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Global Bioweather

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Cool Roofs

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 reflectanceTo definition provided on the glossary page, or albedoTo definition provided on the glossary page. These materials reflect a large portion of the sun's energy. Cool roofs also may have a high thermal emittanceTo definition provided on the glossary page, and thus release a large percentage of absorbed heat. This keeps the material cooler and helps to reduce the heat island effect.

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

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