Solar Reflectance Index (SRI) Definition?

The Department of Energy states the following with respect to Solar Reflectance Index: "The Solar Reflectance Index (SRI) is a measure of the roof's ability to reject solar heat, as shown by a small temperature rise. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. For example, the standard black has a temperature rise of 90 deg. F (50 deg. C) in full sun, and the standard white has a temperature rise of 14.6 deg. F (8.1 deg. C). Once the maximum temperature rise of a given material has been computed, the SRI can be computed by interpolating between the values for white and black."

So, SRI is a gauge from 0-100, with 0 being most likely to absorb and radiate heat (such as a tar roof) and 100 being the most reflective (least likely to get hot) such as a "cool roof" which is a roof painted with white reflective paint.

In LEED, the Sustainable Sites credit category refers to SRI in the requirements for meeting SSc7 "Heat Island Effect - Roof", which describes different SRI levels for roofs of different slopes.

Some other concepts that are related to SRI, and are used commonly in LEED and green building are:

• Heat island: The difference in thermal gradient between developed and undeveloped areas.
• Heat island effect: Absorption of heat by hardscapes such as dark, non-reflective pavement and buildings, and its radiation to surrounding areas.
• Albedo/reflectance: Ability of a surface to reflect sunlight (0 to 1)
• Emissivity: The ratio of the radiation emitted by a surface to the radiation emitted by a blackbody at the same temperature
• Emittance: Ability of a surface to shed thermal radiation (0 to 1)