Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2022.38.0509 Acta Polytechnica CTU Proceedings 38:509–515, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague VERIFICATION OF WINDOW PROPERTIES AFTER 10 YEARS OF EXPLOITATION: RESULTS OF MEASUREMENTS IN THE PAVILION LABORATORY AND THE CLIMATE CHAMBER Marek Bartko∗, Pavol Ďurica University of Žilina, Faculty of Civil Engineering, Department of Building Engineering and Urban Planning, Univerzitná 8215/1, 010 26 Žilina, Slovakia ∗ corresponding author: marek.bartko@uniza.sk Abstract. The article will deal with the analysis of measured data on a plastic window with thermal insulating triple glazing, which is suitable for low-energy or passive houses. The window was installed in 2011 in the test laboratory of the Department of Building Engineering and Urban planning, Faculty of Civil Engineering, University of Žilina (Slovakia), where it was tested under standard indoor climate conditions and real outdoor climate conditions. Surface temperatures on the frame friezes and glass system and heat flux density were recorded at a five-minute time step. In 2020, the window was removed from the laboratory and subsequently tested in a climate chamber. This paper will present the results of these measurements in terms of heat flow density waveforms, heat transfer coefficient, and total solar transmittance through the glazing. Subsequently, a simulation model of this window will be created in the environment of a computational program and its verification based on the measurements will be carried out. A series of calculations will be performed on the tuned model and analyses of the results and comparisons will be presented under the same climatic conditions as during the real measurements recorded by the meteorological station. Keywords: Window, triple glazing, climate chamber, pavilion measurement, building physics. 1. Introduction Windows are an integral part of the envelope of almost all buildings. They are the most used transparent system in building envelopes. The main function of windows is to protect the indoor environment from the external climate. Other important functions are daylighting and insolation of the space [1], fresh air supply by natural ventilation, contact with the out- doors, etc. With increasing demands for thermal pro- tection, window constructions are constantly evolving. Physical criteria such as thermal-technical, acoustic, lighting, and hygiene are important. Another impor- tant aspect of the proper functioning of windows is their installation in envelope construction. Window constructions are the most problematic point of the building envelope in terms of thermal insulation and construction design. Therefore, continuous research and development of window constructions are essen- tial. A significant problem that needs to be addressed more than in the past is overheating in summer [2]. Another problem is the elimination of thermal bridges in window construction. For example, by using triple glazing we limit the formation of water vapor conden- sation at the bottom of the glazing. In Slovakia, thermal protection is dealt with in STN 730540:2019 [3], which follows the EC Energy Perfor- mance of Buildings Directive (EPBD) 2010/31/EU. Valid values for window constructions are Uw = 1.0 W/(m2 K) from 2016 and from 2021 the recom- mended value is Uw = 0.65 W/(m2 K). Since 2011, three different windows have been moni- tored in the laboratory of the Department of Building Engineering and Urban planning. It is a so-called pavilion laboratory. The indoor environment is con- trolled by a heating system and an air conditioning unit. The outdoor environment is represented by the real climate, which is recorded by a weather station [4]. From the research conducted so far, several results related to temperature, heat flux, and U-value mea- surements have been published. Comparisons have been made with the model in the FEM Therm soft- ware and result from different years were analyzed. During autumn 2020, the laboratory building enve- lope was renovated and insulated with a new ETICS system, while one of the measured windows was dis- mantled and taken for measurement in the climate chamber [5, 6]. A comparison of the measured results from the pavilion laboratory and the climate chamber after ten years of window exploitation at the same boundary conditions is described in this paper. 