Cool roofs: A climate change mitigation and adaptation strategy for residential buildings

Cool roofs are roofing systems designed to reflect significant solar radiation, reducing heat absorption and subsequent cooling energy demands in buildings. This paper provides a comprehensive review of cool roof technologies, covering performance standards, material options, energy-saving potential, and hygrothermal considerations. The review examines provisions in current codes and standards, which specify minimum requirements for solar reflectance, thermal emittance, and solar reflectance index (SRI) values. These criteria often vary based on factors like roof slope, climate zone, and building type. Different cool roof materials are explored, including reflective paints and coatings that can be applied to existing roofs as cost-effective solutions. Several studies have demonstrated the energy performance benefits of cool roofs, showing significant reductions in cooling loads, indoor air temperatures, peak cooling demand, and overall cooling energy consumption compared to traditional roofs. However, hygrothermal performance must be evaluated, especially in cold climates, to optimize insulation levels and avoid moisture accumulation risks, as reduced heat absorption can alter moisture migration patterns within the building envelope. While cool roofs provide substantial energy savings in hot climates, further research is needed to validate modeling approaches against real-world studies, investigate the impact of seasonality and green spaces on cool roof efficacy and urban heat island mitigation, and explore energy-saving potential, moisture control, and condensation risks in cold and humid environments.

Climate change mitigation and adaptation in Spanish office stock through cool roofs

Climate change, its causes and effects have become a topical issue in the world today. Universal increases in temperature and sea levels, are evidences of this global phenomenon. In the field of architecture, climate-resilient and climate-responsive domestic buildings are being developed to adapt to changing climates, while being socio-economically suited to their geographical contexts. Much research in this area has been carried out in developing tropical countries such as Nigeria. Most studies concentrate on passive optimisation of building envelopes to promote thermal comfort and reduce reliance on active mechanical controls which worsen climate change. Thus, it may be said that thermal comfort has been established as a link between climate change and housing. In south-west Nigeria, where an abundance of tropical rainforests engenders a hot-humid climate, achieving thermal comfort in buildings is a major need. Studies show that of all house envelopes in south-west Nigeria through time, two housing styles namely the pre-colonial traditional and colonial modernist styles best demonstrate effective use of climatic design principles in achieving thermally comfortable interiors. However, due to cultural changes over time, a third style of housing which is referred to as contemporary south-west Nigerian housing, is currently the most preferred form of housing. South-west Nigerian contemporary house shows considerable influence from the International Style. This housing style features envelopes with minimum use of climatic design principles and maximum reliance on mechanical cooling devices such as air conditioners, in providing indoor thermal comfort. These mechanical devices, however, are known for aggravating south-west Nigerian climate change. To reduce the aggravation of climate change caused by south-west Nigerian contemporary housing envelopes, this study proposes a free-running contemporary house envelope which promotes thermal comfort in present and future south-west Nigerian climates. As such, the study extensively reviewed climatic design principles and thermal standards applicable in tropical climates as well as material on chronological south-west Nigerian housing developments. The study modelled and evaluated the thermal performance of a free-running, base-case contemporary house envelope in present and future climates, using DesignBuilder’s dynamic thermal simulations. South-west Nigerian present and future climatic data used in the simulations, were generated by the climatic data source and calculation software Meteonorm. South-west Nigerian present climatic data was validated by readings from data loggers placed in a real-live contemporary base-case house in the region. The results of this study’s simulations confirmed that the free-running, base-case contemporary house envelope does not promote thermal comfort in present climates, hence the current supplementary mechanical thermal controls. Additionally, the results showed that the contemporary envelope would not promote thermal comfort in future south-west Nigerian climates. As such, this study then proceeded to optimise the contemporary house envelope to improve its thermal performance in south-west Nigerian present and future climates. This optimisation involved adjusting the envelope specifications under the following parameters: headroom height, external wall (thermal mass, insulation and thickness), internal ground floor (thermal mass, insulation and thickness), roof (thermal mass, pitch, structure, overhang length, covering), external window (size, type, glazing thickness and type), azimuth angle (building orientation) and ground floor elevation distance. These parameters were based on Szokolay’s (2014) climatic design principles. The results showed that optimising the south-west Nigerian contemporary envelope under those parameters improved its thermal performance, enabling it to provide indoor thermal comfort in present and future south-west Nigerian climates. Accordingly, this study’s main contribution is the proposal of a free-running climate-responsive south-west Nigerian contemporary house envelope which promotes indoor thermal comfort in present and future climates. It recommends that: 1. without any need for mechanical cooling devices, the specifications of this optimised contemporary house envelope can be used to reform housing design policies in south-west Nigeria, and 2. using the optimised envelope would reduce reliance on mechanical thermal controls and therefore mitigate climate change in the region.

