This step involves the introductory research in the Springer and Direct Science gateways. The best journals containing the “cool roofs” keyword in the title and keywords were picked in this research. The keywords linked to the above topic were also studied, and the associated data was derived from numerous journals.

A four-step research method (Figure 1) was applied to obtain the essays for this review. First, two major scientific databases, Thomson Reuters Web of Science and Scopus, were recognized for the keywords exploration. Next, a combination of keywords and phrases was selected based on the study group’s open scientific data and information. Multiple keywords were chosen in English, including passive solar design-cool roof, cool roof, a building envelop, cool materials.

Afterward, a manual screening method was given out based on the titles, abstracts, titles, and keywords. The center of this research was peer-reviewed journal articles, conference articles.

Lastly, in the fourth step, after studying the complete texts of the resting articles, 90 articles directly and indirectly related to the topic were selected for a thorough review investigation.

The principal objective of this study is to give complete knowledge about the cool roof. This article also explains the challenges that require consideration before applying cool roofs on a large scale. The research recommends the possible pathways for utilizing green energy through the synergy of several renewable energy methods on the building scale to improve power production from the cool roofs and present eco-friendly management gains.

Passive solar techniques, as has been described, are the most feasible measures that can be put into practice for the proper utilization of solar energy and thus for the enhancement of the thermal performance of buildings. However, these measures are often related to some features in the design of the building envelope. The building envelope is mainly designed to limit the heat transfer between the inside and the outside in order to control the thermal characteristics of the interior environment and to reduce the need for heating, cooling, and electrical lighting in buildings (Azari, 2014; Yeganeh, 2015; Yeganeh et al., 2018; Motevallian and Yeganeh, 2020). Thus, as Silva et al. (2016) maintained, the energy efficiency of buildings strongly depends on the thermal performance of their envelope.

Basically, Cool roofs or solar reflective roofs have surfaces that reflect sunlight and emit heat more efficiently than hot or dark roofs, and therefore keeping them cooler in the Sun. That is, roofs with high solar reflectance and high infrared emittance can be called cool roofs. A cool roof is not a new concept; traveling pictures from the Mediterranean and the Middle East also show the scenery of homes with white roofs and walls. These are actually cool roofs, and have for thousands of years been a traditional architectural feature (Vickers, 2017). The solar reflectance (SR) (reflectivity or albedo) and infrared emittance (or emissivity) are two surface properties that affect the thermal efficiency of these roof surfaces (Sadineni et al., 2011).

Solar reflectance, also known as albedo, refers to the reflection of solar energy as it comes into contact with the surface material. Thermal emittance refers to the radiant emittance of the heat of a particular object in the form of infrared or thermal radiation, which relates to how easily the surface cools itself (Zingre et al., 2015a; Vickers, 2017).

A “cool roof” as a roofing system that refuses solar heat and holds surfaces cooler below the Sun is defined by The European Cool Roofs Council. A “cool roof” is ordinarily obtained by using cold materials on the external surface. So, these kinds of materials can decrease solar radiation absorption while releasing the heat received by the roof (Chang et al., 2008). The cool roof technology is more cost-effective and uncomplicated to perform in all domestic and commercial buildings (Rawat and Singh, 2021b). Cool roofs reflect a substantial fraction of incoming sunlight and retain the roof surface lower than conventional roofs, decreasing heat conduction into the building and its cooling load. This cool roof appearance performs it most beneficial when solar radiation’s intensity is tremendous, and the highest daily deviation happens (Razykov et al., 2011). There are somewhat differing descriptions for cold materials. Whereby, cold materials must have a solar reflectance originally equal to or greater than 0.65 for ES-ENERGY STAR (Yang et al., 2020). After 3 years, its requirement is higher than 0.50, while APEC Energy Working Group (De Masi et al., 2018) recognized having at least a reflectance of 0.70 and a thermal emissivity of 0.75.

