasn’t taken the attention ability of human eye into consideration. another method covers a more complex energy refurbishment assessment for historical buildings with solar panels system. with the goal of achieving better visual results, one should aim for coplanarity, respect the lines of the existing building, shape, grouping, accuracy and visibility while installing the solar panels on the building envelope, on a difficulty scale ranging from 1 to 3, with 1 meaning that the goal is easy to accomplish, and 3 meaning very difficult to accomplish (lópez & frontini, 2014). in this case, the evaluation process is different for each architecture, because each historical building has its unique aesthetic difficulties when it comes to solar panel installation. therefore the standard of this method varies from case to case, and the deciding process of the difficulty scale is also subjective. a more sophisticated method sets up a holistic benchmark for ‘good’ bipv integration (yu et al., 2009). the overall bipv evaluation is calculated by the multiplication of the separated evaluation results of electricity generation, building integration and bipv combined benefits, then multiply the result with a weight that comes from the evaluation of the aesthetic evaluation. the weight is higher when the bipv is more appropriately installed, and lower when poorly installed, so that the distinction can be made that even when different bipvs fulfil the same basic function, but by achieving different aesthetic results, each of them can still have distinctive overall results. the aesthetic weight is given based on evaluating the bipv on items such as ‘pleasantness’, ‘uniqueness’, ‘specialness’, ‘obviousness’ etc. those terms are qualified descriptors that do not have explicit nor quantified explanations, and the judgment therefore is flexible and subjective. other similar bipv evaluations are also available (lucchi et al., 2014; vassiliades et al., 2014). 1.4. limitations and challenges from the examples in section 1.2 it can be inferred that even though a better energy efficiency is the ultimate goal, normally it is the appearance of the historical buildings that stands in the spotlight. its importance even outranks the other aspects of the historical buildings. therefore, despite the fact that most of the literature above has not referred specifically to the term ‘not significantly’, the evaluations all have the same goal, which is to make the solar installation exist with the existing building in harmony, or in other words – ‘not significantly affecting the cultural heritage’ in its appearance. it is apparent from the literature review that regarding the ‘significance’ of solar installation on the building facade, it is often tried to transform the grading of linguistic descriptors into numeric ones. usually set 1 with qualified terms is given (such as ‘bad’, ‘medium’, ‘good’), and each one of them is mapped into set 2 with quantified numbers (e.g. bad=0, medium=1, good=2 and similar). there are two disadvantages with this kind of transformation method. one is that each person has his own opinion about the qualified terms. that is to say that every individual has his own subjective, individual and flexible opinion about what is called ‘bad’, ‘medium’ or ‘good’. the other drawback is that a reasonable, unified mapping process is lacking. each author can decide the composition of set 1, set 2 and the mapping method based on his need, without having any theoretic reference for r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 239 the mapping method, making it impossible to objectively compare the visual aspect of solar panel installations of different projects with a unified parameter. therefore considering these two disadvantages mentioned above, two things need to be investigated: a. is there a way to quantify the term ‘not significantly’ for bipv in the context of historical buildings? b. for the visual aspect, is there a reasonable, unified mapping process from set 1 (qualified descriptors) to set 2 (quantified descriptors) available? 2. saliency maps and applications 2.1. saliency the english oxford dictionary defines the adjective salient as ‘most noticeable or important’. saliency refers to the noticeability of an object (pixel, person etc.) within its visual environment. the saliency map is therefore a 3d map indicating the probability of obviousness and noticeability of each point within the human vision field. 2.2. saliency maps humans have the ability to rapidly scan through a certain visual field (25 to 50ms per item) and recognize an object that is standing out from their environment, which relies solely on the basic instinct of human nature and is called the ‘bottom-up’ processing of human attention. this attention mode is developed during evolution so as to detect the potential danger in the surrounding environment. the second attention mode is the search for objects that are of interest to the person. with pre-selection criteria that interest him, it is called the ‘top-down’ processing. in this mode, the human can be in charge of his attention selection criteria. the speed of this visual scanning is much slower – 200ms or more per item (itti & koch, 2001). the concept of saliency map was introduced by koch and ullman (koch & ullman, 1985), where they assume that the higher the conspicuity (clear visibility) of a location in the visual scene, the higher the firing of the neuron in the eyes. this is based on the ‘bottom-up’ theory. later, they have refined their approach where they distinctly define the early visual features of ‘bottom-up’ attention to be the intensity, the colour and the orientation of the input image (see fig. 2) (itti, koch, & niebur, 1998). each of these features then will undergo a process called the ‘centre-surround’ operation, which is an imitation of the way the retinal ganglion cells’ receptive field (a part of the human eye) works – each receptive field is divided into the ‘centre’ area, and a concentric ring, the ‘surround’ area. if the light intensity on those two areas is different, then it will cause the eye to respond. e.g. a single light beam might increase response of the cell (difference in centre and surround areas), while an overall brightness in the scene (less difference between light intensities in centre and surround areas) would weaken the response (hubel, 1995). in this way contrast and hence edges of the objects are calculated. after the normalization processes, so as to globally promote maps that have small values but strong peaks, and discourage maps that have overall high values, the feature maps will be combined into three conspicuity maps. those will be again linearly combined into a final saliency map. on the scale from 0 to 1, the higher the value of a spot, the more likely the human eye is noticing this spot (see fig. 3). 240 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings fig. 2. the general architecture of the itti-koch saliency map. fig. 3. an example of the application of saliency map. left: the original image. right: the generated itti-koch saliency map by using a script written by harel, koch, & perona (2006). they have also proposed a winner take all (wta) mechanism. the ‘bottom-up’ saliency map is generated by iterative calculations of the colour, intensity and orientations value. at the beginning of each iteration, the most salient spot from the last iteration will be left out. this mechanism inhibits the saliency modelling returning to the same conspicuous place in the visual scene repeatedly. instead, it allows the modelling to move to the next salient place in the next round of calculation. r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 241 the itti-koch saliency map is based on the theory of bottom up processing of human attention. many other kinds of models are being developed after that, with itti-koch’s principle as the basis (borji & itti, 2013). a benchmark developed by judd et al. (bylinskii, judd, durand, oliva, & torralba, n.d.; judd, durand, & torralba, 2012) lists all the existing saliency methods to evaluate the accuracy of those models. while e.g. the judd saliency model has achieved a much better accuracy score than the original itti-koch method, the principle of the judd method is adding other pre-selection models on top of the itti-koch model, enhancing the recognition ability to identify cars, persons, faces and horizon within the visual scene (judd, ehinger, durand, & torralba, n.d.). 2.3. current application the original goal of modelling the attention within a given visual scene is to simulate the realistic neural network models and to explain human attentional behaviours. this research area has been very active over the past 25 years (borji & itti, 2013). aside from predicting the most salient spots within the human visual scene, inspired application of these models have also appeared. for example the saliency maps can help save the rendering time in high-fidelity image rendering. high quality rendering will only happen to places in an image where human attention is attached to, while rendering the rest of the image in low quality. this can result in a reduction of calculation time (chalmers & debattista, 2009; longhurst, debattista, & chalmers, 2006). another application is using the saliency map to help blind people in environments that are unfamiliar to them. this is achieved by using the three basic features (colour, intensity and orientation) to detect the indoor signs, after that the information can be translated into vocal or other kinds of information that are accessible to visually impaired people (wang & tian, 2011). humanoid robots can benefit from saliency map information. the robot can be guided by the saliency maps, and its attention is then focused on the most obvious locations (ruesch et al., 2008). the field of application for the saliency map is very broad. however, in architecture, its benefit has not been fully realized yet, even though architects are constantly heavily involved in visual experiments in their design works. 3. saliency map as visual assessment for bipv installations the research papers mentioned in part 1.3 all have the same ultimate goal, which is to make the solar panels exist in harmony with the existing building. to be more specific, the ‘popping out’ of the bipv element is not desired and is to be avoided as much as possible. this is especially the case when it comes to bipv installation on historical buildings, which is also the investigation object in this paper. to answer the first question from section 1.4 of this paper: the saliency map theory can provide a method to measure the ‘significance’ of bipv in the context of historical buildings. the mapping process from what human vision is seeing (and hence feeling) to the quantification of the seeing/feeling is standardized,basedontheoriesfrombiology,psychologyandcomputerscience,henceansweringthesecond question. moreover, a basis for numerical comparison can be provided. the saliency map theory can also objectively calculate the saliency difference with and without the bipv installation in the visual scene. it can generate a comparison mechanism to cross compare several different bipv designs on the same facade – which design is less conspicuous and exists more harmonious with the existing building facade – and decide the best design. this process would avoid the tedious procedure of having the human see 242 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings the design first, feel it and then translate it in linguistic descriptor, transform the linguistic descriptor into numeric ones so as to make the evaluation of solar installation possible. instead, an objective scale is used to decide the strength of change the bipv has done to the building in the human visual scene, with the transformation of verbal descriptors to numeric descriptors omitted. this paper adapts the itti-koch saliency method, because even though other advanced saliency methods, e.g. the judd saliency method, are more accurate, the refinement criteria (face, people, car, horizon identification) may not be essential on architectural facade evaluations. 3.1. method this assessment methodology is based on creating a reference rendering (‘as is’) and design proposal renderings (‘new’). the pre-processing stage includes digitally recreating historical buildings itself and different designs with bipv on the computer as 3d cad model. renderings, which are computergenerated images, are being made from the models. the main processing work is calculating the saliency values of each rendering image. the post-processing step is to compare the saliency values of the pictures of the historical building before (‘as is’) and after bipv installation (‘new’). after that, one can then see whether the bipv are ‘not significantly’ enough to exist harmoniously with the existing building, and how much they have changed the existing building regarding attention-aspect. since the general goal of the historical building protection is to change as less as possible, therefore a good bipv design on the facade should cause as less saliency change in the visual scene as possible. the specific workflow can be divided into five major steps (see also table 1). 1) capture the 2d photo ‘as is’ a photo of the building’s original state is taken. the viewpoint of the photo is set where there is the highest probability of seeing the building. later the saliency of the visual scene will be investigated from this viewpoint. table 1 the summary of work steps and deliverables process stage no. step name description deliverable pre-processing 1) as is capture a photo of the historical building’s original state 2d photo ‘as is’ 2) 3d model rendering of the historical building in its original state. viewpoint must be the same as 2d photo ‘as is’. 2d rendering ‘as is’ 3) bipv designs 3d model renderings of the historical building with different bipv designs. viewpoint must be the same as 2d photo ‘as is’. 2d renderings ‘new’ processing 4) saliency generate itti-koch saliency maps of all 2d renderings ‘as is’ and ‘new’ 3d itti-koch saliency maps ‘as is’ and saliency maps ‘new’ post-processing 5) comparison calculate differences between the saliency map ‘as is’ with each saliency map ‘new’ delta saliency maps r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 243 2) create the 2d rendering ‘as is’ as reference 3d digital models of the building are created. the photorealistic image that is produced from the 3d model is called rendering. all the important elements of the building facade are included in the model, such as the proportion, detail, colour, obvious texture etc. in creating the model, the face normal of the model surfaces always face outwards, so as to enable the later rendering with radiance. the radiance program is an advanced lighting simulation tool (ward & others, n.d.), and suits the purpose of rendering a photo-realistic picture well. the viewpoint of the rendering ‘as is’ is identical as the one of the photo ‘as is’. 3) create the 3d bipv designs and 2d renderings ‘new’ on the basis of the before-mentioned 3d model and viewpoint, building renderings with different designs (2d renderings ‘new’) of the bipv installations are generated. since clear sky condition (cie sky definition) will cause hard shadows and strong reflections in the renderings. they will have strong impact on the saliency maps. in order to avoid this situation, all the photos and renderings are made under overcast sky condition, which won’t result in hard shadows and exaggerated brightness. this enables the cross-comparison of saliency distribution between different designs (even solar installations on different buildings) under a unified condition that is independent of the daytime and location. after the rendering process, identical adjustments/optimization procedures are being applied to each rendering in order to make them more similar to the photo ‘as is’. 4) saliency maps will be produced from the renderings ‘as is’ and ‘new’ itti-koch saliency map (itti et al., 1998) are produced from the renderings ‘as is’ and ‘new’ by using a script written by harel, koch, and perona (2006). the thus generated saliency map from the rendering ‘as is’ acts as the standard saliency map; its saliency values act as the standard saliency values. 5) comparison between ‘as is’ and ‘new’ saliency values delta saliency maps are calculated by the difference between the standard saliency value and the saliency values from different design renderings ‘new’ (see formula (1)). �saliency map = |saliencyas is − saliencynew| (1) after comparison, the design with the smallest change in saliency values equals the least visual change through the bipv installation, therefore the building is most similar to its original state, this design is then the most appropriate design for this historical building. 3.2. application 3.2.1. the building the building is a villa in canton nidwalden of switzerland with the name ‘seerose’. it was built around 1920 and is now under historical preservation level b, which means the house is considered to be integrated into the cantonal historical protection list later, but pending further investigation (kanton nidwalden & fachstelle für denkmalpflege, 2010). the villa is surrounded by a garden, with the entrance of the garden in the southwest and the lake lucerne in the immediate east vicinity (see fig. 4). 244 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings fig. 4. site plan of the building ‘seerose’ (left), photo ‘as is’ (right above) and rendering ‘as is’ (right below) with viewpoint from the garden entrance. the owner of the house, ‘üserhuus’, would like to install bipv on the facade. in total, eight designs have been developed, each using ten solar panels with two variations in solar panel front glazing colour and four variations in layouts (see fig. 5). there are four alternations in layout, which are each five panels below two windows; a horizontal continuous line under two windows, a cross and a vertically continuous line. on top of the layout variation, each also has its own variations in using two different bipvs front glazing colour. namely one is the traditional black photovoltaic (pv) with non-coloured front glazing, the other is black pvs with front glazing colour that is similar with the ‘old rose’ colour of the facade. all the bipvs are installed on the southern facade. since this house is a private property and most people do not get to enter the garden, therefore the viewpoint chosen is when a person is standing at the main entrance and looking at the south facade of the house (fig. 4, right side), which is the highest probability of the way the house is being seen. the photo and all the renderings are made from this viewpoint. 3.3. analysis the visual scene of the human eye is rather rectangular, with the horizontal axis being longer, instead of having a square form and therefore the rendering size follows this aspect ratio. the original rendering pictures are being cropped with the proportion of 2 in length and 1 in height (image size 1181 × 591) due to this reason (see fig. 6). the investigation area is emphasized on the area where the bipv are being installed. the 2:1 proportioned rendering images, suits the human vision better on the one hand, on the other hand fits well into the investigation focus of the research. saliency r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 245 fig. 5. left: different bipv layout variations. right: different front glazing colours for bipvs. resulting in eight different design proposals in total. maps will be generated based on these cropped images. after the processing, the saliency map will be automatically resized to 148 × 74 (equals 10952 pixels in total), which is defined in the processing script (harel et al., 2006). after processing the images with the script written by harel, koch, & perona (2006), saliency maps can be generated for the original building rendering ‘as is’ and all eight design renderings ‘new’ (see figs. 7-9). while fig. 8 provides an overall impression of the saliency value distribution, fig. 9 246 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings fig. 6. the cropped renderings ‘as is’ and ‘new’. fig. 7. from left to right: the photo ‘as is’, the rendering ‘as is’ and the saliency map generated from rendering ‘as is’. can provide a more refined reading of the specific saliency values. on each saliency map ‘new’, the saliency values are given on a scale from 0 to 1, with 0 being the lowest probability of drawing people’s attention and 1 being the highest probability. intuitively, it can be read from figs. 8 and 9 that the highest peaks in saliency values come from the silhouette of the house, with values being approximately 0.6-0.9. the second highest peak comes from the porch area of the house because of its heavy black awning, with values varying from 0.5-0.6. the third obvious peak is caused by the black bipv panels (regardless of their layout). they draw the most of the eye’s attention to themselves and cause the biggest change in attention distribution throughout the visual field, sometimes even topping the saliency value peaks from the building’s silhouette itself. the differences between each saliency map ‘new’ of bipv with ‘old rose’ front glazings are not so straightforward. the numbers above each saliency maps in figs. 8 and 9 show the average saliency value throughout each map. even though it is stated that the average saliency value all stay within the range of 0.28-0.38, implying that the overall saliency value have not changed much, they do not reveal information about the peak values in the maps. r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 247 fig. 8. the saliency maps generated from renderings ‘new’. therefore the delta saliency value between saliency map ‘as is’ and saliency maps ‘new’ is calculated, which can be achieved by the equation (1). on the scale from 0 to 1, 0 means that there is absolute no change in saliency values between the saliency map ‘as is’ and saliency maps ‘new’, and 1 stands for the highest possible difference between these two kinds of maps. this delta saliency maps are shown in fig. 10. the results proved our former intuitive readings from figs. 8-9 to be right, that the black panels cause the largest saliency differences. the rest of the readings are still somehow blurred out, with a few vague white spots visible. to make the deviance clearer, the pixels in the delta saliency maps that have a change in saliency value of more than 0.1 are counted (white area). delta saliency values below the threshold of 0.1 are omitted because they are considered as noise that is produced during calculation: percentage�saliency value>0.1 = pixel number�saliency value>0.1 10952 (2) where 10952 is the total pixel number of each saliency map. 248 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings fig. 9. 3d diagram of saliency values generated from renderings ‘new’. the results are shown in figure 11. the first line above each delta value map shows the percentage of white area in each map. designs with black bipv panels have more than 1000 pixels that have a change in saliency value, causing tremendous change in saliency distribution. the large white areas in the left of the delta value maps of column 1 reveal the special characteristic of saliency calculations – the saliency value of each point is calculated in relation with its visual environment. a change of colour, intensity or orientation will eventually lead to a butterfly effect – the saliency values of other points will also change accordingly. so we can conclude that an obvious change in bipv glazing colour can cause the human eye to invest slightly more attention in the west facade of the villa. regarding the bipv with ‘old rose’ glazing, the design in row 4, column 2 proved to be the one with the least pixel number (100) with changed saliency value. this means compared to other designs, this design will only cause a minor change in saliency value before and after the bipv installation. the other design with similar result is the design in row 3 column 2, with 114 pixels having a change in saliency value. in order to present the result in a clearer manner, the percentages of the pixels that have not changed in the saliency value in the saliency maps ‘new’ are being calculated (black area): r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 249 fig. 10. the delta saliency values between saliency map ‘as is’ and each ‘new’ saliency map, calculated from equation (1). percentage�saliency value<0.1 = pixel number�saliency value ≤ 0.1 10952 (3) where 10952 is the total pixel number of each saliency map. this is shown in the second line above each delta saliency map in figure 11. this kind of result representation suits the purpose of keeping the historical building as original as possible better. the design in row 4, column 2 shows that 99.09% of the pixels in the rendering have stayed the same. bipv designs with ‘old rose’ glazings generally have a rate of unchanged saliency value of more than 95%, while black bipv designs generally have only less than 82% of pixels staying the same in saliency values. the higher the percentage of the unchanged pixel, the better the design is for the preservation of this historical building. this result is objective and the experiment results are reproducible at any time. 250 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings fig. 11. in the first row of each delta saliency map: the pixels that have a change in saliency value (white area) of more than 0.1 are counted (see formula (2)). in the second row of each delta saliency map: the percentage of pixels that do not have a change in saliency value (black area) of more than 0.1 are counted (see formula (3)). 4. conclusion this paper introduces a new way to evaluate the visual aspect of bipv installations that take place on historical buildings. with the historical buildings as cultural heritage, they need to be kept as original as possible. visually speaking, the installation of bipv usually has negative impact on the historical building. in order to keep the negative impact below a certain threshold, many assessment methods are being developed. in common practice, the visual impacts of bipv are mainly described linguistically, and later transformed into numerical descriptors, so as to be able to quantify those impacts. on the one hand, linguistic descriptors are subjective and qualitative, and on the other hand, when humans look at different bipv designs, and compare them with the building’s original state, the differences are visible by the human eyes, but linguistic descriptors are not specific enough r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 251 to describe those differences. also the transformation of linguistic to numerical descriptors are not standardized. in this paper, the authors have found a way to objectively assess and quantify the visual impact of bipv installation, where the transformation between linguistic to numeric descriptors is being omitted, and thus leading to a more reasonable judgment of whether the bipv is ‘significantly’ influencing the existing building or not. since the saliency map is a neurobiological and psychological tool to predict human visual attention, it is used here to detect the conspicuity of the bipv installations. first, 3d cad models of the existing historical building and with different bipv installation proposals are made. renderings of the existing building (‘as is’) and design proposal with bipv (‘new’) are being produced from radiance software. saliency maps are generated for each one of the above-mentioned renderings. the saliency map ‘as is’ is used as the reference saliency map. differences in saliency values (delta saliency values) between the saliency map ‘as is’ and each one of the saliency map ‘new’ are calculated. the thus generated maps are called delta saliency maps. the pixels on the delta saliency maps that have a delta saliency value below ± 0.1 are counted together. the sum of the pixels can represent how much the saliency distribution within the human visual scene has changed after the bipv installation, with the building’s original state as reference. since for historical buildings, the appearance should be kept as original as possible, therefore the larger the sum of pixels in unchanged saliency value, the better it is. 5. discussion the saliency map has shown good potential for the assessment of bipv retrofit designs, but a few issues need to be adjusted in future research. a) energy yield an evaluation method of solar facade design that is based merely on the objective visual aspect would be very incomplete. evaluation of architecture designs usually involves personal preference and subjective opinions from the user or authorities. besides, solar facade is a combination of architectural and energetic design. therefore it is necessary to integrate the objective visual aspect, with personal opinions of different stakeholders, and energy production of bipv into an integrated evaluation method. in the combination of these evaluation aspects, the weights of each aspect need to be developed so as to guarantee a meaningful holistic solar facade assessment. b) the drawback of the existing saliency models when humans look at architectures, empirically speaking their attention is prioritized to entrances and windows of the building, which is actually a top-down processing of attention. but at present, with many kinds of top-down attention models available, none of them are specifically trained to simulate human attention when looking at architectures. in fact, their accuracy is trained and evaluated based on the 300 standard natural pictures provided by the mit saliency benchmark (judd et al., 2012) instead of real life practices (such as using case-based eye-trackers to justify the accuracy of the model). since the appropriate top-down attention model is missing, the only solution to simulate the ‘significance’ of bipv is by completely leaving out the pre-selection criteria of human attention when looking at architectures, and only considering the most basic bottom-up attention criteria (colour, intensity and orientation). the validation of the method in this paper with actual human beings via eye-trackers to identify the most salient spots in the visual field is also desired in the future. 252 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings c) colour calibration the author’s current way of colour calibration is by sampling the colour value of certain pixels on the monitor screen to guarantee that the material colour in the render model is the same as the colour on the photo. for the future, a standardized colour sampling process should be developed, so as to guarantee a concerted preparing and evaluation process for all buildings. d) viewpoint decision in this paper, only one viewpoint of the building perspective has been decided. for later development, several viewpoints should be chosen because a building is usually seen from multiple angles. possible different results will be generated from different view angles, how to combine them is also a research issue for the future. e) daylight setting for this paper, the daylight setting is set to overcast sky condition. however, reflection beams can occur under a clear sky condition. this might also lead to a huge visual impact on the historical building. f) extended application at present, the visual impact of pv installations and wind farms on open landscapes has been a heated discussion. compared to the saliency map method, the existing evaluation methods for visibility of pv installations (chiabrando et al., 2009; torres-sibille et al., 2009, 2010) are less relevant to the biological principle of the human eye. therefore comparisons between different methods in the future could be interesting. acknowledgments the authors would like to thank the projects kti pv fassade (funded by swiss commission for technology and innovation), is optische raster für pv module (funded by lucerne university of applied sciences and arts) and üserhuus p&d aipv (funded by üserhuus) for financial support and opportunities provided. gratefulness to prof. klaus zahn from lucerne university of applied sciences and arts should be expressed for his help and inputs in saliency map discussions. tremendous supports from prof. j.-l. scartezzini from leso lab, epfl are also greatly appreciated. references borji, a., & itti, l. (2013). state-of-the-art in visual attention modeling, ieee transactions on pattern analysis and machine intelligence, 35(1), 185-207. doi:10.1109/tpami.2012.89 bundesamt für energie, & eidgenössische kommission für denkmalpflege (2009). energie und baudenkmal – empfehlungen für die energetische verbesserung von baudenkmälern. bylinskii, z., judd, t., durand, f. d., oliva, a., & torralba, a. (n.d.). mit saliency benchmark. retrieved december 7, 2014, from http://saliency.mit.edu chalmers, a., & debattista, k. (2009). level of realism for serious games. presented at the vs-games 09 proceedings of the 2009 conference in games and virtual worlds for serious applications, coventry, united kingdom: ieee computer society. doi:10.1109/vs-games.2009.43 chiabrando, r., fabrizio, e., & garnero, g. (2009). the territorial and landscape impacts of photovoltaic systems: definition of impacts and assessment of the glare risk. renewable and sustainable energy reviews, 13(9), 2441-2451. http://saliency.mit.edu r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings 253 dessi, v. (2013). methods and tools to evaluate visual impact of solar technologies in urban environment (pp. 1-6). presented at the cisbat 2013, lausanne, switzerland. frontini, f., maturi, l., munari probst, m. c., roecker, c., & scognamiglio, a. (2013). designing photovoltaic systems for architectural integration – criteria and guidelines for product and system developers. (c. farkas) report t.41. a.3/2: iea shc task 41 solar energy and architecture (pp. 1-92). harel, j., koch, c., & perona, p. (2006). graph-based visual saliency. presented at the advances in neural information processing systems 19, proceedings of the twentieth annual conference on neural information processing systems 2006 (pp. 545-552), vancouver, canada. häne, s. (2012). heimatschutz warnt vor “solarwelle” in altstadt. retrieved december 2, 2014, from http://www.tagesanzeiger.ch/ zuerich/stadt/heimatschutz-warnt-vor-solarwelle-in-altstadt/story/14894803 hubel, d. h. (1995). eye, brain, and vision. (2nd ed.). w. h. freeman publishers. itti, l., & koch, c. (2001). computational modelling of visual attention. nature reviews neuroscience, 2(3), 194-203. itti, l., koch, c., & niebur, e. (1998). a model of saliency-based visual attention for rapid scene analysis. ieee transactions on pattern analysis and machine intelligence, 20(11), 1254-1259. judd, t., durand, f., & torralba, a. (2012). a benchmark of computational models of saliency to predict human fixations. computer science and artificial intelligence laboratory technical report. judd, t., ehinger, k., durand, f., & torralba, a. (n.d.). judd saliency model/readme.txt. retrieved december 2, 2014, from http://people.csail.mit.edu/tjudd/wherepeoplelook/code/juddsaliencymodel/readme.txt kanton basel (2013). richtlinie für solaranlagen im kanton basel-stadt (version januar 2013). kanton bern (2012). richtlinien – baubewilligungsfreie anlagen zur gewinnung erneuerbarer energien (version juni 2012). kanton luzern (2014). richtlinien solaranlagen – photovoltaische/solarthermische anlagen (version april 2014). kanton nidwalden, & fachstelle für d. (2010). wegleitung bauinventar (pp. 1-25). kanton thurgau (2009). solaranlagen richtig gut – richtlinien zur anwendung von artikel 18a des bundesgesetzes über die raumplanung (version juli 2009). koch, c., & ullman, s. (1985). shifts in selective visual attention: towards the underlying neural circuitry. human neurobiology, 4, 219-227. longhurst, p., debattista, k., & chalmers, a. (2006). a gpu based saliency map for high-fidelity selective rendering, presented at the afrigraph 2006 4th international conference on computer graphics, virtual reality, visualisation and interaction (pp. 1-9). lópez, c. s. p., & frontini, f. (2014). energy efficiency and renewable solar energy integration in heritage historic buildings. energy procedia, 48, 1493-1502. doi:10.1016/j.egypro.2014.02.169 lucchi, e., garegnani, g., maturi, l., & moser, d. (2014). architectural integration of photovoltaic systems in historic districts – the case study of santiago de compostela, presented at the international conference in energy efficiency in historic buildings (pp. 1-15), madrid, spain. ludin, p. w. (2013). keine solaranlagen auf walliser kirchendächern. retrieved december 2, 2014, from http://www.kirchenblogs.ch/d/blogs/ pwalterludin/m96899 munari probst, m. c., & roecker, c. (2009). photovoltaic vs. solar thermal: very different building integration possibilities and constraints, (pp. 1-6). presented at the cisbat 2009, lausanne, switzerland. munari probst, m. c., & roecker, c. (2011). urban acceptability of building integrated solar systems: leso-qsv approach. presented at the ises solar world congress 2011, kassel, germany. munari probst, m. c., & roecker, c. (2012). criteria for architectural integration of active solar systems iea task 41, subtask a. (2014). energy procedia, 30, 1195-1204. doi:10.1016/j.egypro.2012.11.132 rpg. bundesgesetz über die raumplannung (raumplanungsgesetz, rpg), vom 22. juni 1979 (stand on 1. mai 2014). ruesch, j., lopes, m., bernardino, a., hörnstein, j., santos-victor, j., & pfeifer, r. (2008). multimodal saliency-based bottom-up attention, a framework for the humanoid robot icub, presented at the 2008 ieee international conference on robotics and automation, pasadena, (pp. 1-6), ca, usa. stadt zürich hochbaudepartement (2008). solaranlagen in der stadt zürich erwünscht. retrieved december 2, 2014, from https://www.stadtzuerich.ch/content/hbd/de/index/ueber das departement/medien/medienmitteilungen/2008/august/080822a.html swiss federal constitution (2014). bundesverfassung der schweizerischen eidgenossenschaft vom 18. april 1999 (stand on 18. mai 2014). torres-sibille, a. d. c., cloquell-ballester, v.-a., & artacho ramı́rez, m. á. (2009). aesthetic impact assessment of solar power plants: an objective and a subjective approach. renewable and sustainable energy reviews, 13(5), 986-999. torres-sibille, a. d. c., garcı́a, l., & ayuga, f. (2010). visual impact assessment of human interventions of the landscape: the case of wind farms and solar power plants. universidad politécnica de valencia. vassiliades, c., savvides, a., & michael, a. (2014). architectural implications in the building integration of photovoltaic and solar thermal systems – introduction of a taxonomy and evaluation methodology, presented at the world sustainable building 2014, (pp. 1-7), barcelona, spain. vitelli, j. (2013). interpellation nr. 60 (september 2013) – betreffend fragwürdige richtlinien für solaranlagen. retrieved december 2, 2014, from http://www.grosserrat.bs.ch/dokumente/100376/000000376090.pdf von arx, s. (2013). solaranlage auf kirchendach: “das bauvorhaben kann nicht bewilligt werden”. retrieved december 2, 2014, from http://www.grenchnertagblatt.ch/solothurn/thal-gaeu-niederamt/solaranlage-auf-kirchendach-das-bauvorhaben-kann-nichtbewilligt-werden-127238528. http://www.tagesanzeiger.ch/zuerich/stadt/heimatschutz-warnt-vor-solarwelle-in-altstadt/story/14894803 http://www.tagesanzeiger.ch/zuerich/stadt/heimatschutz-warnt-vor-solarwelle-in-altstadt/story/14894803 http://people.csail.mit.edu/tjudd/wherepeoplelook/code/juddsaliencymodel/readme.txt http://www.kirchenblogs.ch/d/blogs/pwalterludin/m96899 http://www.kirchenblogs.ch/d/blogs/pwalterludin/m96899 https://www.stadt-zuerich.ch/content/hbd/de/index/ueber_das_departement/medien/medienmitteilungen/2008/august/080822a.html http://www.grosserrat.bs.ch/dokumente/100376/000000376090.pdf http://www.grenchnertagblatt.ch/solothurn/thal-gaeu-niederamt/solaranlage-auf-kirchendach-das-bauvorhaben-kann-nicht-bewilligt-werden-127238528 254 r. xu and s. wittkopf / visual assessment of bipv retrofit design proposals for selected historical buildings wang, s., & tian, y. (2011). indoor signage detection based on saliency map and bipartite graph matching, presented at the 2011 ieee international conference on bioinformatics and biomedicine workshops bibmw (pp. 518-525), atlanta, usa. ward, g., others. (n.d.). radiance. retrieved from http://radsite.lbl.gov/ wittkopf, s., yang, x., & xu, r. (2014). reflection from pv facades and roofs – new assessment methods based on annual weather data. presented at the eu pvsec 2014, amsterdam, netherlands. xu, r. (2014). visuelle beurteilung der bipv. presented at the swiss bau, basel, switzerland. yang, x., grobe, l., & wittkopf, s. (2013). simulation of reflected daylight from building envelopes. presented at the 13th conference of international building performance simulation association (pp. 1-8), chambery, france. yu, p. m., cheng, c. l., liao, l. m., & yu, y. t. (2009). an evaluation and benchmarking study of the building integrated photovlatic (bipv), presented at the proceedings of green building towards eco-city, (pp. 1-14), taipei, taiwan. http://radsite.lbl.gov/ journal of facade design and engineering 3 (2015) 289–301 doi 10.3233/fde-160046 ios press 289 an energy-active facade element from mineralized foam (mf) and micro-reinforced, ultra-high-performance concrete (mruhpc) andreas maiera,∗, albrecht gilka-bötzowb and jens schneidera atechnische universität darmstadt, institute of structural mechanics and design (ism+d), darmstadt, germany btechnische universität darmstadt, fachgebiet werkstoffe im bauwesen (wib), darmstadt, germany abstract. today, an increase of the energy efficiency in the manufacturing industry is typically achieved by separate, parallel measures, primarily on the level of the individual machines. energy efficiency can be improved by a holistic, integrated approach, which links the machines, the production process, the technical infrastructure and the building and its envelope. the subject of this paper is the development of a new prefabricated element for facades and roofs, which was developed and built in the context of a research project called eta-fabrik (i.e. energy-efficient factory, www.etafabrik.de) at tu darmstadt, germany. the element consists of purely mineral materials (concrete) and can be energetically activated by thin capillary tubes integrated in the surface layer. this surface layer consists of a micro-reinforced, ultra-highperformance concrete (mruhpc) to achieve a low component thickness due to its high mechanical capacity, resistance against thermal changes, surface quality and low permeability. the core of the element is responsible for insulation. for this, a mineralized protein foam (mf) is used. it provides very good thermal insulation properties due to its eminently low density and thus allows low heat transfer coefficients. therefore, the final facade element combines limiting, bearing, insulating and thermal activation using concrete. keywords: facade element, micro-reinforced uhpc, mineralized protein foam concrete, energy efficiency, thermal insulation, energy management, energy activation 1. introduction the energetic model of a typical factory set-up can be interpreted as shown in figure 1. it is a complex system of different functions that are linked with each other and influenced by external parameters (herrmann, kara, thiede, & luger, 2010). maximizing the energy productivity by increasing the product output only is no longer worthwhile for companies in the industrial sector. therefore, within the coming years, productivity, energy and resource efficiency will have ∗corresponding author: dipl.-ing. andreas maier m.eng., technische universität darmstadt, institute of structural mechanics and design (ism+d), franziska-braun-straße 3, 64287 darmstadt, germany. tel.: +49 6151 16 23014; fax: +49 6151 16 23010 e-mail: maier@ismd.tu-darmstadt.de. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. www.eta-fabrik.de mailto:maier@ismd.tu-darmstadt.de 290 a. maier et al. / an energy-active facade element from mineralized foam (mf) fig. 1. energetic model of the factory, translation after herrmann et al. (2010). to be seen in connection and investigated with the product itself, thus combining the view on the product, the production process, its implementation by machines and workers and the building around it. a separate treatment and separate optimization of the different parameters typically leads to contradictions and imbalances. one example is that workers are frequently exposed to strong heat radiation next to machines with emission of waste heat. in addition to the unused heat losses, the low quality of work environment is another disadvantage leading to reduced productivity. a significant correlation of well-being at work with the quality of the final products was, for example, described by landau, helbig & ferreira (2005). furthermore, typical building envelopes in industrial constructions are often composed of different energetically passive and active components in addition to building technology. currently, there are only few examples that can respond intelligently to typical needs related to heat supply and heat reduction, e.g. switchable glazing or phase change materials (pcm). today, an intelligent linkage of the energetic interdependencies shown in figure 1 is lacking, probably also due to the strong interdisciplinary nature of the problem. from a structural point of view, the typical floor plans of factories must be flexible so that production processes have the potential of change. this requires large areas to be covered with construction elements that are relatively light in relation to their span. the structures should be buildable in a short time and should be cheap, usually ignoring architectural identity and thermal behaviour inside the factory. for these reasons, building envelopes today usually consist of several components and layers according to their functions: bearing and limiting. thermal behaviour and energetic activation are often ignored or of lower interest. this approach leads to multiple interfaces and complex joining techniques. moreover, with this separation, a holistic assessment of resources is often difficult. a. maier et al. / an energy-active facade element from mineralized foam (mf) 291 2. the energy efficient factory (eta-fabrik) within recent years, law in germany has gradually restricted the allowable heat energy demand and the permissible heat loss from buildings. in addition, the non-renewable energy consumption in building operation for new buildings should be reduced to zero by 2020 according to an eudirective (bundesministerium für wirtschaft und technologie, 2005). in contrast to the housing sector, industrial buildings should usually meet two typical requirements of production processes: on the one hand, the above mentioned variability of the plant and, on the other hand, an associated variability in the electrical and thermal energy supply. often, a significant excess of thermal energy is the result of this variability and the result of a lack of holistic designing; therefore, the transfer and re-use of this energy will be considered in the project eta-fabrik. ‘eta’ stands for energy-efficient factory for interdisciplinary and applied research. it is part of the 5th energy research program of the german federal government ‘innovation and new energy technologies’ by the federal ministry of economics and technology (bmwi). the topicality of the project is demonstrated by the participation and support of many industrial companies, both directly participating in the research project as well as acting as advisors in an additional working group. by this, a direct internal and external transfer of knowledge shall be guaranteed. within the project, a 1:1 scale model factory is being built on the campus of the tu darmstadt in 2016 (fig. 2). a production process chain, which is typical for the german medium-sized metal processing industry, will be installed in this model factory containing chipping, cleaning and heat treatment (fig. 3). previous research at the institute of production management, technology and machine tools (ptw) at the faculty of mechanical engineering at tu darmstadt has been successfully aimed at fig. 2. visualization of the model factory on campus lichtwiese, tu darmstadt, germany (abele, 2012). fig. 3. process chain for the production of a tax disc for hydraulic pumps, translation after abele (2012). 292 a. maier et al. / an energy-active facade element from mineralized foam (mf) fig. 4. estimation of the potential savings in a holistic view of industrial production, translation after abele (2012). an isolated energy optimization of individual machines, resulting in 40%–50% in energy savings compared to conventional machines. additional savings of about 30% are already identified for an isolated optimization of the production process, 25% for an optimization of the building and its envelope and 20% for the technical infrastructure (fig. 4). the research project eta-fabrik at tu darmstadt, however, with its holistic approach aims at 40% energy savings compared to conventional manufacturing factories. this shall be achieved by a suitable energy concept, an appropriate energy management with energy-efficient operation and architectural and structural measures. for this holistic approach, the energetic interaction of the building envelope, the environment, the technical infrastructure and the production process chain must be carefully analysed and optimized. one important idea is the thermal activation of the building envelope in combination with thermal storages to allow heating and cooling of the factory. it must be noted that contrary to the housing sector, in germany, cooling during summer time is one of the important issues in many factories. beside plants for metal processing industry like in this research project, the concept can be adapted to any kind of process with waste heat emitting machines, e.g. automotive industry, food industry, chemical industry. however, requirements on heating, ventilation and air conditioning for non-residential buildings has been increased in germany for the last years, thermal activation of wall and roof elements becomes more interesting for any kind of industrial plants, because for using this technique, no high temperatures are necessary (12◦c to 17◦c for cooling and 30◦c to 40◦c for heating are sufficient). a. maier et al. / an energy-active facade element from mineralized foam (mf) 293 summer thermal storage tlow thermal storage thigh heat treatment cleaning machinemachine tool temperature-levels of heatand cooling-flows: tmax heat-flow tmedium heat flow tmediumcooling flowtmin cooling flow fig. 5. energy concept for cooling and the use of waste heat from the production operation in the summer (abele, 2012). 3. the building envelope as part of the energetic concept the building envelope can play an important part to influence the thermal exchange and allow energy optimization in energy-intensive production processes. environmental energy from sun radiation and thermal energy from the process chain can be collected and fed into the system according to their suitability to allow an optimal (re-)use according to figure 5. moreover, cooling and heating by using the building envelope instead of energy-intensive air conditioning system units can further increase energy efficiency and also the well-being of the workers. for cooling the factory interior, cool water passes through a fine and near-surface capillary network (fig. 11) in the inner layer of the facade element, and cooling effect takes place. the gained heat can be used for a suitable step in the production process. it was shown by simulation with trnsys, that a cooling capacity of 60kw is necessary for the production area (fig. 6). if required during the winter period, the ambient air may be heated through the capillary network in the sense of a panel heating, using the stored energy in water tanks or the waste heat of the machines directly. therefore, a heating capacity of 40kw is required in reference to figure 6, and is being charged from solar heat or from cooling of the machines or the room. similar to conventional solar thermal systems like flat plate collectors or evacuated tube collectors, solar radiation present at the outside surface is being absorbed by the concrete and transferred to the capillary system. such a system can also collect the outside environmental energy due to the temperature difference between ambient air and surface temperature. for this principle, a cascaded energy storage (water tanks at different temperature levels), and an intelligent operation of the associated process chain and machinery components are required. two different kinds of water-filled thermal storages are used in this concept. for the high temperature level (80–95◦c), which is connected with the cleaningand heat-treated-machines, a vacuum super insulated heat storage with a volume of 15m3 is used. for the medium and low temperature level (30–45◦c and 12–20◦c) as well as for the outer capillary tube system, three water storages made of high volume fly-ash concrete, with a volume of 25m3 for each one, are used. through its high content of fly-ash 294 a. maier et al. / an energy-active facade element from mineralized foam (mf) fig. 6. trnsys-simulation of heating and cooling capacity in relation to outdoor temperature based on a production process with a three-shift operation and an average power consumption of all machines of 83kw (thumm, 2013). and its stability against leaching, no more coatings of the storage walls and floors are necessary. the storages are useful in any case while two or more processes are running dephased, so no consumer can use the thermal energy that was provided by a generator (e.g. the heat-treatment machine), especially during the transition period. at the outside of the facade and roof elements, the same capillary network can be used to cool down warm water heated from the production process. passively, this occurs whenever the temperature level of the outer surface is above the temperature level of the outdoor air. however, if this temperature gradient is not enough, a demand-dependent sprinkler system on the roof leads to an evaporation of water, which causes a high heat transfer coefficient. in this way, the thermal storage can be quickly discharged, for example in the transition from the cold to the warm season, in that case in order to use it for cooling of the interior or the machines. in addition to the integrated active thermal system, a classical passive thermal insulation must be provided. a centre core of the facade element with good thermal insulating properties is made out of mineralized foam. by using this cement-bonded insulation material, all layers of the wall are basically built from the same material: concrete. thus, among other positive effects such as non-flammability a sustainable overall life cycle from production to disposal is guaranteed. 4. structure, layout and materials of the facade element 4.1. structure and layout the goal of the development is an integrative structural facade element, which combines all properties of individual facade components today, adding thermal activation: limiting = bearing + insulating + thermal activation the energetic interaction of the building envelope and the production process require cladding elements on the roof and facades, a rapid thermal activation and a high reactivity of its individual system components. in terms of sustainability and variability of the entire building, a homogeneous material composition is sought in view of production and disposal. to fulfil all these requirements, concrete is particularly suitable, because a structural and energy-optimized geometry can easily be a. maier et al. / an energy-active facade element from mineralized foam (mf) 295 fig. 7. evaluation of different structural systems (extract). achieved, the installation of the required capillary tube network is possible, fire protection issues can be addressed easily and the surface can be designed architecturally attractive. beyond that, concrete is available easily and almost everywhere. at the project start, the best combination of structural system and cross-section with regard to structural behaviour, ecology and economy had been searched by different kind of calculations: in a first step, 20 different static systems for hall constructions were analysed structurally by fem under the existing spans and under unit loads. those were compared with various static and constructive criteria like the amount of reinforcement and the complexity of joining for example (fig. 7). then 41 combinations of static systems and cross-sections, like beams, plates and t-beams were analysed structurally, constructively (e.g. resistance against bending, shear, torsion and stability, transportation), ecologically (e.g. life-cycle analysis) and economically (cost out of quantity determination) (fig. 8). at last the three best combinations from structural systems and crosssections were chosen and a detailed economic analysis, which considered costs for production, mounting and transport as well as a sustainability analysis based on dgnb criteria, was done. a concluding comparison provided the best combination of structural system and cross-section with regard to structural behaviour, ecology and economy. this combination was finally built for the eta-fabrik. figure 9 shows a layout of such an element for roofs and facades of industrial buildings. the built-up consists of an inner structure from pre-fabricated double-web concrete slabs including a capillary tube network at their inside, which is connected to two outlying pipes for flow and return in order to regulate each element individually and to disconnect the element if it will be necessary. due to their geometry, these slabs combine the characteristics of carrying and providing a formwork for the mineralized foam. pre-stressed concrete elements in the shape of a saddle roof are used for the roof. those provide a uniform rib structure inside the building (fig. 10). slender and thus more economical cross-sections regarding the web width and the web height can be achieved compared to additive systems of trusses, purlins and roof slabs. by a shear stiff coupling of several elements in the plate area, the slabs can easily be used for the bracing of the factory building in longitudinal direction. in transversal direction, the elements are fixed in the foundation, thus there is no need for a secondary bracing. in a second step, the mineral foam with a thickness of about 25cm to 40cm is installed on the hardened pre-fabricated component. very thin micro-reinforced, ultra-high-performance concrete (mruhpc) panels including a capillary tube network connected to two outlying pipes are used on the outside. those are connected to the wall-slabs by special facade anchors, which consist of a new developed screw and nut combination made from stainless steel. the latter is a 2mm thick 296 a. maier et al. / an energy-active facade element from mineralized foam (mf) fig. 8. fe-model of one combination of structural system and cross-section and primary energy of all 41 combinations after life-cycle analysis according to din en iso 14040 and din en iso 14044. perforated plate with a diameter of 8cm and a welded internal thread for screws m 16. the plate is located 2.5cm under the panel surface and helps to spread the punctual forces in the cross-section by activating the micro-reinforcement. therefore the resistance against concrete collapsing grows and the failure mechanism changes from concrete to steel. on the roof, load picked-up spacers, based on spacers for double floors, consisting of stainless steel pipes in combination with vertically adjustable bearings are used. due to high mechanical and thermal resistance of mruhpc, the dimensions of the panels can be very large and they are economically installable with a thickness of only 3cm to 5cm. in the research project eta-fabrik, the panel dimensions are 10m × 1.50m and a thickness of 5cm is planned. the capillary network consists of a fine tube system filled with antifreeze and water, which is demineralized in order to avoid clogging up with impurities. the dense arrangement of pipes with an inside diameter of approximately 3mm (fig. 11) and the a. maier et al. / an energy-active facade element from mineralized foam (mf) 297 fig. 9. roof and facade element from concrete components. fig. 10. interior view of the rib structure of the industrial hall. installation in the surface layer of the thermal highly conductive mruhpc lead to the high necessary thermal dynamic of such a system, which distinguishes it from the conventional systems with an activation of concrete elements, and acts as a large heating or cooling surface that can react quickly to the needs of air-conditioning equipment. to demonstrate that, an outside test station with two kinds of facade panels made of mruhpc was installed. it could be shown by trial and using the finite element method (fem) that the cladding panels can heat up to 14 k within twenty minutes, and the temperature distribution in the cross-section is almost constant (fig. 12) by a flow rate of 0.5m/s, which leads to a laminar flow in the pipes. it was also possible to do a non-steady simulation with fem in a timespan of one day. the resulting values were very close to the values resulting from the measurements (fig. 13). 298 a. maier et al. / an energy-active facade element from mineralized foam (mf) fig. 11. exemplary element of capillary tubes used in the eta-fabrik, translation after beka heizund kühlmatten gmbh (2013). fig. 12. heating-up time of a facade panel from mruhpc. a. maier et al. / an energy-active facade element from mineralized foam (mf) 299 fig. 13. non-steady temperature distribution in a thermal activated building element. measurement data from iwb, universität stuttgart. fig. 14. facade panels made of uhpc (left), insulating layer of mineralized foam (right). 4.2. materials beside the described thermal activation, the new development for the insulation uses special cement-bound protein foam, called mineralized foam (mf, fig. 14). very good thermal insulation effects with low thermal heat transfer coefficients can be achieved depending on the obtained density. it is bound with cement or a mineral cement replacement. the material has a dry density of 200kg/m3 and a related thermal conductivity λdry of about 0.06w/(mk). mineralized foam is not flammable and therefore also suitable for use in places with increased fire protection requirements. in contrast to similar insulation boards made of aerated concrete, the material used here hardens in the air and autoclaving is not necessary. in addition to the absence of this manufacturing step, 300 a. maier et al. / an energy-active facade element from mineralized foam (mf) fig. 15. four-point flexural strength test with a micro-reinforced, ultra-high-performance concrete. it is possible to bring the fresh foam material to the surface-hardened concrete components in a precast concrete plant. contrary to the usual limitations of the cellular concrete production, components can thus be manufactured with almost any desired dimensions. the formulation of the binder matrix is optimized with the aid of additives that have a low input of primary energy. moreover, through research at tu darmstadt, a composition of the binder stone was developed which uncouples a desired minimization of heat transfer by conduction and density reducing of the mineralized foam up to a certain level. while retaining low thermal conductivity, a required minimum strength of 0.4n/mm2 can be ensured. a substantial requirement for implementation of the described concept is a construction material that can resist substantial thermal and hygric loads in addition to high static loads. furthermore, the material must allow the integration of the capillary tubes, and should have as little resistance as possible against thermal-energy flow through high thermal conductivity in order to enable thermal activations of the surface layer. this can be achieved by high-strength and dense concretes with micro-reinforcement (fig. 15). the sd-value of uhpc, the factor to quantify the water vapour density of a material, is about 45 times larger than the sd-value of normal concrete (reineck, greiner, reinhardt, & jooß, 2004). in a facade component, the mruhpc concrete offers high potentials for large elements in terms of its strength parameter. bending strengths of more than 35 mpa as well as compression strengths of more than 115 mpa can be achieved. this results in a low component thickness. from a structural point of view, panels with a thickness down to 1cm could be used. however, the installation of the capillary tube network, the manufacturing method and the tests for durability do not permit this. 5. conclusion energy efficiency in production is an important current topic in industrial processes. an optimization cannot be successful if only individual solutions are considered. only a holistic approach including the machines, the production process, the technical infrastructure and the building envelope can significantly increase energy efficiency. this will be shown by the model factory eta-fabrik at tu darmstadt which is being built in 2016. in this context, thermal activation of facade elements seems to be promising to link the elements with the energy network, to make use of low temperature energy losses within the production processes and to allow cooling of factories without energy-intensive cooling units. a. maier et al. / an energy-active facade element from mineralized foam (mf) 301 the element shown here is realized by purely mineral materials (concrete) and can be energetically activated by capillary tubes integrated in the surface layers. these surface layers consist of a micro-reinforced, ultra-high-performance concrete (mruhpc) to achieve a low component thickness due to its high mechanical capacity, resistance against thermal changes surface quality and low permeability. the core of the element is responsible for insulation. for this, a mineralized foam (mf) was developed. it provides very good thermal insulation properties due to its eminently low density (dry density about 200kg/m3, thermal conductivity λdry about 0.06w/(mk)). the newly developed element for facades and roofs combines limiting, bearing, insulating and thermal activation by using concrete. for the future, the structural capacity reserves of the uhpc within such elements should be activated to further reduce the element thickness. for this, research on the structural stability and the respective influence of the concrete shrinkage should be initiated. this includes also a development of foam production and mruhpc mixture. references abele, e. (2012). skizze des forschungsprojektes η-fabrik. tu darmstadt. beka heizund kühlmatten gmbh (2013). beka cd-rom. produktordner und berechnungsprogramme, p. 15. bundesministerium für wirtschaft und technologie (2005). innovation und neue energietechnologien. das 5. energieforschungsrahmenprogramm der bundesregierung. herrmann, c., kara, s., thiede, s., & luger, t. (2010). energy efficiency in manufacturing – perspectives from australia and europe. in: proceedings of the 17th cirp international conference on life cycle engineering (lce2010), hefei, china (pp. 23-28). landau, k., helbig, r., & ferreira, y. (2005). ergonomie am arbeitsplatz. in j. eisele, & b. staniek (eds.), bürobau atlas, grundlagen/planung/technologie/arbeitsplatzqualitäten, (pp. 208-217). callwey-verlag (hrsg.). reineck, k.-h., & greiner, s. (2004). dichte heißwasser-wärmespeicher aus ultrahochfestem faserfeinkornbeton. forschungsbericht zum bmbf-vorhaben 0329606v, p. 82. institut für leichtbauentwerfen und konstruieren (ilek), universität stuttgart. thumm, f. (2013). simulation zur bestimmung des energiebedarfs der halle. tu darmstadt, 2013. journal of facade design and engineering 4 (2016) 79–89 doi 10.3233/fde-161191 ios press 79 a hybrid data-driven bsdf model to predict light transmission through complex fenestration systems including high incident directions marek krehel∗, lars o. grobe and stephen wittkopf cc ease, lucerne university of applied sciences and arts, horw, switzerland abstract. the transmission and distribution of light through complex fenestration systems (cfss) impacts visual comfort, solar gains and the overall energy performance of buildings. for most fenestration, scattering of light can be approximated as the optical property of a thin surface, the bidirectional scattering distribution function (bsdf). it is modelled in simulation software to replicate the optical behaviour of materials and surface finishes. data-driven bsdf models are a generic means to model the irregular scattering by cfs employing measured or computed data sets. even though measurements are preferred due to the realistic values they provide it is not always possible to measure the light scatter in all incident directions. in contrast, numerical simulations have virtually no limitations; however, at the cost of lower reliability. a hybrid approach, combining both, was therefore proposed. the bsdf of a cfs was measured for incident elevation angles from 0◦ to 60◦. for incident elevation angles from 0◦ to 85◦, the bsdf of the sample was computed. the bsdf acquired by both techniques in the overlapping range of directions between 0◦ to 60◦ was compared and revealed good qualitative accordance. the variance of the direct-hemispherical reflection and transmission based on the two techniques was between 3% and 28%. a hybrid data set was then generated, utilizing measurements where possible and simulations where instrumentation could not provide reliable data. a data-driven model based on this data set was implemented in simulation software. this hybrid model was tested by comparison with the geometrical model of the sample and measurements. the hybrid approach to bsdf modelling shall support the utilization of bsdf models based on measured data by selectively overcoming the lack of reliable measured or extrapolated data. keywords: bsdf, brdf, btdf, geometric optics, scattering, complex fenestration systems 1. introduction complex fenestration systems improve visual comfort, particularly in offices by excluding sunlight and evenly distributing daylight across the occupied zone. negative effects of excessive daylight exposure in the building perimeter, such as glare are avoided (kazanasmaz et al., 2016; mohanty, yang, & wittkopf, 2012). the even supply of daylight can help to reduce the operation of artificial lighting, and the related demand for electrical energy. the development, optimization and successful application of cfss require to characterise their optical properties, particularly the light scattering. planning decisions as well as product design and optimization are often supported by computational simulations. extensions for simulation software can predict the light propagation through complex optical systems such as cfss based ∗corresponding author: marek krehel, lucerne university of applied sciences and arts, technikumstrasse 21, 6048 horw, switzerland. tel.: +41 41 349 36 41; fax: +41 41 349 39 60; e-mail: marek.krehel@hslu.ch. issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). mailto:marek.krehel@hslu.ch 80 m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission on explicit geometric optical modelling (bauer & wittkopf, 2016; grobe, müllner, & meyer, 2015; schregle, grobe, & wittkopf, 2016) but further add to the already high complexity of buildingscale simulations. this can be avoided with the implicit modelling of cfss by their resulting light scattering. as a surface property, the bidirectional scattering distribution function evaluates to luminous coefficients; describing scattering for any pair of incident and outgoing directions (ward, mistrick, lee, mcneil, & jonsson, 2011). emanating from assumptions on common optical characteristics of particular classes of reflective or transmissive surfaces, bsdf models are implemented in simulation software replicating regular diffuse and specular transmission or reflections (kurt & edwards, 2009; montes & ureña, 2012; ngan, durand, & matusik, 2005; renhorn & boreman, 2008; westin, li, & torrance, 2004), applying principles of geometric optics e.g. in micro-facet theory to reflection (cook & torrance, 1981) (ashikmin, premože, & shirley, 2000; he et al., 1991; torrance & sparrow, 1967) and transmission (walter et al., 2007). empirical models aim at the accurate replication of measurements, avoiding the complexity of relating these to underlying principles such as surface parameters. similar to their analytical counter-parts, they are valid for a particular class of reflective or transmissive surface (geisler-moroder & dür, 2010; lafortune et al., 1997; phong, 1975). parameters are found directly as measurable optical properties such as diffuse and specular reflection or transmission, or by fitting the entire model to observation data (ward, 1992). both analytical and empirical bsdf models emanate from prior assumptions of characteristics in the scattering distributions, which are gradually changing with varying incident and outgoing directions (noback, grobe, & wittkopf, 2016). the irregular bsdf of cfss challenges such approaches and led to the development of custom models for particular cfss (reinhart & andersen, 2009). to overcome the lack of a general analytical or empirical description of any cfs, data-driven models allow to make direct use of bsdf data in simulations (matusik et al., 2003; tian et al., 2013). interpolation techniques compute the bsdf for any pair of incident and outgoing directions from discrete sets of measured data (bonneel et al., 2011). data-reduction techniques are employed to allow storing and sharing of the tabular data (ward, kurt, & bonneel, 2015), or to compress it by factorization or employing wavelets (claustres, paulin, & boucher, 2003; kautz & mccool, 1999; rusinkiewicz, 1998). the acquisition of bsdf can be performed by imaging and scanning goniophotometers. an example of imaging goniophotometers are systems utilizing imaging spheres (yu et al., 2012). they comprise a highly reflective hemispherical mirror onto which scattered light is directed and from where a digital camera collects the information. thus, these systems have a very short acquisition time. alternates to imaging spheres are scanning goniophotometers (andersen et al., 2005; andersen & scartezzini, 2005; andersen & de boer, 2006; leloup et al., 2008). they sequentially capture scattered light for different outgoing directions by mechanical movement of a detector. the technique leads to typically longer acquisition times, but allows for a higher dynamic range and configurable directional resolution. regardless of the chosen method, the measurements of light scattering characteristics when the sample is illuminated close to grazing angles is always a great challenge. one of the fundamental problems is the light spot size whose diameter is inverse proportional to the cosine of the light incident angle and becomes infinitely large. that is of high interest for daylight redirecting components (drc) for certain locations on the globe where the sun in the zenith position is close to 90 degrees. the other issue can be caused by the mechanics of the measurement systems. particularly for large samples prevailing in the field of building sciences, measurements of the bsdf for directions close to grazing are impacted by shadowing and edge effects. another way to determine light scattering characteristics is the usage of computational simulation, such as ray-tracing, employing models of the sample’s geometry and surface properties. m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission 81 despite the progress in the accuracy of numerical simulation software, its results always reflect an assumed, idealised geometry, which can significantly deviate from samples due to unknown effects e.g. caused by manufacturing or bending. even if they seem marginally small their influence on the whole light redirection system might be significant (grobe et al., 2015). while measurements are preferred by researchers aiming at realistic values, they are constraint by the measurement geometries of the employed instrumentation. reliable extrapolation techniques are not available due to the irregularity of the bsdf. computational simulation is not limited by such constraints at the cost of lower realism. therefore, in this work we propose to combine the advantages of both and comprise the measured bsdf values with values calculated based on geometrical simulations 2. methods figure 1 schematically presents a combination of the measured and simulated data. data obtained by the goniometric measurements are recorded for irregular directions, while the input to simulations is supposed to be in regular grid. therefore, the measured data was resampled to a regular, tabular format prior to the composition of the hybrid model. fig. 1. combination of bsdf data from measurements and simulations into one hybrid set of data. the hybrid set of bsdf (hbsdf) values, as presented in the bottom block of fig. 1, was implemented in the simulation software. consequently, the light scatter simulations based on hbsdf values were conducted. such an approach featuring the advantages of both (data driven and computationally generated bsdf values) has, to the best of our knowledge, not been reported yet. 2.1. light scattering sample a sample of a retroreflecting drc, based on the principles of geometric optics, was acquired by the manufacturer together with the corresponding construction drawings. the sample was chosen due to its large geometry and periodic structure as both features are a typical problem in bsdf modelling. the measured sample consists of 39 parallel periodically placed slats held in position by an aluminium frame. the slats have a mirror-like top, and are coated with a white paint on the 82 m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission bottom surface. figure 2 shows a 2d profile (on the left side) together with a picture of part of the sample (right side). the 2d profile, as provided by the supplier, was used to conduct simulations, whereas the sample presented on right side of fig. 2 was measured. fig. 2. the sample, retro luxtherm12, was provided on request by retrosolar, kirn, germany. the design by helmut köster is patented. 2.2. bsdf measurements 1 the measurements of light scattering were conducted employing a scanning goniophotometer presented in fig. 3 (apian-bennewitz, 2010; krehel, kaempf, & wittkopf, 2015). fig. 3. sketch of the goniophotometer in which the goniophotometric measurements were conducted.1 it achieves high directional resolutions in areas of interest by means of refined peak scanning. a halogen lamp with bandpass filter blocking near infrared light of wavelengths longer than 700nm was employed as a light source. light was collimated, the optical setup resulting in a sampling aperture of 70mm. the diameter in this range results in a coverage of 6-7 light redirection slats which is a prerequisite for bsdf measurements. the measurements were conducted for the following light incident directions: thetain (�i) (altitude)=0◦ to 60◦ with 15◦ increment for one phiin (φi) (azimuth) orientations=0◦ resulting in total number of five incident directions. the coordinate system is schematically presented in fig. 4. 1images courtesy pab advanced technologies ltd, freiburg. m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission 83 fig. 4. angular coordinate system describing directions to light source θi, φi and receiver θs, φs as used in the parametrization of the bsdf. in configurations where the light scatter could not be measured, for instance for retro reflection at �i =0◦, the bsdf values were interpolated with the software provided with the apparatus. 2.3. computation of bsdf based on simulations 2 based on the geometrical design of the sample the light scatter simulations were conducted in lighttools. lighttools is an optical engineering and design software that allows simulation and analysis of non-imaging optics. the sample was modeled based on its nominal parameters and the light beam parameters were replicated from the light beam at the scanning goniophotometer (d=70mm with 1◦ of beam divergence). the results obtained from geometrical based simulations were utilized in order to obtain bsdf values. the computation was performed according to equation 1 (grobe et al. 2015): bsdfn = dsfn cos θs,n = es,n pi · cos θs,n (1) where: dsf – differential scattering function, pi – power of unobstructed beam, es,n – irradiance on sensor s for measurement n. the bsdf was computed for the following set of incident directions: �i =0◦–75◦ with 15◦ increment and additionally 80◦ and 85◦; all with φi orientation equal to zero. the bsdf values obtained for the range 0◦ to 60◦ were used for comparison with the measurements, while the additional angles were used as supplemental ones to create a hybrid set of bsdf values comprising measured and simulated ones – as schematically presented in the bottom red block of fig. 1. 2.4. hybrid bsdf model in lighttools – 3 to use the bsdf values in lighttools for light scatter modelling they need to be structured as presented fig. 5. since the measured data were recorded for irregular directions the data were binned into the tabular grid (the acceptable format by lighttools). light scatter data were embedded in text files with spherical coordinates where one file represents data for a given light incident direction. additionally, one master file indicates all associated data files. furthermore, a file with specified values of direct-hemispherical reflectance and transmittance as a function of 84 m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission fig. 5. diagram illustrating the approach of inputting the tabular hbsdf scatter data into lighttools. light incidents angles was specified. once the set of data is implemented in lighttools it performs the linear interpolation of the light scatter in between the input data on the fly. 3. results and discussion at first the comparison of measured and simulated data is presented. regardless expected discrepancies between the geometrical simulations and measured data the results should be compared to assess if they remain in the same order of magnitude. only when the data do not exhibit very large discrepancies the measured bsdf values can be combined with simulated ones and utilized to create hybrid set of bsdf values. in fig. 6 bsdf profiles in the scatter plane are plotted. the bsdf values were plotted for each incident direction, for which they had been either measured or computed (i.e. �s 0◦, 15◦, 30◦, 45◦ and 60◦). the horizontal axis presents the outgoing angles ranging from �s −90◦ to 270◦. values ranging from �s −90◦ to 90◦ represent the intensity slice, whereas values from �s 90◦ to 270◦ represent data from the transmission hemisphere. in fig. 6 a) on the reflection side the diffuse signal from measurements corresponds well to simulations; however, one missing peak can be noticed. on the transmission side, on the other hand, the peaks correspond well to each other but the difference in the diffuse light could be observed. figure 6 b) and c) presents the bsdf profile of light scatter for the light incident angles �i =15◦ and �i =30◦. on the side representing reflection (�s −90◦ to 90◦) the most prominent peak of the simulations is systematically shifted about 5◦ towards the negative part on the horizontal axis. the missing peak m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission 85 in fig. 6 a) as well as the shift of the peaks in fig. 6 b) and c) can be explained by different tilt angles of slats between the geometrical design and in the physical sample. even a small difference in the slats positioning significantly influences the light redirection. therefore, the missing peak in fig. 6 a) is not visible in measured values. however, on the transmission side of fig. 6 a) the measurements exhibit an elevated signal in the range from �s 180◦ to 270◦ which originates from the redirected peak from the reflection side. the same tilt of the slats in the samples is responsible for the peak shift in fig. 6 b) and c). when examining the peaks positions on the transmission side (�s 90◦ to 270◦) the peaks perfectly coincide. they are not redirected by the simulated drc, hence any discrepancies originating in sample geometry are not introduced. fig. 6. comparison of measured and simulated data at φi = 0◦ and following θi: a) 0◦, b) 15◦ c) 30◦, d) 45◦ and e) 60◦. 86 m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission in the reflection part at the light incident direction θi 45◦ and 60◦ (i.e. fig. 6 d) and e)) the shift of the peaks is less significant than in fig. 6 a) c). when the sample is illuminated at θi higher then ∼40◦, other surfaces start to reflect the light. thus, the effect of the tilt of the slats is less prominent. additionally, the values for incident angle θi =30◦ on the transmission side are systematically lower. overall, the diffused parts of the scattered light differ more significantly. however, the part where the light intensity is stronger than 1 exhibited lower differences. similarly to the work conducted by grobe et al. a good accordance between computed bsdf and the measurement was found, and the observed discrepancies were as expected (grobe et al., 2015). 3.1. reflectance and transmittance table 1 lists values of reflectance and transmittance for each measured and simulated incident angle. it can be easily observed that the measured reflection values are systematically lower. this systemic discrepancy occurs due to the fact that detector collecting light in the reflection hemisphere, at certain positions, covers the light source. since the drc was designed to retroreflect the light this significant amount of the signal had to be interpolated and is thus underestimated. the values for transmission vary randomly. this, as described in the introduction, can originate in the differences between manufactured samples and simulated ones. table 1 comparison of measured and simulated direct-hemispherical reflection and transmission for φi =0◦ θi [◦] reflection transmission meas. sim. meas. – sim.[%] meas. sim. meas. – sim. [%] 0 0.14 0.15 5% 0.821 0.8 3% 15 0.28 0.36 21% 0.642 0.62 4% 30 0.51 0.55 7% 0.429 0.4 7% 45 0.30 0.42 28% 0.47 0.54 13% 60 0.29 0.33 13% 0.505 0.54 6% all of the exhibited discrepancies that were discussed above lead to the conclusion that, as stipulated in the introduction, data driven models are always of the best choice when a high degree of realism is to be achieved. therefore, it is recommended to conduct light scatter simulation based on measured bsdf data. 3.2. hybrid bsdf in this chapter the hbsdf approach is validated. the light scatter generated by a hbsdf data set is compared with measurements for �i: 15◦, 30◦, 45◦, and presented in fig. 7 a), b) and c), whereas in fig. 7 d), e) and f) the hbsdf is compared with geometrical simulations for �i: 75◦, 80◦, 85◦. both types of the simulations were conducted in lighttools. similar to the comparisons presented in fig. 6 the values for �s ranging from −90◦ to 90◦ represent the reflection data, while values from 90◦ to 270◦ represent the transmission data. on a vertical axis, radiant intensity [w/sr] is given on a logarithmic scale. in both types of comparisons (hbsdf vs measurements and hbsdf vs geometrical simulations) the compared profiles exhibited correlation to a very high degree. all profiles preserve the same number of peaks which are placed at the same position. furthermore, the light scatter intensity is maintained throughout the whole profile. the only noticeable differences are the low-level signals in comparison between hbsdf m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission 87 fig. 7. profiles of a light scatter generated for validation of hbsdf simulations. fig. 7 a), b) and c) compare measured data with simulated data by hbsdf model for φi = 0◦ and the following θi 15◦, 30◦ and 45◦ respectively. whereas fig. 7 d), e) and f) present the comparison of geometrical based simulations with hbsdf at the following θi: a) 75◦, b) 80◦ and c) 85◦. the left-hand side of the chart indicated with the letter r represents reflection, while the right-hand side indicated with the letter t shows transmission data. and measurements. these differences vary from 0.01 to 0.1 [w/sr] and thus do not influence the simulation process significantly. 4. summary and conclusions within the course of this work we have proposed a method to generate hybrid bsdf models comprising both measured and simulated data. such an approach allows the user to perform valid light simulation at close to grazing incident directions. firstly, the comparison between geometrical simulations and data driven bsdf simulation was performed and revealed only minor miscorrelation. 88 m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission consequently, the data obtained from geometrical simulations was utilized to compute bsdf values at grazing incident angles. these values were in turn applied to extrapolate and create the set of hbsdf values. the obtained hbsdf data set was compared to the formerly conducted geometrical simulations and exhibited a high degree correlation. therefore, the proposed method can serve as a valuable tool for modelling the behaviour of drcs and to perform more accurate modelling that features access to light incident directions close the grazing ones. acknowledgments this research was supported by the swiss national science foundation as part of the project “simulation-based assessment of daylight redirecting component for energy savings in office buildings” (#147053) and by cti within the sccer feeb&d (kti.2014.0119). references andersen, m., rubin, m., powles, r. and scartezzini, j. l. (2005). bi-directional transmission properties of venetian blinds: experimental assessment compared to ray-tracing calculations. solar energy, 78(2), 187-198. andersen, m., & scartezzini, j. l. (2005). inclusion of the specular component in the assessment of bidirectional distribution functions based on digital imaging. solar energy, 79(2), 159-167. andersen, m., & de boer, j. (2006). goniophotometry and assessment of bidirectional photometric properties of complex fenestration systems. energy and buildings, 38(7), 836-848. apian-bennewitz, p. (2010). new scanning gonio-photometer for extended brtf measurements. proc spie, 7792, 77920o–1–20. ashikmin, m., premože, s., & shirley, p. (2000). a microfacet-based brdf generator. proceedings of the 27th annual conference on computer graphics and interactive techniques, 65-74. bauer, c., & wittkopf, s. (2016). annual daylight simulations with evaldrc – assessing the performance of daylight redirection components. journal of facade design and engineering, 3(3-4), 253-272. bonneel, n., van de panne, m., paris, s., & heidrich, w. (2011). displacement interpolation using lagrangian mass transport. acm transactions on graphics (tog), 30(6), 158. claustres, l., paulin, m., & boucher, y. (2003). brdf measurement modelling using wavelets for efficient path tracing. computer graphics forum, 22(4), 701-716. cook, r. l., & torrance, k. e. (1981). a reflectance model for computer graphics. acm siggraph computer graphics, 15(3), 307-316. geisler-moroder, d., & dür, a. (2010). a new ward brdf model with bounded albedo. computer graphics forum, 29(4), 1391-1398. grobe, l. o., müllner, k., & meyer, b. (2015). a novel data-driven bsdf model to assess the performance of a daylight redirecting ceiling panel at the calgary airport expansion. in pldc 5th global lighting design convention, 240-243. grobe, l. o., noback, a., wittkopf, s., & kazanasmaz, z. t. (2015). comparison of measured and computed bsdf of a daylight redirecting component. in cisbat 2015 international conference on future buildings and districts, 205-210. he, x. d., torrance, k. e., sillion, f. x., & greenberg, d. p. (1991). a comprehensive physical model for light reflection. acm siggraph computer graphics, 25(4), 175-186. kautz, j., & mccool, m. d. (1999). interactive rendering with arbitrary brdfs using separable approximations. acm siggraph 99 conference abstracts and applications on siggraph, 99, 253. kazanasmaz, t., grobe, l. o., bauer, c., krehel, m., wittkopf, s. (2016). three approaches to optimize optical properties and size of a south-facing window for spatial daylight autonomy. building and environment, 102, 243-256. krehel, m., kaempf, j., & wittkopf, s. (2015). characterisation and modelling of advanced daylight redirection systems with different goniophotometers. proc cisbat 2015, international conference. kurt, m., & edwards, d. (2009). a survey of brdf models for computer graphics. acm siggraph computer graphics, 43(2), 1. lafortune, e. p. f., sing-choong foo, torrance, k. e., & greenberg, d. p. (1997). non-linear approximation of reflectance functions. proceedings of the 24th annual conference on computer graphics and interactive techniques siggraph ’97, 31(3), 117-126. leloup, f. b., forment, s., dutré, p., pointer, m. r., hanselaer, p. (2008). design of an instrument for measuring the spectral bidirectional scatter distribution function. applied optics, 47(29), 5454-5467. matusik, w., pfister, h., brand, m., mcmillan, l. (2003). a data-driven reflectance model. acm transactions on graphics, 22, 759. mohanty, l., yang, x., wittkopf, s. k. (2012). optical scatter measurement and analysis of innovative daylight scattering materials. solar energy, 86(1), 505-519. montes, r., & ureña, c. (2012). an overview of brdf models. university of grenada, technical report lsi-2012-001. ngan, a., durand, f., & matusik, w. (2005). experimental analysis of brdf models. in proceedings of the eurographics symposium on rendering, 117-126. m. krehel et al. / a hybrid data-driven bsdf model to predict light transmission 89 noback, a., grobe, l., & wittkopf, s. (2016). accordance of light scattering from design and de-facto variants of a daylight redirecting component. buildings, 6(3), 30. phong, b. (1975). illumination for computer generated pictures. communications of the acm, 18(6), 311-317. reinhart, c. f., & andersen, m. (2009). development and validation of a model for a translucent panel. information systems journal, 877(2), 1-13. renhorn, i. g. e., & boreman, g. d. (2008). analytical fitting model for rough-surface brdf. opt express, 16(17), 12892-12898. schregle, r., grobe, l. o., & wittkopf, s. (2016). an out-of-core photon mapping approach to daylight coefficients. journal of building performance simulation, 1493(may), 1-13. rusinkiewicz, s. m. (1998). a new change of variables for efficient brdf representation. rendering techniques, 98, 11-22. tian, z., weng, d., hao, j., zhang, y., meng, d. (2013). a data driven brdf model based on gaussian process regression. in proc spie 9042, 2013 international conference on optical instruments and technology: optical systems and modern optoelectronic instruments, beijing. torrance, k. e., & sparrow, e. m. (1967). theory for off-specular reflection from roughened surfaces. journal of the optical society of america, 57(9), 1105. walter, b., marschner, s. r., li, h., torrance, k. e. (2007). microfacet models for refraction through rough surfaces. eurographics, 195-206. ward, g. (1992). measuring and modeling anisotropic reflection. acm siggraph computer graphics, 26(2), 265-272. ward, g., mistrick, r., lee, e. s., mcneil, a., jonsson, j. (2011). simulating the daylight performance of complex fenestration systems using bidirectional scattering distribution functions within radiance. in leukos, 7(4), 241-261. ward, g., kurt, m., bonneel, n. (2015). reducing anisotropic bsdf measurement to common practice reducing anisotropic bsdf measurement to common practice. workshop on material appearance modeling (2014), lyon, france. westin, s. h., li, h., torrance, k. e. (2004). a comparison of four brdf models. proc eurographics symposium on rendering, 1-10. yu, yeh-wei et al. (2012). bidirectional scattering distribution function by screen imaging synthesis. optics express, 20(2), 1268-1280. from city’s station to station city 075 journal of facade design & engineering volume 5 / number 2 / 2017 modelling of active solar building envelopes for cost-effective evaluation christoph maurer, tilmann e. kuhn fraunhofer institute for solar energy systems abstract active building envelopes have provided cost reductions of 40% compared to the separate installation of solar collectors on a building envelope. however, solar building envelopes are more complex than conventional building envelopes due to their additional solar function. firstly, the paper explains this complexity before describing methods of handling it. the focus of the simulation models is to obtain high levels of accuracy at low costs. from the development of innovative solar envelopes to the general planning and construction of solar architecture, this paper provides seven recommendations to optimize the cost-benefit ratio of simulations of active building envelopes. keywords solar building envelopes, building-integrated solar thermal (bist), building-integrated photovoltaics (bipv), building-integrated solar systems (biss), solar architecture doi 10.7480/jfde.2017.2.1740 076 journal of facade design & engineering volume 5 / number 2 / 2017 1 introduction and methodology the active solar building envelopes discussed here are defined as multifunctional building envelopes that can convert incident solar energy, so that it can be used at another time and/or location. such solar building envelopes offer a competitive cost-benefit ratio by providing the additional solar energy function at little extra cost when compared to conventional building envelopes. for buildingintegrated solar thermal (bist) systems, it has been shown that 40% of the investment costs have been saved in two analysed bist building projects (maurer, cappel, & kuhn, submitted). this means that the additional investment cost of building a solar thermal building envelope, as opposed to a conventional building envelope plus a conventional solar thermal collector, can be 40% cheaper per square metre of collector area. therefore, iea shc task 56, for example, is currently investigating solar building envelopes. one challenge for solar building envelopes is that their additional function makes them more complex than conventional building envelopes. this paper first analyses these challenges. from the analysis and the discussion, seven recommendations are derived for the cost-effective evaluation of solar building envelopes. many simulation models have been presented in the past (lamnatou, mondol, chemisana, & maurer, 2015b, 2015a; quesada, rousse, dutil, badache, & hallé, 2012a, 2012b). this paper focuses on how to model active building envelopes in order to evaluate them at low cost. innovations will only succeed in the market, if their benefits can be quantified inexpensively. the methods and recommendations described in this paper are not limited to active solar building envelopes. they may also be helpful for other multifunctional building envelopes that do not convert solar energy. 2 research compared to conventional solar thermal collectors, bist is more complex, not only because the ambient temperature influences the solar thermal performance, but also the temperature of the building interior. figure 1 illustrates this situation, which influences the solar thermal performance. additionally, the energy flux to the building interior needs to be quantified, which goes beyond the conventional measurements of solar thermal collectors e.g. according to (iso 9806). fig. 1 schematic drawings of a building-added solar thermal collector (left) and building-integrated solar thermal collector (right) 077 journal of facade design & engineering volume 5 / number 2 / 2017 compared to conventional elements of the building envelope, bist is more complex because the heat flux through the building envelope without irradiance not only depends on the temperature difference, but also on the operating mode of the collector. this means that a constant u value does not characterize solar thermal envelopes correctly. the energy flux to the building interior with solar irradiance on the collector also depends on the operating mode of the collector. this means that a constant g value (also known as solar heat gain coefficient shgc or solar factor or total solar energy transmittance) does not characterize solar thermal envelopes correctly (maurer & kuhn, 2012). typically, an active solar envelope will not be operated without irradiance. therefore, a constant u value with a variable g value may work in a number of cases. however, in general, active solar envelopes can supply energy to the envelope and therefore influence the u and g values; this applies not only to bist, but also to building-integrated photovoltaics (bipv). to handle this complexity, detailed physical models of active solar envelopes can be generated. in general, a simulation model of an active building envelope includes parameters to determine the characteristics of the envelope as well as inputs from, and outputs to, the rest of the building, including the building services and the surroundings of the building. figure 2 presents a schematic drawing of this interaction. inputs and outputs connected to the building environment typically involve the solar irradiance and the heat exchange between the environment and the envelope. these inputs and outputs can range from simple values to very detailed data. similarly, the inputs provided by the building interior and the output to the building interior typically involve radiance and heat transfer. depending on the functions provided by the simulation model for the active building envelope, additional inputs and outputs, e.g. those relating to acoustics and air handling, can be used. regarding the inputs from, and the outputs to, the building services, typical data includes information such as the control of the active envelope, the exchange of heat and electricity, as well as materials such as the supply and return of air and water. of course, the active building envelope can consist of different kinds of building envelope components that are connected. in general, the performance of an active building envelope needs to be evaluated together with the building interior, the building services, and the surroundings, and compared to carefully chosen reference cases without this active envelope. fig. 2 schematic drawing of the links between the simulation model of an active building envelope and the surroundings, the interior, and the building services of the building. 078 journal of facade design & engineering volume 5 / number 2 / 2017 the energy simulation model of an active building envelope typically consists of an optical simulation, which calculates the effect of multiple reflections, and a thermal simulation with thermal nodes and energy fluxes between these nodes, as illustrated in figure 3. detailed physical models may involve large numbers of parameters in order to characterize all subcomponents accurately (lamnatou et al., 2015b, 2015a; maurer, 2012; sprenger, 2013). one challenge can be integration of the detailed collector simulation model into an existing building simulation, especially if the source code of the building model is not accessible. for trnsys, (hauer & streicher, 2013; maurer & kuhn, 2012; saelens, 2002; saelens, roels, & hens, 2008) have presented different ways to connect simulation models of active building envelopes to the closed-source building model. fig. 3 schematic drawing of a detailed physical model of a semi-transparent solar thermal facade collector with the average temperatures of the surroundings, t_amb, of the layers of the facade t4 to t9, the temperature of the interior, t_room, and the fluid inlet and outlet temperature, tfluidin, and, tfluidout. r indicates the thermal resistances between the layers, and j represents the infrared radiation within the element simple bist models were investigated (maurer et al., 2013; pflug, di lauro, kuhn, & maurer, 2013) and newly developed (maurer, cappel, & kuhn, 2015). figure 4 schematically illustrates all four simple approaches. table 1 provides an overview of the cases for which the four approaches are recommended, the inputs, outputs and parameters of the model and the data needed to calculate the parameters. all four approaches need the irradiance and the temperatures of the ambient and building interior as inputs and provide the solar thermal performance and heat flux to the building interior as ouputs. approaches a and b of (maurer, cappel et al., 2015) are recommended for cases where conventional solar thermal collectors are integrated into the building envelope and where the conventional solar thermal performance parameters for building-added solar thermal (bast) collectors according to (cooper & dunkle, 1980; iso 9806) are available. approach a is recommended for cases where there is a high thermal resistance between the absorber and the building interior. the parameters for the solar thermal performance of the bist case are then calculated from the fraction of back losses and the parameters of the bast case. the heat flux to the building interior is calculated based on the thermal resistance between the absorber and the interior. for bist cases with poor insulation between the absorber and the building interior, approach b uses the bast solar thermal performance parameters and the thermal resistances between the absorber and the back of the bast unit, as well as the thermal resistance between the absorber and the bist interior side, in order to calculate the bist solar thermal performance and the heat flux to the building interior. 079 journal of facade design & engineering volume 5 / number 2 / 2017 approach c is recommended for special cases where data from detailed measurement of the collector performance is available for deriving the solar absorptance and the parameters of the thermal resistances. approach d is recommended, for example, in cases where there are new technologies with measurement data for the collector yield and the energy flux to the building interior. approach d proposes a very simple node model with only four constant thermal resistances and solar absorptance which achieves relatively good agreement with the results of a detailed model. fig. 4 schematic illustration of the four simple approaches to model bist approach a approach b approach c approach d recommended application case building integration of bast collectors monitoring data available new components well insulated poorly insulated inputs irradiance, ambient and building interior temperatures outputs solar thermal performance and heat flux to the building interior data needed to calculate the parameters 3 bast parameters, fraction of back losses, thermal resistance 3 bast parameters, thermal resistances collector yield collector yield + energy flux to interior parameters 3 bist parameters, thermal resistance solar absorptance, thermal resistances table 1 data needed for the simple bist models and recommended cases the modelling approaches mentioned above can be extended for the description of buildingintegrated photovoltaic-thermal (bipvt) elements (maurer, sprenger, lämmle, & kuhn, 2015). simple approaches to include the photovoltaic function consider only the pv efficiency and its temperature dependence as parameters. more accuracy can be reached for low irradiance values by including the three parameters of the heydenreich model (heydenreich, müller, & reise, 2008). very accurate models of the pv function are based on the current-voltage curve of the cells and the electric circuit between them e.g. to model the effects of partial shading. the methods for the modelling of bipvt, described by (maurer, sprenger et al., 2015), can also be used for bipv elements without solar thermal functionality. 080 journal of facade design & engineering volume 5 / number 2 / 2017 3 results the simulation of active solar envelopes should provide the necessary level of accuracy at lowest possible costs. the first step is therefore to analyse what level of accuracy is needed in order to choose the most cost-effective modelling approach. semi-empirical models are often the best choice for the evaluation of custom-built collectors or concepts for new multifunctional building envelope elements that actively convert solar energy. semi-empirical models combine parameterized physical calculation models and specific measurements to determine unknown “fit parameters” or “model parameters”. the experimental determination of the model parameters ensures that non-ideal properties of the building envelope elements are taken into account in the evaluation. the combination of parameterized physical calculation models and measurements for the calibration of the model therefore offers the best ratio of costs to benefits in most cases. for innovative products of active solar building envelopes, calorimetric measurements are often crucial for the validation and calibration of simulation models. to date, there is no standard that defines the criteria for a validated simulation model of a solar building component. such a standard could increase confidence in simulation models that have been validated and calibrated according to the standard, and may also decrease the costs. the mathematical complexity of simulation models for multifunctional building envelope elements is irrelevant as long as the models are easy to use. an important next step is therefore a user-friendly front end, integrated into a powerful whole-building energy-simulation environment. the feasibility of a “plug and play interface” has been shown for the case of semi-transparent solar thermal facade collectors (maurer et al., 2013). here, the semi-transparent solar thermal facade collector was represented by a trnsys type with a similar user interface to other solar thermal collectors or walls. the new trnsys type was used by hvac planners to perform a complex simulation of the whole building energy demand, including the detailed modelling of the control of the technical building systems. accurate simulation models that are easy to use make the evaluation of building envelopes cost-effective because each stakeholder can focus on his main competences. however, different stakeholders in the building process use different simulation environments due to the very specific advantages of each. to address this issue, it is recommended to provide the same simulation model of a solar building component in all necessary (or at least in the most important) simulation environments. this is cost-effective if the models for different simulation environments can be generated automatically. at different times throughout the building process, the multifunctional building envelope components are specified with different levels of detail, and simulations with different accuracy levels are needed. one approach is to combine, for example, a model with few inputs and low accuracy and a model with many inputs and high accuracy within one adaptive multi-environment simulation model that can switch its accuracy depending on the available input data at this stage of the building process. adaptive simulation models reduce costs because one model can be used right from the early planning phases to the commissioning and to the facility management. at each stage, depending on the available input, the desired outputs and the acceptable computing time, the best level of accuracy is chosen. for example, a low accuracy may be acceptable in early planning phases, when only few inputs are available. in later planning phases, when more inputs are available, the adaptive model provides a high accuracy. 081 journal of facade design & engineering volume 5 / number 2 / 2017 building information modelling (bim) aims to improve the exchange of data within the building process. to date, the industrial foundation classes (ifc) define a structure of text data which can be exchanged within this format. it is therefore recommended by (maurer, sprenger et al., submitted) to include functions in machine code in this data exchange. this could lead to greater accuracy and lower costs for the planning, construction and facility management of buildings due to the increased knowledge exchange. 4 conclusions active building envelopes offer exciting advantages such as significant cost reduction when compared to the installation of building-added solar elements. however, they are more complex than conventional building envelopes with additional conventional solar elements. this paper first explained this complexity before describing methods to handle this complexity by choosing costeffective simulation models: 1 if a simple model is accurate enough, no detailed model needs to be developed. 2 if some parameters cannot be calculated accurately enough, measurements should be performed to derive them. 3 it is recommended that a standard for the validation of simulation models of building envelopes be developed. it is then easy for companies to choose the cheapest simulation model that performs to the level of accuracy that they need. 4 models can be detailed and accurate as long as they are easy to use. such models save costs for the planners as well as for the owner because the simulations are more realistic. 5 the model of an innovative building envelope should be available in all relevant simulation environments, because it reduces the costs for planners to get used to a new simulation environment. 6 adaptive models can provide an initial estimate with little input data even at early planning stages and a higher accuracy with more input data at later stages of the building process. this reduces costs by making the planning more realistic. 7 the next version of the ifc should include the possibility of exchanging models as machine code. the exchange of knowledge in the form of compiled models reduces the costs because the stakeholders can focus on their main competences. the recommendations no. 1, 2, 4, 5 and 6 are ready to be used for simulation models, which make the evaluation of solar building envelopes cheaper. the development of a standard for simulation models of solar building envelopes is a challenge that will require effort and input over many years. efficiently exchanging simulation models in machine code in building processes, from the early planning stages until the end of the lifetime of the building, is a challenge that starts today but will not be solved soon. rather, it is a continuous task of making the building process progressively more efficient. with these recommendations, the evaluation of solar building envelopes based on simulation models can already be made more cost-effective, with the potential for additional cost reductions in the future. 082 journal of facade design & engineering volume 5 / number 2 / 2017 acknowledgments the research leading to these results has received funding from the european community’s horizon 2020 programme under grant agreement no. 680441. the authors thank helen rose wilson for proofreading this paper. references cooper, p. i., & dunkle, r. v. (1980). a non-linear flat-plate collector model. solar energy, 26(2), 133–140. doi:10.1016/0038092x(81)90076-1 hauer, m., & streicher, w. (2013). gebäudegekoppelte simulation fassadenintegrierter kollektoren mit trnsys. otti 23. symposium thermische solarenergie. heydenreich, w., müller, b., & reise, c. (2008). describing the world with three parameters: a new approach to pv module power modelling. 23rd european pv solar energy conference and exhibition (eu pvsec), 2786–2789. doi:10.4229/23rdeupvsec2008-4do.9.4 iso 9806 (2013): international organization for standardization. quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012a). a comprehensive review of solar facades. opaque solar facades. renewable and sustainable energy reviews, 16(5), 2820–2832. doi:10.1016/j.rser.2012.01.078 quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012b). a comprehensive review of solar facades. transparent and translucent solar facades. renewable and sustainable energy reviews, 16(5), 2643–2651. doi:10.1016/j.rser.2012.02.059 saelens, d., roels, s., & hens, h. (2008). strategies to improve the energy performance of multiple-skin facades. building and environment, 43(4), 638–650. doi:10.1016/j.buildenv.2006.06.024 saelens, d. (2002). energy performance assessment of single storey multiple-skin facades (ph.d. thesis). katholieke universiteit leuven, leuven. retrieved from https://bwk.kuleuven.be/bwf/phds/phdsaelens sprenger, w. (2013). electricity yield simulation of complex bipv systems. stuttgart: fraunhofer-verlag. jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 119 journal of facade design & engineering volume 5 / number 1 / 2017 thermal optimization of curtain wall façade by application of aerogel technology david appelfeld1 1 dow corning europe sa, parc industriel zone c, rue jules bordet, b-7180 seneffe, belgium, tel. +32 499 049 500 david.appelfeld@dowcorning.com, abstract the paper illustrates the use of aerogel technology in façades to reduce thermal bridging and limit condensation. additionally, by mitigating local thermal bridges the indoor climate, especially draught and molds creation, can be eliminated as the surface temperature increases and prevents these negative factors. curtain walls, in comparison to opaque wall, are often not designed in an energy efficient way and can be large contributors to heat loss of buildings. this is common for curtain walls in asia and north america, where the energy requirements are not as demanding as in europe. building envelopes have many details which can easily introduce thermal bridge due to limited space of insulation or incorrectly solved construction detail. the heat flow through a poor thermally-performing detail, e.g. exposed concrete slab, could account for over 40% of the heat loss of the façade. the contribution of a well-insulated slab could be less than 10%. unfortunately, traditional insulation techniques are often not suitable due to limited installation space. this paper discusses several case studies whereby the performance of a reference case is compared to a case with a thermally optimized façade implementing building insulation blanket (bib), which uses silica based aerogel technology. the thermal conductivity of bib can be as low as 0.015w/mk, and together with its high flexibility and fire resistance enables new design possibilities. the use of bib in the case studies, contributed to the reduction of overall curtain wall thermal transmittance up to 30%. additionally, condensation risk was significantly reduced. keywords aerogel, building insulation blanket, façade thermal breaks, high performance façade, energy efficiency, curtain wall thermal optimization doi 10.7480/jfde.2017.1.1430 120 journal of facade design & engineering volume 5 / number 1 / 2017 1 diverse usage of building insulation blanket this paper discusses aerogel based building insulated blanket (bib) through several case studies in north america and asia, architecture details, and performance data from simulations and real-life applications. furthermore, low thermal conductivity and physical properties of aerogel technology are discussed and demonstrated on unique application of bib into a building envelope (casini, 2016). thermal bridges can be found in many different details within the building envelope for example; the junction between wall – floor slab – balcony/overhang, curtain wall framing system, stem walls, jamb between walls and windows/doors, spandrel or areas with limited space available for insulation. solutions for addressing thermal bridges already exist on the market and have been successfully used in the past and present. in the field of energy efficiency research, many different solutions have been already developed to improve the building envelopes performance (brunner, wernery, & koebel, 2015). however, there is no universal solution addressing all types of thermal bridges, therefore we selected four very different case studies where the usability and performance of bib are demonstrated. in most of the cases the insulation applications are limited to small amount of bib, which is in contrast with traditional use of an insulation on entire wall area. the optimized application improves significantly a ration between added value and cost of the material, which consequently increase economical usability and accessibility of such a technology. continuous building envelope insulation is one of the most important criteria for achieving sustainable and continuous energy performance of buildings. using conventional insulation materials in tight spaces or for insulation of complicated details is often challenging. application of dow corning® hpi-1000 building insulation blanket (bib) at fenestration transitions and slab edges can effectively contribute to the enhancement of the thermal performance of the building envelope. aerogel technologies, including bib, offer new opportunities for design of building envelopes combining high insulation properties, flexibility, durability as well as aesthetics and design freedom (aegerter, leventis, & koebel, 2011) (koebel, rigacci, & achard, 2012). additionally, the condensation on cold internal surfaces, caused by thermal bridges, can be effectively addressed in cost efficient ways. the bib material is based on the silica based aerogel which allows good compatibility with silicone based adhesives which can provide good adhesion to most building substrates. due to bib’s simple implementation into construction details, field work on detail insulation can be executed without impacting schedules while keeping high level of quality. in this paper, we address four cases in which the main issue is the limited space available for sufficient insulation and whereby the flexibility of insulation material is required in order to insulate non-standard and unique shapes and conditions. the focus is mainly on new construction, nevertheless we are also aware about large potential of the technology in retrofits, which is illustrated in the first case. 121 journal of facade design & engineering volume 5 / number 1 / 2017 2 methodology for optimization of façade performance the performance of a building envelope is evaluated as a whole construction, including all details, therefore emphasis has to be placed on an optimally configured solution. this includes heat loss through panels (vision and non-vision), framing and connections between panels and framing systems. the heat flow through a poor thermally-performing condition, could account for over 40% of the total heat loss through the building envelope and eliminate the energy improvement of thermal efficient solutions, such as thermally broken framing systems or igu. in comparison, a thermally efficient detail, such as an insulated slab edge, could contribute for less than 10% (lawton & norris, 2013). typically, poorly performing details are in areas where there is space limitation and traditional insulation techniques cannot be used. thermal bridges can be categorized into following: – planar thermal bridge: panels, spandrels, igus (u-value) – linear thermal bridge: façade framing systems, spacer bars (ψ-value) – point thermal bridge: point fixing hardware (χ-value) all the above mentioned thermal bridges have to be added together to define the overall u-value of curtain wall/façade (en/iso 12631, 2012), (nfrc 100, 2004). individual construction details and their heat loss contribution can be calculated accordingly to various iso or astm standards (nfrc 100, 2004), (en iso 10077-2, 2012), (iso 15099, 2003) with finite element method (fem) and simulation programs, such as therm and flixo (flixo pro), (mitchell, kohler, arasteh, 2006). in this paper we look also at other thermal bridges beyond curtain walls and windows, and thus methodologies for thermal bridges in constructions can be used as well (iso 10211, 2007). the structure details studied in this paper are unique and their evaluation is sometimes outside the mentioned standards evaluation method, therefore we adapted the procedure to fit the individual needs. 2.1 high performance insulation aerogel technology building insulation blanket (bib) has significantly improved thermal conductivity, see table 1, as compared to conventional insulation products, see table 2, (dowcorning, 2014). bib material has high compression resistance which allows to compress the material without losing its insulation properties, compared to insulation materials, such as mineral or glass wool. this is due to closing micro structural air cavities within the material under compression which further limit convection. the blanket is also fire-resistant, hydrophobic and does not settle over time, which ensures long term uninterrupted performance. building insulation blanket hpi-1000 thickness 10 mm thermal conductivity 0.0146 w/mk weight 0.013 kg/m2 maximum service temperature 200 °c compression strength si units (10 psi) fire resistance class a; fsi 5, sdi 10 hydrophobic yes water vapour resistance factor (μ-value) 4.7 (-) color gray table 1 material properties of bib 122 journal of facade design & engineering volume 5 / number 1 / 2017 material thermal conductivity [w/mk] bib 14.5 polyisocyanurate 23.6 xps 28.4 mineral wool 33.8 eps 37.3 glass wool 40.5 table 2 comparison of thermal conductivity of conventional insulation materials 3 application of aerogel technology based insulation in curtain walls advanced building envelope design has good potential to lower energy demand and reduce the capital costs of heating and cooling systems, as the need for heating and cooling can be reduced by up to 60% (ornl, 2012) (iea, 2014). large portions of these savings can be addressed by energy improvement of building envelope and the rest by efficient mechanical equipment (iea, 2014). four different case studies with various application purposes within a building envelope are described in this article. the results are discussed based on a comparison between the original base case scenario and the optimized scenario utilizing bib in order to clearly demonstrate the improvement. 3.1 retrofitting façade with focus on point thermal bridges this case study demonstrates a usage of bib and its benefits when extremely tight space needs to be insulated. the refurbished structure does not allow any increase of the space for insulation. a canopy above the entry level of a commercial shopping center was constructed without any thermal breaks and insulation 15 years ago. not-thermally broken structure resulted in excessive condensation during colder months and made the usage of the interior commercial space unpleasant. the space of the shops was undertaking an upgrade and renovation, and the contractor and architect were charged with removing the condensation issue while keeping the original structure and cladding. at the fig. 1 and fig. 2 you can see a 3d sketch and section through the metal frame which is going through the thermal envelope of the building without any thermal break. in this case the focus was poorly on elimination of the condensation. this thermal problem can be considered as 3d, however after initial validation of 2d model by 3d model we continued only with 2d modeling as the results were nearly identical. the design temperatures for the building were -8.6°c and 23°c. from the fig. 3 and fig. 4 it is clearly visible that the condensation occurs on an interior side of the metal frame structure, when interior humidity is 50%. the condensation areas are highlighted by thick red line in fig. 4. 