Cool roofs have become popular in the past ten years due to hotter summers, higher electricity costs, and government and utility promotion efforts. A cool roof is simply a roof that reflects more solar heat than conventional asphalt shingles. Cool roofs save energy by reducing the solar energy that enters your home, thereby saving on the cost of air conditioning. Roofs account for 50 percent or more of the excess solar heat removed by air-conditioning systems.
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, called the solar reflectance index (SRI), which is a number between 0 and 110. Higher numbers are better.
Most residential cool roofs are metal, clay tile, concrete tile, or synthetic rubber membrane. White is the best color for a cool roof, though not all white roof materials are the same. Standard white asphalt shingles have a SRI of only 20 to 25 when new, and that will degrade significantly over time due to 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, but typically they are only used on low-sloping roofs that are out of view. White tile is a close second to the white membranes with an SRI of around 90. Red clay tiles have an SRI of 35. White metal roofs, which are becoming 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 it heats up it does a poor job of emitting heat back into the air, resulting in SRIs of only 46 for bare galvanized steel and 56 for bare aluminum.
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 website at http://eetd.lbl.gov/CoolRoofs/ for more information.