Consequently, when the surface material has higher albedo and emissivity, the thermal fluctuations of the surface will be lower. Moreover, the thermal emittance, i.e. the capability of the roof surface for infrared radiations, is another significant factor in the thermal performance of roofs. That is, the higher their thermal emittance, the faster the roofs will lose heat and cool down (Suehrcke et al., 2008). Cool roof and green roof approach afforded and aided conceive thermal comfort for inhabitants and the energy saving of buildings (Lamnatou and Chemisana, 2015) (Shafique et al., 2018; Gilabert et al., 2021). A sustainable initiative and attracting energy scientists, architects, and urban planners to create better thermal comfort conditions and energy conservation and urban property growth are provided by Cool roofs and Green roofs (Saadatian et al., 2013). Beom-SoonHan examines the effects of cool roofs on turbulent coherent structures and ozone air quality, and analysis shows that cool roofs weaken the effects of turbulent coherent structures on O₃ concentration. (Han et al., 2020).

The effect of solar radiation on sunny days, the lack of infrared heat during the night, and the complications of heavy rainfall affect the roof more than any other building component (Al-Obaidi et al., 2014; Vellingiri et al., 2020). Moreover, as reported by Akbari and Matthews in their 2012 study (Akbari and Matthews, 2012), roof surfaces of buildings account for 20% in less urban areas, while this is up to 25% in cities with high density levels, and therefore the key effects of roofs on air and surface temperature in urban areas can be understood.

On this account, roofs can be used as specific enveloping elements for which revolutionary technology can offer considerable energy savings and lead to the enhancement of indoor thermal conditions. As a consequence, the implementation of efficient passive solar architecture measures into roofs would not only contribute significantly to the use of solar energy on a building scale, but would also help significantly to minimize the negative environmental impact on the city scale.

Cool roofs, i.e. roofs with a high level of solar reflectance and a high level of infrared emittance, are one of the most feasible and efficient passive techniques that can be applied to equip new buildings as well as to retrofit old ones. Additionally, green roofs reduced solar radiation receiving 60% radiation and diminished air conditioning energy among 25–80% (Besir and Cuce, 2018). Taizo Aoyama researches explain that a self-cleaning paint, based on an acrylic silicone polymer, efficiently keeps a high solar reflectance and limits dirt from adhering (Haberl et al., 2004). Some papers have focused on cool roof martials (Akbari and Levinson, 2008; Urban and Roth, 2010; Zinzi, 2010). Tim Sinsel studies the super cool materials. The results also revealed that super cool roofs could lower pedestrian-level air warmth in some areas by up to 2.4 K (Konopacki et al., 1998).

Among the limited studies on the form of a review article, reference may be made to the study conducted by Testa and Krarti (2017), in which the literature on cool roofs and switchable roofing materials are compiled and summarized as a tool for energy savings in buildings. While their study focused on energy savings and penalties for cool roofs, they nevertheless delivered a concise report on additional advantages and limitations of this technique. In the same direction, the 2014 Al-Obaidi et al. (Vickers, 2017). study provided a review of passive cooling strategies, including reflective and radiative roofs in tropical houses in Southeast Asia, and discussed the physical attributes of these approaches. However, as a more comprehensive study, the literature review by Haberl and Cho (Haberl et al., 2004) analyzed seventy-two articles from various sources, both quantitative and qualitative research on cool roofs, and provided a valuable summary of the potential energy performance of this passive technique based on different roof configurations.

While typical roofing materials have an SR of 0.05–0.25, applying reflective roof coating may raise the SR to more than 0.60. Many roofing materials have an infrared emittance of 0.85 or greater, apart from metals with a low infrared emittance of around 0.25. Hence, while metals are highly reflective, namely their SR is greater than 0.60, bare metal roofs and metallic roof coatings appear to get hot as they cannot efficiently emit the absorbed heat as radiation. However, adding special roof coatings may elevate the infrared emittance of bare metal roofs (Sadineni et al., 2011). Albeit, some studies have revealed that a roof with lower thermal emittance such as metal surfaces but extremely high solar reflectance can still remain cool in the sun (Akbari and Levinson, 2008). Lighter-colored surfaces have greater reflectance values than dark surfaces. In fact, the solar reflectance is close to 0 for black surfaces and close to 1 for white surfaces. Besides color, the roughness of the surface and the existence of contaminants can also influence the solar reflectance of the roof. Clean and polished materials have higher SR values (Testa and Krarti, 2017).