123 journal of facade design & engineering volume 5 / number 1 / 2017 several options of insulating this detail were modeled and evaluated, also the feasibility of different applications were discussed. due to the space limited areas around the water gutter, there were places allowing only 10-20 mm of available space under the existing cladding. modelling with conventional mineral or glass wool showed that the condensation would remain as not enough insulation could be used. considering problematic and very limited space only flexible high performance insulation would solve the condensation problem. also an option with insulating internal part of the structure was considered, as there was more space for insulation. however, this solution would have high requirements on making the entire structure vapour tight, and prevent warm interior air from movement to the areas close to the façade where the metal structure would be cold and thus condensation would occur again. fig. 1 3d illustration of metal structure penetrating façade continuously from interior to exterior fig. 2 detail section of the canopy structure including water gutter 124 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 3 simplified model of canopy structure before renovation. fig. 4 view at the section of canopy structure and wall, with highlighted areas with condensation riskbefore renovation when the detail was modeled with bib, see fig. 5 and fig. 6, then the structure and its insulation wrap could achieve required performance to remove the condensation. in fact, according to the model, the condensation would not appear until the relative humidity would reach around 57%; this gives a safety margin for small thermal bridges which were not accounted for due to the simplified 2d model. the temperature of the area with highest risk of condensation increased from 4.87˚c to 14.05˚c. the warmer metal structure would not facilitate warm internal air condensation and hence the condensation would be eliminated. this solution was approved and implemented, and the results from simulations were confirm by fact that the condensation was removed. fig. 5 simplified model of canopy structure with bib after renovation fig. 6 view at the section of canopy structure and wall, with highlighted areas with condensation riskbefore renovation 3.2 thermal optimization of the curtain wall framing system when a limited space needs to be highly insulated, as could be the case for a non-thermally broken curtain wall system or opaque walls with not sufficient thermal resistance and without adequate space for conventional thermal insulation, then the traditional insulation techniques often do not fulfil the demand. in these cases an aerogel based technology building insulation blanket (bib) can be used to mitigate thermal bridges in curtain wall framing systems and other structures. the thermal conductivity of the blanket can be as low as 0.0146 w/mk. the performance of bib is illustrated on an optimization of a curtain wall façade where a base case is compared to an optimized case with bib. the façade is a traditional unitized mullion/transom type with opaque rectangular spandrel, with vision area thermal transmittance of u g = 1.45 w/ 125 journal of facade design & engineering volume 5 / number 1 / 2017 m2k and unitized module dimensions of 3.8 m x 1.5 m. fig. 7 illustrates the curtain wall framing elements on the building before and after design optimization, including the temperature field, which indicates thermal bridges and their reduction (u g thermal transmittance of glazing). the thermal transmittance of the system is presented in a form of thermal joints (u tj ), which represents both thermal transmittance of frame and edge of the glazing, see table 3. the use of bib reduced the overall thermal transmittance of the façade by 26% and the thermal transmittance of framing itself by an average of 50%. in some cases this reduction was up to 65%. the risk of condensation was removed as the temperatures of internal façade surfaces raised above the dew point. the proposed design solution allowed to reduce u cw from initial 2,7w/m2k to required 2,0w/m2k (u cw – thermal transmittance of curtain wall). the benefits of the curtain wall performance improvement contributes additionally to investment optimization as e.g. triple insulated glass units may not be needed and cheaper double insulated glass units can be used. this consequently reduces loads on structures and thus more subtle framings can be used. this is a very large project, and the implementation did take around 2 years as the several levels of validation were needed. starting with conceptual design in early stage with façade consultant until production design with the fabricator. this process included validation by full scale mockup, before the product was integrated in final solution. fig. 7 illustration of design and performance of curtain walling system optimized by bib 126 journal of facade design & engineering volume 5 / number 1 / 2017 transom stacked transom mullion u cw value u tj before [w/m2k] 11.3 8.0 5.8 2.7 u tj before with bib [w/m2k] 3.8 5.6 3.2 2.0 improvement [%] 66% 30% 44% 26% table 3 results of the uvalue of frames reducing by bib optimization of thermal bridges 3.3 thermal separation of a roof slab and a stem wall every building has unique features and needs individual approach to solve thermal bridges. in this case study a casted steel reinforced concrete stem wall was used on a roof slab to create a roof garden seedbed. the reinforcement re-bars continuing from the roof slab to the stem wall create linear thermal bridge. as the re-bars are local thermal bridges, two models for each option were evaluated and averaged (first only concrete, second – concrete with re-bars). the bib material consists mainly of silica which creates excellent insulation properties due to its nano-pours in the silica particles. the nano-pours encapsulated in the silica prevent pressing out air when the insulation is compressed. for this feature, bib can be exposed to a load generated by e.g. stem wall standing on top of a roof. two options of insulating the stem wall were evaluated. the option 1 removes the thermal bridge by placing bib between the roof and stem wall. the option two solves the thermal bridge by wrapping the entire stem wall by bib. the illustration of the reference case and two options are shown in fig. 8. this stem wall can be considered as liner thermal bridge and ψ-value of the individual wall can be derived by the calculations. the reference case which is not insulated has a ψ-value of 1.39 w/mk, option 1 with bib under the stem wall gives a ψ-value of 1.05 w/mk, and the option 2 reduced the ψ-value to 1.00 w/mk. the reduction of the heat flow through the liner thermal bridge is slightly under 40%. by comparison of the option 1 and the option 2, the difference is around 5% in a favor of option 2, complete wrap of the stem wall. however, if the economic aspects and consumption of the material are considered, then the option 1 would be the most reliable from added value perspective. table 4 base case of a stem wall (left), option 1 with bib under a stem wall (middle), option 2 with bib wrap of a stem wall 127 journal of facade design & engineering volume 5 / number 1 / 2017 3.4 increasing wall thermal resistance in space limited structures new buildings as well as renovated building in some cases may have a limited space for an insulation to reach required performance. not sufficient insulation levels may lead to thermal bridges and other major issues with building envelope. in this case we will demonstrate an example where wrong communication led to situation when cladding system was not designed with sufficient space for insulation. this situation could escalate into a large time delays and construction cost increase. as the construction was already delayed, a solution with high performance insulation was sought, while keeping the existing cladding system. the building envelope consists of large amount of windows, therefore the façade has large window-to-wall ration and requires high demand thermal insulation in the walls. the requirement of a local building code for the thermal insulation of the exterior wall assembly specifies that u w -value of wall assembly should be max. u w = 0.36w/m2k (u w thermal transmission of a wall). the design of the building did not allow to have sufficient amount of conventional fiber insulation and thus entire façade would not comply with the regulation. one of the wall window connection detail is shown on fig. 9, including indication of the available space for insulation, which was just around 40mm. by thermal modelling it was calculated that 3 layers would not be enough. to achieve required thermal resistance of the wall, 4 layers of bib would be needed, as each layer is 10mm thick. this solution would give an u w –value of the center of wall 0.3 w/m2k, compared to a situation where there would be 40mm of mineral wool with thermal conductivity λ=0.04w/mk, which would provide u w –value = 0.56 w/m2k. using 4 layers of high performance insulation may seem too excessive, however considering the given situation and potential risks, it gives the most optimal solution for cost vs. performance. in this situation none of the structures need to be adapted or removed to accommodate more insulation. fig. 8 wall window connection detail with indicated available space for insulation 128 journal of facade design & engineering volume 5 / number 1 / 2017 4 conclusion through this paper we demonstrated the usage of aerogel insulation blanket in four projects in north america and asia in applications in curtain walls, façade penetration, roof and perimeter wall. each of these projects has different application, which indicates the versatile and broad application of bib in construction. the low thermal conductivity of 0,0146 w/mk allows to insulate space limited details and structures with sufficient efficiency. also, the flexibility of the material allows to insulate complicated details where the use of rigid insulation would not be feasible. due to the high thermal insulation performance and flexibility it was possible to optimize the curtain wall framing system by 50% on average which contributed to overall façade u cw reduction by 26%. additionally, the condensation risk was removed at wall penetration by large metal structure during a refurbishment. furthermore, bib can be used to limit heat loss through walls, parapets or spandrels by insulating whole planar area which has limited available space and does not allow to have bulky amount of conventionally used fiber-based or rigid foam insulation. this can be done at the moment of the construction or even during building operations in a quick and efficient way. there is also large potential for bib in retrofit, as there, the space is usually given and cannot be adapted to large amount of bulky insulation. another important finding was that the attention to the critical details in mitigation of thermal bridges is crucial. by optimal use of bib, the thermal performance of window-wall façade could fulfil building code requirements and the façade of the new building would not need to be expensively changed in order to accommodate large amount of standard insulation. references aegerter, m. a., leventis, n., koebel, m. m. (2011) handbook of aerogel. hardcover isbn978-1-4419-7477-8 brunner, s., wernery, j., koebel, m. (2015). high performance thermal insulation examples from the swiss built environment. cisbat 2015 lausanne, switzerland, september 9-11, 2015, pp. 39-44 casini, m. (2016). smart buildings advanced materials and nanotechnology to improve energy-efficiency and environmental performance, chapter “advanced insulation materials”, woodhead publishing 2016, isbn:978-0-08-100972-7 dow corning (2014). dow corning® hpi-1000 building insulation blanket, thin-profile, flexible, high thermal resistance blanket, form no. 62-1728b-01 en iso 10077-2 (2012). thermal performance of windows, doors and shutters-calculation of thermal transmittance, part 2: numerical method for frames, european committee for standardization, brussels, belgium, 2012. en iso 12631 (2012). thermal performance of curtain walling calculation of thermal transmittance, european committee for standardization, brussels, belgium, 2003. flixo pro, simulation software, http://www.infomind.ch iea (2014), technology roadmap energy efficient building envelopes, 2014 iso 10211 (2007). thermal bridges in building construction heat flows and surface temperatures, european committee for standardization, brussels, belgium, 2007. iso 15099 (2012). thermal performance of windows, doors and shading devices -detailed calculations, european committee for standardization, brussels, belgium, 2003. koebel, m., rigacci, a. & achard, p. j. (2012). aerogel-based thermal superinsulation: an overview, sol-gel sci technol, 63: 315. doi:10.1007/s10971-012-2792-9 lawton, m., & norris, n. (2013). “thermal bridging: ignorance is not bliss”, journal of building enclosure design, winter 2013 mitchell, r., kohler, c., arasteh d. (2006). therm 5.2/window 5.2 nfrc simulation manual, lawrence berkeley national laboratory, university of california, berkeley, ca, 2006. nfrc 100 (2004). procedure for determining fenestration product u-factors, national fenestration rating councsil ornl (2012). energy savings potential of building envelope and windows technologies, for u.s. department of energy, sub-contract ornl-4000106215 of ornl, winbuild inc., fairfax, virginia, 2012 journal of facade design and engineering 4 (2016) 67–76 doi 10.3233/fde-160052 ios press 67 enhanced flame retardancy of flax bio-composites for the construction market nayra uranga loredo and javier sacristan bermejo∗ polymers and composites group, acciona technological center, valportillo ii n8, madrid, spain abstract. bio-composites made of natural fibres are an attractive alternative to conventional composites with synthetic fibres such as glass or carbon fibres not only because of their intrinsic properties but also because of their contribution to sustainability. however, natural fibres are flammable, and bio-composites need to be protected against fire for safety reasons but also to meet the strictest eu regulations for the transportation and construction sectors. thermosetting composites need high loadings of flame retardant additives to achieve satisfactory results in terms of flammability which may lead to significant deterioration in their mechanical properties. this work explores the possibility of reducing the flammability of flax bio-polyester composites with potential use in the transportation and construction sectors through the combination of several novel fire retardant additives, which are halogen-free and considered environmentally friendly. cone calorimeter tests indicate that proper combinations of fire retardant additives such as alumina trihydrate, ammonium polyphosphate and exolite 740 reduced heat release rate and flammability up to a 60%; delaying the ignition time with respect to the unfilled material. these results were achieved at concentrations much lower than those with traditional solutions. however, the addition of dimethyl propyl phosphonate to the resin formulation with alumina trihydrate and ammonium polyphosphate failed to demonstrate any significant synergistic effect at reducing the heat release rate. keywords: bio-fibres, bio-resins, fire retardant additives, bio-composites nomenclature ath aluminium trihydrate app ammonium polyphosphate dmpp dimethyl phosphate dma dynamic mechanical analysis tga thermogravimetric analysis dtg first derivative of the tga curve phrr peak heat release rate thr total heat release rate tti time to ignition marhe maximum average of heat emission ∗corresponding author: javier sacristan bermejo, polymers and composites group, acciona technological center, valportillo ii n8, madrid 28108, spain. e-mail: javier.sacristan.bermejo@acciona.com. issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). mailto:javier.sacristan.bermejo@acciona.com 68 n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market 1. introduction fibre reinforced polymer composites are made of fibres embedded in a polymer matrix that holds the fibres together and transfers the load among them. the fibres are usually glass, carbon, or aramid, although other fibres such as paper or wood have also been used. the utilization of natural fibres in composites represents a very valuable alternative contributing to reduce the consumption of synthetic fibres and thus minimizing the environmental impact of composite materials in the construction sector. thermosetting and thermoplastic composites made of renewable products such as natural fibres have lower embodied energy than those reinforced with mineral fibres. they are also safer for operatives to handle than traditional products and, finally, they can be composted or burnt for energy recovery at end of life. with the growing concern for fossil fuel depletion and climate change, there is a strong interest in exploring renewable biomass materials as substitutes for petroleum-based feedstock (goutianos, 2006; aziz, 2005). in that context, bio-resins represent an environmentally friendly alternative to traditional resins. from an industrial point of view, the cost is one of the more significant barriers to the development of renewable materials. however, during the last few years the production of some bio-resins has become viable as technologies evolve, and economies of scale come on stream, along with increasing consumer awareness on the subject of recycling and the impact that materials have on the environment. bio-polyester resins can now be used as sustainable alternatives to traditional petro-chemical derived materials in the manufacture of composite products. however, despite the numerous advantages that bio-composites would provide to society in everyday life, there is an obvious disadvantage related to the high flammability of bio-resins and natural fibres such as sisal, jute or flax (matkó et al., 2005). in fact, bio-composites’ fire retardant performance represents one of the biggest challenges to the widespread use of these materials in the construction market (troitzsch j., 2004). recent studies have shown that it is feasible to improve the fire retardant behaviour of natural fibre reinforced polymer composites through the incorporation of fire retardant additives, such as halogen, nitrogen, and phosphorous-based compounds (hull, witkowski, hollingbery, 2011; bismarck, mishra, lampke, 2011; lazko et al., 2013; kandare, chukwunonso, kandola, 2011). halogenated additives prevent flame spread, but have significant disadvantages producing dense smoke and corrosive combustion by-products which can have a negative impact on the environment and fire safety. the composition of these gases depends on the polymer chemistry, additives, and fire conditions. among these gases, hydrogen chloride, (hcl), hydrogen fluoride (hf), hydrogen cyanide (hcn), and carbon monoxide (co) which is produced by almost all samples in varying amounts may be very toxic at relatively low concentrations (une 45545, railway applications – fire protection on railway vehicles –, 2005). other commonly used fillers are alumina trihydrate or aluminium trihydroxide (ath) and ammonium polyphosphate (app). both are considered as ‘greener’ fr’s, but their effectiveness is somewhat limited since relatively large amounts of these fillers are needed for adequate flame retardancy, which has a detrimental effect on processing and leads to possible alterations of product mechanical properties. however, the combination of ath with other fire retardant additives such as app, melamine pyrophosphate, or expandable graphite due to their synergistic behaviour have shown to be a very promising route to improve flame retardancy of composites with low mechanical/structural requirements (hapuarachchi & peijs, 2009; el-sabbagh et al., 2014; dorez et al., 2013; ricciardi et al., 2012). depending on the chemical composition of the polymer matrix, combinations of ath and app have shown some synergistic or antagonistic behaviour with respect to polymer flammability (schartel, braun, & reinemann, 2003; levchik et al., 1995). the aim of this work is to enhance the fire retardant performance of flax fibre composites by means of combinations of four different fire retardant additives: app, ath, dimethyl phosphonate n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market 69 (dmpp), and exolit 740. these additives were chosen because halogen-free products will be key to the future of the fire-retardant additive industry which is seeking for greener and safer solutions with greater flame retardant performance and lower smoke generation. to date, these additives have not been used together to reduce flammability of a bio-polyesterflax composite system. fire retardant performance is evaluated through cone calorimeter studies. special attention is paid to finding the most appropriate combination and concentration of additives which would lead to bio-composites with enhanced fire retardant performance while, at the same time, maintaining optimum processability. 2. experimental 2.1. materials the unsaturated polyester resin used was a bio-polyester, enviroguard i 93271a, provided by ccp composites with a bio-content of nearly 25%. a dual catalytic system was used to cure the resin: a low active oxygen content methyl ethyl ketone peroxide (supplied by pergan inc. under the commercial name peroxan me-50 la3 x) was selected to avoid damage on natural fibre due to the exothermic peak produced during the curing reaction with a 1% cobalt octoate (coo) solution (provided also by pergan inc. under the commercial name pergaquick c12x). biotex flax 2 × 2 twill 400 gr/m2 fabric was provided by composites evolution uk. in order to preserve the environmentally friendly character of these composites a series of “green” fire retardant additives was selected. the flame retardant fillers used were ammonium polyphosphate, app-422, and an intumescent mixture of additives known by the name exolit 740, both supplied by clariant, ath from hüber and a liquid fire retardant additive, dimethyl propyl phosphonate (dmpp), supplied by lanexess. materials used for the preparation of various laminates are given in table 1. table 1 composition of additives added to the bio-polyester resin additive composition 1 caco3 calcium carbonate, non fr additive, average particle size 10.5 �m 2 app fr422 ammonium polyphosphate (phase ii), average particle size 17 �m 3 ath aluminium trihydrate with average particle size 15 �m 4 exolite 740 intumescent system based on ammonium polyphosphate with average particle size 8–12 �m 5 dmpp dimethyl propyl phosphonate composite laminates were prepared by hand lay-up, see figure 1. hand lay-up is the simplest open moulding method of the composite fabrication processes. reinforcing mat or woven roving was positioned manually in the open mould, and resin was brushed over and into the flax plies. entrapped air was removed manually with rollers to complete the laminate’s structure. the laminates were prepared using 4 plies of flax twill impregnated with the polymeric resin. the fire retardant additives were previously added to the bio-polyester resin and mixed with a high speed mixer for 5–10min. the theoretical fibre volume fraction of these laminates is in the 40–50% range. composite laminates were obtained by compression moulding of resin impregnate flax plies in a heated press. flax fabric impregnated with the resin was cut into 10cm × 50cm rectangles, and staked unidirectionally into a 3mm height steel mould with release 70 n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market (a) (b) (c) fig. 1. images a) and b) taken during hand lamination of 2 × 2 flax twill/bio-polyester resin, image c) taken after curing. agent. the thermal curing cycle was: 60min at 120◦c. the dimensions of each laminate were 10 ± 0.5cm × 50 ± 0.5cm × 3 ± 0.3cm. 2.2. characterization the fire performance was measured with a cone calorimeter (fire testing technology, uk) following (iso 5660–1:2002. reaction-to-fire tests, 2002). in the present study all tests were conducted in the horizontal orientation. the cone calorimeter heat flux was set at 50kw/m2, which relates to the more severe fire tests for building products (babrauskas, 1995). ignition was spark induced and specimens were run without a retainer frame. specimen size was 100 mm2 × 100 mm2 and 3–3.3mm thick. the sample was insulated at its back surface using a 46mm thick ceramic paper in order to reduce heat losses as prescribed in the iso 5660 standard. the reported results are average values obtained from testing three specimens. the given tolerances are standard deviation of three results. the important parameters used to assess fire performance are as follows (bakhtiyari, taghi-akbari, & ashtiani, 2015): heat release rate (hrr), time to ignition (tti), total heat release (thr, the integral of the heat release rate), peak of heat release rate (phrr), and the maximum average of heat emission (marhe). thermal gravimetric analysis was conducted using tga q500 machine. samples weighing approximately 10mg were subjected to pyrolysis in nitrogen and air environment to a maximum temperature of 900 ◦c at a heating rate of 10◦c/min. the weight loss was recorded in response to increasing temperature, with final residue yield on set of degradation temperature and number of degradation steps reported. viscosity was measured using a brookfield viscometer equipped with a low volume kit for measurements (model lvf, spindle no. tr10). all tests were run triplicate (three specimens) and the results expressed as the obtained average. 3. results polymer combustion is not a simple and easily understandable process; it occurs as a series of coupled events. the polymer is first heated to a temperature at which it starts to decompose and yields gaseous products that are usually combustible. these products then diffuse into the flame zone above the burning polymer. if there is an ignition source, they will undergo combustion in the gas phase and liberate more heat. under steady-state burning conditions, some of the heat is transferred back to the polymer surface, producing more volatile polymer fragments to sustain the combustion cycle. flame retardants work to stop or delay fire, but depending on their chemical composition, they interact at different stages of the fire cycle. app and ath can provide flame n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market 71 retardant properties by various mechanisms in either condensed or gas phase (schartel, 2010). app422 contains a high percentage of nitrogen and phosphorous which, in the presence of a heat source or fire, acts as a shield coating precursor because of the formation of a continuous cross linked vitreous phase called ultraphosphate (shen, stahlheber, & dyroff, 1969). on the other side, ath starts to break down in the temperature range of 180–200◦c, releasing water in the vapour phase of combustion. the elimination of water results in an endothermic reaction, removing heat from the substrate, and thus, fewer pyrolysis products are formed. the water vapour liberated has a diluting effect in the gas phase and forms a protecting layer over the condensed phase hindering the access of oxygen to the substrate (camino & luda, 1998). however, in spite of all the advantages of using natural fibres in composites and more specifically flax fibres, when exposed to a heat source their presence causes an early ignition because the fibres’ thermal degradation starts at a lower temperature (∼210 ◦c) than that of the bio-polyester (>250 ◦c), as it can be seen from the thermogravimetric curves and first derivative of the weight loss curve (dtg curves) shown in figure 2. 120 (a) (b) 100 80 60 w e ig h t l o ss ( % ) w e ig h t l o ss ( % ) d e ri v. w e ig h t (º c /m g ) d e ri v. w e ig h t (º c /m g ) 40 20 0 25 125 225 325 425 temperature (ºc)temperature (ºc) 525 625 72525 125 225 325 425 525 625 725 825 0.0 0.5 1.0 1.5 2.0 2.5 0 1 2 120 100 80 60 40 20 0 fig. 2. tga (dashed lines) and derivative tga (dtg) curves (full line) of neat bio-polyester resin (red) and flax fibre (blue); a) in air and b) in nitrogen. in air atmosphere (fig. 2a), flax fibre shows an initial weight loss starting at 60◦c, having a maximum at 80–90◦c associated to the loss of the moisture content; it represents about 5% in weight (van de velde & kiekens, 2002). in flax bio-composites, the total elimination of water is not possible because of the hydrophilic characteristic of the fibre. next, it can be noted that a shoulder appears between 325 and 400◦c that is associated to the thermal depolymerization of hemicelluloses and pectin. in an oxidative atmosphere there is partial overlapping of this peak with the exothermic peak corresponding to the decomposition of the �-cellulose at 340◦c (wielage et al., 1999; van de velde, 2002). a third weight loss step occurs at 450◦c, which in a thermooxidative atmosphere leads to the complete degradation of the carbonaceous material. regarding the degradation of bio-polyester, it is evident from figure 2a that there are also more than one peak occurring between 225 and 625◦c in the dtg curve, which shows that the degradation is a multi-stage process. the first process started just before 250◦c and reached its maximum rate at 350◦c. the second weight loss process developed later and showed a higher maximum rate of decomposition at approximately 550◦c. the last stage leads to a complete degradation of the material. this behaviour is determined in the initial stages by chain scission reactions followed by 72 n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market the formation of free radicals that promote further decomposition and scission of the polyester backbone. (ricciardi et al., 2012). in nitrogen, (fig. 2b), it can be noticed that the flax fibre decomposition is also a three-stage process characterized by a first step in the temperature range 30◦c–90◦c associated to the loss of moisture content, followed by a second decomposition step located in the range 225◦c–400◦c and characterized by a mass loss of 79 wt% and a shoulder at 420◦c with a mass loss of 7 wt%. the final residue of the flax fibre is 10 wt%. in the case of the resin, the decomposition follows a two-stage process characterized by a main decomposition peak centred at 364◦c, with a mass loss of 95 wt%. these two peaks are very close corresponding to the scission of the polystyrene bonds and to the oxidation and the breakage of the secondary bonds. the high temperature decomposition peaks observed in the tga carried out in air are missing. in nitrogen decomposition reactions were much slower, which resulted in higher temperatures for peak 2 in comparison to air measurements. reactions do not only happen much faster in air, but the actual reactions may also differ (e.g., the formation of hydroperoxide in air and not in nitrogen). when tested in air, flax fibre and resin are found to leave no residue at temperatures above 600◦c, indicating that all products of decomposition have been oxidized, whereas in nitrogen nearly 5–10% of carbonaceous residue remains. so, taking into consideration these results the combination of fire retardant additives selected should decompose at lower temperatures than those of the fibre and polymer in order to maximize fire retardant protection performance of the flax fibre bio-composites compared to unprotected bio-composites. the flammability characteristics of flax bio-polyester composites were analysed by cone calorimeter; samples were run in triplicate to minimize experimental error. the results of maximum peak heat release rate, total heat release rate, ignition time, and maximum average rate of heat emission, are summarized in table 2. it has been shown that the ignition time represents the flame-spread process surprisingly well (babrauskas, 1987). the shorter the ignition time, the easier the material ignites and the flame spreads on the surface of the material with a higher velocity. good repeatability was seen for the tti results for the three tested specimens per formulation. fire retardant bio-composites exhibit larger tti than that of the unfilled laminate. a marked improvement can be seen in the ath/app table 2 the maximum rate of heat emission (marhe), the peak heat release rate (phrr), total heat release (thr) and ignition time of the fr bio-composites [fr] thickness marhe thr phrr ti(s) content∗ (mm) (kw/m2) (mj/m2) (kw/m2) f0 – – 2.78 297 52.5 586 17 f0b caco3 50 2.82 290 51.5 575 18 f1 app fr422 25 2,66 258 48.3 559 15 f2 app fr422 50 2,60 210.9 48.8 419.8 19 ath f3 app fr422 175 2.60 131,50 39.0 314,06 41 ath f4 app fr422 185 2.70 203.2 40.7 452.1 21 ath dmpp f5 exolite 740 >75 2.65 99,33 38 134,4 19 ∗formulation total fr content is specified in percentage relative to the total amount of resin. n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market 73 filled formulations. the f0 specimen had a tti of 17 seconds and combusted violently with large flames during testing. as the loading of app/ath increases, the tti was prolonged reaching a maximum value of 41s (f3). this may be due not only to the barrier effect of the fire retardant additives, but also to the fact that there is a lower amount of natural fibres exposed at the surface. the primary result from the cone calorimeter test is the measurement of heat energy released over a specific unit of time – that is the heat release rate. a high heat release rate translates to rapid temperature changes, faster flashover times and an increase in the production of the products of combustion. after ignition, surface temperature and the hrr rapidly rises and reaches its peak value, which depends upon the type of the material and its combustible content. the average and peak values of hrr of the burned specimens are given in table 2. the combustion of the reference sample (f0) which does not contain fr additives is the most exothermic with 52.5 mj/m2 and 586kw/m2 as thr and phrr respectively. the addition of app even at a low concentration, 25 phr, reduced the phrr to 559kw/m2. these results can be explained by a combination of two effects: as more inorganic filler is introduced into the resin, less resin is available for combustion and to the shielding effect of the ultraphosphate layer formed during the combustion. the introduction of ath to that formulation, with an overall fire retardant content of 50 phr lead to a considerable reduction in its flammability with up to a 25% reduction in the thr and phrr values. as the ath loading increases from 25 to 125 phr (f2 and f3) the phrr decreases from 419.8 to 314.1kw/m2, and the thr from 48.8 to 39 mj/m2 respectively. the results show that the increase of ath concentration in combination with app has a very positive effect in reducing both the phrr and thr values. this was thought to be due to a combination of three effects; (i) fuel replacement effect, (ii) the ath endothermic water cooling effect which contributes to reduce the combustion of the specimen. this was evident from the phrr values of the samples f3 with ath and app and the sample f0b with 50 phr caco3, which were 419.8 and 586kw/m2 respectively and (iii) when ath and app reacts, produce a more thermally stable p–al–o surface layer instead of the less thermally stable p–o layer formed in specimens loaded only with app. results from specimens f3 and f2 are a good example of that behaviour. as the fire retardant loading increased from 25 wt% to 175 wt%, a 35% enhancement on the flame retardancy is clearly observed through the reduction on the phrr, thr and increase tti values. however, addition of dmpp did not produce any positive effect on the phrr and thr. in fact, phrr and thr increases to 452kw/m2 and 41kw/m2, with respect to the formulation f3. next, an intumescent fr blend of additives with synergists for unsaturated polyester resins, exolite 740, was added to the resin formulation. the f5 system reached the lowest phrr and thr among all the samples. phrr and thr values, which were 134.4kw/m2 and 38 mj/m2 respectively, are approximately 80% and 50% lower than the unfilled laminate. compared to the fire retardant compositions f1, f2 f3 and f4, it also has lower phrr and thr values which are 75% and 2% lower for f1 and 57% and 20% lower for f3 respectively. that intumescent fire retardant additive is a mixture of different additives that work together under fire conditions to form a protective barrier (carbon foam) that “rises up in response to heat”, protecting more efficiently the underlying substrate. the addition of fire retardant additives to the bio-polyester resin has shown that it is feasible to produce bio-composites with lower thr and phrr and higher time to ignition than the unprotected sample. however, whilst phrr, thr and tti are good indicatives of the fire retardant performance of materials, one of the most important factors in evaluating fire retardant behaviour of materials is the marhe, a parameter directly linked to the heat level given off. it can be considered as a good measurement of the tendency for a fire development in the case of a real situation. according to literature, materials with marhe values of about 60–75kw/m2 would be equivalent to class b materials according to une-en 13501-1, which is the european standard for evaluating reaction 74 n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market fig. 3. the effect of fr additive, type and concentration in marhe values of flax bio-composite laminates. to fire behaviour of construction products. figure 3 illustrates the effect of fire retardant fillers on reducing the marhe value for the protected specimens. all the fire retardant systems investigated display lower marhe values compared to the unprotected system. even though flammability of these bio-composites has been successfully reduced through the incorporation of several fr additives, the marhe values are still above the estimated value needed to achieve euro classification b. however, current normative for the railway market (une 45545, railway applications – fire protection on railway vehicles –, 2005) mention a maximum marhe of 90kw/m2 for certain applications which would open up new possibilities for formulation f5. the combination of app and ath (f3 sample) leads to a notable reduction on the marhe value (55%) with respect to the unfilled sample, but addition of dpp (f4) to the resin did not show any further improvement but worsens the result compared to f3. samples containing exolite 740 performed the best, with marhe values up to 66% lower than those corresponding to the reference sample. on the other hand, samples containing only app performed the worst with marhe values similar to the unfilled sample. finally, resin viscosity is a key parameter from a processing point of view and it was taken into account during the preparation of the different fr formulations. in fibre reinforced polymer composite manufacturing, fibre wetting by the resin is of paramount importance. if the wetting is not good, voids occur at interface between the two that result in polymer composites most susceptible to failure. fibre wetting is directly related to chemical compatibility between resin and fibres and resin viscosity; thus, lower values will result in easier production and improved quality of the product. brookfield viscosity, see figure 4, increases upon addition of either ath or app to the resin. formulations containing only app showed a larger increase in viscosity than formulations containing both app and ath. it is important to keep in mind that formulations with ath also contain a low viscosity dispersing agent from byk which would contribute, not only to improve ath dispersion within the resin, but also to reduce the viscosity of the mixture. in this work, 175 phr of fr’s was the maximum concentration achievable which was considered processable by either resin infusion or pultrusion. surprisingly, formulation f4, with an additional 12 wt% dmpp (liquid fr additive, η = 250cps) did not show any viscosity reduction compared to f3. that result suggests that some unwanted reaction might take place between dmpp and the resin leading to an increase in the viscosity. formulation containing exolite 740 has the largest viscosity but at least theoretically it is still possible to pultrude it accordingly to the resin viscosity range established by several authors that goes from 500–3000 cps. n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market 75 fig. 4. brookfield viscosity measurements at 25◦c. 4. conclusions aiming at the production of more sustainable low flammability composites, “greener” non-toxic flame retardants additives were added to commercial unsaturated bio-polyester resin and combined with flax fibres. this study has shown that enhancing the fire retardant performance of flax biopolyester composites can be done through the proper combination of fr additives. it was possible to identify several processes that take place in the fire retardant bio-composite laminate during the exposure to a fire/heat source; (i) on one side, water moisture vapour from the decomposition of app and ath leads to a reduction of combustible gas concentrations slowing down the burning rate of the laminate, (ii) incorporation of high amounts of fr additives (50 to 150phr) results in a reduction of the amount of burnable material, and consequently enhanced fr performance, and finally (iii) thermal decomposition of app and ath produces a protective layer at the surface blocking the available oxygen and delaying flame propagation by adiabatic effect. main results can be summarized as follows; the use of ath in combination with app imparts an improved flame retardant effect on the laminate. a combination of both fr’s showed a decrease in the peak heat release rate, total heat release and marhe values. reductions of up to 55% are achieved with loading degrees of 175phr. however, the most promising result was obtained with exolite 740, the synergistic behaviour among app and other intumescent additives present in the formula, produces a large additional reduction in the phrr, thr and marhe without affecting to its processing performance. on the contrary, a further increase on the concentration of app/ath does not justify its use due to the resultant difficulties with increased viscosity, which will result in major processing issues in adopting these materials in fibre reinforced composites. acknowledgments the research leading to these results has received funding from the european community’s seventh framework programme (fp7) under grant agreement no. 609067 (osyris forest based composites for façades and interior partitions to improve indoor air quality in new builds and restoration) and grant agreement no. 285689 (biobuild – high performance, economical and sustainable biocomposite building materials). the authors also thank s. neira and s. allue from gaiker for carrying out the cone calorimeter tests. 76 n. u. loredo and j. s. bermejo / enhanced flame retardancy of flax bio-composites for the construction market references aziz, s. h., ansell, m. p., clarke, s. j., & panteny, s. r. (2005). modified polyester resins for natural fibre composites. composites science and technology, 1(3-4), 525–535. babrauskas, v., & parker, w. j. (1987). ignitability measurements with the cone calorimeter. fire and materials, 11(1), 31-43. babrauskas, v. (1995). speciman heat fluxes for bench-scale heat release rate testing. fire and materials, proceedings of interflam, 19(6), 243-252. bakhtiyari, s., taghi-akbari, l., & ashtiani, m. j. (2015). evaluation of thermal fire hazard of 10 polymeric building materials and proposing a classification method based on cone calorimeter results, fire and materials, 39(1), 1-13. bismarck, a., mishra, s., & lampke, th. (2005). plant fibres as reinforcement for green composites. in ‘natural fibers, biopolymers and biocomposites. boca raton: taylor and francis, 37-108. camino, g., & luda, m. (1998). mechanistic study on intumescence. fire retardancy of polymers: the use of intumescence. cambridge: royal society of chemistry. dorez, g., taguet, a., ferry, l., & lopez-cuesta, j. m. (2013). thermal and fire behavior of natural fibers/pbs biocomposites. polymer degradation and stability, 98(1), 87-95. el-sabbagh a., steuernagel l., ziegmann g., meiners d., & toepfer o. (2014). processing parameters and characterisation of flax fibre reinforced engineering plastic composites with flame retardant fillers. composites part b: engineering, 62, 12-18. goutianos, s., peijs, t., & skrifvars, m. (2006). development of flax fibre based textile reinforcements for composite applications. applied composite materials, 13(4), 199-215. hapuarachch, t. d., & peijs, t. (2009). aluminium trihydroxide in combination with ammonium polyphosphate as flame retardants for unsaturated polyester resin. express polymer letters, 3(11), 743-751. hull, t. r., witkowski, a., & hollingbery, l. (2011). fire retardant action of mineral fillers. polymer degradation and stability, 96(8), 1462-1469. iso 5660–1:2002. reaction-to-fire tests, h. r. (2002). reaction-to-fire tests, heat release, smoke production and mass loss rate. part 1: heat release rate (cone calorimeter method). international standard organization. van de velde, k., & kiekens, p. (2002). thermal degradation of flax: the determination of kinetic parameters with tga. journal of applied polymer science, 83(12), 2634-2643. kandare, e., chukwunonso, a. k., & kandola, b. k. (2011). the effect of fire-retardant additives and a surface insulative fabric on fire performance and mechanical property retention of polyester composites. fire and materials, 35(3), 143-155. lazko, j., landercy, n., laoutid, f., dangreau, l., huguet, m. h., & talon o. (2013). flame retardant treatments of insulating agro-materials from flax short fibres. polymer degradation and stability, 98, 1043-1051. levchik, s. v., levchik, g. f., camino, g., & costa, l. (1995). mechanism of action of phosphorous-based flame retardants in nylon 6. ii. ammonium polyphosphate/talc. journal of fire sciences, 13(1), 43-58. ricciardi, m., antonucci, v., giordano, m., & zarrelli, m. (2012). thermal decomposition and fire behavior of glass fiber–reinforced polyester resin composites containing phosphate-based fire-retardant additives. journal of fire sciences, 30(4), 318-330. matkó, s., toldy, a., keszei, s., anna, p., bertalan, g., & marosi, g. (2005). flame retardancy of biodegradable polymers and biocomposites. polymer degradation and stability, 88, 138-145. schartel, b. (2010). phosphorus-based flame retardancy mechanisms—old hat or a starting point for future development? materials, 3(10), 4710-4745. schartel, b., braun, u., & reinemann, s. (2003). fire retardancy of polypropylene/flax blends. polymer, 44(20), 6421-6250. shen, c. y., stahlheber, n. e., & dyroff, d. r. (1969). preparation and characterization of crystalline long-chain ammonium polyphosphates. journal of the american chemical society, 91, 62-67. troitzsch j. (2004). plastics flamability handbook: principles, regulations, testing and approval. munich: hanser publishers. (2005). une 45545, railway applications – fire protection on railway vehicles –. wielage, b., lampke, th., marx, g., nestler, k., & starke, d. (1999). thermogravimetric and differential scanning calorimetric analysis of natural fibres and polypropylene. thermochimica acta, 337(1-2), 169-177. journal of facade design and engineering 3 (2015) 237–252 doi 10.3233/fde-160043 ios press 237 simulation study on energy conservation performance for integrated external louver facades yutaka misawaa,∗, keisuke azumab, wanghee choc, shizuo iwamotoc and mamoru iwatac aove arup & partners japan limited, tokyo, japan bcourse of architecture and building engineering, graduate school of engineering, kanagawa university, kanagawa, japan cdepartment of architecture, faculty of engineering, kanagawa university, kanagawa, japan abstract. the authors previously proposed integrated-facade-systems comprising different types of external louvers integrated with buckling restraint braces. the systems can be applied to both new and existing buildings, paying attention to the building’s appearance, upgraded structural performance and indoor environment. the authors have studied indoor daylight conditions with louver facades and a research paper was published in this journal (misawa, hikone, nakamura, iwamoto, iwata, 2014). this research study reports on the energy conservation performance of louver facades and proposes an external shading coefficient (esc) for external louver systems. situations with five external louver facades and no louvers, facing three facade orientations (east, south and west) are examined. the year-round esc is simulated every hour using radiance software for transient solar radiation calculations. furthermore, by using the proposed esc, case studies for typical japanese offices are carried out to verify reduction effects on the annual thermal load and electrical energy consumption of an air source heat pump for each louver set-up. simulation results are calculated in hourly increments using trnsys software; the results show that both thermal loads and electrical energy consumption are reduced regardless of louver types and facade orientations when integrated external louver facades are installed. keywords: facade, external louver system, sun shading, office building, annual thermal load, electrical energy consumption 1. introduction in recent years, it is becoming increasingly important not only to replace buildings, but also to renovate and improve the performance of existing buildings. this is particularly true in japan, where building lives are short, and it is expected that extending a building’s use can reduce materials consumption for new construction, waste from demolition, and co2 emissions. furthermore, there are existing buildings that do not satisfy the latest earthquake-resistance standards and that have to be refurbished. in this situation, structural reinforcements are often added in front of the building’s facades, as it is appropriate to focus on the building’s exterior skin for anti-seismic strengthening. however, many of those reinforcements are designed only from the perspective of structural performance, and they have a negative effect on the overall appearance of the building and its surrounding environment. because the facade is an important aspect that contributes to the urban landscape aesthetics, as well as influences the energy consumption of the building, an integrated design of seismic reinforcement with the facade is preferred. ∗corresponding author: yutaka misawa, ove arup and partners japan limited, 3f tobu fuji building 24-4 sakuragaokacho shibuya-ku tokyo 150-0031, japan. tel.: +81 368616320; fax: +81 334761377; e-mail: yutaka.misawa@arup.com. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:yutaka.misawa@arup.com 238 y. misawa et al. / simulation study on energy conservation performance fig. 1. configuration of bucking restrained brace. an integrated louver facade is a solution that addresses issues of appearance, structural performance and energy consumption. it consists of louvers and buckling-restrained-braces (brb), and can be applied in new construction and in refurbishment (takeuchi, koyano, & iwata, 2005). figure 1 shows the brb configuration. this brb is the same as in the authors’ previous research (kaneki, hikone, yamashita, & iwata, 2008; takeuchi, koyano, yasuda, yuasa, & iwata, 2006a; takeuchi, yasuda, yuasa, okayama, miyazaki, & iwata, 2006b), however, the summary is described again in the following sections. the brb has two main components. one is a cross-shaped steel plate and the other is a steel mortar plank, which serves to keep the core steel plate from buckling. the brb is a structural element that manages both tension and compression forces. generally, bracing elements used for structural retrofit are usually ‘x’ or ‘v’ shaped because they are designed only to take tension forces. however, a diagonally arranged bracing system is possible when brbs are installed. hence brbs have the potential to integrate with facade elements as diagonally arranged louvers, camouflaging brbs as a structural intervention. brbs do not need to be installed in all the diagonal louver elements. one or two are deployed in one beam and one pillar span, as required for structural performance. as reinforcing elements, brbs require minimal materials, just core steel plates and steel mortar planks; therefore, they can readily harmonize with facade components. for this reason, it is possible for brbs to become an aesthetic and environmental feature like external louvers, in addition to being structural elements. this study supports brbs as multipurpose environmental devices/structural elements and proposes diagonally arranged louvers as one of the options for integrated external louver facades. figures 2 and 3 show the components and the environmental appearance of facades. external louvers reduce energy consumption by shading windows and external walls from the sun in summer and by allowing the solar radiation to penetrate into the room in winter. they also improve illuminance distribution, remove glare by blocking high brightness areas, and improve the indoor thermal environment. fig. 2. composition of integrated louver facade. y. misawa et al. / simulation study on energy conservation performance 239 fig. 3. environmental aspect of louver facade. the authors have studied the arrangement of the louvers and the position of the brbs, and assessed them from the point of view of appearance, structure, indoor daylight influence and cost (nakamura, hikone, misawa, iwamoto, & iwata, 2010). although previous research has been conducted on the shading effects of louvers in front of windows, especially in summer, there are few studies considering year-round shading effects of louver facades covering both windows and external walls. there is also little research that demonstrates the effects of louver types and building orientations. for refurbishment, it is necessary to apply a new facade in various arbitrary orientations, because louver and brb installation is based on existing facades. therefore, to reduce thermal loads, it is important to understand the impact of the louvers’ arrangement and the building’s orientation on the shading effect and to conduct an assessment based on year-round environmental changes. this study quantifies over time the shading effects for different building orientations and louver arrangements, and assesses the amount of solar radiation reduction throughout summer (from june to september) and winter (from december to march). the study proposes a method to represent the louvers’ shading effect in a simulation of annual thermal load and energy consumption of an air source heat pump (ashp), using trnsys. this study goes further, and compares simulation results on an office building with different orientations and louver arrangements. 2. outline of daylight simulation the method and results of solar radiation simulation are the same as in the authors’ previous research (kubo, hikone, misawa, sato, fukazawa, iwamoto, & iwata, 2011); however, the summary is provided again in the following sections. proposed external shading coefficient (esc) in section 2.3 is different from solar heat gain coefficient. louver facades are installed not only outside of windows but also external walls. in this research, it is necessary to consider solar shading effects of louver facades for windows and external walls. the esc is a ratio of solar radiation reaching windows and external walls of louver facades. after the esc calculation, the cumulative amount of solar radiation of louver facades is calculated using the esc. figure 10 in section 3.2 shows the results of the cumulative amount of solar radiation of louver facades. these are calculated by thermal simulation using trnsys software. heat gains from windows and external walls (transmission, absorption and reflection), were calculated with integrated tools in trnsys software. 2.1. radiance set-up condition radiance, a software program for analyzing lighting environment, was used to simulate the annual amount of daylight. figure 4 shows the flow of the simulation. as the monte carlo method is used 240 y. misawa et al. / simulation study on energy conservation performance fig. 4. simulation flow. to calculate inter-reflection, more accurate results are expected to be obtained by increasing the number of daylight rays. therefore, the number of daylight rays was set to the maximum value allowed in the program, at 4,096, and the number of reflections was set at eight (8), also the highest level allowed in the program. additionally, other parameters were set to the maximum allowable level in each setting range. reflectivity was set at 68.8%, which was the same value used in the authors’ previous daylight research (misawa, hikone, nakamura, iwamoto, & iwata, 2014). 2.2. application of all sky model-r radiance implements cie-standard-clear-sky, cie-standard-overcast-sky, intermediate sky, and uniform sky. igawa proposed the all sky model-r as a sky-model that expresses a time-oriented luminance distribution in the sky (igawa, matsuzawa, & koga, 2004). this study used the all sky model-r to obtain the sky radiance distribution, and reflected the result into radiance (yoshizawa, kobayashi, inanuma, & takeda, 2005). to set climate conditions for the all sky modelr, the expanded amedas meteorological data in standard years (from 1981 to 2000) was used. figure 5 shows the result for an overcast sky (3rd, january 12:00) and a clear sky (11th, january 12:00). y. misawa et al. / simulation study on energy conservation performance 241 fig. 5. all sky model-r (w/sr m2). fig. 6. measuring method. 2.3. external shading coefficient this study suggests and introduces an esc in order to define the shading effect of louver facades to universally calculate thermal loads. figure 6 shows the method for measuring the effect of external shading. daylight measurement points were arranged in a matrix at intervals of 20mm on an external wall and a window with dimensions of 950mm wide by 1.800mm high. the esc is defined in formula (1) in which ‘lo’ represents the amount of daylight that reaches the outside surface of the non-louvered facade, and ‘lr’ expresses that with louver facades. note that, in this study, the calculation did not consider any temperature rise of the louver facades due to the solar radiation and re-emission to the indoor space. esc = lr/lo (1) louver members were installed in the same positions as with structures for anti-seismic reinforcement, and the distance between the window/external wall and louver members was set at 200mm as shown in fig. 6. 242 y. misawa et al. / simulation study on energy conservation performance fig. 7. louver arrangement. 2.4. louver outline figure 7 shows the arrangement of the louvers. based on the size of the brb, the regular size of louver members was set to 100mm thick by 250mm deep. the size of the unit was determined by the distance between columns, 4.5m, and the floor height, 3.8m. louver members were assembled in frames giving five types: horizontal, vertical, right-up diagonal, right-down diagonal, and ‘bound’. to maintain the view from the interior, each louver’s opening rate was set to approximately 75%. the ‘bound louver facades’ proposed by the authors are attached randomly to a frame that is fitted between the columns of an existing building for anti-seismic reinforcement. the random alignment of louver members is intended to effectively block solar radiation from all orientations. the authors proposed the geometry of the ‘bound louver facade’ by studying a prototype that was made to optimize the constructability of the facade, the transportability of a unit, and the number of joints between the louver members (takeda, hikone, misawa, & iwata, 2010). in the integrated louver design, brbs can be louvers as well as reinforcements, creating ‘integral types’ of diagonal and ‘bound’ louver facades. however, horizontal and vertical louvers cannot be brbs; they are called ‘separated types’ and cannot offer bracing. in this study, ‘separated types’ were made without brbs, and the same opening rate was used in the frames to compare the shading effect of each louver arrangement. figure 8 shows the concept of ‘opening rate’. it was calculated using the external dimensions of the unit area, excluding the louver member and the frame width. 3. calculated result of the amount of solar radiation 3.1. hourly fluctuation of the esc the esc is zero when windows and external walls are entirely covered by louvers and is 1 when windows and external walls have no louvers. the esc before dawn and after sunset is 1, because there is no solar radiation. figure 9 shows the average values of calculated esc against time between 5 a.m. and 6 p.m. in summer (from june to september) and in winter (from december to march). the esc did not change for eastward orientation in the afternoon nor west-facing before noon, since direct solar radiation did not reach the building surface in those cases and only global radiation was measured. y. misawa et al. / simulation study on energy conservation performance 243 fig. 8. opening rate. it is assumed that when windows and external walls do not receive direct solar radiation, the external shading effect is not determined by the louver’s arrangement, but by its opening rate. horizontal louver facades achieved a minimum-shading coefficient in summer for south-facing surfaces; however, they may not be appropriate for east and west-facing surfaces. some shading against incident solar radiation was expected in winter, as the esc is larger in winter than summer. it was observed that in southerly orientations, the values did not change much with time, and the esc tended to be large in winter, just as in east and west orientations. for vertical louver facades, little change was observed in summer for east and west orientations. in winter, values tended to change widely depending on time of day. south-facing surfaces had a large change of shading values around noon especially in winter, and it is suspected that the thermal load would increase during the insolation cycle. for diagonal louver facades, the esc was relatively small for right-up direction in east-facing direction and by right-down in west-facing. in south, the effectiveness of the louvers switched from right-down to right-up around noon. esc value change behaviour for bound louver facades was identical to that for diagonal ones, and transitioned between characteristics associated with right-up and right-down types. it was confirmed that the bound louver facades are comparable to other types of louver facades in their shading effect. overall, in summer there was a smaller difference between different louver arrangements compared to winter. in winter, the esc showed larger values generally when the solar altitude was low and solar radiation was expected to reach the indoor space. additionally, all the types of louver facades showed large changes over time and a large difference was observed between different louver arrangements in winter. 3.2. comparison of the amount of solar radiation in order to compare the amount of solar radiation transmitted, the authors simulated the amount of vertical global radiation in each orientation and for each instant of time in a tokyo standard year. then using the values, the cumulative amount of solar radiation was calculated by multiplying the esc by these values. figure 10 shows the cumulative amount of solar radiation in six cases: in one case the facade has no louvers, while in the other cases the facade has five different arrangements of louvers. summer (from june to september) and winter (from december to march) cases are covered. for eastward orientations, all louver arrangements had a greater solar radiation reduction effect in summer compared with winter. in summer, horizontal louver facades reduced the solar radiation 244 y. misawa et al. / simulation study on energy conservation performance fig. 9. comparison of the esc. y. misawa et al. / simulation study on energy conservation performance 245 fig. 10. cumulative amount of solar radiation. by 66%, the largest amount of reduction; vertical louver facades reduced the solar radiation by 62%, diagonal and bound louver facades reduced it by 55%. in winter, the diagonal (right-down) louver facades reduced solar radiation by 47%, the lowest reduction value, and the vertical louver facades reduced it by 66%. horizontal louver facades reduced solar radiation by 60% and the diagonal (right-up) and bound louver facades reduced it by 50%. in south-facing situations, all louver arrangements demonstrated less reduction in summer than in winter. in summer, the horizontal louver facades achieved the largest reduction of 66%, while the vertical louver facades achieved 62% and the diagonal/bound louver facades 55%. in winter, the diagonal (right-down) louver facades showed the lowest reduction effect of 47%, while the vertical louver facades reduced solar radiation by 66%, the horizontal louver facades, 60%, and the diagonal/bound louver facades by 50%. the shading effect was large in winter, and it was suspected that installing louver facades would cause an increase in the heating load. in west-facing orientations, as was the case with the east-facing, all louver facades had a greater reduction effect in summer than in winter. in summer, the effect of the horizontal louver facades was 66%, the largest reduction of all types of louver facades. the effect of the vertical louver facades was 62% and that of diagonal/bound louver facades was 55%. in winter, the effect of the diagonal (right-down) louver facades was 47%, the lowest value, while the vertical louver facades had the highest value, 66%. the shading effect of the horizontal louver facades was 60% and that of the diagonal (right-up) and bound louver facades was 50%. 4. simulation outline of thermal load and electrical energy consumption 4.1. simulation method in order to simulate the annual indoor thermal load taking shading effects into account, trnsys ver.16, a dynamic heat simulation program for multiple rooms, was used. 4.2. objective building this study undertook a simulation using the japanese standard office model (the model) proposed by takizawa (takizawa, 1985), subject to renovation. figure 11 and table 1 show a standard floor plan of the model together with assumptions concerning material composition. the model was created with reference to standard issues for office proposed by the environmental committee of the architectural institute of japan. some u-values in table 1 are lower than given in the latest building specifications, but these values were used in this study because the model (proposed about thirty years ago) is referenced in many environmental research studies as using these material 246 y. misawa et al. / simulation study on energy conservation performance fig. 11. standard floor plan of the model (unit mm). table 1 material composition of the model part material composition u-value <material (thickness, mm) form interior side> (w/m2k) office floor/ceiling rock wool board (acoustic absorbent, 12), gypsum board (9), concrete (150), tile (3) 1.92 roof rock wool board (acoustic absorbent, 12), gypsum board (9), form polystyrene (25), concrete (150), asphalt (10), lightweight concrete (60) 0.74 exterior wall gypsum board (12), form polystyrene (25), mortar (20), concrete (150), mortar (20), tile (8) 0.95 interior wall mortar (20), concrete (120), mortar (20) 3.54 window single glass (3) 5.72 machine room floor/ceiling glass wool (acoustic absorbent, 25), concrete (150) 1.15 roof glass wool (acoustic absorbent, 25), form polystyrene (25), concrete (150), asphalt (10), lightweight concrete (60) 0.58 exterior wall glass wool (acoustic absorbent, 25), mortar (20), concrete (150), mortar (20), tile (8) 1.11 interior wall glass wool (acoustic absorbent, 25), mortar (20), concrete (120), mortar (20) 1.14 others door metal door with insulation defined in trnsys 1.67 entrance single glass (8) 5.53 exterior wall of stairs and restrooms mortar (20), concrete (150), mortar (20), tile (8) 3.23 y. misawa et al. / simulation study on energy conservation performance 247 compositions. tokyo was selected as the target location. the eight story high building embodied a reinforced concrete structure. as almost all existing buildings require seismic retrofitting and their windows face many different orientations, the windows in the model are on all elevations and represent 30% of the external wall area of the office. 4.3. conditions of calculation model simulation conditions are outlined in table 2. the standard year’s weather data from the expanded amedas weather data (1981∼2000, tokyo) was used to define meteorological conditions. after conducting fore-flow calculations for a month, calculations were made for every hour. 4.3.1. indoor heat generation (1) human heat generation heat generation from the human body was set based on information provided by the shase handbook 14th vol.1 fundamental (the society of heating, air-conditioning and sanitary engineers of japan, 2010). for the model, the heat generation was calculated at 121w/person (sensible heat and latent heat were noted for each month) at an occupancy rate (0.1 person/m2), and seated light work, typing. (2) lighting heat generation lighting heat generation for the model was set at 20w/m2, referring to the standard office lighting energy consumption provided by ibec (institute for building environment and energy conservation, 2010). (3) heat generation from equipment for the model, the heat generation from equipment was set at 20w/m2. 4.3.2. schedule of air-conditioning air-conditioning in the model operated from 7 a.m. to 9 p.m. on weekdays and did not work on saturday and sunday. time schedule was set for human, lighting and equipment percentages of heat generation in table 2. there were no special other controls. heat generation was the same in all cases. the cooling and heating period was set as constants throughout the year. in the comparison study of louver facades, loads caused by air leakages and ventilations were the same value in all cases. this study did not count the loads in the comparison of the results. 4.3.3. settings for air source heat pump (ashp) and indoor air temperature control this paper calculated the electrical energy consumption of an ashp. the specification of selected air conditioners is given in table 3. generally, cop is given in suppliers’ catalogues, but it is only a rated value, not the actual cop in application. ueno proposed a calculation method for the actual cop of working air conditioners. the actual cop was calculated using the heat source model. the proposed method of calculating actual cop does not depend on any individual device (air conditioner) characteristics, and has been verified through many experiments. this calculation method can be applied generically to any air conditioners. (ueno, kitahara, & miyanaga, 2013, 2014) in this study, the methodology was applied to actual cop calculation. the indoor air temperature for simulation studies was set to 22◦c from december to march, 24◦c in april and november, and 26◦c from may to october. the fan power needed to deliver fresh air was not considered in this calculation. 248 y. m isaw a et al. / sim ulation study on energy conservation perform ance table 2 conditions of calculation of air-conditioning heat generation human (w/person) 121 time schedule percentage of heat generation (%) sensible latent human lighting equipment from january to march 78 43 0–8 0 0 25 april 73 43 8–12 100 100 100 from may to october 69 53 12-13 60 50 80 november 73 48 13–18 100 100 100 december 78 43 18-19 50 100 100 lighting (w/m2) 20 19-20 30 100 50 equipment (w/m2) 20 20-21 20 80 50 21–24 0 0 25 air-conditioning schedule weekdays 7 a.m. to 9 p.m. (pre-heating/cooling 7 a.m. to 8 a.m.) saturday and sunday off temperature (deg. c) from january to march 22 air-conditioning set point april 24 from may to october 26 november 24 december 22 indoor relative humidity (%) 50 y. misawa et al. / simulation study on energy conservation performance 249 table 3 specification of the selected air source heat pump cooling electrical energy cop (-) capacity (kw) consumption (kw) cooling rated value 5.0 1.59 3.14 minimum value 0.6 0.13 4.61 maximum value 5.2 1.66 3.13 heating rated value 6.3 2.04 3.08 minimum value 0.5 0.12 4.16 maximum value 8.6 3.23 2.66 fig. 12a. annual thermal load (south), fig. 12b. annual electrical energy consumption (south). 5. result of thermal loads and electrical energy consumptions figure 12 and table 4 show the thermal load and electrical energy consumption simulated with the model facing east, south and west. in this study, an integrated facade system was installed in one building elevation indicated in fig. 11 and the model was rotated to direct the system to east, south and west. facades not covered by the integrated facade system were arranged to face other directions. for example, if the system faced south, no louver facades were faced to east, west and north. under such conditions, the result of three orientations: east, south and west, showed the same qualitative tendency. the results of the south-facing model are graphed in fig. 12 as a representative outcome; table 4 provides detailed figures for all results. 5.1. representative outcome of orientation (the result of the south–facing model) figure 12 shows the results for thermal load (fig. 12a) and electrical energy consumption (fig. 12b) that were simulated with the model for south-facing louver facades. it was found that all models with louver facades, regardless of type, reduced both thermal load and electrical energy consumption. almost all thermal load was cooling load and every louver facade type reduced total load and electrical energy consumption. the reduction ratio of annual thermal load in the horizontal and the vertical louver facades was over 10%, however others were almost 9%. the reduction ratio of annual electrical energy consumption was similar to the load reduction, at around 7.5 to 250 y. misawa et al. / simulation study on energy conservation performance table 4 thermal load and electrical energy consumption louvers no horizontal vertical diagonal diagonal bound (right-up) (right-down) east thermal load (mwh/year) 335.7 291.9 293.9 299.0 298.7 300.1 reduction ratio of thermal load (%) – 13.04 12.44 10.93 11.03 10.61 electrical energy consumption (mwh/year) 81.81 73.07 73.75 74.61 74.34 74.79 reduction ratio of electrical energy consumption (%) – 10.69 9.86 8.81 9.14 8.59 annual performance factor (apf) (-) 4.10 4.00 3.99 4.01 4.02 4.01 south thermal load (mwh/year) 330.2 295.2 296.5 300.3 300.5 301.5 reduction ratio of thermal load (%) – 10.60 10.20 9.06 9.00 8.70 electrical energy consumption (mwh/year) 80.67 74.14 74.53 74.97 75.10 75.28 reduction ratio of electrical energy consumption (%) – 8.09 7.61 7.06 6.91 6.68 annual performance factor (apf) (-) 4.09 3.98 3.98 4.01 4.00 4.01 west thermal load (mwh/year) 333.5 311.2 312.0 314.9 314.8 315.6 reduction ratio of thermal load (%) – 6.68 6.45 5.56 5.59 5.35 electrical energy consumption (mwh/year) 81.54 76.70 76.95 77.49 77.52 77.68 reduction ratio of electrical energy consumption (%) – 5.94 5.63 4.97 4.93 4.73 annual performance factor (apf) (-) 4.09 4.06 4.05 4.06 4.06 4.06 8% in the case of the horizontal and vertical louver facades, and around 6.5 to 7% in other cases. the annual performance factor of the air conditioner (apf) was almost 4 in all louver facades. 5.2. thermal load it was found that all models with louver facades, regardless of their type, reduced the cooling load, although horizontal and vertical louver facades created a slightly larger reduction than others. the most effective case was the east-facing horizontal louver facade. however, other types of louver facades reduced the cooling load by more than 4%. for all orientations, the heating load increased after installation of louver facades. the ratio of heating load increase for horizontal and vertical louver facades was larger than the others. furthermore, east-facing and south-facing louver facades had a relatively higher heating increase than others did. the annual thermal load decreased for all orientations and louver facades in this study. it was observed that all louver facades were effective in achieving a 5 to 13% reduction in total thermal load. east-facing models were most effective in reducing annual thermal load. east-facing louver facades performed as shading devices from sunrise and they reduced starting loads for cooling. 5.3. electrical energy consumption of heat pump it was found that all models with louver facades, regardless of the arrangement of louvers, reduced the electrical energy consumption required for cooling. the electrical energy consumption required for heating increased after installation of louver facades, but the overall annual electrical energy consumption decreased for all orientations and louver arrangements. it was observed that all louver facades were effective in reducing annual electrical energy consumption by between y. misawa et al. / simulation study on energy conservation performance 251 10.69 and 8.59% for eastward orientations, 8.09 and 6.68% for south and 5.94 and 4.73% for west. it was noted that east-facing louver facades could reduce electrical energy consumption by more than other orientations. 5.4. annual performance factor (apf) the apf of the selected air conditioner was calculated for each of the simulation cases. the resulting apf was a reasonable value, almost 4, and if the thermal load is larger, apf increases. it is assumed that the performance of the selected air conditioner is better when the thermal load is large. 6. conclusion this study conducted research on the effect of integrated facade systems, which combine louvers and brbs. the following are the major findings: (1) external shading coefficient: the esc, proposed by the authors using radiance simulations and all sky model-r, was used for each case. five types of louver facades were examined: horizontal, vertical, diagonal (right-up and right-down), and bound louver facades. three orientations were set: east-, southand west-facing. (2) characteristics of louver facades by type were analysed using the esc calculation method. it was confirmed that, even when the opening rate was constant, the esc changed due to louver arrangements, orientation, time, and season. (3) quantified values of solar radiation reduction resulting from the installation of louver facades were obtained. they were as follows (by orientation/season): east/summer 52–76%, east/winter 47–66%, south/summer 58–72%, south/winter 47–60%, west/summer 53–66%, west/winter 47–66%. (4) thermal loads by orientation, (east, south, and west), were simulated using trnsys, using the above-mentioned esc. as a result, it was observed and confirmed that louver facades were effective in reducing the total thermal load by between 13.04% and 5.35% for all three orientations. (5) the electrical energy consumption for air-conditioning for all three orientations was also simulated using trnsys, using the air source heat pump: ashp, and actual cop model proposed by ueno (ueno et al., 2013, 2014). as a result, it was confirmed that five types of louver facades reduced the electrical energy consumption for all orientations. specifically, the electrical energy consumption was decreased by between 4.73% and 10.69%. (6) both thermal load and electrical energy consumption can be reduced by applying an integrated facade system to existing buildings undergoing refurbishment. these buildings may need seismic retrofitting and appropriate thermal insulation of external walls. an integrated facade system provides sun shading and seismic refurbishment, and the system reduces electrical energy consumption without thermal insulation improvements to the external wall. therefore an integrated facade system can be an appropriate energy saving measure for building in phased refurbishment, such as when seismic retrofitting is scheduled prior to improvements in external wall thermal insulation. both seismic safety and a reduction of electrical energy consumption are achieved using integrated facade systems. for future applications, it will be necessary to carefully study the building types where large energy savings are expected; building types in which indoor heat generation is high and cooling is 252 y. misawa et al. / simulation study on energy conservation performance used throughout the year. it is also expected that the size and pitch of the louvers and the distance between the louvers and the window/external wall surface will need to be adjusted appropriately because this study determined them by simply using the dimensions of the main structural frame and the brb. finally, it will be important to measure the efficiency of integrated facade systems in real buildings and to accumulate data for improving building stocks. acknowledgments this study was carried out using japanese ministry of land, infrastructure, transport and tourism grant aid for leading technological development for housing and construction. subsidy title: development of regeneration technologies for existing stock by means of an ‘integrated façade’ that simultaneously attempts performance improvement in design, structure and the environment. we would like to express our thanks for the excellent support. references igawa, n., matsuzawa, t., & koga, y. (2004). comparison of all sky model-r and all sky model-l between previous models. proceedings of architectural institute of japan, d-2, 57-58. kanaki, y., hikone, s., yamashita, t., & iwata, m. (2008). seismic strengthening by the buckling restrained braces arranged diagonally. journal of structural and construction engineering, (634), 2215-2222. kubo, k., hikone, s., misawa, y., sato, t., fukazawa, t., iwamoto, s., & iwata, m. (2011). a study on indoor thermal environment in integrated facade system; sun shading effect of louver and case study of annual air-conditioning load. proceedings of the society of heating, air-conditioning sanitary engineers of japan, 23(2), 927-930. misawa, y., hikone, s., nakamura, m., iwamoto, s., & iwata, m. (2014). diagonally arranged louvers in integrated facade systems – effects on the interior lighting environment. journal of facade design and engineering, 2(3-4), 163-182. nakamura, m., hikone, s., misawa, y., iwamoto, s., & iwata, m. (2010). the integrated facade consists of louvers and buckling-restrainedbraces as a building system. journal of environmental engineering (transactions of aij), 75(647), 121-129. takeuchi, t., koyano, k., & iwata, m. (2005). studies on integrated facade engineering. analyses on existing facades. journal of environmental engineering (transaction of aij), (592), 97-104. takeuchi, t., koyano, k., yasuda, k., yuasa, k., & iwata, m. (2006a). studies on integrated facade engineering. proposal for integrated facade and its evaluation. journal of environmental engineering (transaction of aij), (601), 81-88. takeuchi, t., yasuda, k., yuasa, k., okayama, s., miyazaki, k., & iwata, m. (2006b). seismic retrofit of aged building with integrated façade. aij journal of technology and design, (24), 161-166. takeda, h., hikone, s., misawa, y., & iwata, m. (2010). a study on bound louver of the integrated façade. proceeding of architectural institute of japan, d-2, 1239-1240. takizawa, h. (1985). proposal of standard issues for office building. proceedings of architectural institute of japan, 15th heating issue symposium, 35-42. the society of heating, air-conditioning and sanitary engineers of japan (2010). shase handbook, 14(1) fundamental, 414. ueno, t., kitahara, h., & miyanaga, t. (2013). development of heat source characteristic model of home use air conditioner: (part 1) characteristic model during cooling period, the society of heating, air-conditioning sanitary engineers of japan, (190), 41-49. ueno, t., kitahara, h., & miyanaga, t. (2014). development of heat source characteristic model of home use air conditioner (part 2) characteristic model for heating period and evaluation of applicability of model to various appliances, the society of heating, air-conditioning sanitary engineers of japan, (204), 85-92. yoshizawa, n., kobayashi, k., inanuma, m., & takeda, h. (2005). typical annual weather data (twd9302/l) in tokyo area and its application to the daylighting simulation. journal of environmental engineering (transaction of aij), (598), 23-29. journal of facade design and engineering 3 (2015) 129–141 doi 10.3233/fde-150036 ios press 129 assessment of five control strategies of an adjustable glazing at three different climate zones volker ritter∗, christoph matschi and dietrich schwarz university of liechtenstein, institute for architecture and planning, chair of sustainable design, vaduz, liechtenstein abstract. the energy demand for operating modern office spaces is often driven by either the annual heating demand, cooling demand or the demand for electrical lighting. the irradiation of the sun directly and indirectly affects the demand of all three. consequently, the glazing of higher office buildings is often treated with coating that allows a fixed transmittance. due to changing exterior conditions and interior needs, a fix-transmittance value is a compromise and most often doesn’t provide optimal thermal and visual conditions. the team in the research project named fluidglass develops a new glazing in which the transmittance of the glazing can be adjusted. this is possible by colouring a fluid, which is circulated in chambers of the glazing. the concentration of the colorant can be infinitely adjusted. in addition, this window allows collecting heat in the exterior fluid and allows the interior fluid chamber to operate as heating panel. this paper presents a first assessment of different control strategies for adjusting the colorant concentration with a simplified model. the assessed control strategies result in considerably different overall energy demands. certain control strategies have high potential for reducing the energy demand for heating and cooling depending on the locations (munich 20–30%, madrid 50–70%, dubai 50–60%). however, certain control strategies increase the electricity demand for lighting, which needs to be considered in the further development. in general, control strategies that only consider the solar irradiation are less promising strategies in temperate climate than strategies that also take the interior temperature into account. the results of controls that also respect the thermal comfort based on a predicted mean vote (pmv) index can achieve low energy demand, presuming that a deviation from the highest level of comfort is acceptable. at this stage of research, none of the studied control strategies shows to be optimal for all climate conditions to achieve highest energy reductions. further research is necessary in the development of a control strategy that can universally be applied. keywords: adaptive facade, shading control, control strategies, energy reduction 1. background the driving idea of this research project is developing an adaptive glazed facade element that allows controlling the solar transmittance within the glazing element to benefit from higher solar heat gains when needed during the cold season and to reduce solar heat gain during the hot season while ensuring enough daylight (stopper, 2013). up to now, the technologies used to achieve adjustable transparency are electro-chromic materials, liquid crystals and electrophoretic or suspended-particle ∗corresponding author: volker ritter, university of liechtenstein, institute for architecture and planning, chair of sustainable design, vaduz, liechtenstein. tel.: 423 265 11 11; e-mail: volker.ritter@uni.li. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:volker.ritter@uni.li 130 v. ritter et al. / assessment of five control strategies of an adjustable glazing fig. 1. diagram of a section of the currently considered configuration of fluidglass in mm. green=pane of glass, blue=fluid in fluid chambers, dashed red=lowe coating, grey=inert gas filling (krypton), vsg=laminated safety glass. devices (baetens et al., 2010). in addition to those technologies, the technology used in this research project allows adjusting the concentrations of a colorant in a fluid that is circulated in a chamber between two panes of glass, which affects the solar transmittance. the basic idea of this project is shown in the diagram of figure 1, which is adding panes of glasses to the exterior and interior face of a regular triple glazing to create the necessary fluid chambers. contrary to regular solar glazing, the glass used in this configuration should allow for maximal transmittance, as the solar transmittance is adjusted by the according fluid chambers. the lowe coating on the glasses towards the inert gas fillings is required to achieve high thermal resistance. the coating is not required in terms of solar irradiation. a laminated safety glass is required in the centre to ensure safety. the overall dimension of this configuration, which is called fluidglass, is about 60mm, which will require an adjusted framing. in addition, a configuration with only one fluid chamber will also be assessed in this research project. the fluid chambers (fce and fci) are each 2mm wide. fluid (water plus additives) is circulated in the chambers. heat is transferred between fluid and interior or exterior space. a heat exchanger is connected to each circuit, which allows connecting the fluid circuit to the regular building heating and cooling system, as shown with the diagrams in figure 3. the fluid in the two chambers is operated separately. antifreeze is added to the fluid of the exterior chamber. adjusting the temperature and mass flow rate of the fluid inlet affects the efficiency of the exterior fluid chamber as solar thermal collector and the interior fluid chamber as panel heating and cooling respectively. the mass flow rate varies between 1 to 3 kg·min–1 per running meter of fluidglass. since the hydrostatic pressure of the fluid in the fluid chamber would damage the glass, the fluid is set to negative pressure. this requires spacers to keep the minimal distance between the panes of glasses, as shown in the sketch of figure 2. nozzle bands at top and bottom of the glazing ensure a laminar flow. furthermore, fluidglass requires a so-called fluid circuit box, which contains a separator and a dispenser. these components adjust the required concentration of the colorant in the fluid, as shown in figure 3. (gstoehl et al., 2011). v. ritter et al. / assessment of five control strategies of an adjustable glazing 131 fig. 2. section of the currently considered components of fluidglass, showing the nozzle bands at top and bottom, four inlets and four outlets and the required spacers to keep the distance between the pane of glasses. fig. 3. diagram of the fluidgass facade element, showing the glazing and the fluid circuit box (illustration on the left) and the fluidglass facade connected to the building hvac core system on the right. 2. operation modes the considered configuration allows operating the fluidglass in different modes, depending on the location, the time of the day and the installed building system. the basic modes are shown in figure 4. the transmittance of the facade facing the sun can be adjusted during the summer day to ensure that 132 v. ritter et al. / assessment of five control strategies of an adjustable glazing fig. 4. schematic of the basic operation modes of fluidglass. high concentration of particles in the exterior fluid chamber prevents from overheating during the summer day (top left), while minimal concentration of particles during the winter day allows for high internal heat gains (bottom left). heat surplus can be transferred to the ambient during the night (top right). medium concentration of particles in fluid allows for optimal conditions during the seasons in between (bottom right). minimal illuminance is provided, but overheating is avoided (as shown in figure 4 top left). the first prototype proves that changing the concentration of the particles in the fluid of the chamber can be achieved quickly, which allows reacting quickly to changes of the solar irradiation due to clouds etc. this layer also acts as solar thermal collector, especially if the concentration of the colorant is high. collected heat can be used to support space heating directly or indirectly as heat source for a heat pump system, to generate hot potable water, for regenerating thermal energy storages like boreholes or in district heating networks. at locations with high solar irradiation and exterior temperatures, heat gains generated in the exterior fluid chamber (fce) reduce the interior cooling demand. if the building cooling system is able to store this collected heat during daytime and to release it during the night, instead of releasing it to the ambient with a chiller right away, the energy demand for cooling is expected to decrease, as the temperature of the heat sink during night-time is lower than during daytime. this could be realized by circulating the heated fluid during the night in the exterior fluid chamber (as shown in figure 4, top right). the fluid in the chamber facing the interior (fci) can be operated during the hot season as cooling panel by circulating a fluid with low temperature. during the cold season, the exterior fluid chamber (fce) is less often operating as solar thermal collector (as shown in figure 4, bottom left). solar heat gains are collected directly in the interior space. the interior fluid chamber (fci) acts during this season as heating panel. during spring and autumn, when v. ritter et al. / assessment of five control strategies of an adjustable glazing 133 the solar irradiation is already relatively high in combination with a solar altitude close to the horizon, unwanted interior solar heat gains can cause overheating in modern office spaces. consequently, the shading system is active on the critical windows (as shown in figure 4, bottom right). the gained heat surplus can be transferred for heating in other parts of the building that are not directly exposed to the solar irradiation. the fluid inlet temperature and the mass flow rate define the power of the panel. during the hot season, the interior fluid chamber panel is circulated with cold fluid for cooling the interior space with inlet temperatures of 18◦c, while the fluid inlet temperature is raised to 30◦c during the cold season for heating. this allows for a cooling power of 50 w·m–2 per pane of glass and for a heating power of 70 w·m–2 respectively. certainly, the heating power can further be increased with higher inlet temperature, but increasing the cooling power with lower inlet temperature than 18◦c would risk condensation at the glass panel. typical modern office spaces are built with large areas of glazing. the large areas of glass allow panel heating and cooling with temperatures relatively close to the room temperature. this is an important criterion for operating buildings with renewable energies (e.g. heating and cooling with ground water or geothermal heat exchangers). in general, a surface area of the glazed facade closer to the temperature of other components of the building (floor, ceiling, interior walls) also reduces the imbalance of long wave radiation in the space and raises the thermal comfort of the user. 3. method the full potential of the fluidglass can only be assessed in a dynamic building simulation. fluidglass allows controlling the following five parameters: inlet fluid temperature and mass flow rate of the interior fluid chamber (fce) and inlet fluid temperature, mass flow rate and colorant concentration of the exterior fluid chamber (fce). the model developed for the first analysis, is designed in trnsys 17. the model allows comparing the annual energy demand of a generic office space when controlling the concentration of the colorant of fluidglass in comparison to a reference case with a typical glazing. the results of the annual heating and cooling demands and the electricity demand for lighting are based on an ideal system at three different locations: munich to represent a cold-temperate climate condition, madrid to represent a hot-temperate climate condition and dubai to represent a subtropical climate condition. the model focuses on the colorant concentration in the exterior fluid chamber. it is assumed that the inlet fluid temperatures and mass flow rates of both chambers are operated in ideal mode. in particular, it is assumed that the interior mass flow rate and temperature will always be adjusted so that the required heating or cooling demand will be reached, which is an assumption of an ideal heating and cooling element that is only limited by the previously mentioned maximal cooling power of 50 w·m–2 of pane of glass and maximal heating power of 70 w·m–2. in addition, it is assumed that the mass flow rate and inlet temperature of the exterior fluid chamber are set to allow for efficient heat gains. obviously, this needs to be assessed more in detail in following studies, which are currently in design stage. the model space is set up according to the reference space described in the technical standard vdi 2078:2012-03, which reflects an office space of 17.5 m2, as shown in figure 5. this model space is further differentiated in terms of thermal mass with five classes, varying from extremely heavy (indicated with xh) to extremely light (indicated with xl). besides the fully glazed vertical facade that is orientated to the south, all remaining walls, floor and ceiling are designed to be adiabatic. the 134 v. ritter et al. / assessment of five control strategies of an adjustable glazing 3 .0 m 3.5m 5.0m fig. 5. schematic of the model space used in the simulation. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 1 2 3 ts ol , t vi s concentration [%] tsol colorant tvis colorant fig. 6. tsol and tvis of the studied colorant product at different concentration in percentage of the solution. overall heat transfer coefficient of the glazing in all models is 0.7 w·m–2·k–1 at standard reference conditions. the area of the frame equals 15% of the overall opening, which has an overall heat transfer coefficient of 2.3 w·m–2·k–1. the transmittance of the fluidglass configuration with clear fluid in the fluid chambers is calculated in the software optics6 and window7.2. relatively low concentrations of the colorant in the exterior fluid chamber (fce) affect the visual and solar transmittance already quite considerable. the visual and the solar transmittance of the colorant change proportionally with rising concentration of the colorant as shown in figure 6. the visual transmittance always stays above the solar transmittance, which is beneficial for the visual comfort. v. ritter et al. / assessment of five control strategies of an adjustable glazing 135 table 1 shgc and tvis of the reference case angle 10 20 30 40 50 60 70 80 90 tvis 0.436 0.438 0.433 0.427 0.415 0.391 0.337 0.236 0.095 shgc 0.222 0.222 0.221 0.220 0.218 0.215 0.206 0.178 0.105 the internal load is set according to din 4108-2:2013-02 (din 2013) with 13 w·m–2 representing the load from user, appliances and lighting. further settings for infiltration and ventilation are also according to this standard, with typically an air exchange rate (including infiltration) of 1/h if users are present and a rate of 0.24/h if users are absent. the model also includes a mode with increased ventilation rate of 3/h representing night cooling during the hot season. the reference model is identical to the model described above, besides the type of glazing. this model is equipped with solar glazing that has a solar transmittance of tsol =0.177 in combination with a visual transmittance of tvis =0.436. furthermore, the overall heat transfer coefficient of the glazing with 0.7 w·m–2·k–1 matches the heat transfer coefficient of the fluidglass at standard reference conditions. the electricity demand for lighting is in all considered cases based on the assumption that six fluorescent light bulbs are required to illuminate the space, which cause a power demand of 13.7w per square meter office space. table 1 shows the shgc and tvis according to the angle of incidence. it is planned to realize windows with the dimensions of 1.5m width by 3.0m height. the weight of 80kg per square meter for the fluidglass version with two fluid chambers requires a reinforcement of the framing, especially in the corners. furthermore, the overall thickness of the glazing package with 60mm is at the limit that can be inserted in standard profiles from the consortium partner alcoa. 4. settings of the control strategies the model allows comparing five control strategies of the colorant concentration. the nature of rule based control strategies is to be reactive and not predictive. each control strategy is active the entire year. in general, all control strategies set the concentration of the colorant to maximum if the interior air temperature, tint, rises above 25◦c, since first approximation regarding the cooling power of the fluidglass indicates that this power is limited and cannot provide enough cooling for the considered office space if low transmittance allows high solar irradiation to overheat the space during the hot season. the solar irradiation is key parameter of the first control strategy (cs1). the standard din 41082:2013-02 provides recommendations for simulating shading controls for office spaces. according to this standard, shading is required if solar radiation exceeds 200 w·m–2 global irradiation on the glazing. the purpose of this is to avoid overheating during the hot season. however, this strategy sets the colorant concentration to a maximal or minimal set point. the second control strategy (cs2) sets the colorant concentration based on the illuminance in the interior space. according to din 15251:2007, a minimal interior illuminance of 500 lux is required at the centre of the space on working desk level to guarantee comfortable working conditions. simulations with the software relux-pro show that minimal illuminance of 1700 lux is required 136 v. ritter et al. / assessment of five control strategies of an adjustable glazing table 2 benchmarks of the second control strategy, controlled by the solar radiation [i0] i0 >26w m–2 i0 >55w m–2 i0 >214w m–2 i0 >258w m–2 colorant concentration 0% 1% 2% 3% solar transmittance tsol [-] 0.34 0.17 0.05 0.04 visual tansmittance tvis [-] 0.65 0.31 0.09 0.07 table 3 benchmarks of the third control strategy, controlled by the interior air temperature [tint] tint < =23◦c tint >23◦c tint >24◦c tint >25◦c colorant concentration 0% 1% 2% 3% solar transmittance tsol [-] 0.34 0.17 0.05 0.04 visual tansmittance tvis [-] 0.65 0.31 0.09 0.07 directly behind the fluidglass to allow for the required 500 lux at the working desk level (böing, 2013). depending on the colorant concentration, this level of illuminance is reached at different levels of solar irradiation. while 26 w·m–2 are enough if the fluid of the fluidglass is uncoloured, more than 258 w·m–2 are required if the concentration of the colorant is 3%. table 2 lists the colorant concentrations at the resulting tsol and tvis that ensure the required 500 lux. the disadvantage of cs1 and cs2 is that the interior air temperature, which accounts for the heating and cooling demand, is at best indirectly considered. this is taken into account in the third strategy (cs3), where the colorant concentration is gradually changed with the rising interior air temperature [tint]. this ensures that solar irradiation generates interior heat gains if the room temperature is below the benchmark of 21◦c, but also reduces the solar irradiation gradually if higher temperatures are reached. the colorant concentration is set to maximum if tint exceeds 25◦c. this, however, also reduces the visual transmittance considerably. at days with low solar irradiation and high concentration of the colorant in the fluid of the fluidglass, electrical lighting is required to ensure 500 lux interior illuminance at the centre of the space at the height of a work desk. table 3 lists the controls of the colorant concentration and the according transmittances. the fourth control strategy (cs4) aims for high level of thermal comfort. this is provided by controlling the predicted mean vote index (pmv) which p.o. fanger has developed (fanger, 1970). this index complies with ansi/ashrae standard 55 (ashrae, 2013). the pmv is a measurement for the thermal sensation, given as pmv-index. the pmv-index is scaled by a seven point psycho-physical scale, ranging from –3 to +3. negative values indicate the thermal sensation as too cold, positive values as too warm and the zero point as neutral. the pmv-index is calculated by the pmv-equation, which is more or less an energy balance of the human body. the control strategy cs4 allows a deviation of the pmv in the range of –0.2 to +0.2. presuming this is provided, the colorant concentration is chosen based on tint, as shown in table 4. if the pmv exceeds the benchmark +0.2 or falls below –0.2, the colorant concentration is set to 0% to allow for maximal solar heat gains. the fifths control strategy (cs5) is basically the same as the control strategy cs4; besides this control aims for medium level of thermal sensation by allowing for a pmv ranging from –0.5 to +0.5. table 4 lists the criteria of control cs4 and cs5. v. ritter et al. / assessment of five control strategies of an adjustable glazing 137 table 4 colorant concentrations for the control strategy cs4 and cs5, combining interior air temperature [tint] and solar radiation [i0], presuming the pmv index is within the according range of –0.2 to +0.2 for cs4 and within the range of –0.5 to 0.5 for cs5 presuming pmv within range of –0.2. . . +0.2 i0 >26 w·m–2 i0 >55 w·m–2 i0 >214 w·m–2 i0 >258 w·m–2 with cs4 and of –0.5. . . +0.5 with cs5 tint >25◦c 3% 3% 3% 3% tint =22◦c. . . 25◦c 0% 1% 2% 3% tint <22◦c 0% 0% 0% 0% besides reducing the cooling demand, the purpose of the exterior fluid chamber fce is to operate as solar thermal collector. the efficiency of this collector depends on the concentration of the colorant. a separate model has been developed with the software ees to calculate the outlet temperature of this fluid chamber. based on the temperature differences between fluid outlet and inlet, the efficiency of the exterior fluid chamber as collector is determined. this information is used within trnsys as input for the simulation of an unglazed solar thermal collector. the model calculates the heating and cooling demand based on ideal building systems with maximal efficiency. it is assumed that a hot water tank exists in the building that allows storing the daily gained heat from the fluidglass collector, which is used for reducing the daily space heating demand. this is a simplification of real heating systems. electrical power is also required for circulating the fluid in the interior fluid chamber for heating and cooling, but also in the exterior fluid chamber for shading. this power demand will be compared to the power demand required for operating a heating/cooling panel in the reference case. the concentration of the anti-freeze in cold climate is 30% vol, which equals a density of 1040kg/m³. munich madrid dubai u se fu l e ne rg y de m an d [k w h m -2 ] 0.00 15.00 30.00 45.00 60.00 75.00 90.00 x l l m h x h building types cooling elec. ligh ng hea ng elec. pump 15 30 45 60 75 90 x l l m h x h building types cooling elec. ligh ng hea ng elec. pump 0 30 60 90 120 150 180 x l l m h x h building types cooling elec. ligh ng hea ng elec. pump fig. 7. resulting annual useful energy demand [kwh/m2] for heating, cooling, electrical energy demand for lighting and auxiliary pumps of the reference building with solar glazing (tsol =0.177) in munich, madrid and dubai for extremely light (xl), light (l), medium (m), heavy (h) and extremely heavy (xh) buildings. 138 v. ritter et al. / assessment of five control strategies of an adjustable glazing munich madrid dubai u se fu l e ne rg y de m an d [k w h m -2 ] 10 20 30 40 50 r ef c s 1 c s 2 c s 3 c s 4 c s 5 control strategies cooling elec. ligh ng hea ng elec. pump 0 10 20 30 40 50 r ef c s 1 c s 2 c s 3 c s 4 c s 5 control strategies hea ng elec. ligh ng cooling elec. pump 30 60 90 120 150 r ef c s 1 c s 2 c s 3 c s 4 c s 5 control strategies hea ng elec. ligh ng cooling elec. pump fig. 8. resulting annual useful energy demand [kwh/m2] of heating, cooling and electrical energy demand for lighting and auxiliary pumps, generated with the different control strategies (cs1 to cs5) compared to the reference building (ref) of the building type m in munich, madrid and dubai. with a height of 3m and a hydraulic efficiency of 0.77 and electrical efficiency of 0.9 of the auxiliary pumps, the electricity demand is roughly 0.75w, which is a simplification that only reflects peak demand. more relevant are pressure losses that the nozzle bands and other elements within the fluid circuit cause (e.g. heat exchanger, pumps, etc). measurements at the prototype show that the total power demand is about 18w. the power is multiplied with the number of hours the exterior and interior fluid is circulated in the chamber to determine the energy demand shown in figures 7 and 8 in purple. 5. results in a first step, the assessment shows that different thermal mass of the five building types of the reference case result at the three considered locations in considerably different annual useful energy demand for heating and cooling, as shown in figure 7. the building type ‘xl’ denotes an extremely light building, while the type ‘xh’ denotes a building that is extremely heavy. in between is the building type ‘m’ that denotes a building with medium thermal mass. while rising thermal mass is beneficial in the coldand warm-temperate climate of munich and madrid, high thermal mass is counterproductive at locations like dubai, where stored heat in the building mass is less often possible to be released by night cooling etc. the electricity demand for lighting is generally lower at hotter locations. accordingly, the location and building type affects the results of the simulations with the fluidglass model. in the following chapter, the results of the control strategies are only shown for the building type m, because the results of the other building types basically follow the same trend. the bar diagrams in figure 8 show the annual useful heating demand, cooling demand for space heating and electricity demand for lighting the space for the type m, which is referred to as reference case (ref) for each control strategy (cs). v. ritter et al. / assessment of five control strategies of an adjustable glazing 139 already the simple rule based strategy (cs1), which only considers the solar irradiation, results in lower energy demand. although the total energy demand is lower than that of the reference case, this strategy considerably reduces the cooling demand, but increases slightly the heating demand. the second control strategy (cs2) gradually increases the colorant concentration according to the solar irradiation instead of setting the colorant concentration to maximal at 200 w·m–2 as in the first control strategy (cs1). the result of the cs2 shows that the cooling demand in madrid and dubai can further be reduced, but this strategy also increases the heating demand in munich and madrid slightly. a more successful strategy regarding the reduction of the heating and cooling demand is the third strategy (cs3). considering the interior temperature as the relevant control value increases the possibility to benefit from passive heat gains during the cold season. this strategy increases the number of days with low colorant concentration in the exterior fluidglass chamber, which allows for more solar radiation to pass during the cold season. although cs3 is more successful in terms of the cooling demand in madrid and dubai, this strategy is less optimal at these locations compared to cs2 in terms of electricity demand for lighting. operating the fluidglass with cs3, there are more days during the hot season where the interior temperature tint exceeds the critical 25◦c and the colorant concentration reaches maximum. as a result, the number of hours increases when electrical lighting is required, especially in dubai. consequently, controlling the colour concentration based on the solar radiation is a more successful strategy at these locations. the fourth and fifths control strategy (cs4 and cs5) generate similar results at the location munich and madrid compared to cs3. allowing for a medium comfort with the cs5 even results in the lowest total of the heating and cooling demand. however, the electricity demand for lighting cannot considerably be reduced compared to cs3. the results with cs4 at the location dubai are even the worst of all considered strategies. although the control strategy cs5 allows for the lowest cooling demand, this strategy cannot considerably reduce the electricity demand for lighting. 6. conclusions this paper studies a new type of glazing, which is titled fluidglass. this glazing allows circulating fluid in chambers of the glazing. increasing the concentration of a colorant can change the transmittance of the glazing. the studied models set the colorant concentration according to different control strategies. as expected, the control strategy considerably determines the success of the fluidglass element. this paper only studies the effect that changing the colorant concentration of the exterior fluid chamber has on the heating, cooling and electricity demand for lighting. other controls of this new type of glazing, which are fluid inlet temperatures and mass flow rate, have not been studied. in general, the results of the studied control strategies show that fluidglass can reduce the cooling and heating demand at all three different locations compared to a reference building with good solar glazing. in particular, fluidglass has great potential to reduce the cooling demand in a hot climate. a reduction of the overall heating and cooling demand of about 50% seems possible in madrid and dubai without considerably increasing the electricity demand for lighting. however, fluidglass will require additional electricity for pumping power for circulating the fluid. while no antifreeze will be required in the fluid of the exterior fluid chamber in hot climate, some concentration of antifreeze will be necessary at locations like madrid, where exterior temperatures below 0◦c are possible. next assessments will take into account that the viscosity also affects the power demand. certain control strategies further reduce the cooling demand but simultaneously increase the electricity demand for 140 v. ritter et al. / assessment of five control strategies of an adjustable glazing lighting and pumping considerably. this is mainly because of the limited cooling power of the fluid glass, which requires to increase the colorant concentration of the exterior fluid chamber to maximum to ensure that the interior temperature can be kept below 26◦c. as a result, the number of hours where electrical lighting is required increases. consequently, next studies will focus on more optimal control strategies in a hot climate with the following improvements: first, due to the limited cooling power of the fluidglass, additional cooling components are accepted, which will presumable allow for lower concentration of the colorant and as a result will reduce the electricity demand for lighting. second, it is currently assumed that the lighting can only be turned on or off. the following models will allow for diming the lighting according to the minimum needs. in addition, fluidglass has certain potential to reduce the heating demand compared to the reference model. at best the heating demand at the location in munich is reduced by 20%. the major unknown factor is the actual efficiency of the exterior fluid chamber in terms of heat gains. since the collector efficiency of the fluidglass is very complex to calculate, a prototype has been built in this research project and will be assessed during the following months to allow for more precise simulations. furthermore, the heating system designed for this model uses the heat gains from the collector directly for space heating, which is a simplification of real systems. in the following studies, the heating system will be designed to allow storing heat for more than one day. in addition, a heat pump system will be implemented in the models that allow using the heat gain from the collector as heat source. it is expected that especially during the milder and colder season in the temperate climate of munich the latter improvements to the models will affect the energy demand for heating. in summary, the following models will allow to determine more precisely the heat gains of the exterior fluid chamber, the electricity demand for lighting, the electricity demand for auxiliary pumps and the electricity demand of the heat pumps system. this will allow balancing the energy reduction in cooling and heating demand with potentially rising electricity demand for electricity demand. in addition, changing the solar transmittance of the glazing will affect visual transmittance. consequently, the next assessment of control strategies will not only focus on low energy demand, but on a higher level of comfort, aiming for an adjustable facade system that allows for an optimal operation to serve the actual needs of the user. acknowledgments the team of this research project is a consortium of research institutes in austria, cyprus czech republic, france, germany and switzerland. we thank all partners within the fluidglass project for supporting this paper and working on the development of this new type of glazing. a complete list of partners can be found online on the website www.fluidglass.eu. we thank the european union for gratefully supporting this project with by the seventh framework programme for research and technological development and demonstration activities (fp7) under grant agreement no. 608509. the project is scheduled until september 2017. references ashrae standard committee (2013). ashrae handbook: fundamentals 2013. baetens, r., jelle, b. p., gustavsen, a. (2010). properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: a state-of-the-art review. solar energy materials & solar cells, 94, 87-105. v. ritter et al. / assessment of five control strategies of an adjustable glazing 141 böing, f. (2013). energiebilanzierung eines raumes mit fluiddurchströmten glasfassadenelementen. bachelor thesis, technical university of munich. fanger, p. o. (1970). thermal comfort. analysis and applications in environmental engineering. thermal comfort. analysis and applications in environmental engineering. copenhagen, denmark: danish technical press. gstoehl, d., stopper, j., bertsch, s., schwarz, d. (2011). fluidised glass facade elements for an active energy transmission control, world engineers’ convention. geneva. stopper, j., böing, f., gstoehl, d. (2013). fluid glass façade elements: energy balance of an office space with a fluid glass façade. proceedings of the conference sb13 munich implementing sustainability barriers and chances. munich. from city’s station to station city 035 journal of facade design & engineering volume 5 / number 2 / 2017 directions for the design of energy efficient kinematics in adaptive solar building envelopes paolo basso1, andrea e. del grosso2 1 d’appolonia s.p.a., via s. nazaro 19, genova, italy, paolo.basso@dappolonia.it, michele.mililli@dappolonia.it 2 university of genova, italy, andrea.delgrosso@unige.it abstract the development of adaptive building envelopes is receiving increasing interest in contemporary architecture, as it strives to cope with several requirements such as energy saving and harvesting (or mitigating environmental actions), improving performance and, finally, aesthetics. actual implementation fundamentally concerns external “skins” (i.e. adaptive facades), but internal “skins” (e.g. adaptive ceilings) may also be developed. the engineering aspects related to the above developments are quite complex and involve different behavioral models to be merged within the adaptive strategy. in the present paper, a study is presented that concerns the conception of an adaptive origami-like solar skin. the main design issues in managing the kinematics of the envelope are then identified and the envisaged solutions, to be developed in the next stage of the research, are discussed. keywords building facade, adaptive skins, morphing skins, optimization, energy harvesting doi 10.7480/jfde.2017.2.1737 036 journal of facade design & engineering volume 5 / number 2 / 2017 1 adaptivity concepts in civil structures when the concept of adaptivity is related to the field of civil structures, specific issues, such as the factors of scale and time, have to be considered. contrary to what happens in mechanical engineering, automotive engineering or space engineering where structures are often much lighter and considered to be in motion, it is unlikely that a civil structure is able to change its configuration in a very short time. dimensions, mass and the presence of people are typically going to constrain the possible range of accelerations and velocities, not to mention displacements and trajectories of the moving elements. the human threshold of motion perception is a consistent example of such a limitation (iannucci & fontanazza, 2010). consequently, the adaptive behavior cannot belong to, nor come from, all the elements of the structure at the same time. specifically referring to buildings, the existing proposals involving structural adaptivity are in fact usually focused on the internal and/or external envelope – i.e. the “building skin” (del grosso and basso, 2010; trubiano, 2013; karanouh and kerber, 2015). restricting adaptivity to the building skin means that mechanisms develop exclusively at the boundary, thus reducing kinetic inconsistencies with the internal space and possibly allowing a main static structure to be the core of the building. this, in turn, implies that adaptivity tends to come from mechanisms distributed all over the envelope in order to provide a better change of shape, and a consequent distribution of the actuators is also expected. the envelope then plays an interface role in the most of the environmental actions, both externally (e.g. wind) and internally (e.g. people walking). more generally, this interface role is exploited with respect to all those excitations, other than loads, which have effects that are “shape-dependent”. such an excitation is, for instance, the sunlight, as shown in fig 1. this aspect makes it possible to take advantage of the same adaptive system to improve different performances of the building and it perhaps represents the most relevant feature when comparing structural adaptivity with more traditional control approaches. another specific aspect in the civil field is related to the morphology of the adaptive structure which is implicitly constrained by necessities of the building skin. in particular, the building skin is always required to be a watertight and rigid surface, but it is not trivial to constrain a multi degree of freedom (mdof) system to maintain these two properties throughout the morphing process. besides the main functionality aspects, it is also worth noting the potential of adaptive structures to complement the aesthetics and the organic nature criteria of modern architecture, and thus provide contextual motivations for their implementation. these considerations are not of secondary importance as they indirectly enforce the adoption of sensing and actuation devices in buildings and make their costs more attractive to contractors and stakeholders. in the present paper, a finite-state control (fsc) strategy, recently developed by the authors, is presented for the design and control of mdof-adaptive envelopes. starting from the fsc strategy general description (section 2), a suitable representation of the building skin structure for its application is provided in section 3. section 4 of the paper instead focuses on specific aspects for the efficient application of the fsc strategy to solar envelopes, namely the solar energy harvesting potential and the energy minimization of the kinematics actuation. finally, the paper draws some conclusions concerning the achievements and the future plans of the research. 037 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 1 the “building skin” as the interface to non-structural design drivers (e.g. sunlight). 2 final state control the fsc is a general procedure for the design of adaptive structural envelopes (del grosso & basso, 2012; del grosso & basso, 2013). the procedure is a basic combination of two main parts: a meta-heuristic optimization process, which aims to discover new optimal configurations – i.e. finite states – according to some defined purpose, and a gradient-based optimization process that acts as a constraint for the kinematic compatibility maintenance. finally, a topology optimization process, which aims to decrease the number of degrees of freedom (dofs) of the structure while retaining its ability to achieve the optimal configurations, is proposed as an integration of the finite states selection. the main advantage of the fsc is that a set of optimal configurations (i.e. finite states) are investigated during the design phase to partially or totally avoid the computational cost of the realtime control. the strategy can handle any kind of system that can be associated with the framework representation as provided in section 3 of the paper. the framework representation is central to the strategy development, mainly because the matrix analysis of frameworks is used to control the kinematic properties of the envelope. a scheme of the fsc is summarized in fig. 3, with emphasis on the key steps of the constrained optimization process. the key steps are, namely, the optimization of the different configurations of the envelope (finite states selection), the post-optimization of the framework topology and the management of the actuators’ location. the initial framework to optimize g0 has to be equivalent to a triangular mesh. the choice of this initial mesh is determinant to achieve a good result. the most important parameter in this sense is the density of the mesh – i.e. the number of nodes and edges. a coarser mesh has, in fact, fewer possible configurations compared to a denser one but more dofs to manage. moreover, the mesh should represent a real envelope made with real panels which always have limits in the range of possible measures. the topology of the initial mesh – i.e. how the edges are connected – is another important factor because it constrains the “folding” process. it is, for instance, important to start with a symmetric pattern if the mesh is expected to fold symmetrically. topology and density of the initial mesh could then be part of the whole optimization 038 journal of facade design & engineering volume 5 / number 2 / 2017 process, though on the other hand these two elements are also fundamental to the definition of the envelope appearance. consequently, regarding a building facade, roof or internal ceiling, these turn out to be architectonical parameters in the majority of cases. therefore, the topology and density of the initial mesh are left outside the optimization process here since they are considered a direct choice of the designer. 3 framework representation a framework, from the structural engineering point of view, can be defined as a discrete set of onedimensional elements in three-dimensional space, connected at their ends to points called nodes. frameworks are therefore general enough to represent a huge variety of structural systems such as, for instance, trusses, cable-nets, tensegrities, membranes, reciprocal frames, etc. in order to make the fsc applicable, two more characteristics have to be associated with the generic framework definition provided above. the first is related to the internal restraints that are assumed to be pin-joints. the second is a further specification of the topology of the framework. specifically, the framework will be assumed to be “single-layer”, which means that whatever projection on a plane would result in no intersections of the edges, except in correspondence of the nodes. a pinjoint single-layer framework, like the one in fig. 2, is usually a multi degree of freedom (mdof) system, which allows different compatible configurations to be obtained by reciprocally rotating edges around nodes. such a particular system has thus a great potential for structural applications as an adaptive building envelope since: – the pattern of the system can be filled with rigid panels to achieve a watertight and rigid surface; – mechanisms are purely geometric, i.e. they do not rely on the elasticity of materials and robust kinetic structure in a larger scale under gravity can be realized; – the transformation from one configuration to another can be controlled by a limited number of degrees of freedom enabling a semi-automatic deployment of the structure. in a system of this kind, the number of dofs depends on the topology and geometry of the pattern. the triangular pattern, for instance, is the most frequently used but with such a pattern the number of dofs increases along with the number of nodes. on the contrary, a quadrilateral pattern results in no dofs or a maximum of 1 dof when the pattern is singular (koiter, 1984). a main issue is then to design a pattern which optimizes the number of dofs and then to define their location and, consequently, the optimal type, number and location of the actuators. a limited number of dofs simplifies the mechanism control and can result in an actuation process with reduced energy requirements. on the other hand, too few dofs could overly restrain the adaptability of the structure, depending on the set objective. moreover, a high number of dofs that are based on reciprocal kinematic relations among the system parts makes the real time control of this kind of structure not insignificant. 039 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 2 mdof pin-joint single layer framwork. fig. 3 flowchart of the optimization process. 040 journal of facade design & engineering volume 5 / number 2 / 2017 4 development of a cost-effective solar skin the key point in the design of an adaptive envelope is to quantify its benefit by comparing it to a static approach. however, due to the complexity of the subject, to date research on mdof adaptive envelopes has mostly focused on the viability of the different analyzed solutions and advantages and disadvantages are usually reported only qualitatively (tachi, 2010; tachi, 2013). since the research developed by the authors currently focuses on the conception of a new kind of adaptive solar envelope, i.e. an envelope with the characteristics of a rigid origami mechanism and an integrated solar energy harvesting process, the quantification of the achievable energy gain becomes the measure of the product’s effectiveness. therefore, the energy balance is the difference between the energy consumed by the actuation system and the exploited solar energy. the final aim is thus to overcome the main barriers identified for the integration of solar technologies in the building envelope, i.e. performance and aesthetics (prieto, 2017), through the conception of a dynamic skin which could handle an increment of the energy harvesting potential and a major freedom of design. starting from these considerations, the proposed actions that enable this energy efficiency advantage are discussed hereafter. 4.1 lightweight multi-pane glazing units with energy harvesting potential to achieve lightweight envelopes is the first step in reducing the energy consumed by the actuation process. on the other hand, an energy harvesting process should be included in the envelope design in order to obtain a positive energy balance from the actuation. finally, the envelope insulating and transmittance properties have to be comparable to the best alternatives on the market. using these considerations as a starting point, based on the combination of transparent and translucent curtain walls with etfe membranes and optional pv membranes or advanced solar thermal concepts, the idea is to develop a new range of higher optical, thermal performance and a lower weight component. some internal glass layers are replaced with lightweight transparent foils expressly treated (e.g. plasma and low-e coatings) in order to exceed the current level of performance of glass panels through a cost effective solution and a minimum quantity of material. the use of foils can drastically reduce the risk of breaks in the intermediate layers, realized using standard float glass, due to a high temperature difference (over 55k) within an individual glass pane. in addition, multi-pane (more than 3-pane) glazing units are built of complex glazing systems with outer, inner, intermediate, and expansion glass panes that vary optically. reflective coatings and transparent pv membranes are integrated to enhance energy harvesting; an example of such a functionality can be seen in the high insulating solar glass (hisg) developed at the national taiwan university of science and technology (fig. 4). in this case, the principle is that incoming radiation encounters the pv membrane to generate energy and then reflects back on to the pv membrane to maximize energy harvesting. this contributes further to heat insulation. the concept can be further integrated into a high-efficiency thermodynamic facade, comprising an air-driven heat pump and an external air-gap which would act as a solar thermal collector, for the distributed supply of heat and air conditioning to buildings. one of the innovations of the proposed concept is the capability to use the heat from the surrounding environment to fulfil the heating and air conditioning requirement of the rooms. this system could incorporate a heat recovery device to generate an all-in-one device for the hvac needs of a medium sized room. the external aesthetics & appearance of the system will be similar to a ventilated facade, and a small-sized internal terminal unit. this modular approach can be broken down into a range of semitransparent components introducing different finishes and levels of transparency for the cell, moving from membranes to crystalline cells integrated in a thinner outer glass layer. 041 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 4 scheme of the high insulating solar glass (hisg) concept. image courtesy of prof. young national taiwan university of science and technology. 4.2 actuation energy minimization low-energy actuators are important in minimizing the total energy consumption during actuation. however, an even more problematic but neglected issue is represented by the actuators network topology, i.e. the number and placement of the actuators in the framework. the difficulty with this issue is that it is directly related to the framework dimensions, to the envelope mechanism, and to the optimization of the energy harvesting process. moreover, an optimal actuators distribution: a) minimizes the trajectories leading from one configuration to another; and b) minimizes the stress ranges in the skin’s structural elements, for improving fatigue life. consequently, the actuators network topology must be part of the whole optimization process of the envelope (steps 5 to 7 in the fsc flowchart – fig. 2). the number of actuators needed to manage the morphing process of an adaptive framework depends on the number of finite mechanisms within the framework and the type of actuators used. here, the actuators are assumed to be linear and to be linked both to the adaptive envelope and to a supporting structure through pin-joints and through fixed joints respectively, as shown in fig. 5. every actuator belonging to this type that is correctly placed on a node of the framework constrains a maximum of three mechanisms. an incorrect placement may generate self-stress states leaving some of the mechanisms untouched. therefore, the location of the actuators must be chosen carefully in order to control all the mechanisms. 042 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 5 scheme of an adaptive envelope linked to a supporting structure by linear actuators. actuators are pin-jointed both to the supporting structure and to the envelope. this concept is better illustrated by comparing fig. 6a and fig. 6b. note that the two frameworks have the same number of nodes, the same number of edges and also the same edge lengths. moreover, the number of mechanisms (m = 11) is exactly the same when assuming the framework is unconstrained in the 3d space. but in fig. 6b, if no actuators are placed at node 7 and node 7 is unconstrained, it is not possible to manage the face 4-7-8 rotation around the edge 4-8. this does not happen in fig. 6a because of the different topology of the pattern. the above example leads to the first important conclusion, which is that no nodes with connectivity ≤ 2 can be left unconstrained or not actuated. fig. 6 (a) and (b): two frameworks with the same number of nodes and edges but with a different pattern topology. in (b) node 7 has connectivity ≤ 2 and needs to be controlled directly. the second important observation is that actuators and constraints should be placed first on nodes that are not directly connected to one another. for instance, still looking at fig. 6, it turns out that a minimum of four actuators/restraints are needed to control the 11 mechanisms and at least one self-stress state is expected (3 dofs ´ 4 actuators – 11 mechanisms = 1). the only group of four nodes which can be selected avoiding nodes inside the same group to be connected to each other is the one composed by nodes 1, 3, 7 and 9. controlling these four vertices through pin-joint restraints or actuators results in zero mechanisms and only one self-stress state of the framework. every other combination of four nodes, assuming the same kind of restraints/actuators would end up with more self-stress states. 043 journal of facade design & engineering volume 5 / number 2 / 2017 inverse kinematics can then be used both to plan and to verify the actuation process. the inverse kinematics of the framework can be controlled by the moore-penrose generalized inverse of the jacobian of the non-linear vector equation representing the geometry constraints (constraint equation). the constraint equation for a pin-joint framework reads: (1) where l is the vector of the edge lengths at the current step and l0 is the vector of the initial edge lengths. eq. (1) can be written in terms of the cartesian nodal coordinates: (2) where c is the incidence matrix of the framework and x,y and z are the vectors of the nodal coordinates. eq. (2) yields an underdetermined system, by exploring the solution space for which it is possible to obtain variations in the configuration. valid shapes are found by perturbing the nodal coordinates according to the nullspace of the jacobian . the solution is calculated using the pseudoinverse of the jacobian as follows: (3) where du0 represents the initial perturbation and i is the identity matrix. eq. (3) finds the valid perturbation closest to du0 by orthogonal projection to the solution space. an integration of this infinitesimal motion has to be executed and, for each step, the residual has to be eliminated (e.g. newton-raphson method). it is worth noting that considerations are limited to geometric ones and elastic or plastic behavior of the structure with specific materials is not analyzed. with reference to eq. (3) the perturbation vector du0 = [dx1, dx2, …, dxn] t1xn is then built from the zero vector by substituting the zero values corresponding to the “actuated” nodes with the distance between the node at the current position and the node at the target position. 5 conclusions differences concerning the applicability of adaptive structures to engineering problems of different fields have been discussed. of particular interest for potential civil applications are mdof envelopes which are generally characterized by a high reciprocity in the behavior of their parts. the resulting constrained mdof kinematics are not inconsequential when attempting to achieve optimal configurations through real-time control. the proposed fsc strategy focuses on adaptive envelopes which can be associated to single-layer frameworks and is based on the “a priori” exploration of the design space in order to reduce the computational cost during the real-time control of the structure. in order to apply the fsc to the design of a new kind of solar building envelope, the paper has presented the identified issues and the next key-steps that will be addressed. 044 journal of facade design & engineering volume 5 / number 2 / 2017 references del grosso a. e. and basso p. (2010). adaptive building skin structures. smart materials and structures, 19, 2010, 124011. del grosso a. e. and basso p. (2012). multi-dof single layer truss structures control by means of a finite-state strategy. proc. 5th european conference on structural control, genoa, 18-20 june 2012. del grosso a. e and basso p. (2013). adaptivity criteria for the design of building envelopes. proceedings of the international conference on adaptation and movement in architecture. toronto, 11-12 october 2013, pp. 2-14. iannucci l. and fontanazza a. (2010). design of morphing wing structures. 3rd seas dtc technical conference, edinburgh, uk. karanouh a. and kerber e. (2015). innovations in dynamic architecture the al-bahr towers design and delivery of complex facades. journal of facade design and engineering, vol. 3 (2015) 185–221, issn 2214-302x koiter w.t. (1984). on tarnai’s conjecture with reference to both statically and kinematically indeterminate structures. (report no.788) delft: laboratory for engineering mechanics prieto a., knaack u., auer t., klein t. (2017). solar facades main barriers for widespread facade integration of solar technologies. journal of facade design and engineering, vol 5, no 1 (2017), issn 2213-3038. tachi t. (2010). geometric considerations for the design of rigid origami structures. in iass symposium, shanghai. tachi t. (2013). designing freeform origami tessellations by generalizing resch’s patterns. journal of mechanical design, 135(11), 111006. trubiano f. (2013). performance based envelopes: a theory of spatialized skins and the emergence of the integrated design professional. buildings 2013, 3, 698-712; doi:10.3390/buildings3040689. jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 023 journal of facade design & engineering volume 5 / number 1 / 2017 optimised parametric model of a modular multifunctional climate adaptive façade for shopping centres retrofitting riccardo pinotti1, stefano avesani2, annamaria belleri3, giuseppe de michele4, philip ingenhoven5 1 unibz free university of bolzano / eurac institute of renewable energy, 39100 bolzano/bozen – italy, riccardo.pinotti@eurac.edu 2 eurac institute of renewable energy, 39100 bolzano/bozen – italy, stefano.avesani@eurac.edu 3 eurac institute of renewable energy, 39100 bolzano/bozen – italy, annamaria.belleri@eurac.edu 4 unibz free university of bolzano / eurac institute of renewable energy, 39100 bolzano/bozen – italy, giuseppe.demichele@eurac.edu 5 eurac institute of renewable energy, 39100 bolzano/bozen – italy, philip.ingenhoven@eurac.edu abstract a modular multifunctional façade for the retrofit of shopping malls, capable of adapting to different climates and to the existing building features both by the presence of movable components and by proper sizing of the fixed ones, is under development within the european fp7 project commonenergy. in particular, this curtain-wall façade is equipped with a fixed shading system, a photovoltaic panel with a battery feeding the automated openings for natural ventilation. the aim of this work is to define a reliable parametric model for a multi-functional façade system, to support designers with a set of useful data for the holistic design of the façade configuration depending on climate, orientation and building use. firstly, a reference zone model for the façade was devised; this had to be both representative of reality and smartly defined for simulation software implementation. besides the definition of the façade model parameters, all unknown design parameters were identified with their minimum and maximum values, depending on different possible applications and environmental conditions in which the façade could be applied. the inputs for the model were defined in a parametric matrix and included: facade module size, façade orientation, climate, window typology (thermal transmittance and g-value), distance between the shading lamellas, tilt angle, and openable window size. the simulation engine was decoupled: visual comfort and artificial lighting use were assessed with radiance, while the façade thermal behaviour was evaluated by means of building energy simulations in trnsys, taking into consideration the daylight assessment results. for each simulated configuration, a set of relevant outputs fields for indoor air quality, thermal and visual comfort, and energy performance were derived. the main considered performance indicators were the long-term percentage of people dissatisfied, the number of hours when co2 concentration was within the recommended values for each of the categories defined by en 15251:2007, the illuminance provided by daylight, the energy consumption due to lighting, ventilation, heating and cooling, and the energy generated by the pv panel. moreover, all outputs were collected in a pre-design support tool comprised of a database accessible through a filtering system to gather the desired performances. this work highlights the role of thermal and daylighting simulation in the design of an adaptive multifunctional façade through the definition of a methodology for the support at the pre-design phase. keywords façade, multifunctional, parameterization, commonenergy, trnsys doi 10.7480/jfde.2017.1.1421 024 journal of facade design & engineering volume 5 / number 1 / 2017 1 introduction “shopping malls are sometimes perceived as ‘icons of a consumerist society’, because of their high energy demand, high co 2 emissions and waste, despite the increasing, mostly individual, ‘green’ initiatives in the field. the commonenergy project seeks to transform shopping malls into lighthouses of energy efficient architecture and systems.” (commonenergyproject.eu, 2016). wholesale and retail buildings represent 28% of the eu non-residential building stock and present the highest specific energy demand (bpie, 2011). therefore, existing shopping centres offer a great retrofit opportunity for the reduction of their energy consumption. the eu fp7 project commonenergy (commonenergyproject.eu, 2016) aims to re-conceptualize shopping malls through deep retrofitting, developing a systemic approach comprised of technologies and solution sets, as well as methods and tools, to realise their implementation. modern shopping centres tend to include glazed envelopes in their design ensuring good day-lighting and offering a more seamless connection between the indoor shopping space and the outdoor environment. however, glazed envelope features need to be carefully evaluated in order to limit the energy consumption for air conditioning. within the commonenergy project, among other retrofitting solutions for shopping centres, research and industry partners (acciona, bartenbach, eurac, sunplugged) are developing a modular multifunctional climate adaptive façade system. the newly developed modular climate adaptive façade concept outlined by (attia, favoino, loonen, petrovski, & monge-barrio, 2015), is based on an optimally designed natural ventilation and daylight control, lightweight substructure, enriched by rapid assembly possibilities. thanks to its flexibility and modularity, this façade system is suited for retrofit applications offering the opportunity to adjust the façade design according to climate and building features. the high number of design possibilities raises the need for a tool that enables designers to make informed decisions about façade configuration, glazing materials and shading geometry depending on the building design constraints, such as climate, façade orientation, facade module size and indoor space usage. the aim of this work is to define a parametric simulation model to evaluate the performance of a variety of configurations of the modular multifunctional climate adaptive façade from both an energy and indoor environment quality perspective. moreover, a preliminary design tool, based on a user-defined filtering process, has been developed in order to inform designers towards the optimal façade configurations depending on the design requirements and targets. the first section of the paper describes the façade concept. then, the methodology applied for the parametric model set up is reported: the input settings, the parametric matrix and the key performance indicators. finally, an example application of the design tool is presented. 