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ISSN (Print: 2357-0849, online: 2357-0857)

International Journal on:

Environmental Science and Sustainable Development

DOI: 10.21625/essd.v1i1.32

Coupling of Solar Reflective Cool Roofing Solutions with
Sub-Surface Phase Change Materials (PCM) to Avoid

Condensation and Biological Growth

Alberto Muscio1
1Department of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia

Abstract
Cool roofs are effective solutions to counter the overheating of building roofs, inhabited spaces below and urban ar-
eas in which buildings are located thanks to their capability of reflecting solar radiation. Nonetheless, the relatively
low surface temperatures that they induce can cause condensation of humidity and leave the surface moistened
for a long time during the day, thus, promoting the growth of bacteria, algae and other biological fouling. This
can cause a quick decay of the solar reflective performance. Biological growth is countered by surface treatments,
which may be toxic, forbidden in many countries and may vanish quickly. It can also be countered by lowering
the thermal emittance and thus decreasing heat transfer by infrared radiation and consequently leading to night
undercooling. However, this can decrease the performance of cool roofs. An alternative approach, which is ana-
lyzed in this work, is to embed in the first layer below the cool roof surface a phase change material (PCM) that
absorbs heat during the daytime and then releases it during nighttime. This can increase the minimum surface
temperatures, reduce the occurrence of humidity condensation and biological growth. In this work, preliminary
results on the coupling of a cool roof surface with a PCM sublayer are presented. Results are obtained by carrying
out a theoretical investigation on commercial materials and taking into account the time and patterns of evolution
of the environmental conditions.

© 2018 The Authors. Published by IEREK press. This is an open access article under the CC BY license
(https://creativecommons.org/licenses/by/4.0/). Peer-review under responsibility of ESSD’s International Scien-
tific Committee of Reviewers.

Keywords

cool roof; phase change material; PCM; condensation; biofouling

1. Introduction

1.1. Cool Roofs and Their Lifespan

Cool roofs are roofing solutions reflective of solar radiation thanks to their high solar reflectance, or albedo. They
can prevent overheating of both individual buildings and entire urban areas. Their potential has been quantitatively
investigated in the USA since the 1980s in response to both the urban heat island (UHI) effect and the need of
reducing electric energy and peak power absorption for air conditioning (Taha, Akbari, Rosenfeld, & Huang,
1988). Many studies have followed evidencing the effectiveness of gradually increasing the albedo of a city by
choosing high-albedo surfaces to replace darker materials during routine maintenance of roofs. Proven as well

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Muscio / Environmental Science and Sustainable Development, ESSD

was the usefulness of establishing sponsored incentive programs, product labeling, and standards to promote the
use of high-albedo materials for buildings (Rosenfeld, Akbari, Romm, & Pomerantz, 1998). Surveys on cool
roofing materials were completed (Berdahl & Bretz, 1997) and strong savings of cooling energy and peak power
were shown (Akbari, Bretz, Kurn, & Hanford, 1997). Additionally, the researcher paid attention to the long-term
performance of high-albedo roof coatings (Akbari et al., 1997). Steps were then taken by cities in the warm half
of USA towards the incorporation of cool roofs in the revised ASHRAE building standards and the inclusion of
cool surfaces as tradeable smog-offset credits in Los Angeles (Rosenfeld et al., 1998). Eventually, this resulted in
the culmination of prescriptive requirements such as the inclusion of cool roofs in energy codes like Title 24 of the
California Code of Regulation (Levinson, Akbari, Konopacki, & Bretz, 2002).

