Cool Roofs Research Articles

Cool Pavement Strategies for Urban Heat Island Mitigation in Suburban Phoenix, Arizona

Urban areas are characterized by a large proportion of artificial surfaces, such as concrete and asphalt, which absorb and store more heat than natural vegetation, leading to the Urban Heat Island (UHI) effect. Cool pavements, walls, and roofs have been suggested as a solution to mitigate UHI, but their effectiveness depends on local land-use patterns and surrounding urban forms. Meteorological data was collected using a mobile platform in the Power Ranch community of Gilbert, Arizona in the Phoenix Metropolitan Area, a region that experiences harsh summer temperatures. The warmest hour recorded during data collection was 13 August 2015 at 5:00 p.m., with a far-field air temperature of about 42 ∘ C and a low wind speed of 0.45 m/s from East-Southeast (ESE). An uncoupled pavement-urban canyon Computational Fluid Dynamics (CFD) model was developed and validated to study the microclimate of the area. Five scenarios were studied to investigate the effects of different pavements on UHI, replacing all pavements with surfaces of progressively higher albedo: New asphalt concrete, typical concrete, reflective concrete, making only roofs and walls reflective, and finally replacing all artificial surfaces with a reflective coating. While new asphalt surfaces increased the surrounding 2 m air temperatures by up to 0.5 ∘ C, replacing aged asphalt with typical concrete with higher albedo did not significantly decrease it. Reflective concrete pavements decreased air temperature by 0.2–0.4 ∘ C and reflective roofs and walls by 0.4–0.7 ∘ C, while replacing all roofs, walls, and pavements with a reflective coating led to a more significant decrease, of up to 0.8–1.0 ∘ C. Residences downstream of major collector roads experienced a decreased air temperature at the higher end of these ranges. However, large areas of natural surfaces for this community had a significant effect on downstream air temperatures, which limits the UHI mitigation potential of these strategies.

Evaluating the Operational Potential of LRV Signatures Derived from UAV Imagery in Performance Evaluation of Cool Roofs

It is quite difficult to find studies regarding area-wide data from UAV (Unmanned Aerial Vehicle) remote sensing in evaluating the energy saving performance of a cool roof. Acknowledging these constraints, we investigated whether LRV (Light Reflectance Value) signatures derived from UAV imagery could be used effectively as an indicator of area-wide heating and cooling load that distinctively appears according to rooftop color. The case study provides some quantitative tangible evidence for two distinct colors: A whitish color roof appears near the edge of the highest LRV (91.36) and with a low temperature (rooftop surface temperature: (38.03 °C), while a blackish color roof shows the lowest LRV (18.14) with a very high temperature (65.03 °C) where solar radiation is extensively absorbed. A strong negative association (Pearson correlation coefficient, r = −0.76) was observed between the LRV and surface temperature, implying that a higher LRV (e.g., a white color) plays a decisive role in lowering the surface temperature. This research can be used as a valuable reference introducing LRV in evaluating the thermal performance of rooftop color as rooftops satisfying the requirement of a cool roof (reflecting 75% or more of incoming solar energy) are identified based on area-wide objective evidence from UAV imagery.

The Effectiveness of Cool and Green Roofs as Urban Heat Island Mitigation Strategies: A Case Study

The Effectiveness of Cool and Green Roofs as Urban Heat Island Mitigation Strategies: A Case Study

Cool roofs and hollow microspheres

Cool roofs and hollow microspheres

Study of the Impact of Cool Roof on Urban Thermal Comfort in Hot Arid Climate, Biskra – Algeria

The Uncontrolled urban sprawl and the unadopted building to the environment are the main causes that generate the urban heat island (UHI) phenomenon. This phenomenon contributes to the degradation of the environmental quality of the interior and exterior spaces and increases the energy demand of buildings. In hot and arid regions, the cooling needs of space during the hot season are much greater. This situation imposes the use of electrical air-conditioning equipment for longer periods, which leads to a drastic consumption of electrical energy, implicitly, rise the greenhouse gas emissions, which in turn accentuate the phenomenon of the urban heat island. In this context, one of the most effective strategies to moderate heat stress at the urban scale is the adoption of the cooling roof technique "cool roof”. In fact, solar reflectivity (SR) and high emissivity (IE) of cool materials allow them to absorb less solar energy than conventional concrete roofs, reducing the amount of heat transferred to the atmosphere. This research aims to evaluate the impact of cool roof on the mitigation of the urban heat island phenomenon and to investigate its effect on reduction of energy consumption. Experimental work was carried out in Biskra hot and arid climate.

Investigating the Urban Air Quality Effects of Cool Walls and Cool Roofs in Southern California.