1.1 the modular multifunctional climate adaptive facade the modular multifunctional climate adaptive façade system is a general replicable concept, adjustable for different applications and designed to be used in modular construction methods aiming for a high level of prefabrication (treberspurg & djalili, 2010). the facade modularity and its light weight substructure allows its application as envelope retrofit solution for most existing retail buildings, while its multifunctionality gives the opportunity to adjust the system to the particular local climate conditions and indoor space usage. 025 journal of facade design & engineering volume 5 / number 1 / 2017 the façade system was developed to respond to the following functions, listed in order of design priority: – to protect against overheating through solar gain control by combining a glazing system with a shading element; – to provide fresh air and to cool indoor space by natural ventilation by means of automated façade openings; – to supply energy for window automation with integrated pv modules; – to maintain transparency between indoor and outdoor while providing daylighting and attract customers. the façade system consists of a modular frame made of mullions and transom with flexibility in their position, enabling easy integration of possible technologies such as: shading systems, automated openable windows and photovoltaic panels. in principle, the anchorage system allows double screen installation, and may be easily adapted to multiple designs, creating different geometric, aesthetic and energy solutions. in the model discussed in this work, the concept of multi-functionality is ensured by the presence of automated openings located in the lower and upper part of the façade, enhancing single-sided stack ventilation (see figure 1). glazed façades are commonly used in retrofitting for aesthetics and communicative reasons but a problem due to energy performances arises, due to the transparent nature of the glass that critically characterizes the thermal performances of the enclosure. so, the proposed façade has been carefully designed for the appropriate selection of glass characteristics, considering climates and façade orientation needs, taking into account also shading system effects of a fixed lamellas system. a thin-film pv panel was integrated into the façade to generate the electricity needed for automatic window actuation. the thin-film photovoltaic panel included in the module was the same length as the façade and is 0.3 meters high. the energy provided by the pv supplies the power needed for automated windows; moreover, in order to store power when not directly needed by the actuators, a battery is integrated in the façade module, behind the pv panel. fig. 1 main components in the façade designed by acciona. 026 journal of facade design & engineering volume 5 / number 1 / 2017 2 methodology the model parameterization consists of running the selected model “n” times, changing the “m” values of “p” parameters, with n=m*p. this process allowed a set of outputs for each simulation to be gathered so that the façade performances for each desired configuration could be determined. the parametric simulation model was based on a single-zone model, intended to be representative of typical shopping mall environments. the reference zone was 15-meters deep with adiabatic boundary conditions on all the walls, except for the one that incorporated the multifunctional façade. the depth of the reference room was selected to consider the effects of ventilation rates on a representative volume of typical malls environments. the multifunctional façade module covers the entire external wall of the reference zone model. as far as the façade module is concerned, it was drawn considering a distinction between the fixed mullion and transom and the assigned frame’s percentage of windows installed in the module. as proof of concept, three volumes of zone were considered in the study: 60 m3, 90 m3 and 270 m3 for the façade module dimensions of 2[m]x2[m], 2[m]x3[m] and 3[m]x6[m], respectively. the fixed glazed part of the window was provided with a fixed lamella shading system. thermal transmittance of façade elements, such as mullions and window frames, were set in accordance to data provided by the façade designer (u-value ‘frame & mullion’=3.59 w/m2k). the first part of the process that led to the model definition was setting the reference zone parameters and the application of the following building physics reference points: – set point temperature values for the heating and cooling system are the ones recommended by the en 15251-2007; – the natural ventilation rate has been assessed using the single-sided, two vents, buoyancy driven model (cibse,2005) – the infiltration rates depend on indoor-outdoor temperature difference and wind speed according to coblenz & achenbach, (1963); – internal gains due to people, appliances and artificial lighting system have been provided by an italian shopping malls design company. – secondly, the input data were defined in order to include all the possible simulation choices for the desired conditions in each considered configuration and were divided into three categories: – climatic conditions and façade orientation; – application, depending on the indoor space usage; – façade module size and the characteristics of the glazing system and shadings configuration. finally, the simulation results were post-processed in order to represent indoor air quality, thermal and visual comfort and energy performance of the reference zone. the main considered performance indicators were the longterm percentage of people dissatisfied (lpd) (carlucci, 2013), the number of hours in each iaq category, the energy consumption due to lighting, ventilation, heating and cooling demand and, the energy generation from the pv panel. the developed tool produced simulation results in different plots guiding users towards the optimal selection of facade configuration. 027 journal of facade design & engineering volume 5 / number 1 / 2017 2.1 building energy simulation model the first step of the work concerned the implementation of the façade model in the trnsys simulation environment. trnsys was chosen for its flexibility as energy simulation software through its compatibility with other software used during this study. within the type 56 model, different possible algorithms were available for the modelling of the heat transfer and solar radiation exchange between the façade and the indoor and outdoor environment. the model geometry was drawn with sketchup using a trnsys3d plug-in and then imported into the trnsys simulation environment (klein et al, 2010); in particular, the thermal behaviour of the reference zone was modelled by type 56. given the high number of simulations for the parametric analysis, a trade-off between computation time and model accuracy had to be considered. thus, two geometry modelling approaches (standard and detailed model geometry) and two radiation calculation modes (detailed (gebhart, 1971) or ‘standard’ (seem, 1987) were analysed and compared in order to quantify the influence of the different model approaches on simulation results, in particular concerning windows and frame geometry inputs. the deviation in output trends showed the need to use a detailed model geometry that considered the geometrical distinction between the frame of the windows and the mullions and transoms of the module. furthermore, the standard radiation model led to a reduction in the computational time of the simulation (71.93 seconds down to 29 seconds) compared to the detailed radiation model, without affecting the results in any critical way (pinotti, 2016). therefore, the standard radiation model (seem, 1987) of trnsys was used through the simulation process. the pv power production was evaluated using the trnsys type 94 model (klein et al, 2010). this component allowed the electrical performance of a photovoltaic array to be modelled in a detailed way, knowing all the parameters of the pv module. it was chosen in the simulation because of its easy implementation in the whole model and thanks to the possible interaction with other components, such as batteries or regulators. 2.2 inputs definition table 1 reports the facade configurations used in the parametric analysis. façade module size and configuration resulted in different proportions between openable windows and fixed ones, with a consequent change in the percentage of frame in each window. different module sizes, assumed to be representative of a real possible application case for a shopping centre façade module, were investigated because the different percentage of frame area over the whole façade module influenced significantly the thermal performance of the envelope. the dimension of the openings and louver was calculated in order to ensure three levels of pre-defined air change rates (4, 6 and 8 ach) when the outside temperature is 25°c and there is 1 k difference between indoor and outdoor temperature (cibse, 2005). 028 journal of facade design & engineering volume 5 / number 1 / 2017 façade width façade height zone depth frame % openable window frame % fixed window openable window width openable window height openable window area fixed window area pv area [m] [m] [m] [-] [-] [m] [m] [m2] [m2] [m2] 3 6 15 22% 9% 1.5 0.60 0.90 6.40 0.9 3 6 15 17% 9% 1.5 0.90 1.35 5.97 0.9 3 6 15 14% 9% 1.5 1.20 1.80 5.54 0.9 2 3 15 31% 15% 1 0.42 0.42 1.74 0.6 2 3 15 24% 16% 1 0.63 0.63 1.54 0.6 2 3 15 21% 17% 1 0.85 0.85 1.33 0.6 2 2 15 36% 21% 1 0.35 0.35 0.85 0.6 2 2 15 27% 23% 1 0.52 0.52 0.69 0.6 2 2 15 23% 27% 1 0.69 0.69 0.53 0.6 table 1 possible configuration of the facade module three different typologies of building use have been considered in the parameterization: ‘shops’ (shp), ‘common area’ (cma) and ‘restaurant’ (rst); different building use implies different lighting, appliances and occupancy density and profiles, and, therefore different internal gains. it must be noted that, in the case of ‘shop’, no shading system were applied on the façade because each façade module was supposed to be a shop window. all orientations (north, south, east and west) for each configuration of the reference zone were simulated but for north-oriented façades no shading system was applied on the façade. a review of minimum requirements for national regulations and standards set by energy efficient building certification schemes (see references in table 2) as well as on available products on the market was carried out to define the most likely ranges of u-values and g-values for glazed components in several european countries. the result of this part of study gave realistic thermal transmittance and their respective solar gain values, representing the state of the art in the field of windows and glazing technologies that complied with the current regulatory framework. upper limits for u-values referred to the minimum requirements for national regulations. the lower limit referred to the minimum u-value recommended by the standards set by energy efficient buildings certification schemes. feasible ranges of g-values were assigned to each u-value, taking into account the state of the art of the glazing industry (agc-glass.eu, 2016). window glazing system models were developed using the window 7.4 database (lawrence berkeley national laboratory, 2011). table 2 reports the glazing u-value and g-value ranges for several locations. 029 journal of facade design & engineering volume 5 / number 1 / 2017 country reference city heating degree days (hdd) uw-value [w/m2 k] (max) g-value (max) g-value (min) uw-value [w/m2 k] (min) g-value (max) g-value (min) norway trondheim 5211 1.201 0.67 0.20 0.802 0.63 0.25 uk london 2800 1.804 0.52 0.29 0.853 0.63 0.25 austria wien 2844 1.905 0.73 0.25 0.853 0.63 0.25 italy modena 2529 2.207 0.52 0.29 1.308 0.67 0.22 italy palermo 585 3.007 0.77 0.40 1.308 0.67 0.22 spain seville 1460 4.206 0.61 0.37 1.253 0.67 0.20 1 (kommunalog moderniseringsdepartementet, 2010) 2 (norsk standard, 2012) 3 (passivhaus institut, 2016) 4 (british department for comunities and local government, 2013) 5 (austrian institute for building technology, 2007) 6 (ministerio de fomento, 2013) 7 (dipartimento di energia del ministero per lo sviluppo dell’economia, 2013) 8 (casaclima, 2014) table 2 glazing u-values and g-values ranges. in order to prevent direct sunlight from entering the zone, a fixed shading system was evaluated for all the large central windows in the façade modules, except for the ‘shop’ application and for northoriented cases. among the parameterization variables the tilt angle and spacing of the lamella were also considered. in order to evaluate the effect due to the shading system, a dedicated model, and the related parameterization (in figure 2 all the combinations of inputs for the shading system parameterization are presented), were undertaken using ladybug+honybee (sadeghipour roudsari & pak, 2013) plug-in for grasshopper (mcneel, rutten, & associates, 2007). ladybug+honybee allows for the use of well validated radiance software (ward, 1989), within a parametric environment such as grasshopper, to predict the reduction of solar radiation entering the zone and the daylight availability within the zone. fig. 2 all combinations for the shading system parameterization – lines link the available inputs for the configurations (trondheim, modena and sevilla cases) 030 journal of facade design & engineering volume 5 / number 1 / 2017 essentially, the hourly shading effect induced by the fixed lamella has been translated in the trnsys model as reduction of direct and diffuse solar radiation entering the zone. moreover, the hourly level of internal illuminance predicted by the daylighting simulation has been used within trnsys to control artificial lighting dimming. the parametric analysis was run through the je+ software (zhang & korolija, 2010) in full-factorial mode. je+ software allowed parameterization simulations on the trnsys input file. therefore, for each available combination of inputs, one simulation had to be run. the parametric analysis led 8424 façade configurations, or simulation runs (figure 3), 2808 for each climate. fig. 3 summary of parametric analysis variables (trondheim, modena and seville climate) 2.3 outputs definition table 3 reports the key performance indicators from the simulation results post-processing. these outputs were chosen arbitrarily in order to allow considerations both for the indoor environment quality (indoor air quality, thermal comfort, daylighting) and on the energy consumption of the reference room. all the outputs and their trends were analysed using matlab-based filtering methods. starting from the huge amount of available combinations of inputs, each leading to different outcomes, a series of significant graphical layouts were predefined within matlab, in order to have a general view of all the simulated configurations and to map their outputs; then, keeping the layout of these general graphs, the need for more specific selection through the available configurations arose, in order to easily display on those graphs only the desired ranges of outputs, while excluding the unwanted cases. so, a series of filters on the input and output parameters were implemented and users could select their own preferences excluding undesired ranges for specific variables, thus obtaining the optimal facade configuration for any given boundary conditions. users could define the optimisation parameter, depending on their design targets. for instance, designers may decide to give priority to the comfort of occupants over energy consumption. by filtering their selection, users can set their order of priorities. this simple design tool to support façade designers, based on filtered graphs, is meant to be the foundation for a more user-friendly and accessible tool, that will be developed in the future. table 4 summarizes two different filtering selection procedures available in the tool. so, following one of the two filtering procedures, designers can go through all the available configurations for the façade and end up with just a few cases, whose characteristics depend on the filters applied. the graph for the lpd filtering relates the percentage of hours with an iaq in categories 1 or 2 (y-axis) with the percentage of lpd (x-axis); moreover, the colour of the indicator gives information on the characteristics of the type of glazing used in each facade configuration (u-value, g-value and visible transmittance). 031 journal of facade design & engineering volume 5 / number 1 / 2017 as far as the light consumption filter is concerned, it was decided to use data on artificial lighting consumption as indirect indicators of the value of available daylighting inside the zone. obviously, the lower the artificial lighting consumption results, the higher the daylighting is. the second filter was used to ensure good levels of daylight. by applying this filter, the light consumption (y-axis) was related to the configuration of the shading system: distance between lamella (xaxis) and lamella angle degree (colour of the indicator). output unit description reference specific heating demand kwh/m2 specific cooling demand kwh/m2 long-term percentage of dissatisfied the necessity of using such an indicator instead of the most known ppd is due to the will of having an output for each simulated configuration, summarizing the result of all the considered periods. (carlucci, 2013) light consumption kwh calculation of the lighting consumption has been possible thanks to combined parameterizations regarding the shading system, giving as result the overall luminous flux entering the zone from daylight and considering a designed enlightenment value. mechanical ventilation consumption kwh the electric energy required by fans for providing airflows required to keep an acceptable iaq, considering a specific fan power of 0.75 wh/m3. n° hours with natural ventilation h number of hours over the occupied period when natural ventilation can be activated. n° hours with effective natural ventilation h number of hours over the occupied period when natural ventilation can be activated and provides same or higher airflows than mechanical ventilation n° hours iaq category 1 h co2 concentration in the air has been calculated and indoor air quality categories have been assigned to the environment (en 15251,2007) n° hours iaq category 2 n° hours iaq category 3 n° hours iaq category 4 n° hours thermal category 1 h thermal categories have been assigned to the room environment after having compared the running outdoor mean temperature with the operative temperature inside the room (en 15251,2007)n° hours thermal category 2 n° hours thermal category 3 overheating hours h overheating and overcooling number of hours exceeding thermal categories limits (en 15251,2007) overcooling hours overheating degree °c estimation of the severity of overheating and overcooling (en 15251,2007) overcooling degree pv power generated kwh pv power generated by the façade pv module pv power directly to load kwh power generated by the pv being directly used by the actuators of windows power supply from grid kwh electric energy supplied from the grid to accomplish window automation demand when battery is not charged and no energy is generated by pv table 3 output from the simulation remaining configurations were filtered on the base of total consumption available, choosing cases with the lowest energy demand: heating system, cooling system, mechanical ventilation, light consumptions were considered; moreover, power demand from the grid was taken into account in the total amount of energy consumption. therefore, in the total consumption filter’s graph, the total energy consumption (x-axis) was related to the percentage of hours with an iaq in categories 1 or 2 (y-axis) and lpd value being identified using the colour scale. 032 journal of facade design & engineering volume 5 / number 1 / 2017 comfort priority low consumption priority 1. lpd filter 1. total consumptio+power from grid filter 2. light consumption filter 2. light consumption filter 3. total consumptio+power from grid filter 3. lpd filter 4. final design choice 4. final design choice table 4 different filtering procedures depending on the design priority of the designer finally, a few cases remain, all with very similar comfort and consumption characteristics; therefore, the ultimate selection is related to designer’s preferences on shading configurations (slat orientation angle and distance), proportions of the façade module and type of the glazing to be used. 3 results 3.1 design tool methodology given the parametric model and its output database, a methodology has been set up to support the choices of designers under a performance based approach. a filtering method has been chosen in order to pass from all the database configurations to the target ones. the process is based on preferences of designers; in the tool, these preferences were represented by filters applied to the displayed results. by doing this, the configurations with undesired values for a certain input or output could be excluded from the filtering procedure. by the end of the process, only combinations with similar designed performances remain and the user may visualise the results of their preferences applied on all the possible configurations, facilitating their final choice. 3.2 example application of the design tool one applied example of the façade configuration selection process is shown below: it was used to design a 3m x 6m south-oriented façade in a ‘restaurant’ building application in seville. the priority in the choice of allowed ranges was given to the occupants’ comfort level. 033 journal of facade design & engineering volume 5 / number 1 / 2017 step 1: lpd filtering fig. 4 lpd % seville_3x6_rst_s the first filter selected was the one regarding thermal comfort (lpd). so, after evaluating the available range for the lpd in this specific case while trying to keep lowest values (ppd<10% as recommended by ansi/ashrae standard 55-2013), filters on lpd were applied. in figure 4, all the configurations for ‘restaurant’ 3x6 south-oriented seville facade were reported and a filter on lpd<11 was selected. step 2: light consumption filtering fig. 5 light consumption and shading configuration seville_3x6_rst_s among the façade configurations with higher thermal comfort, a filter on artificial lighting consumption was applied setting a threshold of 26 kwh/m2 (figure 5); this value left many cases for the final choice, while simultaneously reducing available configurations. 034 journal of facade design & engineering volume 5 / number 1 / 2017 step 3: consumption filtering in this example, on the base of consumption range showed in figure 6, total energy consumption filter will be set to 147 kwh/m2 as maximum value. the remaining configurations are reported in figure 7. fig. 6 total consumption related to iaq and lpd seville_3x6_rst_s fig. 7 remaining cases after filtering on consumptions, id simulation displayed seville_3x6_rst_s 035 journal of facade design & engineering volume 5 / number 1 / 2017 step 4: design choice table 5 summarizes the filtering process for the applied example described and lists the resulting optimal façade configurations among which the designer could choose. starting condition seville 3x6 module “rst” application south-oriented design priority comfort filter lpd < 11% filter light consumption < 26 kwh/m2 filter total consumption < 14 kwh/m2 final design choices shading configuration and facade proportions id simulation slat angle degree distance between shadings [cm] area openable window [m2] 499 15° 12 small 715 15° 18 small 716 15° 18 medium 931 15° 24 small 932 15° 24 medium 1579 30° 18 small 1795 30° 24 small 1796 30° 24 medium 2659 45° 24 small table 5 table 5: summary of the filters applied in the example described and available configurations 4 conclusions this paper presents a parametric design tool, suited to support the design process of a modular multifunctional façade allowing the façade itself the possibility of being climate-adaptive. the tool is based on a filtering procedure, generating graphs and driving the user to identify the most optimal façade configuration(s). the available configurations of the façade module – i.e. façade orientation, façade proportions and dimensions and glazing characteristics were firstly modelled in trnsys and secondly simulated through a fully-factorial parameterization. therefore, a smart graphical organization of data has been fundamental in order to manage the high number of results. the choice of the optimally performing configuration for a specific condition depends on the priority of the designer that was assumed to be low-energy consumption or high-thermal comfort oriented. although the tool is at an early and prototypical stage (the knowledge of the adopted matlab code is required), it has the potential to support the designer in the process of defining the most appropriate façade module for a particular climate condition; the tool becomes very useful when filtering the huge amount of configurations given by the parameterization, selecting the most relevant cases depending on the designer’s preferences. further improvements should be undertaken to develop the filtering design tool, enriching it with an intuitive interface, making it more user-friendly. 036 journal of facade design & engineering volume 5 / number 1 / 2017 acknowledgements the research leading to these results has received funding from the european community seventh framework programme (fp7/2007-2013) under grant agreement n. 608678. references ansi/ashrae, standard 55. (2013). thermal environmental conditions for human occupancy. attia, s., favoino, f., loonen, r., petrovski, a., & monge-barrio, a. (2015). adaptive facade system assessment: an initial review. advanced building skins, 1265-1273. austrian institute for building technology. (2007). leitfaden energietechnisches verhalten von gebäuden. bpie. (2011). europe’s buildings under the microscope. british department for comunities and local government. (2013). energy performance of buildings directive. carlucci, s. (2013). thermal comfort assessment of buildings springer. london. casaclima. (2014). regolamento 01.0 concessione del sigillo finestraqualità casaclima. cibse. (2005). am10 natural ventilation in non-domestic buildings. coblenz, c., & achenbach, p. (1963). field measurement of ten electrically-heated houses. comité européen de normalisation. (2007). en 15251 indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. commonenergyproject.eu. (2016). retrieved from commonenergyproject.eu. dipartimento di energia del ministero per lo sviluppo dell’economia. (2013). applicazione della metodologia di calcolo dei livelli ottimali in funzione dei costi per i requisiti minimi di prestazione energetica. ente nazionale italiano di unificazione. (2006). uni en iso 7730 ergonomics of the thermal environment -analytical determination and interpretation of thermal comfort using calculation of the pmv and ppd indices and local thermal comfort criteria. gebhart, b. (1971). heat transfer. klein, s. a. (2010). trnsys 17: a transient system simulation program. madison, usa: solar energy laboratory, university of wisconsin. kommunalog moderniseringsdepartementet. (2010). byggteknisk forskrift (tek 10). lawrence berkeley national laboratory. (2011). window 7.4. berkeley, california. mcneel, r., rutten, d., & associates. (2007, september). grasshopper3d. ministerio de fomento. (2013). documento basico db-he “ahorro de energia. norsk standard. (2012). ns 3701:2012 criteria for passive houses and low energy buildings non-residential buildings. passivhaus institut. (2016). criteria and algorithms for certified passive house components: transparent building components. robert mcneel & associates. (2015, february 26). rhinoceros. seattle, washington. sadeghipour roudsari, m., & pak, m. (2013). ladybug: a parametric environmental plugin for grasshopper to help designers create an environmentally-conscious design. (p. o. lyon, ed.) seem, j. (1987). modelling of heat in buildings. treberspurg, m., & djalili, m. (2010). state of the art report multifunctional façade systems. boku: sci-network. ward, g. (1989). the radiance lighting simulation and rendering system. (b. t. laboratory, ed.) berkeley, california. zhang, y., & korolija, i. (2010, august). performing complex parametric simulations with jeplus. pinotti, r. (2016). parametric model for a multifunctional façade system (master’s thesis, free university of bolzano, bolzano, italy). from city’s station to station city 019 journal of facade design & engineering volume 10 / powerskin / 2022 influence of automated façades on occupants: a review pedro de la barra*1, alessandra luna-navarro1, alejandro prieto2, claudio vásquez3, ulrick knaack1 * corresponding author, p.delabarraluegmayer@tudelft.nl 1 delft university of technology, netherlands 2 universidad diego portales, chile 3 pontificia universidad católica de chile, chile abstract several studies performing building simulations showed that the automated control of façades can provide higher levels of indoor environmental quality and lower energy demand in buildings, in comparison to manually controlled scenarios. however, in several case studies with human volunteers, automated controls were found to be disruptive or unsatisfactory for occupants. for instance, automated façades became a source of dissatisfaction for occupants when they did not fulfil individual environmental requirements, did not provide personal control options, or did not correctly integrate occupant preferences with façade operation in energy-efficient controls. this article reviews current evidence from empirical studies with human volunteers to identify the key factors that affect occupant response to automated façades. only twenty-six studies were found to empirically investigate occupant response to automated façades from 1998 onwards. among the reviewed studies, five groups of factors were found to influence occupant interaction with automated façades and namely: (1) personal factors, (2) environmental conditions, (3) type and mode of operation, (4) type of façade technology, and (5) contextual factors.. overall, occupant response to automated façades is often poorly considered in research studies reviewed because of the following three reasons: (i) the lack of established methods or procedures for assessing occupant response to automated façade controls, (ii) poor understanding of occupant multi-domain comfort preferences in terms of façade operation, (iii) fragmented research landscape, on one hand results are mainly related to similar contextual or climatic conditions, which undermines their applicability to other climates, while on the other hand the lack of replication within the same conditions, which also undermines replicability within the same condition. lastly, this paper suggests future research directions to achieve a holistic and more comprehensive understanding of occupant response to automated façades, aiming to achieve more user-centric automated façade solutions and advanced control algorithms. in particular, research on the impact of personal factors on occupant satisfaction with automated controls is deemed paramount. keywords automated control, automated façades, occupant-façade interaction, occupant acceptance, occupant comfort, dynamic façades doi http://doi.org/10.47982/jfde.2022.powerskin.2 020 journal of facade design & engineering volume 10 / powerskin / 2022 1 introduction in buildings, façades act as a buffer and connector between indoors and outdoors (knaack et al., 2014) and affect building energy consumption and occupant multi-domain environmental comfort (luna-navarro et al., 2022). in particular, façades can affect occupant satisfaction with the thermal environment (carmody et al., 2004), acoustic (tang, 2017), air quality (izadyar et al., 2020), daylight, and view out (boyce et al., 2003; heschong et al., 2013). dynamic façade technologies, identified as building systems or façades that can move by forces acting on an object, can vary the visual or solar transmittance (e.g. switchable glazings or movable blinds) or the level of airflow through them (e.g. openable vents) (barozzi et al., 2016) to effectively respond to changes in outdoor or indoor conditions. dynamic façades can be manually controlled by occupants (reinhart & voss, 2003), or react to changes in environmental conditions, either by passively responding to them (e.g. phase change materials (balocco & petrone, 2017)), or by automatedally being controlled by actuators and sensors (bakker et al., 2014). several automated façades are controlled by a semi-automated logic, which also allow occupants to override the system when they disagree with the control logic (gunay et al., 2017). previous work showed that automated controls can assist occupants and overcome the limitations of manual operation by reducing energy consumption (sullivan et al., 1994; tzempelikos & athienitis, 2007) or improving thermal or visual comfort (hosseini et al., 2019). contrariwise, the automated control can also negatively impact occupants’ satisfaction and behaviour, when the control action does not match individual requirements (day et al., 2019; grynning et al., 2017). in scenarios with automated façades, the type of control logic and the occupant-façade interaction strategy (i.e. the level and mode of interaction) affect occupant behaviour and satisfaction, indoor environmental quality, and energy consumption (luna-navarro et al., 2020). several studies showed that occupant requirements are subjective and individual, affecting occupant response to the control system (cheng et al., 2016; gunay et al., 2017). these variances in occupant responses may be explained by a different personal significance of environmental comfort domains (meerbeek et al., 2014; cheng et al., 2016) or differences in the level of knowledge of users with automated control (lee et al., 2012). therefore, the adaptation of the control logic to individual occupant requirements can be important to achieve occupant environmental comfort and satisfaction, acceptance of automated control strategies, and energy performance of office buildings (kim et al., 2009). four previous studies have performed a literature review on automated controls for automated façades. konstantoglou & tsangrassoulis (2016) reviewed automated control strategies of dynamic shading systems and their effects on building energy performance and indoor environmental comfort. this literature review concluded that, even though automated control strategies can enhance energy performance and occupants’ comfort, their high level of complexity makes them prone to failure and therefore they often do not achieve the predicted performance. jain & garg (2018) analysed the feasibility of various daylight prediction methods and their application in controlling dynamic shading and lighting systems, coming to the conclusion that modified and improved closed loop systems, which include and adapt to user feedback, are better than open loop control strategies based on sensor measurements. however, luna-navarro et al. (2020) examined interaction strategies and requirements for satisfactory occupant-façade interaction, pointing out that achieving effective closed-loop operations by satisfactorily engaging the occupant, is challenging since several factors play a role. tabadkani et al. (2021) reviewed the state-of-theart regarding occupant-centric control strategies, showing that current interaction strategies are ineffective in improving both user satisfaction and energy efficiency. ultimately, there is a need to 021 journal of facade design & engineering volume 10 / powerskin / 2022 comprehensively review existing studies on occupant-automated façade interaction and highlight the current evidence of the factors that influence individual occupant response to automated façade controls. this will facilitate the design and operation of automated façades in an occupant-centred manner. to achieve this, the aim of this work is to review previous experimental work that evaluates human volunteers’ responses to automated façades, either in lab experiments or field studies, and to evaluate current evidence to indicate the directions of future research. section 2 explains the review methodology, including selection criteria and the classification scheme that structures this article. section 3 describes the results of the review, including the discussion of the evidence collected. finally, section 4 draws the conclusions, and highlights potential future challenges and investigations based on the review conducted. 2 methods in order to review previous work on the factors that influence occupant preferences regarding automated façade operation, a systematic review was conducted. this section provides a detailed explanation of the inclusion and exclusion criteria and keywords. advanced queries in all databases based on terms definition were conducted. therefore, a searching protocol through defined keywords has been used, as shown in table 1. as a result, all the papers must meet the following requirements: only papers on automated dynamic façade control strategies and that monitor actual occupant response through experiments and monitoring with human volunteers were considered. occupant response was considered by including the following keywords: user interaction, comfort, satisfaction and acceptance. the following studies were excluded from this literature review: – studies that only considered manually controlled systems that do not incorporate any automated feature; – studies that only considered façades that passively respond to changes in environmental conditions but do not have active control strategies; – studies without human volunteers. keywords were divided into four groups (table 1): (1) façade operation, (2) façade technology, (3) experiment placement, and (4) façade control. consequently, references were searched (wos (2.328), scopus (2.795)). only 127 studies were selected by title and abstract, reduced to 106 after removing duplicates. full-text revisions assessed the eligibility of articles, applying the inclusion and exclusion criteria described previously. finally, we ended up with 26 studies that met the requirements for being examined for this literature review, published between 1998 and 2022. 022 journal of facade design & engineering volume 10 / powerskin / 2022 table 1 search keywords database date of search inclusion searching criteria in title, abstract and keywords number of articles 1) façade operation (2) façade technology (3) experimental testing web of science 24-2-2022 (adaptive or responsive or dynamic or kinetic or intelligent or advance or smart or interactive or active or automated or switchable or climate or control ) w/3 (façade or envelope or skin or shading or glazing or glazed or window or venetian or roller or blind) and (laboratory or on-site or field or experimental or post-occupancy or testbed or test room or campaign or monitoring) 2.328 scopus 22-2-2022 2.795 studies were not restricted in terms of geographical location since the scope of the review is also to contextualise the research results and evaluate whether any geographical location is missing in the research landscape to inform future research directions accordingly. fig. 1 the classification scheme used in this review to group the factors influencing occupant response that were identified through the literature review (after (luna-navarro et al., 2020) 023 journal of facade design & engineering volume 10 / powerskin / 2022 2.1 factors that influence occupant response the classification scheme from luna-navarro et al. (2020) was used in this review to group the factors that affect users as follows (shown in fig. 1): 1. personal factors, 2. environmental conditions, 3. type and mode of operation, 4. type of façade technology, and 5. contextual factors (e.g. type of building etc.). 3 results and discussion 3.1 type of occupant response to af studied through previous work before going to the evidence on factors affecting occupant response, the review results are analysed to identify what type of user response has been considered by previous work. occupant response was evaluated in terms of behaviour (13 studies), environmental comfort (20 studies), environmental satisfaction (18 studies), environmental sensation (6), acceptance of the control system or of the indoor environmental conditions (5 studies), and overall satisfaction with the automated control (4 studies). table 2 shows the type of occupant response considered by each study. these different types of occupant response were studied by previous authors thorugh questionnaires, surveys, and interviews. in addition, occupant behaviour was monitored by tracking occupant override actions (bakker et al., 2014; cheng et al., 2016; goovaerts et al., 2017; gunay et al., 2017; lee et al., 2012; luna-navarro et al., 2022; motamed et al., 2019; sadeghi et al., 2016), or occupant actions to deactivate the control logic (meerbeek et al., 2014), and set-points (clear et al., 2006; guillemin & morel, 2001, 2002; vine et al., 1998). some studies used the term “comfort” and “satisfaction” interchangeably (cheng et al., 2013, 2016; lee et al., 1998; sadeghi et al., 2016), while other studies used these terms to describe different states of mind. for instance, comfort was intended as the threshold or set-point that defines comfortable environmental conditions, which was often contrasted by surveys of the occupant perception to the environmental quality (bakker et al., 2014; cheng et al., 2013, 2016; clear et al., 2006; guillemin & morel, 2001, 2002; kim et al., 2009; lee et al., 2012; meerbeek et al., 2014; motamed et al., 2017; sadeghi et al., 2016; taniguchi et al., 2012; vine et al., 1998). satisfaction was used to indicate occupant contentment with the visual environment (cheng et al., 2013, 2016; choi et al., 2019; clear et al., 2006; day et al., 2019; guillemin & morel, 2002; karlsen et al., 2015; kim et al., 2009; lolli et al., 2019, 2020; luna-navarro et al., 2022; meerbeek et al., 2014; sadeghi et al., 2016; vine et al., 1998), thermal environment (choi et al., 2019; clear et al., 2006; day et al., 2019; lolli et al., 2020; lunanavarro et al., 2022; meerbeek et al., 2014; sadeghi et al., 2016; wu et al., 2020), acoustic environment (clear et al., 2006; lolli et al., 2019; luna-navarro et al., 2022), air quality (luna-navarro et al., 2022), or overall satisfaction with the automated façade (cheng et al., 2013, 2016; clear et al., 2006; day et al., 2019; goovaerts et al., 2017; gunay et al., 2017; karlsen et al., 2015; lolli et al., 2020; luna-navarro et al., 2022; meerbeek et al., 2014; painter et al., 2016). three studies incorporated acceptance as a descriptor of the level of agreement with the control system implemented. acceptance was studied in terms of the different modes of operation applied to venetian blinds (vine et al., 1998) by registering occupant override actions that were intended as a lack of acceptance of the control logic operating the façade (goovaerts et al., 2017; gunay et al., 2017). only one study considered occupant acceptance of the indoor environment (acceptance of the overall indoor environment (lolli et al., 2019)). 024 journal of facade design & engineering volume 10 / powerskin / 2022 table 2 summary of type of occupant response reported by studies: overall response to the control strategy & façade technology (cs); occupant response to the indoor environmental quality (ieq); none (n). occupant response to the indoor environment b eh a vi o u r / in te ra ct io n b eh a vi o u r c o m fo rt s a ti sf a ct io n a cc ep ta n ce p er ce p ti o n s en sa ti o n (vine et al., 1998) cs ieq cs / ieq cs n n (guillemin & morel, 2001) cs ieq n n n n (guillemin & morel, 2002) cs ieq ieq / cs n n n (clear et al., 2006) cs n ieq / cs n n n (kim et al., 2009) n ieq n n n n (lee et al., 2012) cs ieq cs n n n (taniguchi et al., 2012) n ieq n n n ieq (cheng et al., 2013) n ieq cs n n n (bakker et al., 2014) cs ieq ieq / cs n cs n (meerbeek et al., 2014) cs ieq ieq / cs n n n (karlsen et al., 2015) n ieq ieq / cs n n n (cheng et al., 2016) cs ieq ieq cs n n (painter et al., 2016) n ieq cs n n n (sadeghi et al., 2016) cs ieq ieq n cs n (goovaerts et al., 2017) cs ieq cs n ieq n (gunay et al., 2017) cs ieq cs cs n n (motamed et al., 2017) n ieq n n n ieq (choi et al., 2019) n n ieq n ieq n (day et al., 2019) n ieq ieq / cs n n n (lolli et al., 2019) n ieq cs ieq n ieq (motamed et al., 2019) cs ieq n n n n (wu et al., 2020) n ieq cs n n ieq (bian et al., 2020) n ieq n n n ieq (lolli et al., 2020) n ieq ieq n n ieq (korsavi et al., 2021) n ieq cs n n n (luna-navarro et al., 2022) cs ieq ieq / cs n n n a few studies also assessed perceived health (choi et al., 2019) and productivity (choi et al., 2019; sadeghi et al., 2016). the least studied aspect of occupant response was sensation. regarding the visual environment, glare sensation vote (gsv) and illuminance rating (ir) were used to capture visual sensation. thermal sensation vote was the subjective rating scale to capture occupant thermal sensation (tsv), which was assessed by using a 5-point likert scale (from cold to hot) (lolli et al., 2019, 2020). 3.2 contextual factors affecting occupant response to af all of the studies provide information about the context in which the experiments or the field measurements took place. table 3 describes the contextual factors summarised from articles, classifying them into location, climate, orientation, testing facility, and floor layout. regarding location, the studies were conducted in five european countries (14 studies), two north-american 025 journal of facade design & engineering volume 10 / powerskin / 2022 countries (7 studies), and three asian countries (5 studies). despite the variety of locations, the climates were limited to temperate and continental conditions (fig. 2). table 3 summary of contextual factors described by previous works to assess the influence of façades on occupant response. climate orientation layout location n o rt h w es t s o u th w es t s o u th s o u th ea st e a st n o n -d ec la re d o p en p la n 2 -3 p er so n s o ffi ce s in g le o ffi ce (vine et al., 1998) oakland, california us √ √ (guillemin & morel, 2001) lausanne switzerland √ √ (guillemin & morel, 2002) lausanne switzerland √ √ (clear et al., 2006) berkeley, california us √ √ (kim et al., 2009) seoul south korea √ √ √ √ (lee et al., 2012) berkeley, california us √ √ (taniguchi et al., 2012) hiratsuka japan √ √ (cheng et al., 2013) beijing china √ √ (bakker et al., 2014) eindhoven the netherlands √ √ (meerbeek et al., 2014) eindhoven the netherlands √ √ (karlsen et al., 2015) aalborg denmark √ √ (cheng et al., 2016) beijing china √ √ (painter et al., 2016) leicester u √ √ (sadeghi et al., 2016) west lafayette, indiana -us √ √ (goovaerts et al., 2017) brussels belgium √ √ (gunay et al., 2017) ottawa canada √ √ (motamed et al., 2017) lausanne switzerland √ √ √ (choi et al., 2019) toronto canada √ √ (day et al., 2019) charlotte/richmond/virginia us √ √ (lolli et al., 2019) trondheim norway √ √ (motamed et al., 2019) lausanne switzerland √ √ (wu et al., 2020) lausanne switzerland √ √ (bian et al., 2020) guangzhou china √ √ (lolli et al., 2020) trondheim norway √ √ (korsavi et al., 2021) plymouth uk √ √ √ (luna-navarro et al., 2022) cambridge uk √ √ in terms of the relevance of the weather conditions, clear et al. (2006) pointed out that two parameters were strongly correlated to occupant behaviour, such as the variation of the sky conditions and the outdoor vertical illuminance. korsavi et al. (2021) have also reported that occupant behaviour can be impacted by building-related features such as orientation and floor level on automated window operation. lolli et al. (2019) and luna-navarro et al. (2020) showed that orientation and sky condition affect blind occlusion. moreover, depending on the hemisphere, some orientations can be more challenging. for example, the west and east orientation in the northern hemisphere is challenging due to the low-angle sun situations during the late winter and early spring (day et al., 2019), while south orientation can be more challenging for overheating. in most cases, the studies were conducted with south-oriented façades (14 studies). 026 journal of facade design & engineering volume 10 / powerskin / 2022 fig. 2 climate where the reviewed studies were performed (classification according to the koppen climate). the climates are: humid sub-tropical climate (cfa); temperate oceanic climate (cfb), cold-summer mediterranean climate (csc), warm-summer humid continental climate (dfb), monsoon-influenced hot-summer humid continental climate (dwa). concerning where the study took place, two main locations were found: laboratory and real office building (fig. 3). laboratory refers to a room fully equipped with sensors and other instruments and that can be adjusted to create the desired experimental conditions and collect relevant data from the indoor environment and occupants. in addition, laboratories are occupied by users only for the purpose of conducting an experiment. in contrast, real office building includes real-world occupied buildings. the studies were split almost eqally between field and lab environments. fig. 3 pie chart diagram with the number of studies performed on laboratory and real buildings. some studies showed that the number of occupants and room characteristics may impact occupant response. clear et al. (2006) mentioned that visual dissatisfaction reported by occupants was not only produced by the level of sun exposure of the windows but also by the interior walls and object reflection. additionally, occupants indicated that the room’s colour was a source of dissatisfaction. 027 journal of facade design & engineering volume 10 / powerskin / 2022 the weather conditions impact occupant response, and the magnitude of its impact depends on other factors such as orientation, building characteristics, obstructions, window size, and indoor features (karlsen et al., 2016). also, indoor room characteristics, such as type of layout, wall colour, amenities, and office features have been proven to affect comfort perception, satisfaction, and occupant response (bakker et al., 2014; clear et al., 2006). regarding the number of occupants in the same room, cheng et al. (2016) and bian et al. (2020) stated that the situation of multiple persons in the general space, performing different tasks should affect the occupant’s response to the automated control, even changing throughout the day. however, only a few studies have studied occupant response in shared office spaces. 3.3 personal factors affecting occupant response to af personal factors might affect occupants’ behaviour and perception, varying from person to person and depending on specific occupants’ attributes (clear et al., 2006). based on the articles reviewed, personal factors that might affect occupant response are shown in fig. 4 and are grouped into: “general characteristics”, “personal attitudes”, and “personal significance of the environmental quality”. general characteristics refer to the group of features that describe each individual, such as age, gender, profession or work performed, use of glasses, visual disability, handedness, eye colour, and ethnicity (karlsen et al., 2015). attitudes refer to the predisposed state of mind of occupants, including habituation to the laboratory or test room, enjoyment of task, pleasantness of the indoor space, rest, and mood (clear et al., 2006). finally, personal significance of the environmental quality defines the level of importance that an occupant attributes to a specific environmental domain, such as visual aspects, thermal aspects, air quality, acoustic aspects, privacy, personal control, and room quality, e.g. amenities or services in the room (sadeghi et al., 2016). fig. 4 the number of studies that investigated personal factors in previous works as affecting occupant response to automated façades are shown per type of personal factor studied: general characteristics, attitude, and personal significance of the indoor environmental condition. 028 journal of facade design & engineering volume 10 / powerskin / 2022 the most reported personal characteristics were age and gender, while the level of rest was rarely considered. five studies included occupants’ attitudes (clear et al., 2006; karlsen et al., 2015; lunanavarro et al., 2022; motamed et al., 2017; sadeghi et al., 2016), with luna-navarro et al. (2022) being the study that took into account the most attitude descriptors. a few studies considered the personal significance of environmental characteristics, such as visual environment, thermal environment, air quality privacy, personal control and room quality (clear et al., 2006; karlsen et al., 2015; sadeghi et al., 2016). clear et al. (2006) gave a detailed summary about all of them. even though most of the studies gathered personal information, the data was often not used to differentiate the results and provide evidence about the importance of occupants’ characteristics, attributes, and personal significance in response to automated façades. overall, three out of twentysix studies differentiated the data on one or more personal factors to evaluate their impact on occupant response. clear et al. (2006) reported that age, gender, and other characteristics affected occupants’ responses to the electrochromic window operation. this was determined by finding correlations between characteristics of the subjects and appraisals of the different test modes. the main findings were a significant correlation (explained by the level of fitness r2) between the importance of quiet and sensitivity to environmental noise (r2 = 0.48), the importance of access to outdoor view and the importance of windows (r2 = 0.25), the importance of good lighting and the importance of light and window control (r2 = 0.22), and the importance of good temperature control and the sensitivity to both heat and cold (r2 = 0.26). karlsen et al. (2015) analysed the percentage of males and females who selected one of the two control strategies (simple and detailed) or the option ‘no preference’. using a fisher test, the analysis showed no significant dependence between gender and preferred control strategy. painter et al. (2016) examined the data for studying user interaction, considering that one out of the four participants had a visual condition that affected her vision at times and increased her sensitivity to light. however, no evidence was reported about the effect of the visual conditions in the responses provided by the occupant. some authors pointed out that personal factors may determine whether the selected control threshold would lead to a satisfactory indoor environment (lee et al., 2012; painter et al., 2016). for instance, personal significance to specific surroundings impacts occupant tolerance to indoor environmental conditions. karlsen et al. (2015) suggested that the participants might tolerate some glare disturbance depending on the relative importance of access to the outside view. even the occupants’ knowledge (regarding habituation) about the system functionality may impact their ability to interact with the automated façade (bakker et al., 2014; lee et al., 2012; sadeghi et al., 2016). additionally, specific users’ characteristics, such as wearing glasses (lee et al., 2012) and visual conditions (painter et al., 2016), could explain why some occupants are more likely to prefer different lighting conditions. several studies did not report information on personal factors, both in the laboratory and in field studies. this includes a lack of clear information about general characteristics (e.g. wearing glasses, vision disability, handedness, eye colour), attitude (e.g. habituation, enjoyment, pleasantness, rest, mood), and personal significance (regarding the visual, thermal, air quality, personal control, room, and acoustic environment). 029 journal of facade design & engineering volume 10 / powerskin / 2022 3.4 impact of occupant response to indoor environmental conditions on occupant overall satisfaction with af occupant response to indoor environmental condition was taken into account in 26 studies when evaluating the performance of af. the indoor environmental conditions were evaluated by capturing a wide range of comfort domains, particularly in the visual and thermal domains (see table 4). table 4 summary of environmental domains measured by sensors and occupant responses captured by questionnaires investigated in previous works. visual environment t h er m a l e n vi ro n m en t a co u st ic e n vi ro n m en t in d o o r a ir q u a li ty b eh a vi o u r a n d in te ra ct io n c o m fo rt s a ti sf a ct io n a cc ep ta n ce p er ce p ti o n s en sa ti o n o u ts id e vi ew d a yl ig h t g la re (vine et al., 1998) √ √ √ √ √ √ √ (guillemin & morel, 2001) √ √ √ √ √ (guillemin & morel, 2002) √ √ √ √ (clear et al., 2006) √ √ √ (kim et al., 2009) √ √ √ (lee et al., 2012) √ √ √ √ √ (taniguchi et al., 2012) √ √ √ (cheng et al., 2013) √ √ √ (bakker et al., 2014) √ √ √ √ √ √ √ (meerbeek et al., 2014) √ √ √ √ √ (karlsen et al., 2015) √ √ √ √ (cheng et al., 2016) √ √ √ √ √ (painter et al., 2016) √ √ √ √ (sadeghi et al., 2016) √ √ √ √ √ √ √ √ (goovaerts et al., 2017) √ √ √ √ √ √ √ (gunay et al., 2017) √ √ √ √ (motamed et al., 2017) √ √ √ √ (choi et al., 2019) √ √ √ (day et al., 2019) √ √ √ √ √ (lolli et al., 2019) √ √ √ √ √ (motamed et al., 2019) √ √ √ √ (wu et al., 2020) √ √ √ √ (bian et al., 2020) √ √ √ √ (lolli et al., 2020) √ √ √ √ (korsavi et al., 2021) √ √ √ (luna-navarro et al., 2022) √ √ √ √ √ √ √ √ total 4 24 12 11 3 1 13 20 18 4 4 6 the visual environment was evaluated by measuring daylight levels (24 studies), glare probability (12 studies), and access to outside view (4 studies). daylight was very often measured on the work plane in terms of horizontal illuminance (18 studies) and vertical illuminance (10 studies). glare probability was calculated by measuring vertical illuminance at eye level (6 studies) and luminance distribution from the occupant’s point of view by hdr imaging (6 studies). access to outside view was monitored by estimating the visible unobstructed window area (1 study). the thermal environment was 030 journal of facade design & engineering volume 10 / powerskin / 2022 captured by measuring indoor air temperature (9 studies), window surface temperature (2 studies) and vertical irradiance at the window plane (3 studies). the acoustic environment (1 study) and indoor air quality (1 study) were not extensively described since the articles reviewed are talking about dynamic shading devices. although several studies captured occupant response to indoor environment, only a few reported that occupant response to indoor environmental conditions affected occupants’ response to af (fig. 5), either in terms of the visual environment, thermal environment, privacy and acoustic comfort. several studies showed that occupant response to automated control strategies was significantly driven by occupant dissatisfaction with indoor illuminance control (21 studies). regarding visual occupant requirements, office occupants tended to prefer higher indoor illuminance levels when the af was activated (bakker et al., 2014; cheng et al., 2013; clear et al., 2006; guillemin & morel, 2002; lee et al., 2012; vine et al., 1998). sadeghi et al. (2016) and goovaerts et al. (2017) reported that override actions to open the façade were carried out when increasing daylight was needed, while motamed et al. (2017) described that the preference for the automated mode was driven by the discomfort produced on excessive daylight indoor conditions. additionally, it was told that occupants’ illuminance requirements differ with tasks and areas (cheng et al., 2016), changing even throughout the day (bian et al., 2020). vine et al. (1998) indicated that occupants were satisfied not only with the ability to control the blinds to adjust the amount of daylight but also to adjust the direction and distribution of the daylight in the indoor space. fig. 5 the number of studies that showed that environmental factors affect occupant response with the af operation. glare discomfort is the most frequent factor affecting occupants’ responses to the automated control (16 studies). when the automated control did not effectively protect against glare, occupants overrode (goovaerts et al., 2017; gunay et al., 2017; lolli et al., 2019; sadeghi et al., 2016) or adjusted the control parameter as allowed (bian et al., 2020). glare competes with daylight provision. when the automated control was operated based on glare, occupants intervened to improve daylight quality (gunay et al., 2017; meerbeek et al., 2014). when the automated control avoided discomfort from direct sun and or glare, occupants preferred more daylight (lolli et al., 2019; meerbeek et al., 2014; motamed et al., 2017). 031 journal of facade design & engineering volume 10 / powerskin / 2022 other studies also suggested that the outside view impacts occupants’ environmental preferences (9 studies), influencing even the choice of the preferred control strategy (karlsen et al., 2015; lunanavarro et al., 2022). clear et al. (2006) and meerbeek et al. (2014) pointed out that the outside view was an important comfort factor for the occupants, who were operating the façade not only to improve the connection with the outside but also to decrease the level of visual stimulus from the exterior. gunay et al. (2017) described that occupants interfered with the automated control mainly to improve the outside view when the system worked to avoid glare. a few studies also mentioned that occupant response was impacted by privacy (3 studies). sadeghi et al. (2016) mentioned privacy as the most important factor affecting lowering blind actions together with glare discomfort. however, privacy depends on contextual characteristics such as the surrounding environment and position in the building. for instance, meerbeek et al. (2014) explain that the subjects surveyed were not worried about privacy because the office was located on the third floor, far away from the street level. dissatisfaction with the thermal environment was mainly related to the ability of the façade to control the incoming solar radiation (lolli et al., 2019; sadeghi et al., 2016) or to provide air flow, as suggested by korsavi et al. (2021) and lolli et al. (2020). a few studies surveyed occupants to calculate the predicted mean vote (pmv) (kim et al., 2009). a few studies also reported acoustic environmental conditions and acoustic satisfaction (4 studies). luna-navarro et al. (2022) pointed out that acoustic discomfort was the main driver of occupant dissatisfaction with the façade system. studies have also reported that metrics used to capture occupant requirements presented problems when implemented into the automated façade control system. goovaerts et al. (2017) informed that dgp underestimated the impact of direct sunlight, which generated the set-point lowered by users when direct sunlight was present. a similar problem was reported by taniguchi et al. (2012) when the algorithm to evaluate indoor luminance overestimated glare sources in the afternoon. other authors have said people’s glare sensation increases gradually from morning until midday but becomes stable or more sensitive to glare in the afternoon (bian et al., 2020). the majority of the studies investigated the impact of control strategies on occupant’s visual domain. on the thermal domain, articles did not report conclusions on how an automated façade affects the thermal environment. how distance from the façade affects occupant interaction with the façade is still undetermined. only day et al. (2019) mentioned that the window’s proximity improves occupants’ satisfaction. moreover, the impact of indoor environmental conditions on the occupant response to the automated façade has not been researched sufficiently, making it difficult to extrapolate results, throughout different façade technologies, control logics, and under different weather conditions, to improve current control strategies. what the main drivers of occupant satisfaction with automated façades are, remains undetermined, in particular whether or not there is an inherent order of importance among different environmental domains. for example, it has been reported that occupants significantly value daylight access (lee et al., 2012) and outside view (choi et al., 2019; wu et al., 2020) and that these factors are often the main reason for overriding an automated façade control system (meerbeek et al., 2014). the personal level of control also influences occupant environmental requirements. thus, occupant preferences may be different depending on the interaction level provided by the façade controller (luna-navarro et al., 2020). however, there is no clear evidence on whether or not a hierarchy of comfort domains exists. 032 journal of facade design & engineering volume 10 / powerskin / 2022 3.5 the effect of control and interaction logic on occupant response to af the control and interaction strategy influences occupant response to the automated façade (bakker et al., 2014). as a way to improve occupant satisfaction with the automated façade, studies have tested different control strategies. table 5 summarises the main characteristics of the control logics studied up to now. additionally, the table gives information on the sensor position (interior/exterior). table 5 summary of environmental domains measured by sensors and occupant responses captured by questionnaires investigated in previous works. control loop source of information control algorithm control algorithm sensor place o cc u p a n t in te ra ct io n c lo se d -l o o p o p en -l o o p s en so rb a se d m o d el -b a se d o th er s r u le -b a se d a d a p ti ve p re d ic ti ve visual environment t h er m a l e n vi ro n m en t a ir q u a li ty e xt er io r in te ri o r o n o cc u p a n t o u ts id e vi ew d a yl ig h t g la re (vine et al., 1998) √ √ √ √ √ √ √ √ (guillemin & morel, 2001) √ √ √ √ √ √ √ √ √ √ √ (guillemin & morel, 2002) √ √ √ √ √ √ √ √ √ √ √ (clear et al., 2006) √ √ √ √ √ √ √ (kim et al., 2009) √ √ √ √ √ √ (lee et al., 2012) √ √ √ √ √ √ √ √ (taniguchi et al., 2012) √ √ √ √ √ √ √ √ (cheng et al., 2013) √ √ √ √ √ √ √ √ √ √ (bakker et al., 2014) √ √ √ √ √ √ √ √ (meerbeek et al., 2014) √ √ √ √ √ √ √ (karlsen et al., 2015) √ √ √ √ √ √ √ √ √ √ (cheng et al., 2016) √ √ √ √ √ √ √ √ (painter et al., 2016) √ √ √ √ √ √ (sadeghi et al., 2016) √ √ √ √ √ √ √ (goovaerts et al., 2017) √ √ √ √ √ √ √ √ √ (gunay et al., 2017) √ √ √ √ √ √ √ √ (motamed et al., 2017) √ √ √ √ √ √ √ √ (choi et al., 2019) √ √ √ √ √ (day et al., 2019) √ √ √ √ √ (lolli et al., 2019) √ √ √ √ √ √ (motamed et al., 2019) √ √ √ √ √ √ √ √ √ (wu et al., 2020) √ √ √ √ √ √ √ √ √ (bian et al., 2020) √ √ (lolli et al., 2020) √ √ √ √ √ √ √ (korsavi et al., 2021) √ √ √ √ √ (luna-navarro et al., 2022) √ √ √ √ √ √ 033 journal of facade design & engineering volume 10 / powerskin / 2022 sadeghi et al. (2016) reported a dependency between façade configuration (shade position or window transmittance) and occupant satisfaction with the indoor environment. clear et al. (2006) and day et al. (2019) pointed out a similar situation for the switchable glazing operation. when electrochromic glazing became opaque, occupants felt more dissatisfied with that configuration, leading to override actions to improve daylight and outside view. regarding control loops, studies have described two types: open-loop (12 studies) and closed-loop (14 studies). the control logics used three different sources of information: sensor-based (25 studies), model-based (7 studies), and others (e.g. time, sun profile, weather file, and schedule). the low number of model-based control cases is explained by the fact that this method is computationally intense, lacking in algorithms to develop occupant models inside building controllers (gunay et al., 2017). the control algorithm implemented in the façade control system was classified into three categories: rule-based (20 studies), adaptive (5 studies), and predictive (3 studies). most studies implemented rule-based algorithms to control automated façade systems. the adaptive algorithms found were q-learning (cheng et al., 2016) and recursive learning (gunay et al., 2017). only three studies implemented a predictive algorithm to analyse and integrate outdoor weather and indoor lighting conditions into a model-based system (guillemin & morel, 2001; 2002) to anticipate occupant interaction with the automated façade system (gunay et al., 2017). automated façade control can improve indoor environmental quality (clear et al., 2006; kim et al., 2009; lolli et al., 2019; motamed et al., 2019), although the effect on occupant satisfaction varies from case to case. for instance, lolli et al. (2020) reported that automated control improved the desired indoor environmental quality. similarly, luna-navarro et al. (2022) showed that, when the control strategy is properly designed, automated control can provide greater satisfaction than a manually controlled environment. however, if the automated control is disruptive to users, manual controls outperform automated ones. on the contrary, motamed et al. (2017) showed that the subjects’ visual performance was not improved by automated control strategies. therefore, the type of control strategy is an important factor for occupant satisfaction. the impact of façade control operation affects the indoor space zones differently. day et al. (2019) reported that occupants placed in the interior, far away from the window did not receive enough daylight when the switchable glazing became dark, and occupants were ultimately displeased with their workspaces. in regards to determining what aspects of the control strategy most affect occupants, current evidence is fragmented. in terms of control thresholds, goovaerts et al. (2017) showed that different controls could achieve equal indoor illuminance levels on a desk in the same context but still affect satisfaction among occupants differently. therefore, personalising the control threshold may not be sufficient to meet individual occupant requirements. in this context, it seems well-established that occupants have individual comfort preference (cheng et al., 2013) and behavioural responses under different control algorithms (korsavi et al., 2021). however, to what extent personalisation of control strategies is required is less clear. the automated control’s capability to predict occupant preferences is deemed important to improve occupant satisfaction with automated controls (meerbeek et al., 2014).. a predictive lighting and blinds control algorithm can significantly reduce electric lighting consumption in perimeter office spaces whilst mantaining user comfort (gunay et al., 2017). the predictive control strategy should incorporate as many profiles as there are occupants in the indoor space (korsavi et al., 2021). painter et al. (2016) mentioned that a solution might be to develop tools that allow the system to evaluate comfortable and uncomfortable conditions based on physical measurements and occupant control actions. however, capturing more than one user profile and integrating all that information is one of the challenges that adaptive and predictive control strategies currently face. 034 journal of facade design & engineering volume 10 / powerskin / 2022 few studies advocated for controlling and designing the façade by taking into account the multidomain influence of façades on users (luna-navarro et al., 2022), however, there is still discussion on whether one environmental domain should be prioritised by the control (visual over thermal) or visual aspect (glare over daylight) . this is particularly challenging since adjusting one comfort domain can affect the others. for example, goovaerts et al. (2017) showed that occupants overrode the automated control to increase daylight when it was configured to avoid discomfort glare. karlsen et al. (2015) mentioned that occupants felt more comfortable with the automated control when it considered indoor environmental parameters affecting their satisfaction perception (in this case, thermal aspects). gunay et al. (2017) pointed out that occupants intervened to improve the view quality when the system operated based on glare mitigation or building energy efficiency. regarding the mode of operation, several studies indicated that the automated façade might influence occupant response because it affected not only the physical parameter defining indoor environmental quality but also impacted the fulfilment of the occupant requirements for personal control (meerbeek et al., 2014). for instance, bakker et al. (2014) reported that less frequent, discrete transitions in façade operation are better appreciated than smooth transitions at a higher frequency. however, this topic is largely unexplored. personal control is key to restoring comfort when the system is not efficient in controlling environmental parameters (day et al., 2019). guillemin & morel (2002) reported that occupants interacted with the automated control as often as the manual control system, reinforcing that comfortable indoor conditions are insufficient for occupants.occupant environmental requirements and preferences are influenced by the level of control over the system, being able to accept automated control only if they can control it when they need to. limited indoor environment control has detrimental effects on occupant comfort (lolli et al., 2020). furthermore, interaction strategy could work in the opposite direction, being a source of distraction if occupants are involved in the system’s operation too frequently (bakker et al., 2014). 3.6 the effect of façade technology on occupant response to af the type of façade technology affects occupant response to automated control strategies because each façade technology offers a different range of dynamic performances, such as controlling visual transmittance, blocking incoming solar radiation, and redistributing daylight in the indoor space. additionally, different façade technologies have different performances in terms of their ability to balance conflicting requirements, such as glare versus daylight access, solar transmittance versus surface temperature, and privacy versus outdoor view. table 6 summarises the façade systems and the position of the shading system. regarding façade technologies, the main shading system tested in previous work is that of venetian blinds (16 studies), followed by roller shades (8 studies). switchable glazing has also been evaluated (5 studies), while window opening was the least implemented (2 studies). the automated control controlled a range of façade characteristics, which depended on the technology implemented. in the case of venetian blinds, the system controlled the slats deployment (hold/release) and slat tilt, for roller shades it controlled up and down positions, switchable glazing allowed the modification of glass visual transmittance, while for window opening the window aperture percentage was controlled. the type of façade also defines how disruptive a control strategy will be. for instance, luna-navarro et al. (luna-navarro et al., 2022) reported that placing the blinds within the cavity resulted in more effective control of the solar heat gains and was less disruptive to occupants, especially in terms of 035 journal of facade design & engineering volume 10 / powerskin / 2022 their associated noise. bakker et al. (2014) reported that occupants close to the operation of roller shades were the most disrupted by them. vine et al. (1998) mentioned that the transition from one position to another, the activation frequency, and the sound generated was considered sources of distraction. moreover, wu et al. (2020) also pointed out that the speed of switching also had an impact on occupant satisfaction, who preferred slower and smooth transitions. table 6 table 6. summary of façade technologies included by previous works to assess the influence of façades on occupant response. façade system shading device placement s w it ch a b le g la zi n g r o ll er sh a d e v en et ia n b li n d w in d o w o p en in g in te ri o r in t h e ca vi ty e xt er io r (vine et al., 1998) √ √ (guillemin & morel, 2001) √ (guillemin & morel, 2002) √ (clear et al., 2006) √ √ √ (kim et al., 2009) √ √ (lee et al., 2012) √ (taniguchi et al., 2012) √ √ (cheng et al., 2013) √ √ (bakker et al., 2014) √ √ (meerbeek et al., 2014) √ √ (karlsen et al., 2015) √ √ (cheng et al., 2016) √ √ (painter et al., 2016) √ (sadeghi et al., 2016) √ √ (goovaerts et al., 2017) √ √ (gunay et al., 2017) √ √ (motamed et al., 2017) √ √ (choi et al., 2019) √ (day et al., 2019) √ √ √ √ (lolli et al., 2019) √ √ (motamed et al., 2019) √ √ (wu et al., 2020) √ √ (bian et al., 2020) √ √ (lolli et al., 2020) √ √ √ (korsavi et al., 2021) √ (luna-navarro et al., 2022) √ √ √ √ 4 conclusions this work reviewed twenty-six previous laboratory experiments and field studies that monitored occupant response to automated façades. these studies were reviewed to gather and analyse current evidence on the influence of the following factors on occupant response to af: (1) contextual factors, (2) personal factors, (3) environmental conditions, (4) control logic, and (5) façade technology. 036 journal of facade design & engineering volume 10 / powerskin / 2022 throughout the evidence gathered, this literature review shows how occupant response to the af is captured in terms of occupant behaviour or interaction with the automated control, satisfaction with the interaction strategy, level of acceptance of the automated control logic, perception of the indoor environmental conditions, and sensation regarding specific environmental domains affected by the af operation. the focus of existing studies was limited to a few climatic conditions and similar types of buildings. in most studies, the experiments took place in single office layouts, and data on occupant response to afs in open-plan office spaces is scarce. regarding the aspects affecting occupant response to af operation, studies indicated that personal factors impact occupants’ behaviour and perception of the indoor environment, varying from person to person and depending on the specific attributes of occupants. most of the studies reported personal characteristics, but attitudes and personal significance of indoor environmental quality were missed by most of the articles reviewed. concerning the control strategy, occupant interaction with the automated control is an essential determinant of occupant requirements for the af operation. occupant requirements and preferences are influenced by the level of control over the system, accepting automated control only if they can control it when they need to. additionally, occupant interaction with the af is driven primarily to fulfil personal environmental requirements, such as increasing daylight, privacy, access to views, and avoiding glare discomfort. although af can provide “comfortable” indoor environmental conditions, it does not properly ensure the achievement of individual environmental requirements and preferences. in terms of the impact of façade technology, the type of technology affects how disruptive a façade is and depending on the technology, the overall satisfaction could be higher or lower. in particular, differences in façade effects are noticeable when technologies compromise one environmental domain in favour of another. overall, several barriers still exist to automated façades that can enhance occupant response, and further research effort is required to answer the following gaps: 1 relationship between personal factors and occupant response to af, in particular there is the need to establish common methods for gathering evidence in this domain, since the majority of the studies do not consider personal factors; 2 poor understanding of occupant multi-domain comfort preferences regarding façade operation. unlocking a holistic and more comprehensive knowledge of occupant response to automated façades should be used to achieve more user-centric automated façade solutions. 3 the lack of research to define to what extent learning and personalised control are possible and, in such a case, how to deal with multiple occupants in the same room operating a unique automated façade. in addition, extending the test scenario to different climates or contextual conditions would be very beneficial, since studies were mainly concentrated on a few climates and conditions. this also undermines generalisation, since larger replication within the same conditions would be beneficial to extend the results. ultimately, there is the need for new studies that can demonstrate the benefits of automated façade control strategies and whether personalised controls are necessary to achieve higher occupant satisfaction whilst reducing the energy demand. 037 journal of facade design & engineering volume 10 / powerskin / 2022 references bakker, l. g., hoes-van oeffelen, e. c. m., loonen, r. c. g. m., & hensen, j. l. m. (2014). user satisfaction and interaction with automated dynamic façades: a pilot study. building and environment, 78, 44–52. https://doi.org/10.1016/j.buildenv.2014.04.007 balocco, c., & petrone, g. (2017). numerical modelling for the thermal performance assessment of a semi-opaque façade with a multilayer of nano-structured and phase change materials. buildings 2017, vol. 7, page 90, 7(4), 90. www.mdpi.com/journal/ buildings barozzi, m., lienhard, j., zanelli, a., & monticelli, c. (2016). the sustainability of adaptive envelopes: developments of kinetic architecture. procedia engineering, 155, 275–284. https://doi.org/10.1016/j.proeng.2016.08.029 bian, y., dai, q., ma, y., & liu, l. (2020). variable set points of glare control strategy for side-lit spaces: daylight glare tolerance by time of day. solar energy, 201, 268–278. https://doi.org/10.1016/j.solener.2020.03.016 boyce, p., hunter, c., & howlett, o. (2003). the benefits of daylight through windows sponsored by: capturing the daylight dividend program. carmody, j., selkowitz, s. e., lee, e. s., & arasteh, d. k. (2004). window systems for high-performance buildings. w. w. norton \& company, inc.,. cheng, z., xia, l., zhao, q., zhao, y., wang, f., & song, f. (2013). integrated control of blind and lights in daily office environment. ieee international conference on automation science and engineering, 587–592. https://doi.org/10.1109/coase.2013.6653972 cheng, z., zhao, q., wang, f., jiang, y., xia, l., & ding, j. (2016). satisfaction based q-learning for integrated lighting and blind control. energy and buildings, 127, 43–55. https://doi.org/10.1016/j.enbuild.2016.05.067 choi, j. h., loftness, v., nou, d., tinianov, b., & yeom, d. (2019). multi-season assessment of occupant responses to manual shading and dynamic glass in a workplace environment. energies 2020, vol. 13, page 60, 13(1), 60. https://doi.org/10.3390/ en13010060 clear, r. d., inkarojrit, v., & lee, e. s. (2006). subject responses to electrochromic windows. energy and buildings, 38(7), 758–779. https://doi.org/10.1016/j.enbuild.2006.03.011 day, j. k., futrell, b., cox, r., & ruiz, s. n. (2019). blinded by the light: occupant perceptions and visual comfort assessments of three dynamic daylight control systems and shading strategies. building and environment, 154, 107–121. https://doi. org/10.1016/j.buildenv.2019.02.037 goovaerts, c., descamps, f., & jacobs, v. a. (2017). shading control strategy to avoid visual discomfort by using a low-cost camera: a field study of two cases. building and environment, 125, 26–38. https://doi.org/10.1016/j.buildenv.2017.08.030 grynning, s., lolli, n., wågø, s., & risholt, b. (2017). solar shading in low energy office buildings design strategy and user perception. journal of daylighting, vol. 4, issue 1, pp. 1-14, 4(1), 1–14. https://doi.org/10.15627/jd.2017.1 guillemin, a., & morel, n. (2001). an innovative lighting controller integrated in a self-adaptive building control system. energy and buildings, 33(5), 477–487. https://doi.org/10.1016/s0378-7788(00)00100-6 guillemin, a., & morel, n. (2002). experimental results of a self-adaptive integrated control system in buildings: a pilot study. solar energy, 72(5), 397–403. https://doi.org/10.1016/s0038-092x(02)00015-4 gunay, h. b., o’brien, w., beausoleil-morrison, i., & gilani, s. (2017). development and implementation of an adaptive lighting and blinds control algorithm. building and environment, 113, 185–199. https://doi.org/10.1016/j.buildenv.2016.08.027 heschong, l., wright, r. l., & okura, s. (2013). daylighting impacts on human performance in school. http://dx.doi.org/10.1080/00 994480.2002.10748396, 31(2), 101–114. https://doi.org/10.1080/00994480.2002.10748396 hosseini, s. m., mohammadi, m., & guerra-santin, o. (2019). interactive kinetic façade: improving visual comfort based on dynamic daylight and occupant’s positions by 2d and 3d shape changes. building and environment, 165, 106396. https://doi. org/10.1016/j.buildenv.2019.106396 izadyar, n., miller, w., rismanchi, b., & garcia-hansen, v. (2020). impacts of façade openings’ geometry on natural ventilation and occupants’ perception: a review. building and environment, 170, 106613. https://doi.org/10.1016/j.buildenv.2019.106613 jain, s., & garg, v. (2018). a review of open loop control strategies for shades, blinds and integrated lighting by use of real-time daylight prediction methods. building and environment, 135(march), 352–364. https://doi.org/10.1016/j.buildenv.2018.03.018 karlsen, l., heiselberg, p., & bryn, i. (2015). occupant satisfaction with two blind control strategies: slats closed and slats in cut-off position. solar energy, 115, 166–179. https://doi.org/10.1016/j.solener.2015.02.031 karlsen, l., heiselberg, p., bryn, i., & johra, h. (2016). solar shading control strategy for office buildings in cold climate. energy and buildings, 118, 316–328. kim, j. h., park, y. j., yeo, m. s., & kim, k. w. (2009). an experimental study on the environmental performance of the automated blind in summer. building and environment, 44(7), 1517–1527. https://doi.org/10.1016/j.buildenv.2008.08.006 knaack, u., klein, t., bilow, m., & auer, t. (2014). façades: principles of construction (2., rev. e). birkhäuser. https://doi.org/ doi:10.1515/9783038211457 konstantoglou, m., & tsangrassoulis, a. (2016). dynamic operation of daylighting and shading systems: a literature review. in renewable and sustainable energy reviews (vol. 60, pp. 268–283). elsevier ltd. https://doi.org/10.1016/j.rser.2015.12.246 korsavi, s. s., jones, r. v., & fuertes, a. (2021). the gap between automated building management system and office occupants’ manual window operations: towards personalised algorithms. automation in construction, 132, 103960. https://doi. org/10.1016/j.autcon.2021.103960 lee, e. s., claybaugh, e. s., & lafrance, m. (2012). end user impacts of automated electrochromic windows in a pilot retrofit application. energy and buildings, 47, 267–284. https://doi.org/10.1016/j.enbuild.2011.12.003 lee, e. s., dibartolomeo, d. l., vine, e. l., & selkowitz, s. e. (1998). integrated perfonnance of an automated venetian blind l electric lighting system in a full-scale private office. 038 journal of facade design & engineering volume 10 / powerskin / 2022 lolli, n., nocente, a., brozovsky, j., woods, r., & grynning, s. (2019). automatic vs manual control strategy for window blinds and ceiling lights: consequences to perceived visual and thermal discomfort. journal of daylighting, vol. 6, issue 2, pp. 112-123, 6(2), 112–123. https://doi.org/10.15627/jd.2019.11 lolli, n., nocente, a., & grynning, s. (2020). perceived control in an office test cell, a case study. buildings 2020, vol. 10, page 82, 10(5), 82. https://doi.org/10.3390/buildings10050082 luna-navarro, a., hunt, g. r., & overend, m. (2022). dynamic façades – an exploratory campaign to assess occupant multi-domain environmental satisfaction and façade interaction. building and environment, 211, 108703. https://doi.org/10.1016/j. buildenv.2021.108703 luna-navarro, a., loonen, r., juaristi, m., monge-barrio, a., attia, s., & overend, m. (2020). occupant-façade interaction: a review and classification scheme. building and environment, 177, 371–377. https://doi.org/10.1016/j.buildenv.2020.106880 meerbeek, b., te kulve, m., gritti, t., aarts, m., van loenen, e., & aarts, e. (2014). building automation and perceived control: a field study on motorized exterior blinds in dutch offices. building and environment, 79, 66–77. https://doi.org/10.1016/j. buildenv.2014.04.023 motamed, a., deschamps, l., & scartezzini, j. l. (2017). on-site monitoring and subjective comfort assessment of a sun shadings and electric lighting controller based on novel high dynamic range vision sensors. energy and buildings, 149, 58–72. https:// doi.org/10.1016/j.enbuild.2017.05.017 motamed, a., deschamps, l., & scartezzini, j. l. (2019). eight-month experimental study of energy impact of integrated control of sun shading and lighting system based on hdr vision sensor. energy and buildings, 203, 109443. https://doi.org/10.1016/j. enbuild.2019.109443 painter, b., irvine, k. n., waskett, r. k., & mardaljevic, j. (2016). evaluation of a mixed method approach for studying user interaction with novel building control technology. energies 2016, vol. 9, page 215, 9(3), 215. https://doi.org/10.3390/en9030215 reinhart, c. f., & voss, k. (2003). monitoring manual control of electric lighting and blinds. lighting research and technology, 35(3), 243–258. https://doi.org/10.1191/1365782803li064oa sadeghi, s. a., karava, p., konstantzos, i., & tzempelikos, a. (2016). occupant interactions with shading and lighting systems using different control interfaces: a pilot field study. building and environment, 97, 177–195. https://doi.org/10.1016/j. buildenv.2015.12.008 sullivan, r., lee, e. s., papamichael, k., rubin, m., & selkowitz, s. e. (1994). effect of switching control strategies on the energy performance of electrochromic windows. optical materials technology for energy efficiency and solar energy conversion xiii, 2255(9), 443–455. https://doi.org/10.1117/12.185387 tabadkani, a., roetzel, a., li, h. x., & tsangrassoulis, a. (2021). a review of occupant-centric control strategies for adaptive façades. automation in construction. https://doi.org/10.1016/j.autcon.2020.103464 tang, s. k. (2017). a review on natural ventilation-enabling façade noise control devices for congested high-rise cities. applied sciences 2017, vol. 7, page 175, 7(2), 175. https://doi.org/10.3390/app7020175 taniguchi, t., iwata, t., & ito, d. (2012). blind control method based on prevention of discomfort glare taking account of building conditions. experiencing light 2012 international conference. https://www.researchgate.net/publication/307138640_blind_ control_method_based_on_prevention_of_discomfort_glare_taking_account_of_building_conditions tzempelikos, a., & athienitis, a. k. (2007). the impact of shading design and control on building cooling and lighting demand. solar energy, 81(3), 369–382. https://doi.org/10.1016/j.solener.2006.06.015 vine, e., lee, e., clear, r., dibartolomeo, d., & selkowitz, s. (1998). office worker response to an automated venetian blind and electric lighting system: a pilot study. energy and buildings, 28(2), 205–218. https://doi.org/10.1016/s0378-7788(98)00023-1 wu, y., kämpf, j. h., & scartezzini, j. l. (2020). a survey study of occupants? visual satisfaction on an automated venetian blind based on sky luminance monitoring and lighting simulation. proceedings of the ises solar world congress 2019 and iea shc international conference on solar heating and cooling for buildings and industry 2019, 685–692. https://doi.org/10.18086/ swc.2019.13.05 from city’s station to station city 117 journal of facade design & engineering volume 5 / number 2 / 2017 towards facades as make-to-order products – the role of knowledge-basedengineering to support design jacopo montali1, mauro overend1, p. michael pelken2, michele sauchelli2 1 glass and facade technology research group, department of engineering, university of cambridge, trumpington street, cambridge cb2 1pz, uk, +44 7517 192 160, jm2026@cam.ac.uk. 2 engineering excellence group, laing o’rourke plc, anchor boulevard, dartford da2 6qh, uk abstract building facades are engineer-to-order (eto) products and, as such, they show unique features on a project-by-project basis. the partitioning of design tasks during the design and manufacturing process of these products, however, does not fully capture how specific design decisions influence other stakeholders’ choices. this lack of design integration is most severe at early stages when a large proportion of initial costs, mostly driven by manufacturability aspects, is determined. this paper illustrates a methodology to build knowledge-based engineering (kbe) applications to support earlystage design integration through the development of a facade product model for automatic rule checking and knowledge reuse. the main outcome is a preliminary framework for developing knowledge-based, digital tools to support and integrate facade design as well as different scenarios in which the tool can potentially be used, based on two types of procurement methods. a prototype of the tool is also shown here. the paper proposes a new paradigm where facade systems are considered to be more closely related to make-to-order types, rather than etos, in which the product is ready for fabrication and designers can rapidly explore the subcontractor’s manufacturing capabilities and the implications of their design choices. future work will include tool validation by applying the tool into a specific facade manufacturer’s workflow. keywords facade design, design automation, product configuration, knowledge-based engineering, manufacturability doi 10.7480/jfde.2017.2.1744 118 journal of facade design & engineering volume 5 / number 2 / 2017 1 introduction the unprecedented shift towards prefabrication and the increasing market competition in the aec (architecture, engineering and construction) sector require improved efficiency in the delivery of the final project, while concurrently controlling costs, risks and quality. facades present several challenges for achieving these goals. firstly, a degree of customisation always exists in facades; even a commercially-available system presents an almost infinite domain of possible solutions due to the number of infill products, end uses, climatic zones and orientations that make facades unique products, requiring new analyses each time the client requests it. for this reason, facade products are generally defined as engineer-to-order (figure 1), since the client can influence the definition of the specification early on in the process prior to design stage. secondly, the increasing number of requirements in facade design in recent years, from initial structural safety to a wider spectrum of criteria, has also made the design activity highly interdisciplinary and interdependent; a single optimal solution does not exist, but rather a set of acceptable solutions within the above-mentioned domain that meet different criteria, while respecting constraints, should be evaluated. thirdly, prefabricated facades are highly modular systems, like many industrial products; the panelisation scheme identifies the fundamental unit of the product that will undergo serial production in the factory. this raises the issue of understanding manufacturing constraints, an aspect that, as shown by voss & overend (2012), is seen as the most influential aspect in driving costs in facades, especially during early stages of design. allowing variety while tackling both the interdependent nature and the view as industrial products of facades do, still presents a challenge. collaborative approaches consisting of rapid meetings between specialists on specific design issues (e.g. concurrent engineering) are rarely, if at all, used in the fragmented aec sector. a different approach is design automation through knowledgebased engineering (kbe) applications, digital tools used for automation of design processes and reuse of standard knowledge. these applications have already been successfully used in other industries to reduce design time and errors, while optimising design. kbe applications have been mostly used for supporting the design of parts of so-called make-to-order products (mto – figure 1) such as the design of aircraft’s wing ribs, cars’ body-in-white and headlamps (cooper, fan, & li, 2001). mto products are normally associated with so-called ‘mass customised’ products, and the cost of developing a kbe application for mto product types was made possible by the large number of units produced. kbe in facades should therefore focus on the reusable aspects of design by applying such tools the aim of this paper is to introduce a methodology for developing kbe in facades, where the reusable part of manufacturing knowledge is embedded into kbe tools, thus relieving the design team from the burden of checking the manufacturability of the product while integrating multiple design criteria. the methodology is intended to provide a roadmap to facade designers and fabricators interested in digitising design criteria and knowledge. in this way, the facade product is seen as closer to a make-to-order type, where an existing package of knowledge is available and ready to be used, and the facade is designed for manufacture. the kbe tool therefore acts as an ‘interim product configurator’, where a non-fully defined design solution based on the configurator options is developed by the design team. the design solution is then detailed and completed by including the features that were missing from the kbe configurator, until the design is finalised. following a short background of kbe and its application in section 2, the paper will cover the development methodology and possible use cases for facades in section 3. section 4 shows some initial results from the application of the proposed methodology. discussion and proposals for future work are covered in section 5 and 6, respectively. on multiple projects. 119 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 1 four product variant systems and the proposed approach. (adapted from hansen, 2003; and rudberg & wikner, 2004). 2 literature review and related work knowledge-based engineering (kbe) is an approach aimed at supporting design through the creation of specific tools that automate knowledge-intensive design processes from different and multidisciplinary sources. the main benefits are reduction in design times and errors, and design integration. kbe has been successfully applied in industries such as aerospace, automotive and shipbuilding. a general purpose tool for kbe development is called ‘kbe system’, whereas its actual implementation is called ‘kbe application’. a complete review of kbe can be found in la rocca, 2012. the process of building a kbe application requires an integrated description of the fundamental concepts that govern the engineering problem or product and how they are interrelated. these include product parts and how they relate to the whole product; functional, physical and geometrical attributes; and associated constraints and rules. different types of knowledge (e.g. tacit versus explicit) are integrated here. the resulting overall framework, called ‘product model’ (stokes, 2001) or ‘ontology’, is then implemented into the kbe application. a kbe application works as a standard software application, where input data are retrieved from user interaction or databases, processed, and exported to specific, customised formats (figure 2). the product model includes the product architecture and associated knowledge. methodologies have been developed to support the creation of a kbe application, such as moka (stokes, 2001) and knomad (curran, verhagen, & van tooren, 2010). to the authors’ knowledge, these methodologies have never been applied in the facade sector, mostly due the lack of an economy of scale necessary to repay the development cost. it is therefore necessary to develop a methodology that allows rapid change and reuse of information and knowledge on several facade projects. specific use cases can help identify the design step in which the tool is most effective. 120 journal of facade design & engineering volume 5 / number 2 / 2017 in the facade sector, existing work in knowledge-based engineering applied to facades is limited and lacks a common methodology. karhu (1997) developed a product model of precast concrete facades based on a standard taxonomical structure of the facade panel that included the panel’s main features such as standard panel-to-panel joint types. the focus here was on transferring information about the designed panels to other stakeholders. in more recent years, a digital kbe tool (voss & overend, 2012) was developed by voss to evaluate the manufacturing limitations of the overall facade, such as a maximum cold-bending radius or overall manufacturing dimensions, by means of querying a revit model. aram et al. (aram, eastman, & sacks, 2014) included knowledge about costs and quantity estimation to enrich the .ifc file exchange format for prefabricated concrete spandrels. very recently, some facade fabricators, such as schueco and zahner, have started to create product configurators that inform designers about the manufacturing capabilities of their systems and supply chain availability. schueco’s parametric tool (fuchs, peters, hans, & möhring, 2015) is a plugin for grasshopper/rhino, or for revit, that automatically configures a highly customisable product that has been developed by schueco. the plugin includes knowledge about the main manufacturing limits and preliminary structural design criteria. zahner’s cloudwall (zahner, 2016) is an online platform for configuring bespoke facade patterns through freeform, vertical fins. the user manipulates a series of parameters that modify a 3d model of the facade, and receives a cost estimation for the configured product. these two examples therefore demonstrate a combined commercial need for selling highly bespoke products on the one hand, and digital tools supporting their design while controlling their manufacturing limitations on the other. this evidence is also supported by a survey undertaken by the knowledge transfer network (2016); tools and technologies normally applied to mass customised products should be used for construction-related eto products. those tools should also integrate multiple design criteria and tackle the fragmentation of the construction market by looking at the facade product as a system, rather than the simple sum of its parts and functions. fig. 2 high-level view of a knowledge-based engineering system (reddy, sridhar, & rangadu, 2015) 121 journal of facade design & engineering volume 5 / number 2 / 2017 3 the proposed methodology 3.1 the basis of the methodology the proposed methodology for the development of kbe applications in facades is shown in figure 3. it consists of four main steps that regularly increase the formality of the captured knowledge, from high to low level. the methodology presents the typical features of kbe methodologies, such as moka and knomad, and includes the knowledge storage in standard forms (‘icare’ forms) and the use of uml modelling as an intermediate language. this methodology also presents some unique features that distinguish it from existing methodologies, such as its reduced level of complexity and steps to decrease the development time of kbe application, which is currently seen as one of the major limits. this methodology serves as a starting point for engineering and manufacturing companies that digitise standard knowledge and information for reuse and automation of design processes. it focuses particularly on facade systems and products that require the integration of multiple criteria, where the solution usually consists of a trade-off between those design criteria. fig. 3 knowledge formalisation process, from natural language to raw programming code 3.1.1 knowledge capture the first goal of this step is to understand the type of knowledge that is available and its impact in terms of benefits for the company. if a specific design aspect is impossible to determine due to lack of analyses or experts, and, at the same time, is not relevant for the final delivery of the product, no implementation is needed. for those aspects that are not available but required, further studies might be needed. 122 journal of facade design & engineering volume 5 / number 2 / 2017 unstructured interviews with domain experts provide a sense of the major gaps in the design and manufacturing process and how to approach them. the interviewee must be aware of the future opportunities arising from the development of such applications in order to maximise his contribution. semi-structured interviews can then be conducted to retrieve knowledge more systematically, once the problem has been set and the business case has been defined. document-based research is also useful in the retrieval of knowledge and information that would otherwise require excessive effort if people need to use it frequently (e.g. large pdf documents that contain guidelines and technical datasheets). depending on the company, the availability of such documents varies. a standard methodology for capturing knowledge is illustrated by milton (2007) and an example of aerospace application for fibre metal laminate (fml) panels has been developed by emberey & milton (2007). 3.1.2 structure knowledge through moka icare forms the next step structures knowledge by selectively sorting, storing and linking it into a knowledge base, a structured repository where knowledge information is easily accessible. the creation process of a knowledge base consists of the analysis and categorisation of all the concepts related to the design and manufacture of the product in question. the process of creating the knowledge base requires the identification of the fundamental units representing knowledge. icare forms (stokes, 2001) standard tables representing a type of unit of knowledge can be used for this purpose. table 1 shows the type of knowledge these forms can represent. form represented knowledge illustration experience on past projects constraint physical / geometrical limits on product / processes activity single step in design and manufacturing activity rule design / manufacturing engineering rule entity physical entity, function or change in state of a product table 1 moka icare forms knowledge is thus implemented on tables and stored into these standard forms, which are then cross-referenced (e.g. through hyperlinks, if forms are developed in html), resulting in a network of inter-linked concepts. an example is shown in figure 4 where an ‘entity’ form is referenced to a ‘rule’ form. graphical representations of the network help in the visualisation of the overall network and the correlation between different concepts. the knowledge base is then validated against the opinion of domain experts, which helps to correct or extend it. 123 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 4 moka ‘entity’ form representing the structural layer of a precast concrete single-skin panel, linking to a ‘rule’ form containing a simplified engineering rule for dimensioning the concrete thickness 3.1.3 development of the product model architecture via uml modelling unified modelling language (uml, 2016) is used to define the basic structure of the knowledgebased tool, based on the knowledge base. the approach here is to model each knowledge unit through an object-oriented approach, where each physical object is represented by a class as characterised by the following features: – attributes: these represent all the geometrical and physical features of the physical components, i.e. the associated variables (e.g. height, width, thermal conductivity, cost per cubic metre). – behaviours: these represent the change of the state of the object belonging to the class. they are usually represented by functions, i.e. blocks of code that perform a specific task (e.g. change in insulation type depending on the presence of different fire requirements). uml also represents the type of interrelationship between classes, such as inheritance, association, composition and aggregation. the taxonomy of the product under investigation (defined as the hierarchical classification of the sub-components) is therefore created; figure 5 shows a typical ‘composition’ link between the product and its subcomponents, represented by a black diamond, describing the ‘has-a’ relationship between physical entities. once the taxonomy has been defined, the design and manufacturing knowledge is incorporated into the taxonomy to form a lower-level ontological framework of the product. 124 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 5 simplified uml diagram showing the taxonomy of a facade product. each yellow box corresponds to an ‘entity’ moka form. 3.1.4 build the product model the product model is then translated into a programming code, based on the software architecture defined by the uml diagram. the type of programming language can be either a specific kbe system, such as aml, icad or gdl, or a general-purpose programming language. a standalone software or a plugin can be chosen as platform. the overall process (steps 1 to 4) is iterative and adopts an agile approach, in which new knowledge is included, or replaces outdated concepts. the development of a software architecture that allows quick extensions and modifications is therefore desirable. object-orientation, in this sense, allows the creation of custom libraries of standard objects with associated knowledge that can be reused whenever a new kbe tool for a new product is created (e.g. the insulation material of a single-skin precast concrete panel is identical to that used for a loadbearing, precast concrete sandwich panel in terms of intrinsic properties such as thermal resistance and material cost). 125 journal of facade design & engineering volume 5 / number 2 / 2017 3.2 use-case scenarios the use of the knowledge-based tool is analysed here from the point of view of a facade manufacturer at design stages, in order to support design development. consortia of companies could also be formed to cut development costs while integrating multiple manufacturing capabilities or product data in a single platform. three possible use cases are shown, based on two different british procurement methods (riba, 2013) in which the manufacturer may or may not be appointed to develop the design from the early stages. online process maps in a bpmn notation (‘bpmn,’ 2017) of the above use cases has been developed for clarity (author’s webpage, 2016). – case 1: kbe tool available to download for design teams for use during early-design stages (e.g. riba 3) of a design-bid-build (dbb) procurement method. in this case, a design team developing the design solution is using a manufacturer-specific kbe tool to evaluate the level of early ‘tenderability’ by that specific manufacturer, including preferred materials from the supply chain. existing examples of kbe-like applications used in this sense are the schueco parametric system or zahner’s cloudwall. if the design solution does not comply with the configurator, then the design team should consider a bespoke solution. – case 2: kbe tool used by a facade manufacturer to inform and support a design team during early design stages (e.g. riba 3) in dbb. this case considers a situation where the knowledge of the facade manufacturer is protected by confidentiality. the manufacturer therefore provides a service to the design team by using the kbe tool internally for rapid and quick support activities. – case 3: kbe tool used by the project team across design stages in a design-build (db) environment. in this case, the tool becomes central to the design team, whose activity is the development of solutions within the space defined by the tool. if the kbe developers form part of the design team, the possibility to tailor the tool on-the-go (e.g. including more consideration from the design perspective or increasing the level of details) through agile software development should be considered. cases 1 and 2 require an a priori development of the tool, which is then issued and used. case 3 instead requires ongoing development as the project progresses, based on a pre-constructed base (e.g. a .dll library). 4 preliminary results the expected result from the present research is the development of a prototype kbe tool for a chosen facade type manufactured by a specific company, which will consider manufacturing limits, design constraints and performance indicators. the methodology described above has already been applied to a specific facade typology (montali, overend, pelken, & sauchelli, 2017). the example refers to a case study of a precast concrete unit comprising single skin panels manufactured in a specific facility, the explore industrial park in steetley, uk. knowledge was collected by conducting interviews with experts and by collecting design guidelines. the generated knowledge base has been represented in a graphical form through a design structure matrix (lindemann, maurer, & braun, 2009) as shown in figure 6. the matrix presents the icare forms in rows and columns; if the generic cell of the matrix is checked, then a semantic link exists between the two forms. 126 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 6 design structure matrix representing the knowledge associated to the design and manufacture of single skin precast concrete panels (montali et al., 2017). figure 7 represents the generated knowledge based application, developed in rhino / grasshopper by creating a custom grasshopper component in c#. the application requires geometrical (overall dimensions of the panel and position relative to the primary structure) and physical (insulation type) inputs, and returns a geometry that respects some pre-determined constraints, as well as some performance indicators such as u-value, material cost and embodied carbon. fig. 7 developed kbe application for the design of single skin precast concrete panels (montali et al., 2017). 127 journal of facade design & engineering volume 5 / number 2 / 2017 5 discussion this paper has proposed a preliminary framework for developing digital knowledge-based engineering (kbe), three use cases for two common british procurement methods. the framework can be adopted by companies to develop digital tools that inform design teams about their detailed manufacturing and supply chain capabilities. an example of application to a prefabricated, precast concrete panel has been shown to prove the concept. by using those tools, design teams can start to understand the limitations of designing a solution that will eventually be produced by a specific manufacturer, while expressing their design intent. as product development moves increasingly towards ‘mass customisation’, the use of kbe systems might appear to be counterintuitive, given the reduction in design freedom. however, the authors believe that facades should be considered as highly engineered products, the likes of which have not yet been manufactured, with some a priori design knowledge that takes into account some limitations, be they physical or performancebased. this is the onus for unleashing the ‘mass’ part while not reducing the ‘customisation’ aspect. the shift towards make-to-order types should be therefore regarded as asymptotic, since the design of mto is completed before the client develops the specifications; the space of solutions is also more limited. another fundamental aspect in achieving mass customisation that has not been considered in this paper is the role of an agile and broad supply chain. 6 future work future work will include the development of the kbe tool, its application on case studies, and subsequent validation through the creation of specific merit indices. possibilities for multi-objective optimisation will also be explored. 128 journal of facade design & engineering volume 5 / number 2 / 2017 acknowledgements the authors would like to thank the engineering and physical science research council (epsrc) and laing o’rourke plc for supporting the present research program. references aram, s., eastman, c., & sacks, r. (2014). a knowledge-based framework for quantity takeoff and cost estimation in the aec industry using bim. in the 31st international symposium on automation and robotics in construction and mining (p. 1). vilnius: vilnius gediminas technical university, department of construction economics & property. author’s webpage. (2016). retrieved from http://www2.eng.cam.ac.uk/~jm2026/jm_web/researchwork/kbe-tool-use-cases/ index.html bpmn. (2017). retrieved from http://www.bpmn.org/ cooper, s., fan, i., & li, g. (2001). achieving competitive advantage through knowledge-based engineering. university of cranford. curran, r., verhagen, w. j. c., & van tooren, m. j. l. (2010). the knomad methodology for integration of multi-disciplinary engineering knowledge within aerospace production. 48th aiaa aerospace sciences meeting including the new horizons forum and aerospace exposition, (january), 1–16. http://doi.org/10.2514/6.2010-1315 emberey, c. l., & milton, n. r. (2007). application of knowledge engineering methodologies to support engineering design application development in aerospace. 7th aiaa aviation technology, integration and operations conference (atio), (september), 18–20. fuchs, a., peters, s., hans, o., & möhring, j. (2015). schüco parametric system uniqueness in series. in advanced building skins. graatz. hansen, b. l. (2003). development of industrial variant specification systems. b.l. hansen, development of industrial variant specification systems, ph.d. thesis. technical university of denmark, lyngby, 2003. karhu, v. (1997). product model based design of precast facades. itcon journal, 2(november 1996). knowledge transfer network. (2016). high value manufacturing modelling and simulation best practice (simbest) wp4 state of the art deliverable. la rocca, g. (2012). knowledge based engineering: between ai and cad. review of a language based technology to support engineering design. advanced engineering informatics, 26(2), 159–179. http://doi.org/10.1016/j.aei.2012.02.002 lindemann, u., maurer, m., & braun, t. (2009). structural complexity management an approach for the field of product design. berlin: springer. http://doi.org/10.1007/978-3-540-87889-6 milton, n. r. (2007). knowledge acquisition in practice: a step-by-step guide (1st ed.). springer-verlag london. http://doi. org/10.1007/978-1-84628-861-6 montali, j., overend, m., pelken, p. m., & sauchelli, m. (2017). knowledge-based engineering applications for suporting the design of precast concrete facade panels. in iced17: international conference of engineering design (in print). vancouver. omg. (2016). uml. retrieved from http://www.uml.org/ reddy, e. j., sridhar, c. n. v, & rangadu, v. p. (2015). knowledge based engineering : notion , approaches and future trends, 5(1), 1–17. http://doi.org/10.5923/j.ajis.20150501.01 riba. (2013). riba plan of work 2013 overview. rudberg, m., & wikner, j. (2004). mass customization in terms of the customer order decoupling point. production planning & control, 15(4), 445–458. http://doi.org/10.1080/0953728042000238764 stokes, m. (2001). managing engineering knowledge – moka: methodology for knowledge based engineering applications. (professional engineering publ., ed.). asme press. voss, e., & overend, m. (2012). a tool that combines building information modeling and knowledge based engineering to assess facade manufacturability. in advanced building skins. gratz. zahner. (2016). shopfloor. retrieved from http://www.shopfloorapp.com/ from city’s station to station city 025 journal of facade design & engineering volume 5 / number 2 / 2017 used building materials as secondary resources – identification of valuable building material and automized deconstruction magdalena zabek1, linda hildebrand1, matti wirth2, sigrid brell-cokcan3 1 chair of circle oriented construction, faculty of architecture, rwth university, aachen, germany 2 irr innovationsregion rheinisches revier gmbh, julich, germany 3 individualized production in architecture, faculty of architecture, rwth, aachen university, aachen, germany abstract these days, we are constantly expecting more from the performance of the building envelope with regard to both comfort and ecological compatibility. operational energy has been undergoing significant improvement, which in turn draws attention to the building material: if a standard building does not consume energy in its operation, then it is the building material that impacts the level of environmental compatibility. designing with used building material offers an opportunity to decrease emissions from extraction, preserve primary resources and reduce landfill. on top of that, the eu waste directive requires all new construction to have a recycling concept for 70% of the building mass (commission, 2013). this paper deals with a new approach to deconstructing used building elements and re-introducing them in new construction on a regional scale. a well-connected network of stakeholders increases the regional recycling potential. on a technical scale, the deconstruction process needs to be improved so that it becomes more safe and economically competitive. robotic disassembly shows great potential to simplify the process and restore high valuable recycling. deconstruction processes can be arranged in a mobile way with a wide range of tools. such tools can be equipped to relate to the flexible layout of machines, thus allowing individualized adaptation to the building envelope. keywords circular building economy, sustainable design, resource-efficient building material, stakeholder analysis, robotic disassembly doi 10.7480/jfde.2017.2.1684 026 journal of facade design & engineering volume 5 / number 2 / 2017 1 introduction sustainability and circularity have become a global topic of social and political interest due to the recognizable effects of climate change. moreover, against this background of new climate change goals, and a growing population with an ever-increasing need for space and resources, an emphasis is placed on the need for resource-efficient solutions (wbgu et al., 2016). in order to preserve the natural environment and provide for future generations, the extraction of natural resources has to be minimized. several countries, such as germany, are committed to improving their resource efficiency, which aims for a 200% improvement on 1994 levels. (in 2014, 148.7% was achieved. figures for other countries vary according to the natural resources and economy). currently, the motivation to use secondary resources in the built environment is relatively low. several aspects including cost, liability, and social issues such as customer acceptance and a low level of awareness in relation to preserving natural resources, still restrict circular material flow within the building sector. by focusing on an industrial region in western germany as a case study, these obstacles shall be investigated and proven by surveying regional stakeholders linked to the building sector. the rhenish mining area in western germany plays a leading role in extracting resources for energy, nutrition and material supply. north rhine-westphalia, the county in which the region is situated, is home to 12,544 building industry companies, the highest number in germany (destatis, 2015). only mineral material, and its recycling path towards an equal or higher quality compared to its primary material, were considered in the study. fig. 1 rhenish mining area, rwe power 2014 027 journal of facade design & engineering volume 5 / number 2 / 2017 north rhine-westphalia has rich resources of gravel, the main component for concrete production. the annual production of 65.5 million tonnes of sand and gravel forms the largest constituent within the regional building sector (fig. 2), while 15 million tonnes of building waste is also produced in the county annually (it.nrw 2013). looking more closely at the scale of the rhenish mining area, about 1.5 million tonnes of building waste (largely of mineral origin) is created here annually. an unquantified, though significant, share of this volume results from ongoing resettlement processes, in which buildings are demolished and their inhabitants rehoused, to make way for lignite extraction activities. none of the material resulting from such demolitions has yet been reused or recycled for an equal or higher purpose. these volumes of building waste can be considered a valuable source of secondary raw materials – and could be referred to as an anthropogenic material stockpile (schiller, 2015). if systematically managed, this material could serve as a capital reserve for the future. the reuse and recycling path is specifically developed for each building material, each undergoing an individual process. in addition, it is not only the qualities of the material itself that indicate its suitability for recycling, but also the construction context of the material. fig. 2 mineral material flow diagramm, irr 2015 2 methodology the research consists of two parts. the first presents the results of a survey of stakeholders that aimed to identify obstacles and opportunities. second, the energy dimension of deconstruction is analysed using a method, developed at rwth by the authors. this analysis demonstrates the relevance of deconstruction (rather than demolition) in relation to the technical obstacles within towns that are to be demolished due to resettlement. the analysis is organized into two main interdependent parts, and results in the categorisation of 12 typical facade constructions. 028 journal of facade design & engineering volume 5 / number 2 / 2017 the reason for the limited focus here on the facade is due to the argument that it is a particularly complex building element. the embodied energy of the useful material is the benchmark for determining the energy spend on deconstruction. with a higher amount of deconstruction compared to a primary product, the product is more ecologically reasonable. 2.1 stakeholder survey to identify what potential exists for recycling in the region, a survey of stakeholders from political, industrial, and research backgrounds was undertaken, which analysed their motivations in relation to recycling as well as examining the network of such stakeholders in the region. the stakeholders were chosen for their relevance (influence on policy, industry and research), using a top-down approach (representatives of industry, policy and research). these stakeholders became discussion partners and, in their function as experts, decisively shaped the results of the series of investigations. for this reason, the selection of experts was targeted at demolition and mineral building material industry representatives, since they were regarded as the qualified leaders in the subject. representatives from the political level were chosen due to their experience and their field of expertise, while research representatives were chosen based on their relevant scientific publications within this field. answers were given by multiple choice, but an open discussion allowed the possibility for further elaboration. initially, the survey included questions regarding: 1 stakeholder motivation towards a circular building economy 2 general conditions of the circular building economy 3 network (connections between the stakeholders) 4 demands on other stakeholders 5 intentions to act 2.1.1 results by identifying the main conditions needed for a circular building economy, it became clear that the acceptance of recycling products is important, though the survey results suggested this is absent amongst the target market. the lack of acceptance can be partly attributed to the insufficient body of knowledge on the composition and origin of the material. furthermore, the results indicated that statuory regulations negatively impact the use of recycled products. 029 journal of facade design & engineering volume 5 / number 2 / 2017 table 1 general conditions of the circular building economy, rb 2016 it was shown that a well-formed and regularly-used network exists between the industrial stakeholders. however, further development is necessary to address the lack of the consumers within the network. this lack confirms the consumer’s non-acceptance of recycled building material. in the future, substituting primary building material in street construction with demolition waste will become more difficult due to statutoryrestrictions (mantelverordnung), and substituting primary material for high performance concrete is also difficult due to strict regulations (deilmann, 2014, p.51). however, costs associated with landfilling are set to increase, which will lead to economic advantages for secondary resources. 2.1.2 deconstruction the rhenish mining area is characterized by opencast mines and power plants for energy production. since 1952, mining activities have caused the displacement of 44,064 people (bund nrw, 2016). such resettlements of people are ongoing or planned for the near future. the building material produced by the demolition of these places has, up until now, usually been used for landfill. to reach the goal of upcycling the primary material, its recycling potential needs to be defined, and this depends on defining the quality of the segregated material. the process of extricating the material requires construction site equipment, machine time, transportation etc., and the effort required to do so determines the feasibility of the potential material; the lower the effort to deconstruct, the more sensible it is to do so. (this logic can also be applied to ecological and economical aspects.) the deconstruction phase is therefore of special relevance for two reasons: first, for the architectural planning of future buildings and second, in order to provide circularity. the number of deconstructable buildings in the rhenish mining area is growing following the global trend of migration to cities, peri-urban redevelopments and further resettlement plans. as it loses its function, a building’s raw material becomes accessible as secondary resource. 030 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 3 facade types. rb, 2016 in order to provide a broad overview, regional typical facade types from the 1950s and 1980s are investigated, as these account for the main proportion. the decade in which the facade was built generally indicates the type of construction (knaack, hildebrand, konstantinou, & wieland, 2013). 2010 is chosen as an additional reference year as significant changes in building insulation took place around that period. the facade types can be put into categories of no insulation, minor insulation and full insulation. the major proportion of the chosen facades is made up of mineral materials (insulation and cladding material are exemptions). only the opaque parts are considered. four different stone types are shown, including three cladding variations. the facades have a massive mineral loadbearing shell in common. limestone, brick and hollow brick are glued together by mortar, while the concrete variants are monolithic and enclose the reinforcement. ventilated cladding is held on substructure screwed into the wall. self-supporting cladding (i.e. brick shell) works in a way similar way to that which is loadbearing. the energy invested in a the production of a building material can be referred to as its ecological value. similar to economic value, this indicates high responsibility and, in this case, potential to preserve natural resources and limit emissions. according to the bill of quantities, measuring the amount of non-renewable primary energy helps us to understand the capabilities of the substance.1 031 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 4 non-renewable primary energy used in production, subdivided into the following categories: a) non-destructively detachable, b) destructively detachable (pure material), c) destructively detachable (mixed material same category), and d) critical materials. rb, 2016 in most cases, the load-bearing layer has the highest impact. naturally, by adding layers (facing shell b2 or cladding on substructure ls3/rc3) greater results are yielded. the stored materials including ls3, b2 and rc3 are of great ecological value. for obvious ecological reasons, the deconstruction of the product to be reused cannot use more energy than would be used in the creation of a new product. table 2 shows the embodied energy for reused building elements. this is the maximum output when disassembling a facade element. the energy embodied in the reusable and recyclable products can be used as benchmark for the energy of the deconstruction. in most cases, the complexity of the connection details between facade elements indicates the effort required to deconstruct them. naturally, the more layers or components there are in the facade element, the higher the effort.a facade with insulation that has been glued to its structure requires a higher effort for deconstruction (facing shell ls2, b3, ac2, rc 2), as do smaller building materials like brick demand. limestone, brick and hollow brick are glued together by mortar and demand a higher deconstruction effort than reusable concrete variants that are monolithic. ventilated cladding that is held on substructure screwed into the wall (ls3, rc3) can be dismantled with relative ease. ls1 ls2 ls3 b1 b2 b3 ac1 ac2 rc1 rc2 rc3 benchmark (mj/m2) 452,8 452,8 1.317,8 1.314,8 1.972 311,8 383,1 383,1 475,2 475,2 1.259,9 effort for destruction (mj/m2) 2,8 10,2 5,3 9,1 10 7,3 3,8 9,2 0,8 7,4 2,4 table 2 deconstruction results, rb, 2016 ultimately, it must be ecologically sensible to deconstruct all facade types, and the reused or recycled product should have a lower embodied energy than a new product. it can be assumed that a significant portion of the deconstructed facade elements is not appropriate for direct reuse, in which case recycling with a higher energy input can be considered as an option. 