Cool roof technologies, in the USA, have spread worldwide. Among many others, studies demonstrating their
potentialities were undertaken in Europe (Synnefa, Santamouris, & Livada, 2006; Zinzi, 2010). The Cool Roofs
Project as well was co-funded by the European Union to promote high-albedo surface as a UHI mitigation strategy
and a measure for reducing cooling loads (Synnefa & Santamouris, 2012). Moreover, a hot theme in both Europe
and the USA is the contribution to offset CO2 production that can be achieved by increasing the albedo of urban
surfaces (Akbari, Menon, & Rosenfeld, 2008). The usefulness of cool roofs was also investigated with regard
to cold climates, such as in Montreal (Touchaei & Akbari, 2013). While cool roofs have shown to significantly
reduce the contribution to the UHI in the hot season, the penalization introduced in regions with cold winter often
seems negligible in terms of either energy needs for heating or lower heat released, thus, warming the outer urban
environment (Magli, Lodi, Contini, Muscio, & Tartarini, 2016).

Nomenclature

c specific heat (J/(kg◦C))

cl specific heat of the liquid/high temperature phase (J/(kg◦C))

cs specific heat of the solid/low temperature phase (J/(kg◦C))

dl mass density of the liquid/high temperature phase (kg/m3)

ds mass density of the solid/low temperature phase (kg/m3)

hce external convective heat transfer coefficient (W/(m2◦C))

hci internal convective heat transfer coefficient (W/(m2◦C))

hre external radiative heat transfer coefficient (W/(m2◦C))

hri internal radiative heat transfer coefficient (W/(m2◦C))

Isol solar irradiance (W/m2)

k thermal conductivity (W/(m◦C))

kl thermal conductivity of the liquid/high temperature phase (W/(m◦C))

ks thermal conductivity of the solid/low temperature phase (W/(m◦C))

L total thickness (m)

qsl latent heat (J/kg)

T temperature (◦C)

Tair ambient air temperature (◦C)

Tair ambient air temperature (◦C)

Td p dewpoint temperature (◦C)

Te external effective temperature (◦C)

Ti internal temperature (◦C)

Tme mean absolute external temperature (K)

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Tse external surface temperature (◦C)

Tsky sky temperature (◦C)

Tsl phase change temperature

Tsol/air sol-air temperature (
◦C)

t time (s or day)

vwind wind velocity (m/s)

x coordinate across thickness (m)

Dqi change of the entering heat flow rate per unit surface (W/m2)

DTsl half amplitude of the phase change interval (◦C)

eter (external) thermal emittance (0<eter<1)

rsol (external) solar reflectance (0<rsol<1)

0 Stefan-Boltzmann constant (5.67×10−8 W/(m2K4))

Moreover, cool roofs can in fact be seen as a technological rediscovery of ancient concepts. In Mediterranean areas,
roofs and walls of buildings have been white since thousands of years. On the other hand, today we know that a
white or very light color is not an objective term of evaluation but just a qualitative indicator. Thus, current cool
roof technologies are based on measurement of materials performance and calculation of resulting benefits. More
specifically, we know that a cool roof must have high solar reflectance, i.e. the ratio of reflected and incoming solar
radiation. It should also have high thermal (or infrared) emittance such as the ratio of radiation emission in thermal
(or far) infrared and maximum theoretical emission at the same surface temperature. Both solar reflectance and
thermal emittance are measured as a percentage or a fraction of the unit. The higher the solar reflectance, the lower
the fraction of solar radiation that is absorbed by a surface is. Such an absorbed radiation can then be returned to
the atmosphere by convection with the air and by thermal radiation. In the absence of wind, heat removal mainly
occurs through thermal radiation provided that the thermal emittance is high. In contrast, low solar reflectance may
cause the surface to overheat and, consequently, lead to head transmission to the roof structure and living spaces
below. This contributes to building overheating and to the correlated UHI effect either directly, in terms of heat
transfer to the external air by convection, or indirectly, due to the removal of the transmitted heat from the living
spaces by means of the air conditioning systems. The latter contribution is augmented by the compressor power
absorption which increases with the external air temperature.

