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Urban Heat Island

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There are two types of cool roofs: those used on low-slope or flat buildings (primarily commercial) and those used on steep-sloped buildings (primarily residential). Most cool roof applications for low-slope buildings have a smooth, bright white surface to reflect solar radiation, reduce heat transfer to the interior, and reduce summertime air conditioning demand. Most cool roof applications for steep-slope buildings come in various colors and can use special pigments to reflect the sun's energy.

Another alternative to traditional roofing materials is a rooftop garden or "green roof." Installed widely in a city, green roofs contribute to heat island reduction by replacing heat-absorbing surfaces with plants, shrubs, and small trees that cool the air through evapotranspiration (or evaporation of water from leaves). Planted rooftops remain significantly cooler than a rooftop constructed from traditional heat-absorbing materials. Further, green roofs reduce summertime air conditioning demand by lowering heat gain to the building.

Source: US EPA

Index

On a hot, sunny, summer day, traditional roofing materials may reach summertime peak temperatures of up to 190°F (88°C). By comparison, cool roofs only reach peak temperatures of 120°F (49°C).

The term "cool roof" is used to describe roofing material that has high solar reflectance or albedo. These materials reflect a large portion of the sun's energy. Cool roofs also may have a high thermal emittance, and thus release a large percentage of absorbed heat. This keeps the material cooler and helps to reduce the heat island effect.

Urban Heat Island

Cool Roofs