032 journal of facade design & engineering volume 5 / number 2 / 2017 2.2 results of the case study the way in which materials are connected defines the purity of disassembled elements and components, and impacts the potential for reuse while contributing to a higher recycling value. looking at existing building stock, potential methods of deconstruction were seldom considered in their design. the facade types include various types of connections with specific levels of connectivity, ranging from low (non-destructive detachable) to high (destructive detachable or even hazardous waste). these categories were defined following the sorting process on site. from an energy-use point of view, deconstruction is sensible for all construction types when addressing their ecological dimensions. however, deconstruction is still labour-intensive due to complex disassembly processes. furthermore, health and safety issues need to be addressed. deconstruction has a heavy impact on humans due to the following aspects: 1) noise, 2) dust, 3) vibration, 4) debris. automation addresses a lot of the potential risks, and robotic deconstruction is an option to be considered in order to decrease human health impact and secure safety. further reasons to favour robotic deconstruction are: 1) limited space, 2) contaminated material, 3) risk of fall/collapse, 4) building height (motzko, 2016). 3 conclusion the rhenish mining area indicates what the situation might be at other suburban industrial regions with a high number of buildings that will be demolished in the near future. this building stock can serve as a valuable reservoir of secondary raw materials. to ensure a reuse of this building material, a well-functioning circularity in the building sector must be secured. to achieve this, all protagonists have to cooperate and communicate well. the exchange of information about the possibilities and advantages of reusing building material is vital in increasing awareness of its ecological impact. by reusing building material, primary resources and energy used for production can be saved. economic aspects have not been included in this research, but consideration of these is necessary in order to guarantee a market placement of reused building material and this should be investigated in future research. from the perspective of acceptance, it would be beneficial if the industry provided reused and recycled products nationwide. prototypes increase the acceptance of reused and/or recycled products, resulting in public investors favouring recycled material. to achieve the goal of recycling building material, all protagonists involved in the building process have to include the principle of circularity in their actions. in future scenarios, architects and engineers should include the deconstruction phase in their planning, and design for easy disassembly. this significantly increases the potential to make use of the material, energy and capital stored in buildings. 033 journal of facade design & engineering volume 5 / number 2 / 2017 endnotes the data situation allows for a simplified approach; while the facades shown are meant to demonstrate facades from ‘50s and ‘80s, the values refer to current data. it is added a 10% security tolerance, values retrieved form ökobau.dat references bund nrw. (02.11.2016) umsiedlung im rheinland, retrieved from: http://www.bund-nrw.de/themen_und_projekte/braunkohle/ verheizte_heimat/%20umsiedlungen_im_rheinland/ (02.11.2016) commission, e. (2013). directive 2008/98/ec on waste (waste framework directive) european parliament and the council of the european union (vol. 2008/98). brussels: official journal of the european union. deilmann, c., krauß, n., & gruhler, k. (2014). sensitivitätsstudie zum kreislaufwirtschaftspotenzial im hochbau. retrieved from: https://www.ioer.de/fileadmin/internet/ioer_projekte/pdf/fb_e/endbericht_rep.pdf destatis, (03.11.2016) statistische berichte. retrieved from: https://www.destatis.de/gpstatistik/servlets/mcrfilenodeservlet/ nwheft_derivate_00008963/e213201508.pdf;jsessionid=47ddc4f146d9691e35655abdd1309597 (03.11.2016) it.nrw 2013. statistische berichte daten zur abfallwirtschaft nrw 2013 tab. 1 und tab. 2 knaack, u., bilow, m., auer, t., & hildebrand, l. (2010). imagine 05energy (u. knaack, t. klein, & m. bilow eds. vol. 07). rotterdam: 010 rotterdam. landesdaten bank, (02.11.2016): https://www.landesdatenbank.nrw.de mettke, a., heyn, s., & thomas, c. (2008). rückbau industrieller bausubstanz – großformatige betonelemente im ökologischen kreislauf. cottbus: motzko, c. k., jörg ;, & wöltjen, j. l., daniela (2016). bewertungsmatrix für die kostenplanung beim abbruch und bauen im bestand [press release] schiller, g., ortlepp, r., & krauß, n. s. (2015). kartierung des anthropogenen lagers in deutschland zur optimierung der sekundärrohstoffwirtschaft. dessau rößlau wbgu, schellenhuber, h. j., messner, d., kraas, f., leggewie, c., lemke, p.,schneidewind, u. (2016). der umzug der menschheit: die transformative kraft der städte. retrieved from: http://www.wbgu.de/fileadmin/templates/dateien/veroeffentlichungen/ hauptgutachten/hg2016/wbgu_hg2016.pdf journal of facade design and engineering 4 (2016) 3–18 doi 10.3233/fde-160047 ios press 3 numerical investigation of the effect of vacuum insulation panels on the thermal bridges of a lightweight drywall envelope dimos a. kontogeorgos∗, ioannis a. atsonios, ioannis d. mandilaras and maria a. founti national technical university of athens, school of mechanical engineering, thermal engineering section, laboratory of heterogeneous mixtures & combustion systems, heroon polytechniou 9, zografou campus, athens, greece abstract. this paper addresses the thermal bridges issues of a two storey lightweight steel framed envelope in which the vips are placed in an inner “protected” layer of the external walls. this configuration provides “protection” for the vips, allows flexibility in installation of facade elements and at the same time permits interventions and modifications (e.g. drilling, installation of appliances) on the internal side of the wall. the envelope is extensively analysed in terms of all the different types of thermal bridges utilizing commercial computational tools and standardized methodologies, and their effect on the overall thermal performance is evaluated. a total improvement of 33% on the heat transfer coefficient of the building is calculated. results indicate the junctions between the external and internal walls, the external walls and the ceiling, the internal walls and the roof and the internal walls and the floor, respectively, as the most crucial thermal bridges. different design modifications and solutions are assumed in order to further reduce the impact of the most crucial thermal bridges. the implementation of the modifications resulted in a further reduction of the overall thermal losses by 27.5%, leading to an overall thermal loss reduction by 60.5% as compared to the reference building. keywords: vip, thermal bridges, lightweight building, thermal transmittance 1. introduction nowadays, the building sector has a lion’s share in energy consumption (u.s. eia, 2015). one of the most efficient ways to reduce the energy demands of a building is the installation of insulation at the building envelope. an innovative insulation solution, which reaches thermal conductivity values less than 5mw/(m k) (the value refers to the centre of panel), is the vacuum insulation panel (vip), whose insulation capability is approximately seven times better than that of conventional insulation materials, such as mineral wool or eps. it consists of an evacuated, open pore core material (typically mainly out of pyrogenic silica) surrounded by thin barrier laminates, consisting of a multilayer of metalized films, and a sealing film all laminated together, used to maintain the high level of vacuum. vips are already used in appliances such as refrigerators and deep-freezers (hammond & evans, 2014), however recently many researchers investigate their share and use in the building sector (alam, singh, & limbachiya, 2011, simmler & brunner, 2005, kalnaes & jelle, 2014, mandilaras, atsonios, zannis, & founti, 2014). during the last decade, awareness has grown for the need of advanced building shells which combine high thermal performance with short construction times ∗corresponding author: dimos a. kontogeorgos, laboratory of heterogeneous mixtures and combustion systems, heroon polytechniou 9, zografou campus, athens 15780, greece. tel.: +30 210 772 4002; fax: +30 210 772 3527; e-mail: dimkon@central.ntua.gr. issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). mailto:dimkon@central.ntua.gr 4 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges and maximum constructive simplification. hence, lightweight constructions coupled with vips form an attractive solution due to the advantages they offer, such as the low construction time, structural seismic resistance, the reduction and recyclability of wastes, the decrease of loads and costs on bearing structures and the high insulation capability (de angelis & serra, 2014). the concept of standardized lightweight steel framed residential buildings, aiming to build more buildings in a short period with fewer resources, is currently being investigated in the framework of the fp7 elissa project (www.elissaproject.eu). walls are made of drywall materials (i.e. gypsum and cement boards) anchored on a lightweight steel frame structure. one of the main challenges of the steel framed lightweight constructions is related to the effect of the thermal bridges. crucial areas of the building envelope, such as near the steel components of the construction, where the otherwise uniform thermal resistance is greatly affected by materials with a different thermal conductivity and/or a change in thickness of the elements, establish thermal bridges (zalewski, lassue, rousse, & boukhalfa, 2010). multidimensional heat flows are generated at these regions, which may induce a high risk of condensation and mould growth due to the low internal superficial temperature, with a negative impact on the structure and the indoor environmental quality (evola, margani, & marletta, 2011). many researchers have studied the impact of the thermal bridges on the total energy demands of a building. theodosiou and papadopoulos (2008) showed that the heating need can be 30% higher than the one calculated without taking into account the thermal bridge effects at a typical threestorey building in greece. de angelis and serra (2014) presented that without considering the metal studs the thermal transmittance of a lightweight wall is underestimated by ca. 67–74%. capozzoli et al. (capozzoli, gorrino & corrado, 2013) showed that the thickness of the insulation layer is one of the most important variables affecting the deviation of the linear thermal transmittance. evola et al. (evola et al., 2011) showed that by reducing the effect of thermal bridges using external insulation, the heating requirements are restricted to ca. 17–25%, while the annual energy demand is reduced by ca. 8% in the mediterranean climate. the innovative insulation of modern buildings with vips induces thermal bridge effects, which are related to three levels of thermal bridges: vip level, component and facade levels (quenard, 2015, sprengard & holm, 2014), that should be calculated (alam et al., 2011). the scope of this study is to evaluate the impact of an additional layer of vips at the inner side of the external wall on the thermal bridges introduced in a lightweight building envelope, as well as to propose potential design modifications for further reduction of the thermal bridges. the assessment of the thermal bridges is based on the standard en iso 12011 (en iso 10211, 2007), coupling detailed numerical simulations of the thermal bridges with the standardized methodology. the influence of the thermal bridges due to the metal studs, two and three dimension junctions on the overall thermal transmittance of the envelope is examined both without and with vips at the inner side of the external wall of the building envelope. finally, the impact of the potential design modifications, for the regions with high thermal bridging effects, on the overall thermal performance of the building is also analysed. 2. description of the building in order to assess the impact of the thermal bridges on the overall thermal performance of the construction, a two storey building is analysed. it is a lightweight steel framed construction based on a cavity wall system. the metal skeleton is founded on a cement base and the dry-wall system envelope is anchored on the steel skeleton. the overall dimensions of the building are 4.5m×2.5m×5.3m. www.elissaproject.eu d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 5 fig. 1. schematic diagram of the configurations of the building elements: a) external wall without vip, b) external wall with vip, c) roof and d) floor. the elements of the external walls (ew) are anchored on three different types of metal studs, i.e. c, cw (smaller c type) and i type. an additional vip layer is placed in the internal side of the external walls. the schematic diagram of the external wall configuration is depicted in figure 1 for the cases without (fig. 1a) and with vips (fig. 1b). the layers of the roof (rf) and the floor (fl) elements are anchored on i-type studs, 200mm wide. figures 1c and 1d show the schematic diagram of the roof configuration (fig. 1c) and floor (fig. 1d). it should be mentioned that the suspended part of the roof (i.e. resilient channel) is considered to be filled with conventional mineral wool. in the floor, a 180mm thick mineral wool layer is placed inside the cavity, while a 150mm thick foundation concrete slab and 500mm of soil were assumed for the analysis of the current work. the configuration of the ceiling (cl) (i.e. between the 1st and the 2nd floor) is similar to that of the floor excluding the concrete layer and the soil, with the suspended part utilized in the roof. the internal wall (iw) consists of two gypsum boards on the two sides of the wall and mineral wool (120mm) with air cavity (30mm) at the intermediate, resulting in a total thickness of 207mm. 3. methodology the methodology followed in order to assess the impact of the thermal bridges on the total thermal performance of the building is based on en iso 10211 (en iso 10211, 2007). numerical simulations are performed for the calculation of the thermal transmittance for each configuration. 6 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges fig. 2. partitioning of building into 1d, 2d and 3d geometrical models. 3.1. assessment of building thermal performance the main concept of the standardized methodology is the partitioning of the building into 1d, 2d and 3d geometrical models, as shown in figure 2. the 1d geometrical model stands for the centre of the wall assembly, including the steel studs, while the 2d and 3d geometrical models stand for the 2d and 3d junctions of the walls, respectively (e.g. external wall and roof, external wall and inside wall and ceiling etc.), including the steel studs of the junction. after partitioning the building, the transmission heat transfer coefficient, hd, is calculated by equation 1: hd = ∑ i fiaiui + ∑ k fklk�k + ∑ j fjχj (1), where fi is the factor of temperature correction of the building part i, ai is the area of element i of the building envelope, ui is the thermal transmittance of element i of the building (centre of wall), fk is the factor of temperature correction of the linear thermal bridge k, lk is the length of linear thermal bridges k, �k is the linear thermal transmittance of linear thermal bridges k, fj is the factor of temperature correction of the point thermal bridge j, and χj is the point thermal transmittance of the point thermal bridge j. according to equation 1, in order to calculate the transmission heat transfer coefficient, the individual thermal transmittances of each geometrical model should be calculated. these calculations can be performed by detailed cfd simulations. 3.2. calculation of linear and point thermal transmittance the linear thermal transmittance, �, concerns the two dimension geometries and is calculated according to equation 2: � = l2d − nj∑ i=1 ujlj (2), where, l2d is the thermal coupling coefficient obtained from a 2d simulation of the component separating the two environments being considered, uj is the thermal transmittance of the 1d d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 7 component, j, separating the two environments being considered and lj is the length over which the value uj applies. the point thermal transmittance, χ, of a part of the building envelope is calculated according to equation 3: χ = l3d − ni∑ i=1 uiai − nj∑ j=1 �jlj (3), where, l3d is the thermal coupling coefficient obtained from a 3d calculation of the 3d component separating the two environments being considered, ui is the thermal transmittance of the 1d component, i, separating the two environments being considered, ai is the area to which the value ui applies, �j is the linear thermal transmittance calculated from equation 2, and lj is the length to which the value �j applies. the thermal transmittance of the 1d component, u-value, at each component, is calculated according to en iso 6946 (en iso 6946, 2007) taking into account the physical properties (i.e. thermal conductivity and thickness) of the materials composing the assemblies. it should be noted that in the above equations (i.e. equations 1, 2 and 3), the linear and point thermal bridges refer to the steel skeleton and the geometry of the facade, i.e. level 3 according to quenard (quenard, 2014), and not to the thermal bridges due to the laminate barrier of the vip, i.e. level 1 as reported in brunner, stahl & ghazi wakili (2012a). 3.3. numerical simulation in the current work, the two and three-dimensional thermal coupling coefficients, l2d and l3d, were numerically calculated for all the junctions of the building envelope, covering all the thermal bridges introduced due to the presence of the steel frame. the ansys cfx (ansys cfx, 2009) commercial cfd package was used to simulate all the configurations and the types of thermal bridges. regarding the boundary conditions, at the inner side of the walls the ambient temperature and the heat transfer coefficient were assumed to be tin =20◦c and hin =7.69w/(m2k), respectively, while the respective values at the outside environment were tout =–10◦c and hout =20w/(m2k). in the case of ground soil, its temperature was considered to be constant and equal to tsoil =–10◦c. finally, the factor of temperature correction, f, was assumed to be f=1, apart from the floor assembly and floor’s junctions where it was assumed to be f=0.6 (tichelmann & ohl, 2005). the thermal properties of the materials were assumed to be constant. table 1 summarizes the physical properties and the thickness of the utilized materials. the ventilated air cavities were assumed to be stagnant air with an “effective” thermal conductivity calculated according to en iso 6946 (en iso 6946, 2007) taking into account the convection-radiation phenomena. the equivalent thermal conductivity of the vip layer has been calculated and found to be equal to 0.007mw/(m k) taking into account the edge effect according to brunner et al. (2012b). the total heat flow, �, which passes each configuration, can be calculated by the simulations. hence, the two and three-dimensional thermal coupling coefficients, l2d and l3d, can be calculated via equation 4: l = � tin − tout (4) 8 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges table 1 physical properties of the materials material thickness density thermal conductivity specific heat capacity d (mm) ρ (kg/m3) k (w/m k) cp (j/kg/k) boards and panels cement board 12.5 1150 0.35 1000 gypsum board 15 1033 0.27 990 sound insulation 10 250 0.07 2100 floor load panel 28 1500 0.44 – floor heating/cooling panel 32 1500 0.38 – insulation mineral wool 23/50 0.035 850 vip 20 195 0.007 800 xps 0.035 structural components structure c147/50/1.5 1.5 7854 60.5 434 profile cw50/0.6 0.6 structure dt2xc197/50/2.0 2.0 resilient channel 60/27/0.6 0.6 other roof sealing film – – 0.2 – floor covering – 2000 1.5 1000 – 1200 0.17 1400 concrete – 2300 1.95 900 – 2300 2.3 1000 soil – 2000 2.0 1000 ceramik tile – 2300 1.3 840 4. results all the thermal bridges of the building with and without the additional vip layer were evaluated. in total, eight (8) cases concerning the impact of the metal studs at the centre part of walls, fourteen (14) bi-dimensional intersections and twelve (12) three-dimensional junctions between the building elements were examined. the individual and the overall contribution of the thermal bridges at the transmission heat transfer coefficient of the building were investigated, highlighting the improvement due to the installation of vips. 4.1. central part of the walls the impact of the metal studs within the central part of the wall elements at the thermal performance of the building envelope is depicted in figure 3. figure 3a illustrates the temperature contours of the central part of the external wall for every type of utilized studs, as well as the linear thermal transmittance of each stud, in the cases with and without vip layer. as it is shown, the presence of the metal studs introduces significant thermal bridges, which have to be taken into account in the overall thermal assessment of the building. moreover, it can be seen that d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 9 fig. 3. a) temperature contours of the external wall for all the types of the studs without and with the vips and b) impact of the studs on the u-value for all the building elements. the thermal transmittance of the i-studs is ca. 35–40% higher than the respective of c-type stud (in both cases without and with vips). this is due to the fact that the i-stud is a double c-stud, which means that there is more metal in the same area; increasing the heat losses. additionally, the impact of the cw-type is not significant compared to the other types of the studs. finally, considering the impact of the vips on the linear thermal transmittance it can be seen that it is reduced by ca. 64% to 83% for all the stud types. figure 3b shows the effect of the metal studs, indicated by the u-value for each wall element (i.e. external wall, dry roof and floor) of the building. as it is shown, the presence of the metal studs affects significantly the thermal performance of the wall elements by increasing the uvalue. more specifically, the increase of the thermal transmittance of the external wall is ca. 50% for the external wall without the vip and ca. 27% for the case with the vip. concerning the roof and the floor, the inclusion of the metal studs increases the u-value by ca. 169% and 210% respectively. 4.2. two-dimensional junctions figure 4a illustrates the temperature contours of an indicative 2d junction, i.e. junction between two external walls for the case without and with the vips. it is clearly shown that the metal studs’ junction introduces thermal bridges in both cases defined by the two dimensional temperature field. comparing the two cases shows that the presence of the vips reduces the heat flow through the junction. the linear thermal transmittance of all the 2d junctions of the building elements examined in this work is presented in figures 4b and c. focusing on figure 4b, it is obvious that the vip layer improves the impact of the 2d junctions at the thermal performance of the building, decreasing the �-value by 12% up to 92%. it is observed that the linear thermal transmittance has negative values at the junction between external walls (ew+ew), meaning that this thermal bridge has a positive effect on the total thermal performance of the building envelope. figure 4c presents the 2d intersections that are not affected by the additional insulation. overall, it can be seen that the most important thermal bridges are the junctions that include the internal walls (i.e. ew+iw, iw+rf and iw+fl), as well as the junction between the external wall and the dry ceiling. thus, special design modification should be taken into account in order to further reduce the heat losses through these thermal bridges. 10 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges fig. 4. a) temperature contours of the external wall – external wall junction without and with the vips, b) linear thermal transmittance for the 2d junction affected by the vips and c) linear thermal transmittance for the 2d junction not affected by the vips. 4.3. three-dimensional junctions figure 5a illustrates the temperature contours of an indicative 3d junction, i.e. between the two external walls and the roof for the cases without and with the vips. once again, the metal studs of the junction introduce significant thermal bridges in both cases defined by the three-dimensional temperature field. figure 5b shows the point thermal transmittance for all the 3d junctions of the building examined in this work. it is clearly shown that the presence of the vips generally improves the thermal performance of all the 3d thermal bridges. the point thermal transmittances are decreased up to 138% due to the installation of vip layer at the external walls. 4.4. overall thermal transmittance the contribution of the thermal bridges and the impact of the vip layer on the overall thermal performance of the building envelope is depicted in figure 6. the installed vips decrease the total thermal transmission, hd, by ca. 33%. the contribution of the metal studs of the central part of the wall configurations is ca. 30% of the total transmittance in both cases. although the additional insulation reduces the linear thermal transmittance of all the metal studs at the centre part of the walls, the percentage impact of them remains the same because of the high �-values of the studs at the roof and floor, where extra insulation is not installed. the total impact of the 2d and 3d d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 11 fig. 5. a) temperature contours of the external wall – external wall – roof junction without and with the vips and b) point thermal transmittance of all 3d junctions without and with the vips. fig. 6. impact of thermal bridges on the overall thermal transmittance without and with the vips. thermal bridges is ca. 31% in the case without the vips, while in the case with the vips it is ca. 25%. the 2d and the 3d junctions contribute the same percentage in the case without vip, showing that three-dimensional intersections are as important as two-dimensional intersections, and thus they should be taken into account in relevant calculations. in the case with vip, the transmission of the point thermal bridges is 4 times lower than that of the linear thermal bridges. 12 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 5. design modifications as mentioned above, the installation of vips at the internal part of the external walls resulted in a decrease of ca. 33% compared to the overall thermal transmittance of the building. nevertheless, it was found that specific areas of the building, such as the floor, the dry roof, the junctions of the internal walls and the junctions between the external walls and the ceiling suffer from increased thermal bridges despite the installation of the vips. thus, different design modifications and solutions were examined in order to further eliminate the remaining increased thermal bridges, which are described in detail below. 5.1. roof regarding the roof, two potential design solutions were assumed: the gap of the suspended ceiling was increased in order to increase the thickness of the mineral wool and an xps insulation layer (80mm thickness) was positioned on the top of the roof. figure 7a and b illustrate schematic diagrams of the design solutions described above. figure 7c and d show the calculated u-value of the dry roof assembly, considering the different types of insulation, as well as the calculated linear thermal transmittance (�-value) of the i-stud inside the dry roof assembly. it should be mentioned that the case “rc” means “resilient channel” and the number next to it defines its thickness, fig. 7. design modification of the roof configuration: a) schematic diagram of the wall assembly with different thicknesses for the resilient channel, b) schematic diagram of the wall assembly with an xps layer on the top, c) u-value of the wall assembly for different types of insulation and d) linear thermal transmittance of the wall assembly’s studs for different types of insulation. d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 13 while the case “xps 80mm” defines the xps insulation layer (80mm thickness) positioned on the top of the roof. as it is expected, when the thickness of the suspended ceiling is increased the u-value of the dry roof and the linear thermal transmittance of the i-studs decreases. the 200mm thickness of the resilient channel achieves ca. 67% and 81% reduction of the u and � values, respectively. nevertheless, increasing the thickness of the resilient channel means that the “free space” of the room or else the room’s height decreases. on the other hand, the installation of an xps layer of 80mm thickness on the top of the roof, which is also an easier design solution from a construction point of view, reduces the u and � values ca. 50% and 64%, respectively. this reduction is equivalent to the reduction that occurs if the thickness of the resilient channel is 100mm. 5.2. floor regarding the floor, the potential design solution that was assumed was the installation of an eps insulation layer of two different thicknesses, 100mm and 200mm, respectively, between the cement slab and the ground soil, as shown in figure 8a. figure 8b shows the calculated u-value of the floor assembly, considering the different types of insulation (i.e. eps of different thickness), as well as the calculated linear thermal transmittance of the i-stud inside the floor assembly. as it is expected, when the thickness of the eps insulation layer between the foundation slab and the ground soil is increased, the u-value of the floor and the linear thermal transmittance of the i-studs decreases. moreover, the installation of the 100mm thick eps results in a ca. 60% reduction of the u-value. additionally, a further increase of the eps’s thickness from 100mm to 200mm results in ca. 15% more improvement. considering the ratio between the cost of the eps and the u-value reduction percentage for each thickness, the solution of the 100mm thick eps is a reasonable solution. fig. 8. design modification of the floor configuration: a) schematic diagram of the wall assembly and b) u-value of the wall assembly and linear thermal transmittance of the wall assembly’s studs for different thickness of eps between the foundation and the soil. 5.3. external wall-ceiling junction in order to improve the thermal performance of the external wall-ceiling junction, a potential solution would be the installation of a super insulation material (sim) inside the ventilated part of the external wall near the junction, as shown in figure 9a. in this study, two different sims (aerogel and vip) of two different thicknesses (10mm and 15mm) and five different lengths (308mm, 14 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges fig. 9. design modification of the external wall – ceiling junction: a) schematic diagram of the junction and b) linear thermal transmittance of the junction with different sim protection. 338.8mm, 369.6mm, 462mm and 616mm) were assumed. the 308mm length corresponds to the minimum length, inside the ventilation part of the external wall, which covers the distance between the metal studs of the junctions (see figure 9a). figure 9b shows the linear thermal transmittance for the external wall-ceiling junction for all the examined cases. it is observed that the two examined sims, i.e. vip and aerogel, have significant differences as far as their thermal performance is concerned. the vip is observed to perform better than the aerogel due to the significant difference in the thermal conductivity values. when the length of the sim increases, the effect of the thermal bridge also decreases. the length of 616mm is almost the maximum length that could reach the maximum thermal bridge reduction. moreover, when increasing the thickness of the sim, the effect of the thermal bridge of the junction decreases. the reduction of the thermal bridges by incorporating the proposed solutions ranges between ca. 18% and 61%. it is clearly shown that the aerogel cannot reach the insulation capability of the vip. the bigger aerogel sample examined (i.e. 15mm thickness and 616mm length) reduces the thermal bridge by ca. 32.5%. on the other hand, the smaller vip sample examined (10mm thickness and 308mm length) reduces the thermal bridge by ca. 29%, which is very close to the reduction percentage obtained by the bigger aerogel sample. another significant issue that must be taken into consideration is the thickness of the sim due to regulation restriction for the thickness of the ventilation facade. as it is shown, by increasing the length of the sim, the thickness of the sim could be compensated with respect to the reduction of the thermal bridge effect. thus, the 10mm thick and 616mm length vip has an almost equivalent insulation capability as the 15mm thick and 462mm length vip. 5.4. external wall-internal wall junction the potential design solution that was assumed for the external wall-internal wall junction was the total separation of the two walls, as shown in figure 10a. figure 10b shows the linear thermal transmittance of the junction for the reference and the new design solution. as it is shown, the linear thermal transmittance for the reference design is ca. � =0.153w/m k, while the respective value for the new design solution is ca. � =0.002w/m k, which corresponds to ca. 98% reduction of the specific thermal bridge. the respective values for the heat transmission through the junction were 0.39w/(m k) for the reference design solution and 0.24w/(m k) for the new design solution. the latter indicates that the overall reduction of the thermal losses of this junction was ca. 38.5%. d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 15 fig. 10. design modification of the external wall – internal wall junction: a) schematic diagram of the reference junction, b) schematic diagram of the modified junction and b) linear thermal transmittance of the two design solutions. fig. 11. overall thermal transmittance of the building with different design solutions. 5.5. overall thermal transmittance as mentioned above, different design modifications were assumed in order to further reduce the increased thermal bridges of the building. table 2 summarizes the final solutions that were assumed to quantify the overall thermal transmittance of the building taking into account the applicability (ease of construction), as well as the cost effectiveness of the solution. figure 11 16 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges shows the transmission heat transfer coefficient of the building for the different design solutions. as it is shown, each solution contributes positively to the reduction of the thermal losses compared to the reference case (i.e. building with vips installed at the external walls). more specifically, the larger improvements are obtained due to the design modifications in the roof (ca. 11.5% thermal loss reduction) and the floor (ca. 8.9% thermal loss reduction). this is due to the fact that the two wall assemblies were not insulated in the reference case. moreover, the local design modifications, i.e. installation of a vip strip inside the ventilation facade and the decoupling of the external wallinternal wall junction, have a smaller, but still significant impact on the reduction of the overall thermal losses. the most important conclusion of the analysis is that when all these design solutions are implemented the overall thermal loss reduction reaches ca. 27.5%. it should be mentioned that this reduction is achieved on the building with the vips installed inside the external walls. compared to the initial building (without the vips at the external wall) the overall reduction of the thermal losses reaches ca. 60.5%. table 2 design solutions for decreasing the thermal bridges solution description ref. reference building with vips at the internal side of the external walls sol-1 installation of an xps insulation layer (80mm thickness) on the top of the roof sol-2 installation of an eps insulation layer (100mm thickness) between the foundation cement and the soil sol-3 installation of a vip strip (616mm length – 10mm thickness) inside the ventilation facade near the external wall-ceiling junction sol-4 decoupling of the external wall-internal wall junction sol-all implementation of all the solutions (i.e. sol-1 to sol-4) 6. conclusion this work examined the impact of the thermal bridges on the overall thermal transmittance of a metal framed lightweight drywall building envelope and the effect of an additional vip layer at the inner side of the external wall. the vip layer is placed in an inner “protected” layer, allowing flexibility in installation of facade elements and at the same time permitting intervention and modifications (e.g. drilling of appliances) on the internal side of the wall. three types of thermal bridges were analysed: thermal bridges resulting from metal studs and two and three-dimensional junctions between the elements of the building envelope. moreover, additional design modifications were assumed in order to further reduce the remaining significant thermal bridges. the impact of the thermal bridges on the thermal transmittance was found to be very strong, increasing the thermal transmittance by ca. 61–55%. results revealed that the highest contribution on the total thermal transmittance was owed to the metal frame of the building (approximately 30%). the effect of the two-dimensional junctions was about 15–19%, while the impact of the three-dimensional intersections was found to be up to 16%, in contrast to many researchers who neglect the effect of the point thermal transmittance (evola et al., 2011). the extra internal vip insulation resulted in a reduction of the overall thermal transmittance by ca. 33%. concerning the thermal bridges, the additional internal vip insulation decreased all the linear and the point thermal transmittances up to 130%. despite of the outcomes of other studies (capozzoli et al., 2013), the additional internal insulation of the external wall decreased the thermal bridges, not only in relative terms, but quantitatively, too. d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges 17 the thermal analysis of the examined building taking into consideration the additional vip insulation layer revealed that there were remaining increased thermal bridges. the most crucial thermal bridges were found to be at the roof, the floor, the external wall-ceiling junction and the external wall-internal junction. different design modifications were assumed in order to reduce the impact of these thermal bridges. the installation of an xps layer (80mm thick) on the top of the roof and an eps layer (100mm thick) between the foundation cement slab and the soil resulted in ca. 11.5% and 8.9% reduction of the overall thermal losses, respectively. local modifications, such as the installation of a vip strip inside the ventilation facade near the external wall-ceiling junction and the decoupling of the external wall-internal wall junction resulted in ca. 2.9% and 4.1% reduction of the overall thermal losses, respectively. overall, the assumed design modifications further reduced the thermal losses of the examined building by ca. 27.5%. compared to the reference building (i.e. without the additional vip layer), the total reduction of the thermal losses reached ca. 60.5%. acknowledgments the authors acknowledge the financial support of the european commission within the framework of the fp7-2013-nmp-env-eeb project ‘elissa’ (www.elissaproject.eu). references alam, m., singh, h., & limbachiya, m. c. (2011). vacuum insulation panels (vips) for building industry a review of the contemporary developments and future directions. applied energy, 88(11), 3592-3602. doi: 10.1016/j.apenergy.2011.04.040 ansys cfx (2009). ansys cfx-solver theory guide. canonsburg, ansys, inc. brunner s., stahl t., & ghazi wakili k. (2012). an example of deteriorated vacuum insulation panels in a building façade. energy and buildings, 54, 278-282. brunner s., stahl t., & ghazi wakili k. (2012). single and double layered vacuum insulation panels of the same thickness in comparison, in: proceedings of building enclosure science & technology conference (best3), april 2-4th 2012, atlanta. capozzoli, a., gorrino, a., & corrado, v. (2013). a building thermal bridges sensitivity analysis. applied energy, 107, 229-243. doi: 10.1016/j.apenergy.2013.02.045 de angelis, e., & serra, e. (2014). light steel-frame walls: thermal insulation performances and thermal bridges. energy procedia, 45, 362-371. doi: 10.1016/j.egypro.2014.01.039 en iso 10211 (2007). thermal bridges in building construction – heat flows and surface temperatures – detailed calculations (iso 10211:2007). brussels, management centre. en iso 6946 (2007). building components and building elements – thermal resistance and thermal transmittance – calculation method. (iso 6946:2007). evola, g., margani, g., & marletta, l. (2011). energy and cost evaluation of thermal bridge correction in mediterranean climate. energy and buildings, 43(9), 2385-2393. doi: 10.1016/j.enbuild.2011.05.028 hammond, e. c., evans, j. a., 2014. application of vacuum insulation panels in the cold chain – analysis of viability. international journal of refrigeration, 47, 58-65. doi: 10.1016/j.ijrefrig.2014.07.010 kalnaes, s. e., & jelle, b. p. (2014). vacuum insulation panel products: a state-of-the-art review and future research pathways. applied energy, 116, 355-375. doi: 10.1016/j.apenergy.2013.11.032 mandilaras, i., atsonios, i., zannis, g., & founti, m. (2014). thermal performance of a building envelope incorporating etics with vacuum insulation panels and eps. energy and buildings, 85, 654-665. doi: 10.1016/j.enbuild.2014.06.053 quenard d., (2015). iea-ebc annex 65, long-term performance of super-insulation materials in building components & systems, 3rd international school on sustainable chemistry and energies: challenges in urbanism, housing and transportation, january 20-22, 2015 maison des etudiants aime schoenig – um1, montpellier, france. [pdf]. retrieved from http://infoscience.epfl.ch/ record/213314/files/2 quenard.pdf simmler, h., & brunner, s. (2005). vacuum insulation panels for building application: basic properties, aging mechanisms and service life. energy and buildings, 37(11), 1122-1131. doi: 10.1016/j.enbuild.2005.06.015 sprengard c., & holm a. h. (2014). numerical examination of thermal bridging effects at the edges of vacuum-insulation-panels (vip) in various constructions. energy and buildings, 85, 638-643. doi: 10.1016/j.enbuild.2014.03.027 tichelmann, karsten; ohl, rené (2005). wärmebrücken atlas – trockenbau, stahlleichtbau, bauen im bestand. verlagsgesellschaft rudof müller. www.elissaproject.eu http://infoscience.epfl.ch/record/213314/files/2_quenard.pdf http://infoscience.epfl.ch/record/213314/files/2_quenard.pdf 18 d.a. kontogeorgos et al. / numerical investigation of the effect of vip on the thermal bridges theodosiou, t. g., papadopoulos, a. m. (2008). the impact of thermal bridges on the energy demand of buildings with double brick wall constructions. energy and buildings, 40(11), 2083-2089. doi: 10.1016/j.enbuild.2008.06.006 u.s. energy information administration (eia) (2015). monthly energy review. washington, u.s. energy information administration. zalewski, l., lassue, s., rousse, d., & boukhalfa, k. (2010). experimental and numerical characterization of thermal bridges in prefabricated building walls. energy conversion and management, 51(12), 2869-2877. doi: 10.1016/j.enconman.2010.06.026 jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 051 journal of facade design & engineering volume 5 / number 1 / 2017 solar façades – main barriers for widespread façade integration of solar technologies alejandro prieto1, ulrich knaack2, thomas auer3, tillmann klein4 1 delft university of technology, faculty of architecture and the built environment, department of architectural engineering + technology, façade research group. julianalaan 134 2628bl delft, netherlands. +31 6 48462317, a.i.prietohoces@tudelft.nl 2 delft university of technology, faculty of architecture and the built environment, department of architectural engineering + technology, façade research group. julianalaan 134 2628bl delft, netherlands. u.knaack@tudelft.nl 3 technical university of munich, department of architecture, chair of building technology and climate responsive design, arcisstraße 21 80333 munich, germany. thomas.auer@tum.de 4 delft university of technology, faculty of architecture and the built environment, department of architectural engineering + technology, façade research group. julianalaan 134 2628bl delft, netherlands. t.klein@tudelft.nl abstract solar energy has been actively promoted as a clean energy source since 1973’s oil crisis, evidenced by the emergence of initiatives such as the solar heating & cooling programme of the international energy agency or the us department of energy. nonetheless, solar technologies have not been widely used in the built environment, limiting their operation to industrial and macroscale applications. commercially available products such as building integrated pv panels (bipv) or building integrated solar thermal collectors (bist); and novel prototypes and concepts for solar cooling integrated facades are seen as interesting alternatives for the development of new performance based façade components for high-performing commercial buildings. however, there are barriers to overcome in order to promote widespread application of architecturally integrated solar components. the present paper seeks to discuss perceived barriers for widespread façade integration of solar technologies, in order to define the current scenario and generate guidelines for future developments. in order to achieve this, the paper presents the results of a survey addressed to professionals with practical experience in the development of façade systems for office buildings, situated at any stage of the design and construction process. hence, architects, façade consultants, system suppliers and façade builders were considered. the outcome of this study is the definition of the main perceived barriers for façade integration of solar technologies, discussing the results from the survey along with other related experiences found in the literature. this study is part of the ongoing phd research project titled coolfacade: architectural integration of solar cooling strategies into the curtain-wall, developed within the façade research group (frg) in the green building innovation programme (gbi) of the faculty of architecture and the built environment, tu delft. keywords solar technologies, pv, solar thermal collectors, solar cooling, façade integration, survey doi 10.7480/jfde.2017.1.1398 052 journal of facade design & engineering volume 5 / number 1 / 2017 1 introduction worldwide energy consumption is expected to greatly increase during the next three decades. predictions elaborated by the us energy information administration (eia) show that by 2040, world energy consumption will present an increase of 48%, compared to consumption levels registered in 2012 (doe/eia, 2016). similarly, in the last annual energy outlook released by bp, it was declared that an increase of 35% in total energy usage is expected between 2014 and 2035 (bp, 2016). this panorama clashes with iea reports which state that energy production and use presently account for two thirds of worldwide greenhouse gas emissions, supporting the need to lower them, while sustaining the growth of the world economy (oecd/iea, 2015). as a matter of fact, it has been stated that the expected consumption increase will be mostly supported by non-oecd nations, in order to drive long term economic growth, so even if energy saving campaigns and strategies are enforced worldwide, there will still be a demand for alternative sources of energy. solar energy has been actively promoted as a clean energy source since 1973’s oil crisis, evidenced by the emergence of initiatives such as the solar heating & cooling programme of the international energy agency (iea-shc, 2016) or the us department of energy (doe, 2016). nonetheless, besides scattered examples, the introduction of solar technologies into the built environment has not been massively accepted, which seems relevant considering that the building sector accounts for 40%45% of the total energy demand (ep, 2002). important advances have been made in the development of solar driven building products for façade integration, such as semi-transparent pv glazing (fung & yang, 2008; li, lam, chan, & mak, 2009), pv and solar thermal collector integrated shading devices (frontini, 2011; mandalaki, zervas, tsoutsos, & vazakas, 2012), or coloured glazed thermal collectors by means of multi-layered films (schüler et al., 2005) and selective paint coatings (joly et al., 2013; orel et al., 2007). however, these have been proven insufficient in order to promote widespread application in buildings. this paper seeks to contribute to the field, by presenting the results of an open ended survey addressed to professionals with practical experience in the development of façade systems for office buildings. the main goal of the survey was to identify main requirements and barriers for façade integration of building services, as a way to promote the development of new cost-effective multifunctional façade products for high-performance office buildings. the first part of the survey dealt with design and construction problems related to the integration of building services and was discussed on a previous paper (prieto, klein, & knaack, 2016). the second part was specifically aimed to identify the main barriers for façade integration of solar technologies. hence, this paper presents the perceived barriers and compares the findings along similar research experiences on the topic. among related experiences it is possible to count surveys carried out to identify barriers for the application of building integrated photovoltaics (bipv) and building integrated solar thermal panels (bist). yang (2015) discussed technical barriers for façade integration of pv panels, considering design, construction, commissioning and operation stages. her findings highlighted the need for advanced simulation tools and monitoring platforms to assist the role of designers and building managers. cappel et al. (2014) stated that economic factors are seen as the most relevant barrier, with an important lack of knowledge from architects as the second most important problem for market penetration of facade integrated solar thermal systems. also discussing thermal collectors, munari probst and roecker (2007) stated the need for more design flexibility regarding aesthetical aspects such as shape and colour of integrated building components. 053 journal of facade design & engineering volume 5 / number 1 / 2017 one of the most relevant works about barriers for architectural application of solar technologies was made as one of the outcomes of task 41 of the solar heating & cooling programme of the international energy agency. a web-based multiple choice survey was distributed among architects in 14 participating countries, to assess their use of solar technologies, perceived barriers for implementation, and satisfaction levels of commercially available products. results from 394 valid questionnaires showed that economic aspects are the main issues to overcome, followed by knowledge and information about solar technologies and product availability (farkas & horvat, 2012). the information presented on this paper is organized according to two main aspects. first, barriers were identified and assessed in terms of perceived relevance by the interviewees, comparing the results to the findings of farkas and horvat (2012). secondly, key aspects among the barriers were discussed, giving particular attention to perceived barriers related to the products themselves, in order to draft recommendations for future product development, based on requirements and experience from experts in the field. 2 methodology 2.1 the survey the survey was conducted from mid-september to mid-november, 2015 and was distributed both as an online form and in printed format among several professional and research networks related to façade design and construction. 133 questionnaires were recovered, consolidating a final number of 79 valid questionnaires after filtering empty (40) and half empty forms (14). the questionnaire was structured explored design and construction issues related to the integration of building services in general while also considered specific questions about the integration of solar technologies. this paper focuses on the second part of the questionnaire. multiple choice questions were analysed through descriptive analysis, while open ended questions were processed following content analysis methods. the findings from task 41 were used as reference for the evaluation of the responses, allowing for comparisons. the analysis does not pretend to be exhaustive nor completely representative, however, it is seen as valuable information to understand perceived barriers and detect key aspects to overcome in order to develop architecturally integrated solar façade components. 2.2 the sample the questionnaire was addressed to professionals with practical experience in the development of façade systems for office buildings, situated at any stage of the design and construction process, which meant different backgrounds and experience within the sample. in terms of the background of the interviewees (n=79), the large majority corresponded to engineers (44%) and architects (39%) as shown in fig.1. regarding experience in the field, 67% of the interviewees stated that they have between 5 and 20 years of experience, and 18% claimed to have more than 20 years. only 15% of the professionals approached for the survey had less than 5 years of experience (fig.2). 054 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 1 declared background of the interviewees fig. 2 declared years of professional experience familiarity with solar technologies was evaluated, considering direct experience with pv panels, solar thermal collectors, and solar cooling technologies. the interviewees were allowed to check more than one alternative, in case they had experience with more than one type of technology. almost half of the sample (47%) declared to have had direct experience with photovoltaic cells, while 38% declared the same for the case of solar thermal collectors. as expected, a smaller group of interviewees declared to have particular experience dealing with solar cooling technologies. lastly, 33% declared not to have any experience working with solar technologies. this fact is worth mentioning because it means that around two thirds of the sample does have direct experience working with the cited technologies, which validates their appreciations on the topic. fig. 3 declared experience with solar technologies 3 results and discussion first of all, figures 4 and 5 show the overall perception of the interviewees regarding possibilities for integration of solar technologies in façade systems. figure 4 illustrates the perceived potential for further development of solar integrated façade products. the vast majority of the interviewees (91%) believed that there is potential for integration, while only 6% were not sure and 3% did not provide an answer. it is worth mentioning that nobody declared to believe that there is no potential for further developing façade products integrating solar technologies. the main reason declared by those who were not sure about façade integration potential was the difficulty associated with the development of cost-effective solutions, due to low efficiency and high costs of current systems. 055 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 4 perceived potential for solar façade integration fig. 5 current market for solar façade products furthermore, the interviewees were asked if they believe that there are current market possibilities for commercial application of solar integrated architectural products. as shown in figure 5, 80% of the sample believes that there is current commercial demand for these products, while 15% believes the contrary. the main reasons given to support the latter were the high costs of the systems compared with the low cost of energy, and budget management within the conventional facade design and production process, based on separate trends and contractors, obstructing the development of integrated products. 3.1 main perceived barriers to promote widespread façade integration of solar technologies the interviewees were asked to state the most important barriers to overcome in order to promote widespread integration of solar technologies into the building envelope. this was an open ended question to assess their perceptions on the subject without external conditioning. furthermore, the interviewees were asked to mention up to three constraints in order of relevance, so the order of the mentions was considered on detailed further analyses as well. figure 6 shows a word map of all issues declared by the interviewees, as a first approach to data analysis. the size of the displayed words, represents their frequency within the sample. the word map was made using the exact words from the responses, considering all words mentioned at least two times, after filtering connectors and other auxiliary words without standalone meaning. no distinction was made based on the mention order of the perceived barriers. by looking at the word map, it is clearly noticeable that ‘cost’ stands out as the most used word to refer to integration barriers, which does not come as a surprise. other topics which received a relevant amount of mentions were related to design aspects (‘aesthetics’, ‘design’), performance of the systems (‘performance’, ‘efficiency’, ‘energy’), the knowledge to design, implement or operate them (‘knowledge’), or the need for extra maintenance activities (‘maintenance’). 056 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 6 word map of barriers declared by the experts as a second step for data analysis, the responses were processed and formatted into new limited categories to allow for further evaluation. this step was necessary in order to overcome false conclusions created by different phrasing or word choice by the interviewees. however, detailed information from the original answers was preserved and used when discussing the results, to add depth to the analysis. it was decided to use the categories defined by the iea shc task 41 research project (farkas & horvat, 2012), for the analysis of the sample (table 1). therefore, using categories already validated in the literature, while giving the opportunity to compare and discuss the results against previous experiences. main categories description of the barriers interest lack of interest in solar design by architects and clients/developers economy not economically justifiable and lack of governmental incentives knowledge lack of sufficient technical knowledge by architect, by client/developer and by consultant information lack of architecturally oriented literature about these technologies and useful data for architects in product datasheets product lack of products suitable for quality building integration and complementary building components process lack of tools that support design and sizing of systems / technology is considered too late in the design process (insufficient time and resources) table 1 barriers and categories identified by iea shc task 41 the categorised responses are shown in figure 7. as expected, economy related issues have the most amount of first mentions, being perceived as the main barrier to overcome. secondly, issues about the product itself arose as a relevant barrier, followed by knowledge and information related problems. lastly, problems related with design and construction processes were also mentioned as the main barrier, along with other topics not considered in the previous categories. lack of interest was not identified as a barrier in the first mention by any of the interviewed experts. 057 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 7 main perceived barriers categorised on variables defined by iea shc task 41 considering the total amount of mentions under each category, ‘economy’, ‘knowledge’ and ‘information’ show a frequency increase without surpassing the number of first mentions. however, in the cases of ‘product’ and ‘process’ related barriers, second and third mentions surpass the first one. this shows that even if these are not strongly perceived as the main barrier to overcome, there are important ‘product’ and ‘process’ related issues to solve in order to promote widespread façade integration of solar technologies. this issue is particularly evident in the need for suitable products for integration. as shown in the graph, it becomes the most pressing issue to overcome considering the total amount of mentions. of course, different aspects related to the suitability of said products were considered, which will be further detailed in this document. in general terms, the results obtained from the survey fall in line with the findings from iea task 41. similarly, farkas and horvat (2012) declared that economic issues seem to be the main barriers for implementation of solar technologies, although this becomes clearer in the case of the application of pv panels, compared to the use of solar thermal technologies. discussing possible strategies to overcome the barriers, besides economic measures, the authors declared that even if there have been considerable advancements in the design, look and efficiency of solar components, there is need for new developments and improvements of available products to appeal to architects. the recognition of these two issues by both experiences is seen as a sign of validation for this study, by comparison with a larger sample. the most relevant differences between both experiences, refer to the perceived lack of knowledge and overall interest to architecturally integrate solar technologies into the building envelope. task 41 results showed these issues as the second and third most relevant barriers for the use of both pv cells and solar thermal panels. while lack of knowledge was also declared as a relevant barrier during the present study, lack of interest was barely mentioned in comparison with other barriers, and not stated at all as the main barrier (under first mention). on the one hand, this fact could be seen as a sign of increasing interest on renewable energies since the application of the task 41 survey. on the other hand, another reason to explain this could be the specialisation level of the interviewed experts (professionals from different disciplines with experience in façade design and/or construction) when compared to the surveyed sample from task 41 (mostly architects). furthermore, it is relevant to mention that more than 90% of the interviewed experts believed that there is potential for façade integration, whose probably take interest in this technologies for granted. this notion may be supported by looking at the disaggregated results from the task 41 report, which differentiated between lack of interest by the client and lack of self-interest (by architects). in effect, clients’ lack of interest was declared as an important barrier while architects’ lack of interest was barely stated as an issue, in a similar way to the present study. conjectures aside, the fact that ‘lack of interest’ did not appear in relevant numbers as answer to an open ended question, shows that it was clearly not perceived as a defining barrier for the surveyed sample. 058 journal of facade design & engineering volume 5 / number 1 / 2017 several statistical analyses were conducted to assess potential differences in the perceived barriers between independent groups among the sample, based on background, role, experience with façade integration or solar technologies. even though some discrepancies were found among the groups, particularly between professionals with and without experience with solar technologies, the deviation was never found significant enough to unequivocally state that perceived barriers change according to different groups. thus, the discussed results are valid for the different groups allowing for minor variations in judgement. more detailed studies with a larger sample are advised if there is further interest to explore these variations. 3.2 identified aspects to overcome within main barriers given that the survey was structured in open ended questions, the interviewees had the possibility to freely declare the issues they perceived as the main barriers to overcome for widespread façade integration of solar technologies. this information is regarded as relevant in order to devise focused strategies to cope with the perceived barriers, discussing several mentioned aspects in each category for better understanding. it is worth mentioning that the depth of the discussion is constrained by the level of detail of the gathered responses, allowing for more insight in some topics than others. ‘product’ related issues will be presented separately, due both to its relative perceived relevance within the sample and its potential to inform the development of future architectural products. hence, the discussion will be conducted to the definition of key aspects to overcome for the development of appealing solar integrated façade components. first of all, issues categorised under ‘economy’, largely refer to the cost of solar technologies in terms of the economic feasibility and payback time for the initial investment. an additional concern was the current energy price, which is not high enough to promote the emergence of competitive solar driven alternatives on a larger scale. nevertheless, the energy cost is expected to rise in the future, while at the same time further developments and governmental incentives should encourage the offer of new solar based architectural products for façade integration. regarding knowledge, the main concern was the lack of experience and overall knowledge of architects about technical issues and solar technologies in general, obstructing its widespread application. besides this, there were also mentions of the role of end users, referring to their lack of knowledge on how to operate these systems. finally, some concerns were also expressed about the experience of the workforce on site, required to successfully assemble the components minimising the occurrence of errors. the perceived lack of knowledge is closely linked with the need for useful information about solar technologies. the interviewees declared that information should be focused on showing purposes and associated benefits to end users, while it also should be clear and complete enough to assist architects during design stages. in this regard, there were concerns about proper documentation of some properties, such as efficiency over time and reliable performance data; and also the need for updated standards for building integration, considering reputable references and state-of-the-art available technologies in the market. the main issues categorised under ‘process’ revolved around the perceived need to rethink the overall façade design and construction process to incorporate solar technologies from earlier stages, allowing for closer collaboration between the different disciplines involved. it was stated that integration has to be further considered by both designers and manufacturers, pushing for optimised mass production processes while minimising planning efforts. it was also mentioned that more design oriented tools are needed in order to bring architects closer to technical issues during early design stages. 059 journal of facade design & engineering volume 5 / number 1 / 2017 declared barriers not considered in the main categories were related with building regulations, conflict of interest between different stakeholders, building management problems during occupation, and insurance and liability aspects. safety of the installation and definition of clear responsibilities in case something does not work as expected were particularly stated among these barriers. nonetheless, these issues were not perceived as relevant as other previously discussed in this document. 3.2.1 barriers to overcome in product design and development as stated before, product related barriers received the most amount of total mentions. these barriers were further categorised for detailed analysis of the information, identifying key perceived aspects to guide future developments under the following topics: performance, technical complexity, aesthetics, durability and availability. figure 8 shows the subcategories of product related barriers, based on the gathered responses. performance related barriers seem to be perceived as the most pressing to overcome, considering the amount of first and second mentions. aesthetics is also seen as a relevant barrier, followed by the technical complexity of the systems involved. lastly, the durability of the systems and product availability were also stated by the interviewees as barriers to overcome, although they were not perceived as relevant as the other barriers. in general terms, performance related barriers referred to the need to improve the efficiency of current systems and components. it was also specifically stated that the integration of storage strategies is a requirement to allow for continuous operation, improving the overall performance while supporting a better management of energy flows. fig. 8 main perceived barriers related to current products 060 journal of facade design & engineering volume 5 / number 1 / 2017 regarding aesthetics, the answers were generally somehow vague, stating that the aesthetics of current components are something to improve, without further detailing parameters to be considered. nevertheless, some specific aspects were mentioned by the interviewees, focusing on the necessity to allow for customisation in the design, promoting variety in the development of architectural products for integration. furthermore, transparency was indeed mentioned as a factor to advocate for, minimising visual constraints in façade units. the findings from task 41 support these results, especially in the case of solar thermal components. while it was found that both technologies need to be further developed to fully satisfy architectural integration requirements, farkas and horvat (2012) declared that photovoltaic applications present a higher degree of flexibility in formal characteristics, counting with more variety in terms of shape, colours, sizes, texture and possible translucency. therefore, there is a particular need for incentives for manufacturers and a clear definition of aesthetical requirements, in order to develop architecturally appealing solar thermal components for façade integration; a fact supported by the work of munari probst and roecker (2007) in the field. answers grouped under ‘technical complexity’ focused on problems associated with both overly complex assembly processes and operating modes of solar based façade components. in general terms, it was mentioned that further standardisation is advisable, without of course compromising design flexibility and customisation possibilities. it was specifically pointed out by some experts that the sum of modular components under a ‘plug-and-play’ connection logic should be the aspiration of future developments designed for façade integration, minimising assembly times and avoiding incompatibility issues between components. the main issue regarding the durability of the systems was the need for maintenance activities which would have an impact on the operating cost of the building. the answers focused on two aspects as barriers for the integration of present technologies. first, durability of the components needs to be improved, while detailed information on the performance of aging components is needed to convince stakeholders and minimise economic risks. this issue is greatly connected with information barriers discussed above, being perceived as a knowledge gap within the performance of these technologies. alternatively, it was also stated that even with enhanced durability, these components will need maintenance, so the possibility to easily maintain, repair or even replace several parts should be considered in the design along with end-of-life scenarios for the different components. performance aesthetics technical complexity durability availability increase cost effectiveness of components improve overall aesthetics avoid overly complex assembly and operation modes improve durability of components and systems develop array of products designed for integration improve/consider energy storage strategies allow for customisation of appearance promote modularity and plug and play components to ease integration devise long term maintenance strategies and end-of-life solutions allow for energy management promote variety in design of components (form/materials) standardisation of components allowing for design flexibility allow for retrofitability minimise visual constraints table 2 key aspects to overcome for solar integrated product development 061 journal of facade design & engineering volume 5 / number 1 / 2017 finally, the responses categorised under ‘availability’ considered general complaints about the limited amount of suitable products for façade integration. in this aspect, it is greatly related to the other categories, which focus on particular barriers to overcome in order to provide new architectural products. it is the authors’ opinion that this barrier is a secondary aspect, validated by the fact that it was not perceived as relevant as the rest by the interviewees. the offer of suitable products is expected to increase if product development overcomes the issues discussed under ‘performance’, ‘aesthetics’, ‘technical complexity’ and ‘durability’; provided of course that there is demand for them by tackling non-product related barriers previously discussed in this document. a summary of the main identified topics under each category is shown in table 2. the information in the table is presented in the form of recommendations to overcome the identified barriers and problems related to product development, based on the gathered responses. this is regarded not only as valuable practical information to inform future developments, but also as key aspects to consider for the evaluation of current solar technologies and components in terms of their suitability for façade integration purposes, based on how well they respond to issues related to performance, aesthetics, technical complexity and durability. 4 conclusions this paper discussed perceived barriers for façade integration of solar technologies, by presenting the findings of an open ended survey addressed to professionals with practical experience in design and development of façade systems. the survey aimed to identify the main perceived barriers, and key aspects to overcome, comparing the findings with previous experiences. furthermore, special attention was given to product related barriers, drafting recommendations to drive future product development based on the gathered responses from experts in the field. the vast majority of the interviewees (91%) believed that there is potential for façade integration, while 80% of the sample believed that there is current commercial demand for architectural products. regarding perceived barriers, economy was the most pressing barrier declared by the experts. furthermore, the cost of current systems, energy prices, and the lack of economic incentives were mentioned as key aspects to overcome within this category. product related issues were also identified as highly relevant, based on the total amount of mentions. lack of knowledge and information and process related barriers, although mentioned, were not perceived as pressing as economic and product related aspects. key issues to overcome within product related barriers centred mostly around performance, aesthetics and technical complexity of current systems and components, with durability and product availability aspects also mentioned. relevant recommendations based on the gathered responses are the need to increase the cost-effectiveness of individual components, without compromising aesthetics of the product; and the need to design for integration, avoiding overly complex assembly and operation modes. appearance customisation through a variety of components for façade design was encouraged in order to appeal to architects and clients, while modularity and the use of standardised elements and connections were advised to minimise construction and operation problems. 062 journal of facade design & engineering volume 5 / number 1 / 2017 the findings presented in this paper fall in line with previous experiences on the topic in terms of the main perceived barriers. additionally, the identification of key aspects based on the gathered responses allowed for a detailed discussion of product related barriers, in order to draft recommendations to inform future developments. further research could be done to explore potential divergences in perceived barriers among different groups, expanding the sample to consider other contexts, backgrounds and roles not just in design and construction but also operation of these systems. furthermore, the recommendations obtained from this study could be used to evaluate solar technologies in terms of their current suitability for façade integration. this would not only present the possibility to validate the findings against real experiences and commercially available products, but would also generate valuable feedback in the understanding of current limits for the development of solar integrated façade products. acknowledgements this paper is part of the ongoing phd research project titled coolfacade: architectural integration of solar cooling strategies into the curtain-wall, developed within the façade research group (frg) of the department of architectural engineering + technology, delft university of technology (tu delft). the research project is being funded through a scholarship granted by conicyt, the national commission for scientific and technological research of chile. the authors would like to thank all interviewees and especially the members of cost action tu1403 “adaptive façades network” for their support throughout the study. references bp. (2016). bp energy outlook, 2016 edition. london, united kingdom. cappel, c., streicher, w., lichtblau, f., & maurer, c. (2014). barriers to the market penetration of façade-integrated solar thermal systems. energy procedia, 48, 1336-1344. doi: 10.1016/j.egypro.2014.02.151 doe. (2016). u.s. department of energy. retrieved sept 13th, 2016, from http://energy.gov/ doe/eia. (2016). international energy outlook 2016. washington, dc, usa: us energy information administration, us department of energy. directive 2002/91/ec: eur-lex (2002). farkas, k., & horvat, m. (2012). t.41.a.1: building integration of solar thermal and photovoltaics barriers, needs and strategies: iea shc task 41: solar energy and architecture. frontini, f. (2011). daylight and solar control in building: a new angle selective see-thorough pv-façade for solar control. paper presented at the plea 2011 architecture and sustainable development, conference proceedings of the 27th international conference on passive and low energy architecture. fung, t. y. y., & yang, h. (2008). study on thermal performance of semi-transparent building-integrated photovoltaic glazings. energy and buildings, 40(3), 341-350. doi: http://dx.doi.org/10.1016/j.enbuild.2007.03.002 iea-shc. (2016). iea solar heating & cooling programme. retrieved sept 13th, 2016, from https://www.iea-shc.org/ joly, m., antonetti, y., python, m., gonzalez, m., gascou, t., scartezzini, j.-l., & schüler, a. (2013). novel black selective coating for tubular solar absorbers based on a sol–gel method. solar energy, 94(0), 233-239. doi: http://dx.doi.org/10.1016/j. solener.2013.05.009 li, d. h. w., lam, t. n. t., chan, w. w. h., & mak, a. h. l. (2009). energy and cost analysis of semi-transparent photovoltaic in office buildings. applied energy, 86(5), 722-729. mandalaki, m., zervas, k., tsoutsos, t., & vazakas, a. (2012). assessment of fixed shading devices with integrated pv for efficient energy use. solar energy, 86(9), 2561-2575. doi: http://dx.doi.org/10.1016/j.solener.2012.05.026 munari probst, m. c., & roecker, c. (2007). towards an improved architectural quality of building integrated solar thermal systems (bist). solar energy, 81(9), 1104-1116. doi: 10.1016/j.solener.2007.02.009 oecd/iea. (2015). energy and climate change / world energy outlook special report. paris, france: iea international energy agency. orel, b., spreizer, h., šurca vuk, a., fir, m., merlini, d., vodlan, m., & köhl, m. (2007). selective paint coatings for coloured solar absorbers: polyurethane thickness insensitive spectrally selective (tiss) paints (part ii). solar energy materials and solar cells, 91(2–3), 108-119. doi: http://dx.doi.org/10.1016/j.solmat.2006.07.012 prieto, a., klein, t., & knaack, u. (2016). facade integration: survey-based assessment of the main problems for the integration of building services in facade systems. paper presented at the id@50 integrated design conference, bath, united kingdom. schüler, a., boudaden, j., oelhafen, p., de chambrier, e., roecker, c., & scartezzini, j. l. (2005). thin film multilayer design types for colored glazed thermal solar collectors. solar energy materials and solar cells, 89(2–3), 219-231. doi: http://dx.doi. org/10.1016/j.solmat.2004.11.015 yang, r. j. (2015). overcoming technical barriers and risks in the application of building integrated photovoltaics (bipv): hardware and software strategies. automation in construction, 51(0), 92-102. doi: http://dx.doi.org/10.1016/j.autcon.2014.12.005 journal of facade design and engineering 4 (2016) 131–142 doi 10.3233/fde-171645 ios press 131 biomimetic inspired, natural ventilated façade – a conceptual study michael johann paara,∗ and alexander petutschniggb ainstitute of space and design, industrial design, university of arts and industrial design linz, austria binstitute of wood technology and wood construction, engineering, university of applied sciences salzburg, austria abstract. in this paper, the authors elaborate an adaptable curtain-wall façade design concept that combines façade greening with biomimetic approaches. it begins with an overview of façade greening. next, the paper takes a more in-depth look at prairie dog burrows and the modular growth of barnacle colonies as sources of biomimetic inspiration, including how they may be applied in technology and brought together with façade greening. the concept is expected to act contrary to the urban heat island effect by naturally cooling external building walls and thus reducing the energy needs and greenhouse gas emissions associated with artificial cooling of buildings. this is considered important, as our cities are heating up more and more, resulting in increased energy demand. the promising information gained from lab experiments on scaled façade models, based on continuous wind-generated air circulation between the building’s external wall and the façade, further initial calculations regarding natural convection, and the additional cooling effect of façade greening, can be considered a sound basis for future developments and discussions. keywords: adaptable curtain-wall façade, urban heat island effect, façade greening, prairie dog burrows, modular growth structure of barnacle cultures 1. introduction an urban heat island is an urban area that is significantly warmer than its surrounding rural areas due to human activities. this climate phenomenon was first investigated and described by luke howard in the beginning of the 19th century (howard, 1818). tarmac, concrete and stone shape our cityscape (minke, 2010). this supports the potential for the urban heat island effect, which has been documented over a number of decades, e.g. by the world meteorological organization. one result of the urban heat island effect is increased average temperatures in our cities (santamouris, 2011). occasionally this is due to thermal energy stored in buildings. the urban heat island effect is present in all cities and “is the most obvious climatic manifestation of urbanization” (landsberg, 1981). thus, the energy required for cooling buildings with artificial methods – highly dependent on non-renewable energy sources – is greater in cities, especially during the summer months. studies also show that in the future we can expect a marked increase in greenhouse gas emissions from the use of artificial cooling methods (riviere et al., 2008). over the next 20 years, the figure is set to double for residential buildings, while for non-residential buildings an increase of 25% is expected. ∗corresponding author: michael johann paar, phd student, university of arts and industrial design, hauptplatz 8, 4010 linz, austria. tel.: +43 650 595 65 70; e-mail: michael.paar@ufg.at. issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). mailto:michael.paar@ufg.at 132 m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade “cities are part of the climate change problem, but they are also a key part of the solution” (kamal-chaoui & robert, 2009). they have the ability to act on climate change owing to their responsibilities towards urban structures such as buildings and façades. façades in particular have become an important factor in the regulation and control of the energy demand. the role of the façade is changing from being “a passive protective covering to an active regulator of a building’s energy balance” (gosztonyi et al., 2013). bearing that in mind, the authors ask whether a new elaborate façade design concept combining façade greening with biomimetic approaches might allow the heat island effect in our cities to decrease by naturally cooling external building walls, and also reducing the energy needs and greenhouse gas emissions associated with cooling buildings artificially. combining façade greening with biomimetic approaches represents a sustainable and promising solution to address the issues mentioned above and should therefore be examined in further detail. this paper is divided into three main sections. first, an outline of the topics of façade greening and biomimetic approaches will be given. it will explore the ways in which the two topics might be brought together for use in the construction and architecture sectors. the second part of the paper explains how to convert these approaches into the development of an adaptable curtain-wall façade design concept, related to the segmentation of the building surfaces. finally, the results of laboratory experiments using scaled models of the façade design and initial calculations are shown, and opportunities for future development will be discussed. 2. connecting nature and architecture 2.1. façade greening the use of plants in architecture is well established. in his time, the roman statesman pliny the younger (c. ad 61 – c. ad 115) already described “decorated houses”. there has since been continuous evidence of the use of plants in building design. in the middle ages, vines and roses extended across walls, while in the renaissance and baroque periods, prestigious buildings were decorated with greenery. climbing plants were used to accentuate the architecture of classical villas and country houses. the numerous examples of turf houses such as those found in north america and scandinavia are particularly important in the development of greening in contemporary architecture (minke, 2010). at the beginning of the 20th century, frank lloyd wright, le corbusier and walter gropius in particular, implemented the concept of green buildings in their work. current projects featuring the use of greening in design planning include those by patrick blanc, vincent callebaut and ken yeang. façade greening can be subdivided into two main groups: ground-based systems and those connected to façades (köhler, 2012). the second group is crucial for further observations in this study. this system generally forms the external wall of a building and supersedes other materials such as glass, metal plates or plaster. it does not require a direct connection with the ground and has already been field-tested using various design approaches (fig. 1). there are many reasons for employing green façades on building surfaces. one key factor is that green façades generally retain less heat and therefore present a natural way to cool external building walls (wong et al., 2010). “their utilization is essential and can considerably improve the microclimate of the built environment” (eumorfopoulou & kontoleon, 2009). this is based on socalled phytophysiological processes. important process factors hereby that enhance this cooling effect are evaporation, reflexion, respiration and transpiration of plants and damp plant substrates, which further support the cooling effect (bass, martens & alcazar, 2008; krusche, althaus & gabriel, m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade 133 fig. 1. examples of existing façade greening systems. fig. 2. schematic diagram of energy processes on green façades. illustration: michael j. paar according to per krusche. 1982). of 100% of the incident solar energy on a green façade, approx. 4% is bonded through photosynthesis, 18% reflected, 30% converted to heat, 30% used for evapotranspiration and only about 18% is transmitted through the foliage (fig. 2) (minke & witter, 1982). regarding the context between the reflexion of solar rays and the type of building surface, the albedo effect is also worth mentioning here. 2.2. biomimetic approaches “technology learns from nature” (translated by the authors) (nachtigall & pohl, 2013). nature can act as a source of inspiration for technological thinking. a domain also known as biomimicry or biomimetics. biomimetics is defined as the “abstraction of good design from nature” (vincent et al., 2006) or “mimicking the functional basis of biological forms, processes and systems to produce sustainable solutions” (pawlyn, 2011; rawlings, bramble & staniland, 2012). systems found in 134 m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade nature offer a huge portfolio of strategies and mechanisms that can be implemented in biomimetic designs. therefore, biomimetics has also become increasingly important in the construction and architecture sectors, particularly in recent years. thus, the following section identifies biomimetic sources of inspiration that may be combined with the promising properties of façade greening to allow for the natural cooling of a building’s external walls, thereby reducing the heat island effect in our cities. 2.3. basic design the basic concept behind the façade design is an adaptable curtain-wall system (related to the segmentation of the building surfaces), ensuring continuous air circulation in the slot between the façade and the outer building wall via ventilation modules fitted above and below. it can also be combined with surface greening (fig. 3). specific research in energy and resource saving sources inspired by nature leads to animal constructions, e.g. anthills, prairie dog burrows and termite mounds. all of them have a ventilation system based on a natural induced airflow (nachtigall & pohl, 2013; vogel, ellington jr. & kilgore jr. 1973; korb, 2003). after a detailed comparison and evaluation for an implementation on a façade in accordance with the parameters of the basic design defined above, the next section will provide a more in-depth analysis of prairie dog burrows. 2.3.1. prairie dog constructions the construction of prairie dog burrows is based on saving energy and maximising resources; they have an easy as well as sophisticated system of wind-generated, continuous air circulation. fig. 3. basic concept of the façade design. m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade 135 fig. 4. the principle of air circulation within a prairie dog burrow (applies to cynomys ludovicianus). illustration: michael j. paar according to steven vogel. prairie dogs (cynomys) create a purpose-built system of passages in their burrows, facilitating cooling of the air supply, especially in summer (nachtigall, 2002), taking advantage of the bernoulli principle. by the 18th century, daniel bernoulli had already begun to describe the relationship between the flow speed of a fluid (gas or liquid) and its pressure. he discovered that the increase in the speed of a flowing fluid is accompanied by a fall in pressure. to achieve continuous air circulation, prairie dogs build the entrance and exit to their burrows at different heights (fig. 4). it takes only a light breeze for an area of high pressure to form before the lower opening, thus pulling air through the opening into the burrow. at the higher end of the canal system, on the other hand, there is a low-pressure area that sucks used air out of the burrow. the biologist steven vogel and his team were able to demonstrate through measurements that a wind speed of only 0.4m/s can ventilate the entire burrow within 10 minutes. when the wind speed is 1.2m/s, this takes only 5 minutes (vogel, ellington jr. & kilgore jr. 1973). bernoulli equation for calculating the pressure of perfect incompressible media, equation (1): p + ρ.g.z +(ρ/2).v2 = const. (1) where v stands for the flow speed of the media. p and ρ represents the pressure and density of the media. g is the acceleration due to gravity. z is the elevation above/below a reference plane at the same geodetical elevation. to determine the structure of the ventilation modules mentioned above (see fig. 3), further detailed researches should be undertaken. 136 m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade fig. 5. the modular growth structure of barnacle cultures (applies to semibalanus balanoides). 2.3.2. barnacles following the example of the irregular 3d structure, particularly the different heights of prairie dog burrows’ entrances and exits, such self-contained and closely packed, irregular 3d structures are sought out and analysed regarding the use in a façade design. the flow optimized (thomason et al., 1998), modular growth structure of barnacle cultures is of great interest in that regard. barnacles (balanidae) belong to the crustacean group and are more specifically classed as cirripedia. these are marine, sessile and immobile creatures that live in water. charles darwin had already begun to work extensively with barnacles by the middle of the 19th century. only greyish-white truncated cone shapes are visible externally. their limbs have been adapted into cirri (anderson, 1994). larvae hatch from barnacle eggs and drift in seawater. as they advance in life, they cement themselves onto hard substrates packed close to one another (bertness et al., 1992). the subsequent metamorphosis into an adult barnacle involves forming four to eight calcareous plates, resulting in a polygonal base which surrounds their soft body in the form of a truncated cone (fig. 5). a detailed analysis of the self-contained, closely packed, flow optimized, modular growth structure of barnacle cultures (semibalanus balanoides) allows us to observe that their surface segmentation is very similar to that of the principle behind a voronoi diagram. the degree of overlap between the natural growth of barnacle colonies and partitioning the same plane according to the principles of the voronoi diagram (for the same centres) is approximately 80% (fig. 6). voronoi diagrams are employed in a wide variety of scientific fields. the history of research using voronoi diagrams began in the middle of the 19th century with the ukrainian mathematician georgi f. voronoi. the focus of such research has since been: modelling natural events, mathematical investigations of their geometrical and stochastic properties, and using computers to construct voronoi diagrams (aurenhammer, 1991; zimmer, 2014). regarding irregular 3d structures, it has been observed that recurring geometrical proportionalities exist (fieldwork: measurements of 50 barnacles – semibalanus balanoides, gulf of juan, france, summer 2016) in terms of maximum barnacle width and height and the width of the top opening (fig. 7). m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade 137 fig. 6. similarities between the growth of barnacle cultures and the voronoi diagram. fig. 7. geometric proportionalities of barnacle shell. proportionalities, equations (2) and (3): wmax : h = 1 : 1/3 (2) wmax : w = 1:1/3 (3) where wmax stands for the maximum width of barnacle. h is the barnacle height. w represents the width of the top opening. 3. a gradual construction schema building on the above findings, the following section presents a gradual schema to construct the adaptable curtain-wall façade design concept aimed at minimising the heat island effect in our cities (fig. 8). 138 m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade fig. 8. a gradually construction schema using the example of a square façade surface. 1) determine the border of the façade surface to which the system is to be attached. 2) use a random-number generator to determine an arbitrary number of central points on the chosen surface. use a voronoi diagram as a basis for structuring the surface in a manner similar to the growth of barnacle colonies (see fig. 6), as far as the border previously chosen. additionally, the structuring according to the voronoi diagram supports – from a designer’s point of view – the appearance of the concept. 3) three-dimensional extrusion of the self-contained, closely packed structure, taking account of the geometrical proportionalities observed in the case of barnacles (see fig. 7), thus creating the required ventilation modules. accordingly, the ventilation modules take the form of irregular, flow optimized hollow truncated cones. 4) remove the middle section of this three-dimensional structure and insert an extensive greening system that will retain less heat and increase cooling, as discussed above. a reference for the removal are standard values for vent openings in curtain-wall façades, e.g. norm din 18516 – cladding for external walls, ventilated at rear. thus; a minimum of 200cm2 vent openings per running façade meter, fitted above and below must be guaranteed. 5) install the system thus created as a curtain wall on a solid load-bearing structure using knowledge gathered from analysing the construction of prairie dog burrows, used in a vertical arrangement. this should facilitate a wind-generated, continuous air circulation in the ventilation slot between the building’s outer wall and the façade and cool down the wall surface. a thickness of the ventilation slot of at least 2cm must be guaranteed, also see din 18516. m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade 139 fig. 9. set-up for the experiment with scale model 1:10. 4. laboratory experiments and initial calculations 4.1. laboratory experiments scaled models, constructed according to the previous explained schema, of one running meter of the façade design concept built out of cardboard (scale 1:10), resp. sheet metal (scale 1:2) are used to show wind-generated, continuous air circulation within the ventilation slot between the building’s external wall and the façade. a real size ventilation slot of 40mm and 200cm2 vent openings per running façade meter, fitted above and below are defined. a hairdryer, resp. an axial blower are used on the upper ventilation modules to simulate air masses supplying airflow parallel to the ventilation modules. an incense stick and a thread of wool are positioned in front of the lower ventilation modules to render visible the air circulating within the ventilation slot. continuous circulation of air rising up from below can be observed immediately after the hairdryer, resp. axial blower is switched on (figs. 9 and 10). in the case of a real-life application, it is worth noting here that wind speed, which increases with height (hupfer & kuttler, 2005), and the stack effect (mclean & silver, 2008) can have an additional positive effect on the air circulation occurring in the façade slot. 4.2. initial calculations an initial, simplified calculation model is used to illustrate the wind-generated, continuous air circulation within the ventilation slot and the consequent effect of natural convection (fig. 11). the physical dimensions used for the calculation refer to the above shown scaled models (see figs. 9 and 10). temperatures are based on summer conditions in central european cities. wind-generated laminar flow speed [m/s] above the upper ventilation modules, equation (4): v0.a0 = v1.a1 → v0.a0 = v1.(a0/2) → 140 m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade fig. 10. details of set-up for the experiment with scale model 1:2. fig. 11. settings for the calculation model. v1 = 2.v0 (4) pressure difference [pa] above the upper ventilation modules according to bernoulli (ρ . g . z=0, negligibly), equation (5): p0 + (ρ/2).v20 = p1 + (ρ/2).v21 → �p = (p0 − p1) = (ρ/2).(v21 − v20) = (ρ/2).((2.v0)2 − v20) (5) m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade 141 laminar flow speed [m/s] within the ventilation slot according to hagen-poiseuille, equation (6): �p = (12.η.l.v)/(d2) → v = (�p.d2)/(12.η.l) (6) heat transfer (natural convection) [w] between the airflow and the building’s external wall, equation (7): w = a.a.�t (7) α depends on v and l, according to norm vdi 2055 – thermal insulation of heated and refrigerated operational installations in the industry and in the building services. v0 and v1 stands for the flow speed of the air. a0 and a1 resp. p0 and p1 are the cross-sections of the air columns and the corresponding air pressures. ρ is the density of air. v, l and d represent the laminar flow speed within the ventilation slot and the length and thickness of the ventilation slot. η, � and w are the viscosity of air, the heat transfer coefficient and the heat transfer between airflow and wall. �t and a stand for the temperature differential and the regarded wall surface. a sample calculation for one running meter of the adaptable façade design concept with the below parameters gives a heat transfer between the airflow and the building’s external wall of 375w. parameters: v0 =1.4m/s (according to norm din en iso 6946 – building components and building elements. thermal resistance and thermal transmittance. calculation method), ρ = 1.2kg/m3 (at 20◦c), η = 0.0000171 n.s/m2 (at 20◦c), l = 10m, d = 0.04m → v = 2.7m/s. α = 15w/m2.k (standard value according vdi 2055), �t = 2.5◦c and a = 10m2. the flow speed v=2.7m/s within the ventilation slot is faster than in conventional ventilated façades; here speed values between 0.15m/s and 1.0m/s are common (häupl, 2008). thus, the surface temperature of the outer wall of the building may can reduced more efficiently. the effect of cooling mainly depends on the environmental conditions – wind velocity and the temperature differential between lower and upper ventilation modules. 5. conclusion the findings of the laboratory experiments with the scaled models of one running meter of the adaptable façade design concept and the initial, simplified calculations let us assume that its use can enhance the speed of the continuous airflow within the ventilation slot. that would imply that a higher amount of air could flow through the ventilation slot and the surface temperature of the outer building wall could be reduced more efficiently. combined with the additional natural cooling effect of façade greening, it may represent a natural approach towards minimising the heat island effect in our cities, as hypothesised in the introduction to this paper. to verify the actual effect of the concept on the heat island effect and reductions in energy requirements and greenhouse gas emissions associated with cooling buildings artificially, further measurements on a 1:1 façade model with integrated greening and a detailed computational fluid dynamics simulation are absolutely relevant. owing to the higher flow speed, noise measurements are required as well. adding further methods to intensifying the cooling of the building’s outer wall could be interesting. those might include special circulation channels within the façade’s structure, the use of “wind catchers” in the area of the upper ventilation modules, or introducing cooler air masses from lower strata. to act on special environmental conditions, temporary adjustable ventilation, e.g. barnacle-like flaps integrated within the ventilation modules, could be considered as well. 142 m.j. paar and a. petutschnigg / biomimetic inspired, natural ventilated façade references anderson, d. t. (1994). barnacles: structure, function, development and evolution. london: chapman & hall. aurenhammer, f. (1991). voronoi diagrams – a survey of a fundamental geometric data structure. acm computing surveys journal, 23(3), 345-405. doi: 10.1145/116873.116880 bass, b., martens, r., & alcazar, s. s. (2008). roof–envelope ratio impact on green roof energy performance. urban ecosystems journal, 11(4), 399-408. doi: 10.1007/s11252-008-0053-z bertness, m. d., et al. (1992). components of recruitment in populations of the acorn barnacle semibalanus balanoides (linnaeus). journal of experimental marine biology and ecology, 156(2), 199-215. doi: 10.1016/0022-0981(92)90246-7 eumorfopoulou, e. a., & kontoleon, k. j. (2009). experimental approach to the contribution of plant-covered walls to the thermal behaviour of building envelopes. building and environment, 44(5), 1024-1038. doi: http://dx.doi.org/10.1016/j.buildenv.2008.07.004 gosztonyi, s., et al. (2013). bioskin – research potentials for biologically inspired energy efficient facade components and systems. vienna: bmvit. häupl, p. (2008). bauphysik – klima. wärme. feuchte. schall: grundlagen, anwendungen, beispiele. berlin: ernst & sohn. howard, l. (1818). the climate of london, deduced from meteorological observations, made at different places in the neighbourhood of the metropolis. london: 2. vol. hupfer, p., & kuttler, w. (eds.) (2005). witterung und klima. eine einführung in die meteorologie und klimatologie (11th ed.). wiesbaden: b. g. teubner verlag. kamal-chaoui, l., & robert, a. (2009). competitive cities and climate change. oecd regional development working paper. paris: oecd. köhler, m. (ed.) (2012). handbuch bauwerksbegrünung. planung – konstruktion – ausführung. köln: verlagsgesellschaft rudolf müller. korb, j. (2003). thermoregulation and ventilation of termite mounds. the science of nature journal, 90(5), 212-219. doi: 10.1007/s00114002-0401-4 krusche, p., althaus, d., & gabriel, i. (1982). ökologisches bauen. gütersloh: bauverlag. landsberg, h. e. (1981). the urban climate. new york: academic press. mclean, w., & silver, p. (2008). introduction to architectural technology. london: laurence king publishers. minke, g. (2010). dächer begrünen – einfach und wirkungsvoll. planung, ausführungshinweise und praxistipps (4th ed.). staufen bei freiburg: ökobuch verlag. minke, g., & witter, g. (1982). häuser mit grünem pelz – ein handbuch zur hausbegrünung. frankfurt: verlag d. fricke. nachtigall, w. (2002). bionik. grundlagen und beispiele für ingenieure und naturwissenschaftler (2nd ed.). berlin: springer-verlag. nachtigall, w., & pohl, g. (2013). bau-bionik. natur – analogien – technik (2nd ed.). berlin: springer-verlag. pawlyn, m. (2011). biomimicry in architecture. london: riba publishing. rawlings, a. e., bramble, j. p., & staniland, s. s. (2012). innovation through imitation: biomimetic, bioinspired and biokleptic research. soft matter journal, 8(25), 6675-6679. doi: 10.1039/c2sm25385b riviere, p., et al. (2008). preparatory study on the environmental performance of residential room conditioning appliances (airco and ventilation). draft report for the european commission. santamouris, m. (ed.) (2011). energy and climate in the urban built environment. london: routledge. thomason, j. c., et al. (1998). hydrodynamic consequences of barnacle colonization. recruitment, colonization and physical-chemical forcing in marine biological systems, 132, 191-201. doi: 10.1007/978-94-017-2864-5 16 vincent, j. f. v., et al. (2006). biomimetics: its practice and theory. journal of the royal society interface, 3(9), 471-482. doi: 10.1098/rsif.2006.0127 vogel, s., ellington c. jr., & kilgore d. jr. (1973). wind-induced ventilation of the burrow of the prairie-dog, cynomys ludovicianus. journal of comparative physiology, 85(1), 1-14. doi: 10.1007/bf00694136 wong, n. h., et al. (2010). thermal evaluation of vertical greenery systems for building walls. journal of building and environment, 45(3), 663-672. doi: http://dx.doi.org/10.1016/j.buildenv.2009.08.005 zimmer, h. (2014). optimization of 3d models for fabrication. aachen: shaker verlag. http://dx.doi.org/10.1016/j.buildenv.2008.07.004 http://dx.doi.org/10.1016/j.buildenv.2009.08.005 jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 107 journal of facade design & engineering volume 5 / number 1 / 2017 convective concrete – additive manufacturing to facilitate activation of thermal mass dennis de witte1, marie l. de klijn-chevalerias2, roel c.g.m. loonen3, jan l.m. hensen4, ulrich knaack5, gregor zimmermann6 1 design of construction, delft university of technology, p.o. box 5043, 2600 ga delft, the netherlands, +31 (0)15 27 84094, d.dewitte@tudelft.nl – institute of structural mechanics and design, technical university darmstadt, franziska-braun-straße 3, 64287 darmstadt, germany, +49 6151 16 23035 dewitte@ismd.tu-darmstadt.de 2 unit building physics and services, eindhoven university of technology, p.o. box 513, 5600 mb eindhoven, the netherlands, +31 (0)40 247 2577, m.l.d.klijn@tue.nl 3 unit building physics and services, eindhoven university of technology, p.o. box 513, 5600 mb eindhoven, the netherlands, +31 (0)40 247 2571, r.c.g.m.loonen@tue.nl 4 unit building physics and services, eindhoven university of technology, p.o. box 513, 5600 mb eindhoven, the netherlands, +31 (0)40 247 2988, j.hensen@tue.nl 5 design of construction, delft university of technology, p.o. box 5043, 2600 ga delft, the netherlands, +31 (0)15 27 84094, u.knaack@tudelft.nl institute of structural mechanics and design, technical university darmstadt, franziska-braun-straße 3, 64287 darmstadt, germany, +49 6151 16 23013 knaack@ismd.tu-darmstadt.de 6 g.tecz engineering gmbh, angersbachstraße 12b 34127 kassel, germany, +49 561 86 17 555, contact@gtecz.com abstract this paper reports on the research-driven design process of an innovative thermal mass concept: convective concrete. the goal is to improve building energy efficiency and comfort levels by addressing some of the shortcomings of conventional building slabs with high thermal storage capacity. such heavyweight constructions tend to have a slow response time and do not make effective use of the available thermal mass. convective concrete explores new ways of making more intelligent use of thermal mass in buildings. to accomplish this on-demand charging of thermal mass, a network of ducts and fans is embedded in the concrete wall element. this is done by developing customized formwork elements in combination with advanced concrete mixtures. to achieve an efficient airflow rate, the embedded lost formwork and the concrete itself function like a lung. the convection takes place with separate pipes on both sides of the concrete’s core to increase the charge/discharge of the thermal storage process. the first stage of the research, described in this paper, is to simulate the convective concrete at the component level, whereupon a mock-up is tested in a climate test set-up. the paper concludes with describing planned activities for turning this concept into a real building product. keywords concrete, thermal mass activation, computational design support, additive manufacturing, advanced formwork, concrete, optimization, heat exchange, heat storage doi 10.7480/jfde.2017.1.1430 108 journal of facade design & engineering volume 5 / number 1 / 2017 1 introduction the use of thermal mass is usually considered as an effective strategy for achieving energy efficient building designs with high thermal comfort levels (holmes and hacker, 2008). this is normally done by applying construction types with high thermal storage capacity (e.g. concrete) on the inside of the thermal insulation layer (knaack et al., 2007). such heavyweight constructions have a slow response time. this thermal inertia helps to flatten temperature peaks, but the slow response is not advantageous at all times (hoes et al., 2011; loonen et al., 2013). due to a lack of control possibilities regarding when and how much energy to exchange between interior zones and the constructions with thermal mass, these dynamic effects may actually also increase heating and cooling energy demand during intermittent operation or can cause unwanted discomfort, either due to too cold surface temperatures when the building is already occupied on winter mornings, or because the accumulated heat can sometimes not be sufficiently released, leading to potential indoor overheating issues in summer (hoes and hensen, 2016). another shortcoming of thick concrete slabs is that actually only a small part of the heavyweight material (usually the first few centimetres) effectively plays a role in storing thermal energy. this forms a missed opportunity. the goal of the research project that is reported in this paper is to explore new ways of making more intelligent use of thermal mass in buildings. this is accomplished through the research-driven design process of an innovative building envelope concept: convective concrete. this paper first introduces the underlying principles of convective concrete. in the second phase, the simulationbased and experimental activities that are used to inform the design specifications for convective concrete are addressed. the paper concludes with presenting future most promising application areas and future perspectives for high-performance buildings with convective concrete. 2 the convective concrete concept the idea behind convective concrete is inspired by the concept of hollow-core ventilated slabs, of which its potential has been investigated by zmeureanu & fazio (1988), and a more recent energyactive façade that uses capillary tubes, described by maier, gilka-bötzow & schneider (2015). the hollow-core ventilated slabs are concrete floor elements that play an active role in the thermal comfort of buildings, by being part of the ventilation system in order to precondition the ventilation air. most of these systems have been applied in offices. analyses show that an increased airflow rate through the hollow core slab floors during the night can decrease the inner cooling load by 28.4 – 44.2 w/m2 in comparison to a standard mechanical climate system (zmeureanu & fazio, 1988). warwick et al., (2007) monitored an office building in sheffield, uk with hollow-core ventilated slabs and found that an airflow rate of 0.3 m/s (6.2 air changes per hour) within the slab is sufficient to completely eliminate the occurrence of summer discomfort. simulations show that ventilated hollow core slab floor elements can provide a temperature below 28 degrees centigrade for 90% of the occupied time in an office situated in italy (corgnati & kindinis, 2007). the energy-active façade is an ultra-high-performance concrete slab that is combined with concrete foam for insulation and capillary piping on both sides, to allow energy to be exchanged from inside the building to the outside and vice versa (maier et al., 2015). another design iteration of the hollow-core slab principle has been proposed in the form of ventilated internal double walls (vidw) (fraisse et al., 2006). simulation studies show that the application of such a wall element can be very effective for improving thermal comfort conditions in warm summer periods (fraisse et al., 2010). despite the promising potential of vidw, practical manifestations (e.g. prototypes or mock-ups) cannot be found in scientific literature. 109 journal of facade design & engineering volume 5 / number 1 / 2017 convective concrete initially targets the residential building market. the goal is to mitigate residential overheating during summer periods by reducing the temperature of constructions through active heat exchange between the building construction (hollow-core concrete slabs) and cool outside air at night. even though air has a relatively low volumetric heat storage capacity compared to e.g. water, it is used as a transport medium in this project, because of: – its widespread availability at favorable temperatures; – can be combined with earth tubes; – easy construction an installation process: plug-and-play; – provides standalone elements that do not need to be connected to additional systems; – can function passively without mechanically forced convection due to the buoyancy effect; – no risk for leakages, punctures or frost damage; – low weight and therefore less structural requirements. to accomplish the on-demand charging of thermal mass, a network of ducts with attached fans, as back-up to the buoyancy effect to ensure a sufficient amount of air flowing through the convective concrete, needs to be embedded in the concrete wall element. this is done by developing customized formwork elements in combination with advanced concrete mixtures. additive manufacturing (am) is used, because it is a good method for this kind of rapid prototyping. customized and free-form parts can be produced easily. am of lost formwork differs from the approach of direct concrete printing, but allows for a traditional processing of the concrete itself. to benefit most from am as production technology, the free-form and customized parts needed for the convective concrete are printed in wax, using fused deposition modeling (fdm), an am process based on material extrusion, that can be melted after the concrete is hardened. the building volume and resolution of fdm printers can be adapted to the desired size and layer thickness easily. to achieve an efficient convective flow, the embedded lost formwork and the concrete itself should function like a lung. the convection takes place with separate pipes on both sides of the concrete’s core to increase the charge/discharge of the thermal storage process with help of fans, in the event of lack of buoyancy effect and with the help of valves, to control when the slabs are ventilated. there will not be any openings through the slabs themselves, because that would cause thermal bridges. the concrete mixture with matching characteristics (density, porosity and lambda value) will be fabricated on the basis of input from computational simulations. 110 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 1 convective concrete concept. by integrating these additively manufactured smart voids and piping for convection within the customized concrete elements, it is possible to compensate for the emissions from concrete production during the usage phase. carbonation is a process which binds co 2 (cao and co 2 react to form cao 3 ) to the concrete during the usage phase. with convective concrete, the rate of carbonation can be increased since the surface area doubles in the models used for the first simulations and since the amount of carbonation is directly related to the surface area. the speed of carbonation relates to the way the concrete is exposed and the concrete’s strength class. from those parameters a k – value can be obtained (lagerblad, 2005). althoug the k – value differs with every concrete mixture and only a thin layer of concrete is able to bind the co 2 over a certain time interval, the surface area that is able to bind the co 2 will increase significantly, due to the internal piping in convective concrete. 2.1 mock-up the first stage of the research is to simulate the convective concrete at the component level, whereupon a mock-up is tested in a climate test set-up. if the simulations and the obtained results of the physical mock-ups match with the simpler geometries, the geometry of the tested mock-ups can develop over time. form and routing shall become more complex to increase the efficiency of the heat exchange and to increase the amount of surface that is able to bind co 2 .the research is an iterative process to measure the different geometries of the embedded formwork used. the first layout used is an internal tubing system in which the channels on both sides of the concrete are not connected. there is no cross-ventilation and the element functions as a heat exchanger. the cool outside air ventilates the convective concrete during the night which allows the concrete to absorb unwanted internal heat gain during the day. the geometry is on basis of the computational design described in paragraph 3. in fig. 1 this layout in combination with an abstract principle of convective concrete is shown. 111 journal of facade design & engineering volume 5 / number 1 / 2017 additive manufacturing is in the project used to print the internal voids. for the internal repeating formwork of the first layout, which has a simple geometry, a positive form has been printed to make a mold, which can be used to cast the wax elements. future, more complex, geometries will be printed directly in wax. combining both, direct and indirect production methods by use of am facilitates the desired mass customization. by using am in this beneficial way, to improve performance based on location and its demand, the heat storage and exchange can be optimized. this flexibility in form without additional costs is where the main advantage is, compared to more conventional types of ventilated slabs. 3 computational design support during the design process of an innovative façade element such as convective concrete, many parameters can be considered but not all can be extensively tested via the making of a mock-up. however, computational simulations can help us to make decisions on some variables in order to select the most promising properties to further analyze with the mock-up (loonen et al., 2014). in this case, a two-dimensional dynamic heat transfer model is used via the simulation program energy2d to make decisions on the shape and layout of the air channels in the convective concrete as well as the properties of the concrete mixture itself. the goal is to model the system in a whole building energy simulation program, energyplus, to evaluate the performance of the system in relation to weather and occupant influences, and to determine its most efficient operation mode. 3.1 analysis of dynamic 2d heat transfer the software program energy2d is used to solve the dynamic fourier heat transfer equations for the convective concrete case. energy2d is a relatively new program (xie, 2012) and is not yet widely used as a building performance simulation tool. to gain more confidence in the predictions with energy2d, an analytical validation study was therefore carried out first, inspired by the approach described in hensen and nakhi (1994). those analytical solutions and the simulation results of the dynamic response to a 20°c temperature step change on the surface of a concrete construction with the following properties were compared for this research: properties and symbol value and unit thickness t 0.1 m conductivity λ 0.045 w/(m ×k) density ρ 50 kg/m3 specific heat c 840 j/(kg ×k) convective heat transfer coefficient h c 3 w/(m2×k) table 1 construction properties 112 journal of facade design & engineering volume 5 / number 1 / 2017 the exact solution of the surface temperature of a plane wall with surface temperature was determined as follows according to incropera et al.: where: and the discrete values of ζn (1-4) are positive roots of the following equation: the appropriate values of ζ n for this problem become: with biot number, thermal diffusivity: and the fourier number: as can be seen in fig. 2, the simulation results never divert from the exact solution more than 0.45°c and it is therefore considered acceptable to further use this model. 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 𝑇𝑇𝑇𝑇" = 𝑇𝑇𝑇𝑇% + 𝑇𝑇𝑇𝑇' − 𝑇𝑇𝑇𝑇% (𝐶𝐶𝐶𝐶+exp −𝜁𝜁𝜁𝜁+0𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 cos 𝜁𝜁𝜁𝜁+𝑥𝑥𝑥𝑥∗%+89 ) [eq.] ( 1 ) 𝐶𝐶𝐶𝐶+ = ;"'+<= 0<=>?@a (0<=) [eq.] ( 2 ) 𝜁𝜁𝜁𝜁+𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝜁𝜁𝜁𝜁+ = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 = 3,33. 𝜁𝜁𝜁𝜁9 = 1,217; 𝜁𝜁𝜁𝜁0 = 3,850; 𝜁𝜁𝜁𝜁p =6,739; 𝜁𝜁𝜁𝜁; = 9,751. q∙st u = 3,33 [eq.] ( 5 ) 𝛼𝛼𝛼𝛼 = uwx = 1,07 ∙ 10 yz 𝑚𝑚𝑚𝑚0 𝑠𝑠𝑠𝑠𝑠 [eq.] ( 6 ) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = ^∙_st̀ = 4,29 ∙ 10 y; ∙ 𝑡𝑡𝑡𝑡 [eq.] ( 7 ) [eq.] (3) [eq.] ( 4 ) 113 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 2 validation study, showing a comparison between simulated and analytically calculated results. fig. 3 aligned and staggered distribution of air channels. 3.2 time constant the energy2d model is also used to characterize the thermal time constant of different concrete mixtures in order to select the mixture that will first be used for the initial convective concrete mockup. in this study, the temperature for a point in the middle of a 20 cm thick concrete wall is simulated and the wall is subjected to a 10°c temperature step decrease. the thermal time constant is defined as the time required for this point to change e-1 = 36,8% of the total difference between its initial and final temperature. the thermal time constant was evaluated for 70 existing types of concrete with different density and thermal conductivity. the heat capacity of the 70 types of concrete was always 840 j/kgk. fig. 4 shows the simulation at two stages: t=0 when the concrete is at a temperature of 25°c and the surrounding at a temperature of 15°c and after 3 hours when the temperature in the wall is decreasing. fig. 4 example of time constant simulation. fig. 5 thermal time constant of 70 concrete types. fig. 4 allows us to select a concrete mixture that has a high density for high storage capacity purposes while having a relatively quick thermal response with a time constant in the order of 2 hours. as can be seen in fig. 5, the selected concrete mixture has a density of 3200 kg/m3, a thermal conductivity of 2.3 w/m.k and a time constant of one hour and 56 minutes. fig. 5 presents all the examined concrete types according to their thermal conductivity (x-axis) and their density (y-axis). the diameter of the circle represents the time constant recorded during the simulation for every concrete type while the shades of grey categorizes the results in different sections of time. the smaller the diameter and the darker the grey, the shorter the time constant. 114 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 6 schematic section representing the operation of the ventilated slab model in energyplus. (u.s. department of energy, 2016). 3.3 next step with energyplus while the performance of the mock-up will be measured under controlled conditions, the timespan of the project does not allow to implement and monitor the performance of the system in a real building. this situation can instead be modeled in a whole building energy simulation software. no model pre-exists to simulate the innovative convective concrete system, however, some existing models have enough similarities and flexibility to replicate the effects of the convective concrete. in this study, the ventilated slab model developed by chae and strand in energyplus (chae & strand, 2013) is used. since the convective concrete does not directly circulate the air of the room, the “slab only” mode which is displayed in a schematic manner in fig. 6 is selected. also no heating or cooling coils are implemented as the convective concrete uses outdoor air directly. the first results show promising performance of the system but operation modes and control settings still need to be further investigated. as can be seen on fig. 7 convective concrete has the potential to decrease the operative temperature in a simple square zone of 25 m2 by more than 2⁰c. the zone simulated has all its surfaces exposed to the outside except for its ground floor, a window is implemented on the south oriented wall and the east, north and west wall are equipped with convective concrete. in this example the control to turn on the system is based on the operative temperature and is set to 22⁰c. when both the operative temperature is more than 22⁰c and the outdoor air temperature is lower than the temperature of the convective concrete, outdoor air is circulated in the element and goes out warmer as the concrete is cooled down. 115 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 7 first results of the whole building simulation for the 4th, 5th and 6th of june. 4 future work the simulations show the potential of the convective concrete. future work consists of producing the different concrete mixtures, building mock-ups to test and to compare the results of the simulations and the physical models. 4.1 physical model concerning the concrete mixture, the focus is placed on the density and the thermal conductivity value which need to be the same as in the simulation to be able to compare the simulations and the mock-ups. the test facility is an insulated box with two rooms, one on each side of the sample, a so called hot-box as described in nen-en-iso 8990:1997. a lamp is used to warm up a room while a peltier element is used to cool it down. once the temperature is stabilized, these rooms can represent different conditions on both sides of the convective concrete (e.g. overheated room inside and cold summer night outside). to measure the heat conduction through the convective concrete, heat flux sensors are used in combination with temperature sensors on both sides of the element and in the rooms. with the temperature sensors, the u value can be calculated roughly by: [equ] ( 8 ) dto is the temperature difference between the inside wall surface and the outside air temperature dti is the temperature difference between the inside wall surface and the inside air temperature rse are the surface resistances of the exterior the heat flux sensors will be used for an accurate measurement, which will be compared to the obtained values calculated over the temperature sensors. 116 journal of facade design & engineering volume 5 / number 1 / 2017 4.2 potential applications buildings that require a constant inner temperature benefit most from convective concrete. climates where the night temperature falls below the day comfort temperature are those benefitting most of the system (bernard, 2002). while in his research bernard focusses on cooldeck that eliminates the influence of false ceilings and walls, convective concrete inner walls can be used as well for energy storage, but also the influence of cavities in front of the element activated need to be eliminated. cross ventilation during the night needs to be used to cool down these inner walls, making not only massive façades useful as activated concrete, but internal mass as well. with these innerwalls also the walls in between of row housing. here forced horizontal ventilation can be used to activate the convective concrete. the separated inner and outer channels make convective concrete a closed loop heat exchanger with a thermal buffer in between. to further decrease cooling energy use, the outer channels can also be connected with earth tube heat exchangers. during the summer the lower temperature is used to cool the outside channels and in winter it can be used only when the earth tube’s temperature is higher than the core temperature of the concrete, when heating is needed. cooling will always work as long as the earth tube delivers cooler air as the inside room temperature. 5 conclusions although real mock-ups still need to be tested before the performance can be determined, the convective concrete shows its potential in the simulations. the concrete can store the energy on day/ night cycles allowing a more constant inner temperature, which leads to a better thermal comfort within the built environment. in the next steps of the project, the physical models will be tested and with an iterative process the different geometries will be tested to obtain information on the parameters that influence the thermal exchange; flow speed, internal surface geometry and the geometry of the channels inside the concrete. the technology to print the outcome of parametric models is available. as soon as the outcomes of the simulations match the physical models, parametric models can be designed, after which optimized internal formwork for the convective concrete can be printed and the façade and internal walls can be applied in the built environment. the final product can be in the form of prefabricated concrete slabs, but also in the form of the inserts that are placed in on-site built formwork. 117 journal of facade design & engineering volume 5 / number 1 / 2017 acknowledgements this research is financed with a 4tu.bouw lighthouse grant, and with help of g.tecz engineering gmbh and the technical university darmstadt, faculty of civil engineering and environmental engineering (fb13). references bernard, n. (2002). thermal mass and night ventilation-utilising “hidden” thermal mass. international journal of ventilation, 1(2), 81-90. chae, y. t. & strand, r. k. (2013). modeling ventilated slab systems using a hollow core slab: implementation in a whole building energy simulation program. energy and buildings, 57, 165-175. charles xie, (2012). interactive heat transfer simulations for everyone, the physics teacher, volume 50, issue 4, pp. 237-240. corgnati, s. p., & kindinis, a. (2007). thermal mass activation by hollow core slab coupled with night ventilation to reduce summer cooling loads. building and environment, 42(9), 3285-3297. fraisse, g., johannes, k., trillat-berdal, v., & achard, g. (2006). the use of a heavy internal wall with a ventilated air gap to store solar energy and improve summer comfort in timber frame houses. energy and buildings, 38(4), 293-302. fraisse, g., boichot, r., kouyoumji, j. l., & souyri, b. (2010). night cooling with a ventilated internal double wall. energy and buildings, 42(3), 393-400. hensen, j. l. m. and nakhi, a. e. (1994). fourier and biot numbers and the accuracy of conduction modelling. proceedings of bep ‘94 conference (pp. 247-256). york: building environmental performance analysis club (bepac). holmes, m. j., & hacker, j. n. (2007). climate change, thermal comfort and energy: meeting the design challenges of the 21st century. energy and buildings, 39(7), 802-814. incropera, f. p., bergman, t. l., lavine, s. j., dewitt, d. p., (2007). introduction to heat transfer, john wiley & sons inc., fifth edition. hoes, p., & hensen, j. l. m. (2016). the potential of lightweight low-energy houses with hybrid adaptable thermal storage: comparing the performance of promising concepts. energy and buildings, 110, 79-93. hoes, p., trcka, m., hensen, j. l. m., & bonnema, b. h. (2011). investigating the potential of a novel low-energy house concept with hybrid adaptable thermal storage. energy conversion and management, 52(6), 2442-2447. knaack, u., klein, t., bilow, m., & auer, t. (2007). principles of construction–facades: basel boston berlin: birkhäuser. lagerblad, b. (2005). carbon dioxide uptake during concrete life cycle–state of the art. swedish cement and concrete research institute cbi, stockholm. loonen, r. c. g. m., singaravel, s., trcka, m., cóstola, d and hensen, j. l. m. (2014). simulation-based support for product development of innovative building envelope components. automation in construction 45 , 86-95 loonen, r. c. g. m., trčka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable and sustainable energy reviews, 25, 483-493. maier, a., gilka-bötzow, a., & schneider, j. (2015). an energy-active facade element from mineralized foam (mf) and micro-reinforced, ultra-high-performance concrete (mruhpc). journal of facade design and engineering, (preprint), 1-13. nen-en-iso (1997). nen-en-iso 8990: 1997. thermal insulation–determination of steady-state thermal transmission properties– calibrated and guarded hot box. u.s. department of energy (2016). engineering reference, energyplustm version 8.5 documentation. section 17.6.16 ventilated slab. warwick, d. j., cripps, a. j., & kolokotroni, m. (2007). monitoring and simulation of two operational buildings integrating active thermal mass strategies. 