Two different families of cool roofing solutions can be identified: cool white technologies for flat roof coverings,
such as the example in Figure 1. These, by far, are the ones more commonly used. Cool color technologies for
sloped roofs, as well, are designed to show a reflection spectrum in the visible range (0.4-0.7 µm) as needed to
obtain a desired color. At the same time, a reflection capacity in the near infrared (NIR, 0.7-2.5 µm) where solar
radiation falls by more than 50% remains invisible to the human eye.

Cool white roofing solutions have many types: field applied coatings (paints, fluid applied membranes, etc.), rein-
forced bitumen sheets made of modified bitumen (elastomeric or plastomeric), single-ply sheets and membranes
(thermoset or thermoplastic), tiles (ceramic, concrete, etc.), asphalt or bituminous shingles, pre-painted metal roofs,
and built-up roofing. They can show initial solar reflectance as high as 80-85% and thermal emittance range of
80% to 95% for non-metallic materials (CRRC, 2015; US EPA, 2015; ECRC, 2015). On the other hand, it is very
difficult to retain the initial reflectance value due to chemical and physical deterioration of materials and, above all,
soiling caused by pollutant deposition and biological growth (Berdahl, Akbari, Levinson, & Miller, 2008; Berdahl,
Akbari, & Rose, 2002; Bretz & Akbari, 1997; Sleiman et al., 2014). That being mentioned, the reflectance and
emittance of opaque building elements are of the most superficial matters. These elements consist of a surface
layer or a coating with thickness as low as a few tenths of millimeters with properties generally unaffected by the
underlying substrate. Therefore, a superposed layer of atmospheric suspensions and/or grown up organic matter
may strongly affect the reflective performance. Both initial and aged values of solar reflectance are thus provided in

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Muscio / Environmental Science and Sustainable Development, ESSD

Figure 1. Flatroof with cool white waterproofing membrane (Tecnopolo building of the University of Modena and Reggio
Emilia, Modena, Italy).

the framework of the CRRC rating program which is obtained by natural exposure in three locations with different
climate for at least three years (Sleiman et al., 2011). The development of matrices and white pigments chemically
and physically stable permits avoiding degradation of the reflectance, such as that associated to yellowing of the
surface. However, several approaches are exploited to reduce soiling, such as controlling the surface porosity and
roughness, and possibly applying super-hydrophilic, super-hydrophobic surface treatments or self-cleaning coat-
ings based on photo-catalysis (Diamanti et al., 2013). A few approaches, mentioned in the following section are
also available to limit biological growth.

1.2. Biological Growth and Deterioration of Building Surfaces

As anticipated above, the deterioration of cool roofs and external building surfaces is due to several causes: aging
and weathering, soiling and deposition of atmospheric black carbon, dust, organic and inorganic particulate matter
as well as microbiological growths (Mastrapostoli et al., 2016; Sleiman et al., 2014). It is often difficult to dis-
tinguish between non biological and biologically-mediated weathering of materials: the two processes can occur
concurrently, each one contributing to the overall deleterious effects (Gaylarde & Morton, 2003). The development
of microbial communities on wetted surfaces is called biofilm, and it becomes gradually a more complex system
(Characklis & Marshall, 1990). Biofilms on building surfaces can contain cyanobacteria, heterotrophic bacteria,
algae, fungi, lichens, protozoa, and a variety of small animals (arthropods) and plants (briophyte) (Gaylarde
& Gaylarde, 2005). Biological growth is influenced by both external conditions and intrinsic characteristics of the
building material (Tomaselli, Lamenti, Bosco, & Tiano, 2000). External conditions are represented by rainfall,
wind, sunlight, temperature and humidity as these determinate the water availability, an essential element to the
microbial metabolism: wet surfaces promote autotrophic organism growth, therefore a higher susceptibility to bio
fouling occurs in rainy regions as well as during heavy rain seasons (Tran et al., 2014). The issue arises also
in humid climates due to the low surface temperatures of cool roofs and persistent condensation of atmospheric
moisture. On the other hand, high temperatures induce water evaporation by heating the materials, making wind
essential for the drying phenomenon. Altogether the climatic conditions determine, depending on the geography
position, the moisture and light conditions that define the micro-climate on building surfaces, which is the major
environmental factor influencing biological growth (Ariño, Gomez-Bolea, & Saiz-Jimenez, 1997). If moisture is

