White cars are popular in sunny climates for good reason: they stay much cooler than darker ones, because white surfaces reflect sunlight instead of absorbing its heat. Likewise, U.S. Energy Secretary Steven Chu advocates using white materials for all roofing, to reduce the need for air-conditioning.

Critics counter that in cooler climes, black roofs are actually more energy efficient, because they capture more solar heat in cold weather. Now a group of students and recent MIT graduates has achieved the best of both worlds: a new kind of roofing tiles that would automatically turn black in cold weather and white in hot weather.

Lab measurements show that in their white state, the tiles reflect about 80 percent of the sunlight falling on them. That means they could save more than 20 percent of present cooling costs, according to recent studies. When they turn black, the tiles reflect only about 30 percent of sunlight, but the heating costs they would save over all-white roofs have yet to be quantified.

The team, called Thermeleon (rhymes with “chameleon”), won the $5,000 first prize in MIT’s annual Making and Designing Materials Engineering Contest in October. Team member Nick Orf, PhD ‘09, says the secret is a commercial polymer in a water solution. The solution is encapsulated between layers of glass and plastic, or between flexible plastic layers; the bottom layer is black.

Below a certain temperature (controllable during manufacturing by varying the composition of the solution), the polymer stays dissolved, and the backing shows through. But when the temperature climbs, the polymer condenses to form droplets small enough to scatter light, making a white surface. The researchers are also working on a microencapsulated solution that could be inexpensively brushed or sprayed onto any surface.

But can such roofing endure harsh winters and hailstorms? Time–and testing–will tell. “It’s got to stand up to very harsh conditions,” Orf says.