Solar reflective cool roofs and walls can be used to mitigate the urban heat island effect. While many past studies have investigated the climate impacts of adopting cool surfaces, few studies have investigated their effects on air pollution, especially on particulate matter (PM). This research for the first time investigates the influence of widespread deployment of cool walls on urban air pollutant concentrations, and systematically compares cool wall to cool roof effects. Simulations using a coupled meteorology-chemistry model (WRF-Chem) for a representative summertime period show that cool walls and roofs can reduce urban air temperatures, wind speeds, and planetary boundary heights in the Los Angeles Basin. Consequently, increasing wall (roof) albedo by 0.80, an upper bound scenario, leads to maximum daily 8-h average ozone concentration reductions of 0.35 (0.83) ppbv in Los Angeles County. However, cool walls (roofs) increase daily average PM2.5 concentrations by 0.62 (0.85) μg m-3. We investigate the competing processes driving changes in concentrations of speciated PM2.5. Increases in primary PM (elemental carbon and primary organic aerosols) concentrations can be attributed to reductions in ventilation of the Los Angeles Basin. Increases in concentrations of semivolatile species (e.g., nitrate) are mainly driven by increases in gas-to-particle conversion due to reduced atmospheric temperatures.

Estimation of Energy Savings for Cool Roof in Residential Building in India

Estimation of Energy Savings for Cool Roof in Residential Building in India

Hygrothermal Performance of Cool Roofs Subjected to Saudi Climates

When solar radiation hits a roof surface, a part of solar energy is reflected and another part is absorbed. The absorbed part of solar energy results in an increase of the surface temperature of the roof. Cool (white) roofs use bright surfaces to reflect a significant portion of the incident short-wave solar radiation, which lowers the surface temperature compared to conventional (black) roofs. As such, white roofs help reduce the urban heat island effect during the summer. The questions are: (a) do white roofs lead to moisture-related problems in Saudi climates? (b) what are the amount of energy savings as result of using white roofs instead of black roofs of same insulation amount, and (c) what is the reduction in the amount of insulation in the white roof having the same energy performance level as the black roof? To answer these questions, numerical simulations were conducted for one type of roof that is commonly used in low-rise buildings of Saudi Arabia in order to assess and compare the energy and hygrothermal performance of white and black roofs. The roof was subjected to weather conditions of the Eastern Province of Saudi Arabia. The indoor conditions were taken based ASHRAE recommendations for conditioned space. The main outcome of this study has shown the capabilities of using reflective materials with different short-wave solar absorption coefficients for enhancing the energy performance of roofs and/or reducing the amount of insulation that resulted in same energy performance as black roofs. This study can be used in future for upgrading Saudi Building Code to allow less roof insulation if reflective roof is installed.

Potential benefits of cool roofs in reducing heat-related mortality during heatwaves in a European city

Hot weather can exacerbate health conditions such as cardiovascular and respiratory diseases, and lead to heat stroke and death. In built up areas, temperatures are commonly observed to be higher than those in surrounding rural areas, due to the Urban Heat Island (UHI) effect. Climate change and increasing urbanisation mean that future populations are likely to be at increased risk of overheating in cities, although building and city scale interventions have the potential to reduce this risk.We use a regional weather model to assess the potential effect of one type of urban intervention – reflective ‘cool’ roofs – to reduce local ambient temperatures, and the subsequent impact on heat-related mortality in the West Midlands, UK, with analysis undertaken for the summer of 2006, as well as two shorter heatwave periods in 2006 and 2003.We show that over a summer season, the population-weighted UHI intensity (the difference between simulated urban and rural temperature) was 1.1 °C on average, but 1.8 °C when including only night times, and reached a maximum of 9 °C in the West Midlands. Our results suggest that the UHI contributes up to 40% of heat related mortality over the summer period and that cool roofs implemented across the whole city could potentially offset 18% of seasonal heat-related mortality associated with the UHI (corresponding to 7% of total heat-related mortality).For heatwave periods, our modelling suggests that cool roofs could reduce city centre daytime 2 m air temperature by 0.5 °C on average, and up to a maximum of ~3 °C. Cool roofs reduced average UHI intensity by ~23%, and reduced heat related mortality associated with the UHI by ~25% during a heatwave. Cool roofs were most effective at reducing peak temperatures during the daytime, and therefore have the potential to limit dangerous extreme temperatures during heatwaves. Temperature reductions were dependent on the category of buildings where cool roofs were applied; targeting only commercial and industrial type buildings contributed more than half of the reduction for heatwave periods. Our modelling suggested that modifying half of all industrial/commercial urban buildings could have the same impact as modifying all high-intensity residential buildings in the West Midlands.