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high enough, and lighting and temperature conditions are suitable, colonization of the surface of new buildings can
occur very quickly (Wee, 1992). Also, the building design and the orientation of the building surface influence ex-
ternal factors of bio growth: the north-facing facades, which are wetter and less sunny, get colonized faster (Ariño
et al, 1997). More details on biological growth are given by Ferrari (2015).

In order to limit biological growth, surface treatments based on biocides can be used, but their effects may however
vanish quickly and may be toxic and forbidden in many countries. Biological growth can also be countered by
lowering the thermal emittance and thus decreasing heat transfer by infrared radiation to the sky and the consequent
night undercooling. Nonetheless, this can also decrease the performance of cool roofs. The alternative approach to
counter biological growth on cool roofs and similar building surfaces that is analyzed here is to embed in the first
layer below the cool roof surface a phase changing material that absorbs heat during the daytime and then releases
it in the nighttime. This can increase the minimum surface temperatures, thus reducing humidity condensation and
biological growth. In this work, preliminary results on the coupling of cool roofs and PCMs are presented after
being obtained by theoretical investigation on commercial materials and taking into account the time of evolution
of environmental conditions in a sub-Mediterranean climate with low wind and relatively high humidity.

2. Phase Change Materials and Their Coupling with Cool Roof Surfaces

Latent heat storage technologies that use phase change materials (PCM) embedded in lightweight building elements
are considered an interesting alternative to sensible storage by heavyweight constructions (Zalba, Marın, Cabeza,
& Mehling, 2003; Khudhair & Farid, 2004; Mavrigiannaki & Ampatzi, 2016). These materials can undergo a
phase change, typically melting or solidification, and therefore exchange more heat with the environment in terms
of latent heat rather than sensible heat storage capacity. In recent times the attention has also been drawn by the
coupling of PCMs with cool roofs, studied by either numerical simulation or experiments. Experimental results,
by Karlessi and others (2011), demonstrated that PCM incorporated in building coatings yields lower surface
temperatures than either common coatings or cool infrared-reflective coatings. The numerical investigation by
Aguilar and others AR (2013) extended previous work by considering the impact of PCM embedded in roofing
module on cooling energy and showed that solar reflectance is the parameter with the biggest impact. Nonetheless,
PCMs may be worthwhile in locations where the reflectance undergo a sharp decrease due to soiling. Moreover,
they verified that simulation is a powerful tool for the involved multi-parameter analyses. Roman and others
(2016) showed through simulation that a PCM allows a sharp decrease of through-roof heat gain at a wide range
of albedo. In (Chou, Chen, & Nguyen, 2013) the coupling of a metal sheet cool roofing structure with a PCM
was studied in order to absorb the downward heat flow induced by incident solar radiation and then release it
back to the environment by convection during the nocturnal cycle. Experimental and numerical analyses showed
that the downward thermal flow through the roof into the house can be significantly reduced. Another study
was aimed at the development and prototyping of a cool polyurethane-based membrane with PCM inclusion for
roofing applications (Pisello et al., 2016). In another, PCM cool roof system was created using PCM doped tiles
(Chung & Park, 2016). Experimental results showed that such tiles, during the summer, allow a decrease of surface
temperature while maintaining low room temperatures.

Generally speaking, most of the studies have shown that PCM can even both positive and negative peaks of surface
temperature and thus improve thermal comfort and reduce cooling energy demand. None, however, seem to have
yet paid attention to the specific issue of using PCM to limit condensation and biological growth on cool roof
surfaces. This is therefore the topic of investigation in this work.

