Energy Secretary Steven Chu says painting all the nation's black roofs a heat-reflecting white could cut global warming.

But white isn't that popular a color for rooftops. And what about wintertime, when black roofs help cut home heating bills by absorbing the sun's heat?

A team of graduate students at the Massachusetts Institute of Technology think they might have an answer – the Thermeleon roof tile, which turns white when it's hot and black when it's cold.

Named to rhyme with chameleon, the color-changing roof tiles won first place in MIT's Making and Designing Materials Engineering Contest on Wednesday, MIT's news office reports.

Now the team behind the tiles is seeking to test their durability and find opportunities to bring the technology to market, Nick Orf, MIT post-doctoral student and Thermeleon team member, said Friday.

The tiles use commercial polymers and solvents encapsulated between a clear plastic top layer and a black backing layer, Orf said. When cold, the polymer remains dissolved in the solvent, letting the black backing show through. When it gets hot, the polymer condenses into a white, heat-reflecting surface.

"When light hits this rough surface, it causes a lot of scattering, and that's what you see – white light reflected back," Orf explained. That white surface can reflect 80 percent of the sunlight falling on them, which could cut cooling costs by 20 percent compared to a dark roof.

The Thermeleon tiles are made with "pretty cheap polymers," commonly used in commercial products like hairspray, Orf added.

"The cost of our materials came out to be less than what common roof asphalt shingles sell for," he said.

Now, "we have to start looking into the lifetime of the materials under different environmental conditions," he said. Durability issues will play a big role in whether the Thermeleon technology can be considered for commercial-scale production.

"Cool roofing" materials have been available since the 1990s, though they don't change color. The Environmental Protection Agency, which certifies roofing materials under its Energy Star program, reports that cool roofing materials made up about a quarter of the commercial roofing market and one-tenth of the residential market in 2006 (see Building a Cool World, With New Roofs).

Those cool roof materials tend to be 5 percent to 20 percent more expensive than conventional roofing products, the EPA's cool roofs Web site notes. But they can cut building air conditioning needs by up to 15 percent, according to the Cool Roofing Rating Council industry group.

Orf said that he's seen other research projects looking into color-changing roofing materials, but that those have concentrated on more expensive color-changing dyes.

The Thermeleon website notes that the team is welcoming interest from potential investors or licensing partners, but "We're just right now taking a step back and seeing if there's any interest, and continuing this work on our own," Orf said. "It's too soon to predict what will happen next."

One of the new projects the team is planning to work on is a microencapsulated version of its polymer-solvent mixture that can be painted or sprayed onto existing roofs, he said.

California is one state that has cool roofing regulations in place. Beyond savings on air conditioning bills, there are environmental benefits to take into account, according to a December study by Lawrence Berkeley National Laboratory researchers.

The study found that heat-reflecting roofs, combined with lighter-colored road surfaces, could reduce the "heat island" effect that sees heavily developed urban areas heat up by as much as an additional 8 degrees Fahrenheit as their dark roofs and roadways absorb heat by day and release it into the atmosphere at night.

The wide-scale adoption of heat-reflecting roofs and roadways could lead to a global warming reduction equivalent to cutting 44 billion metric tons of greenhouse gas emissions, the study found.

But given that leading climate scientists are warning that a whole host of immense "geoengineering" projects might be necessary to combat global warming, perhaps a massive project to make roofs and roads more heat-reflective could gain traction (see Geoengineering May Be Mandatory, Royal Society Says).

On a smaller scale, a 1,000-square-foot roof that reflected 60 percent of the sun's heat (compared to conventional roof's 10 percent to 20 percent reflection) would be equivalent to cutting 10 metric tons of emissions, the study found.

Photo via MIT News Office.