State of the art on the development of cool coatings for buildings and cities

Solar Energy 144 (2017) 660–680

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Solar Energy journal homepage: www.elsevier.com/locate/solener

Review

State of the art on the development of cool coatings for buildings and cities Anna Laura Pisello ⇑ CIRIAF – Interuniversity Research Centre on Pollution and Environment ‘‘Mauro Felli” at University of Perugia, Via G. Duranti 63, 06125 Perugia, Italy Department of Engineering - University of Perugia, Via G. Duranti 91, 06125 Perugia, Italy

a r t i c l e

i n f o

Article history: Received 3 November 2016 Received in revised form 28 January 2017 Accepted 30 January 2017

Keywords: Urban heat island Mitigation Cool materials Passive cooling Cool coatings Energy efficiency in buildings Passive cooling Indoor-outdoor thermal comfort

a b s t r a c t Urban systems, from their early origins, were acknowledged to be responsible for both benefits and penalties due to anthropogenic actions affecting human wellbeing. In this view, local overheating exacerbated by urban heat island phenomenon has been identified as a result of anthropogenic actions responsible for citizens’ health issues and other serious socio-economic consequences in urban areas, where almost the 70% of the world population is expected to live in thirty years. This fact imposes to respond to an urgent research question concerning the development and the real-world application of effective mitigation strategies against urban climate change phenomena, for a better population resilience. Among the variety of these strategies, the implementation of ‘‘cool” coatings over urban surfaces exposed to the solar radiation, i.e. cool roofs and cool pavements, represents a proved solution to counteract such overheating effect and its negative consequences on the population living in urban context. In this view, the present work reviews the state of the art about the development of new materials and their main applications as cool roofing and paving systems for passive cooling purpose of buildings and cities, which have been published in more than 260 papers in the last decades. Both indoor and outdoor passive cooling benefits were clearly demonstrated and quantified with varying climate conditions, material characteristics and the built environment context. Despite that, the investigation around this issue is still active worldwide, from chemistry, material science and engineering fields. Additionally, new triggers were highlighted as possible starting points for future scientific focus. In particular, still active discussions give rise to promising research findings expected to clarify (i) the effect of cool coatings on pedestrians’ glare, possibly mitigated by directionally-reflective materials, (ii) the role of cool coatings for HVAC optimization, (iii) the combined benefits of cool coatings and thermal-energy storage techniques for UHI mitigation. Ó 2017 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cool coatings indoor-outdoor behavior. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The cool coating concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Solar reflectance and infrared emittance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Solar reflectance index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials for cool coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Cool roof and cool façade coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Cool roof paintings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Cool waterproof membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Cool tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Cool natural materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multiple effects of cool coatings applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Address: CIRIAF – Interuniversity Research Centre on Pollution and Environment ‘‘Mauro Felli” at University of Perugia, Via G. Duranti 63, 06125 Perugia, Italy. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.solener.2017.01.068 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.

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5.1.

6. 7.

Cool coatings for cooling energy saving. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Residential buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Commercial buildings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Cool coatings for urban heat island mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Cool coatings for indoor thermal comfort optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Cool coatings for outdoor thermal comfort optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Cool coatings for optimizing HVAC and PV performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Cool coatings durability and life-cycle performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7. Cost-benefit analysis of cool coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Statistics on cool coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction From their early origins, urban systems have produced both benefits of penalties due to anthropogenic actions (Pain, 2016). Some risks have been successfully tackled while other new ones are increasingly making urban citizens more vulnerable. Anthropogenic heat release represents an added energy input to the energy balance (Zhao et al., 2014; Ward et al., 2013) in cities, responsible for the urban surface temperature increase, i.e. the world-wide known phenomenon of Urban Heat Island (UHI) Allen et al., 2011; Fischer and Schär, 2010. Lindberg et al. (2013) estimated the quantitative consequences of the anthropogenic heat excess in cities through the global anthropogenic heat flux (QF) model LUCY (Large scale Urban Consumption of energY model), allowing them to identify a general relatively low QF around European territory (i.e. about 1.9– 4.6 W m
2), but with urban maximum peaks up to around 80 W m
2 simulated in London dense boroughs. Such excess of anthropogenic heat showed to be responsible for massive increase of energy need for cooling in summer, up to 13%, and on the other side, the decrease in energy consumption for heating in winter corresponded to around 11%. Such urban energy balance variation and extra-carbon emissions in densely populated areas are identified as key consequences of the urban heat island (UHI) phenomenon. UHI has been widely acknowledged as a direct responsible for citizens’ health issues (Patz et al., 2005; Li et al., 2013) and other serious socio-economic and energy consequences in urban areas (Harlan et al., 2006; Santamouris, 2016; Wang and He, 2014), where almost 85% of the world population is expected to live in 2100 (World Urbanization Prospects, 2014) and where almost 80% of the total CO2eq emissions are produced (Pérez-Lombard et al., 2008). The acknowledged consequences of dense and rapid urbanization, with the following decrease of natural landscape, basically concerns the massive modification of the local energy balance due to the implementation of radiative absorbent and non-permeable surfaces that tend to minimize their capability to reflect heat much more than forests and planted areas and, therefore, being responsible for the dramatic increase of urban surface and air temperature (Santamouris and Kolokotsa, 2015). In this view, Urban heat island (UHI) is the most acknowledged climate change related phenomenon, which has been documented through a wide variety of experimental and simulation studies (Neophytou et al., 2014; Kusaka and Kimura, 2004; Richiardone and Brusasca, 1989) aimed at investigating overheating path of urban temperatures with respect to the relatively colder conditions of suburban areas (Chen et al., 2016; F. Chen et al., 2014; C. Chen et al., 2014). Such phenomenon has been also acknowledged for being responsible for serious socio-economic and health issues making city citizens more vulnerable to climate change and less resilient to its consequences (Santamouris and Kolokotsa, 2015; Pyrgou et al., 2017;

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Heaviside et al., 2016). The situation becomes even more urgent knowing about the rapid gathering of population in urban areas, and the consequent need for cheap and readily available dwellings, where indoor environmental low quality and energy poverty showed to hugely compromise occupants’ health conditions and comfort during extreme climate events (Hatvani-Kovacs et al., 2016a, 2016b, 2016c), e.g. heat waves (Graham et al., 2016; Ward et al., 2016). In this perspective, a massive scientific effort has been dedicated to the proposal of geo-engineering based solutions for effectively mitigating climate change phenomena, such as UHI (Akbari et al., 2016; Akbari and Muscio, 2015). Among these solutions, the proposal of high albedo materials for urban paving and building envelopes has been proving key achievements during the last decades, as effective measure for counteracting urban warming by increasing the solar reflectance of urban surfaces and, therefore, urban solar heat gain (Wang and Akbari, 2015). High albedo surfaces are typically characterized by ‘‘cool” coatings applied over building roofs and outdoor pavements, considering the large potentiality of such surfaces to contribute to UHI mitigation. In fact, roof surfaces in urban areas may correspond to about 20– 40% of the total area exposed to solar radiation, while the paved area correspond to 29–44% of the total (Akbari and Matthews, 2012; Xu et al., 2012b), meaning that the implementation of cool coatings could be massively handled with promising local and global benefits energy and environmental benefits. The key benefits of cool coatings when applied over building envelopes and urban pavements basically consist of the improvement of indoor and outdoor thermal comfort conditions in summer with evident contribution at several scales: such as the indooroutdoor microclimate scale and the mesoclimate scale, together with the global climate one (Cotana et al., 2014b; Rossi et al., 2013; Akbari et al., 2012, 2009b). At the same time, if the cool coating is also applied over building roofs and facades, it can lower the energy need and the CO2 eq emissions for cooling imputable to the HVAC operation or, as passive cooling technique, it can improve indoor thermal comfort conditions in summer (Pisello et al., 2016c; Wang et al., 2016). Therefore, cool coatings can be effective measurements for counteracting the increasing trends of (i) cooling energy in urban areas, (ii) air pollution and ozone concentration in cities (Sheng et al., 2017), and (iii) urban carbon footprint (Rossi et al., 2016a). Despite the consistent agreement about the effect of high albedo solutions for indoor-outdoor overheating mitigation, the effectiveness of cool coatings is sensitive to both UHI characteristics and building features where they are applied. In particular, UHI represents a complex phenomenon, depending on a variety of factors such as: local weather boundary, geomorphology, urban layout, anthropogenic heat intensity, urban vegetation design, characteristics of building skins and, more in general, of the whole

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Fig. 1. The review paper concept and layout.

urban built environment surfaces (Santamouris and Kolokotsa, 2015). In this view, recent literature contributions showed that cooling need reduction due to cool roofs corresponds to 2–44%, with an average benefit around 20%(Kolokotsa et al., 2012; Synnefa et al., 2006a). The same high albedo surfaces demonstrated to mitigate outdoor local overheating by about 0.2 K for each 10% increase of albedo (Santamouris et al., 2012b). In this panorama, this paper is aimed at reviewing the state of the art concerning the development and testing of cool coatings as energy efficiency and local climate change mitigation strategy, when applied to building envelope and, more in general, to the urban built environment (Fig. 1). Therefore, starting from the analysis of the wide literature background and other reviews in this field (Santamouris et al., 2011), this review deals with the key updates about the new generation of materials and products for cool coatings, their applications as high-albedo urban skins, the final discussions around this matter, basically concerning the current still open issues and futures trends in this field, e.g. life cycle assessment implications, cost-benefit analysis, durability and ageing protocols. Finally, a brief statistical analysis about research products published within this framework has been carried out, showing the massive progress achieved in this scientific field during the course of last five decades.