2nd palenc conference and 28th aivc conference on building low energy cooling and advanced ventilation technologies in the 21st century, crete island, greece zmeureanu, r., & fazio, p. (1988). thermal performance of a hollow core concrete floor system for passive cooling. building and environment, 23(3), 243-252. journal of facade design and engineering 4 (2016) 115–129 doi 10.3233/fde-160054 ios press 115 photocatalytic self-cleaning coatings for building facade maintenance. performance analysis through a case-study application a. andaloroa,∗, e.s. mazzucchellia, a. lucchinia and m.p. pedeferrib apolitecnico di milano, department of architecture, built environment and construction engineering, ponzio, milan, italy bpolitecnico di milano, department of chemistry, material and chemical engineering “giulio natta”, mancinelli, milan, italy abstract. facade maintenance has become a key aspect in building management, due the specific actions involved and operation related costs. within this framework, the application of titanium dioxide photocatalytic sol-gel products on facade elements offers a wide range of opportunities to ensure proper functionality maintenance over time. this paper illustrates the self-cleaning performance of titanium dioxide and silicon dioxide based coatings applied to different kinds of cladding materials. all tested samples were opaque. preliminary laboratory tests were performed by means of water contact angle measurements to verify hydrophobic and hydrophilic behaviour prior to outdoor application. afterwards, outdoor tests were performed to monitor colour variation over a 36-month period to verify product effectiveness and durability. results proved that the application of functionalized nanotechnological coating to a facade can significantly facilitate cleaning operations and reduce the necessary frequency over time. in addition, the output provides some preliminary information about the exposure condition influence on self-cleaning performance, which could be further investigated in the future. keywords: facade maintenance, cladding, photocatalysis, self-cleaning coating, retrofit 1. introduction the constant evolution of facade systems is leading to the use of high performance technology within envelope elements, such as the integration of solar active systems in the form of thermal accumulation or photovoltaic modules for electrical production. however, facade material and component cleaning operations are very often delayed both by owners and facility managers as they involve relevant and frequent investments. it is therefore not unusual that pleasant architectural objects are transformed into soiled surfaces that are unpleasant to witness in the urban context, as shown in fig. 1. on the other hand, it is clear that the required effort, in terms of water expense, use of detergents, operation cost and time, becomes more constant as soiling deposits on the facade are harsh. conversely, maintaining the front covers of photovoltaic modules and envelope ∗corresponding author: a. andaloro, politecnico di milano, department of architecture, built environment and construction engineering, via ponzio 31, 20133, milan, italy. tel.: +39 333 3900052; fax: +39 02 2399 6080; e-mail: annalisa.andaloro@polimi.it. issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). mailto:annalisa.andaloro@polimi.it 116 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance surfaces in a clean state is of the outmost importance to ensure that performance levels remain constant over time, and as close as possible to the desired design values. polluted operating environments, such as urban or industrial contexts, cause rapid envelope soiling phenomena which in turn raise durability and aesthetic concerns. in this framework, keeping facade materials in a fairly clean condition is fundamental to preserve the proper functionality of the envelope elements over time (rigone, 2011). for this reason, the integration of building maintenance units (mobile facade access devices moved by electric engine and suspended on ropes or rail, as seen in fig. 2), referred to as bmu from here on, on the facade is becoming more and more diffused for new buildings. in fact, their presence optimizes indirect costs due to maintenance operations during the whole building service life, guaranteeing easy and economically convenient access to envelope surfaces. although bmu’s certainly represent a good option, there are some cases where this strategy is not applicable or economically inconvenient, as in the case of existing or new buildings characterized by particularly complex shapes. in these cases, the use of functional self-cleaning coatings can significantly ease cleaning operations due to the combined chemical-physical behaviour of treated surfaces (diamanti et al., 2013; watanabe et al., 1999; yang et al., 2006; parkin & palgrave, 2005; nishimoto & bhushan, 2013). for example, the photocatalytic activity of titanium dioxide (miyauchi et al., 2002; nakajima et al., 2001; carp et al., 2004) induced by the absorption of ultraviolet (uv) radiation initiates chemical fig. 1. example of significant water stains on a facade, caused by lack of maintenance action. fig. 2. building maintenance unit. source: rostek. a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 117 oxidation of soiling deposits on exposed surfaces. afterwards, preliminary decomposition actuated by the coating allows for their easy washing under the effect of driving rain on the building facade, profiting from the uv-induced superhydrophilic state of the surface (hoffmann et al., 1995; ganesh et al., 2011; sciancalepore & bondioli, 2015; fujishima et al., 2001). more in detail, the advantage of the use of photocatalytic coatings is to prevent massive adhesion of dirt deposits, minimizing the necessary cleaning operations in terms of both intensity and frequency (gladis & schumann, 2011). this reduction implies economic benefits for owners and estate managers and on the other hand contributes to a more sustainable management of the built environment. so, the integration of self-cleaning photoactive functional materials is an effective preventive maintenance strategy, that allows consistent savings and reduces environmental impacts of cleaning operations during the whole building service life. due to their particular behaviour, titanium dioxide photocatalytic particles have found a wide range of applications in the civil and architectural field over the last decades (ritter, 2007; leydecker, 2008; fernandez, 2006; chen & poon, 2009; liu et al., 2008). over the last 20 years or so, research progress led to the combination of active components such as titanium dioxide photocatalytic nanoparticles with various construction materials such as cement, mortar, asphalt, floor tiles and many others (chen & poon, 2009; husken et al., 2009), as seen in figs. 3 and 4. more recently, photocatalytic coatings (paints, plasters or spray transparent coating) to be applied at the end of facade materials production process or on already existing facade surfaces revealed a great potential for air purification and easy cleaning properties (laufs et al., 2010; aguia et al., 2010; maggos et al., 2007; salthammer & fuhrmann, 2007). titanium dioxide photoactive products are often used for this purpose. these kind of coatings can be applied to a substrate with very simple methods, such as spray or dip-coating (sample immersion in a functionalized liquid solution). this post processing alternative represents a particularly effective way to exploit the quantity of photoactive material used, especially when dealing with opaque cladding materials, where only layers directly exposed to sunlight can be photoactivated and perform photocatalytic behaviour. in fact, catalyst particles eventually dispersed in inner layers cannot play a role in soiling decomposition, air purification and surface self-cleaning if they are not activated by light penetrating inside the material. in addition to this, self-cleaning materials have found wide acceptance in the field of historical building heritage preservation, due to their transparency on the substrate and the possibility to maintain the original aesthetical appearance of those buildings at almost no effort and very reduced cost (quagliarini et al., 2012). as a matter of fact, not only does the application of titanium dioxide fig. 3. cité des arts et de la musique, chambery, france. this is the first building where mortars containing titanium dioxide were applied, 2000 (source: http://citedesarts.chambery.fr/). http://citedesarts.chambery.fr/ 118 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance fig. 4. church dives in misericordia, rome, italy designed by architect richard meier who choose to build the structure using cement containing a significant percentage of titanium dioxide, 2003 (source: http://files1.structurae.de/). based products on porous facade materials allow to keep their surface clean over time, but it also minimizes the risk for phototropic bacteria growth, which is usually among the main causes of mould formation related damage in porous cladding materials, both on vertical and horizontal envelope surfaces (roofing) (graziani et al., 2014). as a consequence of the aforementioned issues, current research activities on photoactive selfcleaning materials involve not only skills and competences from the chemistry and material science field, but also from the building and construction sector, in order to develop the most appropriate technical solution able to exploit the functionalizing properties of nano-structured materials at the maximum possible level, in case such products are to be applied to building components. the present work presents the application of titanium dioxide self-cleaning products to different cladding materials. most of them contain titanium dioxide in the anatase form, that is the molecular structure considered the most active in terms of photocatalytic behaviour over time. in particular, the paper aims at discussing the results of a 36 month-long experiment on cladding materials, and the influence of particularly polluting industrial activities on self-cleaning efficiency over time. the work proposes the integration of such coatings within the building facade as a preventive cleaning strategy which allows to postpone and reduce frequency of ordinary maintenance operations related to facade washing in polluted environments. in the results and discussion section, some considerations concerning coating durability issues are discussed. 2. materials and methods 2.1. opaque cladding panels four different materials used for external cladding were tested under normal operating conditions, being installed as external cladding elements (vertical and sub-vertical on a canopy) on an office-industrial building located in northern italy (see fig. 5). http://files1.structurae.de/ a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 119 all the materials tested outdoors were part of an existing building facade and could therefore not be removed for laboratory analysis. all the work performed on these elements had to be carried out in situ. the main characteristics of the four materials are summarized in table 1. samples of the same materials were also subjected to laboratory tests aimed at verifying surface hydrophilic or hydrophobic behaviour prior to outdoor self-cleaning coating applications. such tests involved the use of a uv-lamp for sample irradiation and measurement of the water contact angle over time, as detailed in section 2.3. the main characteristics of the tested specimen are included in table 1. 2.2. deposition of titanium dioxide sol-gel products for surface self-cleaning the materials were treated with three different functionalizing products and with a combination of two out of the three. all the self-cleaning products used for the experimental campaign were produced through sol-gel processing – a very versatile method to produce photoactive sols. a sol is a dispersion of colloidal particles in a liquid, titanium or silicon dioxide in the specific case. after product application, solvent evaporation initiates the creation of a photoactive gel, that is to say an interconnected network of active particles characterized by porosity magnitude below one fig. 5. view of the case study building. source: www.edilportale.com. table 1 tested cladding panels: main characteristics and exposure condition (outdoor refers to in situ exposure, laboratory refers to the procedure described in section 2.3) product name and producer short name material colour test condition esoroccia – mmg srl ce granite powder and cement grey outdoor collection – laminam ly/lw laminated porcelain yellow/white outdoor/lab silbonit ha-hc – sil sc-v silica-calcium yellow outdoor vertical silbonit ha-hc – sil sc-c silica-calcium yellow outdoor canopy cotto rdb – rdb spa co terracotta tile red outdoor cotto – unieco bologna cl terracotta tile red laboratory www.edilportale.com 120 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance micrometre, and polymeric chains with length above one micrometre. all products are commercially available, and the dimension of colloidal particles dispersed in the solutions ranges between 10–100nm (hench & west, 1990). for each cladding material, reference non-treated samples have been kept under constant monitoring to highlight substantial differences in soiling intensity. product compositions and their functional characteristics are briefly summarized in table 2. sol 111 cd (t) is a semi-transparent water based solution. the active photo catalyst is nanocristalline titanium dioxide, and its use is recommended to provide self-cleaning properties to building components, cladding panels and even furniture elements. as the suggested deposition thickness is really limited to maintain film transparency, this product should rather be applied at the end of the production processes as consistent mechanical stress can damage coating photoactive behaviour. sol 232/ox (s1) is a semi-transparent isopropyl alcohol based solution. the active component is nanocristalline silicon dioxide, and its application gives a strongly hydrophobic characteristic to the treated surface (lakshmi, 2011). this means that the material surface is unlikely to be wet even in case of driving rain or direct water pouring, similarly to what happens in the case of more traditional siloxane based hydrophobic treatments. however, in this case the hydrophobic surface behaviour is based not only on chemical but also morphologic characteristics. in fact, the dried coating does not form a completely smooth surface. on the contrary, it is characterized by the presence of numerous peaks and narrow valleys which prevent water droplets to infiltrate. for this reason, wettability of the treated surface is extremely reduced. effects of such modified behaviour are shown in section 3.1. sol 322/ox (s2) is an isopropyl alcohol based solution containing suspended silicon dioxide nanoparticles. this product gives surfaces water-repellent characteristics through the so called lotus effect. sol 111 cd + sol 322/ox (t+s2) represents a combination of the two sols, applied one above the other providing the necessary time frame for surface drying. the deposition method used for this experimental set-up was a spraying technique. sol mixtures were produced from titanium and silicon with a hydrolysis and condensation process to form the final self-cleaning products (hench & west, 1990). when the liquid solution is applied to a surface, a film forms and the subsequent drying (in natural or accelerated conditions) allows for enhanced mechanical resistance of the coating. all of the above mentioned solutions are completely transparent to the human eye after the solvent evaporation process is completed, both in wet and dry condition of the substrate. this feature is very important from an application related viewpoint as it broadens the possibility of self-cleaning product use both in new construction and historic buildings (quagliarini et al., 2012). table 2 chemical base element of the tested sol-gel products and specific functional properties product name short name chemical base functional property sol 111 cd t tio2 1-2% and functionalized silica 1-2% photocatalytic and hydrophilic sol 232/ox s1 sio2 hydrophobic sol 322/ox s2 sio2 water repellent (lotus effect) sol 111 cd + sol 322/ox t+s2 application of two different sprays, one above the other strongly hydrophobic a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 121 2.3. laboratory verification of hydrophilic and hydrophobic properties four different samples treated with the self-cleaning coatings and a non-treated reference specimen were tested to verify the achievement of the desired functionalizing surface properties. the substrate chosen for this preliminary laboratory test was the laminam ly (see table 1) and the four applied products are those listed in table 2. the test method is based on the water contact angle (wca) variation observation through 4 hours of continued uv irradiation, according to what is proposed in the iso 27448 standard. figure 6 shows an example of wca measurement of one of the tested samples. the initial contact angle value was determined on fifteen discreet points for each test specimen as suggested in the uni en 15802:2009 standard, and an average value was calculated. afterwards, the uv source was activated and an ultraviolet light radiometer was used to set the irradiation intensity to 2mw/cm2 (tolerance 0.1mw). the water contact angle measurement was repeated every hour for the five different points of each specimen, with the last measurement after 4 hours of continued uv irradiation. all values were noted in a log book, until the required conditions for the determination of a final contact angle according to the applied test procedure (standard deviation of three consecutive measurements on a specimen lower than 10%) were achieved. 2.4. outdoor exposure test an outdoor experimental activity was carried out on an existing building located in turate (como), northern italy. the different cladding materials listed in table 1 were treated with the sol product varieties described in section 2.2 and listed in table 2, and their self-cleaning performance was monitored over 36 months. sols were applied with pressurized spray coating techniques as shown in fig. 7a–b. for each selected material substrate (see table 1), a non-treated specimen was monitored with the same testing procedure, and maintained as a reference. the parameter used to estimate selfcleaning performance is colour variation measured on opaque cladding elements. this has been performed taking sample colour coordinates on two, four or eight different points for each cladding element depending on the element size (see fig. 8) from which an average value was determined. for the purpose of this study, measurements were taken 5–10cm away from elements borders, in order to avoid biased results due to border effects or to a relative closeness of joints between adjacent cladding panels. this technique was the only means to monitor self-cleaning performance of the cladding elements over time, as it was not possible to remove the facade panels for further laboratory analyses. however, the authors relied on results from previous works on photoactive self-cleaning products, proposing colorimetry as an accountable method for the estimation of self-cleaning capacity when operating outdoors (quagliarini et al., 2012; graziani et al., 2014; fig. 6. water contact angle measurement just after drop pouring. the image is shadowed as the picture was taken before uv irradiation source activation. 122 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance (a) (b) fig. 7a–b. pressurized spray coating on facade cladding panels. fig. 8. scheme of measurement points on cladding elements. gladis & schumann, 2011). colour parameters were extracted by means of a portable konicaminolta spectrophotometer cm-2600d, which measures surface spectral reflectance every 10nm in the range of 360–740nm, using the supporting software spectramagic nx. from reflectance data collected on sample surfaces, the instrument calculates colour coordinates according to the cielab standards, l* brightness, a* hues from red to green and b* hues from yellow to blue. colour variation over time �e was calculated according to the following formula, which measures the difference between two points in the cielab colour space, respectively corresponding to the surface colour at time t (measurement during outdoor weathering) and the same surface initial colour t0 measured after sol-gel application and drying (equation (1)): �e = √( l∗t − l∗0 )2 + (a∗t − a∗0 )2 + (b∗t − b∗0 )2 (1) for each cladding element, six measurements were performed during the outdoor testing period. as the humidity content influences materials reflectance, preliminary laboratory tests were carried out to identify a repeatable colour measurement strategy to be applied for all of the outdoor a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 123 experimental activities. laminated porcelain samples were wetted according to two modes: driving rain simulation or total water immersion. in the first case, rain was simulated keeping samples under a continuous water stream for 4 hours. in the latter, samples were kept inside water for 2 days. colour variation measurements were performed according to equation 1 to benchmark materials drying duration (dry sample has �e ≈0). 3. results and discussion 3.1. laboratory tests during 4 hours of uv irradiation, wca values for the tested samples varied as shown in table 3. the table also shows that the treated samples showed different surface behaviour even before the uv irradiation test started, which was hydrophilic in the case of sol t and hydrophobic in the case of sol s1, sol s2 and the combination of sol t+s2. this happened because the applied sol products immediately provided treated surfaces with the desired characteristics, which occurred prior to actual start of the irradiation test (graziani et al., 2014). however, laboratory performed water contact angle tests showed a consistent enhancement of these properties under the effect of uv light, after a stabilization time interval which was necessary to complete the photo activation of the treated surface, as also required in the iso 27448 standard. as expected, the non-treated reference specimen (n) did not show significant wca alterations with uv light irradiation. sample t, which was treated with titanium dioxide showed hydrophilic surface behaviour from the beginning of the test onward and had a consistent decrease of the water contact angle after the 4 hour uv irradiation time. however, this trend showed up after the second hour of uv irradiation, while it was opposite during the first hour. this may be ascribed to a buffering time during which the photo catalyst goes through the photo activation process. samples s1 and s2 were both treated with different silicon dioxide based sols and showed a rather hydrophobic behaviour, as expected. the last analysed specimen was coated with a combination of two sols: t and s2. the results of this last application are particularly relevant as the global results leads to a strongly hydrophobic surface behaviour, showing a water contact angle which is up to 30◦ higher than the mere hydrophobic surfaces of samples s1 and s2 after 4 hours of uv irradiation. the above mentioned strongly hydrophobic behaviour is caused by the combination of hydrophobic and photocatalytic surface properties provided by titanium and silicon dioxide (boroujeny et al., 2012). this can be particularly relevant for certain outdoor applications, when this behaviour could be more suitable to maintain facade surfaces in a clean condition according to specific operating environments. in fact, soiling deposits are very unlikely to stick onto the surface of hydrophobic table 3 water contact angle measurements for the different treated samples (4 hours uv irradiation test). only the average values of the measurements made for each sample are reported in this table water contact angle measurement (degrees) irradiation time non treated (n) sol 111 cd (t) sol 232/ox (s1) sol 322/ox (s2) sol 111 cd + sol 322/ox (t+s2) 0h – dark 55[st.dev=0.078] 39[st.dev=0.073] 96[st.dev=0.078] 84[st.dev=0.076] 99[st.dev=0.078] 1h 54[st.dev=0.076] 48[st.dev=0.080] 81[st.dev=0.080] 74[st.dev=0.078] 96[st.dev=0.076] 2h 58[st.dev=0.078] 44[st.dev=0.082] 90[st.dev=0.083] 87[st.dev=0.089] 90[st.dev=0.089] 4h 66[st.dev=0.077] 28[st.dev=0.090] 93[st.dev=0.084] 87[st.dev=0.088] 118[st.dev=0.079] 124 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance materials, opposite from what happens with superhydrophilic materials. so, in the latter case rain events are necessary to pursue natural facade washing (ballari & brouwers, 2013). conversely, in the former case the coated area could remain clean for longer periods without any need for rain. of course, real and forecasted operating condition and environmental context should be carefully evaluated before choosing a specific surface treatment. as previously shown in table 3, the five different samples showed dramatically different wettability even before uv irradiation test initiation (graziani et al., 2014). this proves that surface treatments convey a specific behaviour to treated surfaces since the moment of their first application. despite surface functional properties are increased by solar light activation, the preliminary surface state is useful to prevent soiling deposits on the surface (especially in the case of hydrophobic and strongly hydrophobic behaviour). 3.2. outdoor photocatalytic efficiency measurements the self-cleaning performance of applied coatings was measured through a prolonged monitoring of colour coordinates variation for each tested cladding material and type of coating. the measurements were repeated 6 times during 24 months. as an example, fig. 9a–b shows colour coordinates variation on both elevation north and south of the tested building, located in the area of como (italy) for one of the sample cladding. this technique was chosen as it could be easily performed without the need of dismantling facade cladding to bring it to a laboratory facility, which was of course not possible in this case (working on an already existing building). moreover, monitoring �e values on an outdoor experimental campaign has already been proved to be an effective way to evaluate the self-cleaning performance of the applied coatings (quagliarini et al., 2012; graziani et al., 2014; gladis & schumann, 2011). cement cladding elements (ce) had higher �e and �l* values on the northern exposure over time on average as compared to the southern elevation (meaning that they had heavier soiling deposits on their surface). hydrophobic coatings s1 and s2 showed lower colour variation than the photocatalytic ones. in addition, non-treated samples showed a better behaviour than coated cladding elements on this exposure, so it is easy to infer that there is no benefit in integrating selfcleaning coating for the northern orientation of facades in the case of cement cladding elements. global �e values are significantly lower on the southern facade, and this could be partly ascribed to the presence of direct sunlight irradiation, which enhances the photocatalytic activity of coatings containing tio2 (maggos et al., 2007). panels treated with sol t registered higher �e values on fig. 9a–b. �e variation for ce cladding elements (left: northern elevation, right: southern elevation). a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 125 the last measurement day, while samples treated with sol t+s2 registered extremely low colour variation, and the �e value registered is quite similar to the original one at time t0 . this reveals actual soiling affecting the non-treated material, as opposed to the negligible accumulation of darkening particles (such as organic pollutants and particulate matter) on surfaces coated with photoactive tio2 sol-gel (sol t and t+s2). this implies that the application of self-cleaning coatings is much more effective in the case of southern exposure. a supporting explanation for this result is related to the presence of direct light irradiation on this latter orientation, conversely to what happens on the northern side of the building. in fact, it has been proven that the photocatalytic activity of titanium dioxide is higher under the effect of direct exposure to light (watanabe et al., 1999), implying a higher soiling decomposition rate and stronger self-cleaning capability. parameters having an influence on the self-cleaning performance can be related to higher direct rainfall or generally more favourable weather conditions (e.g. dominant wind direction, relative humidity values and the presence of direct solar irradiation) (ballari & brouwers, 2013; moussiopoulos et al., 2008; maggos et al., 2007). further analysis on prevalent wind direction during rain events could be performed in the future to determine the influence of wind driven rain on a specific elevation on the global self-cleaning performance. a brief comparison among all tested materials is proposed, for the purpose of evaluating the influence of specific cladding material on global self-cleaning efficiency, summarizing the results of outdoor exposure tests according to the two tested orientations, north and south. the graphs in the two figures show a global view of the �e values on the last day of measurements according to treated substrate and sorted by coating type. as far as the northern facade is concerned (fig. 10a), sol t has shown low �e values on all tested substrates, except for terracotta cladding elements (co) which have reported a higher variation. the combined sol t+s2 has proved its efficiency on cement based (ce) cladding panels and performed quite well also on silica calcium vertical panels sc-v, but according to test results it is less suitable in the case of terracotta cladding panels (co), where sol s1 showed better performance (fig. 10a). however, the hydrophobic coating had a good self-cleaning performance in just one single case, while photocatalytic coatings generally tend to perform better than hydrophobic products in the discussed results. figure 10b summarizes final colour variation results for the southern elevation. in this case, photoactive materials have shown a satisfactory self-cleaning ability both for cement based materials (ce) and laminated porcelain (ly). in fact, sol t and sol t+s2 were the most performing coatings in terms of self-cleaning activity in both cases (lowest �e values). results for the southern facade shown in fig. 10b suggest that facade materials generally benefit from direct sunlight exposure when treated with functional coatings. fig. 10a–b. summary of �e values reported on tested materials (a: north, b: south) according to material substrate and applied coatings on the last measurement day (namely end of the monitoring period for the scope of this work). 126 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance fig. 11a–b. heavy soiling deposits on the protection canopy (silica-calcium cladding). in addition to this, a quick comparison between the �e values measured on southern and northern facades for the same coating (sol t) on cement substrates (ce) gives evidence of the fact that the photocatalytic coating (sol t) has a greater self-cleaning efficiency on the southern exposure (see fig. 10a–b). in fact, on a northern elevation, less sunlight reaches the material surface compared to other orientations. under this condition, the titanium dioxide needs more time to complete its photo activation (maggos et al., 2007), which makes the presence of the coating less efficient as compared to the same application on a southern facade. one last note concerns silica-calcium cladding elements located on the sub-vertical canopy (scc) which showed satisfying self-cleaning behaviour during the first operating period (12 months approximately), but at the time of the last two colour measurements had soiling deposits clearly visible and spread all over the cladding surface, as shown in fig. 11a–b. further analyses could be done in the future to verify the causes of the limited service life of coatings in this case, as many different alternatives are possible. however, it is possible to state that the reduction in self-cleaning performance is not related to coating ageing, as the rest of the northern facade did not show significant soiling deposits. as a matter of fact, the canopy location is likely to have had an influence on soiling deposit persistence, as it receives consistent amounts of thin iron powders rising upward from the production site underneath. these may have caused precocious decay in self-cleaning performance due to titanium dioxide removal from the surface, caused by mechanical abrasion in combination with weather agents (especially wind, but also rain) this fact raises some concerns related to coating durability issues, discussed by hassan et al., 2010; osborn et al., 2014; mellot et al., 2006; graziani et al., 2014. 4. conclusion the contemporary trend towards a more sustainable management of the built environment suggests the integration of self-cleaning coatings on facades as a viable option for enhanced cleaning capabilities of external surfaces. this possibility becomes even more relevant if taking into account the increasingly frequent inclusion of solar active components in facade systems, which require their front covers to remain as clean as possible in order to guarantee performance design levels. a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 127 this article focused on the self-cleaning behaviour of different opaque envelope materials coated with titanium dioxide based sol-gel products, proposing a convenient and effective alternative to traditional facade washing for maintenance purposes. further research work has also been carried forward regarding the application of the same self-cleaning product to glass, but results are not presented in this article for the sake of simplicity. a comparison with hydrophobic silicon dioxide based surface treatments was conducted in order to determine specific convenience conditions according to the different cladding element materials. both laboratory experimental techniques and an outdoor weathering set-up were used to evaluate self-cleaning coatings performances. more specifically, laboratory wettability tests were performed and an extensive colour variation monitoring campaign was carried out on an existing building for 36 months. at a first laboratory characterization, titanium dioxide based sol showed hydrophilic characteristics, while both the two silicon dioxide based products were hydrophobic. in addition, the combination of titanium and silicon dioxide provided coated surfaces with strong hydrophobicity. this condition may be particularly advantageous in significantly polluted environments with low precipitation rates, where the ordinary self-cleaning of facades through rainfall washing can hardly be achieved. within the scope of this work, outdoor exposure tests outlined differences between northern and southern exposures. the differences can be ascribed to the presence of direct irradiation in the latter case, which is likely to enhance photocatalytic reaction speed and consequently ease spontaneous cleaning during rain events. this suggests that the application of titanium dioxide based self-cleaning coatings is more effective when coupled with the most suitable orientation of a facade. sol t showed significantly high self-cleaning performance on tested cladding materials, such as cement, terracotta and laminated composites, providing very limited colour variations at the end of the monitoring period; mixing a titanium dioxide based sol with silica also allowed to achieve satisfying levels of cleanliness at the end of the monitoring period. finally, some durability concerns on retrofitting coating application have arisen due to the visible soiling deposits on the silica calcium cladding panels located on the northern facade. in this case, the self-cleaning performance was satisfying during the first 12 months but after this period dark stains started to build up on the surface. considering all of the above, the use of self-cleaning coatings on facade surfaces enables owners and designers to act in the direction of maintenance cost reduction in the medium and long-term. so, further research effort in the near future will be dedicated to the investigation of self-cleaning coating compatibility with transparent and active facade components. in addition to this, more detailed climatic analysis within the context of performed experiments could provide interesting insights in triggering causes for limited service life or efficiency for more effective product development on the side of tuning material properties. acknowledgments the authors thank nextmaterials s.r.l. for supplying the nanostructured titanium and silicon dioxide sols and aderma s.r.l. for participating in the experiments providing free access to the company building, which was used as a case study. references aguia, c., angelo, j., madeira, l. m., & mendes, a. (2010). influence of photocatalytic paint components on the photoactivity of p25 towards no abatement. catalysis today, 151(1-2), 77-83. 128 a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance ballari, m. m., & brouwers h. j. h. (2013). full scale demonstration of air-purifying pavement. journal of hazardous materials, 254, 406-414. beldeens, a. (2006). environmental friendly concrete pavement blocks: air purification in the centre of antwerp. proceedings of the 8th international conference on concrete block paving, san francisco, california, usa. boroujeny, b. s., afshar, a., & dolati, a. (2012). photoactive and self-cleaning tio2–sio2 thin films on 316l stainless steel. thin solid films, 520(20), 6355-6360. carp, o., huisman, c. l., & reller, a. (2004). photoinduced reactivity of titanium dioxide. progress in solid state chemistry, 32(1-2), 33-177. chen, j., & poon, c. s. (2009). photocatalytic construction and building materials: from fundamentals to applications. building and environment, 44(9), 1899-1906. chen, j., & poon, c. s. (2009). environmental photocatalytic cementitious materials: influence of the microstructure of cement paste on photocatalytic pollution degradation. environmental science & technology, 43(23), 8948-8952. diamanti, m. v., del curto, b., ormellese, m., & pedeferri, m. p. (2013). photocatalytic and self-cleaning activity of colored mortars containing tio2. construction and building materials, 46, 167-174. fernandez, j. e. (2006). material architecture: emergent materials and issues for innovative and responsible buildings. elsevier architectural press. fujishima, a., rao, t. n., & tryk, d. a. (2000). titanium dioxide photocatalyisis. journal of photochemistry and photobiology c: photochemistry reviews, 1(1), 1-21. ganesh, v. a., raut, h. k., nair, a. s., & ramakrishna, s. (2011). a review on self-cleaning coatings journal of materials chemistry, 21, 16304-16322. gladis, f., & schumann, r. (2011). influence of material properties and photocatalysis on phototrophic growth in multi-year roof weathering. international biodeterioration & biodegradation, 65(1), 36-44. graziani, l., quagliarini, e., bondioli, f., & d’orazio, m. (2014). durability of self-cleaning tio2 coatings on fired clay brick facades: effects of uv exposure and wet & dry cycles. building and environment, 71, 193-203. graziani, l., quagliarini, e., osimani, a., aquilanti, l., clementi, f., & d’orazio, m. (2014). the influence of clay brick substratum on the inhibitory efficiency of tio2 nanocoating against biofouling. building and environment, 82, 128-134. hassan, m. m., dylla, h., mohammad, l. n., & rupnow, t. (2010). evaluation of the durability of titanium dioxide photocatalyst coating for concrete pavement. construction and building materials, 24, 1456-1461. hench, l. l., & west, j. k. (1990). the sol-gel process. chemical reviews, 90, 33-72. hoffmann, m. r., martin, s. t., choi, w., & bahnemannt, d. w. (1995). environmental application of semiconductor photocatalysis. chemical reviews, 95, 69-96. husken, g., hunger, m., & brouwers, h. (2009). experimental study of photocatalytic concrete products for air purification. building and environment, 44, 2463-2474. iso 27448:2009 (2009). fine ceramics (advanced ceramics, advanced technical ceramics) – test method for self-cleaning performance of semiconducting photocatalytic materials – measurement of water contact angle. lakshmi, r. v., bharathidasan, t., & basu, b. j. (2011). superhydrophobic sol–gel nanocomposite coatings with enhanced hardness. applied surface science, 257, 10421-10426. laufs, s., burgeth, g., duttlinger, w., kurtenbach, r., maban, m., thomas, c., wiesen, p., & kleffmann, j. (2010). conversion of nitrogen oxides on commercial photocatalytic dispersion paints. atmospheric environment, 44, 2341-2349. leydecker, s. (2008). nanomaterials in architecture, interior architecture and design, springer science & business media. liu, z., zhang, x., murakami, t., & fujishima, a. (2008). sol-gel sio2/tio2 bilayer films with self-cleaning and antireflection properties. solar energy mater solar cells, 92, 1434-1438. maggos, t., bartzis, j. g., liakou, m., & gobin, c. (2007). photocatalytic degradation of nox gases using tio2-containing paint: a real scale study. journal of hazardous materials, 146, 668-673. mellott, n. p., durucan, c., pantano, c. g., & guglielmi, m. (2006). commercial and laboratory prepared titanium dioxide thin films for self-cleaning glasses: photocatalytic performance and chemical durability. thin solid films, 502, 112-120. miyauchi, m., kieda, n., hishita, s., takefumi, m., nakajima, a., watanabe, t., & hashimoto, k. (2005). reversible wettability control of tio2 surface by light irradiation. surface science, 511, 401-407. moussiopoulos, n., barmpas, p. h., ossanlis, i., & bartzis, j. (2008). comparison of numerical and experimental results for the evaluation of the depollution effectiveness of photocatalytic coverings in street canyons. environmental modeling & assessment, 13, 357-368. nakajima, a., koizumi, s., watanabe, t., & hashimoto, k. (2001). effect of repeated photo-illumination on the wettability conversion of titanium dioxide. journal of photochemistry and photobiology a: chemistry, 146, 129-132. nishimoto, s., & bhushan, b. (2013). bioinspired self-cleaning surfaces with strong hydrophobicity, superoleophobicity, and superhydrophilicity. rsc advances, 3, 671-690. osborn, d., hassan, m., asadi, s., & white, j. r. (2014). durability quantification of tio2 surface coating on concrete and asphalt pavements. journal of materials in civil engineering, 26, 331-337. parkin, i. p., & palgrave r. g. (2005). self-cleaning coatings. journal of materials chemistry, 15, 1689-1695. quagliarini, e., bondioli, f., goffredo, g. b., cordoni, c., & munafò, p. (2012). self-cleaning and de-polluting stone surfaces: tio2 nanoparticles for limestone. construction and building materials, 37, 51-57. rigone, p. (2011). le facciate continue la manutenzione dell’involucro edilizio vetrato. sant’arcangelo di romagna (rn), italy: maggioli editore. a. andaloro et al. / photocatalytic self-cleaning coatings for building facade maintenance 129 ritter, a. (2007). smart materials in architecture, interior architecture and design. birkhäuser. salthammer, t., & fuhrmann, f. (2007). photocatalytic surface reactions on indoor wall paint. environmental science & technology, 41, 6573-6578. sciancalepore, c., & bondioli, f. (2015). durability of sio2–tio2 photocatalytic coatings on ceramic tiles. international journal of applied ceramic technology, 12, 679-684. uni en 15802:200 (2009). conservation of cultural property – test methods – determination of static contact angle. watanabe, t., nakajima, a., wang, r., minabe, m., koizumi, s., fujishima, a., & hashimoto, k. (1999). photocatalytic activity and photoinduced hydrophilicty of titanium dioxide coated glass. thin solid films, 351, 260-263. yang, y. h., han, y. s., & choy, j. h. (2006). tio2 thin-films on polymer substrates and their photocatalytic activity. thin solid films, 495, 266-271. jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 063 journal of facade design & engineering volume 5 / number 1 / 2017 updated urban facade design for quieter outdoor spaces jochen krimm1/2, holger techen1, ulrich knaack2 1 frankfurt university of applied sciences, department 1/architecture-civil engineeringgeomatics, nibelungenplatz 1, 60318 frankfurt , tel. no.+496915333001, holger.techen@fb1.fra-uas.de 2 delft university of technology, faculty of architecture and the built environment, architectural engineering + technology, julianalaan 132-134, 2628 bl delft, tel. no.+31 15 27 88566, u.knaack@tudelft.nl abstract the increasing migration into cities leads to an increasing number of people stressed by noise. more and more people are moving into urban settings comprised of multiple noise sources and hard reflective glass and steel facades. the omnidirectional arrangement of noise sources like airborne noise or car traffic noise and their reflection on the facades neither composes urban arrangements with silent indoor areas nor comfortable quiet areas outdoor. to come up with requirements for silent areas inside and outside of buildings further design parameters have to be introduced. the facade is not only a shelter for the inside it can also provide comfort spaces outside the building. as engineers and architects we cannot change the noise source, but we can influence the impact on the surrounding urban space by controlling the reflection of noise emissions on the urban surfaces like facades. in a facade design the capability of reflecting noise can be tuned by modifying the surface. in order to come up with the acoustical needs no radical new way of facade design has to be introduced. mainly a shift of attention to the acoustic parameters is needed. based on acoustic measurements of basic geometry principles this research presents known facade designs and their acoustic parameters regarding the reflection capabilities and the functions in a facade. keywords acoustics, soundscape, geometry, facades, design parameter, noise, doi 10.7480/jfde.2017.1.1422 064 journal of facade design & engineering volume 5 / number 1 / 2017 1 introduction reflections on huge facades made out of glass, steel and other hard reflective materials are increasing the noise levels in public or private spaces of urban agglomerations by redirecting the sound energy to the urban ground. a field measurement of the authors during the dismantling process of a high-rise demonstrated that the reflected sound energy can exceed the theoretical level addition of 3 db(a) up to 8 db(a) (techen, & krimm, 2014). thus in the vicinity of new or refurbished buildings equipped with hard reflective facades, the noise levels and the number of noise-affected people will increase. beside the measureable and perceivable facade effect the on going migration into major cities is leading to a growing number of noise-affected people in the official statistics of the eu guided noise mapping procedure. based on the noise map regulations defined in the european noise directive (end, the european parliament and the council of the european union, 2002) only cities with more than 100.000 inhabitants have to calculate noise maps. in the framework of the end noise map procedure the migration into major cities becomes an equivalent of a migration from non-statistically reported areas to statistically reported ones. in this growing group of the noise-affected people more and more people are harmed by more than one noise source. this increase is also linked to the migration into major cities and its growing demand for new households. in order to provide more households office or industrial buildings were converted into spaces for living. three exemplarily examples located in the city of frankfurt/main are shown in figure 1 and 2. in these examples at one receiver point two or more noise sources can be detected. receiver points in this context are positions in urban space were people walk or stay. figure 1 is showing the development area “lyoner strasse”. here abandoned office buildings are going to be converted into apartment buildings. due to the business optimized urban planning from 1962 the former called “office town niederrad” is in short distance to all important traffic infrastructures like airport, train station and motorway. the result is a projected conversion area for 3000 apartments surrounded by up to three or more heavy noise sources: the aircrafts approaching frankfurt airport, the motorway a5 and the railroad track. fig. 1 the situation around the conversion project area of “lyoner strasse” in frankfurt/main, drawing by the author. 065 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 2 new housing areas in frankfurt/main close to the railroad and flight tracks, drawing by the author. another redensification strategy is the replacement of workshops located in the courtyards by apartment buildings. figure 2 shows two conversion areas of former industrial or workshop usage. the new housing areas in frankfurt/main sachsenhausen are located very close to the main railroad tracks to frankfurt main station. the nearby flight track with aircrafts flying in 600 m above ground is contributing even more high noise levels to the urban acoustic space there. this noise source noise receiver setting results in perceivable noise levels outdoors or indoors far above the minimum comfort levels of 55 db. the data of the the federal environment agency of germany (uba) are confirming this trend of migration into noisy areas. in 2014 68 % of the people captured by the noise mapping of 2012 are affected by one or more noise sources (myck, 2014). more than half of the 68% group of these inhabitants is affected by two or up to five noise sources. see figure 3. fig. 3 people in germany affected by noise, drawing by the author, based on myck, 2014 . 066 journal of facade design & engineering volume 5 / number 1 / 2017 furthermore these facts have out-dated the classic architectural tooling like the orientation of rooms to the silent face of the building. whenever an urban space is surrounded by noise sources on street level and in the air, the concept of orientating functions like a sleeping room to the silent face of a building is not longer possible. some efforts were made to come up with solutions for buildings in relation to one noise source. among others the research conducted by l. nijs and f. kranendonk “akoestisch optimaleoriëntering van bouwmassa’s nabij verkeerswegen” (nijs & kranendonk, 1979) and “reclaiming land from urban traffic noise impact zones” from arc de ruiter can be named (de ruiter, 2005). the research of martijn lugten “re-sil(i)ence, design patterns for an aircraft noise abating spatial environment” from 2014 was focussing on aircraft noise (lugten, 2014). the ongoing broad research on soundscapes presented in the book soundscape and the built environment edited by jian kang and brigitte schulte-fortkamp is not directly linked to the architectural solution of a facade design (kang & schulte-fortkamp, 2015). a lot of research was conducted throughout the years in order to investigate and determine the influence of a facade on an urban acoustic space. but all these investigations have the limitation that they are dealing with one specific facade in a specific arrangement. a few examples should explain the problem of applying these results to the architectural needs of a metropolitan area. in the early investigations of urban spaces the focus was on the range of acoustic signals in street canyons and on the speech intelligibility over distance. among others wiener malme and gogos can be named (wiener malme and gogos, 1965). lyon investigated the influence of multiple reflections and their influence on the sound propagation in an urban space. he recommended the scaled model measuring technique as a promising tool for a precise sound propagation in three dimensions (lyon, 1973). bullen and fricke introduced the scattering on facades to their sound propagation model (bullen&fricke, 1976). picaut and simon were proofing in 2001 that a given structured facade with its reflection abilities could be replaced by pure geometry (picaut&simon, 2001). van renterghem and botteldooren are treating the green facade or green roofs in a suburban housing setup with different simulation and measurement methods (van renterghem and botteldooren, 2008, 2009, 2011). in the research of schiff, hornikx and forssén, the concept of the noise transmission between shielded canyons was simulated with numerical and measurement methods (schiff, hornikx and forssén, 2008, 2010). these research projects were investigating the acoustic and its methods and not the architectural aspect of it. the scale of the used urban situations is more linked to smaller cities as to major cities and their high-rises. furthermore all researched simulation methods except the scaled model measurement are remaining in two dimensions. from all this research a lot of proposals for investigating an urban space with several simulation or measurements methods can be drawn out but only a few recommendations for a facade design can be found. so up to now the impact of the urbanisation and the influence of the facade on the urban soundscape were neither considered seriously as architectural design parameters nor translated into an architectural language for the design of facades. 067 journal of facade design & engineering volume 5 / number 1 / 2017 2 case studies in the following two case studies the attempt was made to investigate the influence of modified facade geometries to the noise impact on the surrounding urban space. the scale measurement method based on the method of dimensional analysis by lord rayleigh was used in this geometry study (rayleigh, 1915). with this method it became possible to scale measuring setups down to a smaller size applying the same dimension factor for scaling the setup and the signal to be measured. the first scale model investigations in engineering were used in the middle of the 18th century for the analysis of rivers and bridges. later on the method of scale modelling became common for the development of aeroplanes, cars, ships, bridges, and concert halls. scale measurements are widely used in industry and research because they facilitate testing the impact of changed shapes or changes in size of downscaled elements, thus saving time and resources. in a 1:1 scaling it is virtually impossible to change for example the whole construction of a bridge over a valley in order to select the construction which is delivering a better performance due to airflow in this valley. the method of scale model engineering used for the acoustical investigations was developed with recommendations and formulas of d.j. schuring (schuring, 1977). when setting acoustical measurements of an existing urban situation, the building layout has to be scaled by the same factor as the wavelengths of the audio signal emitted from the source. if the building layout is scaled down by factor 10 the frequency has to be scaled up by factor 10 to achieve a wavelength scale down of 1:10. limiting factors to scaling in acoustical measurements are laboratory space and threshold frequencies of the equipment. in order to focus on pure geometry all scale model surfaces were made out of hard reflective materials. all scaled measurements done in the framework of this research were focused on pure geometry because one important limitation of the scaled model measurement method is difficulty of downscaling material properties. nevertheless the measurements and their results stay reliable because whenever a hard reflective geometry is reducing noise levels an introduction of absorbing material is always improving the acoustical performance. both cases were measured in a scaled measurement set-up and in a field measurement. as the case studies should represent the daily practice in engineering or architectural offices the approach to an acoustic facade intervention in both case studies was different. in the case study lyoner strasse 54 mainly design decisions led to a facade design. the possible acoustical qualities were considered in a second step. opposite to this the facade modifications of the henninger turm study were developed for identifying the acoustical effect of adding horizontal or vertical structures to the south facade. 068 journal of facade design & engineering volume 5 / number 1 / 2017 2.1 case study lyoner strasse 54 fig. 4 the abandoned office building in the lyoner strasse 54, picture by the author. in this study, the urban acoustic space around an eight-storey office building was investigated. the building is located on the south part of the lyoner strasse in frankfurt/main, see figure 4. the 100 m long building is orientated nearly perpendicular to the arrival flight track of frankfurt airport. refer to figure 5. in the measurement set-up in the facilities of the german federal research institute for roads and traffic (bast) a 1:100 downscaling of the existing situation was built. the model of the building with the facade modification is shown in figure 6. in this case the facade modification changes the plain hard reflective surface of the model into a triangulated form of facade geometry. the surface quality of the facade modification regarding the acoustics remains hard reflective. the positions of the six measurement points were defined in order to detect the edge effect at the corners of the building in difference to the measureable effect in middle of the 100 m facade of the building. with having on both sides three measurement points it will be possible to determine the emerging effect according to the building face. refer to figure 7. 069 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 5 drawing of the urban space and its noise sources around lyoner strasse 54, drawing by the author. fig. 6 measurement model in grey colour with attached facade modification in white, picture by the author. fig. 7 position of the measurement points around the building lyoner straße 54, drawing by the author. the moving aircraft in reality was replaced by an air pressure noise source in the scaled measurement. because of spatial limitations of the measurement room the original distance of the airplanes to the building has to be shortened. the air pressure noise source was mounted on a moving track system in a height of 150 cm above ground and in a distance of 261 cm to the building in order to meet the geometrical conditions of the existing situation. with this equipment it was possible to measure appearing noise levels at the predefined points around the building. the moving track of the noise source in conjunction with the measurement system delivered a set of frequencydistributed levels in the range between 100 hz and 2000 hz at every 1 cm. the single db values for every point on the moving track were then calculated out of the frequency-distributed levels. the measurements were proceeded with the facade modification and repeated without. subtracting the measured levels without facade modification from the captured data with the facade modification delivers the resulting level change. the result for the facade modification at measurement point west2 and ost2 is shown exemplarily in the graphs in figure 8. 070 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 8 measurement results for lyoner strasse 54 linked to the airplane position, measurements of the author at the facilities of the bast. the graph in figure 8 is impressively depicting that around a solitaire building an effect of a facade modification is delivering level changes more than the theoretical 3 db. for the point t2 the level change at the measuring position ost2 is -7 db. the measurement data give evidence for the possibility of reducing noise levels during an aircraft noise event around a freestanding building. 2.2 case study henninger turm the urban plot around the henninger turm in frankfurt/main was taken as a basic layout for an investigation of facade modifications and their impact on the urban acoustics in the vicinity of a building. measurement and noise source positions are defined by the field measurement positions from 2013 (techen, & krimm, 2014). as there was no moving noise source in the measurement facilities available for the simulation of a flying airplane, the measurements were done using a ring radiator as a point source in two positions. the two positions of the point noise source were representing two points in time of the moving airplane on the flight track. the noise source position “on axis” was perpendicular to the south facade of the tower. the “off axis” noise source position was angled 19 degree away from the normal of the south facade. for an overview of the measurement points and the layout of the set-up refer to figure 9. 071 journal of facade design & engineering volume 5 / number 1 / 2017 h enninger turm o ffic e b uilding p o 1 a c tiv e s ound s ourc e pos ition po2 po1 a c tiv e s ound s ourc e pos ition p o 2 h enninger turm o ffic e b uilding flight track flight track modified facade modified facade pos 1/mp 2 pos 2 pos 3/mp 1 pos 4 microphone positions pos 1/mp 2 pos 2 pos 3/mp 1 pos 4 microphone positions mea s urement “ o n-a x is “ mea s urement “ o ff-a x is “ fig. 9 measurement positions (measurement points) in the scaled measurement setup, drawing by the author. fig. 10 measurement set-up in a 1:50 down scaling at the measurement facilities at tu delft, picture by the author. the measurements were taken in the facilities of the laboratory of acoustical wavefield imaging, department of imaging physics at the delft university of technology by the author. the measurement model in the scale of 1:50 consists out of planed beech wood blocks representing the building on-site of the field measurement around the henninger turm. in figure 10 the set-up in the anechoic chamber at tu delft is shown. on the plain facade of the tower model smaller beech wood blocks could be added in different configurations. for this row of measurement sequences the facade modifications were classified in horizontal oriented modifications and vertical oriented modifications. in these two classes, the implemented variation of density in the arrangement of blocks results in varying sizes of the front face area. additional to that, blocks with one tilted face were used in the class of the horizontal arrangement gaining for an insight on the effect of downward or upward reflection. the sizes of the beech wood blocks were representing facade modifications in the dimension of 0.5 m x 1.5 m x 3.0 m or 1.0 m x 1.5 m x 3.0 m. figure 11 shows the measured facade variations and their abbreviations. 072 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 11 list of the measured surface modifications, drawing by the author. aiming for a more detailed view on the frequency distributed noise levels and the noise coverage of the urban space four measurement points in front of the facade modification were introduced in this setup. with these four points the measurement data was evaluated due to the frequency-distributed level in each measuring point. as the frequency-distributed levels are not so clear readable in terms of the acoustical impact of a facade modification the method of calculating single values out of frequency-distributed levels was used. the average frequency distributed noise level of each of the four measuring points was calculated out of the frequency distributed noise level values of the single measurement points. in order to obtain insight on the effect of a facade modification the change of noise levels was calculated by subtracting the measured level values of the modification from the level values of the reference model ref01 measured in the sequence. the change of noise levels in relation to the reference model was calculated for average noise level for the validated frequency range of 25 hz to 630 hz in one microphone position and in all four microphone positions. the results for the measurement sequence of the facade modifications “ref02 v1 v3” are shown exemplarily in the following. the graphs are showing results from two defined arrangements of noise source and measurement position. if the noise source is on axis with the active measurement point the measurement is defined as a “on axis” measurement. whenever the measurement position is not in the axis of the noise source to the measured object, the measurement is defined as “off axis”. calculating the average out of the frequency distributed level data set for each measuring point delivers a single value on the broadband weighting of one surface modification regarding one measuring point. the results are showing for the “on axis” measurement a level reduction from 0,25 db up to 0,5 db. the values from the “off axis” measurement are detecting a maximum level change to the reference model from 0,15 db, refer to figure 12. 073 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 12 level changes for the facade modifications ref02 v1, refo2 v2 and ref02 v3. measurements on-axis and off-axis conducted by the author. the measurement with the noise source positioned on axis indicates level changes for the averaged noise level of the validated frequency range up to -0,6 db. the values derived from the measurement with the noise source located off axis results in an average noise level change in the range of -0.1 db for the three surface modifications. data of all four positions were used to obtain information on the noise coverage of the area in front of the south facade. therefore the four averaged frequencydistributed levels of each microphone position were averaged resulting in one level value for each surface modification within the validated level range of 25 hz to 630 hz. the result draws an oppositional picture to the averaged value for one microphone position. each surface modification is increasing the level when taking all four points into account, refer to figure 13. the data of this geometry design study represented in the graphs is indicating that the effects of surface modifications on a facade are located in spots. at the locations of the spots a level change of 3 db can be measured. putting this into the perspective of inspecting all four measurement points simultaneously the effect switches from a level increase of 0,6 db to a level decrease of 0,2 db. remarkable is here not the small level change below 1 db but the switching from a level decrease to an increase when the whole area of the four measurement points was taken into account. only with an evaluation of all measurement data in a table a “best modification” can be detected. see table 1. 074 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 13 averaged noise level values for positions 1-4, measured on-axis and off-axis. f a c a d e m o d if ic a t io n o r ie n t a t io n d e n s it y m a x im u m l e v e l c h a n g e f r e q u e n c y o f m a x im u m l e v e l c h a n g e a v g . l e v e l c h a n g e o n a x is f o r 2 5 6 3 0 h z a v g . l e v e l c h a n g e o f f a x is f o r 2 5 6 3 0 h z a v g . l e v e l c h a n g e in f o u r p o in t s o n -a x is f o r 2 5 6 3 0 h z a v g . l e v e l c h a n g e in f o u r p o in t s o f f -a x is f o r 2 5 6 3 0 h z db hz sp1/mp 1 sp1/mp 2 sp 1/mp1 sp2/mp2 ref02 v1 hor. 1 -1.9 db 160 hz -0.3 db 0.0 db +0.2 db +0.1 db ref02 v2 hor. 2 -2.6 db 400 hz -0.4 db 0.0 db +0.2 db 0.0 db ref02 v3 hor. 3 3.0 db 400 hz -0.6 db -0.1 db +0.4 db + 0.1 db ref03 v1 hor. 1 -1.9 db 200 hz -0.4 db 0.0 db +0.3 db +0.1 db ref03 v1u hor. 1 -3.2 db 250 hz -0.2 db 0.0 db +0.3 db +0.1 db ref03 v2 hor. 2 -3.5 db 250 hz -0.4 db 0.0 db +0.3 db +0.1 db ref03 v2u hor. 2 -4.5 db 250 hz -0.9 db 0.0 db +0.5 db +0.1 db ref03 v3 hor. 3 -3.2 db 250 hz -0.4 db 0.0 db +0.4 db +0.1 db ref03 v3u hor. 3 -4.9 db 250 hz -0.9 db 0.0 db +0.5 db +0.1 db ref04 v1 vert. 1 -1.9 db 200 hz -0.2 db +0.6 db -0.2 db -0.3 db ref04 v2 vert. 2 -2.4 db 160 hz -0.2 db +0.5 db -0.2 db n.a. ref04 v3 vert. 3 -2.7 db 160 hz -0.4 db +0.5 db -0.2 db -0.4 db ref04 v4 vert. 4 -4.4 db 400 hz -0.5 db +0.5 db 0.0 db -0.3 db ref04 v5 vert. 5 -3.5 db 400 hz -0.4 db +0.5 db -0.1 db -0.4 db ref05 v1 hor. 1 -1.8 db 400 hz -0.2 db 0.0 db +0.1 db 0.0 db ref05 v2 hor. 2 -3.3 db 400 hz -0.6 db 0.0 db +0.2 db 0.1 db table 1 the calculated results of the measured sequences 075 journal of facade design & engineering volume 5 / number 1 / 2017 3 conclusion both case studies are pointing out that acoustical design of facades has to be individually measured and evaluated. the use of the scale measurement method gives the possibility to engineers and architects to introduce geometric surface modifications to a building design process whenever an acoustical approach is needed. the measurements can be introduced according to the design stage of a project. with the scaled measurement method it is possible to investigate geometric details of an acoustic design in a 1:10 downscaling as it is possible test a building geometry with 1:100 models. the effects on the urban space in relation to the facade can be predicted and tuned to a complete coverage without an overall decreased noise level. if we want to come up with the challenge of creating a lively and comfort environment in the process of an increasing urbanisation the tool of scaled measuring has to be introduced to architectural design processes in order to define the geometrical basis for acoustically comfortable spaces. the even more remarkable outcome of the case studies is that all observed level changes were caused only by a change of geometry. no material properties were yet involved in the studies. all achieved level changes in both case studies could be optimised in further developments. the results of acoustical effective facades could be tuned by introducing specially shaped perforations of the facade surface or by adding materials with acoustical properties to the building envelope. references techen, h., krimm, j., (2014) akustische fassaden, fortschritte der akustik daga 2014, deutsche gesellschaft für akustik e.v. (dega), berlin, pp 846-847 european noise directive, (end) directive 2002/49/ec of the european parliament and of the council of 25 june 2002, the european parliament and the council of the european union, official journal l 189 , 18/07/2002 p. 0012 0026, myck, t., (2015). die lärmsituation in deutschland nach der umgebungslärmrichtlinie (conference contribution). retrieved from https://www.dega-akustik.de/fileadmin/dega akustik.de/dega/aktuelles/symposium/9_symp_myck.pdf, p 15 kranendonk, f & nijs, l. (1979) akoestisch optimaleoriëntering van bouwmassa’s nabij verkeerswegen, delft: z. uitg, de ruiter, e. p. j., (2005) reclaiming land from urban traffic noise impact zones. delft: university of technology, delft, 2005 lugten, m., (2014) re-sil(i)ence, design patterns for an aircraft noise abating spatial environment. delft: university of technology, delft kang, j., schulte-fortkamp, b., (2015) soundscape and the built environment, crc press, taylor & francis group, llc wiener, f., m., malme, c., i., gogos, c., m., (1965) sound propagation in urban areas, j. acoust. soc. am. 37, 738 (1965); lyon, r., h., (1974) role of multiple reflections and reverberation in urban noise propagation, j. acoust. soc. am. (1974), pp. 493–503 bullen, r., fricke, f., (1976) sound propagation in a street, journal of sound and vibration, volume 46, issue 1, 8 may 1976, pages 33-42 picaut, j., simon, l., (2001), a scale model experiment for the study of sound propagation in urban areas, applied acoustics 62 (2001) 327±340 van renterghem, t., botteldooren, d., (2008) numerical evaluation of sound propagating over green roofs, journal of sound and vibration, volume 317, issues 3–5, 11 november 2008, pages 781–799 van renterghem, t., botteldooren, d., (2009)reducing the acoustical façade load from road traffic with green roofs, building and environment, volume 44, issue 5, may 2009, pages 1081–1087 van renterghem, t., botteldooren, d., (2011) in-situ measurements of sound propagating over extensive green roofs, building and environment, volume 46, issue 3, march 2011, pages 729–738 m. hornikx, j. forssén,(2008) noise abatement schemes for shielded canyons, applied acoustics 70 (2009) 267–283 m. hornikx, j. forssén,(2010) excess attenuation for sound propagation over an urban canyon, applied acoustics, volume 71, issue 6, june 2010, pages 510–517 rayleigh, j.w.s.,(1915) the principle of similitude, nature 95, no.2368, nature publishing group, retrieved from: http://www. nature.com/nature/journal/v95/n2368/index.html, at 19-03-2016, 22:00 schuring, d., j., (1977) scale models in engineering, pergamon press, oxford, from city’s station to station city 113 journal of facade design & engineering volume 5 / number 2 / 2017 an innovative app for a parametric, holistic and multidisciplinary approach to early design stages mattia donato, giovanni zemella, gianluca rapone, jak hussain, conor black ove arup & partners, arup façade engineering (afe), 13 fitzroy street, london, w1t 4bq, uk, +44 20 77554344, mattia.donato@arup.com abstract during early project stages, design teams need to explore a wide range of possible envelope configurations in order to identify those that best address the project constraints and objectives. crucial aspects such as control of solar gains, use of blinds and renewable energy production are typically the subjects of extensive discussions among architects and facade, mechanical, electrical and ph engineers. traditional methodologies used to inform the design on such matters are neither flexible nor time efficient, failing to meet the expectations of the team. arup solar is an innovative app developed to overcome such inefficiencies and to provide a user-friendly way to aid the discussion between architects and engineers. the validated app aims to investigate the relationships between envelope features (e.g. window to wall ratio, g values, etc.) and cooling strategies, as well as identify potential opportunities for renewable solar energy production. it allows for the exploration of a large number of design options instantaneously, visualizing results by mapping them on the 3d model of the building. the process of building any instance of the app includes a first step where the nurbs modeler rhino/grasshopper is utilized to run a radiance & daysim solar analysis on any complex geometry. the resulting data (on each surface mesh) is then exported to the unity gaming engine, where a set of pre-programmed features is automatically implemented and the graphic interface is created. the outcome is a stand-alone parametric application that can be potentially run on any device. keywords total architecture, software development, design tool doi 10.7480/jfde.2017.2.1739 114 journal of facade design & engineering volume 5 / number 2 / 2017 1 introduction decisions made by the design team during early project stages can have a big impact on the final energy performance of the building (granadeiro et al., 2013; negendahl et al., 2015), as well as on its appearance and operation.during the concept stage, design teams typically need to explore a wide range of possible facade options, so that they can feel comfortable that the chosen solutions work within the project constraints. in spite of these demands, the use of building performance simulation in the architectural design process is hindered by three key obstacles (zemella et al., 2014): 1 the resources required to create building models for energy simulation; 2 the time needed to accurately run and validate simulations on different geometrical options; 3 the lack of simple and straightforward communication of results. consequently, in a typical workflow, consulting engineers only simulate buildings later on in the design process, in order to verify a frozen design (greenberg et al., 2013). the downside of this practice is that the amount of time and resources required to assess the impact of changes in parameters including, but not limited to, architectural configuration or mechanical systems, is large. the process is inflexible and does not allow for informative iterations. the typical software tools employed for building energy simulation (bes) such as equest, ies-ve, design builder, openstudio are often used through extrusion of prismatic elements, which are in contrast with the current architectural trends. these models are employed later in the design process and are aimed at a much larger set of analyses. other software tools are usually limited to solar incident radiation assessments due to the slow computational engine. other examples such as sustain (greenberg et al., 2013) are still at the cutting edge of research projects, not yet ready for the market. finally, many of these tools do not offer enough customization, computational precision or transparency. there is a demand for software tools that help and support the whole design team during early design processes, in order to develop true performance-driven solutions. reliability, flexibility, clarity and visual appeal are some of the key features of a software solution for this purpose. these new tools should then act as a platform on which the different members of the design team such as architects, facade engineers, mechanical engineers, electrical engineers and public health engineers can work together, saving time and resources, and striving for the real “total architecture” (arup, 1970). one aspects that has a significant impact on facade design is the control of solar gains through the transparent portion of the envelope. this is typically achieved by varying window to wall ratio and glazing g-value. additionally, shading features of different types are often employed to mitigate gains where there is high solar exposure. moreover, in order to achieve challenging objectives, such as the (nearly) zero energy building, a significant amount of the energy demand of a building should be provided by renewable sources, including energy generation on site or nearby (european parliament, 2010). the sooner these aspects are considered, the more likely it is that facade solutions that are aligned with the architectural intent can be found. with regard to the above parameters, the exploration of the options should be facilitated by appropriate tools in order to inform the design in a proactive way. such tools should be able to manipulate a large amount of data, enabling the design team to derive clear indications and results. however, there is a gap in the typical early stages workflow. there is a lack of tools available to adequately support the whole design team in defining the overall building energy performance strategies. the work presented in this paper was developed with the aim of providing such a tool. arup solar is a new innovative app that supports the design team in early stage development to overcome the inefficiencies described above. 115 journal of facade design & engineering volume 5 / number 2 / 2017 2 arup solar 2.1 previous experiments after testing available software tools for handling complex geometry, solar simulation and interactive data visualization, the conclusion was that none could meet the requirements described in the previous sections. a number of experiments were carried out, including the combination of ecotect results with an interactive data visualizer created by java scripting, and the creation of a tailored tool in excel to post process and interactively combine results from energy plus and radiance. the downsides of these experiments were as follows: 1 the large amount of resources required to create the tools; 2 the long computational time to run the simulation because: a simulations with good sky resolution at the building scale take time; b scripts had to be developed to run energy plus and radiance automatically more than 600 times and those analyses were limited to top floors without any obstructions; 3 the expertise required to use the tools; 4 the results were not expressed in a graphic, intuitive way and required extensive post-processing. grasshopper standardized the direct link between geometry (rhinoceros) and bes (energy plus and radiance) to break a first barrier to parametric assessments. however, despite the powerful visualization, grasshopper was not able to handle such large amounts of data and interactively visualize them. an attempt was made to put results into layers where information was stored and the design team could visualize the results by means of rhinoceros, but the process was neither flexible nor efficient. another possible way was identified and is described in the following paragraph. 2.2 identification of adequate tools as mentioned earlier, the starting point was that an efficient approach to early design stage assessments was required to understand how solar radiation on the facades affects the architectural configuration, the mechanical system and the renewable energy fields. following an analysis of how the main software tools for solar analyses worked, a combination of radiance & daysim was considered to be the most flexible and reliable solution for such applications. in addition, radiance & daysim can be run through the nurbs modeler rhino/grasshopper. this allows the geometry to be linked to radiance & daysim in a few steps by means of specific plug-ins such as ladybug and honeybee. however, radiance & daysim on their own do not allow users to derive clear design indications as their output is limited to radiation values and they do not offer an adequate interactive interface. moreover, as stated above, grasshopper does not offer sufficient computational power to post-process a large amount of data in real time. these points directed research towards more suitable engines that could overcome such limitations. unity was found to be the right software for this use. unity is a game engine widely known for its accessibility. its unique nature allows a fast and responsive software that combines advanced calculation methods and rendering techniques. the algorithms were written using c# language in order to take advantage of its powerful parallel processing capabilities. figure 1 offers an overview of the tools required to build the app. the novel use of a game engine as design tool is one of the unique features of arup solar. the next paragraph explains the workflow used to develop the final app. 116 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 1 required tools to create the app (authors) 2.3 app framework the identification of this suitable software allows users to define the steps and the information to be exchanged through the different platforms. figure 2 shows the main steps required to create the app, which are summarized as follows: 1 create the geometry (step 1, figure 2); this step is very important as the rationalization of the model is fundamental in order to run a successful simulation; 2 run the incident solar radiation analysis only once (step 2, figure 2); this step allows the user to convert all the rhinoceros scenes in a radiance & daysim file and assess the solar incident radiation on the geometry under investigation by defining the main radiance parameters; 3 create the app (step 3, figure 2); when the simulation ends, the whole set of outputs is automatically exported from grasshopper via scripting. the files are then copied into a unity project where all the information is automatically post-processed. one challenge was to overcome the interoperability between the different software used, as they do not read the same file formats and protocols. this led to the introduction of tailored scripts to derive the suitable outputs from grasshopper to recreate the scene along the numerical results. the final step (step 4, figure 2) is to run the app on any device as unity allows the user to choose the operating system on which the app will run (microsoft, mac, android, linux…). 117 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 2 workflow to create the app. 2.4 validation arup solar is a tool that post processes solar incident radiation values. a validation of the tool can be carried out by comparing the radiation levels derived from radiance & daysim (honeybee and ladybug version 0.060 and 0.064) with the results provided by another validated software. energy plus version 7.2.0.006 was identified as reliable software to carry out this validation. a previous validation of radiance & daysim using grasshopper and energy plus was carried out by waibel et al. (2016), showing good agreement on general trends. a further validation was carried out by the authors using a building in central london as case study. (this is described in more detail in section 2.6). the image below (fig. 3) shows the building (in black) and the urban context (in light grey). fig. 3 case study analized (real model) 118 journal of facade design & engineering volume 5 / number 2 / 2017 the validation focused on three representative bays (6m x 3.5m), with different orientations and levels of obstruction as described in the following set of images. the images below (fig, 4 and fig. 5) show representations of a shading mask of the surface and localization of the assessed bay. a shading mask represents a matrix of azimuth and altitude angles at which the percentage shading for the assessed surface was determined (autodesk®, 2010). ecotect analysis 2011 was employed to assess the shading mask of the bays. equidistant sun path polar projections overlap the shading mask to correlate the position of the sun with the shadows due to the obstructions. the key represents the percentage of surface that is shaded. fig. 4 west elevation, shading mask and localization of the assessed bay. fig. 5 south elevation, shading mask and localization of the assesed bay. 119 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 6 east elevation, shading mask and localisation of the assessed bay. the simulation was carried out for a typical year. the weather file used was the design summer year for london (dsy) by cibse (chartered institution of building services engineers). radiance & daysim were run by means of grasshopper and energy plus directly from the latter’s own interface (.idf). the following radiance & daysim parameters were set within the subroutine of grasshopper to perform the simulation: 1 analysis grid = 1.5m x 1.5m; 2 quality = 2; 3 ambient bounces = 6; 4 ambient division = 2048; 5 ambient super sample = 1024; 6 ambient resolution = 256; 7 ambient accuracy = 0.10; 8 site ground reflectance = 0.2. the following parameters were set within the .idf of energy plus: 1 solar distribution assumed as “full interior and exterior with reflection”; 2 calculation frequency = 30 days; 3 maximum figure in shadow overlap calculation = 15000; 4 algorithm for polygon clipping = “sutherland hodgman”; 5 algorithm for sky diffuse modeling = “detailed sky diffuse modeling”; 6 site ground reflectance = 0.2. the correlations between the results obtained from the two models are shown in the graphs below (fig.7, fig. 8 and fig.9). it should be noted that the night hours were not considered to increase the accuracy of the calculations. 120 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 7 correlation west elevation fig. 8 correlation south elevation 121 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 9 correlation east elevation west elevation south elevation east elevation unit r 0.99 0.98 0.97 r2 0.98 0.97 0.95 rmse 25 28 24 wh / m2 δpeak -6.4 -4.8 -15.8 % δcumul. -3.0 1.6 1.5 % table 1 results of the validation table 1 summarizes the correlation coefficient r, the coefficient of determination r2, the root mean squared errors rmse and the difference in terms of % between the two engines (peak levels δpeaks and cumulative levels – δcumul.) found. the results show a good correlation between the two models, with a reasonable fit for most of the orientation. some differences were expected due to the different approaches used to simulate solar radiation adopted by the software. discrepancies were found when the facade bays experience of high levels of shading (i.e. the bay on the east elevation, during early hours of the day in the middle season march and april). the post-processed data was then used to assess solar gains, extra shading requirements and potential for new solar renewable energy as described in the following paragraphs. all of the algorithms were based on validated mathematical models and were developed to provide conservative results to reflect the accuracy of the input typically available during the early design stage. 122 journal of facade design & engineering volume 5 / number 2 / 2017 solar gain was calculated with the static equation in accordance with cibse technical memorandum 37:2006. pvs were described referring to the inoct method (skoplaki et al., 2008) taking into account the overheating of the cell, the mounting typology and the real hourly efficiency of the system (duffie et al., 2013). solar thermals were described by an implicit euler method, which is a first order approximation, accurate in time and unconditionally stable (diorio et al., 2014). the efficiency of the panel was defined by a second order equation function of panel temperature and current incident radiation level (quaschning, 2016; o’hegarty, 2016). in order to increase the accuracy of pv and solar thermal outputs, backwards ray-tracing tracked the relative angle between the surface normal and the incident solar radiation. this allowed for consideration of the angle-dependent optical properties of the glass (u.s. department of energy, 2016) and the angle modifier factor (duffie et al., 2013). the following paragraphs show the core functionalities of the app and describe the case study in detail. 2.5 app functionalities once the app is created it can be run on any platform (unity does not have to be installed on the machine). thanks to pre-coded post processing functions of the solar radiation data, the users can explore different design options. the features offered can: 1 visualize the incident solar radiation (hourly values, cumulative and peak values, in accordance with percentile and/or a certain period of the year); 2 calculate the solar gains according to glazing percentage and g-value, or visualize any of the three variables above as a result of the combination of the other two; 3 extra shading requirements to achieve solar gain target: hourly data plus performance indicators (e.g. amount of hours that extra shading features are typically required per day for each month); 4 estimate the amount of energy that can be produced by means of bipv (buildingintegrated photovoltaic); 5 evaluate the amount of energy that can be produced by means of bist (building-integrated solar thermal system) for hot water and hvac utilities. the final result is an app where the data is visualized. the graphical user interface (gui) presents a horizontal bar that shows core functionalities and a vertical bar with the settings (figure 10). by selecting the buttons, it is possible to visualize: 1 gi, general information (assumptions for the calculations); 2 tsp, toggle sun path (on/off); 3 rm, recenter model; 4 fd, filtering of data (certain hours, months); 5 s, solar gains calculation – the default method assesses the solar gains on 4.5m of perimeter floor area but this can be changed with different floor depths. in addition, instead of assessing them based on floor area, solar gains can be also calculated on facade area; 6 m, geometrical model; 7 ir, incident radiation; 8 sg, solar gains; 9 b, extra shading requirements; 10 pv, photovoltaic calculation; 11 st, solar thermal calculation. 123 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 10 buttons to chose functionality to visualize (top horizontal tab) or general setting (vertical bar). the functionalities are described in more detail in the following section, which describes a case study where the app was tested. 2.5.1 case study arup solar was tested using real project applications during the development of the current version. one of the projects where the tool was employed was a new development in central london where a new office building will be built. the building was characterised by two volumes and surrounded by other buildings. the ambition of the design team was to have low energy requirements, a high percentage of glazing and no external shading devices. the facade design had to reach a compromise among a number of conflicting requirements. the context of the surrounding buildings played a key role in the identification of the areas where clearer glass or more glazed areas were possible. in addition, by allowing more communication within the design team, the tool could save time and avoid misunderstandings, making the design decision process smooth and fast. 2.6 results of the case study once the calculation was finished, the data was loaded and the .exe file selected. the first tab of the app showed the model geometry of assessed scene with the buttons as presented in figure 10. a common feature of all the outputs is that by hovering over a mesh, specific information about that selected function is displayed. by selecting the incident radiation mode, the incident solar radiation (hourly values, cumulative and peak values in accordance with percentile and/or a certain period of the year) for each mesh or for the whole building is visualized (fig. 11). in figure 11, with the option “peaks” being selected, the moments when the peak of incident radiation occurs and the intensity can be shown. 124 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 11 incident radiation section fig. 12 solar gains section in the solar gains mode, the solar gains according to glazing percentage and g-value can be calculated. alternatively, any one of the three variables (i.e. glazing percentage, g-values and target solar gains) can be visualized in relation to the other two. (fig. 12). figure 12 shows a particular setting of g-value of the glass, percentage of window / wall, and depth of the facade zone. these can be easily changed by moving the corresponding sliders. in the extra shading requirements mode, it is possible to identify the amount of time when additional shadings are required. the users need to input the solar gain target, the glass g-value and percentage of window. in particular, when the use of extra shading in the form of blinds is required, clients typically want to know how often these will be deployed. clear indicators support client’s decision by visualizing the amount of time when blinds will have to be used (average daily usage per month and specific daily usage) (fig. 13). 125 journal of facade design & engineering volume 5 / number 2 / 2017 fig. 13 extra shading requirement section fig. 14 pvs sections (authors) in the sections renewable energy, pvs and solar thermal (st), the deliverable energy during a certain period can be assessed. both these functionalities present algorithms based on relative angles of incident radiation, processes based on backwards ray-tracing and hourly temperature of the panels. figure 14 shows pvs mode where the deliverable energy is displayed. 126 journal of facade design & engineering volume 5 / number 2 / 2017 3 results and discussion this paper describes an innovative app for a new approach to early design stage analyses that relies on interactive, flexible and user-friendly tools. arup solar satisfies a requirement in the early stage workflow. the app is built from the model geometry by means of rhinoceros/grasshopper that allows the link with radiance & daysim. for a specific geometry, the radiation analysis is required to run only once and unity is used to post-process the whole set of results (substantially reducing computational times). the process to create the app is straightforward and fast. in the case study, approximately 90% of the processing time was for the computation (step 2, figure 2) of the solar incident radiation and the rest was for geometry creation and app creation (steps 1-3, figure 2). for more complex geometries, the time required to prepare the model could be longer. the innovative use of game engine as a design tool improves the decision-making process with clear results mapped on the 3d model of the building. the current tool is already powerful, stable, and quick. furthermore, it allows the testing of an indefinite number of options (e.g. between g-value, window / wall ratio, target of solar gains / extra shading requirements). the app also enhances the awareness of potential renewable energy generation through envelopes. some developments to improve the app in the near future have already been planned, such as increasing the flexibility based on users’ feedback and functionalities. in the mid to long term, the app will be adapted for later design stages, so that the same approach can follow the development of projects. thanks to its flexibility, the app can easily become a tool where architects and engineers (facade, building services, electrical and ph) work together so that holistic design can become straightforward to achieve. 127 journal of facade design & engineering volume 5 / number 2 / 2017 references arup, o., (1970). the key speech. arup digital publication. retrieved from: http://publications.arup.com/publications/o/ ove_arups_key_speech autodesk®, (2010). autodesk® ecotect® manual. shading masks. san rafael (united states). cibse, (2006). technical memorandum tm 37, design for improved solar shading control. london (united kingddom). retrieved from: http://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008i7elaac diorio, n., (2014). technical manual for the sam solar water heating model. national renewable energy laboratory (u.s. department of energy, office of energy efficiency and renewable energy). retrieved from: https://sam.nrel.gov/system/tdf/ simplesolarwaterheatingmodel_sam_0.pdf?file=1&type=node&id=69521 duffie, j.a., beckman, w.a., (2013). solar engineering of thermal process. john wiley & sons, hoboken (new jersey). retrieved from: http://eu.wiley.com/wileycda/wileytitle/productcd-0470873663.html european parliament, (2010) energy performance of buildings directive 2010/31/eu. official journal of the european union. retrieved from: eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj:l:2010:153:0013:0035:en:pdf granadeiro v, et all. (2013). building envelope shape design in early stages of the design process: integrating architectural design systems and energy simulation. elsevier, automation in construction. retrieved from https://www.researchgate.net/ publication/257371570_building_envelope_shape_design_in_early_stages_of_the_design_process_integrating_architectural_design_systems_and_energy_simulation greenberg, d., et al. (2013). sustain: an experimental test bed for building energy simulation. energy and building. retrieved from . //dx.doi.org/10.1016/j.enbuild.2012.11.026 negendahl k, (2015). building performance simulation in the early design stage: an introduction to integrated dynamic models. automation in construction. retrieved from: https://www.researchgate.net/publication/274736654_building_performance_ simulation_in_the_early_design_stage_an_introduction_to_integrated_dynamic_models o’hegarty, r., et al. (2016). review and analysis of solar thermal facade. elsevier, solar energy. retrieved from: http://www. sciencedirect.com/science/article/pii/s0038092x16301852 quaschning, v., (2016). understanding renewable energy systems. routledge, new york (united states). skoplaki, e., et al. (2008). operating temperature of photovoltaic modules: a survey of pertinent correlations. elsevier, renewable energy. retrieved from http://www.sciencedirect.com/science/article/pii/s0960148108001353 u.s. deparment of energy. (2016). energy plustm version 8.6 documentation. engeneering reference. national renewable energy laboratory (u.s. department of energy). retrieved from: https://energyplus.net/documentation waibel, c. et all. (2016). using interpolation to generate hourly annual solar potential profiles for complex geometries. proceedings of bso 2016. newcastle (united kingdom). retrieved from: http://www.ibpsa.org/?page_id=797 zemella, g., et al. (2014). evolutionary optimisation of facade design. a new approach for the design of building envelopes. springer-verlag. london (united kingdom). retrieved from: http://www.springer.com/us/book/9781447156512 journal of facade design and engineering 3 (2015) 253–272 doi 10.3233/fde-160044 ios press 253 annual daylight simulations with evaldrc – assessing the performance of daylight redirection components carsten bauer∗ and stephen wittkopf cc ease, lucerne university of applied sciences and arts, horw, switzerland abstract. evaldrc is a newly developed daylight analysis tool for the evaluation of daylight redirecting components (drc) in architectural spaces. it focuses on the accurate simulation of light redirection with help of the lighting software environment radiance. it employs various key technologies, among them are: a) the daylight coefficient method, b) characterisation of the light redirection behaviour of materials and specially designed systems with appropriate data models, and c) daylight metrics. we present several enhancements to these key technologies and the currently existing tools. in the context of daylight coefficients, we improve the solar contribution calculation by using realistic 0.5◦ solid angle sun primitives, thus generating true sun coefficients. for simulating light redirection behaviour, we introduce contribution photon mapping, a recent add-on to the radiance environment. in addition, we introduce monthly breakdowns of the established daylight metrics spatial daylight autonomy (sda) and annual sunlight exposure (ase), to provide a more detailed assessment of drc performance throughout the course of a year. the paper gives an overview of the mentioned annual daylight simulation key technologies. it also explains how our enhancements and developments surpass the current approaches and lead to a versatile tool, capable of producing meaningful and detailed simulation results. a description of the implementation and an application example is given, rounded off by a discussion of the current state of the on-going work and a tentative outlook. keywords: cbdm, cfs, daylight redirection, daylight metrics, bsdf, photon mapping 1. introduction daylight redirection components (drc) as part of complex facade or fenestration systems (cfs) can play an important role in reducing the amount of energy used for artificial lighting by increasing the daylight levels in the interior space. however, a quantitative analysis of the impact of drcs on the interior lighting levels is a difficult task. due to the dynamic nature of daylight, the typical static illuminance calculations used in artificial lighting design only provide little information about the performance in the annual average. additionally, drcs exhibit complex light reflection, transmission, redirection and scattering characteristics, which demand elaborate techniques for producing reliable simulation results. mathematical models for the generation of sky radiance distributions based on local weather data are needed to provide a representative input for the general amount of available daylight at a specific location. the lighting simulation step finally has to be followed by suitable data reduction mechanisms, to convert large annual data sets into some meaningful figures describing the useful interior daylight availability. ∗corresponding author: carsten bauer, cc ease, lucerne university of applied sciences and arts, 6048 horw, switzerland. tel.: +41 41 349 36 47; fax: +41 41 349 39 60; e-mail: carsten.bauer@hslu.ch. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:carsten.bauer@hslu.ch 254 c. bauer and s. wittkopf / annual daylight simulations with evaldrc dynamic daylight simulations (dds) have been a topic for on-going research for a long time. in recent years, much progress has been made in this field by the introduction of various key technologies. the daylight coefficient method provides an efficient strategy for handling annual simulation runs (bourgeois, reinhart, & ward, 2008). light redirection characteristics of drcs can be simulated with measured data of the bidirectional scattering distribution function (bsdf) (ward, mistrick, lee, mcneil, & jonsson, 2011). both mentioned technologies are implemented within the radiance 3 phase method (mcneil, lee, 2013) and the daysim (reinhart, 2000, daysim) program suite, which, to a certain extent, constitute the current standard for annual daylight simulations. finally, suitable daylight metrics have been developed, which reduce annual simulation data to a small set of indicators. an example of a commonly established metric is the combination of spatial daylight autonomy (sda) and annual sunlight exposure (ase), proposed by the illuminating engineering society of north america (iesna) (heschong et al., 2012). however, the mentioned key technologies and their implementation in the currently established simulation tools do not perform equally well in all conceivable scenarios. while the actual methods for the daylight coefficient calculation adequately represent the light from the sky hemisphere, they are less accurate for the sun contribution. drcs are designed to respond to precise sunlight positions, thus requiring an accurate treatment of the sun contribution for producing reliable simulation results. in the context of drc simulation, bsdf data sets are not equally appropriate for all available drc system types, both in terms of their measurement and their use in simulation tools. especially distributed drcs, such as light pipes and ducts, call for other, more appropriate simulation methods. finally, the daylight metrics sda and ase respectively produce a very strong data reduction to an annual scalar value. this allows a quick comparison of different installations with respect to their annual daylight performance, but does not provide information about the drc performance in different periods of the year. as the sun path over the sky hemisphere shows a strong seasonal variation in all regions except the equatorial zone, drc response can be expected to follow this variation, which in turn makes it interesting to reflect this in the daylight metrics. to address the limitations described above, our simulation tool evaldrc introduces new concepts and enhancements in selected steps of the annual simulation process, while still building on the mentioned key technologies in a general sense. in the daylight coefficient calculation, a particular emphasis lies in the exact treatment of the solar contribution. additional to the bsdf strategy, light redirection characteristics of drcs are evaluated with photon mapping (schregle, bauer, grobe, & wittkopf, 2015). in contrast to classic backward ray tracing algorithms, photon mapping or forward ray tracing is capable of simulating the light transport through refracting and reflecting media to a high degree of accuracy. finally, enhanced versions of existing sda and ase daylight metrics at a monthly resolution are proposed. a more detailed metric dataset can be very helpful in identifying critical phases and optimization potential during drc analysis and development. 2. strategies for daylight simulations 2.1. the daylight coefficient method for annual simulations annual daylight simulations demand a high number of individual calculations with varying input values due to the dynamically changing sky conditions. in order to handle this task, the daylight coefficient method splits it into two steps. the first step, the coefficient generation, performs the complex light path simulation from the daylight sources (sky and sun) through daylight openings and drcs and finally via interreflections within the interior space. the second step, the result accumulation, then produces illuminance results for all timestamps of a chosen evaluation period. c. bauer and s. wittkopf / annual daylight simulations with evaldrc 255 this is done by adding up the coefficients, weighted with radiance values for the corresponding daylight sources, which are determined from weather data files via mathematical sky models. of course, the different natures of the two daylight sources, sky hemisphere and sun, demand different strategies for the coefficient generation. 2.1.1. sky coefficients for the sky coefficient generation, a discrete representation of the sky hemisphere is used, based on the scheme originally proposed by tregenza (tregenza, 1987). the hemisphere above the horizon is divided into 145 segments or patches, and one additional hemispherical segment represents the ground reflection. the average solid angle of the segments reflects the aperture angle of 11◦, which was a typical value for sky luminance scanners at that time. using a normalized sky hemisphere as input, the lighting simulation calculation with the radiance rcontrib program then produces a separate record for the partial scene illumination produced by each patch. dependent on the chosen output format, such a coefficient can either be a single unitless value or an hdr rendering of the scene, illuminated by one specific sky patch (fig. 1). 2.1.2. solar coefficients as a moving light source with narrow emission angle (0.5◦), the sun initially does not fit into the sky patch discretization concept. to overcome this problem, various ideas have been proposed so far. in the radiance 3 phase method, the solar radiance is added to the three neighbouring sky patches which are closest to the current sun position, thereby distributing it over an unrealistically large solid angle. as the average solid angle of the sky patches in the tregenza scheme is quite high, often a subdivision of the tregenza patches by small powers of 2 is applied. this avoids distributing the solar radiance over unrealistically high solid angles (fig. 2, left). the daysim tool fig. 1. hemispherical projection of the tregenza sky patches (left) and one coefficient hdr rendering of a demo scene equipped with a drc in the upper part of the window (middle), for the patch marked in the projection. the sky patch is visible as white square. the image on the right shows a final result for one specific timestamp, generated by a weighted superposition of all sky coefficients plus one solar coefficient (cf. sect. 2.1.2, 2.1.4) using sky and sun radiances for this date and time. fig. 2. comparison of the radiance 3 phase method sun & sky model (left, with reinhart subdivision factor 2), the daysim model (second from left, schematic), the radiance 5 phase method sun vector (second from right) and the evaldrc model (right). only a section of the sky hemisphere is shown. 256 c. bauer and s. wittkopf / annual daylight simulations with evaldrc uses a different approach. separate solar contributions are calculated for a set of predefined sun positions (currently 65, at approximately 10◦ angular separation in azimuth and altitude). then each actual solar contribution can be determined by averaging the results from four neighbouring predefined positions. (fig. 2, second from left). as the sun contribution to interior illumination is generally much higher than that of the sky hemisphere, both methods suffer from the disadvantage that the most valuable contribution is treated in the least accurate way. this becomes even more important when redirection of sunlight by drcs comes into play. for systems with sharp peaks in the redirection characteristic, the distributed or averaged input will likely produce inaccurate results if the distribution solid angle exceeds the halved peak width (w.r.t. the incoming direction). one approach of addressing this problem is to increase the sky patch or predefined sun position resolution. this is applied in the radiance 5 phase method (mcneil, 2013), where an additional high-resolution sky patch subdivision with reinhart factor of 6 is used with patch-centred predefined suns in a separate step for adding the solar contribution (fig. 2, second from right). 2.1.3. true sun coefficients in the new evaldrc tool, the original tregenza subdivision into 145 patches is used for the sky, while the solar radiance is not distributed or averaged at all. separate coefficients are calculated by using exact 0.5◦ angular sun source primitives for all timestamps of the evaluation period (fig. 2, right). therefore, we introduce the term true sun coefficient, to distinguish them from the approximated solar coefficients described above. in theory, this provides the maximum possible accuracy for the treatment of the solar contribution. however, it demands a considerably increased calculation effort compared to the aforementioned more abstract methods (with exception of the radiance 5 phase method, with its high predefined sun position count). 2.1.4. coefficient accumulation: from coefficients to contributions the coefficients are only intermediate results. they can be understood as potential contributions, indicating how much a source might contribute to the scene illumination. to generate a final hdr rendering of a scene or illuminance values on a sensor plane, one needs to perform a weighted superposition of all coefficients, using the radiances of the sky patches and the solar radiance as weight factors. for daylight sources, the coefficients or potential contributions can be assumed to initially scale with the solid angle of emission (cf. table 1). table 1 solid angles as initial indicator for potential coefficient size of the daylight sources in decreasing order daylight source type solid angle typical radiances [ w m2 sr ] ground reflection � = 2π ≈ 6.28 ≈ 2 − 20 one sky patch � ≈ 2π/145 ≈ 0.40 ≈ 5 − 300 sun � ≈ 6.10−5 ≈ 3 − 5 · 106 c. bauer and s. wittkopf / annual daylight simulations with evaldrc 257 of course, the scene geometry and material properties then determine the quantity and extent of light from these different emission directions reaching the interior scene and how it is further distributed throughout it. based on this information, the radiances used as weighting factors finally determine how much a source (or a source patch) actually contributes to the lighting. thus a high coefficient does not automatically imply a high contribution. in contrast, the high solar radiance (compared to sky and ground reflection) generally outweighs the smaller coefficient size by far, making the solar contribution the substantially dominant component. one set of coefficients suffices to generate results for several different timestamps by simply applying the corresponding time dependent radiance distributions for each of them in the superposition step. this is a key aspect of the coefficient method, and explains why it is very efficient for annual daylight simulations, which otherwise would need a lot of individual simulation runs. evidently, our approach using exact sun positions in the coefficient calculation partly breaks this flexibility with respect to the solar part. the true sun coefficients are fixed to the timestamps they are generated for. 2.2. methods for the simulation of light redirection methods algorithms for general lighting simulation are established and validated already for some decades. the most common strategies are the radiosity and the ray tracing method. the simulation of drcs however poses additional challenges to the simulation algorithms beyond those of general lighting simulations. drcs come in a wide variety of geometric shapes and material combinations (fig. 3), but they commonly deflect the light from its straight path upon passing through the system. only the ray tracing method is generally capable of simulating such complex light redirection behaviour; traditional radiosity only accounts for diffuse light transfer. two ray tracing based strategies have proven themselves a valuable means for this task, notably a) using ray tracing algorithms capable of evaluating bsdf data and b) the photon mapping technique. 2.2.1. bsdf data the bsdf, or bidirectional scattering distribution function, is a means of describing light redirection from incoming into outgoing directions for a specific point on a surface. consequently, bsdf data are dimensionless values, representing the ratio of emitted radiance to incoming irradiance (richmond, hsia, ginsberg, & limperis, 1977). the whole set is often separated into two different components brdf and btdf for reflection and transmission. for drc simulations, usually only the transmission is of interest, so we limit our description to this component here. the functional representation in theory describes the light redirection in an infinitesimal resolution. in practice, often a finite data set is used instead (fig. 4). both incoming and outgoing hemispheres are subdivided into sections, and the bsdf data constitute a tabular representation of the bsdf values for all incoming and outgoing section combinations. bsdf data can be measured with goniophotometers (gps) or calculated e.g. with forward ray tracing programs (grobe, noback, wittkopf, & kazanasmaz, 2015; mcneil, jonsson, applefield, ward, & lee 2013). depending on the material and/or geometric properties, a large amount of data might be necessary to adequately describe the redirection characteristics of the system or material (fig. 5). with scanning gps, measurements can be performed with significantly higher resolution, but the process is more time consuming, compared to image based gps (krehel, kämpf, & wittkopf, 2015; apian-bennewitz, 2010). to a certain extent, the characterisation of a drc with a bsdf data set can be seen in analogy to the description of artificial luminaires with lighting intensity distribution data (ldcs). 258 c. bauer and s. wittkopf / annual daylight simulations with evaldrc fig. 3. examples of drcs and drc installations. top left: interior daylight distribution generated from a functional drc made of curved specular lamellae integrated into a double glazed window. top right: skylight sections of a larger office building with drcs combining sunlight retroreflection and daylight redirection. bottom left: exterior drc installation with movable louvers. bottom right: custom drc installation for staircase lighting, adding aesthetic dimensions to the pure lighting functionality. sky and sunlight is redirected through a rectangular short light pipe with facetted mirrors, to produce lighting patterns varying in appearance over the course of a day. photograph top right: copyright osram, traunreut (germany), photographs top left and bottom row: copyright bartenbach, aldrans (austria). fig. 4. schematic representation of the transmission part of a bsdf data set. one section pair is highlighted, showing the incoming irradiance from direction (θi, φi) contributing to the outgoing radiance into direction (θ0, φ0). often, bsdf data is stored at a much higher angular resolution than shown in this schematic drawing. c. bauer and s. wittkopf / annual daylight simulations with evaldrc 259 fig. 5. left: projection of the scanning paths of a goniophotometer detector head onto a plane. each grey line consists of closely located points, representing measurement points on a hemisphere on the backside of the sample. the grey disc indicates a local region of high resolution data corresponding to a peak. right: 3d mountain plot of btdf data of a redirection system for one incoming light direction, showing light scattering from one incoming direction into different outgoing directions. evidently, the use of bsdf data increases the level of abstraction in the simulation process. the finite resolution of the data set may introduce accuracy problems, if the section or patch size (cf. fig. 4) does not suit the complexity of the redirection characteristic. simply raising the angular resolution leads to an increase of the data amount by a power of two, producing data files which claim a large portion of system memory during the simulation run. however, for the majority of systems and materials, a high angular bsdf resolution is only needed for selected solid angle regions, so variable resolution data storage formats have been developed, e.g. the so-called tensor tree bsdf (ward, kurt, & bonneel, 2012). radiance fully supports the use of bsdf data since version 4.2. the radiance 3 phase method relies on the use of bsdf data representations for drcs to perform annual daylight simulations. evaldrc also builds upon the radiance bsdf support as one of two general methods provided for light redirection simulation (cf. sect. 2.2.2). 2.2.2. contribution photon mapping photon mapping, originally introduced by wann jensen (wann jensen, 2001), is a variant of the general ray-tracing algorithm in which rays (photons) are emitted from the light sources. this is in contrast to the classic backward ray tracing, where rays are initially emitted from the view point. the forward path tracing constitutes the ability to accurately simulate the light particle transport through complex refracting and reflecting media, which is – cum grano salis – principally impossible to achieve with the normal backward ray tracing method. therefore, photon mapping is an interesting technique for drc simulations. a first photon map module for radiance was developed and validated in 2002 (schregle, 2004). recently, this photon map module was enhanced to include support for the contribution coefficient method (cf. sect. 4) and finally was integrated into the main radiance distribution. the new contribution photon mapping (schregle, 2015; schregle, grobe, & wittkopf, 2015) offers the possibility to sort the individual light source contributions into different bins during the final gathering. by introducing the concept of primary photons (fig. 6), which carry the information about 260 c. bauer and s. wittkopf / annual daylight simulations with evaldrc fig. 6. principle of the contribution photon mapping implementation. photons emitted from the light sources are deposited as primary photons upon their first hit in the scene. each spawned photon then references this primary. in the gathering step at point −→x , this primary reference is used to sort all collected photons according to their emitting light source. the emitting light source, additionally a dynamic binning dependent on the emission direction is possible. this connection to its original light source and emitting direction is of course necessary to use photon mapping in the daylight coefficient method (cf. sect. 2.1). coefficient calculation was impossible with the previous photon map version, because the photons were unaware of their origin. 2.3. data reduction with daylight metrics daylight metrics (dm) are special criteria for quantifying the useful daylight availability in interior spaces. they are generated from large sets of lighting simulation results with different reduction techniques with respect to space and time. ideally, not only the amount of daylight, but also discomfort problems due to glare should be considered in the evaluation algorithm. this demands annual simulations, which reflect the current building and room scenario to a sufficient degree of realism, both statically (room geometry, furniture, material properties) and dynamically (location specific weather data, dynamic sunshade operation). evidently, in practice various abstractions might be introduced when exact data are either not available or too complicated to be considered in the simulation. dms can be seen as successor to the former daylight factor, which they surpass by far in terms of information content. the daylight factor has more in common with the coefficients (cf. sect. 2.1), because it is a dimensionless ratio of interior to exterior daylight illumination, being further limited to overcast sky conditions only. among the several dms which have emerged, the spatial daylight autonomy (sda) and the annual sunlight exposure (ase) as defined by the iesna have received a widespread acceptance, and thus were chosen for integration into evaldrc. for both metrics, a defined evaluation period of one year is assumed, consisting out of five-day working weeks with occupancy hours from 8 a.m. to 6 p.m. simulation results and weather data used for generating the sky radiance distributions should be provided at an hourly resolution. the scene should be modelled according to its actual use case, c. bauer and s. wittkopf / annual daylight simulations with evaldrc 261 so e.g. office rooms also should include basic models of the furniture. the metric calculation itself is based on illuminance values on a sensor point grid as input, with defined limits for maximum grid point spacing. for a detailed description of all requirements, definitions and discussions about justification of the chosen strategies and thresholds, see heschong et al. (2012). 2.3.1. spatial daylight autonomy (sda) this metric describes the amount of available and useful daylight for a given sensor plane. it is defined as: “the percent of an analysis area [...] that meets a minimum daylight illuminance level for a specified fraction of the operating hours per year” (heschong et al., 2012). the sda calculation is performed in a two-step process. the first step occurs in the time domain. all grid points are recorded which exceed a given illuminance threshold for a given fraction of the total evaluation time. the second step is then executed in the spatial domain. the resulting sda value represents the fraction of the sensor plane area, which fulfils the criterion of receiving the specified amount of daylight illumination for the given fraction of the total occupancy time. the recommended default settings are 300 lux for the illuminance threshold and 50% for the temporal fraction threshold. final sda values above 55% are categorized as nominally acceptable, values above 75% as preferred daylight availability. with the default thresholds one takes into account that daylight will not be available to a sufficient extent throughout the whole year. but the thresholds may be adjusted individually. dependent on the tasks performed in the room, a higher or lower illuminance threshold might be reasonable, and dependent on different occupancy scenarios a higher or lower temporal fraction for attaining and exceeding the illuminance threshold might be justified. in order to distinguish sda metrics with different thresholds, these are appended as subscripts. the default sda is thus correctly written as sda300,50% when we assume a grid of n points, and define a function s(j), which is 1 for each grid point j which receives a sufficient illuminance for more than the given fraction of total occupancy time, else 0, the sda can be expressed as: sda = ∑ n j=1s (j) n with s(j) = { 1 : sj ≥ τty 0 : sj < τty } , (1) where sj = occurrence count of exceeding the sda illuminance threshold at point j ty = annual timestamp count τ = temporal fraction threshold in the simulation, sun shading has to be considered for – at least – those timestamps, in which direct sunlight illumination on the sensor plane exceeds a specified amount. calculating these threshold levels for sunshade operation is closely related to the second part of the metric, the ase value (cf. sect. 2.3.2). the consideration of sun shading finally justifies the sda’s meaning as indicator for the amount of useful available daylight. 2.3.2. annual sunlight exposure (ase) the aim of this metric is to quantify both glare problems and possible solar gains. it is defined as: “the percent of an analysis area [...] that exceeds a specified direct sunlight illuminance level more than a specified number of hours per year” (heschong et al., 2012). 262 c. bauer and s. wittkopf / annual daylight simulations with evaldrc the interesting aspect is that, again, grid point illuminance data are used as basis. so no further simulations, such as field of view renderings for identification of regions with disturbingly high luminances, are needed. however, unlike the sda case, the illuminances calculated for the ase metrics must only include the contribution from direct sunlight or purely specular sunlight reflections. for lighting simulations with radiance, this simply means to disable the indirect calculation by setting the ambient bounces parameter to zero. like the sda, the ase metric also operates with illuminance and temporal threshold values, and finally results in a fractional value w.r.t area. with the default settings, grid points which receive at least 1000 lux for more than 250 hours of the annual occupancy time (default thresholds) are recorded, and the fraction of points, or the fraction of the sensor plane area, which fulfils these criteria, determines the ase value. the thresholds are again added as subscripts, so the default ase is correctly denoted as ase1000,250. the mathematical representation is similar to equation 1 ase = ∑ n j=1a(j) n with a(j) = { 1 : aj ≥ ty 0 : aj < ty } , (2) where aj = occurrence count of exceeding the ase illuminance threshold at point j tj = annual absolute hour threshold the illuminances used for the ase explicitly have to be calculated without sunshade, thus the ase acts as indicator for the amount of direct sunlight reaching the evaluation plane and, in consequence, the need for appropriate measures against it (e.g. sunshade operation and glare control). categorizing ase value ranges as acceptable or not is more complicated than in the sda case. due to the deliberate simulation without sun shading, the grid point illuminances do not represent a ‘final’ state. evidently, conditions leading to a high ase value are those which usually trigger sunshade operation, either automatic or manually by the occupants. in fact, this aspect of the ase calculation is used in the metric to determine the dynamic sunshade operation thresholds needed for the sda evaluation (cf. sect. 2.3.1). per default definition, sunshade operation is assumed for those timestamps during which 2% or more of the grid points exceeds the abovementioned 1000 lux illuminance threshold. consequently, the 2% also represents the nominally acceptable ase value for a ‘complete’ simulation scenario including sunshade operation. again, all ase thresholds may be adjusted to fit the actual circumstances. compared to the sda, the ase definition based on grid point illuminances rather than field of view evaluations, including the chosen default thresholds, are both subject to a more complex justification argumentation as well as a still on-going discussion. the iesna reference explores this in great detail. 2.3.3. monthly daylight metric values the sda and ase metric both imply a strong data reduction to one annual value each. when used for comparing different drcs, this can be enough to make a quick decision about which system performs better in total. but no information about the drc performance throughout the course of the year can be extracted from the metric. as seasonal variation of daylight availability plays a great role in all regions of the world except the equatorial zones, breaking the annual metrics down into monthly values is a first step of adding more detail and useful information to the daylight metrics by still maintaining a reasonable number of values. the new equivalents to the annual sda and ase metrics are denoted msda for the monthly spatial daylight autonomy, and mse for the monthly sunlight exposure. their calculation proceeds analogously to the annual values, with the only difference that all temporal thresholds now are c. bauer and s. wittkopf / annual daylight simulations with evaldrc 263 interpreted relative to the timestamp count of one month. similarly to equation 1, the msda for month m can be expressed as msdam = ∑ n j=1s(j, m) n with s(j, m) = { 1 : sj,m ≥ τtm 0 : sj,m < τtm } , (3) with sj,m = occurence count of exceeding the sda illuminance threshold at point j for month m tm = timestamp count for month m because of the different timestamp count for each month, the function s(j) from equation 1 now becomes s(j,m), i.e. a function of grid point index j and month m. analogously to the annual case, the mse can also be expressed by a similar mathematical expression: msem = ∑ n j=1a(j, m) n with a(j, m) = { 1 : aj,m ≥ tm 0 : aj,m < tm } , (4) with aj,m = occurence count of exceeding the ase illuminance threshold at point j for month m tm = absolute hour threshold for month m it is important to note that, due to the two step evaluation algorithm with illuminance and temporal fraction thresholds, there is no straightforward relationship between the annual metrics and the average of the monthly values. this can be seen if we examine the contribution of one single grid point. point j contributes to the annual sda if: sj ≥ τty (5) and it contributes to the msda for month m if: sj,m ≥ τtm (6) now, the annual occurrence count for exceeding the threshold illuminance is the sum of all monthly counts. if, for simplicity, we also assume equal day counts per month, ty = 12tm, then equation 5 becomes: 1 12 12∑ m=2 sj,m ≥ τtm (7) this would be equal to equation 6 only if all monthly occurrence counts of illuminance threshold exceeding for this point j are exactly one twelfth of the annual occurrence count of illuminance threshold exceeding, which cannot be expected in general: sj,m /= 1 12 12∑ m=1 sj,m (8) so each grid point may contribute to a selected monthly msda and not to the annual sda, or vice versa, and thus the annual sda is not just the average of all monthly msda values. a similar deduction can be done for the mse vs. ase relationship. this missing straightforward relation between 264 c. bauer and s. wittkopf / annual daylight simulations with evaldrc annual and monthly metric values can be observed for a concrete example in the application case study in section 4. 3. evaldrc implementation 3.1. overview evaldrc is a set of separate modules, which each perform a specific task of the simulation process chain. it is implemented in python, because this framework provides both the convenience of a scripting language, including important features such as built-in support for parallel processing, as well as a comprehensive set of high level programming constructs. this enables the integration of specific algorithms, e.g. for the metrics calculation, as well scripts for handling program calls and administering a complex data and metadata directory tree. the process flow is depicted in figure 7: the scene description including the drc and the material properties must be provided in valid radiance input syntax. bsdf data must be given in the lbnl xml file format. weather data files must be typical meteorological year (tmy) data sets in the epw format. all other input (location, timeframe, calculation parameters, etc.) can be provided via configuration file settings. for annual simulations the timeframe is set up automatically according to the mandatory sda/ase nominal requirements, followed by the generation of the sky and cumulative sun primitives together with their radiance distributions. when all input is prepared, the photon map generation and the coefficient calculation can be performed. the sky configuration and the lighting simulation steps are accomplished by calling the radiance programs gendaylit, gensky, mkpmap and rcontrib. in the end, the coefficient superposition step follows. based on these results, finally the daylight metrics sda/ase and msda/mse can be calculated. the outputs of evaldrc are hdr images, illuminance values for given sensor point grids and daylight metrics, including graphical representations (false-colour, mountain or section line plots) with help of the opensource data visualization package gnuplot. 3.2. sky configuration the daylight sources are prepared with the module prepsky, including the timeframe setup. the latter can be done automatically for a monthly period or a whole year, for given office hours, working days per week, daylight savings time period, geographic location and meridian, etc. alternatively, a custom timestamp file can be used. based on the timestamps, cumulative sun primitives are generated, followed by the calculation of the corresponding time dependent radiance distributions fig. 7. evaldrc process flow. c. bauer and s. wittkopf / annual daylight simulations with evaldrc 265 for the sun and the sky. three different types of sky models are available: a) a static, generic cie sky (clear, intermediate or overcast) (cie, 1973) b) a dynamic sky consisting of a random combination of the generic cie skies based on monthly sunshine probability data, and c) a true climate based sky derived from tmy weather data (cf. sect. 2.1.4). the static sky of course has no meaning for climate based daylight analysis, but it can be helpful for visually examining the redirection behaviour of drcs in the current installation scenario. 