Duro-Last Cool Zone® Solar Reflectance

The Duro-Last Cool Zone roofing system is an ENERGY STAR® labeled roofing system, with an initial reflectivity of 87.5% and a three-year weathered reflectivity of 86.5%, based on Energy Star testing procedures.  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.  The Cool Zone’s total emittance is measured at 95%.

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. 

Another measurement, called the solar reflectance index (SRI), is beginning to get some attention.  SRI combines reflectivity and emittance to measure a roof’s overall ability to reject solar heat.  The calculation of this index is defined by ASTM E 1980-01 and is based on a mathematical formula that includes values for solar absorptance, solar flux, thermal emissivity, the Stefan Boltzman 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. 

Using the solar reflectance index, a specific value can be calculated for any roofing product.  Materials with the highest SRIs are the coolest choices for roofing.  The SRI for the Duro-Last Cool Zone roofing system is 108. 

Here’s a comparison of other roofing systems as measured by Lawrence Berkeley Labs and the Florida Solar Energy Center:

Although reflectivity is an important measurement, it’s not the only measurement.  Keep that in mind if you’re evaluating roofing systems.