Potential net energy savings from changing roof reflectivity. Savings are measured in dollars. Note that the net savings are the savings of cooling energy use less the penalties of heating energy use.

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. Link to: Roof Heat Transfer

Staying comfortable in under a dark shingle roof often means more air conditioning and higher utility bills. These roofs also heat the air around them, contributing to the heat island effect. Conversely, cool roofs can reduce the heat island effect and save energy. In a study funded by the U.S. EPA, the Heat Island Group carried out a detailed analysis of energy-saving potentials of light-colored roofs in 11 U.S. metropolitan areas. About ten residential and commercial building prototypes in each area were simulated. We considered both the savings in cooling and penalties in heating. We estimated saving potentials of about $175 million per year for the 11 cities. Extrapolated national energy savings were about $750 million per year. Link to: Energy > Ways to Save Energy

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.

The 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. 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. For more detail, link to: Roof Heat Transfer > "Various Materials in Sunlight"

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). Berdahl compiles and measures the solar reflectance and infrared emittance of roofing materials. Reliable summary data and information on heat transfer are posted on our web-based database. Go to: Cool Roofing Material Database We also collaborate with manufacturers to find ways to make roofing materials cooler. Link to: Collaboration with Industry Sample data for some examples of cool roofing materials are available. Link to: Cool Roofing Samples

Manufacturers have recently developed clean, "self-washing" white shingles with even higher SRIs--up to 62%. This is useful because the labor costs of maintaining the high albedo of a roof coating may exceed the cost of conserved energy. Link to: Wash Your Roof

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 an overview of our research on cool roofing materials, link to: "Measurements and Database for Cool Roofing Materials"

To learn more about the instruments we use to characterize roofing samples, link to: Cool Roofs Instrumentation