2. Methods of measurement The analyzed plastic window has been installed in the department testing laboratory since 2011 (Figure 1). The window was oriented to the south with a slight inclination to the west (15°). From the exterior side, the window was exposed to real outdoor climate condi- tions, which are monitored and recorded by a mobile experimental weather station [7], located on the roof of a nearby building. The indoor climate is provided 509 https://doi.org/10.14311/APP.2022.38.0509 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en Marek Bartko, Pavol Ďurica Acta Polytechnica CTU Proceedings Figure 1. External and internal view of the built-in plastic window in the test laboratory. Material Number of Glazing Gas Total solar energy Uw Uf Ug chambers transmittance g [%] [W/(m2 · K)] [W/(m2 · K)] [W/(m2 · K)] Plastic 6 Triple Ar 36 0.80 1.00 0.50 Table 1. Properties of the measured plastic window given by the manufacturer. by an air-conditioning unit, which maintains it based on the Slovak standard boundary conditions: 20 °C and 50 % humidity [8]. The sensors used for the measurements consist of NiCR-Ni [9] thermocouples and heat flux density plates (HFP), also equipped with a correction ther- mocouple (standard 120 × 120 mm) and a half-sized (120 × 60 mm) for window frames and sashes [10]. The monitoring points are the frame, sash, and glazing. The datalogger and both types of sensors are from Ahlborn. The data logging interval in this case was five minutes. The heat flux density was only recorded at the glazing, as measuring the heat flux density through the window frame requires different input conditions – the glazing must be replaced by a full panel. For this reason, the degradation of the glazing system will be mainly evaluated in this paper. Subsequently, the window was dismantled in 2020 during the reconstruction of the facade of the pavilion laboratory. However, it is not at the end of its life. After dismantling, the window was removed and in- stalled in the climate chamber, where it has been and will continue to be tested and measured. The purpose was to compare the parameters of the window mea- sured in the pavilion laboratory with those measured in the climate chamber. For the measurements in the climate chamber, two variants of indoor climate modeling were used, one with a constant indoor temperature and a measure- ment with a hotbox, the other without the application of a hotbox and with the same temperature waveform as was recorded in the pavilion. For the measure- ment of the climate chamber was chosen 1st March 2018. The outside temperature ranges from -3.4 °C on a sunny day to -16.7 °C at night. The indoor air temperature was also used. This method was chosen to compare the behavior of the window under the same boundary conditions in the pavilion laboratory and the climate chamber. Instead of thermocouples, PT100 and NTC sensors were used to measure tem- perature. The HFP used is of similar types to those used in the pavilion measurements. When the window was initially installed in the pavil- ion laboratory, not all the parameters that we now measure in the laboratory were measured. For this reason, the measured values from the pavilion labo- ratory and the climate chamber are compared with the values set and declared by the manufacturer. The declared values of the window parameters from the manufacturer are summarized in Table 1. 2.1. U-value The heat transfer coefficient U of a structural ele- ment describes the amount of heat energy that passes through it from one side to the other per second in a square meter of the area at a constant ambient temperature difference of 1 °C. The relation for cal- culating the heat transfer coefficient U is shown in Equation (1): U = 1 R0 = 1 (Rsi + R + Rse) = 1 1 hi + 1Λ + 1 he = q θai − θae , (1) U heat transfer coefficient [W/(m2 K)], R0 structure resistance to heat transfer [(m2 K)/W], Rsi internal surface resistance [(m2 K)/W], 510 vol. 38/2022 Verification of window properties after 10 years of exploitation . . . Figure 2. Detail of the window in the wall of the laboratory. Marked positions for comparison of surface temperatures. Surface transfer STN 73 0540 Glazing Glazing Glazing coefficient [W/(m2 · K)] pavilion clim. chamber clim. chamber with hotbox without hotbox hi 7.62 10.47 10.92 16.51 he 25 14.56 15.92 15.66 Table 2. The surface transfer coefficient is defined in STN 73 0540 and calculated using Equations (2) and (3). Rse external surface resistance [(m2 K)/W], R thermal resistance of the structure [(m2 K)/W], hi surface transfer coefficient at internal surface [W/(m2 K)], he surface transfer coefficient at external surface [W/(m2 K)], Λ thermal conductance [W/(m2 K)]. The surface transfer coefficient can be calculated according to the Equations (2) and (3): hi = q θsi − θai , (2) he = q θse − θae . (3) 2.2. Solar transmittance For transparent and translucent constructions such as window constructions, besides the thermal quan- tification, the optical properties, especially the solar transmittance, are equally important parameters [11]. There are several ways, to measure solar transmit- tance. In our case, measuring devices in the form of two pyranometers were used, one of which was mounted on the interior side of the measured window and the other on the exterior side as part of a mobile meteorological station. From the values measured in this way, the solar transmittance is finally determined as the ratio of the observed solar radiation intensities behind (from the interior side) and in front (from the exterior side) of the measured window located in the pavilion laboratory and subsequently in the climatic chamber. 2.3. Window simulation in simulation software In the design and planning process of a building, ther- mal bridges are analyzed for several construction frag- ments, including the window frame detail and the window fitting into the opening. The analysis can be carried out using various simulations, however, modeling different window construction is quite time- consuming [12]. In the field of two-dimensional modeling of heat transfer in buildings, several software is available. This article used Therm software, developed by Lawrence Berkeley National Laboratory. The simulation results in the form of surface temperatures were compared with the actual measured surface temperatures in the pavilion laboratory. A detailed view of the modeled window is shown in Figure 2. These windows were placed in the same composition as in the laboratory to achieve the best possible temperature match. The simulation was carried out in the lining and the sill. 3. Results of measurements For the calculation of the surface transfer coefficient were used Equations (2) and (3). Results are sum- marised in Table 2. The results show in some cases (especially outdoors) a large difference between the 511 Marek Bartko, Pavol Ďurica Acta Polytechnica CTU Proceedings Heat transfer Specified by the Glazing Glazing Glazing coefficient [W/(m2 · K)] manufacturer pavilion clim. chamber clim. chamber with hotbox without hotbox Ug 0.5 0.92 0.94 0.96 Uw 0.8 1.12 1.13 1.15 Table 3. Heat transfer coefficient window Uw and glazing Ug . Figure 3. Temperature courses for the Centre of glazing. Very good match of courses at the night (without solar radiation). measured values and the standard values. The differ- ence could be due to the measurement method itself, a non-stationary state, lower accuracy of the thermo- couples, or imperfect contact between the sensor and the surface. In the case of the outdoor coefficient, the non-stationary state of the external environment: so- lar radiation, wind, and rain. The calculated values of the surface transfer coefficient based on the measured heat fluxes are given in Table 2 and the heat transfer coefficient glazing in Table 3. The measured surface temperature values at each position are shown in Figures 3–6. In this case, the temperature waveforms for a selected time interval were compared (from 20:00 28th February 2018 to 23:59 1st March 2018). Although there are 6 (10) positions displayed within the window, only 3 specific positions are compared. In other positions the results are similar. Figure 3 shows the temperature waveforms at the center of the glazing from the inside. Temperatures were measured using HFP. Figure 4 and Figure 5 show a comparison of the different internal temperatures. In the first case, the steady – constant air temperature is measured with a hotbox in the climate chamber. In the second figure, the non-stationary temperature was used as the boundary condition in the chamber. Figure 6 shows the surface temperatures at the window sash location. In Table 4 we can see the solar transmittance cal- culated as the ratio of the observed solar intensities behind (from the interior side) and in front (from the exterior side) of the measured window. Measurements were carried out in the pavilion laboratory and the climate chamber. In Table 5 we can see a comparison of the sur- face temperatures measured in the climate chamber and pavilion laboratory with the surface temperatures from the Therm simulation software (Figure 7). The boundary conditions in the simulation were set ap- proximately according to the real conditions from the measurements in the pavilion. Outdoor temperature -10 °C and indoor temperature 20 °C. 4. Conclusion This article deals with the comparison of measured parameters of a plastic