2. Cool coatings indoor-outdoor behavior As previously mentioned, cool coatings applied over building roofs provide a very effective solution for passive cooling of building indoors and of local outdoor microclimate, when applied over urban pavements. The first generation of cool coatings (Berdahl and Bretz, 1997a) basically consisted of natural materials, intrinsically characterized by high albedo (Doulos et al., 2004; Bretz and Akbari, 1997; Reagan and Acklam, 1979), which is also something recently re-gaining the scientific attention, such as natural stones aggregates having light color and taking also advantage of their permeability, being porous layers for roofs and walkable surfaces (Pisello et al., 2014b; Shokri Kuehni et al., 2016). They typically

present relatively higher albedo compared to standard bitumen layers, e.g. around 0.5–0.7, but not as much as the following generation of artificial cool coatings. Such following trend basically consisted on the development, in-lab measurement and optimization, and field testing of very white coatings, i.e. with albedo higher than 0.8 (Rosenfeld et al., 1995b; Costanzo et al., 2013). In these two phases, the key experimental investigations prepared the ground for the following generation of cool coatings, consisting of nonwhite high-albedo materials also suitable for application in historical contexts or environmentally preserved environments, where the massive application of light color techniques would have massively modified the outdoor visual perception of the built environment (Ferrari et al., in press, 2016a; Levinson et al., 2010a, 2005a). Those materials were characterized by high reflectance within the non-visible region of the solar spectrum, i.e. the near infrared one, where around half of the solar energy is released (Ferrari et al., 2015a). Such cool colored coatings were characterized by a much higher albedo that conventional colors, with a consequent reduction of their surface temperature when exposed to outdoor solar radiation. Synnefa et al. (2007e, 2006a) wrote two reference studies in this field by reporting the test procedures in terms of opticthermal performance of 10 cool colored coatings, elaborated by using high-NIR reflecting pigments achieving the same color of classic ‘‘hot” pigments. Firstly, solar spectrophotometer measurements were carried out in order to identify the SRI optimization, achieving the best performance in the black coating samples where, starting from a SR of 5% the ‘‘cool black” was characterized by a SR of 27%. Then, also thermal emittance tests were performed and they showed negligible differences imputable to the cool pigment inclusion. Finally, superficial temperature has been continuously monitored during the course of 5 months of field experiment, and an acceptable linear correlation was identified in between solar reflectance increase and daily peak surface temperature decrease. Then, the following conceptual step was pursued by analyzing the effect of the developed cool coatings on the energy need or residential buildings, which was achieved in 2006 (Synnefa et al., 2007f). In this work, the authors performed a variety of building thermos-energy dynamic simulation in residential cases with

A.L. Pisello / Solar Energy 144 (2017) 660–680

varying climate conditions and roof solar reflectance, and therefore they found out that cool coatings applied over roofs were able to produce 18–93% decrease in cooling load and 11–27% decrease in terms of peak cooling demand in air-conditioned buildings. At the same time, they also started talking about possible winter penalties, and they identified sort of balanced relation between summer benefits and winter penalties with varying climate conditions. The analysis of such winter penalties was more deeply carried out only several Years later in London, in Canada, in Italy and in the Netherlands (Pisello and Cotana, 2014c; Hosseini and Akbari, 2014; Kolokotroni et al., 2013; Mastrapostoli et al., 2014), where both experimental and numerical studies showed that, in winter conditions or in cold climate areas, the solar forcing is relatively minor given the low sun angle, the short daily sunny time, and the relatively more frequent cloudy sky conditions. Outdoor benefits produced by cool coating applications concern several scale analyses, starting from the benefits of outdoor paving for pedestrians located in close proximity to the cool coating, up to the urban scale and finally to the regional (Savio et al., 2006) and planetary scale, which does not represent the focus of this review but it still deserves to be considered when dealing with cool coating holistic performance analysis. City scale investigations are mainly focused on urban heat island mitigation potential of cool surfaces, such as cool roofs, like it has been studied in Athens (Synnefa et al., 2008b), New York, and many other cities (Mangiarotti et al., 2008; Santamouris, 2015), after demonstrating that more than 400 major cities in the world suffer from increased urban temperatures (Santamouris, 2015). A massive impulse to these studies was originally given by forward-looking policy campaigns, like the ones implemented in the United States more than 20 years ago (Rosenfeld et al., 1995b), where highly promising programs supported by energy utilities and product labeling initiatives started pushing the market, together with the scientific international flow toward high albedo materials application over buildings and streets for urban heat island mitigation. For instance, Levinson et al. (2005b) quantified both energy saving and economic benefits associated to the cool roof implementation in nonresidential buildings in California, highlighting how they are able to decrease cooling electricity use, peak cooling power demand, and cooling-equipment capacity need, while increasing heating energy consumption, with respect to non-cool low-sloped roofs. Their quantitative analysis carried out by means of DOE-2.1E building energy simulation tool prepared the ground for the next implementation of cool-roof technology within the California Title-24 NR building energy efficiency standard for all the building types. The following key progress achieved in US energy policies was reviewed by Levinson and Akbari (2008) in 2008.

3. The cool coating concept A cool coating is a highly reflective covering material that, when applied over a surface exposed to solar radiation, can absorb less heat and stay cooler, compared to a more traditional coating. In this way, it reduces the amount of energy absorbed by a surface where it is applied by means of a selective absorption and reflection of spectral wavelength. This has benefits both at singlebuilding scale and at inter-building scale up to the mesoscale and global climate level, as previously mentioned (Pisello et al., 2016d). In fact, at building scale the application of cool coatings over building skin allows to (i) reduce building cooling loads, (ii) improve indoor thermal comfort, (iii) increase the lifetime of the building roof structure which is less affected by thermal stresses, and (iv) decrease the CO2 emissions in the atmosphere due to the buildings’ HVAC operation especially during summer. At the same time, at city level, cool coatings help mitigating urban heat

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island effect by reflecting most of the incident solar energy back, and therefore, they minimize the heat absorbed and released by irradiated built environment surfaces (Rossi et al., 2016b). High solar reflectance and high thermal emittance, or cool, coatings therefore represent a cost effective, environmentally sustainable and passive solution able to significantly contribute to indoor and outdoor overheating reduction and, at the same time, to the reduction of waste by increasing the lifetime of the roof surfaces where they are applied (Antonaia et al., 2016; Ihara et al., 2016). Furthermore, they can be easily applied in a variety of urban surfaces such as building roofs, paved streets and squares, where they also improve outdoor comfort conditions for pedestrians, which thermal wellbeing is much affected by surface temperature, in severe climate conditions in particular (Santamouris and Kolokotsa, 2015). 3.1. Solar reflectance and infrared emittance As general definition of a cool coating, it is a material which is characterized by combined high solar reflectance (SR) and high infrared emittance (TE) Gentle et al., 2011, where SR is the capability of a textured surface to reflect solar radiation over the hemisphere and the solar spectrum, including the direct and the diffuse component. Having also high infrared emittance, a cool coating is able to re-radiate previously absorbed heat compared to a black body working at the same temperature. Both of these properties are usually measured on a scale of 0–1 or 0–100%. Therefore, when a cool coating is applied over a building envelope or an urban paving, it is able to reflect solar radiation and emit heat and therefore, to reduce its superficial temperature under the sun, resulting in less heat entering the building and less air overheating in close proximity to the urban paving. The standard methods to evaluate solar reflectance of a surface are the ASTM E903-2012, which reports the calculation procedure starting from in-lab experimental data achieved through spectrophotometer with integrating sphere (ASTM E903-12, 2012). Standard ASTM C1549-02 describes the procedure to measure SR by portable solar reflectometer though in-situ method (ASTM C1549, 2002). Also, ASTM ASTM E1918-06 ASTM E1918-06 (2015) describes the test method for measuring SR on horizontal or low-slope large surfaces by means of pyranometers in the field, which is applicable when the sun angle to the normal from a surface is less than 45 degrees. While the reference standard ASTM G173-03-08 (ASTM G173-03, 2008) provide the direct normal and hemispherical reference solar spectral irradiance values with varying coated surface tilt angle, air mass, etc. The standard method to measure thermal emissivity by portable emissometer is the ASTM C1371-2015 (ASTM C1371-15). The best performing cool coatings usually present SR around 80% or more, compared to standard black coatings (e.g. bitumen based ones, etc.) which reflect less than 10% of solar radiation, and they are characterized by thermal emittance values around 0.80–0.90. Both these key properties are sensitive to the most exterior layer of the surface exposed to the solar radiation, that is the reason why they are referred to as ‘‘coatings” for surface applications. Therefore, they hugely affect the thermal balance of a horizontal surface, i.e. roof or urban paving, exposed to the solar radiation, described as follows in steady state conditions (1):

  dT ð1
SRÞI ¼ TEr T 4s
T 4sky þ hc ðT s
T a Þ
k dx

ð1Þ

where SR is the solar reflectance or albedo of the irradiated surface, ranging from 0 to 1. I is the solar irradiance [W/m2].

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TE is thermal emittance ranging from 0 to 1. r is Stefan-Boltzmann constant (5.6685  10
8 [W/m2 K4]). Ts is the surface temperature of the given surface [K]. Tsky is sky conventional temperature [K]. hc is the convection coefficient [W/m2 K]. Ta is the air temperature [K]. k is thermal conductivity of the same surface [W/mK]. dT is temperature gradient in the surface layer, describing temdx perature change in the considered direction x. The previous balance (1) shows that, if the cool coating is applied over a thermally insulated roof exposed to solar radiation, its thermal behavior is mainly affected by SR and TE, being the k component relatively less important. More in details, during daily hours, when the sun forcing represent the key boundary affecting the surface temperature of the coating, SR is the main parameter that, if optimized, can reduce the surface temperature Ts. During nocturnal hours, on the other hand, the night-time performance is much affected by infrared emittance, affecting the surface capability to re-radiate heat toward the sky. 3.2. Solar reflectance index Since cool coatings are exhaustively characterized by considering of both solar reflectance and infrared emittance, a calculated parameter named SRI (Solar Reflectance Index) was proposed for incorporating SE and TE into one unique value. A dedicated international standard was developed in order to report the ‘‘Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces” (ASTM E1980-11, 2011). In particular, SRI describes the passive cooling potentiality of a flat surface compared to a standard black (with SR of 5% and TE of 90%), having a SRI of zero, and a standard white (with SR of 80% and TE of 90%), having a SRI of 100. It is calculated as follows (2):