Hence, one of the easiest ways to apply cool roofing techniques is to shift from dark color finishes to light colors. (Zinzi, 2010). investigated the influence of color on the thermal profiles of the building material by monitoring the average daily surface temperature and temperature range of a number of mosaic and concrete samples. the color has a significant impact on the thermal performance of the material in such a way that there is a difference of about 10°C between the black and white color of the mosaic samples and about 12°C for the concrete samples. Moreover, the light color mitigates the range of thermal fluctuations, i.e. the difference between the maximum and minimum material temperature. (Zinzi, 2010).

That is to say, higher solar reflectance and higher thermal emissions result in lower roof surface temperatures and prevent heat from flowing to the building compared to dark and hot roofs that absorb more than 90% of the incident solar radiation that induces temperatures above 66°C (Urban and Roth, 2010). Table 1 shows the degree of these two factors in some common roof finishes. As can be observed, if the solar reflectance and infrared emission are higher, the temperature of the roof would be lower.

Solar energy intensity ranges from 250 to 2,500 nm in wavelengths. White or light-colored cool roof products reflect visible wavelengths, whereas colored cool roof products reflect in the infrared energy range. In view of the demand for colored roofing products for many buildings, manufacturers have tried to develop cool colored products that reflect near-infrared or infrared wavelengths ranging from about 700 to 2,500 nm. That is, as Figure 2 shows, by employing cool coating material for each particular color, the solar reflectance of the roof tiles has significantly decreased.

Based on the widespread acceptance of cool roofs in recent years, various innovative cool coating materials and implementing technology have been developed and this procedure is also underway. Table 2 illustrates the most common types of cool roofs that are already available on the market.

The subject of passive solar architecture covers a wide range of techniques, among which the application of solar reflective finishes for roofing systems has been of considerable interest over the last few decades. That is, there has been growing concern about the potential benefits of applying cool finishes to roof surfaces of buildings. The use of cool roofs to enhance comfort conditions and minimize energy consumption in dwellings and more commonly used buildings, such as educational, administrative, commercial, and cultural facilities, has therefore been examined in a large number of experimental and theoretical studies. As far as residential buildings are concerned, one may refer to the 1998 study by Parker et al. (1998), which carried out a series of experiments in Florida residences during the summer to measure the effect of increasing solar reflectance on space cooling loads. In the same direction, Synnefa et al. (Vickers, 2017) analyzed the effect of cool roof coatings on cooling and heating loads and the indoor thermal comfort conditions of residential buildings in 27 cities around the world, reflecting various climatic conditions. However, for non-residential buildings, Akbari et al. (2005) monitored the impact of cool roofs on energy usage and environmental parameters in six California buildings at three separate locations. In the same direction, Romeo and Zinzi (2013) recorded the results of a large application in an office/laboratory building belonging to a school campus in Trapani, the location that enjoys Mediterranean climate on the west coast of Sicily in Italy.

Cool roofs have an outer surface covered with specific materials that are capable of minimizing solar absorption and maximizing thermal emittance. Therefore, they are able to sustain lower surface temperatures and minimize the heat transfer to the building. In particular, solar absorption is reduced by increasing the solar reflectance of the roof, defined as a fraction of the solar radiation that is diffusely reflected away from the surface (Testa and Krarti, 2017).

Principally, the benefits of adding cool materials to roofs derive from their ability to reduce the surface temperature. In general, the advantages of cool roofs can be regarded on a building, city, and global scale. At the building scale, the use of cooling materials decreases the usage of cooling energy and the peak energy demand for ventilation, since less heat is transmitted from the cooler roof to the building (Santamouris et al., 2011). The energy performance of cool roofs and their ability to provide thermal comfort in residential and non-residential buildings have been the subject of both experimental and numerical studies.

