Albedo

Solar Reflectance in Roofing

Albedo, or solar reflectance, is a measure of a material’s ability to reflect sunlight.

Albedo includes the visible, infrared and ultraviolet wavelengths on a scale of 0 to 1. An albedo value of 0.0 indicates that the surface absorbs all solar radiation, and a 1.0 albedo value represents total reflectivity. EPA ENERGY STAR specifies an albedo of 0.65 or higher for low-slope roof applications and 0.25 for sloped roofs.

Most roofing industry experts agree that a cool roof is one the exhibits a combination of high albedo and high emissivity.

California’s Title 24, Georgia Energy Code’s White Roofing Amendment and ASHRAE 90.1 mandates cool roofing. Others like the U.S. Green Building Council’s LEED rating system or Energy Star makes it an optional element of overall building design.

These specifications don’t jibe when it comes to the exact emissivity or reflectivity numbers and creates more confusion to the building owner.

Specification	Emissivity	Reflectance
California Title 24	0.75	0.70
Georgia White Roofing Amendment	0.75	0.75
Energy Star	not specified	0.65

LEED version 2.2, released in October 2005, is the first national specification to use a relatively new measure of reporting a cool roof’s properties. LEED 2.2 sustainable site credit 7.2 states that to receive one point, building owners should use a roof with a Solar Reflective Index (SRI) of 78 over at least 75% of the roof surface for roofs with slopes less than 2:12. The new twist is SRI, a unit developed by scientists at Lawrence Berkeley National Laboratory. SRI incorporates reflectivity and emissivity properties into one, easy to read, standardized measure so that roof buyers won’t have to scratch their heads and try to figure our if a high reflectivity and low emissivity is better or worse than a medium reflectivity and high emissivity.

SRI is calculated with a complex formula spelled out in ASTM E1980 and is a scale of 1 to 100 that is a measure of a roof’s combined thermal properties. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. But some hot roofs can have negative values, and some white thermoplastics and white roof coatings have scored as high as 104 to 100.

SRI as a method for reporting cool roof data will probably take a little while to catch on. The Cool Roof Rating Council, an organization that verifies and labels cool roofing products has begun using the measure, while retaining reflectivity and emissivity measurements.