Low emissivity
Low emissivity (low e or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect and emit radiant energy, but here, the primary concern is a special wavelength interval of radiant energy, namely thermal radiation of materials with temperatures approximately between 40 to 60 degrees Celsius.
Definition
Emissivity is the value given to materials based on the ratio of heat emitted compared to a blackbody, on a scale from zero to one. A blackbody would have an emissivity of 1 and a perfect reflector would have a value of 0.
Reflectivity is inversely related to emissivity and when added together their total should equal 1 for an opaque material. Therefore, if asphalt has a thermal emissivity value of 0.90, its thermal reflectance value would be 0.10. This means that it absorbs and emits 90 percent of radiant thermal energy and reflects only 10 percent. Conversely, a low-e material such as aluminum foil has a thermal emissivity value of 0.03 and a thermal reflectance value of 0.97, meaning it reflects 97 percent of radiant thermal energy and emits only 3 percent. Low-emissivity building materials include window glass manufactured with metal-oxide coatings as well as housewrap materials, reflective thermal insulations and other forms of radiant thermal barriers.
