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The rising worldwide temperatures leading to a spike in air conditioning to ensure thermal comfort in buildings is the root cause of the increase in energy demand in India's building sector. The roof of a building significantly adds to the heat gained on the top floor, accounting for 50–60% of the total heat gain. One sustainable way to reduce this heat gain is via cool roofing technology. Through examination and dynamic simulations with the Energy-Plus-based Design Builder software tool, this study explores different cool roof solutions. An experimental setup of a Reinforced Cement Concrete (RCC) roof with cement plaster, solar reflective paint, solar reflective tiles, and earthen pots have been created on the institute campus. Recordings were taken during the peak summer days and peak winter days, alongside simulations carried out to determine various factors such as Albedo, Surface Temperatures, Air Temperatures, Diurnal Heat Ingress, and the Building's Thermal Energy Consumption. The experiments have demonstrated that cool roof layers can considerably improve interior thermal comfort by minimizing heat transfer, which maintains a constant temperature over the bottom surface of the roof. By reducing heat gain by 33–71%, they can save energy by 21-26% compared to traditional RCC roof slab. [ABSTRACT FROM AUTHOR]

Abstract This study systematically examines how the urban heat island (UHI) and urban breeze circulation (UBC) respond to an increase in roof albedo (α r) and its influence on urban air pollutant dispersion. For this, idealized ensemble simulations are performed using the Weather Research and Forecasting (WRF) model. The increase in α r from 0.20 to 0.65 decreases the UHI intensity, UBC intensity, and urban planetary boundary layer (PBL) height in the daytime (from 1200 to 1700 LST) by 47%, 36%, and 6%, respectively. As both UBC intensity and urban PBL height decrease, the daytime urban near-surface passive tracer concentration increases by 115%. The daytime UHI intensity, UBC intensity, and urban tracer concentration nonlinearly change with α r: For 0.10 ≤ α r

The existing roofing system in tropical climates is not adapting to the worsening climate, along with the demand for sustainable building design; this study introduced a pioneering cool roof system to reform the traditional roofing system. The pioneering cool roof system is composed of white reflective coating, hollow foamed concrete roof (HFCR) tile, and a sustainable active reflective insulation (RI) system. Despite the complex integrated roof technologies, the study adopted 3D CFD numerical simulation for an in-depth and frontier thermal distribution analysis and fluid flow presentation. The outcomes of the numerical model were compared with field measurements for validation purposes, which resulted in just a 3.31% error. The internal attic was 35.64 °C despite the roof exterior achieving 54.59 °C after receiving an average 680 W/m2 solar radiation source. After the heat flux was hugely terminated by the HFCR tile and sustainable active RI system, the heat flux magnitude attained in the internal attic was 1.0822 W/m2, and the active RI system removed 96.29% of heat flux excellently. Meanwhile, the proposed roofing design's energy-saving performance was 27.5%.

Several studies have assessed the thermal performance of green and cool roofs. However, few have comprehensively addressed Brazilian buildings and climates, considering indoor and outdoor environments. Considering three Brazilian cities, this study aims to assess the performance of green and cool roofs compared with traditional fibre cement roofs in a typical multifamily residential building. Energy consumption, thermal comfort, and outside surface temperature were assessed using computer simulation. The results show that the cool roofs performed better in cities with warmer climates (e.g., Cfa and Aw), reducing electricity consumption by up to 24.8% compared with traditional roofs. Green roofs are better suited for colder climates (e.g., Cfb), with up to 28.2% energy savings. Green roofs provided the highest percentage of thermal comfort hours in all climates. Cool and green roofs provided hourly reductions in outside roof surface temperature of up to 16.5 °C and 28.4 °C, respectively, compared with the traditional roof. This work reinforces that the choice between these two roof types for each city depends on the parameter used for comparison. Based on the relevant information applied to Brazilian buildings and representative climates presented, this work provided recommendations for urban planning policies and building regulations in Brazil. [ABSTRACT FROM AUTHOR]

The existing roofing system in tropical climates is not adapting to the worsening climate, along with the demand for sustainable building design; this study introduced a pioneering cool roof system to reform the traditional roofing system. The pioneering cool roof system is composed of white reflective coating, hollow foamed concrete roof (HFCR) tile, and a sustainable active reflective insulation (RI) system. Despite the complex integrated roof technologies, the study adopted 3D CFD numerical simulation for an in-depth and frontier thermal distribution analysis and fluid flow presentation. The outcomes of the numerical model were compared with field measurements for validation purposes, which resulted in just a 3.31 % error. The internal attic was 35.64 °C despite the roof exterior achieving 54.59 °C after receiving an average 680 W/m2 solar radiation source. After the heat flux was hugely terminated by the HFCR tile and sustainable active RI system, the heat flux magnitude attained in the internal attic was 1.0822 W/m2, and the active RI system removed 96.29 % of heat flux excellently. Meanwhile, the proposed roofing design's energy-saving performance was 27.5 %. [Display omitted] • A numerical model for a pioneering cool roof system was developed, validated, and evaluated based on temperature distribution and fluid analysis. • The developed numerical model produced a low percentage error (3.31 %) with the field measurement. • The developed numerical model explained the efficiency of a sustainable active reflective insulation system. • Adopting the pioneering cool roof system could save 27.5 % of building energy. [ABSTRACT FROM AUTHOR]

Passive radiative cooling materials are widely recognized as attractive innovations for reducing emissions and expanding life-saving cooling access. Despite immense research attention, the adoption of such technologies is limited largely due to a lack of scalability and cost compatibility with market needs. While paint and coating-based approaches offer a more sensible solution, many demonstrations suffer from issues such as a low solar reflectance performance or a lack of material sustainability due to the use of harmful solvents. In this work, we demonstrate a passive radiative cooling paint which achieves an extremely high solar reflectance value of 98% using a completely water-based formulation. Material sustainability is promoted by incorporating size-dispersed calcium phosphate biomaterials, which offer broadband solar reflectance, as well as a self-crosslinking water-based binder, providing water resistance and durability without introducing harmful materials. Common industry pigments are integrated within the binder for comparison, illustrating the benefit of finely-tuned particle size distributions for broadband solar reflectance, even in low-refractive-index materials such as calcium phosphates. With scalability, outdoor durability, and eco-friendly materials, this demonstrated paint offers a practical passive radiative cooling approach without exacerbating other environmental issues. [ABSTRACT FROM AUTHOR]

