There are many types of roofing materials with a varying range of reflectance and emittance levels. Asphalt shingles, the most common type of roofing material, are the least efficient at reflecting the sun’s heat energy. The asphalt is composed of asphalt-saturated mats made from organic felts or fiberglass. Roofing granules, one-millimeter sized stones coated with an inorganic silicate material, protect the roof from the sun’s ultraviolet light. The coating contains microscopic pigment particles, similar to those used in paint, to provide color. Asphalt’s low solar reflectance can be attributed to several factors:
- First, there is a limited amount of pigment in the granule coating.
- Second, the roughness of the shingle contributes to multiple scattering of light and thus to increased absorption.
- Third, the black asphalt substrate is not 100% covered by the granules, and reflects only about 5% of the light that strikes it.
Solar Reflectance Index
- Solar Reflectance is the fraction of the solar energy that is reflected by a roof, expressed as a number between zero and one. The higher the value, the better the roof reflects solar energy. For example, white reflective coating or membrane has a reflectance value of 0.85 (reflects 85% of solar energy hitting it and absorbs the remaining 15%), while asphalt has a value of 0.09 (reflects 9%).
- 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.
- Solar Reflectance Index (SRI) indicates the roof’s ability to reject solar heat, and is the combined value of reflectivity and emittance. It is defined so that a standard black is zero (reflectance 0.05, emittance 0.90) and a standard white is 100 (reflectance 0.80, emittance 0.90). Because of the way SRI is defined, very hot materials can have slightly negative SRI values, and very cool materials can have SRI values exceeding 100.