SRI ¼


T black
T surface  100 ðT black
T white Þ

ð2Þ

where Tblack, Twhite and Tsurface represents the surface temperatures of the standard black, white, and the tested material surface, respectively, in steady-state conditions. Therefore, high performance cool coatings could have an SRI higher than 100, while dark and absorbent materials have SRI lower or close to zero. Given the complexity of further variables affecting both indoor and outdoor overheating mitigation potential of cool coatings, SRI calculation does not represent an exhaustive parameter for completely determining the cool coating performance, since, as previously studied within a variety of experimental and numerical works, it is much affected by climatological parameters, such as spectral characteristics of the solar radiation, ambient temperature and relative humidity, wind speed and all the other parameters mainly affecting weatherization of cool coatings, e.g. precipitation, pollution, etc. (Levinson et al., 2005c; Levinson and Akbari, 2010). Within this framework, the following section will deal with the analysis of the most promising cool coating materials (Section 3), by presenting the key achievements in this field at both building and city level (Section 4). 4. Materials for cool coatings 4.1. Cool roof and cool façade coatings Most of the available cool roof products and technologies are developed for flat roof application, but many solutions are recently being implemented also in tilted roof surfaces. They are typically

membranes, coatings, paintings, metal roofs, shingles, and tiles. Generally, cool or reflective roofs are single ply or liquid applied. Typical liquid products include white paints, elastomeric, polyurethane or acrylic coatings, i.e. EPDM (Ethylene-Propylenediene-Te trolymer Membrane), PVC (Polyvinyl Chloride), CPE (Chlorinated Polyethylene), CPSE (Chlorosulfonated Polyethylene), and TPO (Thermoplastic Polyolefin) materials (Santamouris and Synnefa, 2011). Generally, a smooth black asphalt roof with an initial reflectance of 0.04 can reach a 0.80 if coated with a smooth white surface (Levinson et al., 2005a; Berdahl and Bretz, 1997b). Additionally, a black single-ply membrane with initial reflectance of 0.04 can reach 0.20 and 0.80 solar reflectance is if gray and white, respectively. Finally, a bitumen membrane with a reflectance of 0.1–0.2 can achieve solar reflectance value of 0.65–0.7 if a white coating is applied on top. On the other hand, cool white coatings, which are typically elastomeric or cementitious, have solar reflectance values ranging from 0.7 to 0.85. Silver aluminum coatings contain aluminum flakes in an asphalt-type resin able to increase the solar reflectance to >0.5 for the most reflective coatings (Akbari et al., 1998; https://www.energystar.gov/). A full list of cool roof materials meeting certain criteria is provided by Energy Star Roof Products (http://coolroofs.org/): - low-slope roofs: initial solar reflectance > 0.65 and aged reflectance > 0.50; - steep-slope roofs: initial solar reflectance > 0.25 and aged solar reflectance > 0.15. Databases with information about the solar reflectance and the infrared emittance of commercial roof products are also provided by the US Cool Roof Rating Council and the EU Project Cool Roofs (http://coolroofcouncil.eu/; Zhang et al., 2007). Experimental studies (Akbari and Taha, 1992; Levinson et al., 2007a) showed that surfaces with low solar reflectance and high infrared emittance (e.g. black coating, asphalt shingle, black gravel surface) reach temperatures up to 75–80 °C, surfaces with medium to high solar reflectance and low infrared emittance (e.g. unpainted metal roofs, aluminum coatings) reach temperatures up to 60– 75 °C and surfaces with high solar reflectance and infrared emittance (cool white coatings, white membranes, etc.) reach average temperatures of 45 °C, depending on local conditions. 4.2. Cool roof paintings One of the most effective cool roof solution is represented by paintings and coatings. In fact, cool coatings are documented to have a superior thermal performance even compared to other cool materials like white marble and white mosaic. Statistical analyses showed differences in the thermal behavior even among coatings of the same type and color, mainly due to the differences in (i) their reflectance, affecting their performance during the day, and (ii) their emissivity, influencing the thermal performance during the night in mid-latitude summer conditions (Synnefa et al., 2006b). The solar reflectance of white coatings is essentially due to the presence of transparent polymers and white pigments. The most commonly used pigments are represented by the zinc dioxide, the titanium dioxide (mostly in the form of rutile), presenting a solar reflectance between 0.70 and 0.85. The use of selective cool ‘‘paints” allows to enhance the thermal-optical properties of materials’ surfaces by modifying their interactions with incident solar radiation. In fact, white and cool selective paints in general can significantly improve the hemispherical solar reflectivity value of a material. The absorption of cool selective painted surfaces is almost never higher than 0.20 compared to the 0.93–0.96 of black surfaces.

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Many research studies investigated the passive cooling potential of such products. For instance, 14 different reflective paintings were monitored by (Synnefa et al., 2007c) in terms of surface temperature, spectral reflectance, and infrared emittance. A reduction by 4 °C and 2 °C was registered in summer conditions during the day and the night, respectively, by demonstrating the major thermal-performance of such cool solutions compared to more traditional non-cool ones. Additionally, near-infrared reflective colored pigments able to maintain lower surface temperatures than conventionally pigmented color-matched coatings were developed in (Uemoto et al., 2010). This temperature difference is indeed mainly due to differences in solar reflectance. These cool colored coatings can be used on buildings (roofs and walls) and other surfaces in the urban environment. Thus, at building scale, the use of cool colored coatings can improve building comfort and reduce cooling energy use, while at city city-scale it can contribute to the reduction of outdoor air temperature due to the heat-transfer phenomena and therefore improve outdoor thermal comfort by reducing the heat-island effect. The thermal performance of cool colored acrylic paints containing infrared reflective pigments in comparison to conventional colored acrylic paints of similar colors (i.e. white, brown, and yellow) applied on sheets of corrugated fiber cement roofing was also investigated according to ASTM D 2244-2016 ASTM D2244-16, 2016), the UV/VIS/NIR, and ASTM E 903-2012. Results demonstrated that the cool colored paint formulations produced significantly higher NIR reflectance than conventional paints of similar colors, and that the surface temperatures were more than 10 °C lower than those of conventional paints when exposed to infrared radiation. Moreover, in Xue et al. (2015) the optical properties and the cooling effect of a bi-functional cool white roof coating based on styrene acrylate copolymer and cement integrating both a good cooling effect and good mechanical properties and impermeability was studied. Such cool white coating has a much higher solar reflectance compared with the conventional polymer-modified cement-based waterproof coatings due to the presence of the titanium dioxide rutile and Altiris 800, and can significantly reduce surface temperatures. Therefore, a significant thermal benefit can be achieved by replacing dark colored materials with materials of the same color including near infrared reflecting pigments. Synnefa et al. (2007d)measured the temperature differences between 10 prototype cool colored coatings/paints by using near infrared reflective complex inorganic color pigments and an acryl based binder and 10 traditionally non-cool pigmented coatings of the same color. The maximum temperature difference observed was 10.2 °C in summer between the cool and the standard black, associated to a solar reflectance difference of 0.22 (i.e. between 0.05 and 0.27 of the standard and cool black, respectively). In winter the temperature difference between cool and standard colored coatings was found to be less than 1 °C. 4.3. Cool waterproof membranes Cool roofing membranes are also a very diffuse way to increase roof solar reflectance and therefore passive cooling potential. It was proved that innovative cool membranes can optimize building year-round energy efficiency by up to 19.3% (Pisello et al., 2016e). An innovative waterproof polyurethane-based membrane was experimentally tested both in laboratory and in-field before and after optimization by means of titanium-dioxide and ceramic microsphere (Revel et al., 2014a). Solar reflectance values between 80.7% and 85.4% were detected compared to the more traditional and darker membrane, i.e. 26%. In other studies (Revel et al., 2014b; Ramamurthy et al., 2015) an innovative white membrane

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based on a mix of metal oxide pigments i.e. CoAl, FeCr, and NiSbTi characterized by an increased NIR solar reflectance of +0.40 compared to dark traditional membranes was developed. Such formula was able to improve the global SRI by 26 points without affecting the thermal emittance. Despite their good thermal-energy performance given their capability to reduce peak surface temperatures, cooling energy need, and offset CO2, highly reflective roofing membranes suffer from weathering, soiling, and biological growth that might affect their solar reflectance. The solar spectral reflectance values of 12 different roofing membranes was measured in Paolini et al. (2014) before the exposure and after 3, 6, 12, 18, and 24 months of natural ageing in different Italian cities. Results showed a decrease of solar reflectance with time by 0.14 in Roma and 0.22 in Milano after two years. A new and aged cool roofing membrane was assessed also in Turkey (Kültür and Türkeri, 2012) by means of lab and field experimental campaign, confirming a slight reduction of solar reflectance capability mainly due to weathering effects. Most recent research efforts in the field of cool roofing membranes are focused on the combination of cool materials and thermal storage effect as passive technique for building thermalenergy efficiency and urban heat island mitigation. For instance, an innovative cool polyurethane based membrane with integrated phase change materials (PCMs) was proposed (Pisello et al., 2016a) for combining the thermal benefits of high-albedo with improved latent heat storage capability in the view of surface peak temperature buffering and durability improvement due to minor thermal stress within the polymer-based membrane (Pisello et al., 2017). The promising results produced highlighted also the relatively simple production process of PCMs inclusion into cool coatings. Consistently with previous works, Lu et al. (2016a) investigated the effect of encapsulated PCMs into cool coatings when applied over building envelopes and they showed promising effects in terms of peak temperature reduction and potential energy efficiency improvement due to the lower heat flow through the envelope. At the same time, given the thermal buffer effect of thermalenergy storage systems such as phase change materials, their effect when applied as urban heat island mitigation technique is controversial, since their delayed heat release may lead to larger air temperature rise, if during nighttime. 4.4. Cool tiles Among the main solar reflective roofing materials, ‘‘cool” tiles can be listed as one of the most original technique, suitable for integration in existing and historical buildings given its low aesthetic and architectural impact. In fact, traditional tilted roofs are generally characterized by ceramic tiles of traditional colors such as brick red with generally low solar reflectance that can cause overheating of the building due to solar gains during summer. Even if polymeric products dominate nowadays the cool roof market, the use of cool ceramic-based tiles can guarantee a very good cool roofing performance by combining great architectural quality and thermal-energy purposes i.e. good optical properties and durability (Ferrari et al., 2015b). Ceramic products in fact are naturally characterized by a high thermal emissivity and chemical-physical durability. In Ferrari et al. (2016b) a traditionally engobed porcelain tile was used as a substrate for a new generation of colored glazes characterized by different surfaces reaching promising values in solar reflectance. Cool-colored tile with relatively high solar reflectance, combined with a thin insulating layer and made of a silicagel super-insulating material with an aluminum foil with very low thermal emittance applied below the insulating layer were also developed in Pisello (2015). Such composite tiles can provide a strong increase of roof thermal resistance, helpful to control either heat loss in winter and building overheating in summer. They can