This study examines the sustainability of roofs subjected to hot climate of Phoenix and cold climate of Montréal by using a validated numerical model that simultaneously solves the heat, air, and moisture equations. In many situations, the initial conditions needed to solve these equations are not available. This paper provides a procedure for conducting energy and moisture modelling so that using different initial conditions would result in no effect on the final results of roof performance. Additionally, the study focuses on: (a) assessing the moisture performance of black and cool roofs in case of using material layer with high initial construction moisture content (Fibreboard, FB), (b) identifying the time period needed so that the moisture content would reach acceptable limit by the building codes, and (c) determining the amount of energy savings as a result of using sustainable cool roofs instead of black roofs. The results show that the moisture content in the FB decreases to permissible levels after 92.6 days for black roof and 175.7 days for cool roof in Phoenix, while in Montréal, it takes 238.6 days and 1597 days, respectively. The effect of using different initial condition vanishes after 196 days for Phoenix’s black roof, 583 days for its cool roof, and 1247 days for Montréal’s black roof and 4590 days for its cool roof. Mold is detected in FB. In Phoenix, mold disappears after 379 days for black roof and 479 days for cool roof, whereas in Montréal, it takes 708 days and 3143 days, respectively.Article highlights: The highest hourly external surface temperature for black roof is about 33 °C higher than for cool roof. The yarely energy load with black roof subjected to hot climate is 77% greater than that with cool roof. The time beyond which initial conditions have no effect on performance depends on roof type and climatic conditions. [ABSTRACT FROM AUTHOR]

Cool (solar reflective) roofs and walls can reduce solar heat gain and decrease unwanted heat flowing into indoor spaces. To explore the potential of these strategies tomitigate energy and thermal comfort challenges in Los Angeles, we conducted a study that used EnergyPlus building energy simulations. Our analysis focused on single-family homes in Los Angeles under various historical and future weather conditions, including both typical meteorological year (TMY) and heatwave weather year (HWY) scenarios, based on the COordinated Regional Downscaling EXperiment (CORDEX) framework under the Representative Concentration Pathway (RCP) 8.5. Our study evaluated the impact of cool envelope strategies on heating, ventilation, and air conditioning (HVAC) primary energy intensity savings and thermal comfort improvements. We employed three thermal comfort models: predicted mean vote (PMV), adaptive, and heat stress. Our findings indicate that in Los Angeles, a package of cool roof + walls (reflective roof and reflective walls) can reduce annual HVAC energy consumption by at least 11% for buildings equipped with mechanical cooling systems. They can reduce occupants’ warm thermal discomfort (thermal-sensation-scale-unit-weighted warm exceedance hours) by at least 28% in air-conditioned buildings and by at least 16% in buildings without mechanical cooling systems. Cool envelopes can also lower daily heatwave heat stress by at least 9%.

This study addresses the global plastic waste crisis and the urban heat island effect by developing an innovative solution: recycled plastic roof tiles embedded with phase change material (PCM) and coated with hollow-glass-microsphere-based white paint. The samples were fabricated with cow pie fibers, OM37 and OM42 PCM materials with different wt./vol. values, i.e., 15/50, 20/50, 25/50, 30/50 ratios. The fabricated tiles were coated with hollow glass microspheres to provide a reflective layer. The tiles' effectiveness was evaluated through morphological examination and thermal analysis. The SEM analysis revealed an excellent bonding ability for the PCM blend, i.e., OM37 and OM42 at a 20/50 ratio (wt./vol.) with cow pie fibers. Adding cow pie fibers to the PCM shifted the melting points of OM37 and OM42, indicating an increased heat storage capacity in both blends. The thermal conductivity results revealed decreased thermal conductivity with an increased cow pie fiber percentage. The recycled plastic roof tile of the PCM composite at a 20/50 (wt./vol.) ratio showed good thermal properties. Upon testing in real-time conditions in a physical setup, the roof tiles showed a temperature reduction of 8 °C from outdoors to indoors during the peak of summer. In winter, cozy temperatures were maintained indoors due to the heat regulation from the roof. [ABSTRACT FROM AUTHOR]

An analytical hierarchy model of the impact of solar reflectance, thermal emittance, heat transfer coefficient, and heat storage coefficient on building energy consumption was established through the implementation of orthogonal design experiments. The EnergyPlus software (v9.0.1) was utilized to simulate building energy consumption across diverse climatic regions in China, providing essential benchmarks for the orthogonal design. The results of the range analysis consistently indicate that, barring regions characterized by extremely cold climates, solar reflectance emerges as the predominant factor exerting an influence on building energy consumption. As geographical latitude increases, the impact of the heat transfer coefficient becomes progressively larger, while the weight of thermal reflectance concurrently diminishes. Drawing upon the principles rooted in the gradient refractive rate theory and the concept of atmospheric window radiation, a range of high-reflectance and high-emittance cool roof coatings in various colors were meticulously developed. A spectrophotometer was employed to precisely quantify their reflectance properties, and simulations were subsequently conducted to scrutinize their energy-saving characteristics. The results demonstrate that the cool roof coatings that were developed using the methodology described in this paper exhibit substantial enhancements in reflectance, with increases of 0.24, 0.25, 0.37, and 0.35 for the yellow, red, blue, and green cool roofing materials, respectively, in comparison to conventional colored coatings. Under typical summer conditions, these enhancements translate to significant reductions in roof temperatures, ranging from 9.4 °C to 14.0 °C. Moreover, the simulations exploring the cooling loads for the roofs of differing colors consistently revealed remarkable energy savings. These savings were quantified to be 4.1%, 3.9%, 5.5%, and 5.4%, respectively, when compared to conventional coatings of the corresponding colors. These findings offer valuable insights into strategies for optimizing the energy efficiency of buildings through the application of high-reflectance cool roofing materials. [ABSTRACT FROM AUTHOR]

