Roofing System Ratings

R-Value

• The R-value measures the ability of a material to resist heat flow. Each material used to construct the roof has an R-value. For example, the R-value for fiberglass is 3.90 and extruded polystyrene (XEPS) has an R-value of 5.00, as defined by ANSI. Municipalities set building code standards for the minimum R-value for roof installations. The combination of materials used must be the minimum R-value. Builders are required to use a combination of roofing materials and insulation to meet local R-value standards. Buildings with roofs that exceed building code requirements from 25 to 50 percent are referred to as High R-Value roofs.

Solar Reflectance

• Solar reflectance measures the proportion of solar energy reflection on a scale from 0.0 to 1.0. A highly reflective roof surface can reflect solar energy away from the building interior and the air surrounding the building and back into the atmosphere. A rating of 0.0 means that the roof reflects no solar energy; a rating of 1.0 means that the roof reflects 100 percent of the solar energy. One of the industry standards for SR measuring and rating is ASTM C 1549, E903 and E 1918, authored by the American Standards Testing Measurements organization.

Thermal Emittance

• Thermal Emittance measures the relative proportion of energy that a roof radiates back into the atmosphere. This is solar energy that is absorbed into the roofing surface materials, as opposed to energy that the roof reflects and “bounces back” into the atmosphere as solar reflective energy. Like SR, thermal emittance is measured on a scale from 0.0 to 1.0, ranging from how little (0.0) or how much (up to 1.0) thermal emittance the roof radiates back into the atmosphere.

Solar Reflectance Index

• The Solar Reflectance Index is a standard measurement that combines SR and TE as one value. SRI quantifies how hot a surface gets. The testing and measurements use standard black and standard white-colored surfaces to measure heat solar reflectance. Black surfaces used for setting the standards were assigned a reflectance value of 0.05 and an emittance value of 0.90, and given a relative value of zero. White surfaces were assigned a reflectance value of 0.80 and an emittance of 0.90 to set the standards for the high end at 100. Therefore the higher the roof measures, on an index rating scale from zero to 100, the higher the rating of the roof for its combined Solar Reflectance and Thermal Emittance capabilities.

Cool Roof Rating Council

• The Cool Roof Rating Council is an independent, nonprofit organization that supervises and sets standards for roof reflectance, emittance and SRI. States such as California assign the CRRC to supervise the measuring and labeling of roofing materials. The CRRC leads a program called “Cool Roofs,” which is a nationwide initiative to promote the installation of roofs on homes and buildings that use special reflective materials to improve SR, TE and SRI ratings for roofs.