Definitions of key roof terms:
Solar reflectance is the fraction of solar energy that is reflected by a surface, such as a roof, and is expressed as a number between zero and one. The higher the value, the better the roof reflects solar energy and the more it keeps cool. For example, a white reflective coating or membrane may have a reflectance value of 0.8 (i.e., it reflects 80 percent of incident solar energy and absorbs the remaining 20 percent), while asphalt concrete may have a reflectance of 0.1 (it reflects 10 percent while absorbing 90 percent). The solar reflectance of a material is similar to its albedo, which is a true field measurement of a material’s reflectivity in sunlight conditions. A material’s initial solar reflectance often weathers over time to a relatively stable aged solar reflectance.
Emittance (also called thermal emittance) is the amount of absorbed heat that is radiated from a roof, expressed
as a number between zero and one. The higher the value, the better the roof radiates heat. Higher emittances
help to keep building interiors cool and to lower energy demands.
Solar reflectance index (SRI) is a measure of a surface’s ability to stay cool in the sun. It is defined so that a standard black surface has an SRI of 0 and a standard white surface has an SRI of 100. SRI is calculated from solar reflectance and thermal emittance.