Reflectivity, or albedo, is the percentage of the sun’s energy that is reflected by a surface. Another important measurement of a roof membrane’s performance is emittance. Emittance deals with how effectively a surface releases heat; it is the percentage of absorbed energy that a material can radiate away.
Most authorities have concentrated on reflectivity as the prime measurement of energy performance of cool roofing. However, with even the most reflective materials some energy is absorbed, and if that absorbed energy is not released efficiently it can cause a roof to heat up.
There is another measurement, called the solar reflectance index (SRI), that is beginning to get some attention. SRI combines reflectivity and emittance to measure a roofs overall ability to reject solar heat. The calculation of this index is defined by ASTM E 1980-01 and is based on some rather complicated math that includes values for solar absorptance, solar flux, thermal emissivity, the Stefan Boltzmann constant, and various other coefficients. Standard black (reflectivity 5%, emittance 90%) has an index of 0, and standard white (reflectivity 80%, emittance 90%) has an index of 100. Very hot materials can actually have negative values and very cool materials can have values greater than 100.
When all is said and done, a specific value can be calculated for any roofing product. Materials with the highest SRIs are the coolest choices for roofing.
Here is a sampling of products measured by Lawrence Berkeley Labs:
|Duro-Last Cool Zone||108||87%||95%|
|Atlanta Metal Products, Kynar Snow White||82||67%||85%|
|White Granular Surface Bitumen||28||26%||92%|
|Trocal Roofing Systems, White||104||83%||90%|
|Light Gravel on BUR||37||34%||90%|