Scale dependence of the benefits and efficiency of green and cool roofs

Cool and green roofs are widely adopted measures for curtailing summertime urban heat islands. Existing numerical studies to assess their effectiveness and cooling benefits usually assume an unrealistic 100% coverage across the entire metropolis. This study investigates the scale dependence of the absolute cooling benefits and efficiency (cooling per adapted roof area) of cool and green roofs in a typical summer when they are installed over 25% of building rooftops at local, city, or regional scales. Six major U.S. cities with active climate action plans in different geoclimatic zones are compared through high-resolution simulations using the Weather Research and Forecasting model. The results reveal that reductions in 2-m air temperature over the urban core increase non-linearly with the intervention area, and the benefits of both roof types scale similarly. This scale-dependence of urban core cooling is not universal, but is rather controlled by the shape of metropolitan areas and wind pattern. The siting of mitigation measures hence plays an important role especially under windy conditions, and some urban cores are not able to achieve a noticeable and consistent cooling by retrofitting their own rooftops. Regional-scale deployments of mitigation strategies, on the other hand, yield a more substantial temperature reduction but with a lower efficiency. The scale-dependence of regional cooling efficiency showed remarkable similarity across studied cities, yielding a potentially generalizable power law. The successful resiliency plans for cities should account for the scale dependence and geoclimatic determinants of the achievable cooling, and identify the target neighborhoods of most interest.

Effect of High Aged Albedo Cool Roofs on Commercial Buildings Energy Savings in U.S.A. Climates

Cool roofs may reduce the temperature of a surface exposed to the sun and decrease the energy consumption inside buildings. This type of technology works under two different principles; solar reflectance and thermal emissivity and there is a wide variety of products in the market with different initial and aged reflectance and emissivity values. Reflectance is the fraction of solar energy that is reflected by the roof, and this value decreases over time. Roofs with high initial reflectance and low aged reflectance are currently the most common scenario. In this paper, simulations were run to determine energy savings when using high albedo cool roof technologies with albedo values fewer than 70%. For this study, we selected the cities with the most solar radiation in the U.S.A. and those with rebates and incentives available and approved by the Cool Roof Rating Council (CRRC). Results show a significant difference between the values of solar reflectance and suggest supporting the application of high albedo coatings for substantial energy savings.

Comparison of Rooftop Surface Temperature and Indoor Temperature for the Evaluation of Cool Roof Performance according to the Rooftop Colors in Summer: Using Thermal Infrared Camera Mounted on UAV

Comparison of Rooftop Surface Temperature and Indoor Temperature for the Evaluation of Cool Roof Performance according to the Rooftop Colors in Summer: Using Thermal Infrared Camera Mounted on UAV

Analysis of Cool Roof Passive Techniques for Residential Buildings in a Subtropical Humid Climate

The objective of this study was to provide the thermal comfort by means of passive cool solar reflective materials which have a high solar reflec­tance and high thermal emittance on the existing roof and thereby reducing the consumption of electrical energy. Experiments were performed by using locally available solar reflective materials to insulate the roofs. In this work, the thermal performance of concrete roof and asbestos sheet roof with and without insula­tion has been analyzed by performing experiments to existing buildings. The re­sults are verified by computer simulation using Computational Fluid Dynamics tools with FLUENT software. The result of using solar reflective paint with high reflective coating shows a fall of 4.8°C in peak hours and saves 303 kWh when considering an energy load with an air conditioner in the summer season, in comparison to non-insulated pitched and flat roof energy loads of residential buildings in Bhopal. An optimum solution of an insulator for both types of roofs is presented. The application is intended for low-rise or medium-rise residential buildings in a subtropical climate like Bhopal.

CLAY ROOFING TILE: A COOL ROOF?

The aim of this study is to determine if the clay roofing tile in its natural albedo can be considered “cool” in the thermal performance through a comparison of its surface temperature to a cool coated clay tile using the exposure rack method. It begins with highlighting the expectation (from review) of the thermal performance of a cool colored tile and comparing the surface temperature (maximum, mean and minimum) of the tiles. Day and night time temperature behaviors were also compared using graphs on excel software. A python t- test of means was also carried out to determine if there was any significant difference in the performance of the materials. It was found that the performance of the clay tile in its natural albedo is a cool tile in its thermal performance. The implication of this is a cost saving strategy. However, further research is needed as a form of coating may be needed in order to enhance its life span. A non- color coat whose only function is to protect the surface of the tile. This does not in any way disregard the use of coated tiles as user preference may still tend towards non- natural clay colored tiles, its purpose is to encourage informed cheaper methods of achieving cool roof strategies

쿨루프 적용에 따른 건물의 레이어별 온도 변화 : 창원대학교를 사례로

쿨루프 적용에 따른 건물의 레이어별 온도 변화 : 창원대학교를 사례로

Systematic Comparison of the Influence of Cool Wall versus Cool Roof Adoption on Urban Climate in the Los Angeles Basin.