Cool roofs are beneficial for most buildings almost everywhere in the world. They are described as an inexpensive method in order to improve the comfort level in buildings in mild and hot climate and to save energy. However, their cost-effectiveness can vary significantly, depending on climate and local factors. Therefore, the use of simulations with local conditions can provide the clarity required in order to deploy cool roofs in a particular location. This paper presents an estimation of the potential energy savings resulting from the use of cool roofs for different climatological conditions within South Africa. Several cities have been selected across South Africa and climatological data have been obtained from the NASA Atmospheric Science Data Center. The DOE Cool Roof Calculator was adapted for the estimation of cooling and heating saving corresponding to specific type of roofs and climatological variables within these selected cities. The preliminary results strongly suggest that cool roofs yield positive potential savings in residences using electrical heating for most of the cities studied.

Utilizing a cool roof is an efficient way to reduce the cooling energy use of a building. Cool roofs, however, may increase heating energy use in winter. In cold climates, during winter the sun angle is low, days are short, the sky is cloudy, and most the heating occurs early in the morning or in the evening hours when the solar intensity is low. In addition, the roof may be covered with snow for most of the heating season. All these lead to a lower (than what is commonly thought) winter time heating penalties for cool roofs. We used DOE-2.1E to simulate energy consumption in an office building in four cold climate cities in North America: Anchorage (AK), Milwaukee (WI), Montreal (QC), and Toronto (ON). The effect of the sun angle, clouds, daytime duration, and heating schedules can be modelled with existing capabilities of DOE-2. Snow on the roof provides an additional layer of insulation and increases the solar reflectance of the roof. To simulate the effect of snow, we defined a DOE-2 function consisting of U-value and absorptivity of the roof on a daily basis to simulate four different types of snow on the roof. We used an average of six years meteorological data from National Oceanic and Atmospheric Administration and Environment Canada to estimate the snow thickness on the roof. Results show that the heating penalties of cool roof are significantly lower (than what is commonly thought) considering snow on the roof. The annual heating energy consumption of the building with dark and cool roof without considering the snow are 85 and 88 GJ/100 m2, respectively (3 GJ/100 m2 penalty for cool roof) in Anchorage, whereas, the annual heating energy for the dark and cool roof considering the effect of late-winter packed snow are 83 and 84 GJ/100 m2, respectively (1 GJ/100 m2 penalty for the cool roof). For a typical office building with electricity as cooling fuel and natural gas as heating fuel, cool roofs save 0.08 $ m–2 in Montreal and in Toronto the saving for cool roof is 0.04 $ m–2 (not accounting for the effect of peak demand savings and potential downsizing of the heating, ventilation, and air conditioning systems).

Human health can be negatively impacted by hot or cold weather, which often exacerbates respiratory or cardiovascular conditions and increases the risk of mortality. Urban populations are at particular increased risk of effects from heat due to the Urban Heat Island (UHI) effect (higher urban temperatures compared with rural ones). This has led to extensive investigation of the summertime UHI, its impacts on health, and also the consideration of interventions such as reflective ‘cool’ roofs to help reduce summertime overheating effects. However, interventions aimed at limiting summer heat are rarely evaluated for their effects in wintertime, and thus their overall annual net impact on temperature-related health effects are poorly understood.In this study we use a regional weather model to simulate the winter 2009/10 period for an urbanized region of the UK (Birmingham and the West Midlands), and use a health impact assessment to estimate the impact of reflective ‘cool’ roofs (an intervention usually aimed at reducing the UHI in summer) on cold-related mortality in winter. Cool roofs have been shown to be effective at reducing maximum temperatures during summertime. In contrast to the summer, we find that cool roofs have a minimal effect on ambient air temperatures in winter. Although the UHI in summertime can increase heat-related mortality, the wintertime UHI can have benefits to health, through avoided cold-related mortality. Our results highlight the potential annual net health benefits of implementing cool roofs to reduce temperature-related mortality in summer, without reducing the protective UHI effect in winter.Further, we suggest that benefits of cool roofs may increase in future, with a doubling of the number of heat-related deaths avoided by the 2080s (RCP8.5) compared to summer 2006, and with insignificant changes in the impact of cool-roofs on cold-related mortality. These results further support reflective ‘cool’ roof implementation strategies as effective interventions to protect health, both today and in future.

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.

