Cool Roof

A cool roof is a roof surface designed to stay cooler in sunlight than a conventional roof. It does this by combining high solar reflectance (reflecting much of the incoming sunlight) with high thermal emittance (efficiently radiating absorbed heat away). Cool roofs can be white, light-coloured, or made with special "cool colour" pigments that reflect invisible near-infrared light while looking darker.

How much cooler

Lawrence Berkeley National Laboratory's Heat Island Group reports that on a typical summer afternoon:

• a clean white roof reflecting 80 percent of sunlight stays about 31 °C (55 °F) cooler than a grey roof reflecting 20 percent;
• a cool-coloured roof reflecting 35 percent stays about 12 °C (22 °F) cooler than a conventional roof reflecting 10 percent.

A study of New York City roofs coated white through the NYC CoolRoofs program found an average surface temperature reduction of about 24 °C (43 °F) compared with black roofs.

Benefits

• Lower cooling energy and peak demand. The US EPA reports that in air-conditioned homes, solar reflectance can cut peak cooling demand by 11-27 percent.
• Cooler indoor spaces. In non-air-conditioned homes, cool roofs can lower maximum indoor temperatures by about 1.2-3.3 °C (2.2-5.9 °F).
• Reduced urban heat. Dark roofs are a major contributor to urban heat islands. A UK study cited by the EPA estimated that citywide cool roofs could offset 18 percent of heat-related deaths linked to the heat island.
• Possible longer roof life, because lower temperatures slow heat aging of some materials.

Trade-offs

In cold climates, cool roofs can slightly increase winter heating energy. LBNL research found this penalty is usually small compared with summer savings across most of the United States, and long-lasting snow cover reduces it further. Dirt and biological growth lower reflectance over time, so low-slope cool surfaces may need cleaning. Products with infrared-reflective pigments tend to cost more.

Products

Low-slope options include white or light single-ply membranes, reflective coatings, cap sheets with reflective granules and light-coloured ballast. Steep-slope options include light or cool-coloured shingles, tile, coated metal and some polymer products.

Policy

Cool roof requirements appear in building codes and green building programs in several US states and cities, according to Cool Roof Rating Council data compiled by the EPA, as well as in programs elsewhere. ENERGY STAR in the United States has required low-slope products to have an initial solar reflectance of at least 0.65 and a weathered value of at least 0.50.