The penetration of increasing solar gain towards the building deteriorates the building's thermal quality and raises cooling demand. In particular, the roof receives the most solar gain. The roof system reformation is urged to improve the roof system's heat capacity and conserve building energy. This paper introduced a novel cool roofing system to replace the traditional roof system. The proposed novel cool roofing applications merged: (i) lightweight foam concrete, (ii) hollow concrete slab, (iii) white cooling paint and (iv) active reflective insulation system powered by solar energy. The discovery of the proposed system was evaluated with experimental chambers exposed to the outdoor environment. Under intense sunlight, the internal roof surface was 30.6 % cooler (50.5 °C to 35.0 °C). The mean attic temperature achieved was 33.9 °C under a field where the daily maximum recorded surrounding temperature is 43.0 °C. In contrast with a bare roof at 41.9 °C, the proposed design demonstrated a 19.2 % lower solar gain, achieving a sub-ambient structure. Besides that, the cooling demand simulation generated similar cooling demand savings (20 %). Significantly, this comprehensive cool roofing system exhibited a considerable and constructive outcome in cooling the buildings by achieving sustainable development to create a better and more environmental future for all.

Passive radiative cooling materials are widely recognized as attractive innovations for reducing emissions and expanding life-saving cooling access. Despite immense research attention, the adoption of such technologies is limited largely due to a lack of scalability and cost compatibility with market needs. While paint and coating-based approaches offer a more sensible solution, many demonstrations suffer from issues such as a low solar reflectance performance or a lack of material sustainability due to the use of harmful solvents. In this work, we demonstrate a passive radiative cooling paint which achieves an extremely high solar reflectance value of 98% using a completely water-based formulation. Material sustainability is promoted by incorporating size-dispersed calcium phosphate biomaterials, which offer broadband solar reflectance, as well as a self-crosslinking water-based binder, providing water resistance and durability without introducing harmful materials. Common industry pigments are integrated within the binder for comparison, illustrating the benefit of finely-tuned particle size distributions for broadband solar reflectance, even in low-refractive-index materials such as calcium phosphates. With scalability, outdoor durability, and eco-friendly materials, this demonstrated paint offers a practical passive radiative cooling approach without exacerbating other environmental issues.

Daytime Radiative Cooling (DRC) is a highly effective passive cooling technique that holds great potential for enhancing energy efficiency and promoting decarbonization. Several Daytime Radiative Coolers (DRCs) are reported in the literature to highly reflect shortwave radiation and emit thermal radiation, either specifically in the Atmospheric Window (AW) wavelengths (8–13 µm) (Selective DRCs) or in both AW and non‐AW wavelengths (Broadband DRCs). This review presents the advancements in the six key categories of DRCs identified in the literature based on their design characteristics: i) multilayer DRCs, ii) metamaterial DRCs, iii) randomly distributed particle DRC structures, iv) porous DRCs, v) colored DRCs, and vi) adaptive DRCs. The review discusses their main attributes in terms of their demonstrated thermo‐optical performance. Research gaps include the need for real‐life studies to investigate scaling up DRCs in the built environment and their effects on indoor thermal comfort and air temperature reduction. Finally, an eight‐step feedback‐loop scheme is proposed to establish efficient design and implementation protocols for DRC materials in the built environment. [ABSTRACT FROM AUTHOR]

The building sector's energy consumption has significantly increased due to climate change, emphasizing the need to develop sustainable low-energy buildings using experimental and computational tools. As a joint project between Kuwait and Australia, two portable cabins with internal sizes of 2 m × 2 m × 2.80 m, made from 75 mm thick sandwich panels and metal frames, were built in Kuwait to investigate their thermal and power consumption characteristics under various energy-saving techniques and different climates. This is the first attempt to analyze the energy-saving aspects of portable cabins made from sandwich panels for future sustainable cities. Each cabin has an indoor air-quality-sensing device and a novel power-monitoring system to measure their thermal and power consumption characteristics. First, shingles and novel finned metal cool roof (CR) techniques were experimentally investigated. Then, a new multi-zone SketchUp model of the portable cabins was created and simulated in TRNSYS. Next, the energy saving data of the portable cabins were investigated by adding PCM on the walls and the roof using built-in models of bio-phase change materials (Bio-PCMs) in TRNSYS. The annual energy performance index (EPI) as an important sustainability index was explored for determining heating/cooling/total demands of the portable cabins in the desert climate of Kuwait and in various climate regions of Australia. The findings reveal that both shingles and finned metal roofs contributed to higher power consumption. Meanwhile, the use of sustainable Bio-PCMs in Kuwait demonstrates a significant energy-saving potential of 30%, with variations ranging from 25% to over 45% across different climate regions in Australia. [ABSTRACT FROM AUTHOR]

Increasing envelope facet albedos considerably reduces solar heat gain, thus yielding building cooling energy savings. Few studies have explored the potential benefits of utilizing cool coatings on building envelopes (“cool-coated buildings”) based on life-cycle cost analysis. A holistic approach integrating the field testing, building energy simulation, and a 20-year life-cycle-based optimization was developed to explore cool-coated building performance and the maximum net savings of optimal building envelope retrofit and design. Experimental results showed that applying cool coatings to a west wall of an office building in Chongqing, China reduced its exterior surface temperature by up to 9.3 °C in summer. Simulation results showed that in Chongqing, making the roof and walls cool could reduce annual HVAC electricity use by up to 11.9% in old buildings (with poorly insulated envelopes) and up to 5.9% in new buildings. Retrofitting old buildings with a cool roof provided the net savings per modified area with present values up to 42.8 CNY/m2; retrofitting a new building with a cool roof or cool walls was not cost-effective. Optimizing both envelope insulation and envelope albedo can achieve 5.6 times the net savings of optimizing the insulation only, and 1.6 times that of optimizing albedo only.

