The Problem: Most American homes sport black roofs -- an aesthetic preference that's costly in terms of both comfort and energy. A standard black roof absorbs 70 to 80 percent of the solar radiation that hits it. In hot climates like Florida and Texas, rooftops can bake to nearly 200 degrees Fahrenheit. Some of that heat is conducted into the building, only to get sucked back out by an air conditioner. And air-conditioning, during peak summer usage, typically guzzles more than half of all household electricity used in the United States. To make matters worse, dark roofs also warm the air around them, creating a "heat island effect" that can further increase the demand for air-conditioning.
The Solution: Hashem Akbari's Heat Island Group at Lawrence Berkeley National Laboratory worked with roofing material manufacturers to create coatings and shingles that use light-colored materials with higher reflectivity, such as titanium dioxide. The new materials they developed can reduce a home's cooling costs by 20 percent. After studying 11 major cities, Akbari's group determined that cool roofs in commercial and residential buildings could cut air-conditioning costs by $750 million nationwide.
Several states, including California and Florida, have revised their building codes to encourage or even require cool roofs in new construction or in the reroofing of commercial buildings. Still, there's a lingering image problem: Although cool roofs are widely available in all sorts of light colors and materials, residential builders haven't shown much interest. So Akbari's team developed a technology to create pigments that appear dark but, due to the size and shape of their molecules, still reflect 50 percent of the sun's near-infrared energy (invisible thermal radiation). Manufacturers have created metallic and clay-tile roofs using the new darker pigments; affordable shingles are still a few years away. Lower electricity bills will easily make up for the cost of switching over, says Akbari. And time is on his side: "Almost all roofs have to be changed sooner or later."