3.3. coefficient calculation the coefficient calculation is the central and most time consuming process. it is performed by the module runsim, which calls the radiance executables rcontrib and the new photon map generator mkpmap. the calculation parameters are taken from configuration files, shielding the user from assembling the complicated command lines. but knowledge of radiance and the photon map options is necessary, as well as an understanding of the operational principle of the drc(s) used in the simulation and their available data representation, to set the adequate calculation type (bsdf data or photon map). coefficient calculation happens separately for the sky and the sun. the true sun coefficient strategy results in a considerable amount of individual source objects, making it impossible to produce coefficients for all of them in one execution run without dramatic loss of accuracy. so an intelligent mechanism to produce the solar coefficients in groups was implemented, which of course demands a higher calculation effort compared to the more abstract methods explained in sect. 2.1.2. this includes separate photon maps for each solar coefficient group, resulting in a high demand of disk space for storing all temporary data. dependent on the parameters (e.g. photon counts), typically 50 to 100 gb are needed for annual simulations. execution times vary strongly dependent on the scene complexity, the drc simulation type (with or without photon map) and the output type (illuminance values or renderings). 3.4. result accumulation and daylight metrics the result accumulation is straightforward, as it merely involves multiplying the coefficient matrices for the sky and the sun with the corresponding radiance distribution vectors. two modules perform this task, valspp and picspp, for sensor plane illuminances and pictures, respectively. illuminance data can be further processed with the module calsda, which implements the sda and ase daylight metrics algorithm including the additional monthly breakdowns msda and mse. finally, graphical representations of sensor plane illuminances and the monthly daylight metrics can be produced with a small set of helper modules (plotpln, plotval, plotsda). 3.5. standard and advanced applications the usual evaldrc application is the automated annual evaluation of a daylight scenario with final metrics output. for simulations without consideration of dynamic sunshade, this can be conveniently accomplished with the module evlscn, which consecutively calls all modules mentioned above. it also offers an integrity check for the scene input data plus the configuration file settings. besides that, the modular concept allows various individual applications. with a custom timestamp file, arbitrary periods can be simulated, e.g. for animations or a detailed daylighting analysis with sub-hourly weather data. the true sun strategy of course imposes a limit on the flexibility for the coefficient evaluation, as the latter are fixed to the timestamps. but it is possible to reuse existing coefficients in repeated result-accumulation runs with different radiance distributions. fur266 c. bauer and s. wittkopf / annual daylight simulations with evaldrc ther options, e.g. superpositioning sky and solar contributions separately, are helpful in examining detailed aspects of the light redirection. for evaluations with consideration of dynamic sun shading, two geometry inputs need to be prepared, one scene with and one without sunshade. then an annual simulation must be performed for each of them. only the generic sun shade type as defined by the sda metric norm is currently implemented. this means adding surfaces to the daylight openings with pure diffuse visible light transmittance (vlt) between 5% and 20%. the sda/msda values then result from a dynamic combination of the two static result sets, according to the time dependent sunshade operation thresholds described in sect. 2.3.1 and 2.3.2. 4. application example evaldrc was employed in a retrofitting case study for a studio classroom in the department of architecture at the lucerne university of applied sciences and arts in switzerland. this example shows how a concrete daylight optimization task can be analysed with help of evaldrc, and how the combination of annual daylight metrics sda and ase and the new monthly breakdowns msda and mse enables the user to gain a thorough understanding of the impact of different facade configurations upon the daylight availability in the interior space, thus providing a reliable basis for making decisions on the optimal solution for the case in consideration. 4.1. the scenario the room is located in the main building on the campus, at a geographic latitude and longitude of 47.3◦ north and 8.3◦ east. it has a rectangular shape, 8.8 times 9.05m wide, with a south-facing window wall consisting out of 6 glazed segments. the ceiling height is 3.63m, the window sill is at 0.9m above the floor, so the window to wall ratio (wwr) is approximately 75%. the room layout consists of 3 times 5 working desks of 0.94m height surrounded by shelves on two walls. a ground plan of the room and a photograph is shown in figure 8. fig. 8. plan view of the studio classroom used in our case study. the light grey fields indicate the furniture and the dashed line marks the sensor plane for the illuminance calculation. on the right, a photo of the room is shown, taken from the entrance facing the window facade. c. bauer and s. wittkopf / annual daylight simulations with evaldrc 267 the large glazed area of the facade causes very high solar gains, so the main aim of the retrofitting analysis was to find a solution which reduces the wwr, but still provides enough daylight to meet the nominally preferred sda criterion of 75% (cf. sect. 2.3.1). three optimization strategies were compared, in which parts of the existing facade were replaced with a combination of drc, opaque wall and clear glazing segments in varying area proportions. the impact on the resulting daylight availability was assessed based on the annual and monthly daylight metrics (sda/ase, msda/mse). the sensor plane for illuminance values was 7.2 times 7.2m wide, at a height of 1cm above the desk height. grid point spacing was 0.3m, which is roughly half as wide as the nominally maximum spacing threshold of 2ft. (∼0.66m) demanded for sda/ase calculations. because of the visually demanding tasks performed in the studio, the sda illuminance threshold was set to 500 lux instead of the default value of 300 lux. as tmy weather data are not available for lucerne, available data from the nearest geographical location were used, which in this case was geneva, ch. the chosen drc was a micro-structured holographic film, shown in fig. 9, which can easily be applied to the interior side of existing window glass panes without the need of mechanical refurbishing work on the facade itself. its redirecting characteristics were measured at the cc ease goniophotometry lab and exported to a variable resolution bsdf data set in the radiance tensor tree format, which was then used as material model in the simulation. for the simulation, we simplified the geometry model of the facade to facilitate the assembly of the evaldrc resp. radiance geometry and material input files for the several different variants. in the abstract model, each of the six facade segments consisted of a wall section of 0.9m height and a clear glazing area of 2.7m height. frame geometry and external mounts for the blinds were omitted. for the analysis of the proposed optimizations, the clear glazing area was then subdivided into a view window section, extending from the window sill at 0.9m to a height of 1.8m, and an optimization area, reaching from 1.8m to the full ceiling height at 3.6m. the view window fig. 9. sample glass plate with a layer of the micro-structured film used in our case study held against a window on a sunny day, showing both the redirection effect on the ceiling and the reduced transmission for the direct view through the sample. 268 c. bauer and s. wittkopf / annual daylight simulations with evaldrc fig. 10. partitioning of the vertical window segments of the facade in the abstract model used for the simulation. the existing facade is shown on the left, on the right the three optimization variants and their different set-ups with a combination of either wall and clear glazing (o1), drc and clear glazing (o2) or wall and drc sections (o3) are depicted. section remained unmodified, whereas the optimization area was modelled either as increased opaque wall and reduced clear glazing area (o1), as addition of a drc in the upper part and clear glazing in the lower part (o2) and finally as combination of opaque wall and drc in the lower part without any clear glazing (o3). figure 10 shows a schematic drawing of the existing facade and the three optimization variants in the abstract model. in all three variants, the optimization area was always divided into equal sized sections for the respective components. besides the different wwrs, we also denote the different area relations of drc elements to clear glazing sections as drc to window ratio (dwr). in the context of daylight redirection, o1 is not an optimization, as it merely constitutes a wwr reduction. but it is interesting for the overall comparison, as it allows a judgement of the potential to counteract the impact of a simple wwr reduction with the installation of a drc. 4.2. result analyses: daylight metrics the sda500,50% and ase1000,250 metrics (in short sda and ase for the remainder of the text) for the exisiting facade and the three optimization scenarios, as calculated with evaldrc, are presented in figure 11. each graph shows both annual values and our proposed monthly breakdowns msda and mse. the sda/msda data were generated with consideration of dynamic sun shading. as generic sunshade, a 10% purely diffuse vlt material was used, covering all clear glazing sections. the sunshade was considered as active when 2% or more of the sensorplane area received an illumination by direct sunlight of 1000 lux or higher, as explained in sect. 2.3.2. the percentages of the timestamps with active sunshade are printed in the graphs, too. 4.2.1. annual daylight metric analysis with an sda of ∼70%, the existing facade meets the nominally accepted (55%), but misses the preferred sda criterion (75%). sunlight exposure on the sensor plane (ase) is high, which could c. bauer and s. wittkopf / annual daylight simulations with evaldrc 269 fig. 11. daylight metric graphs for the existing facade (top left) and the optimization variants o1–o3 (top right and bottom). each graph shows the annual sda and ase in a bar diagram and the new monthly msda and mse as line plots. the dashed line shows the monthly percentages of timestamps with active sunshade. comparing the left and right graph in each row shows the influence of reducing the wwr, whereas comparing the top and bottom graph in each column shows the influence of introducing the drc. be expected from the large glazed area. shading is required throughout the year. so the existing facade does not provide adequate daylight, creates overheating and requires measures for active shading, and may cause visual discomfort through glare. reducing the wwr in the variant o1 reduces the ase from 34% to 21%, causing lower solar gains and fewer requirements for active shading, but the sda falls below the acceptance level. introducing a drc in variant o2 greatly raises the sda, exceeding the preferred criterion considerably. the ase is reduced, but still quite high. it is similar to the o1 case, which can be understood from the fact that both variants have the same height of the clear glazed window area. in the absence of purely specular reflections, the height of the glazed area alone determines the cut-off angle for sunlight reaching the sensor plane. the most interesting variant finally is o3, especially when compared to o1. it shows that adding a drc section in the lower part of the optimization area contributes – together with other effects, see below – to a significant increase of the daylight performance, turning an inacceptable solution into one meeting even the preferred sda criterion. the stronger effect of the drc in o3 vs. o1, compared to o2 vs. the existing facade, evidently coincides with the higher dwr (50% for o3 in contrast to 33% for o2). additionally, with an ase of around 5–6%, sunlight exposure on the sensor plane is the lowest of all scenarios thus requiring the least active sun shading period. it is also quite close already to the threshold of 2%, which is considered as the acceptable limit (cf. sect. 2.3.2). 270 c. bauer and s. wittkopf / annual daylight simulations with evaldrc fig. 12. comparison of msda graphs (left) and mse graphs (right) of the variants o1–o3 (continuous and dashed lines) vs. the graph for the existing facade (shaded in grey). 4.2.2. monthly daylight metric analysis the monthly metrics msda and mse give a detailed insight into the effect of reducing the wwr and introducing a drc on the daylight distribution over the course of the year. figure 12 shows a direct comparison of the msda and mse graphs for the existing facade and the variants o1–o3. reducing the wwr (comparing o1 vs. the existing facade) results in an approximately 20% decrease of msda almost evenly across the year, except for june and july, where the decrease is less. in contrast, comparing variants with equal wwr, but with and without drc (i.e. o2 vs. the existing facade, and o3 vs. o1) shows that the drc results in increased msda values mainly in spring and autumn, and, to a lesser extent, in summer. in winter, the msda values more or less converge. comparing o3 against the existing facade finally shows that the drc can overcompensate for the msda reduction produced by the reduced wwr for spring and autumn and can compensate for it in summer. only in winter, msda values for o3 fall below those of the existing facade. so the higher annual sda (∼80% for o3 compared to ∼70% for the existing facade) does not imply increased daylight availability over the whole year. instead, it is a net effect resulting from a strong to moderate increase of msda during spring, summer and autumn and a decrease in winter. a further important factor is the sunlight exposure and the resulting need for dynamic sun shading which can be deduced from the mse graphs in figure 12. only the o3 variant leads to a significant reduction of sunlight exposure during winter and spring/autumn, and makes sun shading completely unnecessary for four months from april to july. in fact, the reduced need for sunshade operation adds to the contribution of the drc, and it is this combination of two effects which finally is responsible for the significantly better daylight performance of the o3 variant compared to all others in the comparison. 5. conclusion and outlook the new simulation tool evaldrc is a valuable frontend for detailed annual drc simulations based on the radiance lighting simulation program suite. it shields the user from the complexity of the individual commands, by still providing a great amount of flexibility. the use of two key c. bauer and s. wittkopf / annual daylight simulations with evaldrc 271 simulation technologies for light redirection, bsdf data and the newly developed contribution photon mapping, make it possible to simulate a wide range of different drc types, from simple systems like micro-structured films to complex, spatially extended setups such as light ducts. the various outputs (hdr images, illuminance plots and daylight metrics) provide all necessary data for assessing the drc performance both visually and quantitatively, either directly or by using them as input to further analysis tools (e.g. field of view image-based glare evaluation). especially the use of the photon map allows more realistic renderings of the true visual appearance of the light redirection in architectural spaces, compared to other current methods in which all complex drc geometry is represented by abstract polygonal layers with attached bsdf data. in the context of the evaldrc development, we have presented an enhancement to the established daylight metrics sda and ase. generating monthly variants msda and mse provides more detailed information about the drc performance throughout the course of the year, which greatly helps in identifying critical phases and optimization potential for both daylight design and drc development and optimization. in a concrete application example it was shown how the monthly metric breakdowns provided valuable deeper insights into the performance of a drc installation than the annual values alone, and how this info could be used as basis for determining the optimal solution to a specific daylighting optimization task. however, the monthly breakdowns msda cannot be directly judged against the annual nominally required criteria accepted and preferred, as they reflect both seasonal variation of daylight availability in general and the different drc performance throughout the year. so, in a future task, monthly varying equivalents to the annual accepted and preferred criteria need to be established to subtract the effect of general seasonal daylight availability. the use of true sun coefficients by simulating exact 0.5◦ sun primitives guarantees a high level of accuracy for the solar contribution within the daylight coefficient method, but it demands a high calculation effort. it also reduces the flexibility for coefficient evaluation. this reduced flexibility can be accepted, as generally, the timeframe and temporal resolution used for annual simulations is defined by the nominal requirements. the high calculation effort plays a greater role. the calculation is already optimized by processing cumulative sun positions at once, but this can only be done to a certain extent. developing more sophisticated and scalable interpolation algorithms for the solar contribution thus remains a promising task for future development. ideally, the degree of optimization should be adjustable according to the demands posed by the complexity of the scene and drc in consideration. additionally, improvements of the contribution photon map are a topic of currently on-going work, with the aim of increasing the performance for the cumulative true sun coefficient calculation as well as the achievement of higher rendering quality for hdr images. although evaldrc has already proven its robustness in the described application example, it is still considered as being under development, and thus not yet completely ready for making it available to the daylighting community. first of all, it will be applied and enhanced in various case studies to assess the performance of different drcs in different climate zones within the frame of a projected visiting scholar programme. acknowledgments this research was supported by the swiss national science foundation as part of the project “simulation-based assessment of daylight redirecting components for energy savings in office buildings” (snsf #147053). we thank visiting scholar t. kazanasmaz and l. grobe for preparing the simulation model. 272 c. bauer and s. wittkopf / annual daylight simulations with evaldrc references apian-bennewitz, p. (2010). new scanning gonio-photometer for extended brtf measurements. in spie optical engineering+applications. international society for optics and photonics, 77920o-77920o. bourgeois, d., reinhart, c. f., & ward, g. (2008). standard daylight coefficient model for dynamic daylighting simulations. building research & information, 36(1), 68-82. cie (1973). standardization of luminance distribution on clear skies. publication no. 22, paris, cie. daysim, reinhart, c. institute for research in construction, national research council of canada, url: www.daysim.com. grobe, l., noback, a., wittkopf, s., & kazanasmaz, t. (2015). comparison of measured and computed bsdf of a daylight redirecting component. in proceedings of international conference cisbat 2015 future buildings and districts sustainability from nano to urban scale (no. epfl-conf-213324 ). leso-pb, epfl. 205-210. heschong, l., wymelenberg, v. d., andersen, m., digert, n., fernandes, l., keller, a., et al. (2012). approved method: ies spatial daylight autonomy (sda) and annual sunlight exposure (ase) (no. epfl-standard-196436). ies-illuminating engineering society. jensen, h. w. (2001). realistic image synthesis using photon mapping. natick, ma: ak peters, ltd. krehel, m. p., kämpf, j., & wittkopf, s. (2015). characterisation and modelling of advanced daylight redirection systems with different goniophotometers. in proceedings of international conference cisbat 2015 future buildings and districts sustainability from nano to urban scale (no. epfl-conf-213326). leso-pb, epfl, 211-216. mcneil, a. (2013). the three-phase method for simulating complex fenestration with radiance. lawrence berkley national laboratory. mcneil, a., & lee, e. s. (2013). a validation of the radiance three-phase simulation method for modelling annual daylight performance of optically complex fenestration systems. journal of building performance simulation, 6(1), 24-37. mcneil, a., jonsson, c. j., appelfeld, d., ward, g., & lee, e. s. (2013). a validation of a ray-tracing tool used to generate bi-directional scattering distribution functions for complex fenestration systems. solar energy, 98, 404-414. reinhart, c. f., & herkel, s. (2000). the simulation of annual daylight illuminance distributions – a state-of-the-art comparison of six radiance-based methods. energy and buildings, 32(2), 167-187. richmond, j. c., hsia, j. j., ginsberg, i. w., & limperis, t. (1977). geometrical considerations and nomenclature for reflectance. washington, dc, usa: us department of commerce, national bureau of standards, 160. schregle, r. (2004). daylight simulation with photon maps. phd thesis, universität des saarlandes, saarbrücken, 2004. http://scidok.sulb.uni-saarland.de/volltexte/2007/1171. schregle, r. (2015). development and integration of the radiance photon map extension. technical report, lucerne university of applied sciences and arts. http://dx.doi.org/10.13140/2.1.3332.9449. schregle, r., grobe, l., & wittkopf, s. (2015). progressive photon mapping for daylight redirecting components. solar energy, 114, 327-336. schregle, r., bauer, c., grobe, l., & wittkopf, s. (2015). evaldrc: a tool for annual characterisation of daylight redirecting components with photon mapping. in proceedings of international conference cisbat 2015 future buildings and districts sustainability from nano to urban scale (no. epfl-conf-213332). leso-pb, epfl, 217-222. tregenza, p. r. (1987). subdivision of the sky hemisphere for luminance measurements. lighting research and technology, 19(1), 13-14. ward, g., mistrick, r., lee, e. s., mcneil, a., & jonsson, j. (2011). simulating the daylight performance of complex fenestration systems using bidirectional scattering distribution functions within radiance. leukos, 7(4), 241-261. ward, g., kurt, m., & bonneel, n. (2012). a practical framework for sharing and rendering real-world-bidirectional scattering distribution functions. techn. report lbnl-5954-e, lawrence berkeley national laboratory. www.daysim.com http://scidok.sulb.uni-saarland.de/volltexte/2007/1171 http://dx.doi.org/10.13140/2.1.3332.9449 journal of facade design and engineering 3 (2015) 143–163 doi 10.3233/fde-150039 ios press 143 switching from static to adaptable and dynamic building envelopes: a paradigm shift for the energy efficiency in buildings marco perino∗ and valentina serra department of energy, politecnico di torino, corso duca degli abruzzi 24, torino, italy abstract. the key role of the building envelope in attaining building energy efficiency and satisfactory indoor comfort has long been established. nevertheless, until recent times, all efforts and attention have mainly been focused on increasing and optimizing the thermal insulation of the envelope components. this strategy was a winning approach for a long time, but its limitations became obvious when users and designers started to consider the overall energy demand of a building and started to aim for zero energy building (zeb) or nearly zeb goals. new and more revolutionary concepts and technologies needed to be developed to satisfy such challenging requirements. the potential benefits of this technological development are relevant since the building envelope plays a key role in controlling the energy and mass flows from outdoors to indoors (and vice versa) and, moreover, the facades offer a significant opportunity for solar energy exploitation. several researches have demonstrated that the limitation of the existing facades could be overcome only by switching from ‘static’ to ‘responsive’ and ‘dynamic’ systems, such as multifunctional facade modules (mfms) and responsive building elements (rbe). these components are able to continuously and pro-actively react to outdoor and indoor environment conditions and facilitate and enhance the exploitation of renewable and low exergy sources. in order to reduce the energy demand, to maximize the indoor comfort conditions and to produce energy at the site, these almost ‘self-sufficient’, or even ‘positive energy’ building skins frequently incorporate different technologies and are functionally connected to other building services and installations. an overview of the technological evolution of the building envelope that has taken place, ranging from traditional components to the innovative skins, will be given in this paper, while focusing on the different approaches that have characterized this development. examples of innovative solutions for responsive and dynamic components and the future trends of development will also be described. keywords: adaptive building envelope, responsive building envelope, multifunctional facade, phase change materials (pcms), thermotropic glazing, advanced integrated facades 1. introduction since their origins human beings frantically sought for confined spaces within which they could be safe and comfortable. if safety and security were the earliest concern, it was soon realized that a confined space could also be an effective measure to create a thermally comfortable environment. to enhance such function, the attention was focused on the building envelope, perceiving this construction element as a set of sub-systems and/or components which was used to separate the outdoor ∗corresponding author: marco perino, tel.: +39 011 0904423; fax: +39 011 0904499; e-mail: marco.perino@polito.it. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:marco.perino@polito.it 144 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes b a fig. 1. typical example of building envelope optimization in traditional architecture, based on the classical approach of the ‘energy conservation’ – hhd=2894◦c · d, winter design temperature –8◦c, summer design temperature 32◦c, cdd=386◦c · d – (ashrae, 2013) (a: north exposition; b: south exposition). from the indoor. the underlying and common idea was therefore that of creating a shield, able to protect the built environment – which was climatised – from the weather, and from the strains of an external climate, which was considered to be hostile. as a result, the ultimate goal of designers and engineers was, and has been for centuries, to optimize the ‘separation effect’. this was typically achieved by improving properties like those of – broadly speaking – ‘insulation’, ‘tightness’ and ‘waterproofing’ of the building envelope components. the shape and the structure assumed by the building facades has been the mirror of such conception. moreover, keeping the indoor air temperature at a sufficient level in the cold season was assumed to be the unique relevant requirement for assuring a proper ieq (indoor environmental quality), while air-conditioning in the summer period, artificial lighting and plug-loads1 were not of concern, either because they were deemed optional gimmicks or because the energy demand related to them was comparably lower. such vision and concepts supported the assumption that the energy efficiency in buildings could be achieved just by minimizing the heat transmission losses through the building envelope and maximizing the free gains. these were the foundations of the so-called ‘energy conservation approach’ (goia, 2013) that has driven the building and facade design and development until recently. as a consequence, in the past – at least in the industrialized countries – the building envelope assumed the shape of massive opaque walls with only few and small transparent openings. the difficulty of finding suitable energy sources, to make them available on site and to convert them through efficient processes, also led to optimize the building envelope in relation to the opportunities offered by the local climate conditions. these characteristics are easy to recognize just looking at the traditional architectures, where the envelope was designed according to the exposition, adopting different ratios of transparent and opaque surfaces (see the example in figure 1 – maximization of the solar gains, by means of windows on the southern walls, and of the thermal insulation, using thick masonry walls, on the northern side). such design and construction philosophy allowed, through the centuries, to satisfy the requirements of mechanical resistance and those of a basic thermal protection. the presence of an opaque structure with a high thermal mass could provide an effective thermal storage, and the thermal resistance – even though it was not optimal – was at least adequate to the indoor environmental quality that was expected by the occupants at that time, especially if compared to the very bad performance 1the plug-loads are the loads deriving from electric devices such as computers, televisions, electric appliance etc. m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 145 that the available transparent components back then could provide. in this period, the technological evolution developed along the improvement of the construction techniques and of the properties of the materials, but it stayed crystallized on the energy conservation approach. indeed, these innovations, the use of new materials (steel, concrete, glass etc.) and the improvement of the design procedures allowed to ‘disembody’ the surfaces of the building envelope and to split the function of structural resistance from the others (like e.g. thermal, protective, aesthetic). the facades of traditional buildings – solid, compact, massive – can now be made lighter by introducing ‘airy’ and transparent elements, whose extension grew more and more, to such an extent of replacing – in certain cases – almost completely the opaque surfaces. an era of enthusiasm and confidence about the technology started, which was inspired by the cultural movement of modernism and supported by the availability of reliable and relatively energy efficient conversion systems and of energy vectors at a rather low cost. the combination of all these favourable factors led to the development of projects where the local climatic conditions were no longer taken into account in a proper way. many designers and architects started to focus the attention mainly on formal and aesthetic issues, rather than on energy implications and comfort conditions, and to uncritically follow the style proposed by the international ‘trend setter’ architects. as a consequence, a flourishing of buildings in which the poor performance of the facades was counterbalanced by the installation of oversized hvac systems took place. these designs were invariably characterized by an unacceptable high energy consumption and disappointing indoor environmental quality. a stigmatic example is represented by the fashion of using large glazed surfaces having a size that is excessive with respect to the need for daylighting. such facade configuration gives origin to significant transmission heat losses during the winter and to huge solar heat gains in the summer period. the obvious, and just apparent, remedy of installing oversized hvac systems has the only effect of counterbalancing, at least under the mere energy balance point of view, the excessive free gains, but is not capable of solving the local discomfort problems in a satisfactory way (e.g. draft risk, noise, radiant asymmetry). this is due to both excessively high and low radiant temperatures, and to the draft risk consequent to the need of introducing large flow rates of conditioned air (required to counterbalance the ‘massive’ thermal loads). the energy crisis in the early seventies, the increasing cost of the energy, a growing consciousness towards environmental problems and, last but not least, the dissatisfaction of the occupants (who were living in nice looking but not so comfortable buildings) represented the boost towards the next steps of the evolution. the first law on the energy conservation in buildings dates back to the early 1970s. for the first time, and practically in the whole industrialized world, minimum requirements on the thermo-physical performance of opaque and transparent building envelope components were set. however, also this new phase of development was driven by the ‘energy conservation approach’ and the attention was focused, almost completely, on measures aimed at limiting the energy demand for space heating – through the reduction of the transmission and ventilation losses – and to maximize the solar free gains. from a technological point of view that meant: increasing the thickness of the insulation layers in the opaque walls, improving the thermal transmittance of the glazing and the air-tightness of the buildings, installing heat recovery units and enhancing the harvesting of the solar energy (either ‘passively’, through transparent building envelope components and/or solar greenhouses, or ‘actively’, by means of solar thermal systems). proofs of such conventional wisdom are the studies done at the mit in boston (hottel, 1989), during that period. the facades that today ‘dress’ the majority of the buildings are the result of such technological evolution and even if the awareness for the energy sustainability and indoor environmental quality 146 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes (ieq) has grown, still many designers pay little attention to the ‘optimization’ of the building envelope with respect to the needs of the indoor environment, to the exposition and to the climatic context, as one can see in figure 2. certainly, in the course of 20–30 years, the widespread and continuous application of this strategy led to a drastic reduction of the energy demand for space heating and allowed to cover a relevant portion of the demand through the use of renewable sources (mainly solar). the obtained benefits were unquestionably relevant and allowed to realize more sustainable buildings, especially at the beginning of the application of this strategy. however, as the progress went on, the limits of this approach started to show up in all their severity and the drawbacks worsened over the time. specifically, three major issues can be highlighted: – overheating of the built environment, caused by the ‘unbridled’ and uncritical application of the ‘energy conservation’ principles. if on the one hand the previously discussed measures allowed to reduce the energy demand for space heating, on the other, they often increased the cooling loads to a significant extent. such phenomenon is particularly relevant in case of non-residential buildings, where the improvement of the thermal insulation came along with the rise of the endogenous thermal loads. it is common knowledge that today many modern office buildings need to be cooled also during the winter season, even in cold climate locations. – change in the relative weight of the heating, cooling, lighting and electric energy demand. the application of an optimization process aimed at solely addressing the problems of space heating and domestic hot water production, for a period of some decades, led up to a significant reduction of the heating demand. at the same time, the expectations of occupants for higher levels of indoor environmental quality, either from the thermo-hygrometric point of view during the summer season or for the visual comfort, combined by an increasing use of electric appliances, caused the other entries of the energy balance of a building to extend their influence. the energy consumption related to cooling, artificial lighting and plug loads is today comparable with the heating energy demand. in particular, it is worth mentioning, that the optimization of natural hong kong cina tdesign, winter ≈ 9 °c hdd ≈ 170* °c⋅d tdesign, summer ≈ 34 °c seoul – south korea tdesign, winter ≈ -11 °c hdd ≈ 2684* °c⋅d tdesign, summer ≈ 32 °c helsinki finland tdesign, winter ≈ -24 °c 4786* °c.dhdd ≈ tdesign, summer ≈ 25 °c tokyo japan tdesign, winter ≈ 1 °c hdd ≈ 1569* °c⋅d tdesign, summer ≈ 33 °c fig. 2. typical examples of building envelopes which were not designed in relation to exposition and climate: different expositions and climatic contexts, same design philosophy of the facade. m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 147 lighting has often been a neglected element in the building design. therefore, even considering the energy improvements of the light sources (of about an order of magnitude), the overall energy consumption for artificial lighting has increased in percentage. – consequences due to the so-called ‘law of diminishing returns’ (shepard, 1974). this rule is common to many engineering and economic sectors, and states that in all processes, keeping to improve just one of the factors, while holding all others constant (‘ceteris paribus’), will at some point yield lower incremental per-unit returns (samuelson & nordhaus, 2001). in the case of the building envelope, this principle translates into the fact that the prolonged application of ‘the energy conservation approach’ has allowed to attain – so far – high performing solutions, but pushing this strategy any further will only provide marginal improvements, with ever growing costs. torcellini, pless, judkoff & crawley (2007), for example, demonstrated that even optimizing the technological solutions so far explored to their best limits, the maximum achievable improvement, compared to the current energy performance of buildings, can be at most of about 50%. a level that is far worse than the target imposed by the epbd recast directive of 2010 (european commission, 2010). a clear proof of such situation is represented by the trend of the loss coefficient for transmission, ht, and ventilation, hv, over the time. these two quantities are defined by european and national standards (uni en 12831, 2006) and are the ratio between, respectively: – the heat flux cumulatively lost by transmission through the facades and the outdoor-indoor air temperature difference, – the enthalpy flux lost with the ventilation airflow rate and the outdoor-indoor air temperature difference. figure 3 shows the evolution of these two parameters for an exemplary, ideal, building. ht has been calculated considering an archetypal building of 100-m2 floor area and assuming that all the transparent and opaque surfaces would have the u-value mandatorily set by the law at the corresponding 0 50 100 150 200 250 300 350 400 450 500 1940 1960 1980 2000 2020 2040 2060 lo ss c o effi ci en t [ w /k ] year ht hv fig. 3. transmission and ventilation loss coefficients versus time for a typical residential building (100 m2 of floor area, window area 1/8 of the floor area. ht =loss coefficient for transmission, hv =loss coefficient for ventilation). 148 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes year (or the presumable value at the future time). the hv coefficient has been assessed assuming a reasonable air permeability for the building envelope at a certain time and the corresponding average air change rate by infiltration. a minimum limit value of 0.5 1/h for the air change rate (ach) was however kept, that is the one typically required by the technical standards in order to provide an adequate iaq in residential buildings. as it is possible to see from figure 3, before the seventies, when thermal insulation was not mandatorily required, the energy losses for transmission was more than two times the energy losses for ventilation. by improving the performance of the building envelope, either in terms of thermal insulation and air tightness, it was possible to reduce these losses by a factor of about 3.5–4, in a period of fifty years. nevertheless, in 2006 the ratio between ht and hv was still of about 2. however, the further enhancement of the thermal insulation, enforced by the current and future regulations, is – and will – strongly change this picture. in 2020 the ventilation losses will, likely, exceed the transmission losses, since it will be not possible to reduce the ventilation airflow rate anymore (as mentioned, a minimum ach is always required to assure a satisfactory indoor air quality). this implies that, from now on, it will be unwise to give too much attention to the optimization of the thermal resistance of the facade. it will be far more effective to find a way for saving energy for the fresh air pre-heating. moreover, all these issues are being combined with a deep and rapid strengthening of the requirements regarding the energy efficiency in buildings. the new european legislative framework (european commission, 2010) in fact, sets the ambitious objective that all the new buildings will have to satisfy, within 2020 (2018 for public buildings), the ‘nearly zero energy building’ (nzeb) target. this picture reaffirms the key role played by the facade in the energy design of a building and demands for a revolution of the traditional constructive habits. a radical change in the dogma on which facade engineers and architects have based their professional certainties for decades is, nevertheless, required. in this paper this change of concepts will be analysed from a theoretical point of view, highlighting those key elements that need to be considered for the future technological development of facades. moreover, after this general overview, some practical examples of innovative solutions for responsive and dynamic building envelope components will be presented and discussed. they refer to a research activity that is running since the beginning of the year 2000 at the department of energy, politecnico di torino. the main operative features of the components and their measured performance will be presented and critically analysed in the perspective of their dynamic behaviour. 2. from a static envelope to dynamic and multifunctional facade modules – a paradigm shift ‘paradigm shift’ is today a trendy term, all too often abused in the community of architects and designers. besides its greek original meaning, a ‘paradigm’ in the philosophy of science identifies a disciplinary matrix for a given scientific community. in this matrix a globally shared vision is crystallized. probably khunz gave the best definition (1970): “it is a scientific result that is universally recognized and that, for a period of time, provides a model and solutions for a given scientific community.” therefore, a ‘paradigm shift’ (or revolutionary science) is a radical change in the basic assumptions that has ruled a certain research and/or technological environment for a long time. this definition perfectly fits in the current framework of the building envelope. as highlighted in the previous section, limitations and emerging requirements are demanding for it. m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 149 the principles of the ‘energy conservation’ approach, which saw the apex of its maturation in the idea of the ‘passive house’, appear today to be out-dated. to be able to fulfil the requirements set forth by the nzeb and/or zeb buildings, a wider view must be adopted. first of all, heating, cooling lighting and plug loads must be simultaneously addressed and the following ‘changes of mind’ must be achieved: – from ‘shield, barrier, separation’ to ‘interface, host location, management’. today it is mandatory to exploit the opportunities offered by the local climate and by the natural resources, instead of working against them, as suggested in the past by the energy conservation beliefs. we must switch from an attitude of denial to a positive approach. the envelope should be no more perceived as a ‘problem’, a ‘difficulty’, but it should be considered as an ‘opportunity’, a ‘potentiality’ and, of course, a ‘challenge’. the building envelope, more than a ‘construction component’, will have to be seen as a ‘place’, a spatial location. it represents the surface that divides the conditioned and controlled environment from the external environment, and through it all the mass and energy transfers take place. hence, the building envelope offers a huge interface that can be effectively used to manage the mass and the energy balances of the built environment and to host technologies for the exploitation of renewable (non-carbon) sources and/or low quality energies (low exergy). – from ‘invariable, static, generic’ to ‘dynamic, adaptive, responsive, customized’. the idea that the constructive elements of a building are components that do not change their structure and aspect, and whose features, properties, functions and behaviour are immutable, starts to be strongly limitative and unsatisfactory. moreover, the frequently adopted ‘one-sizefits-all’ approach, i.e. a generic facade configuration is suited for all the conditions (see e.g. figure 2), is not functional at all. the innovation of the building envelope that appears far more promising consists in passing from the concept of insulation to that of adaptability. the best facade is not necessarily the one that shows the higher air tightness, thermal insulation and the one that maximizes the solar heat gains. depending on the season, working conditions and user preferences the building envelope could be asked to allow a higher/lower heat flux to take place, to store/release the energy, or to tune the ventilation airflow rate or, finally, to adjust its transparency. – from ‘single function, single behaviour’ to ‘multifunctional and integration’. the vast majority of building envelope technologies currently available on the market is made of ‘passive’, e.g. ‘resistive’2, elements. even when some active technologies (for example pv systems) are present, they are more incorporated as an ‘addition’ to the facade than being functionally/structurally integrated. a significant improvement of the energy efficiency can only be achieved by conceiving ‘active’ components, where the equivalent electric network becomes a combination of resistors, capacitances and current generators. such multifunctional modules, typically host generation/conversion systems (as for example the technologies for the on-site exploitation of the solar radiation), play a role in the ventilation (being used as air heat exchangers, air pre-heaters, ventilation outlets/inlets, ducts etc.) and are combined with the lighting strategies. in order to be efficiently exploited, these dynamic components must not be integrated just adopting a mere ‘additive principle’ (e.g. ‘sticking’ the new component on a traditional 2that is, adopting the well-known electric analogy for the heat transfer, the traditional building envelope components can be represented by means of a network of resistors and capacitors. 150 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes facade), but they must become an intimate part of the hvac and artificial lighting systems and be controlled in a holistic and coordinated way with them. therefore, according to this paradigm shift, opaque and transparent components become a ‘living’ membrane used to “filter, store and/or modify the mass and heat fluxes, hence managing the external environmental parameters to satisfy the internal needs and to guarantee an optimal iaq” (goia, perino, serra & zanghirella, 2010). the keywords on which their development will have to be based are (van der aa, heiselberg & perino, 2011): responsivity, adaptability, dynamic behaviour, integration/interactivity, harmonization (tuning) with the indoor/outdoor environment, multi-functionality, as schematically shown in figure 4. 3. the different scale of the research translating this revolution of concepts from a vision to the practice is not trivial and a systemic approach is needed. some researchers (goia, 2013) suggested to subdivide and structure the research on three different levels, corresponding – to a certain extent – with the dimensional scales of the building envelope: – the concept level, – the system level, – the material level. clearly, the process is such that each level can ‘cross-fertilize’ the others, transforming in this way an apparently linear flow path into a research development that is iterative. 3.1. the concept level the aim of this level is to explore new ideas and visions, analysing them from a theoretical point of view in order to obtain information on the ‘working principles’. the objective is to identify which fig. 4. conceptual scheme of dynamic, responsive, multifunctional and integrated facades (adapted from van der aa et al., 2011). m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 151 lines to follow to conceive and develop new components and to optimize the strategies of integration with the systems and the building. from a practical point of view, this level tries to give an answer to the question of ‘what’ would be desirable to obtain from a new product, without worrying about ‘how’ to do it (hence no attention is put on the technological feasibility of the solutions or on the actual availability of materials with the desired properties). it is a very theoretical phase and, apparently, far from the applicative reality. nevertheless, it represents the fundamental step to establish, for example, what is the ‘range’ of properties that a dynamic envelope should have to satisfy the adaptability requirements and what is the ‘amplitude’ of their variability (see e.g. favoino, overend & jin, 2015). it is also useful to define control strategies, the degree of integration with other building/systems components, the multi-functionality level and the benefits in terms of energy savings. since the development of this kind of analyses needs a ‘total energy’ approach (at least heating, cooling and lighting must be simultaneously considered) the simulation instruments currently available are often inadequate and the researchers need to develop numerical models ad hoc. such a difficulty explains why a limited number of studies and papers can be found in the literature. a first and interesting attempt in this field was done by goia (2013), who developed a sensitivity analysis to evaluate the influence of various thermal properties of the building envelope on the total energy demand (to identify which are the most influential ones) and the range of variability which is needed to proceed to the energy optimization of the building. an example of product development based on the ‘concept level’ approach is available in loonen, singaravel, trčkal, cóstola & hensen (2014). more recently, kasinalis, loonen, costola & hensen (2014) presented a design and analysis approach for the so called cabs (climate adaptive building shell), exploring the possibility of conceptually ideate and optimize an adaptive facade (on seasonal basis) which allows for the minimization of the energy consumption. the obtained results show how a responsive building envelope component, which can dynamically modify six of its properties (density, specific heat and thermal conductivity of the material, external absorption coefficient of the surface, opaque on transparent surface ratio, typology of glazing), allows an improvement of 16–18% of the performance in comparison with the state-of-the art facades based on the traditional static concept (favoino, jin & overend, 2014b). 3.2. the system level this level is focused on the identification of technological solutions to build modular and multifunctional facade modules. the objective is to obtain building envelopes which are almost self-sufficient from the energy point of view, creating systems which integrate many functionalities (for example: ventilation, thermal exchange and heat recovery, lighting, thermal storage, energy conversion systems, hvac components, solar shading devices etc.). even though the conception of a self-sufficient skin appears to be a very ambitious goal, and its translation into a product ready for the market even more difficult, this idea is considered very promising. proof is that some leader companies in the sector are developing, or have already proposed, multifunctional facade modules inspired on this philosophy (for example the e2 facade from schüco and temotion from wicona). quite numerous are also the examples of studies on these kind of envelopes which are available in the literature (quesada, rousse, dutil, badache & hallé, 2012a and 2012b; saadatian, sopian, lim, asim & sulaiman, 2012). 152 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes even though the configurations can be extremely various, it is however possible to identify some basic features which are common to the multifunctional building facades: – the management of the heat fluxes. this is obtained – in general – minimizing the transmission losses through the component (maximizing its thermal resistance) and controlling the heat transfer by means of the ventilation, or the charge/discharge of thermal storages (in facade) or, eventually, with the use of heat transfer fluids and devices (e.g. peltier cells); – the on-site conversion of solar radiation, done with active systems (for example pv, pvt, etc.) or with passive techniques (managements of free gains); – the reduction of the energy inefficiencies at the building scale (for example avoiding long ducts for ventilation, which cause pressure drops); – the integration with mechanical systems, using the facade as: a terminal device, a heat recovery system, a ventilation outlet/inlet, or as a source/sink of thermal energy; – the optimization of daylighting; – the energy storage capability (using the envelope as a lhtes. latent heat thermal energy storage system). examples of mfm are shown and analysed in xu and van dessel (2008) and in favoino, goia, perino & serra (2014a). a practical example of these multifunctional facade modules will also be presented and discussed in section 4.1. 3.3. the material level this is the phase where most of the detailed studies have been developed. the objective is to identify those materials and/or sub-components whose features are suitable to build multifunctional facade modules and to characterize their behaviour. it is hence a functional and preliminary step for the design at the system level. the aim is to answer the question ‘how’ to practically implement the outcomes obtained in the concept level. the incredible development of the material science that took place in the last decade, and the consequent availability of new products, offer great opportunities for the realization of innovative envelope components. among others, the following materials show the most promising application: – super-insulating materials (especially vips – vacuum insulation panels – and aerogels), – gas-filled panels, – phase change materials (pcms, slurry pcms), – non-conventional glazings (thermotropic, photochromic and electrochromic glazings), – coating and membranes with selective/reflective optical properties, – coatings with specific physical/chemical behaviour (hydrophobic finish, photocatalytic coatings etc.). in section 4.2 some researches performed on transparent components will be presented; they are focused on the integration of phase change materials into conventional double/triple glass units. m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 153 4. practical examples in the following sections an overview of some of the most significant research projects carried out by the tebe research group at the department of energy – politecnico di torino will be given. this field of research started in the year 2000 and, along the time, has been focused on various innovative building envelope components, either transparent or opaque. specifically, the activities were mainly performed at material and system level and concentrated on multifunctional transparent facades (climate facades, hybrid-ventilated facades, naturally ventilated facades. corgnati, perino & serra, 2007; serra, zanghirella & perino, 2010; goia, perino, serra & zanghirella, 2010), smart glazing systems (with a special attention on pcm based configurations. goia et al., 2013; goia et al., 2014; goia, zinzi, carnielo & serra, 2015) and multifunctional facade modules (mfms) (favoino, goia, perino & serra, 2012; favoino et al., 2014a). 4.1. actress, an example of a multifunctional facade module the actress (active, responsive and solar) prototype was designed as a prefabricated unit of one storey high (3,50m). it consists of an opaque sub-module (osm) and a transparent sub-module (tsm) (fig. 5). the window to wall ratio (wwr) was determined on the basis of a preliminary numerical simulation. a wwr of about 50% demonstrated to be one with the lowest total (cooling, heating and lighting) yearly energy demand for the local climate of torino, regardless the orientation of the facade (favoino et al., 2012; goia, haase & perino, 2013). the actress module is characterized by a moderate ‘hardware’ integration with a potential hvac system, but by a strong functional coupling with the mechanical installations. the opaque module is a ventilated facade with the external skin made of three pv panels (amorphous asi, nominal fig. 5. the actress prototype (left) and a sketch of the various components (right). 154 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes power 87w). the ventilated cavity can be operated to provide various ventilation strategies (supply air, outdoor air curtain and exhaust air) and modes (natural, hybrid and mechanical ventilation; a thermal buffer mode, with inlet and outlet grids closed, may also be realized to improve the thermal insulation). the inner skin is made of a gypsum-board panel that provides mechanical resistance to the wall structure, a vacuum insulation panel (vip, thickness 25mm, � 0.005w/mk) and two layers of phase change materials (pcms, melting temperatures of 27◦c and 23◦c). finally, three electric heated carpets (heat output: 50w each), directly powered by the pv panels, are located in between the pcm layers, thus allowing for an active thermal energy storage (lhtes, e.g. activation of the pcm on-demand). the pv panels are integrated in the facade with the specific aim of improving the energy self-sufficiency of the module, rather than ‘generically’ produce electric energy to be sold to the network. according to this concept, the power converted by the pv panels can be directly used in the actress module to power the fans (when the air cavity is mechanically ventilated) and the shading device, or to store thermal energy in the pcm layers. if the electric energy production exceeds the module self-demand, the extra production can be sold to the network. the transparent sub-module presents a lower degree of complexity. it is made of two glazing systems: the lower glazing (about 2/3 of the total transparent surface) is a triple low-e coated glazed unit (6/15/6/15/6) with argon and with the outer cavity hosting a high reflective, low-e coated venetian blind, for solar and light transmission control; the upper part of the transparent sub-module is made of a triple-glazing, whose outer cavity is filled with granular, translucent, aerogel (� aerogel 0.009–0.012w/mk), while the inner cavity is filled with argon. the key aspect of the actress modules lies in its dynamic and active properties, which can be continuously modified to achieve better energy and comfort performance. the highest degree of dynamicity is reached in the opaque module where several combinations of ventilation strategies and thermal energy storage activations may result in very different behaviours. the transparent module instead shows a more limited degree of responsiveness. this surface, in fact, is made of highly performing technologies, but only the solar shading system allows for a dynamic behaviour (which, nevertheless, affects to a great extent the overall performance of the facade). during the summer season, the opaque module typically adopts an outdoor air curtain strategy and operates in mechanical mode (for days with low solar radiation natural ventilation can be used instead). the pv panels provide the power to activate the fans and to control/displace the shading device. the forced ventilation of the air cavity allows to reduce the solar heat gains and, at the same time, lowers the temperature of the pv panels increasing their efficiency. the super insulation layer (vip) helps in thermally disconnecting the indoor environment from the air cavity, while the pcm layers increase the thermal inertia of the room. specifically, the pcm is exploited as a passive thermal storage, smoothing and delaying the thermal wave through the facade and reducing/shifting the cooling loads caused by internal and solar gains. the operation of the transparent module is more straightforward. since in this period the concern is to prevent the overheating of the indoor environment, the shading device is operated in order to avoid the direct solar radiation to enter into the room. the upper glazing, thanks to its light diffusing behaviour, the elevated position and the good thermal insulation properties aim at providing the best opportunities for daylighting, keeping the solar heat gains as low as possible. during the winter season, the opaque module operates either with the supply air strategy or the thermal buffer, depending on the outdoor air temperature and solar radiation levels. if the incident solar radiation is high enough, the air cavity can be used to pre-heat the ventilation air and the facade is configured in the supply air strategy (natural and mechanical ventilation modes can be used, depending on the incident solar radiation). during the night, or during cloudy days, the thermal m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 155 buffer configuration is adopted, since it allows a better thermal insulation and helps in reducing the heat losses through the envelope. the electric energy converted by the pv panels is mainly used to feed the heated carpets and to produce thermal energy that is stored in the two pcm layers. this allows to accumulate the thermal energy during the day, when the heating demand is low, and to use it afterwards, when the heating demand increases (better match between energy demand and production). finally, as far as the transparent module is concerned, the shading device can be operated, both to control solar gains and to prevent glare discomfort, by means of the electric motor powered by the pv layer. a full-scale prototype of the actress module was built, installed on the twins facility (corgnati et al., 2007) and tested for the local climate of torino (45.08n, 7.68e, 2894 hdd) during a period of almost two years. the measurement apparatus consisted of a total of 68 sensors (52 thermocouples, 9 heat-flux meters, 4 pyranometers, sensors to measure fans angular speeds, pv voltage and current), connected to a data logger. a comprehensive and critical analysis of the data can be found in favoino et al. (2014a). in the following, for the sake of brevity, some resumed results will be discussed. as expected, the overall thermal insulation of both the opaque and transparent modules is excellent (table 1). the additional use of a low-e coated solar shading device allows to improve the thermal insulation of the glazing of about 2%. the aerogel glazing, though it has a very good thermal insulation, presented severe drawbacks due to the very high surface temperatures it reached. peak values as high as 45◦c were measured, which can seriously compromise the thermal comfort conditions in the room. the aerogel glazing overheating (due to the absorbed solar radiation), also showed a disturbing ‘thermal tail’ that lasts about four hours and affects the performance of the component also during the late afternoon/night. in relation to the dynamic features of the facade, the adoption of the supply air strategy (with natural ventilation) during the winter demonstrated to be an effective measure to preheat the ventilation air. the measured pre-heating efficiency (corgnati et al., 2007), assessed for the daytime, highlighted the following achievements: – for about 10% of the daytime the air can be effectively preheated in the air cavity and directly supplied to the indoor environment at the indoor design temperature of 20◦c, thus zeroing the ventilation losses; – for 5% of the working time, the air can be heated at a temperature above 20◦c (that is, the facade becomes an active element being able to help heating the room); – for more than 80% of the working time it is possible to have a certain degree of pre-heating of the ventilation air in the cavity (that is: the pre-heating efficiency is positive). during the summer, to limit the heat gains, the outdoor air curtain strategy is used instead. for these operative conditions the dynamic insulation efficiency is of interest (corgnati et al., 2007). its value revealed to be around 1 for about 90 % of the working time (meaning that the opaque module table 1 measured u values for the osm and tsm component −→ osm tsm – aerogel tsm – lower glazing tsm – lower glazing glazing (raised venetian blind) (lowered venetian blind) u – value [w/(m2k)] 0.08 0.58 0.63 0.62 156 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes -200.00 -150.00 -100.00 -50.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 -20.00 -15.00 -10.00 -5.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 0 .0 0 4 .0 0 8 .0 0 1 2 .0 0 1 6 .0 0 2 0 .0 0 0 .0 0 4 .0 0 8 .0 0 1 2 .0 0 1 6 .0 0 2 0 .0 0 0 .0 0 4 .0 0 8 .0 0 1 2 .0 0 1 6 .0 0 2 0 .0 0 0 .0 0 4 .0 0 8 .0 0 1 2 .0 0 1 6 .0 0 2 0 .0 0 0 .0 0 q to t [w /m 2 ] q to t [w /m t [hh:mm] 2 ] q_osm q_actress (tsm+osm) 29/09/2011 summer season sunny 06/10/2011 summer season cloudy 07/10/2011 mid season sunny 14/10/2011 mid season cloudy fig. 6. total heat flux exchanged through the opaque sub module (osm) and the actress modules – time profiles. behaves almost like an adiabatic surface). this excellent result is due to the coupling of a static feature, that is, the very low thermal conductance of the inner skin, and a dynamic behaviour, that is, the cavity ventilation and the presence of a latent heat thermal energy storage (lhtes) that can store thermal energy during the peak period of the heat gains and release it afterwards (e.g. passive activation of the pcm). in particular, the lhtes presents an interesting behaviour and is significantly improving the building element responsiveness. this is clearly shown for example in figure 6, where the time profiles of the specific heat fluxes through the opaque module alone (black dotted line) and through the whole actress module (red line) are plotted versus time, for typical days of the summer season. as expected, the transparent module is responsible of the major part of the energy entering into the room through the facade, but its profiles present peak values that are simultaneous to the peak value of the specific heat fluxes absorbed by the pcm layers. as it is possible to see, in peak conditions, the lhtes is able to remove from the room a certain amount of the specific heat fluxes entering through the facade. unfortunately, the size of these two heat fluxes is quite different (fig. 6) and an improvement in both the amount of the thermal energy that can be buffered in the opaque module and in the heat exchange mechanisms is still needed. also during the winter the activation of the lhtes by means of the pv panels provides significant improvement in the energy efficiency of the system, even if the monitoring campaign highlighted a non-optimal exploitation of the stored energy. the overall average efficiency of the pv+lhtes system, in fact, was about 25%. finally, the overall energy performance of the actress prototype was synthetically evaluated by means of the specific daily energy, e24, exchanged through the facade along the 24 hours. specifically, three quantities have been used, that is: the amount of energy that enters, e+24, or leaves, e – 24, the indoor environment through 1 m2 of the facade module, and the net value e+24 = e+24– ∣ ∣e–24 ∣ ∣, being: e + 24 = ∫ 6 am+1 6 am q̇ + tot · dτ amfm [wh/m2] e−24 = ∫ 6 am+1 6 am q̇ + tot · dτ amfm [wh/m2] (1)3, m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 157 table 2 measured values of the overall thermal energies exchanged between the outdoor and the indoor environment through the actress module average average daily irradiance e−24 e + 24 e24 operating conditions day toutdoor [◦c] [wh/m2] [wh/m2] [wh/m2] [wh/m2] winter –0.3 4649.1 –76.84 236.37 159.53 supply air & natural ventilation. lhtes activated with the pv. lowered venetian blinds. summer 22.2 2537.9 –24.84 212.86 188.02 outdoor air curtain & mechanical ventilation. lhtes passively exploited. lowered venetian blinds mid-season 16.5 2563.9 –48.35 254.63 206.28 outdoor air curtain & mechanical ventilation. lhtes passively exploited. no venetian blinds where q̇+tot and q̇ − tot are the entering/exiting total specific heat fluxes measured during the monitoring (they account, cumulatively, for convective, short and low wave heat exchanges). as an example, table 2 resumes the measured e24 values for an average winter, summer and mid-season day, respectively. as one can see, the heat losses during the winter are very low. the presence of the lhtes, actively charged during the daytime by means of the pv system, can compensate, to a large extent, the heat fluxes exiting during the night. on the overall4, the facade ensures a net positive energy even in the cold season (e.g. e24 > 0). it is worth noting how the energy exchanges through the module can be effectively managed by adopting a proper choice of the operational modes and of the adaptable features of the component; as a result the e+24 is of the same order of magnitude during the whole year. furthermore, during the summer, by means of the passive activation of the pcm layers, it is also possible to have a negative energy exchange (e.g. reject part of the heat gains towards the outdoor environment: e–24 = –24.84 wh/m2), which helps in reducing the energy demand for cooling. roughly considering the heat fluxes to be positive for twelve hours (during the day) and negative for the remaining time (during the night), the thermal energies shown in table 2 would correspond to average specific heat fluxes5 between: • 6.4w/m2 and +19.6w/m2 in winter • 2.1w/m2 and +17.7w/m2 in summer • 4.0w/m2 and +21.2w/m2 in mid-season. 3the superscript ‘+’ means that only positive heat fluxes are taken into account in the integration (and viceversa for the superscript ‘-’). 4care must be taken when net energies are analysed. the integration over a day does not allow to account for possible mismatch problems (that is positive heat fluxes during the day that cannot be exploited to compensate the heat losses during the night). in this case, thanks to the lhtes system, the likelihood of mismatch between energy demand and production is reduced. still problems of non-complete exploitation of the diurnal energies may arise. 5these are mean values over the day and over the whole facade module. it has to be remembered that during the three different seasons the facade is operating adopting different working strategies. 158 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 4.2. smartglass, a responsive glazing integrating pcms a new concept of transparent facade was developed, integrating phase change materials pcms in the air gap of multiple glazing in order to better exploit solar heat gains and to increase the thermal inertia of the glazed system. two configurations were tested (fig. 7): the first adopting a paraffin wax with melting temperature 35◦c inserted in a double glazing, mainly aimed at reducing the cooling loads in the summer period; the second coupling the same pcms, inserted in a triple glazing, with a thermotropic glazing (tt+pcm), so to increase the responsiveness of the system to solar radiation, thus achieving a better control on the ‘charge’ phase of the pcm layer and contemporarily improving the winter behaviour thanks to a higher thermal resistance. a detailed literature review of the developed and tested pcm glazing systems can be found in goia et al. (2013), bianco (2014), goia, bianco, cascone, perino & serra (2014), goia et al. (2014) and goia et al. (2015). the main expected benefits of a glazing integrating pcms are: – during the summer period, a reduction and shift of the solar heat gain through the glazing, which means a decrease in cooling loads and in cooling energy demand; – during the winter period, a reduction in the ‘mismatch’ between the heating energy demand and the solar energy availability thanks to the buffering effect; – an improvement of thermal comfort, due to a better control of the internal glass surface temperatures; – a better control of the visible solar radiation and a reduction in the glare risk. as far as the first configuration is concerned, research has proved that the pcm glazing concept can be effective only when some conditions are realized. some drawbacks can appear and need to be properly managed. in particular the control on the ‘charge’ phase of the pcm layer is necessary to prevent overheating phenomena due to the complete melting of the pcms, as well as the control on the ‘discharge’ phase is also necessary to avoid either unwanted heat loss (in the winter period) or heat gain (in the summer period). the ‘charge’/‘discharge’ phase control of the pcm layer is the main relevant aspect to be faced. the first configuration, even showing a high potential in reducing direct solar irradiance entering the room fig. 7. glazing filled with pcms: double-glazing configuration and triple-glazing configurations (pcm in and pcm out) coupled with a thermotropic. m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 159 (reduction of 1/5 of the entering energies if compared to a conventional clear+clear double glazing), didn’t allow for an optimized control of solar radiation, since in days characterized by high solar irradiance the melting process at 4 p.m. was completed and very high internal surface temperatures were reached thus causing noticeable thermal discomfort. moreover the pcms glazing shows a low performance in the winter period, if compared to a conventional argon filled and low-e coated double unit glazing, due to the higher conductivity of pcms and the impossibility to reduce the ir exchange through the adoption of a low-e coating. starting from this experience a new concept has been conceived and tested: a thermo-tropic (tt) layer was included in the pcm window, so that the complete melting of the pcm layer could be avoided, and a triple-glazed configuration was adopted in order to increase the thermal resistance of the system (pcm+tt). this new prototype was expected to provide a better control on solar energy transmitted and to present an increased thermal inertia, thus acting simultaneously as solar shading device, storage medium and glazing surface temperature moderator. the adoption of the tt layer allows for a dynamic control of the solar transmission, switching from an ‘off’ to an ‘on’ state in the temperature range of 20–40◦c with a corresponding solar transmittance ranging from 0.69 to 0.41. the triple glazing (tgu) had one cavity filled with pcm and the other one filled with argon; moreover the two glass surfaces facing the argon-filled cavity were low-e coated. the thermo-tropic glazing was placed outside, in front of the tgu, and two configurations were tested, placing the pcmfilled cavity towards the outdoor (tt+pcm out) and towards the indoor environment (tt+pcm in) respectively. the coupling between pcms and tt layer was chosen for the very good match between the switching range of the tt layer and the phase change range of the pcms: actually when the pcm starts its phase change, its absorptance decreases and the transmittance increases and, contemporarily, the tt layer starts to reduce its transmittance, thus lowering the solar radiation reaching the pcm layer. the pcm+tt prototypes were tested by means of the outdoor test cell facility twins and compared with a reference tgu (fig. 8). the two samples were installed on the same facade (south exposed) and continuously monitored through a measurement apparatus consisting of thermocouples, heat flux meters and pyranometers. a detailed description of sensors and of the measurement methodology are reported in goia et al. (2014). for the sake of brevity, just the more relevant results are reported here. the role of the increased thermal inertia in the tt+pcm is shown in figure 9, where the time profile of the surface heat flux is plotted for different ‘typical days’, comparing the two configurations (tt+pcm in and tt+pcm out) with the reference technology (tgu). it is evident that both the configurations show the ability in smoothing and shifting (to a certain extent) the peak of the heat flux exchanged by convection and long-wave radiation. the highest reduction can be observed in days characterized by high solar irradiance, when the entering heat flux is reduced to about one third if compared with the reference tgu. during the evening, the ‘discharge phase’ of the pcm layer occurs and the energy accumulated within the pcm layer is released in the indoor environment from approximately 6 p.m. onwards. at that time, the heat flux exchanged in case of the reference technology drops very quickly as soon as the facade is no longer exposed to direct solar irradiation. this phenomenon is particularly relevant when the day is sunny, while it is almost negligible during cloudy days. 160 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes a) c)b) fig. 8. prototypes installed in the test cell (a), pcms partially melted in the lower glazing (b) and different transmittance of the thermotropic layer (c). fig. 9. time profiles of specific ‘surface’ heat flux – tt+pcm in and reference (a) and tt+pcm out and reference (b). the time profiles of the specific ‘total’ heat flux exchanged by the glazing systems is shown in figure 10. the different behaviours of the two tt+pcm configurations can be noticed, especially when high solar irradiance is present: while the configuration with the pcm in the innermost cavity is always able to reduce the total heat flux, regardless the boundary conditions, down to a maximum value of 35w/m2, the other configuration is only able to reduce the total heat flux down to a maximum value of approximately 70w/m2. this lower reduction, which is, nevertheless, relevant if compared with the reference technology (for which the total heat flux reaches almost 250w/m2), is due to the fact that, while in the first configuration pcms do not complete the phase change, in the other one, due to the position of the pcm layer in the outermost cavity, the whole pcm layer melts and the latent heat of fusion is completely exploited. the effect becomes more and more evident when the specific daily net energy e24 is assessed. if compared to the reference technology (tgu), the configuration with the pcm in the innermost cavity (tt+pcm in) is able to reduce e24 by 99%, to 82%, depending on the solar irradiance amount, while the configuration with the pcm in the outermost cavity (tt+pcm out) shows a very similar m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 161 fig. 10. time profiles of total heat flux – tt+pcm in and reference (a) and tt+pcm out and reference (b). performance in case of low moderate irradiance, but when high solar irradiance is present (combined with high outdoor air temperatures) the reduction in e24 is just 68%. the different position of the pcm layer thus affects the thermo-physical behaviour of the system and makes one configuration more efficient than the other. as far as the winter performance is concerned the obtained thermal transmittance for the tt+pcm system (assessed considering nocturnal value and pcms in the solid state) and for the reference tgu shows that the insertion of the pcms do not sensibly increase the heat transmittance of the glazing components, and a u-value of about 0.85w/m2 was found for all the glazing systems. as a drawback in the winter period the melting temperature of the tested pcms (rt35hc), which were identified as the most appropriate for the cooling loads control, is too high to allow the phase change to occur thus limiting a full exploitation of solar heat gain entering the room. the potential of this technology is quite high but the adoption of a pcm layer characterized by a single melting temperature considerably limits the dynamicity of the system and the responsiveness under different boundary conditions. moreover the thermotropic layer shows a slightly translucent behaviour, even when in the off state, thus creating an unpleasant haze effect. in order to overcome the main criticism some other strategies are currently under development, i.e. the adoption of electrochromic devices rather than the thermotropic glazing or polymeric layers to be used in the gap as a glazing spacer to allow for the coupling between two different pcms. 5. conclusion the key role of the building envelope towards highly energy efficient building has been established for a long time. nevertheless, until recent times, all efforts and attention have mainly been focused on increasing and optimizing the thermal insulation of the envelope components. this approach, in the forthcoming period dominated by zero energy building (zeb), reveals to be no more viable since, as the energy efficiency of the whole building is increasing, any further upgrade of building envelopes energy performance provides quite limited effects. 162 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes new and more revolutionary concepts and technologies need to be developed at concept, system and material scale, working on the fact that the building envelope can play a key role in controlling the energy and mass flows from outdoors to indoors (and vice versa) and, moreover, the facades offer a significant opportunity for solar energy exploitation. it appears necessary to switch from ‘static’ to ‘responsive’ and ‘dynamic’ systems, such as multifunctional facade modules (mfms) and responsive building elements (rbe), identifying technological solutions able to continuously and pro-actively react to outdoor and indoor environment conditions and facilitate and enhance the exploitation of renewable and low exergy sources. in this paper an overview of the technological evolution of the building envelope that has taken place, ranging from traditional components to the innovative skins, has been given, focusing on one hand on the different approaches that have characterized this development and, on the other hand, providing some examples of innovative solutions and highlighting the main potential and criticism resulting from an intensive experimental activity on this field. acknowledgments results presented in this paper are the main outcomes of a decennial activity on advanced building envelope components, carried out by the authors mainly in the framework of two research projects (smartglass, polight and prin-miur 2007). the authors are grateful to all the ph.d. students involved in the research activity (fabio zanghirella, francesco goia, lorenza bianco, ylenia cascone, fabio favoino and stefano fantucci) and to all the technicians involved in the experimental set-up. references ashrae (2013). handbook of fundamentals, ashrae, 1791 tullie circle, atlanta. bianco, l. (2014). involucri trasparenti innovativi: modellazione e sperimentazione su componenti dinamici e sistemi di facciata attivi. phd thesis, turin, italy. corgnati, s. p., perino, m., & serra, v. (2007). experimental assessment of the performance of an active transparent façade during actual operating conditions, solar energy. journal of the international solar energy society, 81(8), 993-1013. european commission (2010). directive 2010/31/eu of the european parliament and of the council of 19 may 2010 on the energy performance of buildings (recast). official journal of the european union, 53, 13-35. favoino, f., goia, f., perino, m., & serra, v. (2012, may). energy performance assessment of an advanced responsive multifunctional facade module: first results of an experimental campaign. proceedings of the 5th international building physics conference (ibpc 2012) (pp. 28-31). kyoto, japan. favoino, f., goia, f., perino, m., & serra, v. (2014a). experimental assessment of the energy performance of an advanced responsive multifunctional façade module. energy and buildings, 68 (part b), 647-659. favoino, f., jin, q., & overend, m. (2014b). towards an ideal adaptive glazed façade for office building. energy procedia, 62, 289-298. favoino, f., overend, m., & jin, q. (2015). the optimal thermo-optical properties and energy saving potential of adaptive glazing technologies. applied energy, 156, 1-15. goia, f. (2013). dynamic building envelope components and nearly zero energy buildings – theoretical and experimental analysis of concepts, systems and technologies for an adaptive building skin. phd thesis, trondheim, norway. available in http://urn.kb.se/resolve? urn=urn:nbn:no:ntnu:diva-23867 goia, f., perino, m., serra, v., & zanghirella, f. (2010). towards an active, responsive and solar building envelope. journal of green building, college publishing, 5(4), 121-136. goia, f., haase, m., & perino, m. (2013). optimizing the configuration of a façade module for office buildings by means of integrated thermal and lighting simulations in a total energy perspective. applied energy, 108. goia, f., bianco, l., cascone y., perino, m., & serra, v. (2014). experimental analysis of an advanced dynamic glazing prototype integrating pcm and thermotropic layers. energy procedia, 48, 1272-1281. goia, f., perino, m., & serra, v. (2014). experimental analysis of the energy performance of a full-scale pcm glazing prototype. solar energy, 100, 217-233. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-23867 http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-23867 m. perino and v. serra / switching from static to adaptable and dynamic building envelopes 163 goia, f., zinzi, m., carnielo, e., & serra, v. (2015). spectral and angular solar properties of a pcm-filled double glazing unit. energy and buildings, 87(1), 302-312. hottel, h. c. (1989). fifty years of solar energy research supported by cabot fund. solar energy, 43, 107-128. kasinalis, c., loonen, r. c. g. m., costola d., & hensen j. l. m. (2014). framework for assessing the performance potential of seasonally adaptable facades using multi-objective optimization. energy and buildings, 79, 106-113. khunz, t. (1970). the structure of scientific revolutions, the university of chicago, second edition, enlarged, usa. loonen, r. c. g. m., singaravel s., trčka, m., cóstola d., & hensen j. l. m. (2014). simulation-based support for product development of innovative building envelope components. automation in construction, 45, 86-95. perino, m., & serra, v. (2011). l’innovazione dell’involucro trasparente: oltre il concetto di isolamento termico, 48◦ congresso internazionale aicarr – ‘il recupero energetico degli edifici esistenti: quali soluzioni per un sistema integrato, l’involucro, gli impianti e la regolazione’, atti 48◦ convegno internazionale aicarr (pp. 61-79). quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012a). a comprehensive review of solar facades. opaque solar facades. renewable and sustainable energy reviews, 16, 2820-2832. quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012b). a comprehensive review of solar facades. transparent and translucent solar facades. renewable and sustainable energy reviews, 16, 2643-2651. saadatian, o., sopian, k., lim, c. h., asim, n., & sulaiman, m. y. (2012). trombe walls: a review of opportunities and challenges in research and development. renewable and sustainable energy reviews, 16, 6340-6351. samuelson, p. a., & nordhaus, w. d. (2001). microeconomics (17th ed.). mcgraw-hill. serra, v., zanghirella, f., & perino, m. (2010). experimental evaluation of a climate façade: energy efficiency and thermal comfort performance. energy and buildings, 42, 50-62. shepard, r. w. (1974). the law of diminishing returns. lecture notes in economics and mathematical systems, 99, 287-318. torcellini, p., pless, s. d., judkoff, r., & crawley, d. (2007). solar technologies and the building envelope. ashrae journal, 14-22 uni – en 12831 (2006). method for calculation of the design heat load, cen and uni standard. van der aa, a. heiselberg, p., & perino, m. (2011). annex 44 – final report – designing with responsive building elements. aalborg: aalborg university. xu, x., & dessel, v. (2008). evaluation of a prototype active building envelope window-system. energy and buildings, 40, 168-174 www.schueco.com/web/uk/unternehmen/presse/trade fair innovations/schueco e2 facade system mature. http://www.wicona-int.com/en/product/facade/temotion-intelligent-facade-concept/ www.schueco.com/web/uk/unternehmen/presse/trade_fair_innovations/schueco_e2_facade_system_mature http://www.wicona-int.com/en/product/facade/temotion-intelligent-facade-concept/ jfde journal of facade design and engineering jfde journal of facade design and engineering jfde / journal of façade design and engineering 093 journal of facade design & engineering volume 5 / number 1 / 2017 solar pv building skins – structural requirements and environmental benefits claudia hemmerle1 1 center for sustainable building, technical university of munich, arcisstr. 21, 80333 munich, germany, tel. +49.89.289.23990, fax +49.89.289.23991, claudia.hemmerle@tum.de abstract the majority of the photovoltaic (pv) modules used in building skins contains glass, but does not entirely comply with the product standards and design rules for glass in building. as a result, structural applications are subject to individual approval by the building authorities in many cases. this paper presents experimental research on glass based photovoltaic modules, analysing their mechanical properties in comparison with approved construction products. the focus is on glass-glass modules and on the question whether the most common module configurations can be classified as laminated safety glass. testing included residual resistance testing to study the potential to provide residual load-bearing capacity and shear testing to examine the interaction of photovoltaic cells and interlayer material as well as adhesion characteristics. if approved interlayers are used, glass-glass modules correspond to the safety level of laminated safety glass, because the pv integration does not impair breakage behaviour and improves residual resistance, while the observed reduced adhesive bond does not imply a higher injury risk. formal classification of photovoltaic products within the product and design standards for glass in building could facilitate the use of building-integrated photovoltaics. life-cycle assessments of photovoltaic systems so far concentrated on roof-top and ground-mounted installations. based on these studies, the specific environmental performance of building-integrated systems was analysed. constructive integration of the pv modules associated with the substitution of conventional materials in the building skin reduce the life-cycle environmental impacts like primary energy demand and greenhouse gas emissions, especially in those areas with suboptimal solar irradiation like façades. the net energy payback times calculated for central european range from 0.8 and 5.6 years and the net carbon footprint varies between 12 and 192 g co 2 -eq/kwh. keywords photovoltaics, glazing, laminated safety glass, approval, energy payback time, carbon footprint doi 10.7480/jfde.2017.1.1528 094 journal of facade design & engineering volume 5 / number 1 / 2017 1 introduction solar electricity produced by photovoltaic (pv) systems will play a major role in future energy supply systems. integrating pv modules into buildings’ envelopes can stimulate new architectural applications and improve sustainability of both pv power generation and buildings. the majority of the pv modules used in building skins contains glass as cover and backing material. their mechanical performance as glazing product has not been adequately characterized (dibt, 2012). type approval and0 safety qualification according to the international electrical standards (din, 2006; din, 2009; din, 2007; din, 2012a) as well as quality control in module production are adequate for non-structural applications including so-called building attached pv (bapv) modules in roof-top systems and simple building integrated (bipv) applications like pv modules used as roof covering (dibt, 2012). in contrast, these standards are not sufficient for structural applications like façades or overhead glazing (dimova, pinto, feldmann, & denton, 2014; schneider, kleuderlein, & kuntsche, 2012). here, the basic requirements for building as well as the safety level, product standards and design rules for glass components apply. generally, mechanical resistance and stability as basic requirements for construction works can be verified using regulated construction products with mechanical properties defined in product standards in combination with approved design methods and rules. pv modules do not entirely comply with these standards. as a result, structural bipv applications are subject to individual approval by the building authorities in many cases. this paper presents experimental research on glass based photovoltaic modules, analysing their mechanical properties in comparison with regulated construction products. the aim is to provide a scientific basis for a formal classification of pv modules as regulated construction products, enabling their use without further approval in the future. life-cycle assessment (lca) is a common method to characterize ecological impacts and benefits of products and processes. it describes the energy and material flows in all live-cycle stages. the environmental impacts of photovoltaic systems heavily depend on the cell technology and the associated energy demand for different production procedures, e. g. polycrystalline or monocrystalline type wafers or different deposition methods for thin-film cells. in this context, the electricity mix at the production site has a significant influence. benefits arise from the generation of electricity from solar radiation without resource consumption and pollution. the potential pv electricity production essentially depends on the solar irradiation at the installation site and on the electrical efficiency of the pv components. fig. 1 non-structural bipv with modules as roof covering (left) and structural application as roof glazing or façade glazing. ©marché international, kemptthal/ch (left), bruno klomfar (middle), stefanie flohr, tu dresden (right) 095 journal of facade design & engineering volume 5 / number 1 / 2017 various lca studies already have proved that pv systems produce by far more energy during their life time than was necessary for their production and in most countries reduce greenhouse gas and other emissions. improvements in production technologies, material utilisation and module efficiency have improved the ecological footprint in the past and will continue in the future. yet, these studies concentrated on typical roof-top and ground-mounted installations. therefore, the specific environmental performance of building-integrated modules is analysed considering the lower electricity yields, for example in façades, and the substitution of conventional building elements by the pv modules. 2 pv products and methodology 2.1 module configurations in terms of structural requirements photovoltaic modules used as roof coverings or in roof-top systems typically rely on glass-backsheet configurations. the 0.2 to 0.4 mm thick backsheet typically consists of a combination of several polyethylene terephthalate (pet) or pa films with uv and a coating providing uv and hydrolysis resistance. glass-glass modules are more suitable for integration in roof and façade glazing and can also be processed into insulating glass units. configurations basically differ according to laminated glass with embedded pv cells and thin-film cells as a coating directly deposited on one of the two glass panes. the embedded cells are separate crystalline silicon wafer cells interconnected with metal soldering ribbons or a continuous polymer substrate coated or printed with thin-film or organic pv cells. the embedded flexible thin-film cells shown in fig. 2 c) are polymer substrates coated with copper indium gallium diselenide (cigs) cells, adhesively interconnected and affixed to a carrier mat. another example would be continuous flexible solar films including organic pv cells. common interlayer materials are polyvinyl butyral (pvb), ethylene vinyl acetate (eva), thermoplastic polyolefin (tpo) or silicone-based encapsulants. fig. 2 views and sections of common types of pv modules 096 journal of facade design & engineering volume 5 / number 1 / 2017 compared to regulated glass products, there are some uncertainties and deviations from the product and design standards. glass-backsheet modules at first consist of thermally toughened glass (ttg), but the laminated cell-backsheet bond changes the safety properties: in case of breakage, the laminate sticks together and might fall down as a whole like a wet towel instead of small particles, which are typical for ttg and relatively harmless. glass-glass modules are analogous in structural configuration to laminated glass. pv modules with embedded cells use regular glass products with well-known mechanical strength properties according to harmonized product standards (din, 2012b; din, 2015). an exception are innovative thin glass (tg) products, as there is no standard for thin glass yet, but there are products on the market with a national approval (dibt, 2014). the pv thin-film industry uses float glass (fg) standardized according to din (2012c) as substrate or superstrate glass. although the coating processes influence the mechanical strength properties, the coated panes still can meet the minimum bending strength values of float glass (hemmerle, 2016). in various european countries, many glazing applications require the use of laminated safety glass. thus, classifying glass-glass modules as lsg (din, 2011) is desirable, but the specifications and verification methods are not harmonized in detail. germany’s building codes, for instance, used to narrow the general definition of lsg to the exclusive use of pvb as interlayer with no embedded materials and no coating towards the interlayer, as impacts on the adhesion cannot be ruled out. a major issue is residual resistance. a minor question is whether the integrated cells may affect the shear stiffness of the interlayer. the stiffness of the interlayer is influencing the static behaviour of laminated glass, but not all european countries consider this effect. various pv manufacturers applied for a national technical approval allowing their bipv modules to be used as lsg. the growing number of granted approvals demonstrates that glass-glass pv modules are generally able to provide structural safety equivalent to laminated safety glass. 2.2 residual resistance testing residual resistance is a main safety property of laminated safety glass. post-fracture capacity after breakage of all glasses can only be verified experimentally. however, there are no harmonized test methods to prove residual strength and lsg with pvb is the only acknowledged benchmark. based on previous schedules by various german building authorities (hmwvl, 2012; lfb bw, 2009; stmi, 2012; espich, 2011) as well as studies on the correlation between wind load and glazing temperature (wellershoff, 2006) and temperature increase due to pv integration, a test concept was developed to analyse pv modules in comparison with laminated safety glass of identical dimensions and sections (hemmerle, 2016). table 1 shows the standard load scenario and the increased temperature scenario to take account of the temperature dependent mechanical properties of the interlayer materials. the load level of 0.65 kn/m2 corresponds to half the load-bearing capacity of the unbroken pv modules and lsg references sized 800 mm x 1300 mm and comprising 2 x 3.2 mm float glass. table 2 shows the tested pv module types and the related lsg references. for the pv wafer modules, 3.2 mm float glass conform to din (2012c) and 2.1 mm thin glass were considered. the modules with flexible cigs thin-film cells included thermally toughened glass made of patterned glass conform to din (2000). the samples were taken from commercial production of three manufacturers: pv wafer modules and lsg 1 by lisec austria gmbh, pv superstrate modules and lsg 2 by masdar pv gmbh; and pv flexible cigs cells by solarion ag. 097 journal of facade design & engineering volume 5 / number 1 / 2017 test scenario sample temperature test load test time standard +23 °c 0.65 kn/m2 ≥ 72 h increased temperature +50 °c +68 °c 0.325 kn/m2 0.325 kn/m2 ≥ 24 h ≥ 7 h (sequential after ≥ 24 h at +50 °c) table 1 load scenarios pv cells pv modules: section (see fig. 2) and materials lsg reference embedded b) fg 3.2 mm | polycrystalline pv wafer cells in 1.0 mm pvb | fg 3.2 mm lsg 1: fg 3.2 mm | 1.0 mm pvb | fg 3.2 mm embedded b) tg 2.1 mm | polycrystalline pv wafer cells in 1.0 mm pvb | tg 2.1 mm (lsg 1) embedded c) ttg 3.2 mm | cigs on flexible polyimide in 1.0 mm tpo | ttg 3.2 mm – coating e) fg 3.2 mm superstrate coated with a-si/µ-si | 0.76 mm pvb | fg 3.2 mm lsg 2: fg 3.2 mm | 0.76 mm pvb | fg 3.2 mm table 2 sections of photovoltaic module samples and laminated safety glass samples used as reference testing was carried out on full size components in order to incorporate effects of the cell interconnections. three samples per sample type and parameter were tested in horizontal position and linear support. before the test load was applied, the samples were damaged with the aim of breaking both glass layers. the criterions for residual resistance were no failure within the test time and centre deflection that was measured for differentiated comparison. 2.3 shear testing the influence of integrated pv cells on the shear bond of laminated glass was analysed in exploratory axial shear tests on small, cylindrical specimens cut from laminated modules by means of water jet cutting. their diameter of 34.6 mm resulted in a laminated area a = 27.2 mm2. two pv laminates each, incorporating 3, 4 or 6 mm float glass panes, were combined with 6 or 4 mm secondary float glass panes via auxiliary bonds in order to provide sufficient contact surface for the clamping jaws of the testing machine. a structural epoxy adhesive was used for the 0.5 mm auxiliary bonds. a tensile load was applied to the symmetrical specimens to displace the pv glass panes against each other and, thus, to place the interlayers with integrated pv cells under shear stress. the shearing was carried out at room temperature and relative humidity of 30.7 % with a constant load of 400 n for 10 minutes followed by an increasing load at constant crosshead speed of 2 mm/min until failure. crosshead displacement δx and load force f were measured to calculate shearing angle or shearing strain γ and shear stress τ according to equation (1) and (2). ( 1 ) ( 2 ) the pv module configurations examined were polycrystalline wafer cells integrated in a pvb interlayer with a thickness d = 1.2 mm and cigs thin-film cells on flexible polyimide substrate affixed to a carrier mat and integrated in a tpo interlayer with a thickness d = 0.8 mm. the interlayer thickness was determined as the difference between the pv laminate thickness and the glass thickness measured after failure. the testing series included three specimen of each type. as a reference, specimens of both types without integrated pv cells were tested. 098 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 3 shear test specimens incorporating a) pv wafer cells or b) flexible cigs cells and c) test set-up and apparatus 2.4 amendment to lca studies on bipv recent lca studies on rooftop pv systems served as basis to determine the specific environmental footprint of building-integrated pv systems. common lca indicators are the energy payback time (epbt) and the carbon footprint indicating the life-cycle greenhouse gas emissions per kwh solar electricity generated. de wild-scholten (2013) has calculated energy payback times between 0.68 and 2.3 years and carbon footprints between 15.8 and 81.4 g co 2 -eq/kwh for a wide range of module technologies using 2011 manufacturers’ data with module efficiencies of 14.8 % and 14.1 % (mono-/ polycrystalline), 11.9 % (cadmium telluride – cdte) and 11.7 % (cigs). production in china increases the carbon footprint due to the high share of coal in the country’s electricity mix. the analysis took mounting structures, cabling, power conditioning and grid connection into account, but excluded installation, operation and maintenance and end-of-life phase. the underlying energy generation refers to southern european sites and south-facing module surfaces inclined at an optimum angle, which receive a global solar irradiation of 1700 kwh/(m²a). life-cycle greenhouse gas emissions of these rooftop pv systems, also without considering end-of-life treatment, are reported (de wildscholten, cassagne, & huld, 2014). in contrast to rooftop systems, different parameters apply to building integrated pv systems. the fact that roof or façade glazing usually requires thicker glass sections, but no backsheets and no aluminium module frames, is neglected due to its minor influence. for crystalline modules, production in china is chosen in order to describe the worst-case scenario. even though pv modules from european production are widespread in european bipv applications, china’s large market shares in the solar silicon, wafer and solar cell production (de wild-scholten, cassagne, & huld, 2014) must be considered. moreover, this paper relates to central european bipv installations and a corresponding irradiation of 1294 kwh/(m²a) on an optimally oriented surface. the reduced potential in electricity generation in roofs and façades of non-ideal east, south and west orientations and under unfavourable temperature conditions were calculated using the dynamic simulation software pv*sol. on the other hand, pv modules as building skins substitute conventional building elements, e. g. roof tiles or façade cladding. the environmental impact of these were offset against the primary energy content (pec) and the greenhouse gas emissions of the pv system as a credit. the credits assumed were 47 to 246 mj/m² and -0.1 to 19 kg co 2 -eq/m² for roof claddings or 47 to 1080 mj/m² and 16.6 to 67 kg co 2 -eq/m² for façade claddings considering a life-cyle of 50 years (el khouli, john, & zeumer, 2014). following internationally harmonised approaches, 30 years life expectancy and a power degradation rate of 0.23 % per year were used for the pv modules (fthenakis et al., 2011). 099 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 4 larger amount of loose glass splinters on the pv-coated superstrate glass (left) compared to the uncoated lsg cover glass (right) at the point of impact after ball drop 3 results 3.1 breakage structure and residual resistance of glass-glass pv modules for residual resistance testing, the pv modules and lsg reference samples consisting of float glass and thin glass were damaged through dropping a 4.1 kg steel ball from a height of 2.5 m on the center. neither the integration of wafer cells nor the thin-film coating significantly influenced the breakage structure. however, mechanical failure within the pv layer of the a-si/µ-si superstrate as the impacted side resulted in a larger amount of loose glass splinters. compared to the float glass modules, the thin glass modules exhibited finer fracture patterns. as the pv modules and the related lsg reference samples in each case were produced by the same manufacturer using the same glass products and interlayers, the results concerning the influence of the pv integration can be generalised. the panes of the pv modules with embedded flexible cigs cells were damaged using a prick punch and broke into small particles, typical of thermally toughened glass. none of the samples failed in the standard scenario. in comparison with the respective laminated safety glass reference, the pv modules with embedded wafer cells showed 20 to 25 % less centre deflection and the pv thin-film on glass superstrate modules 13 to 14 % less deflection. there was not much of a difference in deflection between the wafer modules with thin glass and with float glass. cigs pv modules, in spite of comprising only thermally toughened glass, showed considerably lower deflection than any of the float glass samples. thus, the integration of all three types of pv cells improved the residual resistance. 100 journal of facade design & engineering volume 5 / number 1 / 2017 fig. 5 mean values of the measured centre deflections in the standard scenario (test load 0.65 kn/m2 at room temperature, left) and in the increased temperature scenario (test load 0.325 kn/m2 at +50 °c) in the increased temperature scenario, the test temperature corresponded to the sample temperature measured at the outer glass surface. the interlayer thickness emerged as crucial parameter for the samples using pvb. both pv and lsg 2 samples with only 0.76 mm pvb layer failed as soon as sample temperatures slightly exceeded +50 °c. pv and lsg 1 samples with 1.00 mm pvb layer passed the test time without failure as well as the cigs modules with 1.00 mm tpo interlayer. again, the pv modules with embedded wafer cells showed 20 to 25 % less centre deflection than the lsg reference. moreover, none of the pv wafer modules failed during the second test period at +68 °c, while two of the three lsg 1 samples failed. interconnected wafer cells reinforce the broken laminate, because the soldered interconnectors support tensile forces. this also applies to pv thin-film on glass modules, but to a lesser extent, as these only incorporated one cross-bus ribbon along the transverse axis connecting the outmost cells with the junction box in the centre. different cross-bus layouts available in other pv thinfilm modules may result in different reinforcing effects. adhesively connected flexible pv cells significantly stiffen the broken laminate, as fig. 6 shows, because they form an additional layer with good tensile properties. as conventional laminated safety glass made of thermally toughened glass panes becomes flexible once broken and does not provide any residual load bearing capacity, the integrated pv cells do not only improve post breakage behaviour, but are the primary cause of residual resistance. in principle, these findings also apply to other pv technologies using flexible films to be integrated in laminated glass, e. g. organic based photovoltaics. fig. 6 improved residual resistance due to pv wafer cells’ soldered interconnectors (left, here: at +68 °c) and flexible pv thin-film cells stiffening the broken laminate (right, here: in contrast to lsg made of ttg without integrated pv layer) 101 journal of facade design & engineering volume 5 / number 1 / 2017 the pv cells’ solar absorptance results in increased warming of pv modules in the building skin. in order to evaluate whether this might overcompensate the reinforcement effects, the differences in deflection measured at sample temperatures between room temperature and +68 °c have been analysed to calculate a temperature coefficient. as a result, pv modules with embedded wafer cells could heat up to 11 to 18 k above the lsg sample temperature to reach the same deflection. actually, the expected maximum temperatures of pv modules in the building skin are only 5 to 6 k higher compared to transparent laminated glass (hemmerle, 2015). thus, in the worst case, the overall effect of the pv integration on the residual resistance remains positive. 3.2 influence of pv cells on shear bond and adhesion shear testing showed that integrating pv wafer cells in the pvb interlayer did not significantly influence the shear modulus. yet, the specimen with integrated cells failed earlier than those without pv and exhibited adhesive failure at the interface between pvb and the rear side of the silicon wafer cells. fig. 7 shear stress to shear strain diagram at +22 °c and 30.7 % relative humidity consequently, the pv cells reduced adhesive bond within the laminate. all specimens with flexible cigs cells integrated in the tpo interlayer already failed adhesively during the constant load phase at the interface between the self-adhesive rear side of the cigs cells and the carrier mat. thus, the embedded cigs cells also reduced adhesive bond within the laminate. shear bond could only be determined for the specimens without cigs cells with the results confirming the lower stiffness of the tpo interlayer in comparison with pvb at room temperature. 3.3 energy payback time and carbon footprint of photovoltaic building skins for central europe, an average solar irradiation of 1294 kwh/(m²a) on an optimally oriented surface was calculated using country-specific annual irradiation data and country areas presented by de wild-scholten, cassagne, & huld (2014). assuming a performance ratio of 0.77, the electricity generation averages 996 kwh/(kw p a). according to the simulation results, the yield decreases to 85 to 75 % on roofs facing west or east, to up to 67 % on south façades and to up to 53 % on west or east façades. based on this lower electricity generation (see electricity generation and energy payback times (epbt) for central european bipv installations), the energy payback times calculated by de wild-scholten (2013, see section 2.4) for southern europe and optimum orientation were proportionally converted. without considering credits, the energy payback times of pv roof systems installed in central europe with optimally oriented module surfaces increase to values 102 journal of facade design & engineering volume 5 / number 1 / 2017 between 1.9 or 3.0 years for systems with crystalline silicon modules and 0.9 or 1.3 years with thinfilm modules. less favourable east or west orientations result in longer periods of 2.4 to 4.0 and 1.2 to 1.9 years. in south façades, the primary energy balance turns positive after 1.3 to 4.5 years, depending on the module technology, in east or west façades after 1.6 to 5.7 years. fig. 8 energy payback time depending on pv technology and bipv application in central europe and carbon footprint in comparison with country electricity mixes the primary energy content of building elements that are potentially substituted by the pv modules were taken from el khouli, john, & zeumer (2014) and refer to a life-cyle of 50 years. adjusting these values to the pv modules’ life-cyle of 30 years reduced the deductible credits to 28 to 648 mj/m². the energy payback times by de wild-scholten (2013) can be converted into primary energy contents of the pv systems in mj/m2 via multiplication by the annual electricity generation, the primary energy efficiency of the substituted electricity mix and the pv module efficiency. depending on the module technology and the material of the substituted roof or façade claddings, the respective credit amounts to 1 to 55 % of the primary energy content of the bipv system and thus reduces the energy payback time by 0.02 to 0.9 years. the net epbt ranges between 0.8 and 5.6 years. pv integration yield in kwh/ kw p a epbt in years min max (cdte) (mono cn) credit in mj pec/m2 min max credit in years min max (cdte) (cigs) net epbt in years min max (cdte) (mono cn) roof, optimum orientation 996 0.9 3.0 28 148 0.02 0.1 0.8 3.0 roof east or west oriented 747 1.2 4.0 28 148 0.02 0.1 1.0 4.0 façade, south oriented 667 1.3 4.5 28 648 0.03 0.7 0.6 4.4 façade, east or west oriented 528 1.6 5.7 28 648 0.03 0.9 0.7 5.6 fig. 9 electricity generation and energy payback times (epbt) for central european bipv installations available carbon footprint values of pv systems (see section ) were also converted to relate to electricity yields or bipv systems in central europe. the greenhouse gas emissions of roof or façade elements were allocated to the pv modules’ life-cyle of 30 years. the resulting potential credits range from -0.06 to 40.2 g co 2 -eq/m2. this corresponds to 0 to 59 % of the life-cycle greenhouse gas emissions of pv systems per m2 module area, depending on the module technology and efficiency. thus, credits for substituted roof or façade cladding can reduce the carbon footprint of central european bipv systems from 20 to 197 g co 2 -eq per kwh to 12 to 192 g co 2 -eq/kwh. 103 journal of facade design & engineering volume 5 / number 1 / 2017 pv integration life-cycle emissions in g co 2 -eq/kwh min (cdte) max (mono cn) credit in g co 2 -eq/m2 min max credit in g co 2 -eq/kwh min (mono) max (cigs) net emissions in g co 2 -eq/kwh min (cdte) max (mono) roof, optimum orientation 20.2 104 -0.06 10.0 -0.02 3.6 16.8 104 roof east or west oriented 26.9 139 -0.06 10.0 -0.02 4.7 22.4 139 façade, south oriented 29.3 156 11.4 40.2 3.8 18.2 12.0 152 façade, east or west oriented 36.7 197 11.4 40.2 4.9 22.8 15.1 192 fig. 10 life-cycle greenhouse gas emissions of bipv and credits to consider substituted roof covering or façade cladding since own consumption or grid feed-in of the pv electricity substitutes conventional electricity generation, the carbon footprint of the electricity mix of the respective country approximates the greenhouse gas emissions avoided by the pv system. balancing life-cycle emissions against emission reductions, pv and bipv systems significantly contribute to decreasing greenhouse gas emissions in most european countries. in the future, the increase in the share of renewable energy will reduce the potential savings. 4 discussion 4.1 glass-glass pv modules as laminated safety glass evaluating the residual resistance test results from section , integration of all three pv cell types proved to enhance resistance compared to laminated safety glass. thus, the design standards should classify standard pv module configurations to provide the same residual load-bearing capacities as laminated safety glass of the same sections without integrated pv cells. only special designs should require experimental verification. in terms of residual strength of pv modules with wafer cells, 2.1 mm thin glass is comparable to 3.2 mm float glass, as the measured deflections showed. addressing other important properties of lsg, adhesion of interlayer to glass provides sticking of broken glass pieces and minimizes the injury risk. the lower adhesion of pvb to the rear side of the silicon wafer cells than to glass as well as between the self-adhesive rear side of the cigs cells and the carrier mat showed no negative influence on residual resistance or on the sticking of broken glass pieces, as these interfaces with reduced adhesion were enclosed in the interlayer. residual resistance testing of the pv thin-film on glass modules showed very good adhesion of pvb to the pv coating, namely the back contact layer. the critical issue is mechanical failure within the pv layer, which is sensitive to aging; and the resulting larger number of glass splinters associated with increased injury risk when the pv coated glass pane is the impacted side and broken glass pieces may fall down on accessible areas, depending on the structure and position of the glazing. however, splinter size and quantity observed were non-dangerous. in the absence of current reference values, the limits defined in a withdrawn standard (din, 1990) justify this evaluation. further investigations on pv thin-film on glass substrate and superstrate glass modules would be of interest to affirm the low risk potential of broken glass pieces and should consider aging effects. 104 journal of facade design & engineering volume 5 / number 1 / 2017 shear bond depends on the interlayer material used. neither the shear testing presented in this paper nor previous studies, e. g. by weller & härth (2005) or friedman & kirchner (2009), found negative influences of integrated pv cells on the shear modulus of glass laminates. it is not possible to evaluate whether the observed breakage at lower shear stress due to adhesive failure at the rear surface of the pv cells is critical or not. test methods to characterize the time and temperature dependent viscoelastic material properties of interlayer materials have neither been harmonized yet, nor are there any minimum requirements. albrecht & maniatis (2003) made shear tests on glass laminates with and without various pv cells and various interlayers. they also found a negative effect of integrated pv cells on breakage shear stress at room temperature, but a positive effect at +60 °c. in the building skin, pv modules reach higher temperatures than transparent glazing. thus, pvb, which exhibits a drastical loss in stiffness at temperatures larger than room temperature, generally is not the best choice for bipv modules. tear resistance of the interlayer turned out to be essential related to residual strength of pv thin-film on glass modules. lsg moreover requires pendulum testing (din, 2003) to classify the pendulum body impact resistance. the tests also provide information on breakage behaviour. comparative pendulum tests on pre-damaged glass laminates with and without pv wafer cells and eva interlayer by friedmann & kirchner (2013) showed no significant influence of the embedded solar cells on crack propagation and impact resistance. the test results from section also indicated no influence of integrated pv cells on the breakage structure of laminated glass. as a conclusion, the tested glass-glass module configurations are basically evaluated to provide a safety level which is at least equivalent to laminated safety glass, if the interlayer material is approved for use in laminated safety glass. the pv integration does not impair breakage behaviour and improves residual resistance, while the observed reduced adhesive bond does not imply a higher injury risk. thus, a classification in future editions of the european lsg product standard (din, 2011) is recommended. the harmonised standard defines conformity assessment by the manufacturer including initial type testing and factory production control (din, 2011; din, 2005). these procedures are not common in the pv industry’s mass production, but have been established by those manufacturers holding a national technical approval for pv modules for use as laminated safety glass. glas-backsheet modules require differentiation depending on the intended glazing structure. as vertical glazing, the modules can fulfil similar mechanical safety properties as laminated glass. in case of breakage, the backsheet binds the small ttg particles and, supported by own weight, prevents the broken laminate from slipping out of the support structure. glass-backsheet modules might as well be used on par with monolithic ttg, as ttg does not always crumble into small particles at once, but large fragments may fall down in practice. for horizontal glazing, glassbacksheet modules are not recommended without additional structural elements as support for the broken laminate. 4.2 environmental life-cycle benefits of photovoltaic building skins the analysis of energy payback time and carbon footprint of pv modules buildings showed significant differences between the available technologies and the potential applications with similar tendencies of both indicators. compared to rooftop systems, the lower electricity generation of building integrated systems results in reduced environmental benefits, but are partially outweighed by the material savings when the modules substitute conventional roof coverings or façade claddings. thin-film modules with generally low environmental impacts and larger specific 105 journal of facade design & engineering volume 5 / number 1 / 2017 areas even enable overcompensation. heading for climate neutral buildings, all appropriate areas in the building envelope must be used for energy production instead of focussing on those with highest solar exposure. building integration improves the environmental footprint of unfavourably oriented pv installations in particular. active building skins incorporating pv electricity generation aim at compensating environmental impacts of the whole building and life-cycle assessment must be performed at building level. this requires differentiated and up-to-date life-cycle inventories describing the relevant module types and manufacturing used in buildings instead of mass products for large commercial pv power systems. end of life scenarios need to be included, as future module recycling will provide further environmental benefits. 5 conclusions with rising energy standards, the use of photovoltaic systems in buildings becomes mainstream. constructive integration of the pv modules associated with the substitution of conventional materials in the building skin reduce the life-cycle environmental impacts like primary energy demand and greenhouse gas emissions, especially in those areas with suboptimal solar irradiation like façades. up to now, the use of pv modules as construction product often requires individual verification or approval procedures due to the lack for harmonized product qualification and design rules. based on experimental testing, the mechanical performance of the most relevant module configurations was examined in comparison with approved glass products. the research provides systematic and material-based knowledge enabling classification of glass based pv modules as regulated construction products according to the existing standards. according to the presented results, glass-glass modules correspond to the safety level of laminated safety glass. formal classification could significantly reduce the need for additional testing and approval, and thus facilitate the use of building-integrated photovoltaics. references dibt. (2012). hinweise für die herstellung, planung und ausführung von solaranlagen. stand: juli 2012. berlin: deutsches institut für bautechnik din. (2006). din en 61215:2006-02; vde 0126-31:2006-02 crystalline silicon terrestrial photovoltaic (pv) modules – design qualification and type approval (iec 61215:2005); german version en 61215:2005. berlin: beuth din. (2009). din en 61646:2009-03; vde 0126-32:2009-03 thin-film terrestrial photovoltaic (pv) modules – design qualification and type approval (iec 61646:2008); german version en 61646:2008. berlin: beuth din. (2007). din en 61730-1:2007-10; vde 0126-30-1:2007-10 photovoltaic (pv) module safety qualification – part 1: requirements for construction (iec 61730-1:2004, modified); german version en 61730-1:2007. berlin: beuth din. (2012a). din en 61730-2:2012-09; vde 0126-30-2:2012-09 photovoltaic (pv) module safety qualification part 2: requirements for testing (iec 61730-2:2004, modified + a1:2011); german version en 61730-2:2007 + a1:2012. berlin: beuth dimova, s., pinto, a., feldmann, m., & denton, s. (eds.). (2014). guidance for european structural design of glass components. luxembourg: publications office of the european union schneider j., kleuderlein j., kuntsche j. (2012). tragfähigkeit von dünnschicht-photovoltaik-modulen. in b. weller, & s. tasche (hrsg.), glasbau 2012, (pp. 315-325). berlin: ernst & sohn din. (2012b). din en 1863-1:2012-02 glass in building – heat strengthened soda lime silicate glass – part 1: definition and description; german version 1863-1:2011. berlin: beuth din. (2015). din en 12150-1:2015-12 glass in building – thermally toughened soda lime silicate safety glass – part 1: definition and description; german version en 12150-1:2015. berlin: beuth dibt. (2014). allgemeine bauaufsichtliche zulassung nr. z-70.4-194: teilvorgespanntes glas: f | ecofloat tvg, f | solarfloat tvg, f | solarfloat ht tvg, f | dur tvg. berlin: deutsches institut für bautechnik din. (2012c). din 572-2:2012-11 glass in building – basic soda lime silicate glass products – part 2: float glass; german version en 572-2:2012. berlin: beuth hemmerle, c. (2016). structural safety of photovoltaic modules in the building envelope. challenging glass 5, pp. 187-200. ghent din. (2011). din en iso 12543:2011-12 glass in building – laminated glass and laminated safety glass – part 1-6 (iso 12543:2011); german version en iso 12543:2011. berlin: beuth 106 journal of facade design & engineering volume 5 / number 1 / 2017 hmwvl. (2012). informationen für die beantragung einer zustimmung im einzelfall. (information sheet). wiesbaden: hessisches ministerium für wirtschaft, verkehr und landesentwicklung lfb bw. (2009). überkopfverglasungen im rahmen von zustimmungen im einzelfall. (explanatory leaflet). tübingen: regierungspräsidium baden-württemberg, landesstelle für bautechnik stmi. (2012). hinweise zur erlangung einer zustimmung im einzelfall (zie) gemäß art. 18 abs. 1 und art. 19 abs. 1 bayerische bauordnung (baybo). bereich konstruktiver glasbau. (explanatory leaflet). münchen: bayerisches staatsministerium des innern g. espich (personal communication, september 20, 2011 wellershoff, f. (2006). nutzung der verglasung zur aussteifung von gebäudehüllen (doctoral thesis). aachen: shaker verla din. (2000). din en 12150-1:2000-11 glass in building – thermally toughened soda lime silicate safety glass – part 1: definition and description; german version en 12150-1:2000. berlin: beuth de wild-scholten, m.j. (2013). energy payback time and carbon footprint of commercial photovoltaic systems. solar energy materials & solar cells, volume 119, 296-305 de wild-scholten, m. j., cassagne, v., & huld, t. (2014). solar resources and carbon footprint of photovoltaic power in different regions in europe. proceedings of 29th european photovoltaic solar energy conference. amsterda el khouli, s., john, v., & zeumer, m. (2014) nachhaltig konstruieren (detail green). münchen: detail institut für internationale architektur-dokumentation fthenakis, v., frischknecht, r., raugei, m., kim, h. c., alsema, e., held, m. et al. (2011). methodology guidelines on life-cycle assessment of photovoltaic electricity (2nd ed.). iea pvps task 12, international energy agency photovoltaic power systems programme hemmerle, c. (2015). photovoltaik in der gebäudehülle: wertung bautechnischer anforderungen. photovoltaics in building envelopes: evaluation of structural requirements (doctoral thesis). retrieved from http://nbn-resolving.de/ urn:nbn:de:bsz:14-qucosa-20483 din. (1990). din en 1249:1990-09 glass in building – part 12: thermally toughened glass, definition, dimensions, treatments, requirements. berlin: beuth weller, b., & härth, k. (2005). eu project bipv-cis improved building integration of pv by using thin film modules in cis technology: results on shear tests with cis-modules within workpackage 9. (progress report). dresden: technische universität dresden friedmann, m., & kirchner, r. (2009). versuchstechnische untersuchungen an fassadenplatten mit einlaminierten dünnschicht-photovoltaikmodulen. (test report). rohrbach: friedmann & kirchner gesellschaft für materialund bauteilprüfung mbh albrecht, g., & maniatis, i. (2003). the use of laminated glass with photovoltaic elements. proceedings of glass processing days 2003, pp. 433-436. tampere din. (2003). din en 12600:2003-04 glass in building – pendulum tests – impact test method and classification for flat glass; german version en 12600:2002. berlin: beuth din. (2005). din en 14449:2005-07 glass in building – laminated glass and laminated safety glass – evaluation of conformity/ product standard; german version en 14449:2005. berlin: beuth friedmann, m., & kirchner, r. (2013). untersuchungen von verbund-sicherheitsglas mit eingelegten solarzellen. (test report). rohrbach: friedmann & kirchner gesellschaft für materialund bauteilprüfung mbh journal of facade design and engineering 3 (2015) 165–183 doi 10.3233/fde-150037 ios press 165 bridging the gap between selection decisions of facade systems at the early design phase: issues, challenges and solutions mohamad kassema,∗ and donald mitchellb atechnology futures institute, teesside university, middleborough, uk bdepartment of architecture and civil engineering, centre for window cladding technology, university of bath, bath, uk abstract. building facade has a significant impact on the environmental and economic performance of buildings and projects. the specification of their elements at the early design phase depends on numerous technical, environmental and economic factors and involves several stakeholders. the procurement and delivery of the facade work package from the early design phase, through detailed design and manufacture, to installation is a process with several inherent risk factors due to the involved cost, technical and engineering complexities and its position on the critical path in all projects. this research investigates the process of selection and specification of building facade elements at the early design phases with the overarching aim of identifying the issues affecting specification decisions, their root causes and impact on projects. the research utilizes a mixed research approach which combines a retrospective case study and an industry survey as two research methods that build on each other. the findings suggest that the complexity of specification at the early design phases is exacerbated by factors such as the inadequate technical knowledge of stakeholders involved in the decision making process, the non-involvement of building facade consultants, the late involvement of specialist facade subcontractors, and in a few cases by some commercial exclusivity agreements that restricts specification decisions. keywords: building facade, business process, curtain wall 1. introduction building envelope (cwct, 2003), facade (pavitt & gibb, 2003) or building enclosures (tran et al., 2014) are interchangeably used terminologies to denote the physical separator between the interior and the exterior environments of a building. the impact of building facade has become more important than ever in determining the operational and economic performance of construction projects. indeed, the building facade accounts for anything between 15 and 25 per cent of the total construction costs and represents a substantial part of the technical and commercial risk on any given project ∗corresponding author: mohamad kassem, technology futures institute, teesside university, middleborough, uk. tel.: +44 (0) 1642 738300; fax: +44 (0) 1642 342494; e-mail: m.kassem@tees.ac.uk. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:m.kassem@tees.ac.uk 166 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems (kragh, 2011a). building facade is also an area of engineering by itself and its elements such as curtain wall systems are being used in various shapes and types, not only in new buildings but also during the renovation of existing structures (efstathiades et a., 2007). this role is intensified by strict evolving energy performance standards and regulations (kragh, 2011b). procuring building facade is perceived as a process with many risks due to the numerous stakeholders, cost, technical and engineering requirements involved, and its position on the critical path in all projects. in addition, the broad range of commercial options available with varying economic, environmental and technical performances increase the challenges associated with the selection and delivery of building facade projects. indeed, devising an optimal building facade solution is becoming increasingly difficult due to the growing number of building facade components and systems (jin & overend, 2010). research efforts analyzing decision making in design and construction processes are often concerned with analyzing issues affecting the performance of construction projects at industry-wide level as evidenced from the literature review presented later. as a result, there is limited research focused on analyzing specific design and engineering processes such as the selection of building facade elements at the early design phase. hence, this research aims to investigate the process of selecting building facade elements at the early design phase, identifies the issues affecting accurate selection decisions and presents recommendations. in the subsequent sections, related research identifying causes of poor performance in the construction industry in general and current research to improve building facade in particular are first presented to understand both the gap and significance of the proposed research. then, background information about the factors involved in the selection of building facade elements is illustrated to help the understanding of the complexity of selection decisions. third, the research methodology, which consists of a retrospective case study and an industry survey, is explained to justify both the research design and research methods followed by a detailed presentation of results from both the case study and the survey. 