The roof plays a pivotal role as it directly engages with solar radiation absorption and external heat exchange, significantly influencing the building’s overall energy dynamics. To assess the performance of green roofs and cool roofs, a roof performance test facility was established in Nanjing. Time-difference comparison experiments were conducted to measure and analyze the energy-saving effects of cool roofs, green roofs, and conventional roofs during both summer and winter conditions. The study aimed to investigate how their thermal performance impacts building energy consumption. The study’s findings reveal that under summer conditions, the incorporation of a cool roof system leads to a substantial enhancement in energy efficiency, achieving an impressive 13.2% energy savings compared to conventional roofing solutions. In contrast, the implementation of a green roof system results in a more modest energy-saving rate of 4.1%. Transitioning to winter conditions, the adoption of a cool roof system shows a marginal increase of 2.8% in energy consumption compared to conventional roofs. Interestingly, the green roof system stands out as an energy-efficient option during winter, demonstrating a significant 4.9% reduction in energy consumption. This approach ensured reliable and valid results to provide a comprehensive view of how different roof types respond to varying climatic conditions.

• Aerogel-based cool roof compared with ceramic paint and bituminous membrane. • Summer and autumn campaign in a full-scale test-room in a Mediterranean climate. • Aerogel paint determines a surface temperature reduction of 9.7 °C during the day. • During extremely hot day the maximum surface temperature decreases of −36%. • After one year of aging Aerogel shows a solar reflectance 47% greater than traditional cool roof. Passive radiative cooling material for building application show consolidated energy environmental and economic benefit, both to the users than community. Technological progress allows to develop new types of cool materials able, for instance, to emit infrared radiation into the space or to have a dynamic behaviour. There are also some paints based on Aerogel, marketed as insulating paints, which however, achieve a limited insulation, due to the extremely reduced thickness. The study of their spectral characteristics and behavior is limited in scientific literature and, their commercialization lacks independent scientific evidence and relies on non-standard detection methods. This paper aims to investigate, under real climatic conditions, the thermal performance of Aerogel-based cool roof, with a solar reflectance of 84% and a thermal emissivity of 90%. It is compared with a ceramic-based paint and a common finishing roof material of bituminous membrane. Since in literature there are mainly numerical/simulative studies or in-lab based, the analysis covers a whole summer and autumn season of continuous experimental campaign in a full-scale test-room, placed in Mediterranean climate. The application of ceramic paint determines an average surface temperature reduction of 9.0 °C during the day and 2.1 °C at night; while the Aerogel one 9.7 °C and 2.2 °C respectively. During the sunniest day the maximum surface temperature decreases of 25% and 28% for the ceramic and Aerogel coating respectively. This percentage increases on extremely hot day (-36% for both coatings) and decreases on windiest day (-20% for Ceramic coating and −24% for Aerogel coating). [ABSTRACT FROM AUTHOR]

Increasing roof albedo (using a “cool” roof) and night ventilation are passive cooling technologies that can reduce the cooling loads in buildings, but existing studies have not comprehensively explored the potential benefits of integrating these two technologies. This study combines an experiment in the summer and transition seasons with an annual simulation so as to evaluate the thermal performance, energy savings and thermal comfort improvement that could be obtained by coupling a cool roof with night ventilation. A holistic approach integrating sensitivity analysis and multi-objective optimization is developed to explore key design parameters (roof albedo, night ventilation air change rate, roof insulation level and internal thermal mass level) and optimal design options for the combined application of the cool roof and night ventilation. The proposed approach is validated and demonstrated through studies on a six-storey office building in Xiamen, a cooling-dominated city in southeast China. Simulations show that combining a cool roof with night ventilation can significantly decrease the annual cooling energy consumption by 27% compared to using a black roof without night ventilation and by 13% compared to using a cool roof without night ventilation. Roof albedo is the most influential parameter for both building energy performance and indoor thermal comfort. Optimal use of the cool roof and night ventilation can reduce the annual cooling energy use by 28% during occupied hours when air-conditioners are on and reduce the uncomfortable time slightly during occupied hours when air-conditioners are off.

In the domestic sector, buildings contribute to about 55% of the total energy consumption for space heating and cooling. In this study, an experiment was carried out using a square shape building prototype made up of laminated plywood with Reinforced Cement Concrete (RCC) roof. Surface treatment of four types of reflective paints was applied on the RCC roof slab. Their performance was monitored in the summer season (April and May). Based on the peak temperature of interior and exterior roof layers, several important parameters, such as temperature, indoor thermal amplitude, time lag, decrement factor, thermal damping, thermal performance index, peak degree hours and thermal performance of each roof slab, were analysed. The application of cool roofs (CR-1, CR-2, CR-3, and CR-4) shows a significant reduction in indoor temperature (2.1°C–3.2°C), and indoor thermal amplitude (16.10%–27.94%) compared to a standard RCC roof. The drops in peak temperature of exterior and interior roof layers were 4.8°C–6.8°C and 3.9°C–6.3°C, respectively. A significant improvement was achieved in time lag and decrement factor (3–4 h more than the reinforced concrete roof), with energy savings varying from 10.58% to 13.73%.

