Cool Roofs: Reversing the Urban Heat Island Effect, Without Turning on the AC
Contributor: Ava Bertolotti
Cezary Wojtkowski/Getty Images
Cool roofs and pavements are made of or paved with light-colored or high-reflectance materials to reflect more sunlight than a standard dark-colored roof, asphalt or concrete pavement. On average, a flat roof receives about 1,000 watts of sunlight per meter in the midday sun, and absorbs much of that energy as heat; conversely, a white roof reflects approximately 85% of the sunlight hitting it. This translates to nearly 150°F dark roofs and over 50°F cooler light roofs.
When applied to low, sloped roofs,
Single-ply membranes, moderately (to prevent glare) reflective sheets rolled onto dark roofs
Built-up, made cool by using reflective mineral granules instead of asphalt or applying a light coating (painting the roof white)
Hybrid (modified bitumen sheet membranes on a built-up roof)
Spray polyurethane foam roofs (susceptible to mechanical, moisture, and UV damage, so must have a protective coating)
Steep sloping roofs,
Cool asphalt shingle roofs with reflective granules embedded
Glazed tile roofs
Green roofs, which provide the added benefits of insulation and stormwater drainage; they can consist of basic plant cover or dense rooftop gardens.
Cool roofs are also more resilient: an upgrade can add years to the life of a roof, as hot temperatures are especially damaging to roofs with lower thermal emittance due to their dark coloration. Although some research points to moisture collecting more readily on cool roofs (evaporation is more efficient on hotter surfaces) and buildings with cool roofs requiring more heating in the winter, these drawbacks were not shown to offset the gains in roof service life and emissions reductions from lower air conditioning usage, respectively. With smart roof design and protective coatings to limit moisture accumulation and well-sealed insulation, the benefits of installing a cool roof would far outweigh any additional costs in the roof upgrade trade-off.
Urban heat and public health
As temperatures increase, so do heat-related mortalities as a function of ambient temperature.
If installed throughout an urban area, cool roofs have the potential to mitigate the urban heat island effect by increasing the city’s albedo, or broad surface reflectivity. Ambient heat islands can drive peak electricity demand up as summers get warmer and more people turn to air conditioning to keep the heat at bay; for people who do not have air conditioning installed, the heat island effect can be deadly.
Atmospheric heat islands can exacerbate the public health consequences of pollution by driving up photochemical reaction rates and speeding photochemically-generated ozone production, causing dense smog to form over cities. When moderate albedo modification to Sacramento, Houston, and Chicago during the 2011 heat wave was modeled by the Concordia University Heat Island Group in 2018, increasing the surface albedo of roofs (.20 to .65), walls (.20 to .60) and pavements (.20 to .45) decreased ambient air temperature by 2.3°C, particulate matter, ozone, and nitrogen dioxide concentrations by 2.7 µg/m3, 6.3 ppb, and 1 ppb respectively in urban areas.
Saving electricity and cutting emissions
Per the Department of Energy, about 6% of all electricity produced in the United States goes towards cooling buildings, at a monetary cost of around 29 billion annually and a high cost to the environment; approximately 117 million metric tons of carbon dioxide are released to power air conditioners per year, projected to increase with air conditioning demand. The climate change-exacerbated and exacerbating “cold crunch” – a meteoric rise in air conditioning use, with only incremental improvements in efficiency – will result in increased greenhouse gas emissions from electricity and refrigerant emissions, potent hydrofluorocarbons released during vapor compression.
Cool roofs have the potential to reduce air conditioning usage on two fronts, due to their immediate cooling effects on individual buildings and collective mitigation of urban heat islands. While reducing energy bills is always a plus, a lowered peak electricity demand can also improve grid reliability and prevent power outages.