In addition, the heating penalty for this measure is also considered in a variety of papers. While a cool roof can reduce the cooling load of the building during warm months, it can unfortunately raise the heating load in cool months, thereby reducing its overall efficiency (Testa and Krarti, 2017). That is, though applying cool roofs can help decrease cooling loads in warm climates, the same cool roofs with high solar reflectance can also increase thermal heating loads and energy consumption in buildings during heating seasons, particularly in colder climates (Akbari and Levinson, 2008). In the same vein, based on EPA reports, this rise is much less significant than the concomitant decrease in cooling load, resulting in positive net savings in warm/moderate climate conditions. This is supported by the reason that, during the winter, the sun is far lower in the sky and the solar radiation on the horizontal surface is less severe. Hence, there is a greater risk of overcast clouds and less solar efficiency (fewer hours of sunshine) meaning that less overall energy falls on the earth to be stored or transmitted over the same amount of time as during the summer.

A thorough understanding of the energy performance of cool roofs requires a review of the related research, a summary of the findings, and an analogy between them. Haberl and Cho (Haberl et al., 2004) have arguably made one of the most noteworthy attempts in this respect, as they provide a relatively extensive review study on the energy efficiency of cool roofs based on seventy-two articles, although today it can somehow be regarded not to be state-of-the-art. Their findings, however exclusively for typical US buildings, revealed that cooling energy savings in residential and commercial buildings ranged from 2 to 44% and averaged around 20%; in addition, peak cooling energy savings from cooling roofs ranged from 3 to 35%, depending on the level of ceiling insulation, duct placement, and attic configuration. Although their approach could be insightful, their research is not only somehow outdated today, but also limited to a certain location and climatic condition. However, in order to provide a more inclusive and state-of-the-art analysis of the energy performance of cool roofs on a building scale, a review of the most relevant studies of different climatic conditions across the past 2 decades has been reported in Table 3, including their basic principles and major findings.

The application of cool coatings on the roof not only enhances their thermal efficiency but also decreases thermal stress, which helps to extend the life of the roof and minimize the cost of roof maintenance. As a matter of theory, material deterioration is consistent with chemical reactions that proceed quicker with higher temperatures, as cool materials lower the surface temperature, they retard destructive reactions within the roofing materials. Besides, extreme thermal variations have significant damaging effects on roofing materials, and thus, as cool coatings mitigate these changes, the service life of roofs is extended. In a study conducted by Gagliano et al. (2015), a comparison was made between three different roofs, i.e. standard, cool and green roofs, in two scenarios as insulated and without insulation in terms of their dynamic thermal behavior. As their findings revealed (Gagliano et al., 2015), although the cool roof has not shown the capability of the green roof to mitigate thermal variations between the outer and inner surfaces of the roofs, this technology can reduce the outer surface temperature by up to 7°C compared to the standard case.

However, as has been pointed out, the benefits of cool roofs are not limited to building scale, as they have a considerable impact on the neighborhood and city scale and as a consequence on the global environment. Materials utilized in building envelopes and urban structures play an essential role in the urban thermal balance. They absorb solar and infrared radiation and dissipate part of the stored heat into the environment by convective and radiative processes that raise the atmospheric temperature. On a city scale, this effect leads to a rise in urban air temperature, namely the well-known phenomenon regarded as the Urban Heat Island (UHI) effect. This is characterized as the increase in air temperature in densely developed areas with respect to the surrounding countryside, and its major driver is the alteration of the land surface in the urban area, where the vegetation is substituted by paved roads and building surfaces (Costanzo et al., 2016).