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be installed on an existing roof, for instance the sloped tile roof of a historical or traditional building, with no need to modify the roof height and structure. In this same scenario, the optimization of the traditional clay tiles with the purpose of building energy retrofit according to the constraint to preserve architectural heritage was performed by Pisello et al. (2016b) and Miller (2005). Therefore, innovative cool clay tiles with 0.75 of solar reflectance but with the same appearance of traditional historical tiles were implemented. Such tiles are able to guarantee a maximum primary energy saving for cooling of 51% with a heating energy penalty lower than 2%. Therefore, these tiles represent an effective non-invasive strategy to (i) optimize thermal-energy performance of historic buildings even in temperate climate, and (ii) mitigate urban climate. In fact, the heat penetration can be reduced up to 70% by substituting a concrete tile with a cool tile (Miller and Kriner, 2001). Moreover, it was demonstrated that a 14 °C surface temperature difference can be reached with a cool steep-slope roof compared to a conventional dark roof (Mangiarotti et al., 2008). Six cool colored tiles with a solar reflectance difference between 0.15 and 0.41 with respect to the same traditional tiles were developed and tested in 1:10-scale model houses in California with insulation (R = 1.9 m2 K/W). Surface temperatures reduced by 5–14 °C and a heat flux reduction of 13–21% were detected (Levinson et al., 2007b, 2010b). Different techniques to increase solar reflectance of concrete tiles are available. A novel strategy based on a two-layer spray coating process where both layers are pigmented latex paint based on acrylic or PVDF/acrylic technology was developed by Levinson et al. (2007a). Such innovative method was applied by using as first layer a TiO2 rutile white and as second layer cool color topcoat with weak NIR absorption and/or strong NIR backscattering instead of using more traditional thick white acrylic basecoats and cool color acrylic topcoats. A significant increase of the initial solar reflectance was found, ranging from 0.26 (dark brown) to 0.57 (light green) for the cool colored tiles ranged and from 0.18 (dark brown) to 0.34 (light green) for the cool colored shingles. Methods based on one-coat (substrate/topcoat) and two-coat (substrate/ basecoat/topcoat) systems depending on the near infrared reflectance of the substrate were also proposed (Synnefa et al., 2006a; Levinson et al., 2014). For metal and clay tile products that have originally high near infrared reflectance, a topcoat containing near infrared reflecting pigments was used. For graycement concrete tile or gray aggregate with low near infrared reflectance, a cool topcoat or basecoat with high near infrared backscattering was applied. By means of the one-coat (cool topcoat) process to metal and glazed clay-tile roofing products, near infrared reflectance values of 0.50 and 0.75, respectively, were achieved. The application of a thick coating colored by rutile white near infrared scattering pigments on gray-cement concrete tiles lead to near infrared reflectance values of 0.60. A two-coat process (TiO2 rutile white basecoat plus topcoat colored by near infrared transparent organic pigments) resulted in a near infrared reflectance of 0.85. The optical properties and the thermal performance of 14 types of reflective coatings (white and aluminum ones) have been measured by Synnefa et al. (2006a). It was found that a cool coating can reduce a white concrete tile’s surface temperature by 4 °C and by 2 °C in day and nighttime, respectively, during summer. Therefore, given the large application potentiality of cool clay and cementbased tiles, they represent a key cool roof solution for mitigating urban heat island in historical cities or traditional architecture built environment, even if their specific solar reflectance capability is relatively lower than high-performance cool coatings previously dealt with, which can be implemented in a relatively minor range of building typologies and urban contexts.

4.5. Cool natural materials Another kind of cool roof materials is represented by natural materials, which can generate important benefits to the buildings’ thermal-energy performance given their intrinsic optimal thermaloptical properties, both at single-building scale and urban scale. The use of naturally cool materials allows indeed to combine intrinsic cool properties with low-cost and low-embodied energy to achieve a satisfying thermal performance with reduced environmental impact. Gravels (Levinson et al., 2014) are detected to be one of the most commonly used ‘‘natural” cool roof materials for horizontal application, especially in the Mediterranean area, as they can be simply applied over the existing bitumen membranes generally installed on the existing roofs. Such material is very sustainable from both an environmental and economic perspective, especially if locally available and due to the naturally light-colored stones. The passive cooling potential of different types of gravels characterized by different grain size was assessed in Pisello et al. (2014a). It was found out that the albedo increases with decreasing grain size. In this same scenario, also light-colored marble has been designated as a cool natural material due to its intrinsic optimal cool characteristics (Rosso et al., 2014). Results from experimental testing of the cooling potential of such material show solar reflectance values up to 79%, while dynamic simulations allow to detect up to 18% of summer cooling energy saving compared to a traditional non-cool concrete envelope.

5. Multiple effects of cool coatings applications 5.1. Cool coatings for cooling energy saving The application of cool roofs application in new and existing buildings can significantly improve the energy efficiency during the cooling season and throughout the year. Several research studies focused on the quantification of the energy saving achievable my means of cool roof application in both residential and nonresidential buildings (Akbari and Konopacki, 2004). Such energy savings are reported to be between 2% and 44%, with an average of 20%. Such percentages vary depending on the specific boundary conditions in terms of local climate phenomena, building envelope characteristics, sky view factor, building type and use, HVAC systems, etc. Many different approaches can be used to quantify the actual energy saving directly attributable to the application of a cool roof, i.e. experimental assessment by means of in-field monitoring and numerical analysis. The experimental monitoring can be performed by means of dedicated microclimate indoor-outdoor stations, while the numerical analysis can be carried out by means of calibrated dynamic simulation that allows to accurately predict the annual energy saving due to the use of a cool roof system with varying weather conditions and climate scenarios, in both commercial and residential constructions. For instance, the cooling energy savings due to the application of cool materials for 240 regions in the United States was calculated to be between 12% and 25% for residential buildings between 5% and 18% for office buildings, and between 7% and 17% for commercial buildings (Miller et al., 2002).

5.1.1. Residential buildings The most common cool roof solutions consist of high albedo and high-emittance coatings or membranes applied over commercial buildings situated in mild or hot climate areas. Nevertheless, the

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application of cool roof solutions over residential buildings is nowadays becoming more frequent. In fact, significant energy savings can be achieved by applying a cool roof over a residential building, with the intensity varying depending on the specific building characteristics and use, local ambient conditions, type of installed HVAC systems, etc. In fact, cool roofs represent a relatively inexpensive technique to reduce the summer building energy requirements for cooling, and they can usually be easily applied over existing roof also in the context of refurbishment or maintenance processes, by respecting the architectural and aesthetic constraints of the building. Energy savings are usually detected to be more important in ancient houses with little or no roof insulation. Up to 15% cooling energy saving were detected by applying a cool brown coating on traditional brown tiles with a solar reflectance of 0.3 (Pisello and Cotana, 2014a). Additionally, an innovative cool clay tile was developed in tested in (Rosado et al., 2014) on a traditional residential building in central Italy to improve the thermal conditions of the indoor environment that is adjacent to the roof. The year-round experimental observations revealed a maximum decrease if the summer peak indoor overheating of the attic by 4.7 °C. The corresponding winter maximum overcooling reduction was found to be 1.2 °C. Such innovative cool roof solution was therefore identified as suitable for implementation on traditional sloped roofs with a clay tile covering, producing non-negligible thermal benefits in summer and relatively small penalties in winter, even in temperate climates. In the same scenario, a continuous monitoring of two similar single-family, single-story homes built side by side in Fresno (California) was carried out for a year in order to assess cool-roof benefits in terms of temperatures, heat flows, and energy uses of a cool roof (albedo 0.51) compared to a traditional roof (albedo 0.15) Dabaieha et al., 2015. The annual cooling energy saving directly attributable to the cool roof application were detected to be equal to 2.82 kW h/m2 (i.e.26%), and the peak-hour cooling power demand reduction was 0.88 W/m2 (i.e.37%). Moreover, a consequent reduction of 15%, 10%, and 22% CO2, NOx, and SO2 emission due to the annual conditioning were measured. Other works (Dias et al., 2014) analyzed the cool roof passive cooling capability in very hot and dry climates, where almost half the urban peak load of energy consumption is used to satisfy airconditioning cooling needs in summer. A hybrid algorithmic was designed to simulate 37 roof design probabilities in terms of roof shape, material and construction. The results of using a roof with high albedo coating show a fall of 53% in discomfort hours and a summer energy saving of 826 kW h compared to the conventional non-insulated and non-cool flat roof in typical Cairo residential buildings. The impact of cool roof solutions i.e. paints on the thermalenergy behavior of residential buildings was also assessed by means of numerical analysis (Zingre et al., 2015). Maximum indoor free-floating temperature reductions between 2 and 7.5 °C due to an increase from 50% to 92% of the roof solar reflectance was simulated by means of ESP-r for a house in Portugal, with a cooling annual energy saving around 30%. In Akbari et al. (2005a), an innovative cool roof heat transfer model for double skin roofs is developed to predict the heat transfers for a double-skin roof combined with cool roof. The model was applied and tested in the case of ventilated apartments in Singapore. The white-color cool coating on a flat double-skin roof was able to reduce the daily heat gain by 0.21 kW h/m2 (i.e. 51%), and the peak indoor air temperature on a typical sunny day by 2.4 °C. 5.1.2. Commercial buildings The thermal-energy benefits achievable by applying a cool roof over non-residential constructions are detected to be significant