In arid regions, global warming raises indoor air temperatures and consequently energy demand. Innovative cool roofing systems has to be developed to reduce indoor air temperatures and hence building energy consumption. The current study examines the advantages of using polystyrene foam insulation with a radiation reflector as a cool roof in Omani summers, as well as the thermal barrier effect of air circulation between them. The ventilated cool room has lower indoor air and heat flux values than the normal reference roof in field tests. The heat flux through the cool roof was reduced by 14 W/m2, 23 W/m2, and 29 W/m2 when the ventilation between the PsF insulation and the radiation reflector roof was 0, 25 cm, or 50 cm, respectively. The roof's thermal resistance increased due to the ventilation system's dynamic air flow between the PsF insulation and the radiation reflector (enhanced convective heat flux). During hot sunny days, the experimental studies demonstrated that the ventilated PsF insulation-radiation reflector cool roof lowers the temperature of the interior by a substantial amount and decreases heat flux through the roof. [ABSTRACT FROM AUTHOR]

This paper focuses on the impact of three types of roof retrofitting: green, high albedo-coated, also called ‘cool roofs’, and recently developed ‘super cool’ materials—in mitigating the effects of summer urban heat islands (UHI) and enhance overall urban thermal comfort. Human thermal comfort was considered in terms of physiologically equivalent temperature (PET), calculated by combining on-site field measurements with ENVI-met microclimate modelling and the RayMan model. The main environmental parameters addressed were roof surface temperature, ambient air temperature, and mean radiant temperature. The results show that the performance of the super cool roofs surpassed the cool and green roofs at reducing average ambient air and mean radiant temperatures, and the super cool roof had the lowest average PET during the day. However, all the roofs shortened the duration of strong thermal stress by 1 h compared to the conventional roof. This work contributes to the understanding of how roof retrofit technologies can be evaluated at the neighbourhood scale, assisting strategic decisions on building envelope retrofitting for sustainable cities.

The cooling load is the most prominent building energy consumption. The roof's poor thermal resistance effect causes buildings in the tropical climate to receive high solar heat that afterwards transfer to the attic before entering the living space. Eight roof design models were built and evaluated under 90 min of solar radiation exposure to noon-time level solar intensity to determine their cooling effect at noon every day. Significantly, the fantastic cool roof system was proven effective with a structural heat gain reduction of 85.5%. The reduction of concrete's density from 2500 kg/m3 to 1100 kg/m3 with lightweight foamed concrete technology allowed the roof's lower surface temperature to decrease by 21.6% (66.3–52.0 °C). Adding a highly solar reflective insulation system with roof ventilation reduced the structural heat gain by up to 33.3%. The combination effects of lightweight foamed concrete roof tile, hollow longitude sections, thermal reflective coating, reflective insulation system, and renewable solar-powered fans reduce the peak attic temperature from 42.8 °C to 32.2 °C.

Solar-reflective “cool” walls reduce absorption of sunlight by the building envelope, which may decrease cooling load in warm weather and increase heating load in cool weather. Changes to annual heating, ventilation, and air conditioning (HVAC) energy use depend on climate, wall construction, wall orientation, building geometry, HVAC efficiency, and operating schedule. Changes to annual energy cost and energy-related emissions further vary with local energy prices and emission factors. We used EnergyPlus to perform over 100,000 whole-building energy simulations, spanning 10 different building categories, three building vintages, 16 California climate zones, and 15 United States (U.S.) climate zones. Cool walls yielded annual source energy, energy cost, and emission savings in all California climate zones and in warm U.S. (ASHRAE) climate zones. In California, annual whole-building HVAC energy cost savings were 4.0–27% in single-family homes, 0.5–3.8% in medium office buildings, and 0.0–8.5% in stand-alone retail stores. In warm U.S. climates—zones 1A (Miami, FL) through 4B (Albuquerque, NM)—annual HVAC energy cost savings were 1.8–8.3% in single-family homes, 0.3–4.6% in medium office buildings, and 0.5–11% in stand-alone retail stores. California and U.S. fractional source energy and emission savings were comparable to fractional energy cost savings. Per unit surface area modified, cool-wall savings often exceeded cool-roof savings because building codes typically prescribe much less wall insulation than roof insulation.

Solar-reflective “cool” walls reduce absorption of sunlight by the building envelope, which may decrease cooling load in warm weather and increase heating load in cool weather. Changes to annual heating, ventilation, and air conditioning (HVAC) energy use depend on climate, wall construction, wall orientation, building geometry, HVAC efficiency, and operating schedule. Changes to annual energy cost and energy-related emissions further vary with local energy prices and emission factors. We used EnergyPlus to perform over 100,000 whole-building energy simulations, spanning 10 different building categories, three building vintages, 16 California climate zones, and 15 United States (U.S.) climate zones. Cool walls yielded annual source energy, energy cost, and emission savings in all California climate zones and in warm U.S. (ASHRAE) climate zones. In California, annual whole-building HVAC energy cost savings were 4.0–27% in single-family homes, 0.5–3.8% in medium office buildings, and 0.0–8.5% in stand-alone retail stores. In warm U.S. climates—zones 1A (Miami, FL) through 4B (Albuquerque, NM)—annual HVAC energy cost savings were 1.8–8.3% in single-family homes, 0.3–4.6% in medium office buildings, and 0.5–11% in stand-alone retail stores. California and U.S. fractional source energy and emission savings were comparable to fractional energy cost savings. Per unit surface area modified, cool-wall savings often exceeded cool-roof savings because building codes typically prescribe much less wall insulation than roof insulation.

Energy demand is growing significantly worldwide to create thermal comfort in buildings. Air-conditioning is contributing to energy consumption at a massive scale in the residential and commercial sectors. The roof is one of the most critical components of the building envelopes, and it achieved maximum heat gain in summer, and it covered nearly 20–25% of overall urban surface areas. In this respect, cool roofs are considered one of the sustainable solutions to maintain thermal comfort in buildings. The results achieved from the literature review indicate that cool roof application reduced energy use in the buildings and a useful tool to mitigate Urban Heat Island (UHI) effect. This paper summarizes cool roof thermal performance with different types of surface coatings in different climatic zones for buildings with additional benefits, limitations, and recommendations for future research work. The results of this review can be helpful for engineers, researchers, dwellers, and architectures to have a good understanding of the benefits of cool roofs to mitigate energy consumption demand in dwelling in a sustainable, cost-effective, and energy-efficient way. The average energy-saving effect of the roof is expressed from 15% to 35.7% in different climatic zones (Temperate, Tropical, Composite, Hot and Warm-Humid) as per the literature survey results. Also, the average roof surface temperature reduction is possible from 1.4 ˚C to up to 4.7 ˚C using cool roof technology.

