A white roof reflects heat and cools a building; a dark one absorbs it. But until now there hasn't been a material that could do both.
But a new 'smart' roof coating - rather amazingly, made from waste cooking oil from fast food restaurants - can 'read' a thermometer and switch between roles.
When the outdoor temperature crosses a preset point, the roof can change from absorbing heat to reflecting it.
"This is one of the most innovative and practical roofing coating materials developed to date," said Ben Wen, leader of the research project and vice-president of United Environment & Energy.
"This bio-based intelligent roof coating, compared with a traditional cool roof, could reduce both heating and cooling costs as it responds to the external environment. It will help save fuel and electricity and reduce emissions of volatile organic compounds from petroleum-based roofing products. In addition, it will provide a new use for millions of gallons of waste oil after it is used to cook french fries and chicken nuggets."
Tests on the new coated asphalt shingles showed that they could reduce roof temperatures by up to 80 percent in warm weather - and warm it by the same amout when the weather is cold. By changing the coating's composition, Wen and colleagues can tune the substance, so that it changes from reflective to transmitive at a specific temperature.
The coating is produced by processing waste cooking oil into a liquid polymer that hardens into a plastic after application. It apparently won't make your house smell like a burger restaurant, and it can be produced in any shade. It's non-flammable and nontoxic.
The coating can be applied to virtually any type of roof, says Wen, who estimates that it could be ready for commercial use in about three years.