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within different climate boundary conditions. In particular, a positive net energy saving has always been registered for warm and temperate climatic conditions (Akbari et al., 2005a), with very low winter penalties. This is mainly due to the lower, less intense, and less available winter solar radiation. The main benefits are related to (i) reduced cooling energy demand and (ii) improved indoor thermal comfort conditions, depending on the construction features of the building, external weather conditions, and use of the building. Summer daily air-conditioning savings and peak demand reductions of 10–30% are reported for non-residential buildings situated in warm-weather climate conditions (Akbari and Konopacki, 2005). More precisely, the energy savings within the range from approximately 250 kW h/year for mild climates to over 1000 kW h/year for very hot climates were detected by Kolokotroni et al. (2013), by increasing the roof albedo from a 0.1 to 0.4. In the case of moderate and temperate climates with high buildings’ heating demand, the type, operation, and thermal characteristics of the building should be carefully taken into account while predicting the potential benefits of the application of cool roof technology. For the case of a naturally ventilated office building in London (UK) a roof albedo of 0.6–0.7 was selected to be the optimum value to (i) achieve energy savings in a cooled office and (ii) improve summer internal thermal conditions in a non-cooled office by means of combined experimental and numerical approach (Virk et al., 2015). A modeling study carried out on a typical office located in the same London area showed a maximum air temperature reduction by 1 °C (Bozonnet et al., 2011a). The same approach was used to investigate the performance of a cool roof for a public low-rise building in Poitiers (France) by means of dynamic simulation (Romeo and Zinzi, 2013). A decrease up to 10 °C of the surface temperature was registered, with low differences for lower temperatures, but a strong impact on the highest temperatures. The monitoring and modeling of a cool roof applied in an office/ laboratory building belonging to a school campus in Trapani (Italy), was performed in (Kolokotsa et al., 2012). A reduction of the surface temperature of the roof up to 20 °C was found, together with an average reduction of 2.3 °C of the operative temperature during the cooling season. Moreover, a 54% reduction of the cooling energy demand could be predicted. The application of cool roofs’ technology in a laboratory building located in Iraklion, Greece, revealed energy conservation equal to 19.8% for the whole year and 27% for the summer period. The cool roofs’ application was found to be the most effective solution compared to increased insulation or windows improvement for the building (Synnefa et al., 2012). Similarly, a decrease in the air temperature up to 2.8 °C and a decrease in the annual cooling load by 40% was detected for a school building in Athens, Greece (Garg et al., 2016). The corresponding heating penalty i.e. the increase of heating load was calculated to be 10%. The impact of cool roofs in hot and humid climate was also studied (Touchaei et al., 2016). Two classrooms of two unconditioned Indian school buildings of same size, function, and occupancy from both the school buildings were monitored for ten weeks. An average reduction and peak reduction in indoor air, roof underdeck, and roof surface temperatures of 2.1 °C, 5.0 °C and 12.3 °C and 4.3 °C, 10.0 °C and 26.3 °C and 1.5 °C, 4.0 °C and 9.5 °C, and 3.3 °C, 4.2 °C, and 25.2 °C were detected for the white roof and gray roof in one school and in the other school, respectively. Four building prototypes in Montreal (Canada) i.e. small office, medium office, large office, and retail store with two types of

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heating systems i.e. gas heating and heat pump (for a total of 24 cases) were studied by means of DOE-2 building energy model to (i) determine the direct effect of increasing albedo of an individual building, and (ii) predict the effect of albedo enhancement in urban scale (Cotana et al., 2014a). The energy consumption and expenditure showed that, in general, increasing the reflectivity of building skin can save energy and money. The maximum expenditure savings was detected to be about 11% for buildings with dark roof and walls, where the contribution of urban-scale increase in the surface albedo represented 4% in the total energy savings. 5.2. Cool coatings for urban heat island mitigation It is well documented that the increase of roofs’ albedo at larger scale i.e. urban scale can significantly help mitigating local climate phenomena such as urban heat island, by lowering local peak air and surfaces temperatures due to less heat transfer from the cooler roof surface to the ambient air. Several analyses have been performed both at meso-scale and meso-urban level by means of meteorological modeling, with the aim of quantifying the impact of the increase of surface albedo. In particular, both simulation and experimental studies have been performed to investigate the impact of various albedo mitigation techniques on the reduction of ambient temperature. Most of the existing works evaluate the impact of a general increase of the local albedo by taking into account a combination of cool roofs, cool pavements roadways, parking lots, and other urban heat island mitigation strategies i.e. green roofs (Gilbert et al., 2016a; Pomerantz et al., 1999), while still few studies consider just the mitigation impact of reflective roofs at large scale, despite the fact that they represent almost 25% in most cities. Among such studies focusing on the evaluation of passive cooling potential of cool roofs as a urban heat island mitigation technique, there are some (Savio et al., 2006; Synnefa et al., 2008a) focusing on the evaluation of the local impact of cool roofs at city-scale, while others (Menon et al., 2010; Jacobson and Ten Hoeve, 2012) investigate the climate impact of reflective roof on a planetary scale. In (Savio et al., 2006) a decrease of the daily average temperature between 0.18 K and 0.36 K (with peaks ranging between 0.31 K and 0.62 K at 3 pm) at 2 m in the various parts of New York city, (US) was detected by means of simulations performed with the Penn State/NCAR MM5 regional climate model by considering an average solar reflectivity equal to 0.5. The climate impact of cool roofs was also investigated for the city of Athens, (Greece) (Synnefa et al., 2007c) by means of simulation with the MM5 climate model for a typical summer day. It was found out that for a low moderate increase of the albedo, i.e. from 0.18 to 0.63, the ambient temperature depression at 2 m at 12:00 LST varies between 0.5 and 1.5 K. For an higher albedo increase, i.e. 0.85, the ambient temperature reduction varies between 1 K and 2.2 K. Therefore, the expected rate of temperature reduction per 0.1 increase of the albedo of roofs is documented to be between 0.1 and 0.19 K for New York and 0.11–0.33 K for Athens, with an average depression for both studies of about 0.2 K per 0.1 increase of roof albedo. Numerical analyses were also performed by using the CSCRC model (Grell et al., 1994) in order to compute the impact of cool roofs in neighborhoods with medium and high rise buildings. In particular, a street-level temperature reduction of 0.1 and 0.12 K for the areas of high and medium height rise buildings, respectively, was detected in association to a change of the albedo from 0.2 to 0.5. Moreover, it was simulated that a worldwide conversion to cool roofs, i.e. by increasing the overall urban albedo by 0.147) will contribute to decrease populated weighted temperatures by 0.02 K but to increase the overall earth temperature by 0.07 K (Yoshida et al., 2000). Similarly, a diurnal temperature decrease

of 0.3 K was calculated by considering a 0.9 increase of the urban roof albedo by using the urban canyon model CLMU coupled with other models (Chen et al., 2004). Other studies about the quantification of the climate impact of cool roofs application were performed by for the city of Los Angeles (USA) Jacobson et al., 2007; Oleson et al., 2008; Sailor, 1995; Mahrer and Pielke, 1977. In detail, a reduction of 0.5 K with peaks of 1.4 K was detected in association ot an increase of the albedo of 0.14. Additionally, by assuming an increase of the average albedo from 0.13 to 0.26 for an area of 100,000 km2, it was calculated that the peak impact of the albedo change occurs in the early afternoon and the potential cooling exceeds 3 K at 3 pm. The simulated peak summertime temperature reductions were between 2 and 4 K. In the same scenario, a cooling potential of 1.5 K was found to be attributable to a roof albedo (1250 km2) increased from 0.15 to 0.5 for the same area (Rosenfeld et al., 1998a). A summary of the effect of the increased albedo at city scale is provided in Millstein and Menon (2011) for various American cities, by means of the Weather Research and Forecasting model (Skamarock et al., 2008). Based on the assumption that roofs typically represent around 25% of the urban built area, and by considering an albedo increase of 0.25, a decrease of the average afternoon summertime temperatures by 0.11 K was predicted. A temperature depression of about 0.3–0.5 K corresponding to an increase of the local albedo by 0.1 was predicted for the city of Philadelphia (USA) Sailor et al., 2002. Furthermore, average temperature decreases by 1–3 °C were detected during the day with no decreases during the night for several cities in USA, Canada, and Greece (Sailor, 1995; Taha, 2008a, 2008b; Taha et al., 1999; Synnefa et al., 2008a). A 1D urban canopy and building-energy simulation model was implemented in (Akbari and Taha, 1992) to determine the annual environmental impacts of various UHI mitigation measures. It was found out that an albedo increase from 0.2–0.8° could reduce the global number of hours where the air temperature is >30° by 60 h, leading to an important reduction of buildings’ peak cooling loads. A local climate cooling in south-eastern Spain was also provided by (Campra et al., 2008), by means of experimental analyses of white-roofed greenhouse farming. Multiyear observations showed a 0.3 K temperature reduction per decade due to the massive construction of high albedo greenhouses through the Almeria area (Spain). A numerical study showed a maximum UHI intensity of 2.2 °C with peaks of 2.4 °C in early morning in the industrial region of west Singapore (Li and Leslie, 2016). The results of sensitivity tests revealed that the application of cool roofs at city scale could lead to the reduction of the near-surface air temperature and surface skin temperature during the daytime with negligible effects at night. Moreover, the application of cool roof materials at urban scale can lead to a significant reduction of air pollution both directly and indirectly (Rosenfeld et al., 1995a; Bretz et al., 1997). In fact, by reducing the use of HVAC systems to cool down the indoor building environment cool roofs, fewer power plant emissions will be released in the atmosphere (CO2, NOx, and PM10 particles). Additionally, the lower urban air temperatures generated by cool roofs can slow down the smog formation by slowing down the ozone formation (Rosenfeld et al., 1998b). A number of studies (Taha, 1997; Taha et al., 1997) proved that a reduction of 10–20% in population weighted smog (ozone) can be achieved by decreasing the air temperature of Los Angeles by 1.5 °C–2 °C using heat island mitigation strategies. Moreover, an offset of the equivalent of 44 Gt of CO2 emissions was estimated by Akbari et al. (2009a) by increasing the albedo of roofs by 0.1 in low- and mid-latitude cities worldwide, with a significant negative radiative forcing at a global scale.