Energy-efficient retrofitting of building envelopes is necessary to reduce global carbon emissions and to reach net-zero goals. Cooling energy demand-dominated countries in the GCC region require simple and effective strategies to reduce building sector energy loads. One such approach is using high solar reflective index (SRI) paints to retrofit building roofs and walls. However, the hot and desert conditions of the region pose a barrier to maintaining consistent radiative properties throughout their life cycle. To this extent, research is limited in the region. The novelty of this work is to qualitatively assess the aging characteristics of high SRI or cool paints and estimate the energy savings for their application in residential buildings. The work encompasses comprehensive lab, pilot, and real-scale experimental studies combined with theoretical modeling for dynamic evaluation. Dynamic simulations enabled to determine the time-dependent aging effect on the energy savings performance of the building retrofitted with cool roof and wall paints. A case study on a townhouse in UAE showed annual energy savings of 34% considering cool roofs, walls, and window films. Aging studies showed SRI reduction of 36% and 25%, respectively, for cool roofs and walls during the first 3 years. The corresponding energy-saving reductions ranged from 31 to 44% for the white roof to dark wall colors. Using the initial values of SRI in energy models overestimates saving by 10% per year. Considering the aging effects, this work provides insights into cool paint retrofit potential on energy, economic savings, and CO2 reductions for four major cities in the GCC region. [ABSTRACT FROM AUTHOR]

An urban heat island (UHI) is a phenomenon where the temperatures within cities are greater than those of surrounding rural areas due to human activity and physical properties of urban surfaces. One method that has been investigated as a way to mitigate the UHI phenomenon is to increase the surface albedo in cities, which reflects a greater amount of solar radiation away from these surfaces compared to conventional materials (e.g., dark asphalt shingles). In this study, we utilize the Weather Research and Forecasting (WRF) model to investigate the UHI under different scenarios during a July 2012 heat wave event in the Kansas City metropolitan area (KCMA). Two cool roof simulations are implemented to determine the effectiveness of this mitigation strategy on reducing temperatures within the KCMA. The first scenario represents "newly installed" cool roofs with an albedo of 0.8 and the second with "aged" cool roofs with an albedo of 0.5. Results indicate that cool roof materials were able to mitigate the UHI effect by up to 0.64 °C during the evening, causing the onset of the UHI effect to be delayed until later in the day. Cool roofs were also shown to have important impacts on the surface energy balance, affecting both sensible and ground storage heat fluxes, and the planetary boundary layer. [ABSTRACT FROM AUTHOR]

This paper investigates the thermal performance of green roofs, cool roofs, and their combined effects in tropical climates. Although each technology has been studied independently for its potential to reduce cooling energy consumption in buildings, their combined effects have not been thoroughly examined in tropical climates. The study employed experimental and numerical methods, demonstrating that combining green and cool roofs can lead to even greater cooling energy savings. The research involved fabricating four identical cubicles made of mild steel sheets and placing them in an open space for testing under two operational conditions: closed window and open window/door. The combined green and cool roof technology achieved a temperature difference reduction of 4.14 °C compared to the original roof under the closed window and door state, with green and cool roofs achieving 0.72 °C and 0.79 °C, respectively. Combining green and cool roofs led to even more significant cooling energy savings, with 53.57 kWh energy savings compared to 20.1 kWh and 3.68 kWh for combined, green, and cool cubicles, respectively. The study found that combining green and cool roofs led to even more significant cooling energy savings, with 53.57 kWh energy savings compared to 20.1 kWh and 3.68 kWh for combined, green, and cool cubicles, respectively. The research suggests that combining these technologies can lead to greater cooling energy savings and highlights the potential benefits of green and cool roofs for tropical climates. [ABSTRACT FROM AUTHOR]

Green and cool roofs retrofit technologies, aim to mitigate the urban heat island effect and cooling energy demands from buildings. This study reports on parametric energy modelling of energy saving loads and indoor air temperature, as well as microclimate modelling of outdoor air temperature. The work adds new knowledge by evaluating roof retrofit technologies at the building and neighbourhood scale, and the results should guide strategic decisions for building envelope retrofitting. The simulations respectively predicted a 10% and 7.5% reduction in the cooling load for cool and green roofs versus a conventional roof. In summer, the indoor air temperature was similar for these roofs, but in winter the cool roof's indoor air temperature was 0.5°C lower than the green. In the microclimate simulation, average roof surface temperatures were 31.5°C (cool) and 31.3°C (green), versus 40.2°C (conventional); the air temperature difference was −0.8°C (cool) and −0.6°C (green) against the conventional roof. [ABSTRACT FROM AUTHOR]

Passive radiative cooling is an energy-efficient method that reduces indoor temperature and cooling load in buildings, thus contributing to significant energy savings. This study investigates the thermal performance of the passive radiative cooling-based cool roof (PRC-CR) based on experimental and numerical analysis in terms of indoor air temperature and space cooling load indicators. Two experiment rooms constructed from concrete are developed with an indoor air temperature control system. One is the PRC-CR room with the roof covered by a passive radiative cooling metamaterial and the other is the reference room with a concrete roof. Long-term testing results in Hefei show that the indoor air temperature of the PRC-CR room is 5.5 °C lower in maximal and 1.8 °C lower on average than that of the reference room in summer when the indoor air temperature control system is off. Besides, when the indoor air temperature control system is switched on, the PRC-CR room achieves a 29.1% reduction in daily cooling load in summer and a 4.1% increase in daily heating load in winter, which suggests that passive radiative cooling can indeed save energy in cooling season (i.e., hot season) but cause overcooling in heating season (i.e., cold season). In addition, a simulation building model is developed to further investigate the effect of the PRC-CR on the energy consumption of buildings in different locations. The predicted results show that PRC-CR reduces the total load of buildings in cooling load prevailing regions (e.g., tropical zones, subtropical zones, and the vast majority of warm temperate zones), while increasing the total load of buildings in heating load dominant regions (e.g., most middle temperate zones, cold temperate zones, and plateau climate zones). In summary, this work demonstrates both the positive and negative effects of PRC-CR, providing valuable insights into PRC-CR applications. • The effect of passive radiative cooling-based cool roof (PRC-CR) is comprehensively investigated. • The cooling load of the building is reduced by 29.1%, but the heating load is increased by 4.1%. • PRC-CR has both positive and negative effects and varies in different climate zones. • PRC-CR is preferred in cooling prevailing regions rather than heating dominant regions. [ABSTRACT FROM AUTHOR]

