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, or albedo. Solar reflectance is measured on a scale of 0 to 1, where the higher the solar reflectance value the "cooler" the roof. High albedo, more reflective surfaces stay much cooler than low albedo, less reflective surfaces. Energy that is not reflected by the roof is potentially absorbed by it; this is where thermal emittance comes into play.

This diagram shows how the radiative properties of a roof affect the solar radiation hitting the surface. Image courtesy of Cool Roof Rating Council

 

Thermal emittance is the relative ability for 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 be transferred to the building below. Thermal emittance is also measured on a scale of 0 to 1, where 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.

Though most roofing materials have a fairly high thermal emittance, in order to accurately determine a roofing product's "coolness," or its ability to shield the building beneath it from heat, both solar reflectance and thermal emittance must be measured. It is possible for a roofing product to have mixed emittance and reflectance values ranging from very high to very low, although products with either a low reflectance or emittance would not typically be considered "cool" roofs. It is important to note that a high emittance value alone will not result in a "cool" roof nor will a high reflectance value alone. The Solar Reflectance Index can be a useful tool for determining the overall thermal properties of a roofing product.

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. In this manner, SRI measures a material's ability to reject solar energy, based on a scale of 0 to 100.

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 prevent excessive heat gain. 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. It is important to note that materials with particularly poor or good radiative properties can have a negative SRI value, or a value that exceeds 100. Like solar reflectance and thermal emittance, a higher SRI value is synonymous with a cooler roof.

Calculating SRI

Lawrence Berkeley National Laboratory (LBNL) hosts an easy-to-use SRI calculator on their website. All that is required is the solar reflectance and thermal emittance values and the tool will calculate the SRI. The calculator is located at coolcolors.lbl.gov.