What is Solar Reflectivity?
Source: Cool Roof Rating Council
Solar radiation can either be reflected or absorbed when it hits the earth. Solar radiation that is reflected back into space does not contribute to global warming via radiative forcing. Therefore, the surface property of “reflectivity” presents an important opportunity to reduce the amount of solar energy heating up the earth. Materials with higher reflectivity, or higher albedo, reflect more solar radiation back into space, slowing global temperature rise.
The reflective characteristics of a surface are often represented with two related measurements. The first, Solar Reflectance (SR), is a value [0-1] which describes what fraction of solar radiation is reflected by the surface (i.e., a cool roof might have a SR of >0.80, where 80% of the energy from sunlight is reflected off the roof and 20% is absorbed). This is also called albedo and is usually what people mean when they use the term reflectance.
Another metric for describing the reflective characteristics of a surface is the Solar Reflectance Index (SRI). SRI is a calculated score that describes the amount of solar energy returned to the atmosphere by a surface, including solar radiation that is absorbed and then thermally re-emitted (as opposed to reflected). SRI is a whole number, where 0 describes a standard black material and 100 describes a standard white material. SRI also takes into account environmental factors such as air and sky temperature, and wind speed, as well as thermal emissivity, which describes a material’s ability to radiate thermal energy. SRI is specifically useful in assessing how a surface will contribute to the heating/cooling needs of a building. High SRI roofs can reduce the cooling load of a building by reflecting light and radiating heat away from the roof. Generally a high reflectance/albedo correlates strongly with a high SRI.
For more information, see SSC partner Cool Roof Rating Council’s “What is the Solar Reflectance Index?” resource.