Solar Reflectance

Solar reflectance is the share of incoming solar energy that a surface reflects rather than absorbs. It is expressed on a scale from 0 to 1 (or 0 to 100 percent). A value of 0.05 means the surface reflects 5 percent of sunlight and absorbs the rest; a value of 0.80 means it reflects 80 percent. The term albedo is often used for the same property, particularly at the scale of cities or the planet.

Why it matters for roofs

Sunlight absorbed by a roof heats it. The US EPA identifies solar reflectance as the most important property for keeping a roof cool. Lawrence Berkeley National Laboratory reports that a white roof reflecting 80 percent of sunlight stays about 31 °C (55 °F) cooler on a summer afternoon than a grey roof reflecting 20 percent.

Visible and near-infrared light

A large share of solar energy arrives as near-infrared light, which the eye cannot see. This means two roofs of the same visible colour can have very different solar reflectance. "Cool colour" pigments reflect near-infrared strongly, allowing darker-looking roofs, such as brown tile or grey metal, to stay cooler than conventional products of the same shade.

Initial and aged values

Reflectance declines as roofs collect dirt, soot, algae and biological growth, and as surfaces weather. Rating programs therefore report both an initial value and a value after several years of outdoor exposure. For example, ENERGY STAR criteria for low-slope roofs have used an initial solar reflectance of at least 0.65 and a weathered reflectance of at least 0.50. The Cool Roof Rating Council in the United States publishes measured initial and aged values for many products.

Typical values

Reflectance varies with product and condition, but in general:

• black membranes and dark shingles are low;
• conventional coloured shingles are low to moderate;
• cool-coloured products are moderate;
• white membranes and coatings are high when new and clean.

Related measures

Solar reflectance is combined with thermal emittance to calculate the Solar Reflectance Index (SRI), which some codes and green building programs use as a single performance number.