3. Model and Case Study

In order to parametrically investigate the thermal behavior of a cool roof coating coupled with a PCM, a one-
dimensional mathematical model of the roof system was implemented in the Matlab programming environment.
The model, based on a finite-volume approach with implicit discretization of the time derivatives, takes into account

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the cyclical variability of the boundary conditions (temperature, solar irradiance, heat transfer coefficients) and the
temperature dependence on the thermophysical properties of materials. It has been used to identify the time periods
during which the external surface temperature falls below the dew-point and the risk of humidity condensation
occurs. In this way, a comparison of the situation with and without a PCM layer below the cool roof surface has
been carried out considering surfaces with relatively low thermal emittance.

A flat roof was considered in this study. Hourly weather data on air temperature, sky temperature, wind velocity,
solar irradiance and dew-point temperature were obtained over the whole year from the TRNSYS programming
environment (TMY data). Representative time evolution patterns are shown in Figure 2.

Figure 2. Ambient conditions over the year for Bologna, Italy (data from TRNSYS, the black lines represent the daily
average).

Figure 3. Roof structure and surface heat transfer processes.

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The heat transfer process is depicted in Figure 3. More specifically, the boundary condition at the external surface
of the solid matter (x=0), delimiting the external environment from the waterproofing layer, is expressed as

(hce + hre)
[
Tsol/air (t) − T (t)x=0

]
= −k

dT (t)
dx
|x=0 (1)

where the so-called sol-air temperature Tsol/air (◦C) is calculated as follows: from the effective external tem-
perature Te (◦C) and the absorbed fraction of the solar irradiance Isol (W/m2) that results from the surface solar
reflectanceρsol (0 < ρsol < 1):

Tsol/air (t) = Te (t) +
(1 − ρsol)Isol (t)

hce + hre
(2)

The effective external temperature Te (◦C) is in turn the average of air and sky temperatures, Tair and Tsky (◦C),
weighted by the external convective and radiative heat transfer coefficients, hce and hre (W/(m2◦C)):

Te(t) =
hceTair(t)+ hreTsky (t)

hce + hre
(3)

Air temperature and sky temperature, as well as the solar irradiance and, consequently, Te and Tsol/air , are function
of the time t (s). The convective heat transfer coefficient hce is evaluated from the wind velocity vwind (m/s)
according to ISO 6946 (ISO, 2007a):

hce = (t) = 4 + 4vwing(t) (4)

The radiative heat transfer coefficient hre is again evaluated according to ISO 6946 at the mean absolute external
temperature Tme (K), also considering the thermal emittance eter (0<eter<1), as

hre = εter 4σ0T 3me(t) (5)

where 0 is the Stefan-Boltzmann constant ( 0=5.67×10−8 W/(m2K4)), and

Tme(t) =
1
2
[
Tsky(t) + T (t)|x=0

]
(6)

An explicit approach is generally followed to evaluate the radiative heat transfer coefficient from the calculated
surface temperature.

Concerning thermal interaction between the roof structure and the inhabited space below, the indoor temperature Ti
(◦C) was assumed to be controlled by an appropriate air conditioning system and kept constant at a level adequate
to thermal comfort, e.g. Ti=27◦C for the summer period in the analyses presented here. The boundary condition at
the internal surface (x=L), delimiting the roof structure from the indoor space, is expressed as

(hci + hri)[t(t)|x=L −Ti] = k
dT (t)

dx
|x=L (7)

Constant values obtained from ISO 6946 were used for both the internal convective coefficient hci, taken equal to
0.7 W/(m2◦C), and the internal radiative coefficient hri, evaluated equal to 5.5 W/(m2◦C) by a formula analogous
to eq. (5) for the typical value of the thermal emittance of inner building surfaces, equal to 0.9, and an internal
absolute mean temperature of 27◦C = 300 K.

