Dark materials absorb more heat from the sun--as anyone who has worn a black t-shirt on a sunny day knows. Black surfaces in the sun can become up to 70°F (40°C) hotter than the most reflective white surfaces. If those dark surfaces are roofs, some of the heat collected by the roof is transferred inside. Staying comfortable in under a dark shingle roof often means more air conditioning and higher utility bills.

The Heat Island Group has monitored buildings in Sacramento with lightly colored, more reflective roofs. We found that these buildings used up to 40% less energy for cooling than buildings with darker roofs. The Florida Solar Energy Center performed a similar study, also showing up to 40% cooling energy savings.

Heat Island Group continues to monitor buildings and measure or simulate the effects of increased roof reflectivities for:

different types of buildings,

different climate zones and seasons, and

different roof insulation levels, angles and orientations.
This important research is needed to find the best ways to save energy and money using reflective roofing.

Solar reflectivity is measured according to ASTM E903.

Also, materials specialist Paul Berdahl is developing a new rating system called the solar reflectance index (SRI) to measure how hot materials are in the sun. The extremes of white and black paint (on the graph above) define the solar reflectance index (SRI). Traditional roofing materials have an SRI of between 5% (brown shingles) and 20% (green shingles). White shingles with SRI's around 35% were popular in the 1960s, but they lost favor because they get dirty easily. The current trend is to make white shingles more reflective. Manufacturers have recently developed clean, "self-washing" white shingles with even higher SRIs--up to 62%.

Look for the SRI to start appearing on roofing products! Reroofing with shingles rated SRI 50% or higher will keep a home cooler and reduce energy bills. Reroofing offers a quicker and even less expensive method to cool your home than planting trees, as well as making our buildings and cities cooler and more comfortable.

For more information about cooler roofs, please visit our Cool Roofing Materials Database.

Continue to "Darkness Versus Light: Paving the Way to Coolness"

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