window after its ten-year ex- ploitation. The measured parameters were compared only with the data given by the manufacturer. The measured window was installed in the pavilion labora- tory where it was exposed to the real outdoor climate. It was later dismantled and used for measurements in the climate chamber with the same boundary condi- tions set as on the selected winter day in 2018. For comparison, a simulation was also created but in the stationary environment of the Therm software. The results showed a very good agreement in the shape of the temperature waveform and for the posi- tion in the center of the glazing also in the values where the difference is relatively small. In other positions, there is a different temperature approx. 3–4 °C. Daily temperatures in the pavilion laboratory are strongly influenced by solar radiation, wind, and rain. Discrep- ancies in the sill results require further analysis, such as the impact of the masking panel compared to the wall. The glazing heat transfer coefficients calculated from the measured heat fluxes in the pavilion labo- ratory and the climate chamber show similar values but differ significantly from the values given by the manufacturers. The heat transfer coefficient glazing 512 vol. 38/2022 Verification of window properties after 10 years of exploitation . . . Figure 4. Temperature courses for the bottom of glazing. Variant 1 with constant indoor temperature. Difference of about 3 °C in surface temperatures. Figure 5. Temperatures courses for the bottom of glazing. Variant 2 with non-steady indoor temperature. Difference of about 4 °C in surface temperatures. Figure 6. Temperature courses for the bottom of windows sash. Difference of about 3 °C in surface temperatures. Specified by the Pavilion Climatic chamber manufacturer Total solar energy transmittance g [-] 0.36 0.34 0.31 Table 4. Total solar energy transmittance g. 513 Marek Bartko, Pavol Ďurica Acta Polytechnica CTU Proceedings Position Temperature [°C] Therm Pavilion Chamber P la st ic w in do w 1 O ut do or -1 0 In do or 20 17.3 15.1 15.0 2 16.5 14.9 13.3 3 -9.1 -8.3 -8.9 4 -8.5 -8.3 -9.0 5 17.5 9.9 15.4 6 15.9 12.5 16.9 7 -8.7 -10.6 - 8 -8.9 -10.6 - Table 5. Comparison of surface temperatures measured in the pavilion and climate chamber with temperatures from the Therm simulation software. Figure 7. Temperature waveforms according to the Therm software simulation at the lining and sill location. shows a difference compared to the manufacturer’s values of about 90 %. The values of the total solar energy transmittance compared to the manufacturer’s value is quite similar. The value measured in the pavilion laboratory compared to the manufacturer’s value show a deterioration of only about 5.5 %. For the values measured in the climate chamber, a de- terioration of 14 % occurs. Based on these findings, we can say that the thermal-technical properties of the plastic window after 10 years of exploitation show deterioration. This deterioration of the parameters is influenced by possible leakage of filler gas from the glazing system, non-stationary conditions, especially from the exterior side (solar radiation, wind, rain), and airflow in the vicinity of the construction. The surface temperatures obtained from the sim- ulation were then compared with the surface tem- peratures measured in the pavilion and the climate chamber. The results showed a slight overestimation of the temperatures in the lining area, however, there is a large difference between the temperatures in the sill area, which may be due to the airflow in the sill area and the stationary environment of the Therm software. Acknowledgements The research is supported by the grant project VEGA No. 1/0673/20. References [1] M. Glória Gomes, A. J. Santos, A. Moret Rodrigues. 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MATEC Web of Conferences 196:02028, 2018. https://doi.org/10.1051/matecconf/201819602028 515 https://doi.org/10.3390/en14061694 https://doi.org/10.2478/cee-2020-0037 https://doi.org/10.1088/1757-899X/415/1/012020 https://www.ahlborn.com/download/pdfs/kap07/05draehted.pdf https://www.ahlborn.com/download/pdfs/kap07/05draehted.pdf http://www.ahlborn.com/download/pdfs/kap13/WflPlatten.pdf http://www.ahlborn.com/download/pdfs/kap13/WflPlatten.pdf https://doi.org/10.1016/j.egypro.2017.09.694 https://doi.org/10.1051/matecconf/201819602028 Acta Polytechnica CTU Proceedings 38:509–515, 2022 1 Introduction 2 Methods of measurement 2.1 U-value 2.2 Solar transmittance 2.3 Window simulation in simulation software 3 Results of measurements 4 Conclusion Acknowledgements References