When sunlight hits an opaque surface, some of the energy is reflected. The measured fraction of solar energy that is reflected by a roofing material's surface is called solar reflectance (SR), or albedo. Solar reflectance is measured on a scale from 0 to 1. High albedo, more reflective surfaces stay much cooler than low albedo, less reflective surfaces. So the higher the solar reflectance value the "cooler" the roof. Energy that is not reflected by the roof is potentially absorbed by it; this is where thermal emittance comes into play.
Thermal emittance (TE) is the relative ability of the roofing material to re-radiate absorbed heat as invisible infrared light (relative to a black body radiator). This absorbed heat will either be gradually or quickly re-radiated away from the roof; the quicker the better because the longer the heat is trapped at the surface of the roof the more likely it is to penetrate the building below. Thermal emittance is also measured on a scale from 0 to 1, so a roofing material with a higher thermal emittance will re-emit absorbed thermal energy more quickly than a material with a low emittance and will result in a "cooler" roof.
SOLAR REFLECTANCE INDEX (SRI)
Codes, standards and programs that specify cool roofing requirements may also reference an additional calculated value,the Solar Reflectance Index (SRI). SRI allows actual measured solar reflectance and thermal emittance values to be combined into a single value by determining how hot a surface would get relative to standard black and standard white surfaces.
The standard black roofing material has a high emittance value (90 percent) but a low reflectance value (5 percent). This creates a hot roof surface because even though the emittance is high, there isn't enough reflectance to help cool the roof. As such, the standard black roof is given an SRI value of 0.
The standard white roofing material is highly reflective (80 percent) and has the same emittance as the standard black surface (90 percent). Its surface is much cooler and the standard white roof is assigned an SRI value of 100. Like solar reflectance and thermal emittance, a higher SRI value is synonymous with a cooler roof.
With today's paint technology, cool metal roofs do not necessary have to be white in colour. These newly developed paint coatings can reflect high percentages of solar radiation outside the visible spectrum, allowing darker colors to remain much cooler. Click here to learn more about Infrared Reflective Pigments.