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5.3. Cool coatings for indoor thermal comfort optimization

5.4. Cool coatings for outdoor thermal comfort optimization

The capability of cool roof technologies to improve indoor thermal comfort in free-floating conditions has been largely assessed (Hernández-Pérez et al., 2014a; Synnefa et al., 2007d), many of them with the aim of identifying the cooling potential and reduce discomfort hours (Vincent and Huang, 1996; Hernández-Pérez et al., 2014b; Cheng et al., 2005; Akbari et al., 2005b, 2005c). Typically, peak summer indoor temperatures may decrease up to 2 °C in moderately insulated buildings while cooling loads reductions range between 10% and 40%. At the same time, the winter heating penalty may be between 5% and 10% depending on the local climate boundary conditions and building characteristics. The effect of color on indoor temperatures in hot humid climates was assessed in Xu et al. (2012a) by means of test cells. The maximum air temperature inside the black cell was detected to be higher by about 12 °C than that of the white cell. Additionally, the indoor air temperature of the white cell was only 2–3 °C higher than the outdoor. Several experimental and numerical studies showed indoor temperature decrease averagely between 1 and 2.7 °C (Bozonnet et al., 2011b; Boixo et al., 2012). The thermal performance of a concrete roof with and without reflective coating was determined with reference to a building in Mexico (Cheng and Givoni, 2005) by means of a pseudo-transient study solving the steady state heat conduction equation using the finite volume method and hourly averaged climatic data as boundary conditions. The cool roof was detected to be able to reduce the indoor surface temperature up to 28 °C at 12.00 p.m. compared to the original gray concrete roof. It was demonstrated for Southern and Mid Latitude Europe climate conditions that the use of cool materials minimizes the roofs’ heat stress (Kolokotsa et al., 2013). For instance, for roof albedos equal to 0.8, 0.7 for the city of Chania, the sensible heat flux is close to zero while the maximum value varies between 15 and 50 W/m2. On the contrary, conventional roofs present a median sensible heat flux close to 30 W/m2, and a maximum flux value around 270 W/m2 in summer. Therefore, conventional roofs are proved to send between 100 and 170 kW h/m2 additional sensible heat to the atmosphere during summer compared to highly reflective roofs. It was also reported that cool-colored paint formulations with higher near infrared radiation reflectance than conventional paints of similar colors can reduce surface temperatures by more than 10 °C (Sekar et al., 2012). In hot climates, cool roofs guarantee a heating up of the roof structure only up to 43–46 °C (Van Tijen and Cohen, 2008). A study carried out in a school building in Kaisariani, Athens (Greece) showed that the application of white elastomeric cool coating over the existing concrete roof lead to a reduction of the indoor air temperature by 1.5–2.0 °C during summer and 0.5 °C during winter (Latha et al., 2015). Moreover, a reduction of indoor air temperature by 0.9 °C was achieved by applying cool eco-friendly paint of the roof of a public building in Trapani (Italy). The application of a cool roof paint on an office building at Brunel University (UK) allowed to reach and indoor air temperature reduction by an average of 3–4 °C (European Cool Roof Council, 2012). The improvement of indoor thermal comfort conditions in residential buildings in 27 cities around the world for different climatic conditions, including Mediterranean, humid continental, subtropical arid, desert condition was investigated in Synnefa et al. (2007a) by increasing the solar reflectance from 0.2 to 0.85. A decrease of the hours of discomfort by 9–100% was found out, together with a maximum reduction of the temperatures by 1.2– 3.3 °C. These reductions were found to be more important for poorly or non-insulated buildings.

The analysis of cool coating in terms of their passive cooling effect on indoor environment, as previously reported, has been attracting investigators’ attention during the last decades. Important findings refer to the effect of such techniques on outdoor environment and citizens-pedestrians’ comfort conditions, as widely examined in a recent review paper published in this same journal special issue by Santamouris et al. (2016). More in details, there are studies only focusing on the UHI mitigation effect of cool coatings when applied over building roofs and paving solutions, and there are just a few examples where façade and paving applications are investigated for their effects on people thermal and visual perceptions. In this view, Tsitoura et al. (2016), for instance, proposed a combined multiparameter analysis where they analyzed possible interventions that can be implemented during urban design actions and city renovation in the Mediterranean area. More in details, they investigated the effect of key urban design parameters such as (i) height to width ratio of an urban canyon, (ii) sky view factor, (iii) greenery percentage, (iv) pavement material properties. The key results showed the promising effect of the climate sensitive urban design by simulating important reduction of air temperature measured at 1.8 meters above the ground (i.e. up to 1.7 °C) and a surface temperature reduction up to 8.5 °C. Such data also supported the improvement of outdoor comfort conditions by minimizing the degree hours above 26 °C by 46%. In the same view, Yan et al. (2014) studied the ‘‘intra-urban air temperature variability” due to anthropogenic causes in Beijing, where they quantified the effect of landscape and urban planning on the air temperature variation up to 7 °C. An interesting outdoor multi-scale modeling analysis was also recently proposed by Yang et al. (2016) and applied in a case study in Phoenix, where the authors highlighted the further need of research investigation about the thermal interactions between buildings and the ambient environment while investigating cool coating systems. They studied in particular the effect of several thermal-energy and radiative properties characterizing cool paving materials such as solar reflectance, heat capacity, and thermal conductivity. The study helped clarify the need for consideration of building-environment thermal interactions in numerical simulation model at inter-building and urban levels, since they demonstrated to play a crucial role in affecting thermal conditions in urban canyons and dense built areas in general. The scientific interest around the possibility to design outdoor spaces, e.g. neighborhoods, districts, etc. as more resilient to local overheating due to climate change opened the doors to several urban planning applications supported by microclimate analysis as key driver for building the city districts of the future. In this view, Martins et al. (2016) quantified the benefits associated to passive cooling strategies named Urban Cool Island measures, e.g. white and green scenarios, and they identified the massive benefits imputable to water bodies and greenery (average PET drop up to 7 °C) compared to high albedo solutions and aspect ratio (up to PET reduction by 1 °C). Such numerical effort while computing the effect of cool paving surfaces on pedestrians’ perception still needs massive scientific investigation at the moment, while a large variety of experimental studies demonstrated non-negligible benefits produced by the outdoor passive cooling contribution imputable to high-albedo coatings in urban areas (Doya et al., 2012). In particular, Doya et al. reproduced an experimental outdoor physical model of a street canyon morphology in France where they have been continuously monitoring the superficial temperature and thermal flux of building-like facades with varying nearinfrared reflectance, orientation and height above the ground. They experimentally demonstrated how cool coating applications in urban canyons can decrease the superficial overheating of the built environment and therefore, the longwave radiative exchanges

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affecting pedestrians’ thermal stress in hoc climate conditions. Consistent findings were also reported by Synnefa et al. (2007b) who showed that the key property affecting surface thermal behavior of concrete tiles exposed to solar radiation is their solar reflectance capability, able to decrease the superficial temperature daily peaks up to about 6 °C with only comparing standard black tiles (with solar reflectance of 5%) to ‘‘cool” black tiles (with solar reflectance of 27%), in August in Greece. Large scale experiments also confirmed such findings such as the ones published by Santamouris et al. (2012a) where they specifically aimed at developing a new urban park with cool coating implementation for mitigating UHI and improving outdoor thermal comfort conditions in Athens. Experimental field measurements and validated computerized fluid dynamics techniques agreed upon an urban paving surface and ambient temperature reduction up to 12 °C and up to almost 2 °C, respectively. Non-negligible thermal comfort improvement was also quantified in terms of CP, Cooling Power comfort index (CP) Zhang et al., 2015, that after the cool paving implementation, moved from the ‘Extremely Hot’ zone toward the zone of ‘Very Hot’ and close to the ‘Quite Hot’ zone. The same promising findings were also achieved through validated computational fluid dynamics models by PHOENICS software tool in Mediterranean cities such as Tirana (Fintikakis et al., 2011) and Athens (Gaitani et al., 2011). Other numerical and experimental studies, at the same time, were pretty focused on superficial temperature benefits produced by the implementation of directionally reflective coatings and tiles aimed at reflecting solar radiation backward to the sun without producing multiple reflections in urban canyons, with possible glare consequences (Rosso et al., 2016; Rossi et al., 2015a; Sakai et al., 2011). Those materials, also known as ‘‘retro-reflective materials”, are capable to reflect the solar radiation through the same direction of the incoming one and therefore, beyond the urban canyon as demonstrated by Rossi et al. (2015b). Acknowledging this potential penalty of cool coatings, Takebayashi proposed effective installation guidelines for cool coatings when applied over building facades. Promising applications of those solutions demonstrated their effect in producing passive cooling benefits both at single building and interbuilding scale, as also investigated by Han et al. (2017) and Han et al. (2015). 5.5. Cool coatings for optimizing HVAC and PV performance As described in the previous sections, the capability of a cool roof to (i) reduce cooling building energy requirement, (ii) improve indoor thermal comfort, and (iii) mitigate local microclimate as a sustainable passive strategy has already been widely investigated with reference to several climate conditions, building operations, and construction typologies. Nevertheless, cool roofs can also produce an additional cooling benefit, which has been defined as ‘‘active cool roof effect”. Such further benefit arises from the fact that usually in commercial and industrial buildings the external units of the heat pumps, commonly used for cooling purposes, are positioned over the roof. This positioning of the system can significantly affect its energy efficiency, since the roof is strongly and continuously exposed to the solar radiation. Therefore, the roof thermal and optical properties together with thermal environment of the air neat to the roof surface play a primary role in determining heat pump energy efficiency. In fact, the performance of heat pumps for cooling is affected by several factors such as the climate boundary conditions, the temperature of the system, the sizing of the heat pump, etc. (Liu et al., 2013). Additionally, it is demonstrated that the coefficient of performance of heat pumps in cooling mode increases with the increase of the outdoor temperature, given its strong correlation with the temperature lift between the source and the