2. literature review studies investigating the issues that affect the performance of projects have proliferated over the last few years, especially within the construction sector. much of these studies have focused on identifying factors that cause time, cost overruns and quality issues. the majority of these studies is focused at sector level (i.e. construction industry) and is based on quantitative survey approaches. the review of these studies can be classified in terms of ‘domain’, ‘granularity’ and ‘approach’ of investigation: • domain of investigation: represents the sector segment in which the research was conducted (e.g. building, civil, residential, etc.). • granularity of investigation: denotes the depth in the exploration of the issues researched structured into three levels namely, country or sector, project and single discipline or trade. • approach of investigation: refers to the research methods used in the investigation (e.g. case study, questionnaire, interviews). a classification of a non-exhaustive list of studies according to these three fields is reported in table 1. the results show that the majority of existing studies are focused at sector wide level and there is still lack of studies at building discipline or trade level. table 2 reports the issues identified as main causes for poor performance in projects from the same studies classified in table 1. extensive m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 167 table 1 classification of a non-exhaustive list of previous studies according to their domain, granularity and research approach o deh and b attaineh (2002) a ssaf and a l -h ejji (2006) e l-r azek et al. ( 2008) sw eis et al. (2008) t um i et al. (2009) a l -h ajj and h am ani (2011) z oya k pam m a and a djeik um i (2011) shebob et al. (2012) fallahnejad (2013) domain of investigation gas & power industry • building & residential • • • public sector • construction (not specified) • • • • • granularity of investigation sector / country • • • • • • • • • project • single building trade approach of investigation questionnaires • • • • • • • • • interviews • case study gap in existing research gap in existing research existing reviews in this domain have also reached similar conclusions. sun and meng (2009), in their effort to develop a taxonomy for change causes and effects in construction projects, reviewed 101 articles from the same source used in table 1.1 much of the identified articles have either focused at wide industry scale using a quantitative survey-based approach. although this research approach, focusing on industry and market level, is valuable in identifying the main areas of deficiencies in the industry that require improvement, it suffers two limitations. the first limitation lies in the nature of causes identified (see table 2) which are often general statements of the areas where the actual root-causes of poor performance lie (table 2). the second limitation is the lack of empirical evidence of the issues identified and their impact. from research design perspective, this approach could have unknown, remedial or biased population sampling and data collection methodologies (succar & kassem, 2015). as a result, there is a need for complementing the current research approach with a new methodology in which the granularity of investigation is increased from sector and market-level to a single building trade level, combined with case studies to provide the empirical demonstration and support. one study, specific to the domain of building facades, was jointly funded by the korean government and a large industrial conglomerate in south korea (i.e. daelim industrial co ltd; better living space, 1e.g. journal of construction engineering and management; international journal of project management; construction management and economics; journal of management in engineering; and engineering, construction and architectural management. 168 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems table 2 types of issues affecting industry and projects issues identified odeh, & battaineh (2002) inadequate contractor experience client interference clients’ financing assaf, & al-hejji (2006) shortage of labour delays in clients’ payments type of project bidding el-razek, et al. (2008) contractors’ finance delays in client payments clients’ design changes sweis, et al. (2008) contractor poor planning contractors’ finance clients’ change orders tumi, et al. (2009) improper planning lack of effective communication design errors al-hajj, & hamani (2011) lack of awareness excessive off-cuts resulting from poor design rework and variations zoya kpamma, & adjei-kumi (2011) low recognition of sources of waste little awareness of waste reduction tools inadequate familiarity of the firms with lean thinking shebob, et al., 2012 drawing approval delays adverse weather conditions delays to site handover to contractor fallahnejad (2013) problems with importing materials unrealistic contract durations slow delivery of clients’ materials and doalltech co.) and aimed to improve the lifecycle of curtain wall through the integration of the supply chain through information management systems (chin et al., 2004; hwang et al., 2006). factors such as the difficulty of involving the right people at the right time; lack of information sharing and communication; information loss due to the fragmentation of processes; redundancy and inaccuracy in information flow; long lead time between activities in the process, and reworks and errors due to missing and inaccurate information in the documentation (chin et al., 2004; hwang et al., 2006) were considered as issues affecting the performance of the curtain wall industry. however, neither a description of the identification process nor a statistical and empirical evidence of such issues were provided. other related studies to building facade have focused on design methodologies to achieve specific technical performance such as sustainability and buildability (singhaputtangkul et al., 2014; mohsen & elaheh, 2012). this paper aims to fill this identified gap while adopting a new research approach. such an approach consists of increasing the granularity and scale of investigation by focusing on a specific building discipline or trade (i.e. building facade) while considering its interactions with other trades such m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 169 as architectural and structural interactions. in addition, it combines the survey approach with a retrospective case study to provide empirical evidence of the issues and their impacts. 3. the complexity of selection of building facade elements: the case of curtain wall a curtain wall is defined as a thin, usually aluminium framed wall, containing in-fills of glass, metal panels or thin stone in addition to glazed-in window and door openings (vigener & brown, 2012). curtain wall is classified according to the method of manufacture and installation as either stick systems or unit systems (eastman et al., 2011). a stick system consists of a framework of site assembled components which is used to support glass and infill panels (cwct, 2000a). a unit or unitised system is a prefabricated wall which is transported to site as unitised frames, normally pre-glazed (cwct, 2000a). the primary structural elements of curtain walls are mullions (vertical elements) and transoms (horizontal elements) (fig. 1). vertical mullion usually spans the full height of the cladding – in the case of stick systems – and they are connected to the horizontal transom using angle cleats, sleeves, spigots or proprietary brackets (cwct, 1999). the framework of mullions and transoms supports infill panels, which may be glazing units or insulated panels. mullions and transoms are usually made of extruded aluminium but may be made of steel in some cases (cwct, 1999). a number of well established suppliers, mostly large and multinational companies, produce and commercialise numerous curtain wall systems. the commercial meaning of a curtain wall system is a collection of curtain wall products (mullions and transoms) having the same section shape but with different dimensions (length, width, height) in order to cover a range of performance required such as: different spans (distance between two mullions), maximum wind deflection, and different glass or infill weights. fig. 1. main elements of curtain wall. 170 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems table 3 engineering and environmental parameters involved in curtain wall selection water tightness curtain wall systems have two different methods to deal with water tightness, namely: face sealed systems and drained and ventilated systems. face sealed systems have the water penetration line at the front of the system. drained systems allow a certain amount of water to penetrate past the first weather seal gaskets, but a pressure equalised chamber is formed in the system that causes any penetrating water to be drained back to the outside via drainage holes positioned in the exterior face (cwct, 2000b). air tightness adequate air tightness of the curtain walling is required to prevent occupier discomfort that may occur due to draughts and/or noise (cwct, 2000b). the lack of air tightness can result in air leakage leading to heat loss during winter and excessive energy requirements during summer. thermal performance curtain wall systems contribute to building energy efficiencies through their thermal performance such as their conductance, which is a function of the frame material, geometry and fabrication (vigener & brown, 2012). acoustic performance the acoustic performance of curtain walls is mainly determined by their infill materials. the acoustic performance can also be improved by making the construction as airtight as possible and using sound attenuating glazing and panels (cwct, 2003) movement accommodation curtain wall systems are required to accommodate the structural movement of the building they are secured to. the capacity of accommodating building movement depends on the shape and dimensions of the curtain wall section selected (cwct, 2007). wind loading the ability of curtain wall systems to withstand wind loads depends on their shape and the way they are attached to the structure at floor slab levels through brackets that transfer the wind load to the structure. while transferring the wind loads to the structure, the curtain walls are also subject to deflection and therefore, their shapes and dimensions are crucial to insure that the maximum deflection is not exceeded (cwct, 2000c). fire safety the installation of a curtain wall system influences the passage of fire and smoke. the installation usually leaves gaps between floors horizontally and between party walls vertically to allow the passage of fire and smoke. many standard products (i.e. fire break materials) are available and are specified by the amount of time they can withstand the passage of fire and smoke (cwct, 2011). maintenance all facades require maintenance during the building operation phase (vigener & brown, 2012). the degree of maintenance and inspection depends on the facade type and the intended design life. the early detection of defects can mitigate expensive repairs or even replacement. therefore, safe and easy access for conducting maintenance operations and possible disruption to occupiers are factors that are considered in the design and selection process. buildability ease, safety and access methods are all factors associated with buildability. for example, the method of erection must be considered during the design stage by taking into account accessibility and site logistics (hse, 2007). regulations issued by the construction design and management (hse, 2007) dictate criteria that go beyond the construction phase and stipulate that the facade must be accessible for replacement and end of life dismantling of the structure. the curtain walls and other building facade elements are a prerequisite in achieving occupant satisfaction, building efficiency and economic construction strategies. their specification and selection is a challenging process due to the numerous architectural, engineering, economic and environmental parameters (table 3) and stakeholders (i.e. architect, client, engineering consultants, vendors and specialist subcontractors) involved in the decision. m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 171 the thresholds of these performance parameters may vary between different countries’ national building codes. for example, the air leakage rate through a curtain wall for the united states market is limited to 0.3 litres/sec*m2 at 75pa air pressure difference. in canada, the air leakage rate is limited to 0.1 litres/sec*m2 at 75pa air pressure difference (quirouette, 2013). 4. research methodology this research aims to investigate the issues affecting the selection process of building facade elements such as curtain wall systems and the impact of selection decisions on projects. to achieve this aim, the research utilizes mixed research methods consisting of retrospective case study and an industry survey of stakeholders involved in selection decisions. this approach is a form of triangulation in which the weakness in each single method is compensated by the counter-balancing strengths of another (amaratunga et al., 2002) and enables to elaborate or develop analysis, providing richer details (rossman & wilson, 1991). the retrospective case study is used to confirm and introduce, by presenting empirical evidence, some of the issues affecting building facade projects and their impact on project performance. however, case studies are not generalizable to a sampling universe but instead directly confirm or disconfirm theory and hypotheses (yin, 1994). as such, the retrospective case study approach is utilized in this research primarily to confirm the existence of issues in selection decisions and secondly to illustrate the impact of incorrect selection decisions. the case study is complemented with an industry survey followed by face to face and phone interviews with industry experts to identify and analyze the range of issues associated with the early selection of curtain wall systems. following the retrospective case study, the business process for curtain wall selection adopted on the case study project is also mapped to show the deficiencies of current processes. 5. retrospective case study the case study is a multi-million dollar hotel located in london, uk. curtain wall is the main element of the building facade with a commercial value just over eight million dollars. this project is representative of the research problem in terms of products (i.e. curtain wall systems) and stakeholders (i.e. client, contractor, and architect) involved. the project’s data were obtained through ‘retrospective story telling’ through three interviews with the project manager responsible for the delivery of the building facade. the project manager works for the specialist facade subcontractor responsible for the detailed engineering design, manufacturing and installation of facade. the project manager collaboratively works with the architect, contractor, curtain wall vendors and other subcontractors to resolve every facade related issue on site and it is in his best interest that things go smoothly on site. therefore, the storytelling by the project manager can be considered unbiased. the three interviews with the project manager respectively addressed three distinct areas: the original specification and issues encountered, the corrective actions, and the impact on the project. only one part of the building facade, which is at the lower ground floor bar area, is used for this study. it should be noted however, that there were similar issues encountered in other areas of the build. the main contractor for the project was one of the largest contractors operating in the uk and worldwide, and the architects were of a major london-based architectural firm. a medium-sized company was employed as a specialist sub-contractor with design responsibility for the building envelope under contract to the main contractor. the curtain wall system used on the project was specified before the 172 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems specialist sub-contractor was appointed and was supplied by one of the major three vendors who will be referred to in the remaining part of the paper as vendor a, vendor b and vendor c, due to confidentiality of commercial information. the three vendors together have more than 70% of the uk market share valued at $250 million in 2008 companies house − a uk government database where company accounts can be consulted − tax returns and are multinational companies operating worldwide. the selected curtain wall system was also specified, based on a commercial partnership between the main contractor and vendor a, in which the contractor is committed to use vendor’s products on all projects. in addition, there were further constraints relating to aesthetic and architectural aspects imposed by the architect and other structural constraints. these factors will be discussed in more detail in the subsequent section of the case study. 5.1. the design intent and issues phases the design intent and brief, received by the specialist subcontractor for the ground floor bar area from the architects, specified structurally glazed curtain wall screens with a span of 7.7 meters in height with mullion centers at 1.67 meters (fig. 2). a mid transom split the screen at a height of 3.5 meters from the bottom transom and 3.45 meters to the top transom. the selected curtain wall system at the early specification phase was supplied by vendor a. when the design brief and early specification was received by the specialist building facade subcontractor for the detailed engineering design stage, several issues were encountered: fig. 2. drawing of the curtain wall screen. m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 173 • deflection of the selected mullion exceeded the maximum deflection allowed: the selected mullion specifies a maximum deflection as the ratio between the length of the curtain wall screen and ’300’, which gives in this case a deflection of 25.7mm (i.e. 7700/300) that exceeds 15mm − the recommended maximum deflection by bs en 13830:2003. therefore, the product specified at the early design stage does not meet the structural requirement. • maximum glass weight supported: the maximum glass weight that the transoms of the selected system could support was 250 kilograms according to the specification of the vendor. the weight of the glass is usually calculated using the empirical formula that each msq of glass weighs 2.5 kilograms per 1mm thickness. the thickness is a function of the barrier loads. in this case, the barrier loads dictated that the glass thickness required was 10mm outer panes and 13.8mm inner panes. the section size in this case is 5.84 msq (i.e. 1.67m × 3.5m) and the glass weight is 347.5kgs (i.e. 5.84 m2 × 2.5kgs × 23.8mm), which exceeds 250 kilograms − the maximum glass weight that could be supported by the selected product. • mullions length available: the selected off-the-shelf curtain wall system is supplied by vendor a with mullions having a length of 6.5 meters only. therefore, this system cannot satisfy the height to the top transom (i.e. 7.005m) required at the ground floor bar area of the build. it is important to emphasize that the issues encountered were spotted at the construction stage during which any design change affects the overall project delivery time and cost, as is widely known in the literature. the subsequent section describes the systematic effort undertaken to explore options for rectifying the identified issues. 5.2. actions taken a number of actions were systematically undertaken to address each of the encountered issues while simultaneously considering commercial, structural and aesthetic constraints: • deflection issue: sections with larger width supplied by the same vendor (i.e. vendor a) were examined as an alternative. a 65mm wide box section was identified. however, structural analysis showed that this box size could not meet the required deflection even with the inclusion of steel inserts. another alternative section, whose width was 15mm larger than the width of the system initially specified, could be meeting the deflection requirement, but it required a joint in its length to satisfy the maximum length required. the architect involved rejected this solution, as no secondary steel was allowed to be used between the mullion span points. this would be visible and totally unacceptable to the architectural intent. • glass weight: the curtain wall system selected could not support the required glass weight. to overcome this issue, it was proposed to bolt the transom to the mullion’s shear jointing blocks. the architects rejected this option as any face fixings on the curtain wall screen was not allowed. then, an extra transom could be introduced to cut the glass size down and consequently bring the glass unit weight within acceptable limits. the architects rejected this solution also, as it would affect the initial design intent and would require planning re-approval. therefore, it appeared that there were no solutions to this problem without the need to reconsider planning permissions. • mullions’ length available: the mullions of the selected curtain wall off-the-shelf system were available in 6.5 meters lengths only. vendor a was approached to enquire if a special length mullion could be produced. the vendor could not satisfy this requirement. an alternative was to introduce a spigot joint in the mullion at suitable points to achieve the lengths required. the 174 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems architects rejected this option as a seamless mullion span was a key aesthetic requirement. then, the technical department of the curtain wall’s supplier (i.s. vendor a) was requested whether they could grant a concession for using the selected product with the exceeding glass weight. the supplier did not approve of this concession. 5.3. consequences the issues encountered were not resolved after exhausting all possible solution options. at this stage, it was decided to investigate whether alternative systems supplied by other vendors (i.e. vendors b and c), who are not even part of the project’s supply chain, could resolve the issues. an off-the-shelf product, supplied by vendor b, having mullions with standard length of 7 meters, was identified as a potential solution. this product would not also require the use of joints and/or reinforcement. this system was proposed to and accepted by the architects. however, this caused some further commercial issues. the alternative product, supplied by vendor b – main competitor of vendor a and not part of the project supply chain – required a sign-off from the central control office of the general contractor who has commercial exclusivity with vendor a, whose approval was also needed. this process caused program delays as site curtain wall work package is on the critical path of getting the building weather proofed. in addition to the delays associated with this approval, the exploration of the engineering options discussed earlier result in time and cost related issues: • the project’s schedule was delayed by more than four weeks due to the additional design and research time spent on looking for alternative systems and in exhausting all possible options based on the preferred selected system. the additional spent time stretched also the available design resources, thus leading to delays in delivering the design of other parts of the building. • the architects who were directly responsible to the main client for quality control had to be fully and formally convinced and informed that the original system could not be used. this was a time consuming process that meant reissue of drawings and technical data showing and justifying the issues encountered. • the new system identified had to be submitted for approval. this included the issue of samples from a new supplier, drawings, technical data and warranties that eventually needed to be issued to the client for approval. together these consequences, resulting from the inaccurate selection of the building facade element, represented a significant wastage of resources and caused time and cost overruns for all the stakeholders involved (i.e. architect, client, contractor, suppliers, specialist building facade contractor and structural consultant). 6. the current business process for selecting building facade elements the retrospective case study revealed some issues related to the early selection of building enveloped elements of such curtain wall systems. using the results from the retrospective case study and the experience of the project manager – case study storyteller – who has thirty years of experience in managing building facade projects, this section depicts the current business process used to select curtain wall systems in construction projects. this process will be verified with the results from the following industry survey. the selection decisions are represented in a business m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 175 fig. 3. business process adopted for the selection of curtain wall systems. process which is defined as a set of coordinated tasks and activities to achieve a project objective (kassem et al., 2011). the current business process, reflecting current practices, is depicted in figure 3. it shows the key of the issue of the late appointment and involvement of building facade specialist consultants and subcontractors, which is currently made after the selection decisions have been made at the early and design phases. it is known that design decisions have the biggest impact on the project lifecycle phases and building performance (schade et al., 2011) and incorrect design decisions bring adverse impact on project participants and are responsible for many of the construction failures (andi & minato, 2003). rework, which is often experienced in construction projects, is regularly attributed to errors made during the design process (love et al., 2000). these statements were proven in the case study earlier. together, the delayed involvement of facade consultant and subcontractor specialists and the limitations of off-the-shelf building facade elements were very detrimental to projects as evidenced in the retrospective case study. the early opportunity to build for greater flexibility and give broader scope in the selection of building facade elements is missed in current business processes. figure 3 shows the three entry points (shaded boxes) at which facade consultants and/or specialist subcontractors could be involved to overcome such issues in a proactive manner. a further validation of this business process and a comprehen176 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems sive overview of the issues depicting current selection business processes will be the subject of the industry survey. 7. industry survey the results of the case study cannot be generalised to all projects or to the whole sector. an industry survey, followed by face to face and phone interviews with industry experts, was used for explorative purposes to obtain a thorough understanding of the issues associated with selection and specification decisions of building facade elements. two criteria in sampling participants and conducting the survey aimed to respectively increase the internal and the external validity of findings. the first criterion is that all participants must be actively involved in or are stakeholders who can influence the early selection of curtain wall systems. the second criterion is that the sample size must allow the findings to be generalised at sector level. to meet the sampling criteria, participants were selected from renowned and leading architectural, consultancy and contracting organizations and only included in the sample if they were actively involved in large commercial and residential construction projects where curtain wall systems are mostly used as main elements of building facades. according to these sampling criteria, sixty individuals were preselected with the support of two project managers who had more than twenty years of experience in the sector. 54 participants expressed interest in taking part and commitment to give information and came from organisations operating at european and international scale such as laing o’rourke, mace, balfour beatty, morgan ashurst, bovis lend lease, bennett’s architects, cwa architects, axis mason architects, rma architects, galliford try, barr construction, dandara, and berkeley first. both semi-structured questionnaires followed by either a face to face dialogue or telephone interview were used in the industry survey. to increase further the internal validity of the survey, two actions were undertaken. first, a pilot questionnaire was tested with an operation director who had thirty years of experience in the facade industry. this ensured that the questions in the survey were perceived as both clear and relevant. second, the telephones and face to face interviews were used with most participants to gather more information about the open-ended statements given by the participants. to adequately answer the issues researched, the questionnaire was organised into three sections having distinct objectives: • awareness of stakeholders about commercially available curtain wall systems. • knowledge of stakeholders about the engineering and technical performance of commercially available curtain wall systems that affect selection decision. • value of available product selection guidance offered by vendors. the commercially available facade systems considered in the questionnaire, and supplied by the vendors, denoted as vendor a, vendor b and vendor c, could be generalised to the entire uk and eu market as the three companies together have more than 70% of the european market. this was verified in the case of the uk with actual figures from the companies house – executive agency of the department for business innovation and skills – and the three vendors considered in this study had just more than 70% of the uk market valued at $ 250 million in 2008 (companies house 2008 tax returns). the questions asked under each section and answers obtained are respectively summarised in tables 4, 5 and 6 respectively. m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 177 table 4 awareness of stakeholders about available curtain wall systems q. 1 how well you know the facade and curtain wall industry? very well (8%) quite well (70%) not very well (11%) not at all(11%) q. 2 do you employ a facade consultant? yes (6%) no (65%) occasional (11%) never (18%) q. 3 how many curtain wall systems are you aware of? 1 to 3 (24%) 3 to 6 (41%) 6 to 9 (31%) more than 10 (4%) q. 4 how many curtain wall systems have you had experience of working with? 1 to 3 (78%) 3 to 6 (17 %) 6 to 9 (5%) more than 10 (0%) q. 5 if asked to name major curtain wall systems used in the uk, which would you name? system a (54%) system b (42%) system c (4%) q. 6 given the choice, which system would you prefer to work with? system a (48%) system b (39%) system c (13%) q. 7 does the company you work for have a specified system, i.e. the choice is already made due to exclusivity deal with a particular supplier? yes (6%) no (94%) q. 8 what would be your main criteria for choosing a certain system? familiarity and past experience (23%) cost (28%) recommendation (12%) engineering aspects (12%) aesthetic (17%) technical help (8%) lead time (0%) 8. findings and discussion the retrospective case study provided empirical evidence of the impact that inaccurate selection and specification of building facade elements can have on the program’s schedule and costs. the identified issues and challenges are causing time and cost overruns in construction projects in the form of: time for re-producing new drawings or amend existing drawings, suspension of construction works, submission of new planning permissions, delays in procurement and fabrication due to new lead times, and in some cases, commercial issues, when there were exclusivity deals. this is very detrimental, not only to the building facade work package, but to the entire project, as in all construction projects curtain wall completion is always on the critical path for getting the building weather-proof. the case study introduced also some of the preliminary issues causing such a negative impact. the results of the survey provide further evidence by showing that cases where curtain wall systems, specified 178 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems table 5 knowledge of stakeholders about the engineering and technical performance q. 9 when selecting a curtain wall systems at the concept design stage, how confident are you that it will meet the engineering requirements (i.e. accommodate the building movement, deflections and imposed loads etc.)? not confident (15%) fairly confident (15%) confident (61%) very confident (9%) q. 10 who would you rely on to confirm that the system will meet the project needs in terms of engineering capabilities? specialist subcontractor (54%) own knowledge (31%) system vendor (15%) facade consultant (0%) q. 11 have you ever had experience of the specified envelope elements being changed due to their incapability of meeting engineering and technical requirements? never (67%) sometimes (33%) often (0%) q. 12 do you think you are given enough information upfront – before system selection and specification? yes (31%) no (69%) q. 13 is the information required in question 12 readily available from suppliers? yes (37%) no (63%) q. 14 do you think there is a difference in the amount of building movement that can be accommodated between different system supplier’s products? yes (18%) no (15%) not sure (67%) table 6 value of available product selection guidance offered by vendors q. 15 are you aware of any specifier guidance documentation/technical notes? british standard (8%) trade bodies (8%) cwct (31%) others (11%) q. 16 do these technical notes/guidance information give any specific system advice? yes (8%) no (72%) not sure (20%) at the early design stage, had to be changed later on in the project due to their inability to meet engineering requirements are not unusual (question 11). on the one hand, this is partly caused by the limited knowledge of the stakeholder, involved in the early specification, of engineering performance of curtain wall systems (questions 1, 9 and 14). similar findings were identified in other studies, where a survey of architects ranked the ‘lack of in-house expertise’ and ‘lack of industry expertise’ as major limitations at the design stage (jaillon & poon, 2010). on the other hand, the appointment of specialist facade consultants and subcontractors is often left until late in the business process as evidenced in the mapping of the business process (fig. 3) once the opportunity of influencing specification decisions and their impact is already missed. this was also confirmed in the survey, where more than 65% of participants acknowledged that they do not appoint a facade consultant (question 2). m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 179 in follow-up telephone interviews, only three participants confirmed that they generally employ a facade consultant to support the selection of the facade system. interviewees explained that in the majority of cases consultants are only called in to investigate and solve unforeseen problems after they occurred at the construction phase. most interviewees justified this practice on a cost cutting ground and indicated that they do not deem this initial cost value for money. facade consultants were generally seen as a ‘necessary evil’ – as expressed by one participant – once problems had become apparent. facade consultants are either hired later once the issues have occurred or not appointed at all as occurred in the retrospective case study. also, in common with the retrospective case study where an exclusivity deal existed between the main contractor and curtain wall system vendors, the survey showed that in some cases (6%) there are commercial influences such as an inclusivity deal between the contractor and the curtain wall vendor (question 7). participants interviewed confirmed that in those cases, the specification options for architects and consultants are even further restricted and technical issues could become unavoidable in those cases. one specialist subcontractor stated: “we have the most to gain if the right system is selected and we can ensure that the right system is selected. however, we have very little opportunity to influence the decision due to our usual late appointment”. the availability of information from curtain wall vendors is inadequate and difficult to obtain (questions 12 and 13). early specifiers are aware of just a limited number of curtain wall systems and usually adopt the system they know best until a problem arises (questions 3 and 4) or “specify the systems of those suppliers that appear to offer the most secure warranties and technical assistance”, noted one of the participants. thelimitedawarenessofparticipantsofavailablecurtainwallsystemscanhavesignificantcommercial implicationsonthemarketshare.vendorswiththehighestmarketingbudgetandcapabilitiescouldhave their systems specified on more and more projects and their market share could grow increasingly. merging together four of the survey findings (i.e. 1. limited knowledge of stakeholders of engineering performance, 2. technical guidance and information are either unavailable or not user-friendly, 3. the non-appointment of facade consultant and 4. the late appointment of facade subcontractor) gives indications of the root-causes of the challenges and risks affecting the selection process in this considerable and expensive industry. if the four issues are seen as constraints in the current industry business processes, a solution that concurrently addresses them is to bring forward engineering information of building facade elements to the early specification phase in a simplified and integrated manner − to cover all commercially available systems − simplistic and user friendly format − to match the limited knowledge of stakeholders. the first contribution of this research was to provide the empirical evidence, by illustrating a real case study and an industry survey of the major players, of the challenges affecting the selection of facade systems at the early design phase, and to identify the root causes of issues creating wastage in the building facade sector. indeed, together the case study and the industry survey depicted a holistic identification and explanation of both the issues and their implications. the findings from both the case study and industry survey can be used to classify the issues into four distinct categories: • limited understanding by the decision makers, involved in the early specification, of the engineering and technical parameters of facade systems. • tendency or reluctance to appoint specialist consultants and subcontractors early in the business process. 180 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems • lack of tools or methodologies that provide information in a user friendly and simplistic format that match the level of experience of the early decision makers, and • commercial influences that affect the early specification and restrict the available selection options. research and development efforts that aim to address the identified issues need to distinguish between the issues that are rooted in the industry mindset and those that are purely related to technical issues. the latter can be addressed in the short and mid-term compared to the former that require a long-term cultural change. indeed, as noted by three participants in the follow-up interviews, some of the identified issues, such as the delayed appointment of specialist contractors and the reluctance to appoint consultants, are rooted in the construction industry and could persist in the short and mid-term, despite several studies researching and invoking the need for early stakeholder involvement. for example, studies focusing on the importance of early stakeholder involvement (wikstrom et al., 2010; kagioglou et al., 2000) and interaction (tribelsky & sacks, 2011) and their impact on value creation (mitropoulos & howell, 2002) have proliferated since more than one decade and was emphasized in a notable industry report (i.e. egan report (1994) − rethinking construction). however, issues related to the lack of involving stakeholders at suitable decision points in construction projects are still occurring, as this case study and survey have demonstrated. early stakeholder involvement gives projects the opportunity to utilize and exploit a richer knowledge base (ramaswamy & gouillart, 2010). in the case of building facade, as it was demonstrated in the case study and the survey, the specialist knowledge of building facade consultants and specialist subcontractors is not exploited due to their late or non-involvement. a recent study, investigating the state of integration in the aec community concluded that despite the acknowledged importance of integrated practices, integration levels vary among different disciplines (uihlein, 2013). this study unraveled some of the collaboration issues specific to the building facade sector. in the short term, it is challenging to present solutions to rooted issues in the industry such as the culture of non-involving all relevant stakeholders in the early design phase. however, it is possible to address some technical issues, such as the lack of simple and user-friendly technical guidance, the complexity and fragmentation of guidance and the limited knowledge of stakeholders involved in early selection decisions. for example, a solution option is to facilitate specification and selection decisions by developing information management and decision support systems that bring forward engineering and technical information in a simplistic and user friendly manner to the stakeholders involved in the specification decisions (kassem et al., 2012). this solution helps filling the knowledge gap of stakeholders and improves the communication and understanding of engineering performance at the early specification stage. very few studies, aimed at addressing the aforementioned issues with the specific focus on the facade industry, are available. chin et al. (2004) and hwang et al. (2006) presented a conceptual framework with three dimensions: production management, organization management, and information management. the production management dimension aims at clarifying the performance requirements for curtain wall and reducing design reworks through manufacturability and constructability reviews. the organization management dimension tackles the nontechnical issues such as the need to change owner’s and architect’s attitude, and the need for improving contractual arrangement. finally, the information management dimension consists of an ‘alternative information-based solution’ for each of the reported issues. however, subsequent papers published by the authors (e.g. hwang et al., 2006) have focused only on the processes downstream the design stage (i.e. manufacturing, delivery m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 181 and installation) and therefore, did not address the specification decisions at the early design phases. also their framework made no distinction between off-the-shelf and bespoke curtain wall systems. the retrospective case study and survey showed that the use of off-the-shelf systems are a popular choice on projects and due to the limitations of such systems, issues arise in design and construction phases. a proposed decision support system to aid the selection process of off-the-shelf curtain wall systems was developed for the products of three major vendors (kassem et al., 2012). the system enables users to identify products that meet the project and engineering requirements. however, the development of the proposed system revealed further challenges. one major challenge consisted of the need for a taxonomy that can to be adopted across all vendors’ systems to uniquely represent key technical parameters between vendors’ systems. resolving the taxonomy challenge in the representation of engineering parameters and performance of building facade products across different supplies will facilitate information management systems and consequently increase the sharing of information between stakeholders. this will also contribute to unravelling some of the less known building facade systems to the stakeholders involved in selection decisions. the final contribution and implication of this research is to instigate or complement methodological approaches in the subject area of ‘investigating and identifying issues affecting construction projects’ with a new approach in which the granularity and scale of investigation is increased to project and disciplinary level (i.e. building facade), without overlooking the link with other disciplines (i.e. architectural and structural). as evidenced from this research, this approach proved to be effective in identifying the very nature of issues and their root causes. 9. conclusions the overarching aim of this research was to empirically identify the issues and challenges affecting the selection of building facade at the early design, the impact of their specification decisions on construction projects, and potential solutions. the use of the retrospective case study, process mapping and the industry survey helped to achieve this aim. the case study systematically demonstrated some of the issues affecting the early selection and building facade elements and their impact. the lack of involvement of facade consultants and specialist facade sub-contractors results in selection of building facade systems that do not meet the project and engineering requirements. such issues, revealed only at the late construction phase, have adversarial effects not only on the project’s schedule and cost but also on the commercial relationships between stakeholders in some circumstances. the industry survey contributed to identify an exhaustive list of the issues affecting the business process of building facade selection and the root cause of such issues. the root cause of issues were classified into four categories, namely: limited understanding of engineering parameters by stakeholders involved in the early selection, reluctance in the appointment of specialist consultants and subcontractors at early stages of the procurement process, lack of tools or methodologies that provide information in a user friendly way to match the limited technical knowledge of stakeholders, and commercial constraints such as exclusivity deals that restricts the options available. in addition to filling the research gap in literature, this study adopted and instigated a new methodological approach in this research domain. the approach chosen consisted of increasing the granularity of investigation by focusing on a specific building trade and providing empirical evidence of the issues and their impacts. this will warrant an incisive inquiry into the very nature of issues affecting the subject investigated. in such a context, this research instigates: 182 m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems • domain researchers, who are interested either in exploring issues (time, cost or quality related) in construction projects in general or in understanding how to bridge the gap between early design decisions and engineering implications, to increase the depth of investigation from sector-wide level to a more granular level such as a single building trade. • industry players to develop methodologies and systems that bring forward engineering information in a simplistic and user-friendly manner to all the stakeholders involved in the selection process. • researchers and industry players to build a taxonomy of technical terms and concepts across all facade systems’ vendors to facilitate the comparison of engineering performance at the early design process in a systematic manner. references al-hajj, a., & hamani, k. (2011). material waste in the uae construction industry: main causes and minimization practices. architectural engineering and design management, 7(4), 221-235. amaratunga, a., baldry, d., sarshar, m., & newton, r. (2002). quantitative and qualitative research in the built environment: application of “mixed” research approach. work study, 51(1), 17-31. andi, a., & minato, t. (2003). design documents quality in the japanese construction industry: factors influencing and impacts on construction process. international journal of project management, 21(7), 537-546. assaf, s. a., & al-hejji, s. (2006). causes of delay in large construction projects. international journal of project management, 24, 349-357. chin, s., yoon, s. w., kim, c. d., choi, y. k., & chun, j. y. (2004). an analysis of the life-cycle curtain wall process through supply chain management. proceedings of international group of lean construction, iglc 12. copenhagen, denmark, 2004. cwct (centre for window and cladding technology) (1999). introduction to structural design of stick curtain walling. cwct technical note, no. 26. cwct (2000a). cladding types. cwct technical note, no. 14. cwct (2000b). weather tightness and drainage. cwct technical note, no. 17. cwct (2000c). introduction to wind loading on cladding. cwct technical note, no. 2. cwct (2003). introduction to building envelope acoustics. cwct technical note, no. 37. cwct (2007). movement accommodation in building envelope. cwct technical note, no. 55. cwct (2011). fire performance of curtain walls and rain screen. cwct technical note no. 73. eastman, c., teicholz, p., sacks, r., & liston, k. (2011). bim handbook: a guide to building information modeling for owners, managers, designers, engineers and contractors, 2nd ed. new jersey, ny: john wiley & sons, inc. efstathiades, c., baniotopoulos, c. c., nazarko, p., ziemianski, l., & stavroulakis, g. e. (2007). application of neural networks for the structural health monitoring in curtain-wall systems. engineering structures, 29(12), 3475-3484. el-razek, m. e. a., bassioni, h. a., & mobarak, a. m. (2008). causes of delay in building construction projects in egypt. journal of construction engineering and management, 134, 831-841. fallahnejad, m. h. (2013). delay causes in iran gas pipeline projects. international journal of project management, 31(1), 136-146. hwang, d. w., choi, s. w., song, y. w., & choi, y. k. (2006). information exchange of curtain wall work for the application of supply chain management system. proceedings of the 23rd international symposium on automation and robotics in construction, (isarc), tokyo, japan, 2006. jaillon, l., & poon, p. s. (2010). design issues of using prefabrication in hong kong building construction. construction management and economics, 28(10), 1025-1042. jin, q., & overend, m. (2010). a thermal performance analysis model for the design optimisation of high performance glazed facades. engineered transparency. international conference at glasstec, düsseldorf, germany, 2010. kagioglou, m., cooper, r., aouad, g., & sexton, m. (2000). rethinking construction: the generic design and construction process protocol. engineering. construction and architectural management, 7(2), 141-153. kassem, m., dawood, n., & mitchell, d. (2011). a structured methodology for enterprise modeling: a case study for modeling the operation of a british organization. journal of information technology in construction, 16, 677-690. kassem, m., dawood, n., & mitchel, d. (2012). a decision support system for the selection of curtain wall systems at the design development stage. construction management and economics, 30(12), 1039-1053. kragh, m. (2011a). façade engineering and the design team of the future. society of façade engineering. executive boardroom commentary, 30-36. retrieved from http://www.cibse.org/getmedia/28be6591-0d43-429e-a14c-8c64c7ecb315/kragh-paper.pdf.aspx http://www.cibse.org/getmedia/28be6591-0d43-429e-a14c-8c64c7ecb315/kragh-paper.pdf.aspx m. kassem and d. mitchell / bridging the gap between selection decisions of facade systems 183 kragh, m. (2011b). the decade of the façade engineer. society of façade engineering. intelligent glass solutions, 1, 44-51. retrieved from http://www.cibse.org/getmedia/eb0a8348-e754-4e78-bd7e-c8674e861d91/kragh-4.pdf.aspx love, p. e. d., mandal, p., smith, j., & li, h. (2000). modelling the dynamics of design error induced reworking construction. construction management and economics, 18(5), 567-574. mitropoulos, p., & howell, g. a. (2002). renovation projects: design process problems and improvement mechanisms. journal of management in engineering, 18(4), 179-185. mohsen, f., & elaheh, n. (2012) building envelope as an environmental apparatus: integrating architectural and natural systems. icsdec, 2012, 268-275. odeh, a. m., & battaineh, h.t. (2002). causes of construction delay: traditional contracts. international journal of project management, 20, 67-73. pavitt, t., & gibb, a. (2003). interface management within construction: in particular, building façade. journal of construction engineering and management, 129(1), 8-15. quirouette, r. (2013). glass & aluminium curtain wall systems, ontario association of architects. ramaswamy, v., & gouillart, f. (2010). the power of co-creation: build it with them to boost growth, productivity and profits. new york, ny: free press. rossman, g. b., & wilson b. l. (1991). number and words revisited: being ‘shamelessly eclectic’. evaluation review, 9(5), 627-643. schade, j., olofsson, t., & schreyer, m. (2011). decision making in a model-based design process. construction management and economics, 29(4), 371-382. shebob, a., dawood, n., shah, r. k., & xu, q. (2012). comparative study of delay factors in libyan and the uk construction industry. engineering construction and architectural management, 19(6), 688-712. singhaputtangkul, n., low, s. p., teo, a. l., & hwang, b. g. (2014). criteria for architects and engineers to achieve sustainability and buildability in building envelope designs. journal of management in engineering, 30(2), 236-245. succar, b., & kassem, m. (2015). macro-bim adoption: conceptual structures, automation in construction, 57, september 2015, 64-79. sun, m., & meng, x. (2009). taxonomy for change causes and effects in construction projects. international journal of project management, 27(6), 560-572. sweis, g., sweis, r., hammad, a. a., & shboul, a. (2008). delays in construction projects: the case of jordan. international journal of project management, 26, 665-674. tran, d., behr, r., & parfitt, m. (2014). global differences in building enclosures, journal of architectural engineering. http://dx.doi.org/10.1061/(asce)ae.1943-5568.0000146 tribelsky, e., & sacks, r. (2011). an empirical study of information flows in multidisciplinary civil engineering design teams using lean measures. architectural engineering and design management, 7(2), 85-101. tumi, s. a. h., omran, a., & pakir, a. h. k. (2009) causes of delay in construction industry in libya. in: proceedings of the international conference on economics and administration (pp. 265-272). uihlein, m. (2013). state of integration: investigation of integration in the a/e/c community, journal of architectural engineering. http://dx.doi.org/10.1061/(asce)ae.1943-5568.0000139 vigener, n., & brown, m. (2012). building envelope design guide–curtain walls. washington, dc: national institute of building sciences. wikstrom, k., artto, k., kujala, j., & soderlund, j. (2010). business models in project business. international journal of project management, 28(8), 832-841. yin, r. b. (1994). case study research: design and methods (2nd ed). thousand oaks, ca: sage publications. zoya kpamma, e., & adjei-kumi, t. (2011). management of waste in the building design process: the ghanaian consultants’ perspective. architectural engineering and design management, 7(2), 102-112. http://www.cibse.org/getmedia/eb0a8348-e754-4e78-bd7e-c8674e861d91/kragh-4.pdf.aspx http://dx.doi.org/10.1061/(asce)ae.1943-5568.0000146 http://dx.doi.org/10.1061/(asce)ae.1943-5568.0000139 journal of facade design and engineering 2 (2014) 183–200 doi 10.3233/fde-150019 ios press 183 innovative design tool for the optimization of blast-enhanced facade systems marc zobeca,∗, guido loria, raymond lumantarnaa, tuan ngob and cuong nguyenb apermasteelisa group, governor macquarie drive, chipping norton, nsw, australia bdepartment of infrastructure engineering, the university of melbourne, australia abstract. in current blast enhancement design strategies, to resist the effects of an accidental explosion, a facade system is commonly designed to behave in-elastically and undergo large deformations. the large deformation of the facade system leads to high blast energy dissipation, subsequently reducing the blast energy transferred to the main structure. in addition to the blast resistance of the facade system, human injuries due to glass fragmentation within the vicinity of the facade system should also be minimized in order to meet the required safety levels. overall building safety can be optimized by balancing blast energy dissipation and glass fragmentation. recently, permasteelisa group has developed an innovative design tool to optimize blast-enhanced facades using an equivalent mdof approach. a novel fragmentation tool has been proposed to assist this design procedure. this paper presents various critical parameters considered in blast-enhanced facade analysis, the experimental validation of these parameters and their influence in the design optimization process. keywords: blast, facade design, hazard, mitigation, computer software 1. introduction threats to building occupants due to blast loads generally result from a combination of blast overpressure injuries due to a breach of the facade system and glass fragmentation injuries. as witnessed in the oklahoma city bombing in 1995, approximately 40% of laceration and abrasion injuries were attributed to glass fragmentation (norville et al., 1999). current blast design related practices commonly refer to achieving a balanced design that is essentially focused on strength limit states whereby in a structural system subjected to blast, no element is appreciably weaker than the element that it supports, thus minimizing the risk of progressive collapse (hinman, 2011). from the perspective of both structural integrity and human safety, a true balanced design requires a combination of both energy dissipation and glass fragment retention (zobec et al., 2012). the polyvinyl butyral (pvb) interlayer used in laminated glass exhibits substantial fragment retention and post-elastic viscous deformation capacity. given the combination of these two behavioural effects, laminated glass is the preferred glazing solution for blast-enhanced facades. figure 1 describes the various glass hazard classification levels according to iso 16933:2007 (2007). as the glazing is ruptured by the blast shock wave, numerous glazing fragments are projected into the ∗corresponding author: dr. marc zobec, group design, training & knowledge sharing manager, permasteelisa pty ltd, 13-15 governor macquarie drive, chipping norton, nsw 2170, australia. tel.: +61 2 9755 1788; mobile: +61 411 897 836; fax: +61 2 9755 1418; e-mail: marc.zobec@permasteelisagroup.com; www.permasteelisagroup.com. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:marc.zobec@permasteelisagroup.com www.permasteelisagroup.com 184 m. zobec et al. / innovative blast enhanced facade design tool fig. 1. cross-section through witness area for arena test (iso 16933:2007). occupied room. limits for low and high hazard regions have been defined according to the kinetic energy of the glass shards and most frequently occurring physiological damage thresholds. eye injuries are presumed to occur at energy levels as low as 0.06 j/mm2 or shard velocities of approximately 2m/s (low hazard). similarly, the potential for skin tissue penetration is presumed to occur at glass fragment kinetic energy levels of around 0.1 j/mm2 or approximately at shard velocities of 9m/s (marchand et al., 2006), although this value has been found to vary significantly between annealed and toughened glass tests. in blast resistant facade design, the fragmentation hazards or spall is often the limiting factor, whilst the viscous-elastic energy dissipation can be maximized by the use of the pvb laminates. as a result, second generation blast-enhanced facades are usually referred to as being dissipative due to beneficial effects in terms of mitigating impulse transfer, but whilst it is obvious that hazard protection is still the dominant blast performance requirement, conformance to this criterion may in certain circumstances be detrimental for design optimization. none the less, performance criteria definitions are required in terms of impulse and energy reduction as building owners and occupants are increasingly requesting that the facade system is designed such that in the event of a blast the magnitude of peak reactions transferred to the main structure are minimised. currently, there is no consensus on the ultimate failure mode of cracked laminated glass due to large displacements. wingard 5.5 considers the primary failure mechanism for laminated glass to occur when the lateral in-plane edge displacement of the glass membrane exceeds twice the framing rebate depth based on arena tests of non-structurally glazed windows. ellis (1991) and ellis & beak (1992) undertook water-bag testing of laminated glass panes to failure and reported the failure displacements and modes. the most common failure mechanism was cutting of the pvb membrane at the glass-frame edge interface due to excessive end rotations (morison, 1999), generally dependent on the short span of the laminated glass and of the total thickness of the interlayer. to take into account the sensitivity of the glass strength to the strain rate, norvillle & conrath (2001) have proposed a method to equate blast loads with an equivalent three second wind gust, enabling glazing make-up selection based on the glazing design standard astm e 1300–12 (2012). the inclusion of monolithic and laminated glass into a single method assumes that glass sizing is based on surviveability up to or just at the point of cracking (morison, 2007). the method does not take into account large pvb deformations and energy dissipation. thus, the design approach is overly conservative and uneconomical. the design of individual facade elements based on a ‘safe’ equivalent static load is straight forward and allows the analysis to be carried out rapidly. however, this approach is not capable of taking into account the dynamic interaction between the components m. zobec et al. / innovative blast enhanced facade design tool 185 of the facade system. dynamic fem analysis is the most accurate analysis tool but it often requires long computation time. the reliability of results is also limited to the accuracy of the input parameters. under dynamic and impulsive loads, materials used in facades exhibit marked strain-rate dependent behaviour that must be considered and accurately modelled. equivalent sdof methods have been used to analyse glass plates subjected to blast since its inception by newmark (1956). the method requires two important parameters: the mass transformation factor and the resistance function. an accurate displaced shape is required to estimate the mass transformation factor, whilst the resistance function is dependent on stiffness parameters and the ultimate failure displacement. for a true balanced design that considers both resistance and building occupant safety, a rapid design tool capable of incorporating a dynamic analysis as well as accurately assessing the projection of glass fragments is required. the permasteelisa group has recently developed a glass fragmentation hazard tool used in conjunction with a fem code, in order to improve the design optimization process as shown in fig. 2. this paper initially outlines the distinctive background of the analysis method used with current approaches. secondly, the results of original research outcomes derived from testing of laminated glasses of various thicknesses, treatments and aspect ratios and critical parameters affecting performance are established. the findings include equivalent interlayer post elastic properties, transformation factors and allowable post cracked laminated glass deflection limits. during dynamic analysis and subsequent hazard level assessment, the glass fragment size is identified as the key parameter affecting the results of the tests. this parameter also has a significant influence during the various stages of the design process. finally, an original fragmentation model, implemented in the analysis tool is presented. fig. 2. schematic of the analysis process in testudo™. 186 m. zobec et al. / innovative blast enhanced facade design tool 2. sdof approach and mdof models 2.1. sdof & mdof the equivalent sdof method commonly used in dynamic analysis is based on initial research work by biggs (1964). biggs defined transformation factors of several structural elements based on the hypothesis that the behaviour of an element under dynamic conditions can be represented by its deflected shape under static load. for a uniformly loaded plate with centre displacement wc , varying with time, a unique possible configuration for the displacement function u(x,y,wc ) could be derived at any displacement position wc . the instantaneous position of every point on the surface could be derived as a function of the centre position wc , knowing the free parameter or degree of freedom. the approach is a simple variation of the modal analysis method for the solution of the linear dynamic problem. unlike the modal analysis method, one main advantage is that the sdof method can easily be extended to account for inelastic system behaviour. once the proper constraint function is selected, u(x,y,wc) (which expresses, under general nonlinear conditions, the three components of the displacement vector at position (x,y), versus the centre deflection), by the principle of virtual work it can be shown that the equilibrium of the system can be represented by a single second order differential equation: klm [mẅc + cẇc] + r (wc) = f(t) (1) where m=mass of the system, c=total damping coefficient, r=non-linear resistance function and f(t)=external load. klm is the ratio between the mass and load transformation factors: klm = km kl (2) it can be demonstrated that (1) is applicable when damping and mass are distributed homogeneously along the surface or by the same distribution function. as stated by biggs (1964), the resistance function is presented as the resistance of the plate under static load related to the static deflection matching the instantaneous centre deflection wc (t). however, equation (1) is an approximation because a second inertia term has not been taken into account. the described approach for surface elements is used to reduce the continuous glass behaviour into a sdof system. however, the concept can be extended by coupling the structural behaviour of various facade elements resulting in mdof models representing the overall interactive facade behaviour (fig. 3). 2.2. sdof: laminated glass resistance function when applied to laminated glass, the sdof approach described by the equation (1) must consider its various behaviour phases. figure 4 describes the static resistance versus displacement relationship of a typical single laminated glass pane. the glass behaves elastically up to point 1, where the deflection of the inner glass lite reaches its (non-linear) design strength limit. subsequently, resistance is provided by the remaining external glass lite. once the external lite breaks, the remaining polyvinylbutyral (pvb) interlayer resists the external load, in combination with the residual stress capacity and stiffening effect still exerted by the broken glass. in this region of the resistance curve, the experimental investigation plays a fundamental role for two main reasons: m. zobec et al. / innovative blast enhanced facade design tool 187 fig. 3. mdof facade models. • there are limited studies into the post cracking behaviour of laminated glass due to previous focus on the elastic behaviour of the material. • dynamic analysis is usually carried out considering elastic-plastic behaviour in lieu of viscoelastic. bennison et al. (2005), morison et al. (2007), hooper et al. (2012) and kuntsche et al. (2014) undertook testing of pvb samples under various strain rates. these tests showed that for increasing strain rate, the stress-strain behaviour undergoes a behavioural transition from viscous-elastic to bilinear (distinct ‘yield’ transition and subsequent plateau). in order to effectively analyse membrane behaviour after point 2, a solver capable of analysing the bi-linear stress-strain behaviour must be adopted. the bi-linear behaviour of the pvb under high strain rates can be seen in fig. 4, where the translationally restrained membrane resistance exhibits a resistance transition at point 4 due to progressive ‘yielding’ of the pvb membrane along the supports. morison (2007) compared the tensile properties of bare pvb membranes and cracked laminated glass under high strain rate tests. it was noted that the glass fragments have a stiffening effect on the membrane. based on back analysis of blast arena tests of single laminated glass, he derived a mean glass fragment-stiffening ratio of 3.8. based on water-bag testing of laminated glass panels undertaken by ellis (1991) and ellis & beak (1992), morison statistically derived the relative failure displacement (rfd) (point 4), as 27.8% of the short span based on a 90% confidence limit for 7 glass samples. however, given the limited number of glass panes tested including dimensions, thicknesses and glass heat treatment, a need for further investigations remained; in particular the need to accurately define laminated glass resistance functions (particularly in the post-cracked region), which is fundamental for use in sdof analysis. though post-cracked laminated glass behaviour under dynamic conditions differs significantly from the response under static loading, the results of water-bag tests are of major importance for the 188 m. zobec et al. / innovative blast enhanced facade design tool fig. 4. typical laminated glass resistance curve. application of the sdof approach to blast loading of laminated glass: the shape function used for the evaluation of the transformation factors is the deformed shape under static loading. the relative failure displacement under static loading is expected to be a safe estimation of the relative failure displacement under blast loading, as the average size of the fragments is larger and then the probability of cutting at the edge is higher. as shown by morison (2007), an empirical correction method can be applied in order to derive the stiffened laminated glass properties under high strain rates, starting from the knowledge of the pvb high strain rate properties, the static properties of the laminated glass and the estimation of a bonding factor, which gives the percentage of the average surface area still ‘bonded’ by the interlayer during the post-breakage deformation. 3. experimental investigations into the post-cracking behaviour of laminated glass permasteelisa, in collaboration with the university of technology of sydney and the university of melbourne, undertook an extensive water-bag testing program (fig. 5). a statistically broad number of samples of various glass thicknesses and heat treatments have been considered in order to derive relative failure displacement values. furthermore, the deflected shape of the glass was progressively mapped at various displacement intervals in order to derive the non-linear shape as well as the stiffening effects of glass fragmentation. nonlinearity effects as a result of non-uniform water pressure distribution due to the varying m. zobec et al. / innovative blast enhanced facade design tool 189 fig. 5. water-bag test of laminated glass. water head; g, over the glass surface were identified and corrected using a numerical fem calibration procedure (lumantarna et al., 2012). current methods to derive the transformation factors in the post-cracked phase assume the pvb to behave as a pure membrane with no effects of glass fragmentation on shape. 3.1. relative failure displacement the water-bag tests have highlighted the significance of glass fragment sizes on relative failure displacements (rfd). for annealed (ann) glass, two primary modes of failure were noted; edge cutting due to excessive rotation and tearing of the pvb over the surface of the glass. the cumulative distribution functions (cdf’s) for both failure modes displayed close statistical mean and standard deviation correlation. hence, the rfd of the respective panels is derived based on the total number of samples. it was observed that the fragmentation size has a significant influence. smaller fully toughened (ft) glass fragments resulted in greater overall pvb strain distribution and hence lower local stress concentrations. due to limitations in the test rig displacement measuring capacity, no failures were noted for ft glass with minimum rfd values of 36% of the short span. for annealed glass, as fragment sizes increased with thickness, rfd was noted to be inversely proportional to glass thickness. 3.2. shape and transformation factors incremental displacement plots of the broken laminate were mapped using photogrammetry techniques. a numerical fem calibration technique was used to correct the variation in water pressure head across the glass surface and hence the non-uniform water pressure to derive the load-mass (klm ) transformation functions. equivalent stiffness values were derived from the fem models by fitting the membrane displacements to the test displacements. consequently, for uniform pressure, the equivalent cracked laminated glass stiffness values were used to calculate the displaced shape for progressive displacements and compared with a pure pvb membrane. the results show that 190 m. zobec et al. / innovative blast enhanced facade design tool fig. 6. cdf vs. rfd for edge cutting and tearing failure modes. fig. 7. 9.52mm ann klm curves for hydrostatic and uniform pressure models. for increasing displacements, klm for the annealed glass approaches that of ft glass. for small displacements of 50mm, klm ,ann < klm ,ft =0.88 whilst for large displacements of 400mm, klm ,ann =klm ,ft =0.95. with regards to the effects of membrane shape, transformation factors for both cracked laminated ft & ann glass exhibits close correlation with pure pvb membrane properties and hence when deriving post cracked non-linear pvb transformation factors under uniform loading, pure membrane analysis is valid. fig. 7 at this purpose shows firstly how the klm ,ann derived by the model for the water pressure is near to the value from the ann glass experimental results and then how the bare pvb model for uniform pressure and the ann glass model for uniform pressure has a perfect correlation in terms of klm trend versus the glass centre deflection. 3.3. pvb stiffening effect at low strain rates, polyvinyl butyral (pvb), exhibits visco-elastic properties. morison et al. (2007), iwasaki et al. (2006, 2007) and bennison et al. (2005) undertook high strain rate testing of pvb m. zobec et al. / innovative blast enhanced facade design tool 191 and noted that as the strain rate increased the stress-strain relationship changes from viscouselastic to bi-linear, exhibiting a distinct elastic range with a ‘plastic’ tangential plateau. these tests were carried out using bare pvb only without the effect of glass fragmentation. layered laminated glass strip fea models were analysed using ls-dyna. in contrast to morison (2007), who uses an arbitrary bonding factor that describes the percentage of pvb that would remain adhered to the glass fragments under minimal strain, the fea models in the analysis consist of a layered glasspvb tensile model with varying percentages of exposed pvb versus laminated glass ratio c, and is expressed as the initial ratio of pvb that can be effectively strained or mobilized. the cracks were set as gap elements with an initial 0.5mm crack opening, which is consistent with the experimental evidence. hence, c=1 assumes that 100% of the pvb is unconstrained and thus will have bare pvb strain rate dependent properties. at c=0, it was assumed that no pvb can strain and hence the stiffness of the model is that of the glass; eglass =70 000mpa. material properties used in the model are as presented by morison et al. (2007). three (3) pvb strain rate properties were modelled; quasi static 0.07 s−1, 16 s−1 and 30 s−1, with values for 16 s−1 hypothesised by morison (2007) superimposed for bonding factors from 0 to 25% showing good correlation to the model. for the normal range of glass spans and thicknesses loaded close to failure, pvb membrane strain rate is typically in the region of10 s−1. this value can be easily estimated evaluating the elongation of the membrane for a certain peak centre deflection in comparison with the time duration of the centre displacement time history up to the peak. comparing back analysis from blast trials, morison noted that the value of pvb glass fragment stiffening ratio varied from 0.6 to 7.1, with a mean value of 3.8. based on a typical strain rate of approximately 10 s−1 for laminated glass plates under typical blast loads, this equates to mean pvb design properties corresponding to a strain rate of 38 s−1 (40 s−1 for simplicity). this is a rough initial estimation, which requires further research and states that a stiffened post breakage behaviour of a laminated glass can be expressed by means of a pvb equivalent young’s modulus (for the first linear range of stress-strain behaviour) of around 264 mpa. in contrast wingard proposes a value of 345 mpa. for initial conservative analysis a value of 220–250 mpa is recommended. 4. the dissipative role of the facade knowledge of the post cracking behaviour of laminated glass is important in order to design a facade that performs as required in terms of maximizing hazard protection and blast energy dissipation. whilst maximizing hazard protection (which will be discussed in the subsequent chapter of this paper) is well understood and protection strategies and requirements have been defined by various statutory organizations over the past twenty years, blast energy dissipation is not yet clearly defined. although second generation blast-enhanced facades are currently being optimized emphasizing blast energy dissipation, clear guidelines on how to best evaluate dissipative effects do not yet exist. by contrast, building owners are currently defining in project specifications specific performance requirements in terms of reaction reduction, strictly related to actual specific limitations of the main structure in its ability to transfer facade loads. as a result, there is a real need for clarification and design criteria should be specified more effectively in order to classify the dissipative performance of blast-enhanced facades. 192 m. zobec et al. / innovative blast enhanced facade design tool fig. 8. effective pvb stiffened elastic modulus versus c (exposed pvb glass ratio). the initial question that needs to be addressed, is whether or not characterizing the facade in general terms, as being ‘dissipative’ is an appropriate term? how much quantifiable energy does the facade need to dissipate?. it is true that a significant proportion of blast energy is dissipated through plastic and viscouselastic deformations of the facade components as well as damping effects, but the most important role performed by the facade is the ‘postponement’ of energy transfer or rather effectively distributing the blast impulse over a longer time duration. this effect is particularly beneficial with regard to load transfer to the main structure. as a result each mechanical component of the facade is subject to less severe loading when the same impulse can be absorbed over a longer time (as can be evidenced by the iso-damage curves behaviour). inertial effects play an important role in reducing the reaction transfer at the initial facade response phase, whilst the plastic/visco-elastic deformations are the primary factors that result in specific force reductions during the secondary phase, when the inertial effects commence to be detrimental. in general the facade can be seen as a system capable of transforming the blast input duration and peak pressure over a different combination of the two parameters, but for the majority of cases without significant changes to their product (impulse). for this purpose fig. 9 describes the effect of transfer from the blast load (bl) to an equivalent reaction load (erl) of a lower iso-damage level. for instance, fig. 10 and fig. 11 describe the reaction time histories transferred to the structure for two different glazing configurations as well as differences in the complexity of the facade analytical model subjected to blast load condition, gsa d. the difference between the two glazing configurations remains in the use of annealed or tempered glass. the first three analytical models are (multipledegree-of freedom) mdof whose component deformability is (1) only glass is deformable, (2) both glass and supporting mullions are deformable and (3) glass, mullions and bracket (dissipative) are deformable. model 3 involves a bespoke two-step modelling procedure, combining the mdof glazing reaction history analysis and subsequently using this load file to model the facade framing members behaviour using fea. m. zobec et al. / innovative blast enhanced facade design tool 193 fig. 9. facade impulse mitigation effect shown on the typical p-i curve background. fig. 10. reaction per square meter of 1500 × 4200 × 8an-16-3.3.4an (by facade models of different complexity) compared with gsa d blast load. tables 1 and 2 summarize the results in terms of two indices: ir, the impulse ratio, that is the ratio between the blast input impulse and the reaction impulse (per square meter) output transferred by the facade, evaluated over the first half cycle in the event of oscillating reactions. rrr is the reaction root mean square (rms) ratio, that is the ratio between the rms of the input pressure and the rms of the reaction time history (per square meter) evaluated over the first half cycle in the event of oscillating reactions. 194 m. zobec et al. / innovative blast enhanced facade design tool fig. 11. reaction per square meter of 1500 × 4200 × 8ft-16-3.3.4ft (by facade models of different complexity) compared with gsa d blast load. table 1 impulse mitigation effect of a 1500 × 4200 × 8an-16-3.3.4an under gsa d lay-out specific reaction impulse [kn/m2.ms] ir specific reaction rms [kn/m2] rrr deformable glass 570 0.93 4.67 0.23 deformable glass and mullion 641 1.04 3.39 0.17 deformable glass, mullion and bracket 545 0.89 2.73 0.14 two step (mdof+fem) 618 1.01 6.4 0.32 table 2 impulse mitigation effect of a 1500 × 4200 × 8ft-16-3.3.4ft under gsa d lay-out specific reaction impulse [kn/m2.ms] ir specific reaction rms [kn/m2] rrr deformable glass 894 1.46 18.85 0.95 deformable glass and mullion 738 1.02 3.65 0.18 deformable glass, mullion and bracket 559 0.91 3.15 0.15 two step (mdof+fem) 714 1.16 10.42 0.52 the use of annealed glass is interesting to note as the impulse ratio remains around a unitary value, whilst the beneficial effect increases proportionally as other deformable (retardant) components are also simulated. by contrast, the use of tempered or fully toughened (ft) glass reduces the beneficial effect of the facade in terms of rms reaction reduction in addition to a certain degree of impulse amplification. although different outlooks are present in the literature, the authors believe that for this reason tempered or ft glass is detrimental with respect to the mitigation strategies, at least in terms of blast reaction transfer. obviously completely different evaluations of the two types of glass behaviour can be given according to protection performance requirements. m. zobec et al. / innovative blast enhanced facade design tool 195 finally it should be noted that the rrr parameter properly expresses the beneficial effect of the dissipative role of the facade when quasi-static criteria are used for the design of the support structure (main building structure), whilst additional considerations in terms of frequency behaviour must be addressed when dynamic responses are not negligible and critical resonances could be further excited. 5. an original fragmentation model where applied to laminated glass, current flight models for glass hazard assessment have generally proven to be overly conservative (marchand et al., 2006). this results in an uneconomical design with regards to both physical performance and architectural aesthetics. in response to such cost impacts as well as hazard level assessment reliability, a fragmentation tool has been developed based on the stochastic behaviour of glass, fracture mechanics principles for crack tip opening and strain energy density in order to derive the major characteristics of the fracture pattern. the targets of the investigation are to understand the: presence of the average number and initial depth of griffith flaws in the glass properties of the local glass surface strength distribution function crack opening progression bifurcation condition. two factors are required in order to define the most important features for a new hazard level assessment model. they are: average size and distribution of the fragments fragment surface velocity and acceleration conditions. 5.1. fragmentation model fundamentals the fragmentation model can be considered to follow three major phases: crack opening condition, crack progression and finally the checking of fragment detachment conditions. after fragment detachment (should detachment occur), a 4th phase consists of an assessment of shard projectile motion based on gravitational and initial velocity conditions at detachment. the average number and distribution of griffith flaws is a function of the glass surface area as shown in haldimann (2006). during model simulation, the defined number of griffith (1921) flaws is randomly and uniformly distributed over the glass surface. each of these flaws is characterized by design strength, according to a normal strength distribution with mean value and standard deviation (morison, 2007). a dynamic fem analysis is then executed. in contrast to conventional analytical approaches, importance lies in understanding velocity and acceleration conditions as well as the glass surface strain and stress distribution. it should be noted that the accuracy and hence reliability of the sdof method diminishes with the influence of higher vibration modes. during dynamic analysis, if local stresses exceed the glass strength, crack propagation is initiated. propagation is governed by the griffith energy balance criterion: uγ = ua (3) 196 m. zobec et al. / innovative blast enhanced facade design tool fig. 12. fracture pattern and similarity with the stress pattern. fig. 13. typical pdf and cdf of the average fragment size [in mm]. where uγ is the surface energy needed for crack formation and ua is the elastic energy stored in the glass. if during crack propagation, an excess of energy is present in the vicinity of another existing crack, the crack bifurcates (bouchbinder, 2005). the process continues until there is no elastic energy stored in the glass. as an example, for a 1250 x 1520 glass pane, the final fracture pattern is represented in fig. 12. as the bifurcation phenomenon is proportional to the strain energy density, the final pattern will result in the number of fragments per unit area (strain release energy) being proportional to the principal stresses in the glass (wang, 1986; wang & mcdonald, 1986). if one or more fragments have an average inertial force greater than the interlayer-glass adhesive resistance, the fragment detaches with a projectile path in accordance with the translational and rotational components derived from the velocity distribution over the fragment surface. roughly the condition can be expressed as: m. zobec et al. / innovative blast enhanced facade design tool 197 fig. 14. experimental test result comparison. amin = sad tρgl (4) where amin is the minimum acceleration for the detachment condition, sad is the average adhesion surface strength, t is the thickness of the glass lite and ρgl is the volume density of the glass. 5.2. compliance comparison with previous research the accuracy of the fractal approach used to simulate the fracture pattern has been compared to previous research using the fragment distribution versus the average size (root square of the area) (grady, 2008), (neda et al., 1993) and is shown to closely correlate with the typical distribution exponential. 5.3. comparisons a set of 87 experimental blast tests (wingard (ara), 2005; viracon, 2004; beauchamp and matalucci, 1998) of structurally silicone retained single and double glazed units compared the test samples gsa hazard level ratings with the simulated hazard assessments derived using the proposed new fragmentation tool. results show good correlation, and it should be noted that the model was calibrated based on small-medium sized glass window samples. further simulation using larger glass sizes typical of building modulations and floor heights is necessary. glass panel heights of 6m are not uncommon for bespoke commercial buildings particularly foyer and entrance areas. final comparative statistics are described in fig. 14, where the gsa error is the difference between the predicted hazard and tested hazard levels; a negative number denoting under prediction of glass hazard levels. more than 52% of the estimations resulted in no error, with analysis correlating closely to testing. less than 10% resulted in unsafe predictions, whilst less than 5% of the predictions were deemed to be extremely unsafe (−3 levels). those error magnitudes are comparable with the errors reported by wingard technical manual and they appear adequate for a quick assessment tool, especially when considering the large number of random variables influencing the phenomenon. 198 m. zobec et al. / innovative blast enhanced facade design tool fig. 15. isodamage curve high hazard and interlayer failure compared. 6. beyond current standards current standards used for hazard level assessment in commercial buildings, have origins based on common experimental military applications. direct application to continuous facades should consider typical civilian occupancy and usage in terms of the number of persons, relative positions and distances from the facade. there should be a distinction in hazard level classification for podium facades typically characterized by large vertically spanning glass units with crowded though transient movement of persons in contrast to typical office conditions where occupants may be stationary for extended time periods. figure 15 represents the isodamage curves b/s (breaks safely), interlayer rupture and high hazard according to the new fragmentation tool for a 1220 × 1520 × 6.6.4an-glass. the figure shows that the impulsive region of the p-i curves background is the region where the ‘loss of design threat area’ is most significant. the reason is clear. in the impulsive region (constant impulse), acceleration conditions are more favourable for shard detachment from the interlayer with different levels of shard velocities possible, with the consequence of every possible outcome within different hazard levels. for quasi-static behaviour (constant pressure), the high hazard curve intersects the interlayer failure curve. this occurs because ‘no-spall’ high hazard is reached without significant acceleration conditions. in this case, shard detachments are unlikely. the chart highlights how the ‘threat area lost’ region corresponds to the most typical satchel bomb conditions according to iso16933. however in this specific case a more typical vehicle bomb threat such as iso-exv19 lies within the transition area. it can be easily understood that applying overly restrictive limitations on fragmentation within the proximity of the facade (as per gsa, assuming spall effects) results in expensive solutions. these solutions generally exhibit stiffer behaviour with less blast energy dissipation and subsequently higher risk of overall facade structural collapse due to excessive bracket reactions to the main structure. similarly, structural collapse risk is greater due to non-optimisation of facade blast energy dissipation capabilities. as shown in fig. 16, under the blast load iso-exv19, the single glass 6.6.4an would be inadequate as it results in high hazard levels. in order to maintain the glass performance within the hazard m. zobec et al. / innovative blast enhanced facade design tool 199 fig. 16. increase in reactions under non-optimized hazard assessment. region with a high degree of confidence, a 10.10.4an is preferable. in terms of reactions to the main structure through the facade connection bracket, this would result in an approximate 50% increase in reactions assuming rigid quasi-static behaviour of the bracket system. other benefits of the fragmentation model would be to create a new perception of probable hazard levels in order to improve design cost effectiveness without loss of safety. estimation of fragment size and velocity is also an important tool for injury assessment once the specific occupant working (or living) conditions within the vicinity of the facade are defined. given that blast effects on facades are not uniform and high pressure-impulse loads are generally localized, further significant risks of structural collapse can also be found by balancing enhanced levels of facade energy dissipation with acceptable injury probability. 7. conclusions in this paper, the features of the design optimization process assessed by a numerical tool developed by the permasteelisa group have been presented. experimental testing to determine the post-cracking behaviour, failure modes and limits of laminated glass shows that the pvb membrane provides significant levels of protection against breach of the blast wave and hence overpressure injuries. the limiting factor though for the design of blast-enhanced glass is the risk of injury due to glass fragmentation. current models are overly conservative, resulting in not only oversized glass but also higher reactions to the main structure with less possibility of effectively utilizing the energy dissipation potential of the facade. a new fragmentation modelling approach assuming griffith energy balance criteria shows excellent correlation with blast arena tests. a design balance between pvb membrane dissipation levels and acceptable glass fragmentation hazards is required for both sufficiently safe yet economical design of blast-enhanced glazing. references applied research associates. (2005). windows glazing analysis response and design (wingard). property of the united states government. astm e1300-12 (2102). standard practice for determining load resistance of glass in buildings, astm international. 200 m. zobec et al. / innovative blast enhanced facade design tool bennison, s. j., sloan, j. g., kristunas, d. f., buehler, p. j., amos, t., & smith, c. a. (2005). laminated glass for blast mitigation: role of interlayer properties, proc glass processing days: the 9th international glass conference, tampere, finland, pp. 17-20. biggs, j. m. (1964). introduction to structural dynamics. new york: mc graw-hill. beauchamp, e. k., & matalucci, r. v. (1998). dynamics of window glass fracture in explosions, sandia national laboratories report sand98-0598, albuquerque, nm. bouchbinder, e., mathiesen, j., procaccia, i. (2005). branching instabilities in rapid fracture: dynamics and geometry, physical review e 71(5), 056118. ellis, b. r. (1991). static testing of glazing for property services agency, report no. tcr28/91, bre structural performance division garston, watford. ellis, b. r., & beak, m. (1992). static testing of glazing phase ii, client report gio 451, bre structural performance division. grady, d. e. (2008). fragment size distributions from the dynamic fragmentation of brittle solids. international journal of impact engineering, 35, 1557-1562. griffith, a. (1921). the phenomena of rupture and flow in solids, philosophical transactions of the royal society of london series a, containing papers of a mathematical or physical character, 221, 163-198. haldimann, m. (2006). fracture strength of structural glass elements-analytical and numerical modelling, testing and design, these epfl no 3671, ercole polytechnique federale de lausanne (epfl). hinman, e. (2011). blast safety of the building envelope. in: whole building design guide, national institute of building sciences. hooper, p. a., blackman, b. r. k., & dear, j. p. (2012). the mechanical behaviour of polyvinyl butyral at different strain magnitudes and strain rates, journal of material science, 47, 3564-3576. iso 16933:2007 (2007). glass in building explosion-resistant security glazing test and classification for arena air-blast loading, 22. iwasaki, r., & sato, c. (2006). the influence of strain rate on the interfacial fracture toughness between pvb and laminated glass, journal de physique iv (proceedings), 134, 1153-1158. iwasaki, r., sato, c., lataillade, j. l., & viot, p. (2007). experimental study on the interface fracture toughness of pvb (polyvinyl butyral)/glass at high strain rates. international journal of crashworthiness, 12(3), 293-298. kuntsche, j., & schneider j. (2014). mechanical behaviour of polymer interlayers in explosion resistant glazing. challenging glass 4 & cost action tu0905 final conference, 447-454. lumantarna, r., nguyen, c., zobec, m., & ngo, t. (2012). pressure correction in water-bag testings to investigate post cracked behaviour of laminated glass, australasian conference on the mechanics of structures and materials (acmsm) 22, sydney, australia. marchand, k. a., conrath, e. j., stevens, d. j., & meyer, b. (2006). blast induced glass hazards: a comparison of design approaches and recent research. in: structures under shock and impact ix, susi ix, 259-268. morison, c. (1999). response of glazed facades to blast loading. msc dissertation, university of westminster, westminster, uk. morison, c. (2007). the resistance of laminated glass to blast pressure loading and the coefficients for single degree of freedom analysis of laminated glass, phd thesis, cranfield university, cranfield, uk. morison, c., zobec, m., & frenceschet, a. (2007). the measurement of pvb properties at high strain rates, and their application in the design of laminated glass under bomb blast, international symposium on the interaction of the effects of munitions with structures. neda, z., mocsy, a., & bako b. (1993). structures obtained by mechanical fragmentation of glass plates. material science and engineering, a169, l1-l4. newmark, n. m. (1956). an engineering approach to blast resistant design, american society of civil engineers transactions, 121, 45. paper no. 2786. norville, h. s., harvill, n., conrath, e. j., shariat, s., & mallonee, s. (1999). glass related injuries in the oklahoma city bombing, journal of performance of constructed facilities, asce, 13(2), 50-56. norville, h. s., & conrath, e. j. (2001). considerations for blast-resistant glazing design, journal of architectural engineering, 7(3), 80-86. viracon. (2004). laminated glass for blast mitigation. owatonna. wang, j. (1986). fracture and stress pattern correlations in glass plates, msc thesis, texas tech university. wang, j., & mcdonald, j. r. (1986). fracture and stress pattern correlations in window glass plates, submitted to eastern space and missile center (esmc/sem) patrick air force base florida 32925, glass research and testing laboratory, texas tech university, lubbock, tx. zobec, m., lori, g., & ngo, t. (2012). blast enhanced facades for tall buildings – a true balanced integrated design approach, proceedings of the council for tall buildings and urban habitat (ctbuh) 9th world congress; shanghai, prc (pp. 867-87). journal of facade design and engineering 2 (2014) 163–182 doi 10.3233/fde-150018 ios press 163 diagonally arranged louvers in integrated facade systems effects on the interior lighting environment yutaka misawaa,∗, shigeru hikonea, makoto nakamurab, shizuo iwamotoc and mamoru iwatac aove arup & partners japan limited, tokyo, japan bmhs planners cdepartment of architecture and building engineering, kanagawa university abstract. building facades play an important role in creating the urban landscape and they can be used effectively to reduce energy usage and environmental impacts, while also incorporating structural seismic-resistant elements in the building perimeter zone. to address these opportunities, the authors propose an integrated facade concept which satisfies architectural facade and environmental design requirements. in europe, remarkable facade engineering developments have taken place over the last two decades resulting in elegant facades and a reduction in environmental impact; however modifications are needed in japan to take account of the different seismic and environmental situations. to satisfy these requirements, this paper proposes the use of a diagonally disposed louver system. diagonally arranged louvers have the potential to provide both seismic resistance and environment adaptation. in many cases, louvers have been designed but not installed due to concerns relating to restricted external sight lines and low levels of natural lighting in the building interior. to overcome these problems, full-scale diagonally arranged louver mock-ups were created to evaluate illumination levels, the quality of the internal daylight environment and external appearance. interior illumination levels resulting from a series of mock-up experiments were evaluated and correlated with results from a daylight analysis tool. keywords: refurbishment, facade, diagonally arranged louver, daylight factor, full-scale mock-up experiment, daylight analysis 1. introduction in recent years, it has become increasingly important not only to replace buildings, but also to renovate and improve the performance of the existing building stock. especially in japan, where building lifespans are generally short, increasing the duration of the building’s continued use can reduce materials required for new construction, waste from demolition and emission of co2. in upgrading architectural stocks as social assets, it is necessary to improve building functionality whilst carrying out upgrades rather than continuing the existing trend towards ‘scrap and build’. approaches that improve building functionality when upgrading will be more important in future from the perspectives of reducing environmental impact and the continuing use of existing buildings. whilst being important elements that make up the urban landscape, facades provide a means to protect the building ∗corresponding author: yutaka misawa, ove arup and partners japan limited, 3f tobu fuji building 24-4 sakuragaoka-cho shibuya-ku, tokyo 150-0031, japan. tel.: +81 368616320; fax: +81 334761377; e-mail: yutaka.misawa@arup.com. issn 2214-302x/14/$35.00 © 2014 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:yutaka.misawa@arup.com 164 y. misawa et al. / diagonally arranged louvers in integrated facade systems interior from its surrounding environment, and they have a direct impact on the interior environment. hence, while changing the appearance of existing buildings, the appropriate reconstruction of facades in the refurbishment of existing buildings can be expected to offer improved building functionality including the reduction of environmental impacts. however, in japan, many existing buildings do not satisfy the latest structural standards and they must be structurally retrofitted in order to reduce damage that would be caused by a large earthquake. in fact, many buildings which did not have enough seismic strength collapsed in kobe and the great east japan earthquake. in many seismic retrofitting cases, reinforcing elements such as steel braces are installed on the outside of the building as it is appropriate to focus on the building’s exterior skin when installing earthquake resistance elements. however, these reinforcing elements are often not designed in relation to the facade, resulting in poor aesthetic form and a situation contrary to the improvement of environmental functionality and better building stock. as the facade is an important part of a building in shaping the urban landscape and influencing the energy consumption of the building, it is essential to optimize the integrated design of the components used for anti-seismic reinforcement on the facade. 2. purpose of the research based on these circumstances, the authors have proposed criteria for, and carried out analysis on, integrated facade systems that improve various characteristics (primarily appearance, safety and environment) when structural retrofitting is performed on existing buildings and structural elements are applied to actual buildings (kanaki et al., 2008; takeuchi et al., 2007, 2005, 2006a, b). also, the safety performance of structural retrofit systems using buckling restrained braces (brbs) installed as diagonally arranged structural elements with minimum dimensions was confirmed in a previous paper (kanaki et al., 2008). figure 1 shows brb configuration. brbs have two main components. the first is a cross-shaped steel plate and the other is a steel mortar plank that serves to keep the core steel plate from buckling. brbs are a structural element which manage both tension and compression forces. general brace elements used for structural retrofit are usually x or v shaped because they are designed only to take tension forces. a diagonally arranged bracing system is possible when brbs are installed. hence brbs have the potential to integrate with facade elements as a diagonally arranged louver, camouflaging brbs as a structural intervention. brbs do not need to be installed in all the diagonal louver elements. one or two brbs are deployed in one beam and one pillar span as required for structural performance. brbs are designed to connect to box-shaped beams, and the allowable storey drift is (floor height/250) when pillars are designed for a shear fracture modes. alternatively, allowable storey drift is (floor height/150) when pillars are designed for a bending fracture mode. as the reinforcing element, brbs require minimal materials; just core steel plates and steel mortar planks; therefore, they can readily harmonize with facade components. for this reason, it is possible for brbs to become an environmental feature such as an external louver etc. in addition to being a structural element. this research supports brbs as a multipurpose environmental device/structural element and proposes a diagonally arranged louver that integrates structural and environmental functionality. also, the verification of interior illumination through measurements using a full-scale diagonally arranged louver mock-up was performed to determine the optimal design of diagonally arranged louvers. y. misawa et al. / diagonally arranged louvers in integrated facade systems 165 fig. 1. configuration of bucking restrained brace. 3. background facade engineering which takes environment and energy efficiency into account, has progressed in recent years, especially in europe (kwok & grondzik, 2007; thomas et al., 2004). when applying such concepts to japan where the climate is significantly different, modifications and improvements are necessary (architectural institute of japan, 2002; architectural pictorial special edition, 2003; architectural institute of japan, 2004). additionally, the earthquake-proofing of existing buildings is an urgent issue in earthquake-prone japan, and reinforcement through the deployment of braces on the outer surface of the building facade has become typical. figure 2 shows two types of typical structural retrofitting. braces are naturally efficient as structural elements, though installing a structural element on the building’s outer surface does not necessarily improve its aesthetic appearance. on the other hand, there are many examples of the use of external louvers as an environmental installation in facades that both increase environmental functionality and improve visual appeal. figure 3 shows examples of louver facades. the primary objective of louvers in reducing environmental impact is the improvement of sun shading. figure 4 shows the shading mechanism of sustainable facades. by reflecting sunlight via the external louvers, heat penetration into the interior is reduced. louvers installed on the outer surface of facades will outperform systems that exhaust heat via an intermediate buffer space (cavity) as in the recently proposed double skin facades, as they prevent heat penetration at the outside of the building (michael, 2005; oesterle & lutz, 2001). the shape and pitch of louvers – horizontal or vertical angles – are adjusted during design for factors such as direction and solar altitude in order to obtain the required heat shading effect. in general, horizontal louvers are installed on southern elevations and vertical louvers are installed on eastern/western elevations in japan (architectural and design institute of japan, 1963; architectural institute of japan, 1963; urano, & nakamura, 1996). figure 5 shows facade orientation and louver types effective in sunlight control. examples of diagonally arranged louvers proposed in this research are not shown here, however they create a heat shading effect in the south-eastern and south-western elevations for moderate solar altitude, and the heat shading effect can be expected throughout a significant period of the day. on the other hand, there are many concerns in the actual application of louvers due to problems of view obstruction and reduced interior illumination. louvers are a type of system that excels in heat reduction through shading, however there are many cases where they are not used. systems that introduce daylight into an interior such as daylight shelves are proposed for interior lighting. 166 y. misawa et al. / diagonally arranged louvers in integrated facade systems fig. 2. appearance of general seismic retrofits. fig. 3. facades with consideration for environment impact. left: lloyds resister of shipping, london, uk, richard rogers partnership, 2000. center: telefonica, madrid, spain, rafael de la hoz architect, 2005. right: ropemaker place, london, uk, arup associates, 2008. figure 6 shows examples of daylight shelves. although they serve to improve the interior lighting environment, these systems are only environmental devices. cases where louvers are installed in an entire window for the purpose of heat reduction integrated with an interior lighting strategy have not been well studied (architectural and design institute of japan, 1975; architectural and design institute of japan, 1985). for this study, a diagonally arranged louver is proposed and the view from the inside and the interior lighting environment has been evaluated through full-scale mock-up experiment and analysis. 4. methodology full-scale mock-ups of diagonally arranged louvers were created to evaluate illuminance levels and the quality of the internal daylight environment. two types of diagonally arranged louvers (r type and c type) were set up with different shapes (rectangular and square) and louver pitches to evaluate daylight scattering by the louvers. external appearance of the full-scale mock-ups, and outside views from the interior were also checked to judge if louver facades are a possible solution for seismic and environmental retrofit. full-scale mock-ups were painted with finishes to evaluate y. misawa et al. / diagonally arranged louvers in integrated facade systems 167 fig. 4. shading mechanism of sustainable facades. fig. 5. facade orientation and louver types effective in sunlight control. daylight scattering by the louvers and interior finishes. direct daylight factor and daylight factor were measured by illuminometer and fish-eye lens. indirect daylight factor calculated from the measured values was also used to evaluate daylight reflections by the louvers. interior illuminance levels resulting from a series of full-scale mock-up experiments were evaluated and correlated with results from a daylight analysis tool. analytical models were created based on experimental conditions such as louvers pitches, louver shapes and louver/interior finishes. the analysis tool ‘radiance’ was used for the evaluation of interior illuminance levels and these values were compared with experiment values. 168 y. misawa et al. / diagonally arranged louvers in integrated facade systems fig. 6. examples of environmental devices (daylight shelf). 4.1. outline of full-scale experiment diagonally arranged louvers applied to kanagawa university building no. 8 (north latitude 35.29, east longitude 139.37) were considered, assuming refurbishment of the existing building. figure 7 shows applied diagonally arranged louver facades. two different shapes of diagonally arranged louvers (r type and c type) were designed. in deciding louver dimension, the incorporation of a brb (minimum cross section dimensions 100mm × 100mm) was taken into account. r type has a rectangular cross section with the smaller side facing the outside. c type has a square cross section with a corner edge line facing the outside. table 1 shows the appearance of the full-scale mock-ups of r type and c type for a single floor, single pillar span of the building. the authors considered the sun-shading effects of these two louver types and coordinated opening shading rate. two types of louver pitches were arranged to maintain a similar shading ratio. the coverage ratio was 22.68 percent for r type and c type was 28.76 percent. brbs are not deployed in all the louvers, and are only installed where structurally necessary. in this research, illumination measurements were performed using these two types of full-scale mock-ups. 4.2. overview of full-scale mock-up figure 8 shows the configuration of the full-scale mock-up. primary brackets, secondary brackets and edge frames are used to join the louvers and braces to the main frame, which assumes use on an existing building. the louvers and the braces are attached to the edge frames by the primary brackets, with a cover plate attached to the joints. a white paint finish is applied to the mock-up, and the inner surface of the main frame has a matt black finish. in order to evaluate the daylight factor, the mock-up was installed, north-facing, on the rooftop of kanagawa university building no. 12, which has few obstructions to the sky. a north-facing mock-up is the best orientation to obtain reliable experimental values as the luminance distribution of the sky y. misawa et al. / diagonally arranged louvers in integrated facade systems 169 fig. 7. facade with diagonally arranged louvers. depends on weather and climate, and the former condition changes during the course of a day with the position of the sun. figure 9 shows the installation environment and interior aspect of the mock-up. figure 10 shows the full-scale mock-up elevations. the full-scale mock-up assumes a building interior perimeter zone, and has dimensions of: width 4400mm × depth 5400mm × height 3450mm. 170 y. misawa et al. / diagonally arranged louvers in integrated facade systems table 1 overview of the diagonally arranged louvers 4.3. experiment conditions table 2 shows the experimental conditions. in the experiment, the interior illuminance and unobstructed sky illuminance were measured (architectural and design institute of japan, 1977; architectural institute of japan, 1993; ogiso, & koike, 1978). the unobstructed sky illuminance was measured simultaneously with the interior illuminance on the rooftop of kanagawa university building no. 23, adjacent to the mock-up installation location and the highest building in the university. figure 11 shows the unobstructed sky illuminance and interior illuminance measurement environments. the illuminometer was placed in the centre of the pedestal of fig. 11, shaded from direct sunlight for the illuminance measurements. overcast skies are a similar condition to diffuse sky conditions; however, this condition was limited during the experiments. the authors considered direct radiation as the most relevant factor of external illuminance measurements because external daylight conditions are constantly changing. therefore, in the physical model, the authors gave special y. misawa et al. / diagonally arranged louvers in integrated facade systems 171 table 2 experiment conditions item illumination measurement condition weather clear or cloudy measurement dates no louvers: 18th 19th march, 2008 r type: 29th march, 1st april, 2008 c type: 22nd 27th march, 2008 measurement location 35.29◦n, 139.37◦e unobstructed sky illuminance: kanagawa university building no. 23 rooftop interior illuminance: kanagawa university building no. 12 rooftop measuring instruments digital illuminometer: topcon im-2d lens: im-2d scaling factor filter (10m) measurement interval every 1 hour from 9:00 to 15:00 measurement room: item properties shape/dimensions width 4400mm × depth 5500mm × height 3450mm opening direction north opening dimensions width 3900mm × height 1500mm retaining wall height 900mm, spandrel wall height 1000mm ceiling finish white wall paper: munsell color n9.3 wall finish white wall paper: munsell color n9.3 floor finish concrete: munsell color n5.5 opening finish no glass or fittings inserted louver: item properties shape/dimensions r type: (left figure of fig. 6) c type: (right figure of fig. 6) attachment pitch r type: @562.5mm (span/8) c type: @1125mm (span/4) opening shading rate r type: 22.68% c type: 28.76% louver finish white coating: munsell color n9.3 louver support frame rectangular cross-section 100mm × 200mm main frame finish white coating: munsell color n9.3 consideration to avoid direct radiation by preparing an instrument for shading direct-sunlight to stabilize conditions. the external measurement method was the best solution to provide stable lighting conditions and one-hour interval measurements supplemented experiment data. the instrument for shading direct-sunlight is a 100mm diameter iron disk, attached to a 10mm diameter steel bar and covered with black tape like an umbrella. the disk and photo receiver are separated and supported so that a shadow falls upon the photo receiver. the instrument was not used in cloudy weather when a shadow cannot be cast. 4.4. measurement plan two daylight factor measurements were taken. one, the result of measuring illuminance by illuminometer, and the other calculated using a solid angle projection factor from photographs taken via a fisheye lens. figure 12 shows the interior illuminance measurement points. the interior illuminance 172 y. misawa et al. / diagonally arranged louvers in integrated facade systems fig. 8. configuration of the full-scale mock-up. was measured for each condition at 1-hour intervals from 9 am to 3 pm over a 2-day period. also, single measurements were made for each case to relating the solid angle projection factor of the measurement point to the window opening. the average value for the 2-day period was calculated using the hourly daylight factor derived from measured values. in the fish-eye lens photography, a nikon fisheye converter fc-e8 was installed on a nikon coolpix4500 digital camera and photographs were recorded at the points where illuminance measurements were taken. the fisheye lens was adjusted to give a measurement height of 1025mm, photographs were taken, and the solid angle projection factor (sky component) was found for the resulting image using spconv (nagata, 2005). spconv is conversion software, which evolves sky component from digital photo data taken by a fisheye lens. 4.5. experiment results tables 3 and 4 show the results of illuminance measurements and the daylight factor calculated from them. also, fig. 13 shows three graphs plotting the daylight factor at each of the measurement points, with table 5 giving the results of the experiment. daylight factor is the ratio of interior illuminance relative to skylight. when louvers are installed on existing buildings, it is generally assumed that the interior will become darker, due to shading by the louvers; however, on inspecting the measurement results it is evident that the interior brightness is not always reduced by louvers. even if there are louvers, there are cases where the interior is bright and the lighting of the perimeter area is improved. at a location 4500mm from the window, the results show an almost identical illumination level to that with no louvers. also, where louvers y. misawa et al. / diagonally arranged louvers in integrated facade systems 173 fig. 9. surrounding environment and mock-up (view from interior side to outside). are installed, the difference in daylight factor at 900mm and 4500mm from the window is small compared with the no louver case. the illuminance differences in the perimeter area are smaller when using louvers. though illuminance differences are negligible, human comfort factors such as glare reduction are improved (illumination society, 1992; illumination society, 1994; japan industrial standards, n.d.). 174 y. misawa et al. / diagonally arranged louvers in integrated facade systems fig. 10. mock-up elevations (unit; mm). fig. 11. surrounding condition of measurement locations. the direct daylight factor measured with the fisheye lens shows sky component obtained directly from the window opening. this is an index that is not influenced even if the measurement point brightness varies due to light scattering and diffusion from the surrounding environment. the relationship between the direct daylight factor and measured illuminance is shown in fig. 13 as the indirect daylight factor. the indirect daylight factor is a ratio, which measures the influence of the interior walls and daylight scattered by louvers. the indirect daylight factor is larger when louvers are installed compared to no louvers at the point of 2700mm and 4500mm from the window. this is because the direct daylight is reduced and the far back of the room is brightened by daylight scattered and diffused by the louvers. both the r type and c type have this tendency. the capture, scattering and diffusion of skylight by the louvers can also be seen from interior aspects of r type and c type y. misawa et al. / diagonally arranged louvers in integrated facade systems 175 fig. 12. interior illuminance measurement points. in fig. 9. also, as the openings in the c type louver are wide, a free view is obtained of the outside scene (illumination society, 1987). 5. daylight analysis 5.1. analysis conditions ‘radiance’ analysis software is used for interior daylight analysis. radiance is an illumination analysis and visualization tool devised by the lawrence berkeley laboratory in california. table 6 shows the analysis conditions and fig. 14 the analysis models. the analysis reflects full-scale mock-up experimental conditions. in radiance, analytic daylight beams are traced randomly from the measurement points and analysis is carried out using the monte carlo method based on daylight rays that reach the source of daylight, the sun. backward ray-tracing is an evaluation method in which a ray is tracked back from the eye position until it intersects with a surface. in basic ray-tracing, a ray is traced from the intersection point with a surface back to the source. if the surface is specular or transparent the ray follows the geometry of that reflection or transmission to the next surface before it is traced to the daylight sources. if the ray is not obscured by another surface or objects, the light reaching the intersection point from the sources is calculated. the ray-tracing process is repeated until the number of rays specified is reached. if a greater number of analytic daylight beams are configured, more accurate results can be obtained. in this research, analysis time can be reduced because the interior space is a rectangular shape, hence the number of daylight beams analyzed is set to a maximum value of 4096 rays. also, the reflection number of analytic daylight beams is set a maximum number of 8 reflections (cibse, 1999). 176 y. misawa et al. / diagonally arranged louvers in integrated facade systems ta bl e 3 m ea su re m en t re su lt s (i llu m in an ce lx , d ay lig ht fa ct or % ) n o lo uv er s 1s t da y n o lo uv er s 2n d da y 90 0 m m 27 00 m m 45 00 m m a ll sk y d ay lig ht fa ct or 90 0 m m 27 00 m m 45 00 m m a ll sk y d ay lig ht fa ct or 9: 00 25 60 14 72 95 7 16 47 0 15 .5 8. 9 5. 8 9: 00 44 60 14 91 81 6 27 40 0 16 .3 5. 4 3. 0 10 :0 0 30 90 15 70 10 69 25 20 0 12 .3 6. 2 4. 2 10 :0 0 45 00 18 11 10 57 37 10 0 12 .1 4. 9 2. 8 11 :0 0 32 50 16 33 11 41 32 50 0 10 .0 5. 0 3. 5 11 :0 0 21 90 81 1 48 4 90 20 24 .3 9. 0 5. 4 12 :0 0 32 40 16 35 11 55 34 40 0 9. 4 4. 8 3. 4 12 :0 0 83 9 29 5 16 4 52 80 15 .9 5. 6 3. 1 13 :0 0 33 00 17 04 12 03 32 90 0 10 .0 5. 2 3. 7 13 :0 0 16 20 60 2 33 8 10 77 0 15 .0 5. 6 3. 1 14 :0 0 33 00 16 66 10 95 30 80 0 10 .7 5. 4 3. 6 14 :0 0 55 1 20 8 12 1 41 40 13 .3 5. 0 2. 9 15 :0 0 31 70 15 00 99 7 25 40 0 12 .5 5. 9 3. 9 15 :0 0 99 2 33 9 18 1 53 90 18 .4 6. 3 3. 4 a ve ra ge 31 30 15 97 10 88 28 23 9 11 .1 5. 7 3. 9 a ve ra ge 21 65 79 4 45 2 14 15 7 15 .3 5. 6 3. 2 r ty pe 1s t da y r ty pe 2n d da y 90 0 m m 27 00 m m 45 00 m m a ll sk y d ay lig ht fa ct or 90 0 m m 27 00 m m 45 00 m m a ll sk y d ay lig ht fa ct or 9: 00 22 75 11 33 77 6 20 20 0 11 .3 5. 6 3. 8 9: 00 16 03 88 4 65 3 11 21 0 14 .3 7. 9 5. 8 10 :0 0 24 60 12 68 87 1 24 60 0 10 .0 5. 2 3. 5 10 :0 0 16 58 92 8 68 2 12 77 0 13 .0 7. 3 5. 3 11 :0 0 25 70 13 56 98 0 25 30 0 10 .2 5. 4 3. 9 11 :0 0 16 32 92 8 70 4 11 97 0 13 .6 7. 8 5. 9 12 :0 0 33 40 15 57 10 77 40 80 0 8. 2 3. 8 2. 6 12 :0 0 16 39 91 4 70 6 12 90 0 12 .7 7. 1 5. 5 13 :0 0 31 30 14 98 10 10 39 10 0 8. 0 3. 8 2. 6 13 :0 0 17 30 10 42 69 5 12 30 0 14 .1 8. 5 5. 7 14 :0 0 25 90 12 25 88 8 35 70 0 7. 3 3. 4 2. 5 14 :0 0 24 40 93 9 75 6 20 20 0 12 .1 4. 6 3. 7 15 :0 0 31 50 14 77 93 9 26 80 0 11 .8 5. 5 3. 5 15 :0 0 21 50 10 59 77 7 18 90 0 11 .4 5. 6 4. 1 a ve ra ge 27 88 13 59 93 4 30 35 7 9. 2 4. 5 3. 1 a ve ra ge 18 36 95 6 71 0 14 32 1 12 .8 6. 7 5. 0 c ty pe 1s t da y c ty pe 2n d da y 90 0 m m 27 00 m m 45 00 m m a ll sk y d ay lig ht fa ct or 90 0 m m 27 00 m m 45 00 m m a ll sk y d ay lig ht fa ct or 9: 00 19 86 10 93 77 2 15 40 0 12 .9 7. 1 5. 0 9: 00 36 10 13 54 83 7 35 80 0 10 .1 3. 8 2. 3 10 :0 0 19 75 11 18 78 5 17 70 0 11 .2 6. 3 4. 4 10 :0 0 44 50 16 88 10 15 48 40 0 9. 2 3. 5 2. 1 11 :0 0 19 84 11 49 83 3 18 60 0 10 .7 6. 2 4. 5 11 :0 0 39 50 14 26 88 6 46 80 0 8. 4 3. 0 1. 9 12 :0 0 20 30 11 47 83 2 19 70 0 10 .3 5. 8 4. 2 12 :0 0 19 81 11 46 84 7 16 52 0 12 .0 6. 9 5. 1 13 :0 0 21 80 12 27 87 9 20 70 0 10 .5 5. 9 4. 2 13 :0 0 22 20 12 31 90 2 17 14 0 13 .0 7. 2 5. 3 14 :0 0 22 10 12 11 86 0 19 20 0 11 .5 6. 3 4. 5 14 :0 0 34 90 15 61 99 8 26 00 0 13 .4 6. 0 3. 8 15 :0 0 23 40 12 16 83 1 17 53 0 13 .3 6. 9 4. 7 15 :0 0 28 90 12 55 78 0 13 28 0 21 .8 9. 5 5. 9 a ve ra ge 21 01 11 66 82 7 18 40 4 11 .4 6. 3 4. 5 a ve ra ge 32 27 13 80 89 5 29 13 4 11 .1 4. 7 3. 1 y. misawa et al. / diagonally arranged louvers in integrated facade systems 177 ta bl e 4 a ve ra ge an d m ax im um /m in im um va lu es of da yl ig ht fa ct or (% ) n o lo uv er s r ty pe c ty pe 90 0 m m 27 00 m m 45 00 m m 90 0 m m 27 00 m m 45 00 m m 90 0 m m 27 00 m m 45 00 m m 2 da y av er ag e 2 da y av er ag e 2 da y av er ag e 13 .2 5. 6 3. 5 11 .0 5. 6 4 11 .2 5. 5 3. 8 2 da y m ax im um va lu e 2 da y m ax im um va lu e 2 da y m ax im um va lu e 24 .3 9. 0 5. 8 14 .3 8. 5 5. 9 21 .8 9. 5 5. 9 2 da y m in im um va lu e 2 da y m in im um va lu e 2 da y m in im um va lu e 9. 4 4. 8 2. 8 7. 3 3. 4 2. 5 8. 4 3. 0 1. 9 178 y. misawa et al. / diagonally arranged louvers in integrated facade systems fig. 13. measurement results (daylight factor %). table 5 summary of experiment results measurement point average daylight factor variance between average daylight factor and direct daylight factor 900mm no louver case is the highest, r type is the lowest average daylight factor is equal to the direct daylight factor for no louvers and c type. r type is largest since it has a lot of louvers 2700mm no great variation in any cases average daylight factor exceeds the direct daylight factor in all cases 4500mm lover cases are higher than no louver average daylight factor exceeds direct daylight factor in all cases and the variation is large when louvers are installed table 6 analysis conditions sky conditions cie standard ovarcast sky, unobstructed sky illuminance 15,000lx surrounding area no obstructions in the surrounding area number of analytic daylight beams 4096 (maximum value) number of daylight reflections 8 (maximum value) room shape/dimensions width 4700mm × depth 5500mm × height 3475mm opening position/dimensions north elevation /width 3900mm × height 2200mm ceiling, walls, louvers, main frame finish off-white paint; r 0.691g 0.696 b 0.615 (reflectance 68.8%) floor finish concrete; r 0.244g 0.194 b 0.145 (reflectance 20.3%) opening no glass or fittings installed y. misawa et al. / diagonally arranged louvers in integrated facade systems 179 fig. 14. interior illuminescence analysis model. 5.2. comparison of analysis and experiment values figure 15 shows a comparison of daylight factors obtained from experiment and analysis. although the analytical values show a lower illuminance level than the experimental values, a good correlation between the analysis and experiment can be seen in all cases. the difference between the computational simulation model and experiments was caused by varying measurement conditions in the experiments. simulation results were stable but experimental conditions were not as stable as computer simulation models, because weather conditions were constantly changing. additionally, in the experiments external measurement conditions were more unstable than internal conditions because the percentage of diffused light is greater if the measurement positions are in the back of the internal space. diffused light, such as reflection from surrounding walls are stable. because of the changing weather conditions, the daylight factor using external and the backspace illuminance was slightly different compared with computational results. while not many diagonally arranged louver facades have been installed in existing buildings, it can be stated that reliable outcomes can be obtained at the design stage by performing analytical studies using the radiance software. figure 16 shows the interior illuminance distribution for no louvers and for diagonally arranged louver facades. when a diagonally arranged louver is deployed, a small bias in illuminance distribution can be seen over that with no louvers. however the illuminance distribution 180 y. misawa et al. / diagonally arranged louvers in integrated facade systems fig. 15. comparison of experiment and analytical values (daylight factor %). fig. 16. interior illuminance distribution (analytical value, unit: lx). (vertical axis indicates the distance from window; horizontal axis indicates distance from the central axis of the window). of c type louvers shows little bias in illuminance distribution – illuminance bias can be mitigated through the adjustment of louver shapes and attachment pitches. 6. conclusion in order to understand the interior daylight environment of a facade with diagonally arranged louvers, a series of daylight illuminance experiments and analyses were carried out by means of a fullscale mock-up plus ‘radiance’ daylighting simulation software. as a result, the following conclusions are drawn: (1) through the installation of a diagonally arranged louver, the interior illuminance variation in the vicinity of the window and interior of the room is mitigated. this reduces human discomfort y. misawa et al. / diagonally arranged louvers in integrated facade systems 181 caused by glare. illuminance values are even and the daylight environment is improved in the perimeter zone when diagonally arranged louvers are installed. (2) when a diagonally arranged louver is installed, daylight is scattered by the louvers and dispersed far back into the room, away from the window. hence by installing a diagonally arranged louver, a brightness equivalent to the case where no louvers are installed can be obtained at the far back of the room. (3) there is a bias in the interior illuminance distribution due to the diagonally arranged louver; however, this bias can be adequately mitigated through the adjustment of louver shapes and pitches. (4) reliable correlation can be obtained between the illuminance measured in full-scale mock-ups and analytical values. ‘radiance’ provides a practical interior daylight design tool for diagonally arranged louvers. through this research, we conclude that the proposed diagonally arranged louver has the potential to improve lighting environments in the building perimeter zone. the performance is better than nolouvers and installation of diagonally arranged louvers for new and existing buildings is an effective solution for building upgrades. this solution can be applied in locations similar to japan where building lifespans are short and is also less wasteful than scrap and build. moreover, vertical and horizontal louvers do not have the same seismic resistance potential as diagonally arranged louvers. the proposed diagonally arranged louvers are multi-functional in environmental, seismic and aesthetic functions. they can play an important role in a future society concerned with sustainability. acknowledgments this research was carried out using a japanese ministry of land, infrastructure, transport and tourism grant aid for leading technological development for housing and construction; subsidy title: ‘development of regeneration technologies for existing stock by means of an “integrated façade” that simultaneously attempts performance improvement in design, structure and the environment’. we would like to express our thanks to maeda chikanori of maeda first certified architectural office for the guidance received in the design and manufacture of the mock-ups for this research. hirai reiko of arup cooperated in the daylight simulation and tadaki hiroyuki, (then) graduate student of kanagawa university assisted in the manufacture of the mock-ups and the execution of the experiments. we would like to express our thanks for their help. references kwok, a. g., grondzik, w. t. (2007). the greenstudio handbook, environmental strategies for schematic design. enquiry: a journal for architectural research, 4(2). architectural and design institute of japan. (1963). lighting design 16. shokokusha. architectural and design institute of japan. (1975). pamphlet 23, illumination design, shokokusha. architectural and design institute of japan. (1977). pamphlet 24, measurement and study of sunlight, shokokusha. architectural and design institute of japan. (1985). pamphlet 30, daylight illumination planning, shokokusha. architectural institute of japan. (1963). lighting design, gihodo shuppan. architectural institute of japan. (1993). calculation methods for daylight illumination. architectural institute of japan. (2002). recommendations for environmental building. architectural institute of japan. (2004). architecture of glass, gakugei shuppansha. 