A cool roof reflects more solar heat than conventional asphalt shingles. Cool roofs save electricity by reducing the solar heat that enters a building, thereby reducing the cost of air conditioning. Cool roofs have become popular in the past ten years due to hotter summers, higher electricity costs, and government and utility promotion efforts.
Rating Cool Roofs with SRI
Roofs account for 50 percent or more of the excess solar heat removed from a building by its air conditioner. A roofing material’s reflectivity and its ability to shed heat to the environment determine its coolness. Scientists at Lawrence Berkeley Laboratory have created a new index for comparing the coolness of roofing materials. They call this rating the solar reflectance index (SRI), which is a number between 0 and 110. Higher numbers are better.
Types of Cool Roof Materials
Most residential cool roofs that metal, clay tile, concrete tile, or synthetic rubber membrane. White is the best color for a cool roof, though not all white roof materials have the same SRI. Standard white asphalt shingles have a SRI of only 20 to 25 when new, and that SRI degrades significantly from dirt deposits. Darker asphalt shingle roofs have SRIs as low as 1. White rubber membranes have the highest SRI of any roofing type at 95 to 105. However builders install membranes only on low-sloping roofs that are out of view.
White tile is a close second to the white membranes with and SRI of around 90. Red clay tile tiles have an SRI of 35. White metal roofs, which are now more popular in new construction, have an SRI of 70 to 82. Bare galvanized steel and aluminum aren’t nearly as cool as white metal. Even though shiny metal reflects solar heat well, once the galvanized steel heats up, it emits heat very slowly back into the environment. This results in SRIs of only 46 for bare galvanized steel and 56 for bare aluminum.
Cool roofs are an effective energy saver for commercial buildings, but the residential market remains resistant to the appearance of cool roofs.
For flat roofs, consider adding insulation beneath the cool roof coating or membrane.
See Your Home Cooling Energy Guide and our course Low-Energy Cooling have extensive information about maintaining comfort and reducing air-conditioning costs.
If you’d like to reduce the cost of cooling your home, consider a cool roof material next time you re-roof. See the Lawrence Berkeley Laboratory web site at for more information on cool roofs.