The same model used in a previous work was exploited for the heat transfer in the solid matter comprised between
the external surface (x=0) and the internal one (x=L), including the PCM layer (Barozzi, Corticelli, Libbra, Muscio,
& Tartarini, 2009). In fact, the thermal behaviour of a PCM can be mathematically modeled through different
approaches. The one adopted here is based on the definition of a fictitious equivalent material whose specific
heat c (J/(kg×◦C)) is a function of temperature. Heat absorbed or released by such a fictitious material must be
equal to that absorbed or released by the actual material for the same increase or decrease of temperature. This
seems consistent with the literature which shows that currently available PCM do not change phase at a precise
temperature level, but rather over a more or less narrow temperature interval (Carbonari, Grassi, Perna, & Principi,

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2006; Farid, Khudhair, Razack, & Al-Hallaj, 2004; Schossig, Henning, Gschwander, & Haussmann, 2005; Tyagi
& Buddhi, 2007; Zalba et al., 2003). Moreover, the slope of heat absorption or release in the phase change
interval is often similar to a gaussian distribution about the central temperature. In the industrial practice, either
the central value or the amplitude of the phase change interval are modulated through an appropriate formulation
of the PCM (ex: the phase change temperature of paraffin depends on the length of the molecular chains, which
can be modulated in terms of statistical distribution about an assigned average value in order to obtain the desired
properties).

Going into detail, the equivalent specific heat c of a PCM with latent heat qsl (J/kg), specific heat cs (J/(kg×◦C))
of the solid phase (or the low temperature phase in solid-solid transitions) and specific heat cl (J/(kg×◦C)) of the
liquid phase (or the high temperature phase) is represented in this study by a gaussian distribution in a range with
amplitude 2×DTsl about the nominal phase change temperature Tsl (◦C) (see Fig. 3). This is described by the
following relationship:

c(T ) = cs +(c1 −cs)·
(

T −Ts1
24Ts1

+
1
2

)
+

qsl
4Ts1

·
2
√

π
·ex p

[
−
(

T −Ts1
4Ts1/2

)2]
(8)

Figure 4. Effective specific heat of the PCM.

The thermal conductivity k (W/(m◦C)) and other thermophysical properties which may vary significantly during the
phase transition can also be modelled by assuming, over the phase change temperature interval, a linear variation
between the values for the solid and the liquid phases, e,g. ks and kl (W/(m·◦C)):

k(T ) = ks +(k1 −ks)·
(

T −Ts1
2·4Ts1

+
1
2

)
(9)

Nonetheless, for the time being, certainly reliable data on the PCM are missing, so the properties of the solid and
liquid phases are assumed to be equal for sake of simplicity, in view of their relatively low differences.

The properties of common building materials as reported in ISO 13786 (ISO, 2007b) for a thick concrete structure
with thermal insulation were used for the different layers (see Tab. 1), whereas a commercial PCM board, DuPont
Energain (Energain, 2016), was considered for the PCM to be introduced between the waterproof coating and the
thermal insulation layer below with a thickness up to 10 mm. More specifically, a product developed by DuPont
and called Energain® was considered, that is a composite PCM wallboard constituted of 60% of microencapsulated
paraffin included in a polymeric structure; such a mixture is laminated by aluminium. Since the literature raises
doubts on the material specifications (Energain, 2016), these were integrated with experimental data from a third
party (Du et al., 2012) (see Tab. 2).

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Table 1. Properties of the reference roofstructure – Raw data from (ISO, 2007b).

Layer Th. conductivity
k (W/(m◦C))

Mass density
d (kg/m3)

Specific heat
c (J/(kg◦C))

Thickness
s (m)

Waterproofing 1.00 1200 1500 0.005
PCM See Tab. 2 1400 0 – 0.010
Thermal insulation 0.04 30 0.100
Concrete 1.80 2400 1000 0.200

Table 2. Properties of the PCM panel – Raw data from Du et al. (2012); Energain (2016)

Latent heat qsl 70 kJ/kg
Phase change temperature
Tsl

24.5◦C

Half-amplitude of the phase change interval 4Tsl 9.5◦C
Specific heat
cs ∼= cl

2.3 kJ/(kg◦C)

Thermal conductivity
ks ∼= kl

0.16 W/(m◦C)

Mass density ds ∼= dl 1001 kg/m3

4. Results

The model has been used to identify the time periods during which the external surface temperature Tse (◦C) falls
below the dewpoint Td p (◦C), thus explaining the risk of humidity condensation. The process is depicted in Figure
4, where the surface temperature is plotted for a short mid-summer period in case of absence of PCM and presence
of a PCM layer with 10 mm thickness.