182 y. misawa et al. / diagonally arranged louvers in integrated facade systems architectural pictorial special edition. (2003). façade engineering, 39. cibse. (1999). lighting guide lg10. daylighting and window design. illumination society. (1987). lighting handbook, ohmsha. illumination society. (1992). technical standards office illumination standards jiec-001. illumination society. (1994). technical standards residential illumination standards jiec-005. japan industrial standards (n. d.). illumination standards, jis z 9110-1979. kanaki, y., takeuchi, t., miyazaki, k., iwata, m. (2008). studies on integrated façade engineering. structural performance of integrated façades, journal of technology and design, 27, 137-142. kanaki, y., hikone, s., yamashita, t., iwata, m. (2008). seismic strengthening by the buckling restrained braces. journal of architecture and building science, 634, 2215-2222. michael, j. c. (2005). curtain walls. birkhäuser. nagata, a. (2005). building simulation resources library. application and database. spconv. oesterle, l., lutz, h. (2001). double-skin façades. munich, germany: prestel. ogiso, s., koike, t. (1978). concerning all factors with influence on the actual measurement of daylight. collection of lecture outlines from the architectural institute of japan mass meeting. takeuchi, t., iwata, m., yasuda, k. (2007). concrete engineering 500th commemorative edition, the environment and concrete, l45(5). takeuchi, t., koyano, k., iwata, m. (2005). studies on integrated façade engineering. analyses on existing façades, journal of environmental engineering, 592, 97-104. takeuchi, t., koyano, k., yasuda, k., yuasa, k., iwata m. (2006a). studies on integrated façade engineering. proposal for integrated façade and its evaluation, journal of environmental engineering, 601, 81-88. takeuchi, t., yasuda, k., yuasa, k., okayama, s., miyazaki, k., iwata m. (2006b). seismic retrofit of aged building with integrated façade, journal of architecture and building science, 24, 161-166. thomas, h., roland k., werner l. (2004). façade construction manual, birkhäuser. urano, y., nakamura, y. (1996). architectural and environmental engineering, morikita publishing co., ltd. journal of facade design and engineering 3 (2015) 105–127 doi 10.3233/fde-150038 ios press 105 assessing the performance of an advanced integrated facade by means of simulation: the actress facade case study fabio favoino∗ glass and façade technology research group, engineering department, university of cambridge, uk abstract. the growing demand for both building energy efficiency and indoor environmental comfort is leading to a substantial evolution of the traditional concept of the building envelope. the future building skin is required to be responsive and dynamic, actively regulating the flows of heat, light, air and water from outdoor to indoor and vice versa, in order to effectively respond to ever-changing climatic conditions, occupant comfort and energy efficiency requirements. in the framework of a decade-long research activity on advanced integrated facade, aif, a new multifunctional facade module called actress has been conceived: the active, responsive and solar envelope is designed to play different roles through its ability to change its thermo-physical behaviour in order to suit the different environmental conditions. this paper briefly illustrates the actress mfm concept and its functional strategies, focusing on the simulation and the assessment of the performance of such a dynamic envelope. the numerical study was conducted in order to evaluate the potential energy savings achievable with such a facade and to evaluate different functional strategies and options. the evaluation of the performance in terms of energy savings was done at both component and whole-building level. moreover this work presents an example of the applicability of building performance simulation tools to the design of an innovative and dynamic facade system, discussing the capability of bps software in simulating and evaluating the performance of such systems. the results show that the actress mfm can effectively reduce the total primary energy consumption of an office building up to 55% compared with a reference facade complying with national regulations. on the other hand modelling assumptions and simplifications are needed in order to evaluate the performance of such a system with bps software, representing a barrier to the design and the adoption of advanced facade systems in the building industry. keywords: double-skin facade, advanced integrated facade, adaptive building envelope, building performance simulation 1. introduction “an environmental diode, a progressive thermal and spectral switching device, a dynamic interactive multi-capability processor acting as a building skin [. . .], trading energy surplus for energy need [. . .], adapting itself to provide best possible interior conditions [. . .].” (davies, 1981) this is what mike davies, roger stirk harbour partner architect, envisioned, more than forty years ago, on the basis of the latest technological developments in the field of material engineering. at that time, currently well-developed technologies using photovoltaic effect, photo, thermo and electrochromic effect were just at their outset, but already inspiring architects and the building industry. although davies’ vision of ‘polyvalent wall’ could seem now much closer, as much more technological ∗corresponding author: fabio favoino, glass and façade technology research group, engineering department, university of cambridge, uk. tel.: +44 01223 766681; fax: +44 01223 332662; e-mail: ff279@cam.ac.uk. issn 2214-302x/15/$35.00 © 2015 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license. mailto:ff279@cam.ac.uk 106 f. favoino / assessing the performance of an advanced integrated façade by means of simulation development and research has been carried out since then, how far are contemporary building envelopes from the environmental diode concept? and how far have design tools been developed in order to help designers at delivering and evaluating the performance of these concepts? nowadays the stricter environmental regulation for building energy efficiency brings designers back to a situation similar to the one that generated such a vision. in fact the idea of the environmental diode was partly generated by the designers’ attempt to answer with building envelope designs that were more conscious of the increased cost of energy caused by the 70’s oil crisis. the relevant interests generated by the facade is due to the key role that the building envelope plays in controlling the energy and mass flows from outdoor to indoor (and vice versa) and the possibility offered by the facade for solar energy exploitation. in fact there was a growing awareness that indoor environmental comfort could not be achieved, in an energy efficient way, demanding the whole need for indoor climate control to heating, ventilation and air conditioning (hvac) systems, but that an energy conscious design of the building envelope was needed, in order to decrease the energy use of the hvac systems, providing at the same time a comfortable indoor environment, in contrast with the uncomfortable indoor conditions and high energy consumptions created by modernist fully glazed building skins. one of the first steps towards davies concept was the introduction of double skin facade (dsf), which became very popular in the nineties (oesterle, lieb, lutz & heusler, 2001). the superposition of a second external transparent skin, to the first one, materialized an air cavity, which could be exploited in different ways (i.e. thermal and sound insulation, ventilation, thermal buffering and sun shading purposes) (compagno, 2002; saelens, 2002; knaack, klein, bilow & auer, 2014). according to how the airflow and/or the shading devices are operated and managed, the facade can dynamically adapt to changing outdoor environmental conditions and occupant requirements, and be optimized for heating, cooling and/or lighting purposes (poizaris, 2006). moreover the introduction of dsfs allowed the designer to rethink the facade as part of the hvac system itself (stec & van passen, 2000) in order to reduce the energy consumption of the building. nowadays the epbd recast (epbd 31/2010) requires all new construction to be nearly zero energy building (nzeb) by 2020 (2018 for public buildings). in order to achieve the nzeb objective, new concepts and technologies that can further improve the energy efficiency in buildings need to be developed and adopted by the building industry. iea–ecbcs annex 44 (2009) on responsive building elements (rbe) demonstrated that the limitations given by the existing technologies could be overcome by revisiting the concept of the building as a ‘static’ system. in fact, especially as far as temperate climates are concerned, the requirements posed by the nzeb target cannot be simply fulfilled by means of an increased thermal insulation and air tightness of the building envelope. for that a ‘responsive’ and ‘dynamic’ behaviour is required, with the aim to continuously change the interaction between the building elements (e.g. walls, roof, windows etc.) and the outdoor (and indoor) environment, on different time scales (i.e. seasonal, diurnal, hourly and instantaneous). among the rbe concepts, identified by the iea-ecbcs annex 44, advanced integrated facades (aifs) are probably one of the most promising technologies (quesada, rousse, dutil, badache & hallé, 2012a, b). this picture is supported by the fact that important players in the field of the facade have started to develop advanced integrated modular facade systems (multifunctional facade modules – mfms). these are almost ‘self-sufficient’ building skins that present a dynamic behaviour, incorporating different technologies (e.g. ventilation systems, decentralized heating/cooling units, heat exchangers, energy supply devices, energy storage, lighting equipment, shading devices, ventilated cavities) and interacting with the other building services in order to reduce the energy consumption and maximize the indoor comfort conditions (favoino, 2010). f. favoino / assessing the performance of an advanced integrated façade by means of simulation 107 another step towards the adaptive building envelope concept was done, thanks to new developments in material science applied to the building envelope, by introducing in the building skin new materials and devices with adaptive and responsive features, as well as energy producing materials and devices, like photovoltaics. these new materials are often referred to as ‘smart’ or ‘intelligent’. ritter (2007) defined as ‘smart’ those materials that show reversibly changeable physical properties in response to some physical (thermal, optical, mechanical) or chemical influence. among these are electro, thermo and photo-chromic, thermoelectric and thermostrictive materials, shape memory alloys (sma) and phase change materials (pcm). while aschehoug, andresen, kleiven and wyckmans (2005) refer to ‘intelligent’ to describe any material or device that has the ability to adjust to changing of boundary conditions due to a subsystem that is able to sense and control the device. in fact the adaptive or responsive feature itself does not assure effective operations, consequently increasing energy efficiency and occupant comfort. in fact to reach an effective performance, different components of an advanced facade need to adjust to changing environmental conditions synergistically, cooperating together and with other building services. this is a non-trivial task, as conflicts and trade-offs need to be addressed in order to provide energy efficiency and a comfortable environment at the same time. different examples of climate adaptive building skins using smart materials and/or intelligent devices could be found in loonen, trcka, cóstola and hensen (2013) and in favoino (2010). even if it is intuitively understandable that a higher performance is achievable by using a building skin which is able to adapt to changing environmental conditions and occupants’ comfort requirements, designing and operating such a complex system is a challenging task. moreover the quantification of the performance of the different components, and of the facade as a whole, is required on a real time basis when operating it. these issues imply two sets of problems. the first one concerns the definition of the performance of dynamic components, as ‘static’ features like uvalues and g-values are no longer meaningful for this kind of building envelopes. the second one concerns the correct simulation of the performance of such systems in order to effectively design and operate them. using building performance simulation (bps) enables to simulate all the possible configurations of the large number of components included in the facade, given a set of boundary conditions (environment and occupants) across the boundaries of multiple physical domains (thermal, luminous, acoustic etc.). by doing so, the virtual model of the building created within the software can be used to predict building performance for different purposes (i.e. research, design, diagnosis, operation), provided that the tool is used in the right way. while the first set of problems has already been explored for some aif designs (i.e. double skin facade by dimaio & van paassen (2001) and corgnati, perino & serra, 2007), the application of bps in relation to adaptive building envelope is still largely unexplored (loonen, singaravel, trcka, cóstola & hensen, 2014). in fact, inspired by davies’ vision, many new concepts of multifunctional and adaptive facades have been developed (favoino, 2010; loonen et al., 2013), their adoption in the building industry is restricted by many barriers, one of them being the inadequacy of bps tools in representing their performance (loonen et al., 2014). in the frame of a research activity aimed at the development of a solar and active building skin of the future, a new concept of mfm has been conceived and a prototype realized to experimentally assess its energy and comfort performances. the new facade module, named actress – active, responsive and solar – was conceived starting from a decade-long first-hand experience and a literature survey (goia, perino, serra & zanghirella, 2010). the main aim of this paper is to quantify the energy performance of the actress mfm facade when it is integrated in the building; this is achieved by means of building performance simulation. the secondary aim is to map and analyse the limitations of current bps tools in the evaluation of this 108 f. favoino / assessing the performance of an advanced integrated façade by means of simulation kind of innovative aif technologies, integrating adaptive building envelope components. to this last aim the actress mfm can serve as a case study, given the high level of integration and the adaptive behaviour of some of its components, and the complexity of its operating strategies. in the second section of the paper the actress mfm and its operating strategies are detailed. in section 3 the software framework for building performance simulation of the actress mfm is presented, while in section 4 the expert modelling needed for the dynamic component in the mfm is described. in the fifth section the metrics to assess the components’ thermal performance and the energy performance of the building integrated actress mfm are detailed, and the results are presented in the sixth section. finally the limitations and requirements for bps at modelling this kind of advanced integrated building envelope are discussed. this numerical study was done to verify and optimize the design of the actress module prior to prototyping. this case study, although not representing all the panorama of possible aif and mfm designs, and all the panorama of bps tools, can give a valuable insight in the methods, possibilities and restrictions of bps, related to the evaluation of the performance of more advanced, integrated and responsive facades. 2. the actress multifunctional facade module 2.1. actress mfm design and components starting from first-hand research experience on aifs and a literature survey, a new facade module has been conceived, named actress – active, responsive and solar – facade module (favoino, goia, perino & serra, 2014). the main goal of the actress facade module is to overcome the limitation given by the current aif technologies and to assess the effects of an adaptive multifunctional facade module (mfm) on building energy savings and indoor environmental quality. the aim of the mfm is to realize an (almost) self-sufficient building skin that integrates different components and interacts with other building equipment to maximize the energy and environmental performance by adjusting to changing environmental conditions and occupants’ requirements. in order to develop an effective/efficient mfm system that represents a step forward in the field of aifs, the weak spots of present-day aif technologies have been used as a starting point during the concept phase (goia et al., 2010; favoino et al., 2014). the actress mfm has been designed as a prefabricated unit of one storey high, of total frontal dimensions 1500mm x 3500mm. it is made of two different sub-systems (fig. 1): a transparent submodule (tsm) and an opaque sub-module (osm). conventional aifs present a large transparent surface, usually equipped with a double skin facade (dsf) (aschehoug, 2009; poizaris, 2006). opaque surfaces are usually avoided or reduced to a minimum extent. on the contrary, mfm prototypes usually adopt a different balance between transparent and opaque surfaces. after some preliminary investigation, carried out by numerical analysis with the software energyplus (goia, haase & perino, 2013), results showed that a 1:1 ratio is a good balance between the glazed and opaque envelope surface in order to minimize energy consumption for heating, cooling and lighting in temperate climates. this strategy allows the heat capacity of the building skin to be increased and the total heat transfer coefficient of the mfm to be decreased, compared to highly glazed mfms and aifs. f. favoino / assessing the performance of an advanced integrated façade by means of simulation 109 fig. 1. actress module concept and prototype realized on the roof of politecnico di torino, italy. the osm is constituted by an opaque ventilated facade (ovf), equipped with four axial fans for the mechanical (fan-assisted) ventilation of the cavity (dimensions of the cavity: 600mm width x 120mm depth x 3400mm height). airflow within the cavity can be managed according to the desired integration with the building services and operation, as explained in paragraph 2.2. the outer surface of the osm is made of three glass laminated pv panels (amorphous silicon pv, nominal power=29w, g-value=0.27, u-value=5.0w/m2k) for power production within the mfm. the back of the cavity is made of a multi-layer opaque wall panel. along with some gypsum-board layers, providing mechanical resistance to the opaque sandwich, a highly insulating vip layer (thickness of the vip panel=25mm, λ =0.005w/mk) is included. it is coupled with two layers of phase change materials (pcms). the two pcm layers are placed between the vip and the indoor environment. an electrically heated foil (ehf) is positioned in between the two pcm layers for pcms on-demand activation (heating strategy). the transparent sub-system is made of two glazing systems. the larger bottom one is a triple low-e coated glazing with argon (u-value=1.0w/m2k, g-value=0.3). the outer cavity of the triple-glazing hosts a high reflective, low-e coated venetian blind, to control the solar and light transmission. the smaller top one is a triple-glazing, whose outer cavity is filled with aerogel (λ =0.009–0.012w/mk). this material improves the performance of the glazing in terms of thermal resistance and allows exploitation of natural light avoiding glare discomfort (due to its translucent appearance). in figure 1 the concept design of the mfm is shown together with a prototype realized for experimental assessment of its performance in real world environmental conditions. first results from the winter experimental campaign are presented in favoino et al. (2014). 110 f. favoino / assessing the performance of an advanced integrated façade by means of simulation 2.2. operating strategies the mfm has been designed to enable different adjustments in respect to ever changing climate and indoor comfort requirements, on a seasonal, daily and hourly basis. moreover the module enables different interactions with the hvac system. in particular the ovf, within the osm, can be operated as a supply air (sa), thermal buffer (tb) or outdoor air curtain (oac) facade (poizaris, 2006), depending on the season, the building location, the orientation of the facade and on the degree of integration between the facade module and the building services. when talking about the osm, the following operating strategies have been proposed: • in winter, the actress mfm could work both as a sa facade (1, fig. 2) or to provide a thermal buffer tb (5, fig. 2) to reduce respectively either ventilation losses and heat losses, or heat losses only; • in summer, the actress mfm could perform as an oac facade, in order to remove the unwanted solar gains during summer (5, fig. 2). oac mode could be performed by means of natural ventilation (nv) or is mechanically ventilated (mv) when higher solar irradiance heats the facade (1, fig. 2). the decrease of the temperature of the pv layer caused by the ventilated cavity of the osm will reduce the power losses of the pv due to the temperature effect, thus improving their performance (2, fig. 2). as far as the power produced by the pv panels and the role of the pcms are concerned, the following hypotheses about the operations have been considered: • in winter, the power generated (2, fig. 2) can be used to heat, by means of the electric heating foil (ehf), the pcm contained into the sandwich wall panel. pcms would then act as a solar latent heat thermal energy storage lhtes (4, fig. 2). pv generated energy could also balance the energy used to operate the shading devices and the lighting system in the enclosed room (3, fig. 2); fig. 2. actress operating modes (summer and winter day, summer and winter night). f. favoino / assessing the performance of an advanced integrated façade by means of simulation 111 • in summer, the pv panels could provide additional power to activate the fans for the mechanical ventilation of the osm (3 and 1, fig. 2). the extra current can be released to the electrical system, since no power storage devices have been adopted in summer mode. pcms could operate in summer as a ‘simple’ and ‘passive’ indoor environmental climate moderator (i.e. smoothing the peak temperatures of the indoor air and reducing the cooling loads) (4, fig. 2). the lower part of the tsm (in particular the venetian blind in the cavity of the tgu) can be operated in winter in order to avoid glare whilst maximizing the entering solar heat gain; in summer to avoid glare, minimize the solar energy entering the environment and maximize the amount of natural light through the glazed portion of the facade and view to the occupant. the control of the facade during mid-season is one of the hardest tasks as far as the building management system is concerned. different strategies either characterizing the summer or winter mode could be necessary during the same day, so that the management system should be able to simulate the energy and comfort performance of the facade and decide which operating condition to adopt depending on climate conditions. the detailed design of the control system for the mfm is out of the scope of this paper; therefore it will not be presented. 3. building performance simulation of actress mfm facade a major challenge faced during the design process of this kind of building envelope components is the lack of integrated design tools that could enable the effective prediction of the thermo-physical behaviour of the mfm, together with the energy performance of a building that adopts this technology (loonen et al., 2014; goia & cascone, 2014). commercially available software products are either too much specific (like most of the cfd codes and fem codes) or too much generic (like whole building performance simulation tools, whole-bps tools). the first typology of software could be used to obtain detailed information on the behaviour of some sub-components, but is not suitable to obtain the overall picture of the performance of the aif as a system. on the contrary, whole-bps tools are useful for assessing the overall performance of the building in terms of energy consumptions and occupants comfort, but present serious limitations when complex components (like an aif, or an adaptive building envelope system in general) need to be described and integrated into the model. as a result energy assessment obtained by means of whole-bps tools can be quite far from the actual performance of a building that integrates aifs and rbes, giving at the same time very limited information about single mfm components performance. moreover whole-bps tools are rarely integrated with building information modelling (bim) tools, thus making the modelling and performance evaluation a costly and laborious process and subject to modellers’ assumptions and errors (bazjanac, 2008). the assessment of the performance of the actress mfm facade, in terms of whole building energy performance and components thermal performance, can be used as a case study in order to evaluate the ability of bps tools at assessing the performance of an advanced mfm and to investigate the main requirements and limitations of these tools for this purpose. the energy performance of the actress mfm and of its components was investigated through a numerical analysis, carried out with the help of two integrated software tools: autodesk revit (bim tool) and ies ve (whole-bps tool). a bim parametrical model was developed with revit for the actress mfm together with the prototype manufacturer in order to provide sufficient flexibility in the architectural design and 112 f. favoino / assessing the performance of an advanced integrated façade by means of simulation manufacturing of the facade, and supporting information to the other stakeholders in the facade design process, among the other facade consultants and building energy modellers (voss, jin & overend, 2013). full details of the actress bim modelling could be found in favoino (2010). a preassessment of different commercial bps tool capabilities at evaluating aif energy performance based on crawley, hand, kummert and griffith (2008) indicated ies ve as one of the most complete bps tools available on the market to be validated for this task, while differently from other validated whole-bps tools it was interoperable with bim software in general and with autodesk revit in this particular case. figure 3 details how the actress mfm was divided into different subcomponents for modelling and performance evaluation purposes in the two different software systems. it was later verified that the two software systems were not fully interoperable on a certain level of information, determining a waste of resources in an accurate modelling of the actress facade in revit. the level of interoperability between revit and ies ve is only related to geometry information (surfaces and volumes), while all the different osm and tsm sub-components (air cavity and opening, wall layers, blind etc.) and material properties were not imported into ies ve (favoino, 2010), therefore re-modelling of lost information was required in the two different software systems. as far as the actress energy modelling and performance evaluation activity is concerned the main limitation of ies ve software was represented by the lack of accurate models regarding the osm sub-module, in particular to the ovf and to the lhtes system facing the indoor environment. therefore equivalent dedicated models were required for both components and some off-line calculations fig. 3. actress mfm components and modeling flow-chart. f. favoino / assessing the performance of an advanced integrated façade by means of simulation 113 were needed in order to assure an accurate modelling and to integrate the operations of some components. the modelling assumptions for the components of the osm (ovf and lhtes) and the method for the energy performance evaluation is detailed in the next sections and mapped in figure 3. 4. modelling of actress mfm 4.1. opaque ventilated cavity different methods are available to accurately model cavity ventilation in double skin facades, according to the level of accuracy needed, the kind of ventilation and the different elements integrated in the cavity. according to hensen, bartak and drkal (2002) the simulation of the air flow in the cavities can be done by means of computational fluid dynamics (cfd) or of the airflow network method (anm). the cfd code is able to perform many tasks that the network modelling will never achieve. however, due to the complexity of the cfd simulation, the amount of calculation power needed, the restricted steady state boundary conditions, the absence of movable shading device in the air cavity in the actress case and the nature itself of the performance assessment needed, its use is not necessary at this design stage. furthermore anm is much more easily integrated into whole building energy simulation models, although much progress has been made in the coupling of cfd and whole building energy simulation models (zhang, lam, yao & zhang, 2013). for anm modelling different approaches are possible. saelens (2002) performed an investigation of the accuracy change with stepwise enhancement of the network model, from the simplest case of a single zone model, to the more complex case of the cell centred control volume method with the convective and radiative flows treated separately. according to saelens (2002) the best anm is based on a cell centred control volume method, in which the cavity is vertically subdivided and in which the temperature of the cavity control volume is represented by a bulk temperature. it is assumed that enthalpy flows only occur along the vertical direction, this restricts the use of this model to multiple-skin facades with roller cavity blinds only, or no cavity shadings. in this case the anm control volume method was used in order to model the ovf cavity ventilation. an additional room, vertically subdivided, was created for the cavity, and its geometry had to be modified in relation to the calculation method used by ies ve for natural ventilation. in particular it adopts a cell centred control volume method, where the control volume is the thermal zone defined in the software, represented by a bulk temperature. the calculation of the airflow between two connected zones is based on the flow characteristics of openings that are small in relation to the spaces they connect (ies ve ltd., 2010). therefore in order to achieve a good model reliability, adjustments to the opening dimensions and cavity thickness were necessary. in particular, the sectional area of the cavity was set accounting for the frictional resistance of the cavity walls and obstructions using the colebrook-white equation for rough duct in turbulent regime: k = f l d (1), where f is the duct frictional resistance, l the duct length and d the hydraulic diameter. using the coolebrook-white equation for rough duct in turbulent regime 114 f. favoino / assessing the performance of an advanced integrated façade by means of simulation fig. 4. pcm fictitious cavity heating (red) and cooling (blue) loads in the hottest week of the year. fig. 5. simulated office test rooms. k = l( 8w { log10 [( ε w ) 7 .4 ]}2) = 1( 8 · 0.12 { log10 [( 0. 030 .12 ) 7 .4 ]}2) = 0.48123 (2), given l=1m, as the cavity vertical sections are 1m high, w=0,12m is the cavity width, � =0,05m is the characteristic obstruction length inside the cavity. in reality, the frictional resistance is evenly distributed along the duct, but in macroflo, as in all the simulation tools adopting the anm model, it has to be aggregated into a discrete opening, connecting the two thermal zones. in order to achieve a good model a subdivision of the cavity in different interconnected zones, vertically separated, is necessary. the subdivision of the cavity consisted in dividing the actress ventilated cavity in five sub-volumes (fig. 5), in order to measure the bulk air temperature at the air cavity inlet and outlet, as well as a bulk surface temperature for each one of the 3 pv panels of the outer glazing confining the cavity. to calculate the equivalent cavity opening area we can refer to the flow conservation principle: f. favoino / assessing the performance of an advanced integrated façade by means of simulation 115 a0u0 = adu (3), given �p = k12 ρu2 and k= (1/0.62)2; when the opening is much smaller than the room we can calculate the equivalent area as: a0 = ad 0.062k0.5 = 0.12 ∗ 0.75 0.0620.481230.5 = 0.21 m2 (4) therefore the ventilated cavity thickness is increased from 12 to 28cm in order to account for the capacity losses due to the frictional and obstruction resistance of the cavity. as far as mv is concerned, four fans were placed at the cavity exhaust section, with a peak capacity of 94,5 l/s, according to data from manufacturers. when the fans are powered, both mechanical and natural ventilation are accounted for into the cavity. the opening schedule of the cavity inlet and outlet for natural ventilation follows the occupation schedule, while as far as mechanical ventilation is concerned, since the fans are directly powered by the pv, the minimum solar irradiance (gfan) able to power a fan at peak capacity was calculated (i.e. gfan =20w/m2). for values higher than gfan each fan is progressively powered at peak load, while for lower values the capacity is proportionally modulated. 4.2. the lhtes system the lhtes system is composed by the two pcm layers with the interpose ehf, which is heated by the pv harvested energy during winter, while it is separated from the air cavity by the vip panel (figs. 1 and 3). as far as the pcm layer is concerned, few commercially available software products allow, as part of the standard routine, to model directly a pcm integrated with whole building energy simulation. the key feature of pcm is to absorb energy as latent heat rather than sensible heat over a small range of temperatures; hence, a pcm exhibits few temperature rises when in the melting process. different modelling problems need to be solved in order to correctly simulate the pcm behaviour, i.e. hysteresis between latent heating and cooling cycles, different melting and freezing peak temperatures and variation of the heat capacity with temperature, sub-cooling phenomena (zalba, marı́n, cabeza & mehling, 2003). nowadays different validated models are available but few of them are implemented on commercial software (al-saadi & zhai, 2013; castell, medrano, castellón & cabeza, 2009). in this case no building envelope integrated pcm model exists in ies ve, since it is not possible to set the specific heat capacity of a material as a function of the temperature, and the latent heat storage of the pcm cannot be modelled unless simplifying assumptions are taken. according to ibanez, lazaro, zalba and cabeza (2005) and kendrick and walliman (2007) it is possible to simulate the pcm layer with no need to model the state change within the material, although with some limitations. both authors model the pcm as an ‘active layer’, whose temperature and heat transfer rate is controlled by means of a fluid, water or air, having the same temperature of the melting temperature of the pcm. this model is able to simulate only ultra-pure pcms, that is pcms not showing a region of temperatures where melting takes place, but a unique defined temperature. according to (ibanez et al, 2005; castell et al., 2009), good agreement with experimental data is achievable with such a model, provided that laboratory characterization of the material properties is done, and provided that the pcm is pure paraffin. 116 f. favoino / assessing the performance of an advanced integrated façade by means of simulation the ‘active layer’ method, or so-called ‘conditioned cavity method’ (kendrick & walliman, 2007), has been adopted to model the pcm layer in ies ve. in this method, a virtual cavity, that simulates the pcm layer, is maintained at a set-point temperature (which corresponds to the nominal melting temperature of the chosen pcm) by means of an ideal hvac system. controlling the airflowdelivered energy according to the total heat capacity of the pcm layer is very time consuming, as an iterative process is required. this restrains the simulation to no more than one month, as a schedule controlling the fictitious hvac management system needs to be set for each different day. that is why it was chosen, in order to perform a yearly simulation, to control the air temperature inside the active layer always at the pcm melting temperature. when such an approach is adopted, an infinite latent heat storage capacity is assumed for the pcm layer, requiring further verification to ensure a good reliability of the results. this verification implies an energy balance to be performed on the pcm: i.e. the sum of the ‘conditioned cavity’s’ heating and cooling loads, on a daily basis, needs to be equal or lower than the actual latent heat storage capacity of the pcm. or, in other words, the areas below the heating (red line in fig. 4) and cooling (blue in fig. 4) power profiles of the fictitious hvac system simulating the pcm, are required to be equal or lower than the pcm total latent heat capacity on a daily basis, taking into account the balance between the two areas of the previous day as well. the heating loads represent the heat which is supplied from the pcm to the room, while the cooling loads represent the heat which is absorbed by the pcm from the room. in fact if the amount of heat supplied or released to the conditioned cavity exceeds the total latent heat capacity of the pcm designed, it means that the real temperature of the pcm should be higher than the equivalent model simulated. this verification can also be used as a design tool to estimate the optimal quantity of pcm to be used, in fact the energy balance of the pcm layer is a way to verify that the amount of pcm designed is sufficient to provide the required performance. the proper approach that provides the foundation of the conditioned cavity method implies the modelling of an infinitesimal volume cavity, whose wall surfaces have a (almost) null thermal resistance (at least lower than 10−4 m2kw–1) and no heat capacity (the sensible heat capacity is neglected and the entire latent heat capacity is accounted for in the cavity loads); the cavity boundaries themselves exhibit a thermal resistance equal to the one of the pcm layer. the ‘conditioned cavity method’ approach can be used during summer operation, with the pcm used as a passive storage media (lhtes), or during winter operation, in which the pcm stores the energy harvested by the pv (solar lhtes). in winter operation also the energy harvested by the pv must be included in the daily energy balance of the pcm required by the ‘conditioned cavity method’. this energy balance on the fictitious layer of the pcm cannot be done in ies ve, so that an off-line calculation is performed. 5. energy performance assessment methodology the energy performance of the actress mfm module is evaluated on two levels: at component level and at whole building level (fig. 3, bottom part). at component level the indicators to measure the thermal performance of dynamic facade sub-components are evaluated (ovf and lhtes) according to the different possible operating strategies. the thermal performance of the tsm components is not detailed in this section, as it can be easily done in terms of thermo-optical properties of the glazing system itself (which is already detailed in section 2.1). at whole building level the total primary energy f. favoino / assessing the performance of an advanced integrated façade by means of simulation 117 consumption of an office reference room adopting the actress facade (including osm and tsm, with different operating strategies of the osm) is compared to an office with a reference facade. 5.1. component thermal performance assessment besides the overall building energy demand, the performance of the single components of the actress mfm needs to be evaluated. as far as the ovf is concerned, different ventilation strategies are assessed and compared: thermal buffer (tb), natural ventilation (nv) and mechanical ventilation (mv). natural stack ventilation (nv) is used in heating mode, for tb or to preheat outside air (sa strategy); nv and mv is used in cooling mode, to remove the excess heat from the facade (oac strategy). in the mid-season both strategies can be used, and the performance and the correct control strategies are evaluated for both cases. the performance of the ventilated cavity was estimated by means of dynamic parameters (dimaio & van paassen, 2001 and corgnati et al., 2007), i.e. the ‘dynamic insulation efficiency’ �, and the ‘preheating efficiency’ η. the dynamic insulation efficiency � measures the ratio between the amount of heat load removed by the cavity ventilation qr and the total heat load impinging the facade qin, being a measure of how the facade is able to remove the unwanted external heat gains in the indoor environment in the cooling season: ε = qr qin (5), while the preheating efficiency η is the ratio between the enthalpy of the external air flowing in the air gap and the enthalpy required to heat up the external air to the indoor environment air temperature, giving a measure of how much the facade is able to reduce heat losses through the facade and ventilation losses, if the air cavity is used with sa strategy: η(t0) = texh − t0 tamb − t0 (6), where texh is the bulk exhaust air temperature, t0 is the temperature of the outdoor air, tamb is the temperature of the indoor environment. in the osm, the pcm layer is coupled with an ehf in order to have an ‘on-demand activation’ available during winter (solar lhtes), while it can be used ‘passively’ during summer, reducing the cooling load in the building (lhtes). in winter the ehf is powered primarily from the pv panels placed on the outer surface of the osm, but connection with the electrical grid can be available. a complete evaluation of the lhtes system is carried out according to the energy balance of the pcm needed to implement the ‘conditioned cavity method’, i.e. the daily energy balance between heating and cooling energy in the pcm ‘conditioned cavity’ is not violated (cfr. section 4.2). during summer heating and cooling energy of the pcm ‘conditioned cavity’ are accounted in the energy balance, while in winter the pv converted energy is included as well. the performance of the system as well as the accuracy of the model is measured by the time percentage of the pcm being in the melting phase; the higher this percentage the more reliable the model and the more effective the lhtes design is in summer and in winter. 118 f. favoino / assessing the performance of an advanced integrated façade by means of simulation 5.2. whole building energy performance assessment the overall building specific primary energy demand, ep [kwh/m3y], of an office equipped with the actress facade module was estimated and compared against the one of an office equipped with a reference facade. the two identical 30-m2 office rooms (6m × 5m × 3,5m) were located in turin (lat. 45◦n, long. 7.65◦e), italy. the actress facade (four modules are used to cover the entire office room elevation) faces south and the other walls are adiabatic (i.e. the office room is adjacent to other identical rooms on all the other surfaces, except the exposed facade). the facade of the reference office room is made of a heavy-weight wall and presents the same glazed area as the actress one (fig. 4). the main physical properties of the reference envelope comply with local building regulations for the climatic zone of turin (uopaque =0,35 wm–2k–1, uwindows =1.97 wm–2k–1). the internal heating loads are 20 wm–2 (including people, lighting and appliances) and 2 wm–2 in a non-occupation period, considering an occupation density of 0.1 person/m2. the occupation period is 8:30–6:30 pm, monday to friday. the indoor temperature set-point is 21◦c during winter and 26◦c in summer (with 14◦c and 40◦c night set-back respectively), with 1.4 l/s per person as primary ventilation. 6. results 6.1. opaque ventilated facade thermal performance the performance of the following cavity ventilation strategies of the osm is evaluated: thermal buffer, air supply in natural ventilation mode and outdoor air curtain in natural and mechanical ventilation mode. the comparison between the exhaust air temperatures under the different ventilation strategies (fig. 6) shows a relevant reduction of the cavity central air bulk temperature of nearly 20◦c comparing tb and mv ventilation. this reduction affects the energy performance of the facade during both cooling and heating season and the pv efficiency. in fact the pv modules’ temperatures are reduced up to 10◦c in winter and in mid-season, and up to 15◦c in summer, when mv is adopted rather than nv. moreover, if compared to a non-ventilated pv panel placed on a wall, the peak temperature is reduced to about 40–50◦c. comparing the actress pv system and a conventional pv module without any integrated ventilation (fig. 7), both in the heating and cooling season, it can be fig. 6. osm cavity exhaust air temperature: mv (blue), nv (red) and no ventilation (green). f. favoino / assessing the performance of an advanced integrated façade by means of simulation 119 fig. 7. improved pv efficiency due to rear ventilation. fig. 8. cumulated frequency of dynamic insulation efficiency. observed that the ventilation improves the pv efficiency up to 10%, with an average value of 5%. mv shows a better performance than all other ventilation strategies, both in the cooling and the heating season, increasing pv efficiency of 2% compared to nv and 3% to tb, on the average (fig. 7). figure 8 shows the cumulated frequency (during the occupation period) of the dynamic insulation efficiency of the two ventilation modes in oac strategy (cooling and mid-season). it measures the amount of heat that is removed from the facade by means of the ventilation. during the cooling season, the facade is able to reduce the entering heat gain by more than 60%, on the average (50% of the occupation period), while mv profiles an average improvement of 10% compared to nv. 120 f. favoino / assessing the performance of an advanced integrated façade by means of simulation fig. 9. cumulated frequency analysis of preheating efficiency during the heating season. the cumulated frequency analysis for the heating season (fig. 9) quantifies the effectiveness of the cavity in preheating the air during winter. in tb mode, the air is confined into the cavity; therefore, even with a very high preheating efficiency (30% of the time higher than 100%), no air can be supplied to the indoor environment, but the heat losses can be consistently reduced in this way. on the contrary, in nv mode the preheating efficiency is not sufficient to supply the air directly into the indoor environment (preheating efficiency higher than 100%), but if used as supply air for the hvac system it is able to provide nearly 20% of the heating plant load during wintertime. the best ventilation strategy during the heating season will be evaluated looking at the overall energy consumption during the heating season of the office room, as the preheating efficiency alone is unable to measure the reduction of heat losses through the envelope and by means of ventilation, in terms of total energy consumption. during the mid-season heating and cooling loads often occur within the same day. for this reason the cumulated frequency distribution is analysed for both parameters � and η. figures 9 and 10 reveal that the preheating efficiency increases during the mid-season with respect to winter (in nv mode with sa strategy), as the outdoor air temperature and the solar irradiance are higher than in winter. when η reaches an adequate value (e.g. η ≈100%), it is possible to supply fresh air to the indoor environment directly from the cavity. a dedicated control is necessary to prevent an increase in the cooling loads from occurring (e.g. when η � 100% and cooling demand is present). 6.2. lhtes thermal performance in order to select the optimal quantity and type (nominal melting temperature, tpcm) of the pcm to be used in the actress facade module, several simulations were run to test the performance of the following nominal melting temperatures: tpcm =23◦c, 25◦c, 26◦c and 27◦c. f. favoino / assessing the performance of an advanced integrated façade by means of simulation 121 fig. 10. cumulated frequency analysis of preheating efficiency during the mid season. the results of the simulations performed to select the proper amount and the nominal melting temperature of the pcm (and to test the model reliability) are shown in figure 11 (cooling season, ‘passive’ use of pcm). the tpcm =25◦c is able to completely absorb (nearly 100% of the time) the factitious heating/cooling load of the virtual cavity, if at least 2.5kg/m2 are used. this combination represents the optimal configuration of the system, as a pcm with a higher tpcm has a worse behaviour than the optimal solution (it is not completely melted for a longer period); a pcm with a lower tpcm is always melted as the sum of the heating/cooling daily energies exceeds the latent heat storage capacity of such a pcm. as it is explained in paragraph 3.1 a measurement of the model reliability is also a proper method to select the best performing pcm melting temperature related to a certain strategy (use of pcm in a passive or active way), so that the best melting temperature choice in summer is 25◦c, with fig. 11. time percentage in which the pcm is within the melting range (cooling season). 122 f. favoino / assessing the performance of an advanced integrated façade by means of simulation fig. 12. time percentage in which the tpcm=25 ◦c pcm is within the melting range. a temperature set point of the hvac system in the room of 26◦c. a higher or lower pcm melting temperature would result in a higher pcm quantity required for good model reliability and effective pcm design. in figure 12, a detailed analysis on the behaviour of the optimal solution is shown for different seasons. it can be noticed that, during the occupational period, the 2.5kg/m2 pcm is maintained within the melting range for about 91%, 95% and 97% of the time in summer, mid-season and winter, respectively. the coupling between the pcm layer and the pv power production has been investigated during the winter season only, since the aim of this system is to store a certain amount of energy available during the daytime for heating purpose (heating season, ‘active’ use of pcm). the influence of the different amount of pcm (mpcm) with different pv panel surface was investigated too, i.e. mpcm from 0.5kg/m2 to 4kg/m2 (in steps of 0.5kg/m2), in combination with three possible pv panel surfaces (i.e. spv =1.67 m2, 2.50 m2, 3.34 m2). considering the optimal summer design solution (pcm with tpcm =25◦c, mpcm =2.5kg/m2), on seasonal basis, it can be noticed that the pcm layer is able to store, by means of latent heat, 82% of the energy produced by a 1.67 m2 pv surface, which is 50% of the total actress surface (fig. 13). if the pv surface would be increased (i.e. spv =2.50 m2, or 3.34 m2, 50 and 100% additional equivalent area respectively, to be placed on the roof), a lower percentage of the pv-converted energy could be stored within the pcm layer (nearly 75% and 40%, respectively). if mpcm =4kg/m2 would be used instead, the entire amount of energy generated by the 1.67 m2 pv surface could be stored, in the latent heat capacity of the pcm layer. the evaluation of the proper amount of pcm against the pv surface is mainly based on three parameters, which relationship is rarely linear, so that the step-wise variation of pcm amount or of pv surface is not proportional to the variation of the time percentage in the melting phase. the three main variables affecting these curves are the daily amount of latent heat storage potential available for energy storage in the pcm layer, the amount of solar energy converted by the pv, and the indoor conditions (internal loads and solar free gains) affecting the amount of heat that could be discharged daily from the pcm. 6.3. whole building energy performance (pe) the total and the break-up primary energy consumption of the office equipped with the actress facade and with the reference facade are compared in fig. 14. adopting the outdoor air curtain f. favoino / assessing the performance of an advanced integrated façade by means of simulation 123 fig. 13. percentage of the energy produced by the pv panel that is stored in the pcm layer. 11.68 6.71 6.17 3.41 2.24 2.24 4.00 4.00 4.00 -3.86 -3.86 0.00 0.07 0.07 -6.00 -1.00 4.00 9.00 14.00 19.00 reference façade actress mfm oac/tb actress mfm oac/sa primary energy balance [kwh/m³y] pe hea ng [kwh/m³y] pe cooling [kwh/m³y] pe ligh ng [kwh/m³y] e pv [kwh/m³y] pe fans [kwh/m³y] fig. 14. break up and total specific primary energy consumption of the office room with the different facade options. strategy during summer and tb in winter, the overall reduction of the primary energy demand eptot is 52%, i.e. from 19.1 kwh/m3y to 9.1 kwh/m3y. a considerable decrease in heating loads eph is also observed (almost 53%); this can be associated to the use of pv energy to activate the pcm, together with the reduce of heat losses due to the thermal buffer strategy in the osm. furthermore, it is remarkable how the electric consumption for lighting and appliances, on an annual basis, can be nearly completely covered (ca. 95%) by the solar energy converted by the pv with the designed area of 1.67 m2 (the energy released into the pcm layer and the energy required for the cavity fans are already included in the energy balance). the possibility to exploit the air that is preheated by the osm cavity as a supply air for the hvac system (during winter and mid-season) was assessed by means of a dedicated simulation. with this configuration, a further reduction in the petot can be achieved (from 9.1 kwh/m3y to 8.5 kwh/m3y), resulting in a total energy saving of about 55% in comparison to the reference room just by substituting the reference building envelope with the actress mfm. 7. limitations of bps tools for aif energy performance evaluation modelling an advanced mfm concept integrating different rbe components, in order to evaluate the benefit from the building energy consumption point of view, is a non-trivial task. this is demonstrated by means of the actress case study. in fact different aspects of the bps tool used, that could be 124 f. favoino / assessing the performance of an advanced integrated façade by means of simulation easily generalized to other bps tools, can be listed as either a barrier or an advantage to model an aif. these aspects can be classified in four main issues: 1) interoperability, 2) model abstraction, (2a) geometrical and 2b) thermo-dynamical), 3) post processing of results, 4) building systems integration and controls. from the present case study these issues are detailed as follows: 1) loss of information and lack in backward interoperability are related to the first class of issues. when converting the geometrical bim model to the whole building energy model different information is lost. this relates to biand tri-dimensional details of openings and windows, to building envelope build-up and to material thicknesses and properties. this resulted in double input definition in the two different software systems. moreover a change in the building energy model (i.e. ventilated cavity thickness, window to wall ratio, layer thicknesses etc.) is not translated into a change of the bim model (backward interoperability). therefore a design variation of the mfm module in the building energy model (i.e. thickness of the cavity, area of openings, wwr etc.) requires re-modelling the mfm system in the bim software. 2a) the level of detail of a mfm model needs to be simplified from a geometrical point of view: only surfaces are defined, distinguished between opaque, transparent surfaces and openings, while any detail of tri-dimensional geometry is lost; the volume of the ventilated cavity needs to be divided into smaller interconnected volumes, in order to have a vertical temperature distribution in the ventilated cavity; the thickness of the volumes of the ventilated cavity and the area of the openings between the volumes need to be adapted in order to compensate for thermo-physical model assumptions (section 4.1). all geometrical abstractions could be reflected into inaccuracies of the model (i.e. error in the vertical temperature distribution in the cavity and thus in the heat exchange across the ventilated cavity, lack in evaluating the thermal bridges effect of the openings on the air entering the ventilated cavity etc.). 2b) no material properties could be varied according to a control function, or as a function of other model input/output variables, this is limiting the possibility to model dynamic/responsive building envelope elements. therefore a surrogate model was needed for this case for the pcm layer, and the accuracy of this surrogate model was assessed in a post-processing stage. the integration of pv with other active systems could not be modelled directly (i.e. fans of the ventilated cavity, pcm), but preand post-processing of inputs and results are needed. the model for the ventilation used by the software was not able to reproduce the dynamics of the cavity ventilation of a double skin facade, unless the geometry of the cavity is divided into smaller volumes and adapted in order to compensate for modelling approximations. anyway in no case the approach presented could be used if cavity-shading devices were present. 3) no metrics for assessing the performance of advanced facades (i.e. η, �) could be calculated directly into the software, although many outputs were generated and presented. in fact a time consuming post-processing task was required, in order to calculate the meaningful performance metrics needed. this issue could be easily solved by allowing algebraic operations, using the outputs and inputs of the simulation in the results’ interface subroutine. 4) although the building envelope system simulated was not traditional, the capability of the software of easily integrating different systems, operating modes and controls with a graphical user interface is a big advantage. in fact different control rules and strategies were easily implemented: cavity ventilation of the mfm rule-based opening and closing of vents according to cavity/indoor/outdoor temperatures; rule-based operation of tsm cavity shading devices according to indoor/outdoor temperatures and incident solar radiation; integration between natural and mechanical ventilation; variation between tb, sa and oac strategy. f. favoino / assessing the performance of an advanced integrated façade by means of simulation 125 a big effort is needed to design and evaluate the performance of a mfm module, for whole building integration. this is not only due to the physiological level of model simplification required when representing a physical system with computer aided engineering tools, but also to the inadequacy of current bps tools at representing more innovative and integrated building envelope components and systems. this inadequacy, though, does not necessarily mean inability. in fact in some cases, as demonstrated with this work, it is possible to properly model more advanced building envelope systems, although assumptions and simplifications may be needed. these represent a higher risk of inaccuracy in correctly designing and operating this kind of integrated building envelope systems, therefore representing a barrier to their adoption by designers, building industry and thus clients. in order to overcome the issues presented, whole-bps commercial tools could focus on addressing the following limitations: 1) backward interoperability and more exchange of information between cad/bim software and bps tools; 2) availability of different validated physical models for different purposes (i.e. different ventilation models for different cavity configurations), or allowing co-simulation with other software; 3) possibility to change materials properties according to changing boundary conditions or state of the material (i.e. temperature), in order to model dynamic and responsive materials, elements and systems; 4) higher level of integration between building service systems, and between building service systems and building envelope, as well as more integrated controls; 5) possibility to actively use outputs as new inputs for control of active systems, and more flexibly allowing calculation using outputs and inputs in the bps tool environment; some of these issues (2, 3, 5) are already being addressed in more advanced bps tools, usually used in academia for research purposes (ellis, torricellini & crawley, 2007; pang, bhattacharya, o’neill, haves, wetter & bailey, 2008; zhang et al., 2013). 8. conclusions the numerical evaluation of the energy performance of the actress, active responsive and solar, mfm was presented in this paper. this is an innovative multifunctional facade module conceived by the tebe group of the polytechnic university of turin. the purpose of this work was two-fold: to evaluate the energy performance of actress design and to evaluate the issues and requirements of whole-bps tools related to the performance simulation of advanced integrated facades. as far as the actress energy performance is concerned the assessment carried out by means of dynamic energy simulation, showed a good level of integration of the different components within the module operating strategies. in fact the design improved the single components energy performance, achieving a high result as regards overall energy consumption. the actress module showed a high performance in terms of dynamic insulation efficiency, pre-heating efficiency and ability to reduce heat losses; furthermore the integration of pv modules with the cavity ventilation improved the electric conversion efficiency of the pv; the integration of pv and pcm into the lhtes system is demonstrated as crucial during the winter season in reducing heating energy demand during winter; different design options for the lhtes were analysed during winter season, taking into account the amount of pcm and the amount of pv surface 126 f. favoino / assessing the performance of an advanced integrated façade by means of simulation available, and during summer, as far as the amount of pcm and melting temperatures are concerned. the actress potential in reducing the overall building energy demand was evaluated in comparison with an office building adopting a traditional facade complying with local national standards. by replacing the reference building envelope with the actress mfm more than 50% of the total primary energy can be saved if tb strategy is adopted, and up to the 55% if sa strategy is adopted. the highest reduction in the primary energy consumption can be found in the heating primary energy demand, as a result of the integration between the ventilated cavity and the lhtes system. the electric energy consumption of the fans, the lighting system and the pcm stored energy can be completely supplied by the pv produced electricity with a pv surface of 50% of the facade module. future work is needed to compare the numerical results with the experimental characterization carried out on the actress prototype. the whole modelling and performance evaluation process by means of the bps tool was mapped and analysed in order to evaluate the requirements and limitations of bps tools at evaluating advanced integrated facade performance. it was demonstrated that whole-bps software is hardly able to provide designers with suitable tools in order to design and evaluate innovative facade components and solutions. the main limitations of the software tool used are presented and discussed; these can be classified into interoperability, model simplifications, control integration and post-processing of results. these limitations could represent a source of error and inaccuracy, and expert modelling is often required. in fact accurate modelling of innovative and integrated solutions still needs tips, shortcuts and assumptions not usually known unless supported by extensive literature review and experience; these represent a source of error and can jeopardize the energy simulation reliability and accuracy if not provided with solid building physics knowledge and supported by verification. future work will deal with mapping requirements and limitations related to the specific issues described in this paper, from the perspective of different bps tools instead of from the point of view of a specific innovative facade design. acknowledgments the actress facade concept was conceived in the framework of the prin 2007 by the research group lead by professor perino in the energy department of the polytechnic university of torino, italy. the development of some sub-components (advanced glazings and pcm panels) has been done in the framework of the polight projects ‘smartglass’ and ‘si2’. the author is grateful to professor marco perino and professor valentina serra for providing the concept design of the system and the support given for the numerical evaluation. references al-saadi, s. n., & zhai, z. (2013). modeling phase change materials embedded in building enclosure: a review. renewable and sustainable energy reviews, 21, 659-673. aschehoug, ø., andresen, i., kleiven, t., & wyckmans, a. (2005). intelligent building envelopes fad or future? in proceedings of the 7th symposium on building physics in the nordic countries, reykjavik, iceland. aschehoug, o., & perino, m. (2009). annex 44—iea—ecbcs report, expert guide—part 2: rbe. http://annex44.civil.aau.dk/ bazjanac, v. (2008). impact of the us national building information model standard (nbims) on building energy performance simulation. lawrence berkeley national laboratory. http://annex44.civil.aau.dk/ f. favoino / assessing the performance of an advanced integrated façade by means of simulation 127 castell, a., medrano, m., castellón, c., & cabeza, l. f. (2009). analysis of the simulation models for the use of pcm in buildings. in the 11th effstock conference proceedings, thermal energy storage for efficiency and sustainability 2009. stockholm, sweden. http://talon.stockton.edu/eyos/energy studies/content/docs/effstock09/posters/125.pdf compagno, a. (2002). intelligente glasfassaden: material, anwendung, gestaltung=intelligent glass facades: material, practice, design. basel, switzerland: birkhäuser. corgnati, s. p., perino, m., & serra, v. (2007). experimental assessment of the performance of an active transparent façade during actual operating conditions. solar energy, 81(8), 993-1013. crawley, d. b., hand, j. w., kummert, m., & griffith, b. t. (2008). contrasting the capabilities of building energy performance simulation programs. building and environment, 43(4), 661-673. davies m. (1981). a wall for all seasons, riba journal, 88(2), 55-57. dimaio, f., & van paassen, a. h. c. (2001). modelling the air infiltrations in the second skin façade. in proceedings of the 4th iaqvec conference, changsha, china (pp. 873-880). ellis, p. g., torcellini, p. a., & crawley, d. b. (2007). simulation of energy management systems in energyplus. in proceedings of building simulation, beijing, china. epbd 31/2010, directive on energy performance of buildings recast, european parliament and european council, brussels. favoino, f. (2010). zero energy building: evaluation of innovative actress façade through dynamic energy simulation. msc thesis report, politecnico di torino, italy. favoino, f., goia, f., perino, m., & serra, v. (2014). experimental assessment of the energy performance of an advanced responsive multifunctional façade module. energy and buildings, 68, 647-659. http://dx.doi.org/10.1016/j.enbuild.2013.08.066 goia, f., perino, m., serra, v., & zanghirella, f. (2010). towards an active, responsive, and solar building envelope. journal of green building, 5(4), 121-136. doi: 10.3992/jgb.5.4.121 goia, f., & cascone, y. (2014). the impact of an ideal dynamic building envelope on the energy performance of low energy office buildings. energy procedia, 58, 185-192. goia, f., haase, m., & perino, m. (2013). optimizing the configuration of a façade module for office buildings by means of integrated thermal and lighting simulations in a total energy perspective. applied energy, 108, 515-527. hensen, j. l. m., bartak, m., & drkal, f. (2002). modeling and simulation of double-skin facade systems. ashrae transactions, 108(2), 1251-1259. ibanez, m., lazaro, a., zalba, b., cabeza, m. l. (2005). an approach to the simulation of pcms in building applications using trnsys. applied thermal engineering, 25, 1796-1807. ies ve ltd (2010), macroflo calculation methods, virtual environment 6.0. http://www.iesve.com/downloads/help/thermal/ reference/macroflocalculationmethods.pdf kendrick, c., & walliman, n. (2007). removing unwanted heat in lightweight buildings using phase change materials in building components: simulation modelling for pcm plasterboard. architectural science review, 50(3), 265-273. loonen, r. c. g. m., trcka, m., cóstola, d., & hensen, j. l. m. (2013). climate adaptive building shells: state-of-the-art and future challenges. renewable & sustainable energy reviews, 25, 483-493. loonen, r. c. g. m., singaravel, s., trcka, m., cóstola, d., & hensen, j. l. m. (2014). simulation-based support for product development of innovative building envelope components. automation in construction, 45, 86-95. knaack, u., klein, t., bilow, m., & auer, t. (2014). façades: principles of construction. basel, switzerland: birkhäuser. oesterle, e., lieb, r. d., lutz, m., & heusler, w. (2001). double-skin façades-integrated planning. munich, germany: prestel. pang, x., bhattacharya, p., o’neill, z., haves, p., wetter, m., & bailey, t. (2011). real-time building energy simulation using energyplus and the building controls virtual test bed. in proceeding of the 12th ibpsa conference (pp. 2890-2896). poizaris, h. (2006). double skin facades: a literature review, a report of iea shc task 34 ecbcs annex 43, department of architecture and built environment, division of energy and building design, lund university, lund institute of technology. quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012a). a comprehensive review of solar facades. opaque solar facades. renewable and sustainable energy reviews, 16(5), 2820-2832. quesada, g., rousse, d., dutil, y., badache, m., & hallé, s. (2012b). a comprehensive review of solar facades. transparent and translucent solar facades. renewable and sustainable energy reviews, 16(5), 2643-2651. ritter a. (2007). smart materials in architecture, interior architecture and design. basel, switzerland: walter de gruyter. saelens d. (2002). energy performance assessments of single storey multiple-skin facades. phd thesis, laboratory for building physics, department of civil engineering, catholic university of leuven, belgium. stec, w., & van paassen, a. h. c. (2000). integration of the double skin façade with the buildings, energy in built environment, energy technology, tu delft, delft, the netherlands. voss, e., jin, q., & overend, m. (2013). a bpmn-based process map for the design and construction of façades. journal of facade design and engineering, 1(1), 17-29. zalba, b., marı́n, j. m., cabeza, l. f., & mehling, h. (2003). review on thermal energy storage with phase change: materials, heat transfer analysis and applications. applied thermal engineering, 23(3), 251-283. zhang r., lam k. p., yao s., & zhang y. (2013). coupled energyplus and computational fluid dynamics simulation for natural ventilation, building and environment, 68, 100-113. http://talon.stockton.edu/eyos/energy_studies/content/docs/effstock09/posters/125.pdf http://dx.doi.org/10.1016/j.enbuild.2013.08.066 http://www.iesve.com/downloads/help/thermal/reference/macroflocalculationmethods.pdf http://www.iesve.com/downloads/help/thermal/reference/macroflocalculationmethods.pdf journal of facade design and engineering 4 (2016) 91–113 doi 10.3233/fde-161146 ios press 91 increased thermal induced climatic load in insulated glass units thomas wüest and andreas luible∗ hochschule luzern, technik & architektur, technikumstrasse, horw, switzerland abstract. in the mid-1990s the climatic loading of double-glazed units (dgu’s) was investigated and design rules were developed for use in practice. even though at the time those rules were developed for dgu’s, they have recently also been used for the design of modern triple-glazed units (tgu’s) and multilayer facade systems. so far, no research has addressed the question whether this design process, and in particular the temperature differences that have been determined for dgu’s, can be applied to other systems. it is therefore the aim of this paper to determine the governing temperatures in insulated glass units (igu’s) under up-to-date boundary conditions. to do this, the thermal behaviour of tgu’s, multilayer facade systems and solar shading is investigated in detail. a simple calculation model is developed, which can be used to determine the gas temperatures in various facade configurations in accordance with the relevant standards. the results show that while parts of the existing requirements are still valid, there are distinct differences in the requirements for more contemporary applications. the gas temperatures affect the climatic load and therefore also influence the structural design of igu’s. keywords: climatic loads, igu design, igu temperature, facade design, insulated glass units igu nomenclature a thermal diffusivity [m2/s] d material thickness [mm] hi, he internal and external heat transfer coefficients [w/m2k] hce convective heat transfer coefficient for external surfaces [w/m2k] hci convective heat transfer coefficient for internal surfaces [w/m2k] hs convective and radiative surface heat transfer coefficient [w/m2k] n amount of substance [mol] p gas pressure [n/m²] p0 isochoric pressure [kn/m²] q̇ heat flow density [w/m2] rj thermal resistance of glass [mk/w] s cavity width [m] c specific heat capacity [j/kgk] hh cavity height [m] htp temperature penetration distance [m] i� radiation intensity at irradiation angle � [w/m2] ij radiative flux to the inside [–] i’j radiative flux to the outside [–] r general gas constant [j/(mol·k)] rsi, rse internal and external thermal resistance [m2k/w] ∗corresponding author: andreas luible, msc, hochschule luzern, technik & architektur, technikumstrasse 21, ch-4048 horw, switzerland. tel.: +41 41 349 33 11; e-mail: thomas.wueest@hslu.ch. issn 2214-302x/16/$35.00 © 2016 – ios press and the authors. all rights reserved this article is published online with open access and distributed under the terms of the creative commons attribution non-commercial license (cc by-nc 4.0). mailto:thomas.wueest@hslu.ch 92 t. wüest and a. luible / increased thermal induced climatic load in igus r thermal surface resistance [m2k/w] t temperature [k] tm mean thermodynamic absolute temperature of the surface and environment [k] text absolute external air temperature [k] ts absolute surface temperature [k] tg equivalent temperature in cavity [k] tin inlet temperature [k] tout outlet temperature [k] v volume [m³] x material property [–] z pressure drop factor [–] �h elevation difference [m] �pmet, air pressure difference [kn/m2] �p0 isochoric pressure difference [kn/m²] �t temperature difference [k] � radiative absorption [–] � irradiation angle [◦] �q safety factor [–] � surface emissivity [–] λ thermal conductivity [w/mk] ρj radiative reflectivity from the outside ρ′j radiative reflectivity from the inside ρl20 air density at 20◦c [kg/m3] σ boltzman constant [w/m2k] ν air speed [m/s] νh rear ventilation air speed [m/s] τ thermal time constant [s] τj radiation transmissivity [–] ϑ temperature [◦c] � dynamic viscosity [pas] 1. introduction modern construction is unimaginable without doubleor triple-pane insulated glass windows. triple-pane glass windows in particular are becoming more and more popular due to their excellent thermal insulation properties. up to the late 1970’s double windows were used in most buildings in germany (vff/bf, 2014). they consisted of two separate glass panes with a cavity in between, which could be accessed for cleaning. due to the oil crisis, the awareness of energy efficiency increased and new window types were developed. in the last forty years, singleand double-glazed windows gave way to a wide range of specialized high-performance insulated glass products. 1.1. climatic load for insulated glass the climatic load is a specific internal load of insulated glass. the climatic load, also called insulated-glass effect, acts on the panes due to changes in the climatic conditions surrounding it. during the production of insulated glass units (igu’s) each glazing cavity (gc) is sealed hermetically and the current state of the gas in production (volume, pressure, and temperature) defines the t. wüest and a. luible / increased thermal induced climatic load in igus 93 fig. 1. effect of climatic loads on double glazed units. properties of the igu. if the gas temperature or atmospheric pressure changes, a pressure difference between the gc and the environment occurs. the pressure balance can be restored by an increase of the volume in the gc, according to the ideal gas law (p×v=n × r × t=>(p × v)/(n × t)=constant. the expanding or contracting gas volume causes deformation (see fig. 1), which in turn results in mechanical stress due to the climatic load. this effect is known for quite some time and has been the topic of various research projects and publications. practical approaches for the appropriate handling of this phenomena were developed by feldmeier (1991), feldmeier (1995) and feldmeier (2000). the magnitude of the climatic load is expressed by the isochoric pressure p0 . the isochoric pressure is defined as the pressure difference between the gc and the immediate environment that develops if the glass panes are rigid and cannot deform. it can be calculated by adding up three separate terms: the elevation difference �h, air pressure difference �pmet, and temperature difference �t between the conditions at production and installation (equation 1). p0 = 0.012 kn m2m · �h − �pmet + 0.34 kn m2k · �t (1) in equation 1, the meteorological air pressure difference �pmet (kn/m2) can be calculated directly from the meteorological air pressure difference, expressed in hpa (10hpa=1kn/m2). the coefficient 0.012 in the term for the elevation difference is based on the linearization of the international barometric formula, which yields very accurate results for altitudes up to approximately 1000m above sea level (feldmeier (1995)). amonton’s law (or gay-lussac’s second law) is used to calculate the isochoric pressure based on a change in temperature. this law stipulates that the pressure of ideal gases at a constant volume and a constant number of moles is directly proportional to the temperature. at normal pressure (1013.25hpa) and an initial temperature of 25◦c the factor for the change in isochoric thermal pressure is 0.34kn/m2k. 1.2. state of the art equation 1 for calculating the climatic load has been included in the technische richtlinie für linienförmig gelagerte verglasungen trlv (1998) since 1998 and in din 18008-2:2010 since 2010. two load cases are considered: production in summer/installation in winter, and production in summer/installation in summer. to ensure a uniform design of insulating glass, the relevant standard parameters as well as the increase or reduction in temperature for non-standard situations are given in din 18008-1:2010 (see table 1). besides providing an approach for calculating the isochoric pressure p0 , din 18008-1:2010 also contains an extensive number of 94 t. wüest and a. luible / increased thermal induced climatic load in igus table 1 climatic load cases according to din 18008-1:2010 combination of actions �t k �pmet kn/m2 �h m “summer” +20 –2.0 +600 part in equation 1 0.34*�t=6.8kn/m2 –�pmet =2kn/m2 0.012*�h=7.2kn/m2 “winter” –25 +4.0 –300 part in equation 1 0.34*�t=–8.5kn/m2 –�pmet =4kn/m2 0.012*�h=3.6kn/m2 formulae to determine the load distribution on double-glazed units (dgu’s), which take into account the coupling effect between the glass panes. the coupling effect describes the specific feature of igu’s to transmit external loads from one pane to the next. the approach by feldmeier (2006) is generally used to estimate the load distribution on triple-glazed units (tgu’s). specialized software has been developed for this approach due to the complexity of the calculations necessary. the currently valid calculation values for �t were determined for double-glazed units with ug =1.8w/m2k. because of a lack of rules to the contrary, the values are currently also being used for the design of triple-glazed units with ug<<1.0. despite the rising popularity of tgu’s no studies exist with respect to the applicability of the dgu calculation values to tgu’s. only feldmeier (7/2009) mentions that the climatic load for triple-glazed units is slightly higher in summer. in practice, however, calculations using the standard cases according to the trlv:2006, with p0 = ±16hpa, are still allowed. in specific cases these calculations need to be checked and reviewed thoroughly. the european pre-standard pren 16612:2013 provides analytical formulae to determine the temperatures in tgu’s that do not have any additional elements such as solar shading and glare protection. this standard, however, contains an example calculation for tgu’s which still uses the standard value p0 =–16kpa as given in the trlv 1998. the design provisions in the trlv:2006/din 18008-1:2010 apply only to dgu’s with ventilated or non-ventilated internal glare protection, higher absorption classes, and thermal insulation behind the dgu. according to equation 1 and table 1, temperature change constitutes a significant percentage (43% in summer 6.8/(6.8+2+7.2) and 53% in winter, 8.5/(8.5+4+3.6) of the climatic load for the standard cases in din 18008, which is why a more in-depth investigation is justified. the mean gas temperature in igu’s has the largest impact on the climatic load and is also the parameter that is most influenced by its immediate environment. the temperature strongly depends on the selected igu system (pane thicknesses and coatings) as well as the climatic and façade construction conditions. hence, this is the focus of the present investigation. the standard values for calculating the climatic load have been adopted in german technical reference books and standards since 1998 (see for example trlv (1998)). even when the deterministic design approach from the trlv was replaced with the semi-probabilistic design approach in din 18008-1:2010 (the same as in the eurocodes), the values remained unchanged. for the latter approach, load safety factors needed to be considered at climate loads. since the mean duration strength of float glass virtually remained the same, the load case “climate” became very important in the design of insulated glass. only two publications regarding the climatic load for igu could be found in a literature survey. two contributions by penkova et al. (2013, 2014) deal with the thermal loading of igu’s, with a particular focus on fe modelling of flat and bent dgu’s. the authors currently do not know of any general observations or estimates for gas temperatures of igu’s in advanced facade types, for example for tgu’s and double-skin facades (dsf’s). t. wüest and a. luible / increased thermal induced climatic load in igus 95 1.3. objectives the present study deals with the climatic loads for igu’s, focusing on the impact of different layer configurations in modern facade systems. the boundary conditions for igu’s used in modern naturally, mechanically, or non-ventilated double-skin facades are not the same as the boundary conditions for igu’s used in windows. therefore, a detailed review of current standards will be conducted. the objectives of this study are – to analyse how dgu’s and tgu’s heat up under standard conditions in summer. – to apply the gained knowledge to modern facade systems. the behaviour of different systems will be illustrated, and suggestions will be made on how they can be dealt with in practice. a user-friendly and powerful calculation model is to be developed which will allow the determination of the temperature evolution in igu facade systems under different climatic and construction conditions. the model is expected to illustrate all energetic processes in facade systems in accordance with the current standards. it is not required to yield results of the highest accuracy – a reasonably accurate representation of the processes under the given conditions, accurate enough for practical applications, are sufficient. the results of the igu-temperature study will be discussed in consideration of current design standards for igu’s and those limitations. suggestions for further improvements of structural igu design will be made, if necessary. 2. method 2.1. model in this section, the physical basis and relevant provisions from the relevant standards required for the development of a calculation tool are presented. first, the physical principles of heat transfer are presented, as well as the calculation approach to determine the thermal resistances of gases and surfaces. thermal as well as solar characteristics calculations are required to determine the solar energy flow in igu’s. the implementation of a simplified optical model is also presented. finally, the implementation of the iterative calculation tool is shown and illustrated with a flow chart. the following calculations refer to the overview flow chart shown in fig. 3. 2.1.1. heat flow balance the fourier differential equation (equation 2) combined with the law of conservation of energy is used for heat conduction calculations. for facade elements, it is sufficient to consider onedimensional heat transfer in through-thickness direction. by eliminating the time dependency, the differential equation can be simplified further, and the expression in equation 3 for the heat flow density q̇ is obtained. ∂t ∂t = λ ρ · c · ( ∂2t ∂x2 + ∂ 2t ∂y2 + ∂ 2t ∂z2 ) (2) q̇ = λ d · �t (3) because of the relatively small thermal time constant τ, it is justified to consider the thermal processes in igu as stationary. the thermal time constant indicates how quickly a material reacts to changes in environmental temperature. according to equation 4 with a=53 × 10–8 m2/s for float 96 t. wüest and a. luible / increased thermal induced climatic load in igus glass, the thermal time constant is approximately 3 minutes for a 10mm thick glass pane. the meteorological data available for simulations is generally collected in a one hour interval, a time period that far exceeds the thermal time constant of glass. further, the temperature increase due to solar radiation does not only take place at the glass surface but also over the entire material cross-section. in practice, the temperature adjustment due to solar radiation will be less time consuming than described in equation 4 from (manz h., 2010): τ ≈ d 2 a = 0.01 2 53 · 10−3 = 189s ≈ 3 min (4) the finite difference method is well suited for solving the thermal conduction equations numerically. to do this, the facade is discretized into a suitable number of nodes. because of the linear nature of thermal conduction, it is not necessary to carry out a highly detailed discretization, which reduces the computational effort. the nodes are located at the internal and external boundary conditions, the mid-points of each layer, and at the interfaces between two layers, with the two layers sharing the interfacial area. the number of nodes n for n layers of material is given by equation 5. n = 2 · n + 3 (5) the sum of all heat flux q̇ must be zero for each node i:∑ q̇i = 0 (6) therefore, the heat flows from node i to the neighbouring nodes i-1 and i+1 are:∑ q̇i = q̇i,i−1 + q̇i,i+1 = 0 (7) it can be assumed that the incoming heat flows carry a positive sign and the outgoing heat flows carry a negative sign. equation 7 can be extended to include external thermal flows by considering heat sources or heat sinks due to ventilation or solar gain:∑ q̇i = q̇i,i−1 + q̇i,i+1 − q̇ex,i = 0 (8) ∑ q̇i = 1 ri,i−1 · (ti − ti−1) + 1 ri,i+1 · (ti − ti+1) − q̇ex,i = 0 (9) the linear system of equations can be written as the following matrix (equation 10): ⎡ ⎢⎢⎢⎢⎢⎢⎢⎢⎣ −1 r1,0 ( 1 r1,0 + 1 r1,2 ) −1 r1,2 0 0 . . . 0 −1 r2,1 ( 1 r2,1 + 1 r2,3 ) −1 r2,3 0 . . . 0 0 −1 r3,2 ( 1 r3,2 + 1 r3,4 ) −1 r3,4 ... ... ... ... ... ... ... ⎤ ⎥⎥⎥⎥⎥⎥⎥⎥⎦ . ⎡ ⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣ t0 t1 t2 t3 t4 ... ⎤ ⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦ + ⎡ ⎢⎢⎢⎢⎢⎣ q̇ex,1 q̇ex,2 q̇ex,3 ... ⎤ ⎥⎥⎥⎥⎥⎦ = ⎡ ⎢⎢⎢⎢⎢⎣ 0 0 0 ... ⎤ ⎥⎥⎥⎥⎥⎦ (10) due to the direction dependence of the thermal conductivity the thermal resistances are: 1 ri,i−1 = 1 ri−1,i (11) the system of equations (equation 10) can be solved numerically for any number of nodes n. the thermal resistances of the gas and air layers are dependent on their respective temperatures. the heat transfer coefficients and cooling efficiency of rear ventilations also refer to the system t. wüest and a. luible / increased thermal induced climatic load in igus 97 temperatures. hence, the system of equations must be solved iteratively. alternatively, the individual node temperatures can be included into the calculations by direct iteration. by rewriting equation 9, an equation for the temperature of node ti (see fig. 3) is obtained, where all heat flows and thermal resistances in dependence of their neighbouring nodes ti–1 and ti + 1 are taken into account. equation 12 can easily be solved iteratively. ti = 1 ri,i−1 ti−1 + 1ri,i+1 ti+1 − qex,i( 1 ri,i−1 + 1 ri,i+1 ) (12) 2.1.2. thermal resistances of gases and surfaces the thermal behaviour of the gases inside insulated glass is well known, and the approach for calculating the ug-value under standard conditions is set out in en 673:2011. if the temperature in the gc changes, the gas properties (xgas) also change, which in turn influences the thermal resistance of the gc (rgas) and the temperature calculations. this can be taken into account iteratively by using the temperature-dependent gas properties (xgas(t)) according to en 13363-2, appendix d, and equation 13 in en 673:2011 calculations. compared to fig. 3, this is used in nodes t4 and t8. xgas (t ) = xgas,20 + ∂x ∂t . (t − 293) (13) the internal and external heat transfer coefficients of the glass surfaces are given in en 673:2011. the contribution of the radiation to the surface heat transfer coefficients (hi, he), however, is determined according to en iso 6946:2007 (equation 15 and 16, instead of en 673) as the temperature-dependent radiation of a black body (equation 14). en iso 6946 states that the external and internal radiation temperatures can be assumed to be approximately equal to the respective air temperatures. hs (t ) = hc + ε · 4 · σ · t 3m (14) hi hs = hs (t ) + hci = hs (t ) + 2.5 hs (t ) + hce = hs (t ) + 4 + 4 · υ (15) tm = text + ts 2 (16) 2.1.3. optical model in order to carry out calculations for different facade configurations, an optical model needs to be integrated into the calculation model. the solar and optical properties are independent of the intensity of the solar irradiation and the prevailing system temperature. the solar characteristics of igu’s can be calculated according to en 410:2011; those of igu’s combined with solar protection devices can be determined according to en 13363-2:2005. the equation system in equation 17, developed by stoll (2005) and visualized in fig. 2, can be extended to any number of layers n. with the initial conditions i0 =1 and i’n =0 the system of equations can be solved to determine the layer absorptions �li (see fig. 3). 98 t. wüest and a. luible / increased thermal induced climatic load in igus i ′ j−1 = ρj · ij−1 + τj · i ′ j ij = τj · ij−1 + ρ′j · i ′ j aj = ( 1 − τj − ρj ) ∗ ij−1 + (1 − τj − ρij) · i ′j (17) fig. 2. schematic of multiple reflection in a facade with two transparent layers. en 13363-2 allows the use of integral values for the calculation if no spectral values are available, which is often the case for solar protection devices. in practice, often no differentiation is made between the luminous and solar characteristics. in this study, typical characteristic values for solar protection devices according to en 13363-1, appendix a, are used for a general examination of this issue. the values given are integrated values assuming that the difference between the properties of light and radiation are negligible. to minimize the computational effort and the size of the included material library, it was investigated whether integrated characteristic radiation properties could also be used for glass layers. the integrated values for different glass layers were calculated with the web application glace (glas trösch ag, 2015) and included in the material library. a comparison of the results for different igu configurations showed very small differences between the calculations with spectral values in glace and integrated values with this calculation model, as can be seen in table 2. the results for the total absorption using the integrated values according to en 13363-2, which was used in this publication, are marginally higher than those determined with glace, using spectral values. these higher absorption values also result in higher temperatures in igu’s. even with this small difference, however, the results are on the safe side and therefore acceptable. the differences in radiation transmission are somewhat higher, but they are of less importance for the present investigation. 2.1.4. natural ventilation model in double-skin facades (dsf) with a connection to the external air, thermal pressure differences create an air flow called the stack effect (at node t4 in fig. 3). assuming that the air current is a uniform piston flow, air velocity, heat transfer, and heat loss can be calculated with the model given in en 13363-2. the given formulae can be solved iteratively or as a system of equations (equation 18). t. wüest and a. luible / increased thermal induced climatic load in igus 99 0 = v2h · ( ρlzo 2 + ρlzo · z ) + vh · 12·μ·hhs2 − ρlzo·to·g·hh·|tg−te|tg·te tg = tm − htphh · (tout − tin) tout = tm,j − ( tm,j − tin ) · e−hh/htp htp = ρl(tg)·cl·s·υh2·hch = ρ(tg)·cl·s·υh 2·(2·hg+4·υh) (18) the temperatures and cooling capacity are determined for the mid-section of the ventilated area and are given in equation 19: qh = hch · ( 2 · tg − tj − tj+1 ) (19) for a naturally ventilated double-skin facade extending over one story, the height of the ventilated space and the gap width for ventilation are assumed to be 3m and 150mm, respectively. these are very common values used in state of the art facade construction. the ventilation of interior glare protection can also be designed with this model. to model interior rear ventilation, a height of the ventilated space of 2.5m and a ventilation gap width of 20mm are used. in both cases, the lower and upper rear-venting openings are considered with 20mm. 2.1.5. iterative implementation as described in the previous sections, different calculation models are combined for this study and, due to the multifaceted temperature-dependent properties, calculated iteratively. combining iterative calculations with explicit analytical formulae is a demanding challenge, which is why it was decided not to complicate the implementation further by using programming languages. a powerful implementation was achieved using microsoft excel with visual basic for applications (vba), with up to 32,767 iterative steps per calculation. a further advantage is the flexible and user-friendly user interface which makes it possible to include both preand post-processing within the same software environment. figure 3 shows how the elements of the excel tool are related to each other, using the example of a simple facade – a naturally ventilated double-skin facade with a single glass pane on the outside and a dgu on the inside. the nodes are shown as temperature nodes (circles), and the thermal resistances are shown as squares. the arrows represent dependencies in the calculation, and double arrows signify an iterative calculation. the required input data includes the facade layers, the internal air temperature as well as global radiation, and external air temperature. the facade configuration can range from a single glass pane with external solar protection or internal glare protection to open/closed double-skin facades with solar protection in the cavity, a tgu, and internal glare protection. the material thickness, thermal conductivity, optical properties, and surface emissivity of each solid layer are taken from table 2 comparison of spectral and integral radiation transmission and absorption glazing calculation radiation transmission total absorption dgu with low-e, pos. 3 glace 50.0% 18.5% calculation model 49.4% 18.3% tgu with low-e, pos. 3&5 glace 37.2% 26.5% calculation model 31.0% 27.7% 100 t. wüest and a. luible / increased thermal induced climatic load in igus fig. 3. schematic of nodes and iteration loops. the material library and used in the calculation. the properties of the gas layers are also taken from the material library or calculated from data in the library. the calculation steps are as follows: 1. radiation absorptions �li based on the solar characteristics of all layers. 2. solar heat gain calculation in layers i. 3. radiative transitions based on the emissivity of opposite surfaces. 4. thermal conductivity of the gas layers. 5. rear ventilation in the dsf cavity and resulting cooling performance in the dsf rear-ventilated space. 6. rear ventilation between facade and inner solar protection and resulting cooling performance in the rear-ventilated space of interior blind. 7. balance the temperature nodes, using the internal heat sources or heat sinks from step 2, 5 and 6 as well as the surrounding thermal resistivity values. 8. repeat steps 3–8 until the convergence criterion of step changes <0.00001 is met. t. wüest and a. luible / increased thermal induced climatic load in igus 101 2.1.6. accruals 2.1.6.1. emissivity in the literature, for example in frank (2010), the emissivity of a diverse range of materials and paints is generally assumed to be 0.95 (=95%). the surface emissivity of uncoated glass is 0.837 (=83.7%) according to en 673:2011. the emissivity of glass coatings can be determined with standardized calculations given in en 673:2011. depending on the product, the resulting emissivity is 0.01–0.02 (=1–2%). as worst case for overheating the lower value of 0.01 is used in the present contribution (see chapter 3.2.1). 2.1.6.2. fresnel the angle of incidence of the radiation influences the transmission properties of glass panes. according to fresnel’s law, the transmission decreases with increasing angle of incidence with respect to the surface normal. the properties are almost constant up to an angle of approximately 45◦. with increasing angle of incidence, the transmission decreases and the reflection increases. the absorption values, which are of major importance for this research, are constant up to 70◦. hence, it is not deemed necessary to implement fresnel’s law. 2.2. facade studies 2.2.1. existing knowledge the goal of this investigation is to increase the existing amount of temperature data of gases in igu’s and to optimize the calculation of the climatic load. to do this, the basis of the calculation for the temperature values according to din 18008-1:2010 has to be taken into account. the requirements of the trlv:2006/din 18008-1:2010 are to form the basis for assessing the influence of different igu’s in different facade configurations. the winter conditions do not include solar radiation, and the stipulated external and internal air temperatures are ϑe =–10◦c and ϑi =19◦c, respectively. the external and internal heat transfer resistances are rse =0.04m2k/w and rsi =0.13m2k/w, and the u-value of the dgu is supposed to be 1.8w/m2k. the summer conditions are based on an irradiation of 800w/m2 at an angle of incidence of 45◦ and with an absorptivity that is 30% for a dgu. external and internal air temperatures of ϑe = ϑi =28◦c are used. the heat transfer resistances are set at rse =rsi =0.12m2k/w. the applicability of these conditions is discussed later in this paper. 2.2.2. extensions of the system even though the trlv assumes single-skin facades (ssfs) with dgu’s, this assumption no longer represents the use of igu’s in modern facade construction. over the last years there has been a tendency to use tgu’s in facades. when determining the climatic load, tgu’s are often likened to dgu’s: “removing the middle glass pane, one ends up with double glazing!” (feldmeier, 7/2009, quote translated from german). this simplification is valid for loading of the external panes under identical isochoric pressure. whether it also applies to tgu’s will be investigated in this study. many modern office buildings have large glass facades, sometimes double-skin facades. doubleskin facades consist, simply put, of a facade with an additional glass skin in front of it. the external solar protection elements are located in the cavity between the external and the internal glass panes, which affords them with significantly improved wind and weather protection. double-skin facades are generally ventilated naturally, so that an air current develops between the two facades due to the natural, thermal driven air currents. various investigations and publications have dealt with the thermal behaviour of dsf’s. balocco (2002) and manz and frank (2005), for example, both 102 t. wüest and a. luible / increased thermal induced climatic load in igus fig. 4. investigated facade configurations containing doubleand triple-glazed units. state that buildings with dsf’s show a significant tendency to overheat in summer. according to pasquay (2003), the temperatures in the cavity rise to 10–15k above the ambient temperature even in well-ventilated dsf’s. therefore, the energy loss from the igu to the outside is hampered severely, and heat flows towards the interior of the building. the closed dsf, called closed-cavity facade (ccf’s), is a relatively new facade type. one of the first ccf’s was built in 2009 by josef gartner gmbh at roche headquarters in rotkreuz, switzerland (burkhard + partner ag, 2015). in this type of dsf the cavity between the interior and exterior skins is completely closed. a small current of air-conditioned air is fed into the cavity, which prevents the entry of humidity and dirt. because of the missing strong air current, the cavity can heat up even more, in particular when the solar shading louvers are closed. the objective of this study is to extend the thermal investigations to dgu’s and tgu’s in different types of vertical facades. to do this, the different combinations of layers are subsumed in a combination matrix; cf. figure 4. all the combinations of internal and external constructional boundary conditions are represented. the internal constructional boundary conditions are shown t. wüest and a. luible / increased thermal induced climatic load in igus 103 in the first column (vertical); the external constructional boundary conditions are shown in the first row (horizontal). internal constructional boundary conditions, from top to bottom: 1) none; 2) ventilated glare protection; 3) non-ventilated glare protection. external constructional boundary conditions, from left to right: a) none; b) external solar protection; c) ventilated dsf without solar protection; d) ventilated dsf with solar protection; e) ccf’s without solar protection; f) ccf’s with solar protection. the combinations of transparent external solar protection and internal solar protection devices have not been considered. 2.2.3. approach in a first step, the standard conditions according to din 18008-1:2010 are employed in chapter 3.1. they serve as reference conditions against which the impact of changes to the systems can be assessed. furthermore, the properties of insulated glass are updated to reflect the state of the art. chapter 3.2.1 describes the extension of the calculations to tgu’s. in 3.2.2 dgu’s and tgu’s of different thicknesses are investigated. the influence of the glass thickness and absorption are then compared with the stipulations from din 18008-1:2010. in chapter 3.3 the impact of changes to the constructional boundary conditions according to fig. 4 is investigated. initially, single-skin facades with internal and external solar protection devices are examined, followed by double-skin facades. investigations are carried out for the different facades containing dgu’s or tgu’s. comparing the results with those from the single-skin calculations obtained in step 1, it can be seen if there is a significant increase in temperature. 3. results 3.1. comparison of current glazing with stipulations from relevant standards first, the gas temperatures for igu’s stipulated in din 18008-1:2010 are considered. to verify the suggested approach and allow a comparison with current boundary conditions, a detailed analysis of the boundary conditions from the standard is carried out. following this, the boundary conditions are modelled with the calculation program. by incrementally altering the standard boundary conditions to the actual boundary conditions, the influence parameters are determined. it is possible to model the “standard situations” according to the trlv/din 18008-1:2010, section 2.2.1. while a lot of information is available, the emissivity of the coating inside the glazing cavity is not known. it is permitted to choose the emissivity value so that the u-values for both summer and winter load case are 1.8w/m2k. when also considering the temperature-dependent gas properties, the required emissivity of the coating in summer is determined to be approximately � =0.2. the stipulated limit of 30% absorption for summer is exceeded slightly when 2×10mm float glass is used (32.4%). it is not even possible to reach an absorption value of >50% in a dgu consisting of 2×19mm float glass (radiation absorption according to glace 44.8%). the trlv prescribes a mean gas temperature at the construction site of 2◦c in winter; the calculation with the presented model yields the same result (2.2◦c). in a modern dgu with a lowe (low-emissivity) coating (� =0.01) a temperature of 3◦c is reached, which deviates less than 1k from the stipulated value in the standard situation. the situation is somewhat different for summer conditions, which include the impact from radiation. the radiation intensity of 800w/m2 and the irradiation angle must be converted in order to be used in the energy balance. the intensity of the solar radiation on an inclined surface can be calculated with a simplified approach, using the cosine of the irradiation angle (equation 20). this irradiation is used for all cases; the influence of the angle-of-incidence is negligible (see section 2.1.6.2). 104 t. wüest and a. luible / increased thermal induced climatic load in igus fig. 5. comparison of glazing according to the trlv under summer conditions with a) 2×10mm, b) 2×4mm and c) 2×4mm with he/hi according to en 673:2011. ia = itrlv · a = 800w / m2 · cos (45◦) = 565.7w / m2 (20) the external and internal heat transfer resistances are given as re =ri =0.12m2k/w in the trlv, resulting in a mean gas temperature of 39◦c (see fig. 5a). this result can be confirmed by a quick calculation by hand using equations 21–23: qin = qout (21) 30% · 800w / m2 · cos (45◦) = 2 · 1 0.12m2k/w · �t = 8.33w / m2k · (tgas − tenv) (22) 0.3 · 800w/m2 · cos (45◦) 2 ∗ 8.33w/m2k + 28 ◦c = 38.2◦c (23) in modern dgu’s with low-e coatings (� =0.01) 10mm float glass is rarely used. it is more common to use 2×4mm float glass, for which the total absorption is approximately 18.3%. therefore, the temperature in the glass cavity drops to 34.3◦c; see also fig. 5b. the difference to the ambient temperature hence decreases from 11k to 6.3k. using the internal and external heat transfer resistances according to en 6946 (approximately rse =0.04m2k/w and rsi =0.13m2k/w according to en 673:2011) the temperature decreases even further, to 32.3◦c (see fig. 5c). this value is approximately 7k lower than the stipulations found in the trlv:2006/din 18008-1:2010. for the climatic load in summer according to equation 1, a temperature difference �t of +20k is used (39◦c – 19◦c). using the presented calculation, however, this difference is reduced to 13k (32◦c – 19◦c). as a consequence, the isochoric pressure resulting from the change in temperature decreases by an impressive 35%, and the total climatic load for summer drops by 15%. 3.2. extension of the calculations to tgu’s the igu system changes due to the addition of another glass pane and gas cavity. according to a publication by feldmeier (2011), it is well known that the climatic loads are higher for tgu’s due to their larger gas volumes. in this section, it is investigated how the additional glass cavity and middle glass pane change the thermal behaviour of tgu’s compared to dgu’s. t. wüest and a. luible / increased thermal induced climatic load in igus 105 fig. 6. comparison of modern doubleand triple-glazing units under trlv:2006 summer conditions, with he/hi according to en 673:2011. 3.2.1. behaviour of simple tgu’s for all further calculations, the thermal transfer resistances are calculated according to section 2.1.3, using 4mm float glass. the gas cavities are assumed to be 14mm and filled with 90% argon. a low-e coating with � =0.01 is assumed in position 3 in dgu’s and in positions 2 and 5 in tgu’s. a tgu with 3×4mm float glass has a total radiation absorption of 29.3%, which is just below the 30% threshold. because of the two gas cavities, tgu’s exhibit a temperature gradient with two different mean gas temperatures in winter. while the mean gas temperature in a dgu is 2.9◦c, the mean temperatures in a tgu are –2.6◦c and 10.4◦c. the winter load case according to the trlv stipulates a temperature of 27◦c at production and 2◦c at installation. instead of this temperature decrease of –25k, two temperature drops of –29.5k and –16.5k are considered for a tgu. this results in an isochoric pressure difference of �p0 =4.4hpa between the two gas cavities. tgu’s also react differently under summer boundary conditions than dgu’s. as can be seen in fig. 6, the additional middle pane also absorbs solar energy and heats up. further, the two gas cavities and low-e coatings make it more difficult for the middle pane to release energy, which must be compensated with a greater �t according to fourier’s equation for thermal conduction (equation 3). therefore, the middle pane experiences a significantly higher temperature increase than the external panes, which leads to the higher mean gas temperatures in tgu’s compared to dgu’s. 3.2.2. influence of the glass thickness the influence of the glass thickness on the temperatures in igu’s is clearly illustrated in fig. 5 for different configurations of dgu’s. the glass thickness and the resulting absorption determine the temperature increase according to physical laws. the behaviour of dgu’s and tgu’s with glass panes of different thicknesses has not been investigated so far. for winter conditions, such an investigation is not necessary, as the thermal resistance of glass (rj =1.0mk/w according to en 673:2011) only adds a minimal contribution to the total thermal resistance of igu’s. to illustrate the influence of the glass thickness, a comparison of differently configured dgu’s and tgu’s under summer conditions is carried out with the presented calculation program. the dgu’s are compared with a symmetrical configuration consisting of x mm float glass/gc/x mm float glass. for tgu’s two different configurations are used: thicker exterior glass pane: x mm float glass/gc/4mm float glass/gc/x mm float glass thicker middle glass pane: 4mm float glass/gc/x mm float glass/gc/4mm float glass 106 t. wüest and a. luible / increased thermal induced climatic load in igus fig. 7. comparison of temperatures in the glass cavity of dgu’s and tgu’s. figure 7 shows that under identical conditions the temperatures of tgu’s are significantly higher than those of dgu’s. the curves are approximately linear. the results become even clearer if the linear slope coefficient of a linear equation of the form y=m * a + b is considered. the mean slope m of the temperature curve for the dgu is 0.36k/mm, and that of the curve for the tgu with the thicker exterior glass panes is 0.27k/mm. this shows that thicker exterior panes have a smaller influence on the temperatures of a tgu with respect to a dgu. in contrast, the middle pane has an extremely strong influence on the temperature of the tgu; the slope of 1.06k/mm is more than three times greater than that of the other configurations. 3.3. influence of layer configuration just as temperature gradients are related to the characteristics of dgu and tgu systems, modifications in the facade system itself also lead to differences in temperature distributions. this section deals with the impact of internal and external solar protection devices. an investigation of the temperatures in modern double-skin facades with and without internal solar protection follows. both dgu’s and tgu’s and their temperatures are examined with respect to igu’s without solar protection. 3.3.1. solar and glare protection devices the influence of internal solar protection devices is considered in the trlv. the temperature to be added to �t for summer conditions is 9k for ventilated solar protections (=48◦c in the gas cavity) and 18k for non-ventilated solar protection (=57◦c in the gas cavity). the comparative calculations show that these values can indeed be reached with opaque, black internal solar protection (based on the trlv summer scenario including the boundary conditions). the investigations using the presented calculation program show that the results are strongly dependent on the chosen transmission and colour properties of the solar protection. with increasing solar absorbance, a temperature increase can be observed in dgu’s. without solar protection, temperatures of 32◦c are reached. however, if solar protection is considered, temperatures of 37– 42◦c and 40– 46◦c can be reached in systems with ventilated and unventilated interior solar protection devices, depending on the colouring. the temperature increase is hence significantly smaller than that stipulated in the trlv because of the distinctly higher energy transfer to the outside, which is due to the heat transfer coefficient for external surfaces he given in en 673:2011. t. wüest and a. luible / increased thermal induced climatic load in igus 107 fig. 8. tgu with (a) interior and (b) exterior black, opaque solar protection. there are no stipulations for exterior solar protection devices in the trlv. depending on the facade configuration, however, they can also cause a temperature increase in dgu’s. depending on the colour intensity of the solar protection, the temperatures can be 0–5k higher in single-skin facades than the 32◦c shown in figs. 5 and 6. as exterior solar protection devices are more often opaque than not, a temperature no higher than 37◦c should be used in the calculation, even for glass with higher absorption. because this temperature is lower than the original 39◦c stipulated in the trlv, no relevant rules can be found in this guideline. the influence of exterior solar protection devices in double-skin facades is discussed later in this article. the thermal behaviour of tgu’s with interior solar protection devices is somewhat different than that of dgu’s. the presence of internal solar protection leads to higher gas temperatures, with lighter-coloured materials inducing higher temperatures. the reflection of the solar protection increases with lighter colours, thereby increasing the absorption of the middle pane, which is already subjected to higher thermal loads. depending on the colouring of the solar protection, the mean gas temperatures increase from approximately 39◦c (cf. fig. 6b) to 44–51◦c and 45–56◦c (cf. fig. 8a) in systems with ventilated and non-ventilated interior solar protection devices, respectively. the inner gas cavity heats up slightly more than the outer one, which leads to asymmetric climatic loading of the two gas cavities. the calculations for a tgu with external solar protection yield different results. in the case of opaque protection, except for black opaque elements according to fig. 8b, the gas temperatures are lower than the reference temperatures of 38.7◦c/39.3◦c of the uninfluenced tgu according to fig. 6b. in systems with elements of medium and high transparency, the temperature in the outer glass cavity increases by 1–7k, while the temperatures in the inner glass cavity remain below 39◦c. a distinct temperature gradient is present in the facade, where the lowest temperatures are found towards the interior of the building. figure 8 shows the temperatures under summer conditions in a tgu with exterior and interior solar protection devices. 3.3.2. double-skin facades (dsf) when calculating the u-value of glazing in a dsf it is common practice to set the heat transfer coefficient for the external igu surfaces equal to that for the internal surfaces (he =hi =0.13m2k/w). this approach also yields good approximate values for layer temperatures without any other influences like shading devices. temperatures of 34.6◦c and 46.2◦c/42.3◦c are determined for the dgu and tgu, respectively. the exact calculations with the model of natural rear ventilation according to en 13363-2 yield temperatures of 35.7◦c and 46.7◦c/41.3◦c for the dgu and the tgu, respectively. the results differ merely by 1.1k (dgu) and <1k (tgu). this simplification, however, is only applicable to the simplest case of a dsf. if solar protection in the dsf is taken into account as well, the system changes significantly and the simplified approach no longer applies. the following calculations are based on external glazing 108 t. wüest and a. luible / increased thermal induced climatic load in igus table 3 mean gas temperatures in the igu of a naturally ventilated facade, including exterior solar protection devices solar protection gas temperature in dgu gas temperature 1 in tgu gas temperature 2 in tgu none 35.7◦c 46.7◦c 41.3◦c white colour 38.1◦c 43.3◦c 34.2◦c pastel colour 42.7◦c 50.1◦c 37.0◦c dark colour 46.8◦c 56.2◦c 39.6◦c black colour 50.7◦c 61.9◦c 41.9◦c table 4 mean gas temperatures in the igu of a ccf’s with exterior solar protection devices solar protection gas temperature in dgu gas temperature 1 in tgu gas temperature 2 in tgu none 36.7◦c 49.6◦c 42.4◦c white colour 40.6◦c 47.2◦c 35.8◦c pastel colour 46.3◦c 55.9◦c 39.5◦c dark colour 51.6◦c 63.8◦c 42.8◦c black colour 56.5◦c 71.1◦c 45.8◦c consisting of 8mm float glass, a cavity of 150mm, and internally installed solar protection. when the solar shading louvers are closed, only the outer cavity is ventilated, and the air volume between solar protection and igu is considered unventilated, containing a resting air layer (see also fig. 4). the calculated mean gas temperatures of the dgu inside the dsf range from 35.7◦c (no solar protection) to 38.1◦c (white solar protection) and 50.7◦c (black, opaque solar protection). if the dgu is replaced by a tgu, the temperatures increase to 46.7◦c/41.3◦c (no solar protection) and 61.9◦c/41.9◦c (black, opaque solar protection). these temperatures are considerably higher than those of a single-skin facade according to fig. 6b. the temperatures for systems with differently coloured solar protection according to en 13363 are summarized in table 3. 3.3.3. closed-cavity facades (ccf’s) in a closed-cavity facade the cavity is not ventilated. the mechanical aeration is very small and is thus not capable of cooling the facade cavity. therefore, higher temperatures are to be expected in this closed system. the air volumes between the exterior and interior glazing are considered as layers of resting air. even without solar protection devices in the cavity, the gas temperatures are marginally higher than those in a ventilated dsf – 36.7◦c in the dgu (+1k) and 49.6◦c/42.4◦c (+3.1k/+1.1k) in the tgu. the temperature increase is significantly higher if the louvers of the solar protection are closed. for the extreme case of black, opaque sun protection in the ccf’s the temperatures in the dgu and tgu are 56.5◦c and 71.1◦c, respectively. further results can be found in table 4. 3.4. summary of the results this section presents a summary of all calculated temperatures. next to showing the temperatures for the previously analysed cases, figs. 9 and 10 also contain results for further cases, such as external and internal, partly transparent, and differently coloured solar protection devices. t. wüest and a. luible / increased thermal induced climatic load in igus 109 fig. 9. calculation results overview for dgu’s in different facade systems. 3.4.1. double-glazed units the mean gas temperatures in a dgu consisting of the commonly used 2×4mm float glass panes are significantly lower than those stipulated in din 18008-1:2010. this includes taking into account the heat transfer resistances according to en 6946/637:2011. the presented calculations also show that the influence of ventilated and non-ventilated interior shading elements is smaller than that estimated from din 18008-1:2010. the differences are most pronounced, however, for facade systems that have not been included in the trlv:2006/din 18008-1:2010. if exterior solar protection is installed on a single-skin facade, significantly higher temperatures can be reached than in an individual dgu. those temperatures, however, are still below the current design temperature of 39◦c. it is shown that the temperatures are lowest in the single-skin facade and higher in the double-skin facade, and that the highest temperatures occur in the closed-cavity facade. dgu’s with thin glass panes in double-skin facades without solar protection devices are not relevant for climatic load design. for dsf’s and ccf’s, the presence of exterior solar protection can be relevant for the design, and the temperatures can be even higher than those for the case of interior solar protection. the results for dgu’s consisting of 2×4mm float glass panes are shown in fig. 9. 3.4.2. triple-glazed units the trlv:2006/din 18008-1:2010 do not deal with the mean gas temperatures in tgu’s. the results of the presented calculations show that, under din 18008-1:2010 boundary conditions, the temperatures in a tgu consisting of 3×4mm float glass (<30% total absorption) are the same or higher than those stipulated in the current trlv (39◦c for <30% absorption). although the effect of exterior solar protection devices is insignificant in single-skin facades, it is considerably relevant for dsf’s and ccf’s. the results for tgu’s used in different facade configurations are shown in fig. 10. the same markers are used for both gas cavities – the outer cavities are shown on the left-hand side, the inner cavities on the right-hand side. for a distinct overview, associated pairs are not shown here. it can clearly be seen that only 22 of the 222 calculated temperatures in the 111 different facade systems are below 39◦c, mainly those of systems with exterior solar protection. generally, the temperatures in tgu’s are significantly higher than those in dgu’s (see fig. 9). this agrees with the previous results, which indicate that multilayer facades tend to exhibit considerably higher temperatures. 110 t. wüest and a. luible / increased thermal induced climatic load in igus fig. 10. calculation results overview for tgu’s in different facade systems. 4. discussion 4.1. facade configurations in the present study, many facade configurations were examined which are either not considered, or explicitly excluded from consideration in din 18008-1:2010 (see fig. 4). the trlv:2006 and din 18008-1:2010 do not provide values for tgu’s; however, in 18008-2:2010 the reader is referred to feldmeier (2006) for information on the load distribution for tgu’s. as tgu’s are in absorption class 30–50% according to din 18008-1:2010 (=48◦c gas temperature), many examined facade systems are nevertheless covered in this standard. as mentioned in section 3.2.2, however, this classification does not apply to all tgu’s and results in temperatures that are much too high or too low for some facade systems. according to fig. 4, 18 combinations of an igu with exterior or interior solar protection devices exist in modern facade construction. three of them can be dealt with using the stipulations in the trlv:2006/din 18008-1:2010 as mentioned above, and one can be neglected because its results are always less severe than those of other systems. figures 9 and 10 show that an igu is subjected to different loads depending on whether it is part of a singleor double-skin facade. it is hence necessary to treat the two facade systems separately. 4.2. safety factors passing from the trlv:2006 to din 18008-1:2010, the calculation of the climatic load did not change – the relevant values were transferred without any change. due to the transition to the semi-probabilistic design concept load safety factors �q are now applied to the climatic load. the design resistances of float glass according to din 18008-1:2010, however, are still at the level of the permissible stresses used in the deterministic safety concept. this has resulted in the climatic loading being the governing design load case since din 18008-1:2010 was introduced in germany. reick (2015) claims that the climatic load according to din 18008-1:2010 is being judged too harshly, as the complaints rate has been very low in the past. in this study, this claim is confirmed for standard dgu’s. it is shown, however, that the safety factor �q =1.5 probably saved many tgu’s from damage, which had been designed using +20k for the summer load case. in pren 16612:2013 and pren 13830:2013 lower safety factors �q are suggested for infills with class of consequences cc1 (low consequence for loss of human life, and economic, social or environmental t. wüest and a. luible / increased thermal induced climatic load in igus 111 consequences small or negligible en 1990:2010). this reduction makes sense due to the relationship between temperature and global radiation, as a 1.5-fold temperature increase also leads to a 1.5-fold increase in radiation intensity. the study shows, however, that the thermal load cases for tgu’s are significantly more complex than those for dgu’s. a closer look at these relationships and the influence of the layer configuration is hence necessary. reducing the load safety factors without doing so could lead to non-conservative results. 4.3. load cases as mentioned in section 4.1, the expected temperatures depend strongly on the facade system and the meteorological boundary conditions. further, the temperature gradient in tgu’s causes a pressure difference between the glass cavities. direction and magnitude of the gradient depend on the facade configuration and installation situation. it was not part of this study to investigate whether this asymmetric loading is or can become relevant for the design. therefore, no conclusion has been reached on whether the temperature difference is as relevant for design as are the maximum and minimum temperatures. the number of load-duration-dependent design resistances in pren 16612:2013 is higher than that in din 18008-1:2010. whereas din 18008-1:2010 contains three load-duration values, pren 16612:2013 will contain eight in total. as a consequence, the climatic actions (change in air pressure + change in temperature) also have different load durations, which increases the number of design load cases. the withdrawn standard pren 13474-3:2009 also contained a similar suggestion. the large calculation volume can easily be managed by using computer programs. hence, even extended load cases can be considered without taking much more time, and the glazing design can be optimized. even so, only professionals with the required knowledge should run the programs and carry out the required design checks. 4.4. boundary conditions the climatic boundary conditions according to the trlv:2010/din 18008-1:2010 were used for all calculations. according to section 2.2.1 and equation 20, the solar radiation of the summer boundary conditions is 565.7w/m2, and the ambient air temperature is ϑe = ϑi =28◦c. meteonorm was used to generate a standard year for a south-facing facade in zurich (zh). external air temperatures of up to 31.5◦c and global radiation of 840w/m2 on the facade surface were predicted for the period between 1st july and 30th september. a similar result was obtained from a simulation for a south-facing facade in berlin, for which radiation of up to 830w/m2 and an air temperature of 34.5◦c were predicted. the original boundary conditions stipulated in the trlv are presumably not the worst-case scenario for the heating up of igu’s. according to fourier’s equation for thermal conduction (equation 3), it can be assumed that the excess temperature in the igu with respect to the ambient air temperature is linearly dependent on the solar radiation. hence, for a south-facing facade in zurich the correction factor is 840/565.7=1.48 ≈ 1.5. this approach can be used for all the temperatures in igu’s calculated in this article. its results show that the thermal loads in igu’s are even higher in practice than assumed so far. by considering the local situation as well as the meteorological and construction boundary conditions, the verification of the facade for climatic loads loses the simplicity of its trlv counterpart. the complexity arising from an increasing number of requirements and the many areas of application of igu’s is difficult to navigate for designers not possessing specialized knowledge. 112 t. wüest and a. luible / increased thermal induced climatic load in igus 4.5. classification of igu configurations the classification of insulated glass units into broad absorption classes does not lead to economical designs for insulated glass units, even for dgu’s. section 4.1 explained why the classification of tgu’s into these absorption classes does not comply with the findings seen in fig. 7. further issues need to be addressed, such as how solar control coating can be integrated into this system. pren 16612:2013 presents a set of analytical equations, which allows the calculation of temperatures in dgu’s and tgu’s. again, the boundary and construction conditions need to be considered correctly. however, the current version of pren 16612:2013 does not contain any information on meteorological boundary conditions or additional structural elements except for tgu’s. according to figs. 9 and 10, dsf’s with tgu’s or a tgu combined with solar protection elements are design-relevant facade configurations. in order to use the formulae according to pren 16612:2013 correctly, detailed boundary conditions for different facade configurations and installation situations are required. 4.6. suggested solution the intention of this document is not only to highlight problems but also to suggest solutions. the design approach for insulated glass units according to din 18008-1:2010 needs to undergo a detailed revision so as to be brought up to the current state of the art. the climatic boundary conditions must be checked and a standardized process for determining the design values must be developed. feldmeier (1995) suggests collecting more precise meteorological data for determining the boundary conditions. such data would include air pressure changes, air temperatures, and solar radiation. unlike the climatic conditions, facade systems and their behaviours are international. the excess temperature for different configurations could be shown as linearly dependent on the solar radiation. by listing the excess temperature due to a reference radiation for different igu configurations in different facade systems, any radiation intensity can be interpolated or extrapolated. it should be noted that this is a work in progress, and the suggestions will be investigated more thoroughly in future research projects. 5. conclusion due to economic and ecological considerations, it is every engineer’s goal to design optimized structures while meeting the safety requirements. there is also continuous technical progress regarding insulated glass units. an “open” solution for the design of insulated glass units as suggested in pren 16612:2013 is capable of dealing with further developments in facade construction. even so, it is necessary to meet a minimum number of requirements and boundary conditions. with increasing complexity of the load cases and material resistances, the effort required for an optimized design of insulated glass units increases as well. hence, for tenders and preliminary design it is necessary to also provide simple and conservative design values. both pren 16612:2013 and pren 13830:2013 suggest smaller safety factors �q as recommended by reick (2015), amongst others. however, the load safety factor can only be reduced after more in-depth analysis of the loads. since almost no new knowledge has been published in the past 17 years regarding climatic load, guidelines for determining the design-relevant climatic boundary conditions are required. for the development of future guidelines and standards it is imperative that the climatic boundary conditions and the different facade systems are analysed more thoroughly and the limits of t. wüest and a. luible / increased thermal induced climatic load in igus 113 applicability should be defined clearly. otherwise, insufficiently high loads and reduced load safety factors could result in the use of higher design stresses, which would lead to a significant deterioration of the safety level in the design of insulated glass units. before pren 13830:2013 or pren 16612:2013 is put into practice, an in-depth analysis of the design of insulated glass units under consideration of extended boundary conditions must be carried out. references balocco, c. (2002). a simple model to study ventilated facades energy performance. energy and building, 34(5), 469-475. burkhard, & partner, a. g. (19. oktober 2015). referenzliste. retrieved from http://www.burckhardtpartner.ch/tl files/content/images/ referenzliste all-gf d 131206.pdf cen. (2003). en 13363-1 sonnenschutzeinrichtungen in kombination mit verglasungen berechnung der solarstrahlung und des lichtransmissionsgrades teil 1. brüssel: european comitee for standarization. cen. (2005). en 13363-2 sonnenschutzeinrichtungen in kombinaton mit verglasungen. brüssel: european comitee for standardization. cen. (2007). en iso 6946 bauteile wärmedurchlasswiderstand und wärmedurchgangskoeffizient. brüssel: european comitee for standardization. cen. (2011). en 410, glas im bauwesen, bestimmung der lichttechnischen und strahlungsphysikalischen kenngrössen von verglasungen. brüssel: european comitee for standardization. cen. (2011). sn en 673 glas im bauwesen bestummung des wärmedurchgangskoeffizienten (u-wert). brüssel: european comitee for standardization. cen. (juni 2013). pren 16612:2013 glas im bauwesen bestimmung des belastungswiderstandes von glasscheiben durch berechnung und prüfung. brüssel: european comitee for standardization. dibt. (1998). technische regeln für die verwendung von linienförmig gelagerten verglasungen. berlon: dibt. dibt. (2006). technische regeln für die verwendung von linienförmig gelagerten verglasungen. berlin: dibt. din. (2010). din 18008-1 glas im bauwesen -teil 1: begriffe und allgemeine grundlagen. berlin: deutsches institut für normung. din. (2010). din 18008-2 glas im bauwesen -teil 2: linienförmig gelagerte verglasungen. berlin: deutsches institut für normung. feldmeier, f. (4/1991). belastung von isolierglas durch wind und klimaänderung. fenster und fassade, 89-97. feldmeier, f. (1995). entwicklung eines vereinfachten verfahrens zur berücksichtigung der klimabelastung bei der bemessung von isolierglas bei überkopfverglasungen. stuttgart: frauenhofer irb verlag. feldmeier, f. (2000). die klimatische belastung von isolierglas bei nicht trivialer geometrie. vdi berichte, (nr. 1527), 185-201. feldmeier, f. (2006). klimabelastung und lastverteilung bei mehrscheibeninsolierglas. stahlbau, 75, 467-478. feldmeier, f. (4/2009). kleine dreifach-isoliergläser. fassade, 5-7. feldmeier, f. (7/2009). klimabelastung von dreifach-isolierglas. glas+rahmen, 32-34. feldmeier, f. (2011). bemessung von dreifach-isolierglas. stahlbau spezial, 75-80. frank, t. (2010). bauphysik, bau & energie. zürich: hochschulverlag eth zürich. glas trösch ag. (16. oktober 2015). glace. retrieved from http://www.glastroesch.ch/services/berechnungsprogramme/silverstarglace/glace.html manz, h. (2010). bauphysik ii. manz, h., & frank, t. (2005). thermal simulation of buildings with double-skin facades. energy and buildings, 37(11), 1114-1121. meteotest. (2015). meteonorm 7. retrieved from http://meteonorm.com/de/downloads pasquay, t. (2003). thermisches verhalten von doppelfassaden an drei gebäuden. bauphysik, 25, 220-224. penkova, n., iliev, v., & neugebauer, n. (2013). thermal-mechanical behaviour of insulating glass units. cost tu0905, mif-therm conference on structural glass. london: taylor & francis group. penkova, n., iliev, v., zashova, l., & neugebauer, j. (2014). thermal load analysis of cylindrically bent insulating glass units. in c. g. 4 (hrsg.). london: taylor & francis group. reick, m. (26. oktober 2015). din 18008: mehr leid als freud? retrieved from http://www.glaswelt.de/gentner.dll/pl 30003 642507 stoll, j. (mai 2005). fenstermodell. hlh, 56(5), 32-39. http://www.burckhardtpartner.ch/tl_files/content/images/referenzliste_all-gf_d_131206.pdf http://www.burckhardtpartner.ch/tl_files/content/images/referenzliste_all-gf_d_131206.pdf http://www.glastroesch.ch/services/berechnungsprogramme/silverstar-glace/glace.html http://meteonorm.com/de/downloads http://www.glaswelt.de/gentner.dll/pl_30003_642507