Properties
When the sunlight strikes a rooftop, most of it gets reflected back into the sky but some of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect more sunlight and absorb less heat than traditional dark-colored roofs. There are two properties that are used to measure the effects of cool roofs.
- Solar reflectance, also known as albedo, is the ability to reflect sunlight. It is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation, and a value of 1 represents total reflectivity.
- Thermal emittance is the ability to emit absorbed heat. It is also expressed either as a decimal fraction between 0 and 1, or a percentage.
Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI measures the roof's ability to reject solar heat, defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100.
Read more about this topic: Cool Roof
Famous quotes containing the word properties:
“The reason why men enter into society, is the preservation of their property; and the end why they choose and authorize a legislative, is, that there may be laws made, and rules set, as guards and fences to the properties of all the members of the society: to limit the power, and moderate the dominion, of every part and member of the society.”
—John Locke (1632–1704)
“A drop of water has the properties of the sea, but cannot exhibit a storm. There is beauty of a concert, as well as of a flute; strength of a host, as well as of a hero.”
—Ralph Waldo Emerson (1803–1882)