The urban heat island, coupled with increasingly intense, frequent and long heat waves, leads to a significant risk of excess mortality and increased energy requirements in urban areas during the summer season. Various cooling techniques are therefore being studied to moderate the intensity of this phenomenon during the hot season. The aim of this study is to quantify the contribution of roofs to urban climate during heatwaves, depending on the position of the insulation in the structure and its radiative properties. This study is carried out in the laboratory, using an experimental set-up to submit roof samples to heatwave conditions. The experimental set up includes a temperature and humidity-controlled climate chamber with a solar simulator. Two roofing configurations are studied: roofs with external thermal insulation (ETI) and roofs with internal thermal insulation. Each configuration is studied for two types of roof: standard (low albedo), and reflective (high albedo). It appears that ETI roofs heat the outside air during the day, but not at night. While ITI roofs heat the ambient air less during the day, they release some of the stored daytime heat during the night. Whatever the structure, increasing solar reflectivity significantly reduces the contribution of roofs to urban heating, although the impact is stronger for internally-insulated roofs, since it reduces the amount of heat released. Finally, for the externally insulated structure, the increase in solar reflectivity reduces daytime convective exchange by 41% and heat entering the building by 75%. For the internally insulated structure, daytime convective heat exchange is reduced by 43%, and the heat stored in the concrete layer is reduced by over 50%. [ABSTRACT FROM AUTHOR]

An analytical hierarchy model of the impact of solar reflectance, thermal emittance, heat transfer coefficient, and heat storage coefficient on building energy consumption was established through the implementation of orthogonal design experiments. The EnergyPlus software (v9.0.1) was utilized to simulate building energy consumption across diverse climatic regions in China, providing essential benchmarks for the orthogonal design. The results of the range analysis consistently indicate that, barring regions characterized by extremely cold climates, solar reflectance emerges as the predominant factor exerting an influence on building energy consumption. As geographical latitude increases, the impact of the heat transfer coefficient becomes progressively larger, while the weight of thermal reflectance concurrently diminishes. Drawing upon the principles rooted in the gradient refractive rate theory and the concept of atmospheric window radiation, a range of high-reflectance and high-emittance cool roof coatings in various colors were meticulously developed. A spectrophotometer was employed to precisely quantify their reflectance properties, and simulations were subsequently conducted to scrutinize their energy-saving characteristics. The results demonstrate that the cool roof coatings that were developed using the methodology described in this paper exhibit substantial enhancements in reflectance, with increases of 0.24, 0.25, 0.37, and 0.35 for the yellow, red, blue, and green cool roofing materials, respectively, in comparison to conventional colored coatings. Under typical summer conditions, these enhancements translate to significant reductions in roof temperatures, ranging from 9.4 °C to 14.0 °C. Moreover, the simulations exploring the cooling loads for the roofs of differing colors consistently revealed remarkable energy savings. These savings were quantified to be 4.1%, 3.9%, 5.5%, and 5.4%, respectively, when compared to conventional coatings of the corresponding colors. These findings offer valuable insights into strategies for optimizing the energy efficiency of buildings through the application of high-reflectance cool roofing materials.

Building energy, particularly air conditioning energy, makes for a significant fraction of overall societal energy usage. The heat island effect is a common urban environmental concern that threatens human sustainable development. Roofs, which cover more than 20% of the total metropolitan area, play an essential role in mitigating the urban heat island effect and lowering building energy use. To examine the triple beneficial benefits of cool roofs and green roofs on building energy conservation in different temperature zones in China, a simulation model based on a typical residential community is established. The results reveal that minimizing direct heat acquisition from the roof, reducing heat transfer from the enclosure, and enhancing the coefficient of performance have a surprising triple effect on building energy saving. In cold areas, hot summer and cold winter areas, and hot summer and warm winter areas, cool roofs may lower the regional ambient temperatures by 2°C, 2.3°C, and 2.6°C, respectively, whereas green roofs can reduce the regional ambient temperatures by 1°C, 1.1°C, and 1.2°C. The triple saving impact of cool roofs and green roofs may accomplish 11.0%, 11.5%, 12.6%, and 9.4%, 8.1%, and 9.3%, respectively, for building energy conservation. Because of the increased solar radiation, cool roofs perform better in low-latitude zones, whereas green roofs function consistently.