output. Therefore, the ‘‘active cool roof effect” consists in an extra-increase of the energy performance of the heat pump in cooling mode usually installed over the roof of the buildings, since it generates the decrease of the temperature lift between the source and the output (Pisello et al., 2013). The literature still lacks of studies focusing specifically on the analysis of such ‘‘active cool roof effect”. The coupled passive–active effect produced by such a technology was mainly analyzed, where the cool roof capability to decrease the suction air temperature of heat pump external units located over the roof is quantified. To this aim, an industrial building with an office area located in Rome, Italy, was continuously monitored in summer 2012. In order to investigate the ‘‘active” contribution, suction air temperature was monitored and a new simple analytical model was proposed and used to estimate the cool roof active effect. The results showed that the cool roof allows to decrease the roof overheating up to 20 °C. The energy requirement for cooling decreased by about 34%. Interesting results were also achieved by Meggers et al. (2016) who investigated the influence played by urban air conditioning system on their surrounding microclimate. In particular, they identified a Coefficient of Performance (COP) variation due to the ambient temperature increase produced around the air-conditioning systems when located in dense overheated urban environment, and they found out an energy need increase up to 7%–47% due to increases in environmental temperature. At the same time, considering the wide variability of the boundary conditions affecting such phenomenon, other authors produced valuable analyses of the same possible further cool roof benefit. More in details, they (Wray and Akbari, 2008) explored the behavior of several configurations of air entering rooftop air-conditioner condensing units in a rigorous parallel experimental measurement where they compared cool and hot roof installations. They found out that the hot roof was characterized by inlet air temperature only up to 0.3 °C warmer than the cool roof configuration on average, in sunny peak times, corresponding to less than 1% benefit in terms of energy efficiency ratio of the air conditioning systems due to the possible installation over a cool roof instead than a hot roof. Together with classic air-conditioning plants, cool coatings demonstrated also to be able to optimize the energy production capability of renewable systems such as photovoltaics plants when positioned over the roof where they are applied, in building integrated photovoltaic (BIPV) applications. The first focus was aimed at quantifying the UHI increase impact of large PV applications such as the investigation by Taha (2013). In this study, the author models the potential atmospheric effects of solar PV deployment at large-scale, as responsible for modifying the radiative balance at the surface-atmosphere interface and he demonstrated how a possible optimization of urban roof albedo by 0.05 may be able to balance the potential adverse impact large PV-surfaces. While several authors investigated the interaction of green roofs, air ventilation and PV performance increase due to the convective action and/or greenery evapotranspiration around the panels (Ogaili and Sailor, 2016; Valeh-E-Sheyda et al., 2014), just a few paper deeply clarify the effect of cool coatings when coupled to BIPV. One of these works is by Scherba et al. (2011). This interesting research compares the effect of a variety of sustainable roofing systems, e.g. cool roofs, green roof, BIPV-roofs, on the rooftop energy balance compared to standard black roofs. Such old-style systems demonstrated to produce toe highest impact in terms of heat research toward the environment (i.e. about 331–405 W/m2), while cool and green roofs reduced such heat by about the 50%. The combination of PV and cool roof effect was also investigated by Salamanca et al. (2016) who modelled their passive cooling through sensitivity experiments with varying the roof coverage rates by cool coatings and PV-panels, in a 10-day clear-sky hot interval metropolitan areas or Arizona, US. Their findings showed that cool roofs behave

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better for reducing daily thermal peaks and UHI, together with active cooling demand (by 13–14%), but during the night, PVinstallations produce the best benefits in terms of cooling energy need reduction, up to 8–11% to add to the further energy savings imputable to their electricity production. 5.6. Cool coatings durability and life-cycle performance In order to pursue the sustainable development of building technologies it is necessary to adopt tools to quantify the environmental impacts of human activities for the provision of products to reduce emissions and consumption of resources. Life Cycle Assessment (LCA) is a widely known methodology for investigating by means of a ‘‘cradle to grave” approach the environmental impacts of products and technological lifecycles (Hauschild et al., 2005). It was developed indeed to calculate indicators of the potential environmental impacts linked to products, by identifying opportunities for pollution prevention and resource consumption reductions by considering the entire products’ life cycle (Rebitzer et al., 2004; Pennington et al., 2004). Nevertheless, only few studies apply this methodology for promoting the sustainable development of the building sector. Such studies mainly focus on the total energy use during buildings’ life cycle with the aim of identifying phases of largest energy use to develop strategies for its reduction. In fact, minimizing the use of energy is a central issue in the perspective of encouraging the design of sustainable constructions. Moreover, in low-energy buildings, the embodied energy typically accounts for a significant part of the total energy use. Therefore, it is also imperative to pay attention to the choice of building materials. In Asdrubali and Baldassarri (2013), the LCA was applied to three standard Italian buildings i.e. two residential buildings and an office building, by including all the life stages, from the production of the construction materials, to their transportation, assembling, lighting, appliances, cooling and heating usages during the operating phase, to the end of life of all the materials and components. The results show that the operation phase has the greatest contribution to the total impact from 77% (for a residential building) to 85% (for an office building), whereas the impact of the construction phase ranges from about 14% (office building) to 21% (residential building). A review of the life cycle energy analyses of both residential and office buildings resulting from 73 cases across 13 countries is provided in (Ramesh et al., 2010). Results show that operating (80– 90%) and embodied (10–20%) phases of energy use are significant contributors to building’s life cycle energy demand. Building’s life cycle energy demand can be reduced by reducing significantly its operating energy through use of passive and active technologies even if it leads to a slight increase in embodied energy. However, an excessive use of passive and active features in a building may be counterproductive. The LCA has been performed also specifically at materials’ level. In fact, the selection of building materials with high content in embodied energy implicates an initial high level of energy consumption in the building production stage and consequently determines future energy consumption in order to fulfil heating, ventilation, and air-conditioning demand. The capability of building materials’ to affect both embodied energy and recycling potential of buildings has been studied in (Thormark, 2006) for a high-energy efficient housing projects in Sweden. The embodied energy was originally estimated to be 40% of total energy need for a lifetime of 50 years. It was demonstrated that such embodied energy could be decreased by approximately 17% or increased by about 6% by substituting the materials. The life cycle assessment of five main construction materials i.e. wood, aluminum, glass, concrete and ceramic tiles of a house in Scotland is carried out in Asif et al. (2007) to determine their embodied energy and associated environmental

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impacts. Embodied energy of various construction materials involved has been estimated to be equal to 227.4 GJ. Concrete, timber and ceramic tiles were identified as the three major energy expensive materials involved. Concrete alone was detected to consume 65% of the total embodied energy of the home while its share of environmental impacts is even more crucial. A LCA study comparing the most commonly used building materials with some eco-materials using three different impact categories is provided by Bribián et al. (2011). The impact of construction products was found to be significantly reduced by using the best techniques available and eco-innovation in production plants, substituting the use of finite natural resources for waste generated in other production processes. LCA has also been used to compare the effects of three different shading materials on building energy consumption and their impacts to the environment within five major climate zones (Babaizadeh et al., 2015). LCA approach can also be applied over roof systems. An original life cycle environmental cost analysis is performed on green roofs compared to traditional roofs in Kosareo and Ries (2007) in order to analyze the similarities and differences in the environmental impacts of the fabrication, transportation, installation, operation, maintenance, and disposal of roof systems for the Pittsburgh, PA climate. The less environmental impact of the green roof with respect to the standard roof was mainly linked to the lower thermal conductivity due to the vegetation presence, which is the parameter mainly influencing the environmental impact of the roof system. Only a few and recent studies focused on the life cycle assessment of cool roofs. In Susca, 2012 an enhancement of the LCA methodology has been proposed through the development and use of a time-dependent climatological model for including the effect of surface albedo on climate, since traditionally LCA does not consider the surface solar reflectivity. A black and a white roof were used as case study to quantify in the impact of albedo on global warming potential in the time-frames of 50–100 years. Surface albedo was found to be responsible for a CO2 eq decrease of 110–184 kg and 131–217 kg in 50 and 100 years, respectively. Moreover, an annual energy use decrease of 3.6–4.5 kW h/m2 was registered with the white roof compared to the black roof. The long-term hygro-thermal performance was also performed for a cool bitumen roof in Toronto (ON), Montreal (QC), St John’s (NL), Saskatoon (SK), Seattle (WA), Wilmington (NC) and Phoenix (AZ) climate conditions (Saber et al., 2012). Results showed that for the outdoor climates of St John’s and Saskatoon, the white roofs could lead to longer-term moisture-related problems. However, for the outdoor climates of Toronto, Montreal, Seattle, Wilmington and Phoenix, buildings with white roofs were shown to have a low risk of experiencing moisture damage. Finally, the LCA of different natural and sustainable cool ‘‘skins” for building roofs characterized by different albedo values, i.e. polyurethane waterproofing membrane, natural fine sand, and white clay roof tiles, has been carried out in Pignatta et al. (2016). Both the environmental impact and the economic evaluation reveal that the sand id the most promising sustainable and cost-effective cool roof solution. Furthermore, the cool clay roof tiles solution has been demonstrated to have the highest environmental impact among the investigated cool roofing skins, even if it is made with a natural and recyclable material. By focusing now on the cool coating performance during the course of their service life, despite this is not the focus of this review, some brief analysis is here dealt with about the successful scientific effort aimed at studying the cool coating performance during the course of the time since around twenty years. In particular, several key papers and international projects brought wide acknowledgment about weathering, soiling, dusting agents affecting the efficacy of cool coating under natural exposure. Firstly, Berdahl et al. (2008) identified the key aspects affecting cool

