Around dawn on July 15, 2003, residents of Phoenix awoke to an astonishing morning low of 96° F. It was brutally palpable evidence of the urban heat island (UHI) effect, which over the last half-century has raised the desert city’s average temperature by more than 5° F, and where summer monthly electric bills can easily top $400 for an average 2,500-square-foot home.
One Cool Roof:
At the new Macallen Condominiums in Boston, the architectural firms Burt Hill and Office dA fought the urban heat island effect – and the lack of local green space – with a sloped green roof, light-colored concrete and gravel, and a 20,000-square-foot outdoor terrace.
Among the prime culprits in warming Phoenix, as other cities, are rooftops drinking in the heat, conveying it into the buildings below, and radiating the leftovers into the atmosphere at night. But the rapidly growing technology of cool roof materials offers a painless way to defuse UHI and save energy. Moreover, a new generation of dark-pigmented coatings overcomes the common objection of glare from white sloping roofs.
The environmental dividends of cool roofs are dramatic. A new study by Hashem Akbari, senior scientist at the Lawrence Berkeley National Laboratory (LBNL), Berkeley, Calif., and his colleague Ronnen Levinson, projects annual savings of 7.69 kilowatt-hours and $1.14 per square meter for a cool roof in Arizona. In a real-world test, the Southwest Energy Efficiency Project monitored a 28,000-square-foot municipal building in Tucson that gained a white elastomeric coating for its existing dark aluminum roof. Average year-round roof temperatures dropped by 17°F and cooling energy needs fell 49 percent, saving $4,000 annually – recouping the $24,993 retrofit cost in just over six years.
Cool roof materials and coatings have two properties, and it’s essential to understand both. The first is solar reflectance or albedo, which is the instantaneous rejection of solar radiation. The second is thermal emittance, the ability of a surface to radiate whatever it does absorb in the infrared spectrum. The faster the radiation, the better.
Not Just White
White or light-colored roofing materials still perform best, but hues such as iron-oxide red and perylene black reflect near-infrared radiation well. Coatings can be applied to many materials, including asphalt shingles, concrete tile, cedar shakes, and painted metal. Some products, such as Cooltile IR Coatings from American Rooftile Coatings, Fullerton, Calif., can improve existing roofs. Even cool asphalt shingles now come in muted tones, like GAF-Elk’s Cool Color Series in “antique slate” and “barkwood.” A good source for new materials is Oakland, Calif.’s nonprofit Cool Roof Rating Council, whose database of more than 1,300 products includes performance data after three years of real-world use.
Cool roofs aren’t right for every location, and computer modeling at LBNL and Oak Ridge National Labs confirms it. In moderate to cold climates, increased winter heating cancels the summer savings. In Boulder, the gain and loss are about equal, and Burlington, Vt., would actually penalize a cool roof.
Akbari, who grew up in Tehran, observes that warm-climate cultures have long deployed cool roofing materials—such as terra cotta tiles in the Mediterranean or white limestone in Bermuda. “This tells me two things,” he says, “that these materials are economical, and that they’re well accepted.”