In the domestic sector, buildings contribute to about 55% of the total energy consumption for space heating and cooling. In this study, an experiment was carried out using a square shape building prototype made up of laminated plywood with Reinforced Cement Concrete (RCC) roof. Surface treatment of four types of reflective paints was applied on the RCC roof slab. Their performance was monitored in the summer season (April and May). Based on the peak temperature of interior and exterior roof layers, several important parameters, such as temperature, indoor thermal amplitude, time lag, decrement factor, thermal damping, thermal performance index, peak degree hours and thermal performance of each roof slab, were analysed. The application of cool roofs (CR-1, CR-2, CR-3, and CR-4) shows a significant reduction in indoor temperature (2.1°C–3.2°C), and indoor thermal amplitude (16.10%–27.94%) compared to a standard RCC roof. The drops in peak temperature of exterior and interior roof layers were 4.8°C–6.8°C and 3.9°C–6.3°C, respectively. A significant improvement was achieved in time lag and decrement factor (3–4 h more than the reinforced concrete roof), with energy savings varying from 10.58% to 13.73%. [ABSTRACT FROM AUTHOR]

Radiative cooling has been demonstrated as an effective way to reduce building energy consumption especially for the recently developed diurnal radiative cooling materials. The most convenient application of radiative cooling materials is configured as the cool roofs. However, cool roofs will produce cooling energy throughout the year which will lead to the increase of heating penalty in the heating season. This phenomenon will offset part of the cooling benefit from radiative cooling. The ventilated roofs are also commonly employed in buildings for achieving energy-saving by reducing the heat gain from the roof to the building. To reduce the influence of radiative cooling in the heating season, a radiative cooling-based ventilated roof (RCVR) is proposed in this study by integrating the ventilated roof with the radiative cooling-based cool roof. The process of the heat transfer through RCVR is analyzed, and the simplified heat transfer model is developed. And then the effect of RCVR configuration on the room temperature is discussed in detail through CFD simulation. Finally, the annual electricity consumption of the building with RCVR is derived using EnergyPlus. The results show that the annual comprehensive performance of RCVR is superior to the cool roof and original roof since the total annual electricity consumption of the building with RCVR can be decreased by 21.8% and 16.9% compared with that of cool roof and original roof. Practical application: A radiative cooling-based ventilated roof (RCVR) is proposed by integrating the ventilated roof with the radiative cooling-based cool roof. The results show that the annual comprehensive performance of RCVR is superior to the cool roof and original roof. This study can provide the guiding significance for the application of RCVR, in which the building energy consumption can be further decreased for the application of radiative cooling in buildings. [ABSTRACT FROM AUTHOR]

• The impact of cool roofs/walls in street canyons (AR = H/W = 1,5) is studied using CFD. • Coupling cool roofs and walls has better cooling effects than solely applying them. • Cool materials mainly cool surface temperature instead of the in-canyon air. • Cool walls mainly cool the partial walls that are directly exposed to sunlight. • In H/W = 5 canyons, 25% / 50% cool walls achieve similar cooling as 100% cool walls. Adopting cool (high-reflective) materials on building envelopes is an effective mitigation strategy for urban heat island effects. However, most previous studies emphasized cool roofs, which can cool low-rise urban areas but have limited effects in high-rise cities. As a novelty, this study numerically investigates the impact of various cool coatings—cool roofs only, cool walls only, and the combination of cool roofs and cool walls—on urban wind and thermal environments. Computational fluid dynamics simulations with standard k-ε model and P-1 radiation model are adopted. Idealized street canyons with two aspect ratios of 1 and 5 and three local solar times (9:00am, 12:00noon and 15:00pm) are considered. We found that all cool coatings mainly cool direct sunlight-exposed building surfaces, but marginally decrease in-canyon air temperature and negligibly affect wind velocity. Specifically, cool roofs significantly decrease roof surface temperature, while cool walls primarily cool the upper-level wall surfaces directly exposed to solar radiation, particularly evident in deep street canyons. Moreover, upper-level partially-cool-coated walls with area fractions of 50% and 25% can achieve comparable cooling effects to entirely-cool-coated walls. This study provides effective methodologies and suggests the requirements for quantitative evaluations of appropriate cool-wall proportions tailored to specific street scenarios. [ABSTRACT FROM AUTHOR]

Subtropical climate is characterized by high solar altitude angle in summer which causes the roof get more heat through solar radiation. GR, DSR, and CR all can decrease solar radiation heat gain of the roof. However, few researches have been done to the comparison of the thermal performance of these three roofs, especially in subtropical climate. In this study, four rooms were built separately with GR, DSR, CR, and ordinary roof (OR). The experiment was done from July 23 to August 4. Results showed that stabilities of the indoor air temperature of the four rooms were: DSR room > GR room > CR room > OR room. The GR, CR, and DSR can reduce the external surface temperature by 13.7°C, 12.0°C, and 4.8°C during the day while bring a temperature rise of 2.3°C, 1.9°C, and 0.9°C at night. Correlation analysis results showed that the internal surface heat flux of GR and DSR were negative correlated with weather factors while internal surface heat flux of OR and CR were positive correlated with weather factors. This study can give support to the selection between GR, DSR, and CR. [ABSTRACT FROM AUTHOR]

The management of thermal emittance always possess challenges in both military and civil domains. The low emissive materials in form of paints offers viable solution for minimising thermal emittance of military objects for camouflage and enhancing infrared reflectivity of roof to reduce cooling loads. Binder apart from pigments is one of the critical components of low emissive paint formulation to decide its ultimate performance. For selection of appropriate resin and binder, a comparative study of five prevalent polymers viz. polyurethane (PU), poly vinyl chloride (PVC), polyethylene (PE), polypropylene (PP) and poly methyl methacrylate (PMMA) has been carried out. Polymeric film of thickness 100µm of each of the polymer is prepared by Elcometer 4340 motorized film applicator. Infrared spectra and thermogravimetric profile have been obtained by Bruker make Tensor 27 FTIR in spectral range 4000 - 600 cm-1 and TGA Q 500. Film formation of PU is excellent and transmission in 3-5 µm is almost 100%. Hence, it may be chosen as binder in low emissive paint for MWIR band. PVC and PMMA among the polymers offer excellent transmission 87-99% in both thermal transmission windows 3-5 µm and 8-12 µm. Thermogravimetric (TG) profile of polymer films evident of thermal stability of PVC, PMMA and PU polymers up to 200 °C. Infrared transmission, TG profile and film formability manifest that both binders-PVC and PMMA are suitable for development of low emissive paints applicable up to temperature 200 °C for both IR transmission windows to evade thermal detectors working for detection of military objects and reducing cooling load in buildings by enhancing reflectivity of roof by application of low emissive paint. [ABSTRACT FROM AUTHOR]