This study for the first time assesses the influence of employing solar reflective "cool" walls on the urban energy budget and summertime climate of the Los Angeles basin. We systematically compare the effects of cool walls to cool roofs, a heat mitigation strategy that has been widely studied and employed, using a consistent modeling framework (the Weather Research and Forecasting model). Adoption of cool walls leads to increases in urban grid cell albedo that peak in the early morning and late afternoon, when the ratio of solar radiation onto vertical walls versus horizontal surfaces is at a maximum. In Los Angeles County, daily average increase in grid cell reflected solar radiation from increasing wall albedo by 0.80 is 9.1 W m-2, 43% of that for increasing roof albedo. Cool walls reduce canyon air temperatures in Los Angeles by 0.43 K (daily average), with the peak reduction (0.64 K) occurring at 09:00 LST and a secondary peak (0.53 K) at 18:00 LST. Per 0.10 wall (roof) albedo increase, cool walls (roofs) can reduce summertime daily average canyon air temperature by 0.05 K (0.06 K). Results reported here can be used to inform policies on urban heat island mitigation or climate change adaptation.

A REVIEW OF MATERIAL COVER FEATURES FOR MITIGATING URBAN HEAT ISLAND

AbstractUrban Heat Island (UHI) is related to the increase of urban compared to rural temperature as the result of global phenomenon. The increase of temperature is predicted to be intensified along with the extend of urban activity in the near future. Therefore, the discussion on UHI becomes significance. This paper discusses the result of literature studies on thermal characteristics of materials that potentially used to reduce Urban Heat Island, especially in utilizing pavement and roof cover. The result of the study concludes that the reduction of UHI is determined by: 1) the high-level albedo (the ratio between the reflected heat and the absorbed heat) of material that is influenced by the color and texture of its surface; 2) The high level thermal emittance of material; 3) The lower capacity of material to store the heat. 4) The capability thermal conductivity of material surface. Recommended pavements to reduce UHI are cool pavement, reflective pavement, porous pavers, permeable pavers, pervious pavement, water retaining pavement. Roof cover materials that reduce Urban Heat Island are Cool Roof and Green Roof. The design and toughness of materials should be considered to reduce UHI. Keywords: Urban Heat Island, material feature, thermal performance.

Use of Cool Roofs and Vegetation to Mitigate Urban Heat and Improve Human Thermal Stress in Melbourne, Australia

AbstractThe ability of cool roofs and vegetation to reduce urban temperatures and improve human thermal stress during heat wave conditions is investigated for the city of Melbourne, Australia. The Weather Research and Forecasting Model coupled to the Princeton Urban Canopy Model is employed to simulate 11 scenarios of cool roof uptake across the city, increased vegetation cover across the city, and a combination of these strategies. Cool roofs reduce urban temperatures during the day, and, if they are installed across enough rooftops, their cooling effect extends to the night. In contrast, increasing vegetation coverage reduces nighttime temperatures but results in minimal cooling during the hottest part of the day. The combination of cool roofs and increased vegetation scenarios creates the largest reduction in temperature throughout the heat wave, although the relationship between the combination scenarios is nonsynergistic. This means that the cooling occurring from the combination of both strategies is either larger or smaller than if the cooling from individual strategies were to be added together. The drier, lower-density western suburbs of Melbourne showed a greater cooling response to increased vegetation without enhancing human thermal stress due to the corresponding increase in humidity. The leafy medium-density eastern suburbs of Melbourne showed a greater cooling response to the installation of cool roofs. These results highlight that the optimal urban cooling strategies can be different across a single urban center.

Cool roofs: High tech low cost solution for energy efficiency and thermal comfort in low rise low income houses in high solar radiation countries

Cool roofs are most effective in reducing cooling loads and alleviating overheating in locations with high solar radiation and external air temperature. This paper presents results of an experimental study of a low income house in Jamaica and a computational study in three countries around the equator: Jamaica, Northeast Brazil (Recife) and Ghana. A case-study typical of single storey houses in Jamaica was monitored before and after the installation of a cool paint on the roof; on days with average solar radiation intensity of ∼420 W/m2 and ambient air temperature of ∼28 °C, internal ceiling surface temperature is reduced by an average of 6.8 °C and internal air temperature by 2.3 °C. Monitoring results were used to calibrate successfully an EnergyPlus model; similar models were developed for Ghana and Brazil differing in size and/or construction to reflect country specific practices. Annual simulations indicate that internal ceiling surface temperatures are reduced on average by 3.2–5.5 oC and internal air temperatures by 0.75–1.2 °C. Cooling demand simulations (setpoint 24 °C) indicate similar annual potential savings in the three locations (∼190 kWh/m2/year) although estimated CO2 emissions reduction differ reflecting electricity generation fuels. Aging of the cool roof has an impact reducing load savings by 22–26 kWh/m2/year.

Cool Roof Coating - Increasing Concern towards Energy Consumption, Carbon Emissions

Cool Roof Coating - Increasing Concern towards Energy Consumption, Carbon Emissions