Effect of cool roofs on commercial buildings energy use in cold climates

Quantifying the direct benefits of cool roofs in an urban setting: Reduced cooling energy use and lowered greenhouse gas emissions

To generate projections of wave climate associated to tropical cyclones is a challenge due to their short historical record of events, their low occurrence, and the poor wind field resolution in General Circulation Models. Hence, synthetic tropical cyclones provide an alternative to overcome such limitations, improving robust statistics under both present and future climates. We use synthetic events to characterize present and future wave climate associated with tropical cyclones in the Gulf of Mexico. The NCEP/NCAR atmospheric reanalysis and the Coupled Model Intercomparison Project Phase 5 models NOAA/GFDL CM3 and UK Met Office HADGEM2-ES, were used to derive present and future wave climate under RCPs 4.5 and 8.5 scenarios. The GFDL model shows less bias in the present climate with respect to NCEP/NCAR results. Furthermore, the numerical results suggest an increase in wave activity for the future climate in the Caribbean Sea and Northern Gulf of Mexico, whereas some areas are expected to decrease the wave energy, as the stretch of the Gulf of Mexico between Yucatan and Southern Texas. The results have practical implications on the design of offshore structures. The 100-year design wave based on the present climate might result in under/over design of structures, owing to the lifespan of a structure that is within the future wave climate period.

95/04062 Opportunities for the solar design of low rise suburban office buildings

William Solecki,1,a Cynthia Rosenzweig,2,a Reginald Blake,3,a Alex de Sherbinin,4 Tom Matte,5 Fred Moshary,6 Bernice Rosenzweig,7 Mark Arend,6 Stuart Gaffin,8 Elie Bou-Zeid,9 Keith Rule,10 Geraldine Sweeny,11 and Wendy Dessy11 1City University of New York, CUNY Institute for Sustainable Cities, New York, NY. 2Climate Impacts Group, NASA Goddard Institute for Space Studies, Center for Climate Systems Research, Columbia University Earth Institute, New York, NY. 3Physics Department, New York City College of Technology, CUNY, Brooklyn, NY; Climate Impacts Group, NASA Goddard Institute for Space Studies. 4 Center for International Earth Science Information Network (CIESIN), Columbia University, Palisades, NY. 5New York City Department of Health and Mental Hygiene, New York, NY. 6NOAA CREST, City College of New York, CUNY, New York, NY. 7CUNY Environmental Crossroads, City College of New York, CUNY, New York, NY. 8Center for Climate Systems Research, Columbia University Earth Institute, New York, NY. 9Department of Civil & Environmental Engineering, Princeton University, Princeton, NJ. 10Princeton Plasma Physics Laboratory, Princeton, NJ. 11New York City Mayor’s Office of Operation, New York, NY

Potential energy savings from cool roofs in Spain and Andalusia

Tropospheric ozone (O3) can damage vegetation, affecting productivity and quality of the crops. Vines, in particular, have an intermediate sensitivity to ozone. Moreover, an increase of ozone levels is foreseen under climate change scenarios. The Douro Demarcated Region is one of the most productive wine areas in Portugal; thus studying the ozone deposition over this region and assessing its potential effects is a nowadays concern. This work aims to evaluate the risk of Douro vineyards exposure to ozone in present and future climates. The chemical transport model CHIMERE, with a spatial resolution of 1 km2, fed by meteorological data from the WRF model, was applied for the years 2003–2005 (present climate), for 2049 and 2064 (mid-term future) and for 2096 and 2097 (long-term future). The assessment of the potential damage in terms of productivity and quality was done through the analysis of ozone deposition and the application of concentration-response functions. The exposure indicator AOT40 (accumulated concentration of ozone above 40 ppb) for the period established in the Air Quality Framework Directive 2008/50/CE was also estimated. The model results show, for present and future climate, that the AOT40 levels in the entire Douro region are above the target value for the protection of vegetation. The results of the exposure-response functions suggest that the tropospheric ozone levels in the future, in the region, would influence the quality and productivity of the wine.

Building envelopes play a pivotal role in influencing building energy consumption. Among its components, the roof, as a critical element, directly absorbs solar radiation and serves as a primary medium for external heat exchange. Its thermal performance significantly impacts the overall energy consumption of buildings. This study focuses on cool roofs as the research subject to investigate their thermal performance and its effects on building energy consumption. Drawing on the principles of life cycle assessment (LCA), a novel concept of environmental payback period is introduced. By comparing cool roofs with photovoltaic roofs, this research employs energy consumption simulation and life cycle assessment to evaluate their performance across three dimensions: economic, energy, and environmental impacts. A comprehensive 3-E (Economic, Energy, Environmental) static payback period theoretical framework based on LCA is established. Within this framework, the concepts of economic static payback period, energy static payback period, and environmental static payback period are explicitly defined, and corresponding calculation formulas are provided. A case study in Nanjing is conducted to validate the proposed framework. The results indicate that the economic payback periods for cool roofs and photovoltaic roofs are 1.75 years and 10.90 years, respectively; the energy payback periods are 13.6 years and 43.7 years, respectively; and the environmental payback periods are 2.2 years and 7.6 years, respectively. In terms of energy savings, photovoltaic roofs outperform cool roofs significantly, with an annual energy saving of 139 kWh/m2 for photovoltaic roofs compared to 6.5 kWh/m2 for cool roofs. However, cool roofs demonstrate clear advantages in the comparison of payback periods.