Figure 5. Time evolution patterns of dew point surface (with and without PCM) temperatures: the risk of condensation exists
where the surface temperature falls below the dew point temperature.

The frequency of the risk of condensation is represented in Figure 5-a) for a cool roof with thermal insulation but
without PCM below the surface. It is then shown in Figure 5-b) and Figure 5-c), respectively, that the risk becomes
lower with a 5 mm PCM layer, and much lower with a 10 mm PCM layer. In Figure 5-d) the risk of condensation
is represented for a cool roof with thermal insulation and without PCM, but with thermal emittance 0.6 instead
of 0.9; in this case one can observe that the reduced thermal emittance limits heat loss toward the sky during the
night-time and, consequently, yields a risk of condensation similar to that provided by a 10 mm thick layer of PCM.
Nevertheless, it was also found that an always positive change Dqi (W/m2) of the heat flow entering the inhabited
space is obtained with respect to a cool roof surface with typical emittance of 0.9, that is an increased entering heat
flow (see Fig. 6), whereas a cool roof surface with emittance 0.9 coupled with PCM causes an oscillating change
Dqi of the entering heat flow but with null average.

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Figure 6. Risk of condensation for a) a typical cool roof, b) a cool roof coupled with 5 mm PCM, c) a cool roof coupled with
10 mm PCM, d) a cool roof with thermal emittance lower than usual.

Figure 7. Change of the heat flowentering the inhabited space with respect to a typical cool roof with rsol =0.7, eter =0.9.

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5. Concluding Remarks

In this work, it was shown that a PCM can actually increase the nighttime surface temperature of a thermally-
insulated cool roof without affecting the overall performance of the cool roofing product. Heat is accumulated
in the daytime by the PCM and then released in the night-time, increasing the minumum temperature and thus
reducing the risk of falling below the dewpoint temperature, with the start of humidity condensation. Limiting
humidity condensation my help preserve the cool roof surface from biological growth, especially for insulated
roofs. A similar result can also be provided by a decrease of the thermal emittance at the external surface, but
with a penalization of the cool roof performance in terms of heat transmitted to the inhabited space below and also
maximum surface temperatures.

A relatively high mass fraction of PCM must be installed in the outer layer, either integrated in a waterproofing
membrane or, as a board, placed just below the membrane, or below an outer metal sheet in insulated sandwich
components. Effective results were in fact found only for a PCM board with thickness 10 mm and percent content
of PCM around 60%. A lower amount of dispersed PCM may lead to an ineffective contribution.

In upcoming work, ambient data for different climatic conditions will be taken into account, considering either arid
or very humid climates. Moreover, the analysis will focus on the choice of the phase change temperature, which
must be slightly higher than the expected dew point temperature and should probably be optimized depending
on the location. A more detailed model of the PCM is also under development, to be supported by experiments.
Eventually, integration of the proposed solution in comprehensive dynamic models is a long-term objective.

6. Acknowledgments

The author wishes to acknowledge Giulia Santunione, Chiara Ferrari, Susanna Magli, Antonio Libbra and all the
other researchers of the Dept. of Engineering “Enzo Ferrari” who provided support and data for this work.

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pg. 68


	Introduction
	Cool Roofs and Their Lifespan
	 Biological Growth and Deterioration of Building Surfaces

	 Phase Change Materials and Their Coupling with Cool Roof Surfaces
	 Model and Case Study
	Results
	Concluding Remarks
	Acknowledgments