As discussed in a variety of studies (Table 4), the application of cool materials on urban surfaces avoids the urban heat island effect (UHI) and slows down the smog formation by minimizing local air temperature. The advantages of cool pavements are due to the fact that raising the solar reflectance of the ground surface makes it cooler under the sun, decreases the convection of heat from the pavement to the air, and thus reduces the temperature of the surrounding atmosphere. In effect, City-scale use of cooling materials will theoretically minimize air pollution, both directly and indirectly. As (Rosenfeld et al., 1998) have clarified, the direct reduction in air pollution is due to the fact that less cooling energy is used; thus, fewer pollutants from power plants, i.e. harmful gases, such as CO2 or NOx, are generated, whereas indirect reductions in air pollution indicate the fact that the ozone-forming reaction that creates smog accelerates at higher temperatures, so the possibility of smog formation is diminished at lower urban air temperatures. Moreover, as Yang et al. (2018) argued, structures, pavements, and car parks keep the heat coming from the earth from dissipating into the cool night sky. As a result, the air temperatures in UHI maintain high even throughout the night, raising the need for air conditioning and the emission of air pollution and greenhouse gasses from fossil fuel plants.

For the sake of brevity and clarity, a concise report on the potential of cool roofs in relation to the city scale as well as the global scale was compiled in Table 2, including the main addressed subject concerning the effect of cool roofs as well as the major results of the studies.

Apart from the benefits of cool roofs on different scales, which have been mentioned in a number of studies, there are, however, some drawbacks that have been attributed to this technology in some texts. The most important issue in this regard is the fact that, since the use of cooling materials on roofs decreases the heat gain throughout the year, they not only minimize the cooling demand but also increase the demand for heating during the cold periods. However, as is typically documented in studies on locations with relatively long hot periods, such as studies by Synnefa et al. (2012)and Pisello and Cotana (2014), the increase in heating load during cold months is not that significant compared to the decrease in cooling load. But, when it comes to areas with longer and severe cold periods, as in the study conducted by Shen et al. (2011), increased demand for heating can have a substantial negative annual impact on total energy use. Another possible adverse consequence of lower surface temperatures from inactive cool roofs is the sensitivity to condensation within the roof assembly. In cold climates with short-tempered summers, lower surface temperatures of inactive cool roofs may increase moisture in the roofing construction by reducing the drying potential and enhancing the uncertainty of interstitial concentration (MoghaddaszadehAhrab and Akbari, 2013). The cool roof payback period is short compared to other methods, which can be 2 months (Zhang et al., 2016). Cool roofs, compared with photovoltaic panel roofs and roof gardens, maintain a lower surface temperature, improving passive cooling during nighttime (Abuseif and Gou, 2018). Accurate choice of this approach is required when heating is highly needed for a building to assess its performance before implementing it on the roofs of a building and avoiding its adverse consequence on heating loads.

Furthermore, as the efficiency of cool roofs is strongly linked to their solar reflectance, the accumulation of dirt and dust might reduce their effect (Urban and Roth, 2010). Thus, in areas where there is a high potential for dust to settle on the surface, the application of cool roofs could lead to some problems over time. Moreover, considering the nature of cool surfaces with high solar reflectance, they provide intense radiation in their surroundings, which may cause visual annoyance and undesired glare, particularly during clear sunny days (Al-Obaidi et al., 2014). Consequently, their use in the vicinity of critical areas that require a clear aerial view, such as airports, is not suggested.

In order to provide an informative detailed summary of the elaborated pros and cons of the cool roofs, one of the best approaches is to provide itemized factors that correspond to the relevant codified references. It should be remembered, of course, that some of these components are not essentially independent of each other and are in a causal relationship. For instance, as cool roofs reduce heat gain, they enhance the thermal comfort of indoor spaces and thus decrease energy demand for air conditioning, which in turn helps to reduce the peak demand for electricity and has a positive effect on lower electricity bills. However, though these components are in close, casual relationships with each other, each study approached the subject from a certain viewpoint and presented its report with a focus on specific components. Ergo, for the purpose of offering a clearer and more instructive summary, they are presented as separate components in Table 5. In addition, while the majority of codified sources pertaining to each item are in the form of research articles, in a few instances, certain review studies which provide insightful information on a particular item are also included in this summary.

Table 4 discusses more deeply the drawbacks and limitations, and advantages of the different methods on various climate conditions.