As summers keep getting hotter because of climate change, it sets up a vicious cycle: By the end of the century, the world may be using more than 30 times more energy for air conditioning, pushing temperatures up even more. But a new variation on cool roofs could help.
A typical dark roof soaks in sunlight, heating up the building underneath and releasing more heat back into the surrounding neighborhood at night. While cool roofs–made from white, reflective material–aren’t new, they still absorb some heat. A new material is the first to actually stay cooler than ambient air.
“What we set out to do was maximize the solar reflectance to see how far it could be pushed…to see the extent of further improvements that are possible with open roofing technologies,” says Angus Gentle, a researcher at the University of Technology Sydney, who developed the material along with physics professor Geoff Smith.
By adding layers of plastic on top of silver, the researchers were able to create a roof surface that bounces sunlight back into space, leaving it as much nine degrees cooler than a state-of-the-art cool roof. “The coating keeps the roof cool by reflecting almost all of the incoming solar radiation,” Gentle says. “A vast majority of this emitted radiation goes directly into space without being absorbed by the atmosphere.”
Compare that to a standard roof–which absorbs as much as 90% of light–or the best cool roofs, which can only reflect 70% to 85% of sun. “This still equates to a summer heat load of 150-300 watts per square meter of heat being absorbed,” says Gentle. In cities, roofs are a major driver of the so-called urban heat island effect, keeping urban neighborhoods several degrees warmer than less developed areas nearby.
Plastered across a neighborhood, the new roofs could help keep local temperatures down, while also directly keeping individual building more comfortable. And if fewer people have to crank up the AC in a city, that helps reduce the chance that the power company will have to flip on an extra power plant–often running on coal–to meet peak demand on the hottest days.
In tests, the new roofing material stayed cool even after it was coated in dust and grime from polluted city air.
Though the material is still in the lab, it uses commonly available, affordable materials. “There are various options for lifting current roofing products towards these results,” says Gentle.