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coating durability when exposed to realistic environmental conditions. Then, Sleiman et al. wrote three key contributions in this field where they analyzed firstly (Sleiman et al., 2011) the products included into the US Cool Roof Rating Council (CRRC) database, showing how the relatively old standard 2008 Title 24 overpredicted their resistance to natural agents. Then they developed an internationally acknowledged ageing method (Sleiman et al., 2014) for experimentally simulating realistic environment exposure for cool roofing materials, with varying the soiling mixtures, characteristics of humid, dry and temperate climates in the US. Finally, they (Sleiman et al., 2015) designed a sort of interlaboratory experimental study with acceptable results in terms of repeatability and reproducibility in 9 participating institutions from all around the world. These efforts, together with other key recent research contributions (Ferrari et al., 2014; Revel et al., 2013) produced the fundamental results of an international standard elaboration such as the ASTM D 7897 (ASTM D7897-15, 2015), where the short-term laboratory method for reproducing cool coating ageing (weathering and soiling) is reported for whatever investigation of natural changes in solar reflectance and thermal emittance of materials in the field. 5.7. Cost-benefit analysis of cool coatings Even if the thermal-energy benefits of cool roof solutions have been largely acknowledged and investigated both at singlebuilding and urban-scale, the current research still lacks in terms of cost-cycle and cost-benefit analyses able to highlight the economic benefits of cool roofs application in addition to the environmental ones. The Life Cycle Cost (i.e. LCC) has been already performed for green roofs systems (Wong et al., 2003). A cost-benefit analysis has been performed in this study to compute and compare the life cycle costs of green roof and standard roofs and to incorporate economic benefits by including energy costs into life cycle costs. It was observed that life cycle costs of extensive green roofs with or without consideration for energy costs, are lower than that of exposed roofs, despite its higher initial costs. In Carter and Keeler (2008), the Benefit Cost Analysis (i.e. BCA) of extensive green roof systems in an urban watershed was carried out by comparing experimental data collected on a experimental green roof to a traditional roofing scenario. The net present value of the green roof was estimated to be between 10% and 14% more expensive compared to the conventional one. However, it was estimated that a reduction of 20% in green roof construction cost would make the net value less than traditional roof value. In the same panorama, Sproul et al. (2014) performed an interesting study where they compared the economic advantages of cool and green roof installation and life cycle cost. Their study highlighted how the cool roof produce a 50-year purely economic saving of about 25$ per square meter compared to the hot black roof. At the same time, they also underlined the further key benefits from a societal and health issue point of view addressed by cool roofs compared to black ones, which should be phased out from the market through dedicated new policies aiming at mitigating UHI. They also considered the green roof opportunity in their comparative study, which are economically less competitive than white cool roofs by about 96$/m2 over the same 50-year life cycle, but for sure are responsible for other key public health advantages together with their poor cost-effectiveness.

applications, architecture and urbanism, etc. this section has been conceived in order to guide the key deductions of the scientific progress around this issue. The source of the following statistics data is represented by Scopus database (https://www.scopus.com/), typically including the key journal and conference proceedings in the subjects of cool coatings. All the researches have been carried out by including the selected keywords ‘‘cool coating”, ‘‘cool coating roof” and the union of data deriving from ‘‘cool coating pavement” and ‘‘cool coating paving” to be found out in the item title, abstract and keywords’ list of each document included into Scopus database. Fig. 2 reports the increasing trend of documents reported in the database, responding to the previously mentioned criteria, where an increasing logarithmic trend is identified with an acceptable correlation level (i.e. R2 = 0.63). The keyword ‘‘cool coating” is much more popular than the two applications (roofs and paving systems) but the roof application reports more than double of products than the paving installation, as demonstrated by the extensive literature reported in this review, and for the key acknowledged benefits produced by cool roofs in terms of indoor passive cooling and energy saving. Fig. 3 reports the source of publication of the research products with at least 5 items, where the first source is represented by an

Fig. 2. Number of papers mentioning the analysed key-words as indexed in scopus (2016-10-22).

6. Statistics on cool coatings Given the wide dissemination activity carried out in the field of cool coatings, by including material science, engineering

Fig. 3. Number of papers mentioning ‘‘cool coating” grouped with respect to the publication source as indexed in scopus (2016-10-22).

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Fig. 4. Number of papers mentioning ‘‘cool coating” grouped with respect to the country of the first author as indexed in scopus (2016-10-22).

Fig. 5. Number of papers mentioning ‘‘cool coating” grouped with respect to the affiliation institution of the first author as indexed in scopus (2016-10-22).

international conference SPIE, having its proceedings indexed by the Scopus database. The second source is represented by ‘‘Energy and buildings” journals, followed by ‘‘Solar energy materials and solar cells” (SEMSC) journal. Both of them are used to publish high quality research findings with a more technical and engineering based perspective for Energy and Buildings, and a more material science perspective in SEMSC. In general, this figure shows how the cool coating scientific development has been achieved a key acknowledged from the international scientific community through publications in high impact factor journals such as Energy and Buildings, SEMSC, Solar Energy. Fig. 4 reports the same collected data organized with respect to the country from where the first author comes from, with 4 or more items. The large prevalence of items coming from the United States confirms the huge effort that such country has been dedicated to the science and the practical implementation of cool coatings, as local climate change mitigation and building energy saving

strategy. Recent items are also increasingly coming from China, while relatively small contribution rates are related to European countries, even if non-negligible acknowledgment is given to Italy, Germany, France and Greece. For a better quality of this analysis, Fig. 5 reports also the Affiliation Institution of the first author for the same collected items. In this figure, the Lawrence Berkeley National Laboratory and the University of Athens show to be the worldwide acknowledged reference for the scientific progress in the field of cool coating for multipurpose analysis. Statistics about the document typology are not clearly showed through the Scopus database, since it collects mostly journal papers. Nevertheless, just as clarification of the high quality scientific value of the research carried out under the framework of cool coating development and testing, it should be highlighted that the large majority with respect to the total (about 64%) concerns Journal papers, while conference papers represent the other 27%, with relatively negligible sources of the remaining documents. At the

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Fig. 6. Number of papers mentioning ‘‘cool coating” grouped with respect to the scientific sector, and number of papers per year of publication in the engineering, material science and chemistry field, as indexed in scopus (2017-01-02).

same time, it is important to point out how the scientific field has been considered as mature enough to deserve several review papers, i.e. 34 items in total. The same database also highlights (Fig. 6), consistently with the source analysis carried out in Fig. 3, how about one third of the total items are considered as ‘‘engineering” products and it shows how the cool coating technical application has been acknowledged as an effective implementation in the field of applied sciences, as a fruitful follow up of the more basic scientific effort carried out in the fields of Material science (21% of the total) and Physics and astronomy (11% of the total). Nevertheless, it is important to highlight that the more basic science concerning cool coatings is still growing its importance and scientific production, since the number of papers about cool coatings in the Engineering category is arising with the same slope of the number of Scopus-indexed contributions in the field of Material Science and Chemistry, demonstrating how the cool coating research and application is still active under the framework of material development and characterization, and also at application and design level. 7. Concluding remarks In this paper, a deep review of the existing literature concerning cool coatings development and their real-world applications is provided. In all the scientific contributions that have been developed since about 1970, such cool coatings demonstrated to have a non-negligible impact both at single-building scale and interbuilding scale by significantly improving indoor and outdoor thermal comfort while reducing at the same time building energy consumption. A large group of reviewed papers concern the analysis of effect of such cool coating when applied to building roofs and/or facades, where they showed to produce up to annulling cooling energy need reduction with negligible penalties even in relatively cold climates. The first works were carried out through dynamic

simulation models where a huge computational effort was dedicated for extending the results to several building typologies and climate conditions. Thanks to the market uptake, also a large and exhaustive group of works was also dedicated to real world applications, where cool coatings demonstrated to play a key role also in optimizing building energy efficiency and indoor thermal comfort conditions. All these results are highlighted in the single scientific papers analyzed in this review and in the final statistics section, where the key observations can be summarized as follows: - Cool coating investigation is still rapidly increasing its impact within the scientific community, described by a logarithmic trend during the last decades. - Cool roofs are more ‘‘scientifically popular” than urban cool paving systems, for their threefold beneficial effect at building, urban and global climate scale. - Cool coating research has attracted publication interest in the best international journals and scientific conferences with a major focus from an engineering investigation, then on the basic material science and chemistry/physics. This element confirms the acknowledged capability of cool coatings to produce a technical and practical effect in reducing energy need for cooling in buildings and for mitigating urban microclimate comfort conditions, as demonstrated by engineering-based studies. - Huge and valuable research effort has been played for investigating the durability and life cycle behavior of cool coating when exposed to weathering, soiling, dusting agents. Such international effort has produced firstly publications, then also reference standards for reproducing realistic environments and simulating accelerated degradation phenomena in the lab. - The key countries involved in cool coatings research, both considering affiliation and nation, are USA, Greece, China and Italy. The key advantage of the US with respect to all the other countries highlights the urgent need of large scale implementation

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Fig. 7. Synthesis of the acknowledged effect of cool coatings: UHI mitigation, energy saving in buildings, passive cooling (Santamouris, 2014; Pisello and Cotana, 2014b).

policies for better investigating and massive application of cool coatings, as a lesson learnt from the huge public and private investment focused on cool materials and the relative technical regulations (Gilbert et al., 2016b; Ban-Weiss et al., 2015a, 2015b; Rosado et al., 2014; Akbari et al., 2005a). - The most investigated effect of cool coating applications concerns their passive cooling effect when applied over building envelopes, where there are many and valuable contributions attracting the positive feedback from the whole scientific community (Fig. 7). At the same time, their effect on outdoor comfort perception by pedestrians is observed in valuable experimental and modeling studies where cool paving systems showed to improve outdoor thermal comfort conditions. - Key promising studies are focused on developing cool coatings able to reflect solar radiation in preferred directions such as retro-reflective coatings. Concluding, at the present day, the application of cool coatings in the urban environment, has been widely acknowledged to be responsible for urban climate change mitigation thanks to its role in counteracting urban heat island and people vulnerability to climate change. Future key research directions are envisaged (i) in the development of other new directionally-selective and spectrally-selective and non-Lambertian cool coatings (e.g. retroreflective (Rossi et al., 2016b, 2015c, 2014) and fluorescent coatings (Berdahl et al., 2016), (ii) in coupling cool coating with

thermal-energy storage techniques (Lu et al., 2016b; Chung and Park, 2016; Karlessi et al., 2011), (iii) in elaborating new and easy-working policy tools aimed at making the cool coating performance assessment ready for the big market (Qin, 2015; Yang et al., 2015; Garman-Kolokotsa and Synnefa, 2013), (iv) in quantifying the cool coating benefits in improving the efficiency of HVAC systems located in close proximity to highly reflective applications. Considering the wide and effective results and real field contributions coming from innovative research all around the world in the field of cool coating development and implementation, the present review contributed to make an instant picture of the current research progress, which is expected to prepare the ground for another a fruitful decade of research at basic and applied level about further material development and implementation of UHI mitigation strategies. Acknowledgements Acknowledgments are due to the ‘‘CIRIAF program for UNESCO” in the framework of the UNESCO Chair ‘‘Water Resources Management and Culture”. The research leading to this review has been supported from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nos. 657466 (INPATH-TES) and 678407 (ZERO-PLUS).

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