Friday, April 6, 2012

White roofs catch on as energy cost-cutters

Returning to their ranch-style house in Sacramento after a long summer workday, Jon and Kim Waldrep were routinely met by a wall of heat. “We’d come home in the summer, and the house would be 115 degrees, stifling,” said Mr. Waldrep, a regional manager for a national company.

He or his wife would race to the thermostat and turn on the air-conditioning as their four small children, just picked up from day care, awaited relief. But all that changed last month. “Now we come home on days when it’s over 100 degrees outside, and the house is at 80 degrees,” Mr. Waldrep said. Their solution was a new roof - a shiny plasticized white covering that experts say is not only an energy saver, but also a way to help cool the planet.

Relying on the centuries-old principle that white objects absorb less heat than dark ones, homeowners like the Waldreps are in the vanguard of a movement embracing “cool roofs” as one of the most affordable weapons against climate change. Studies show that white roofs reduce air-conditioning costs by 20% or more in hot, sunny weather. Lower energy consumption also means fewer of the carbon dioxide emissions that contribute to global warming. What is more, a white roof can cost as little as 15 percent more than its dark counterpart, depending on the materials used, while slashing electricity bills.

Art Rosenfeld, a member of the California Energy Commission who has been campaigning for cool roofs since the 1980s, argues that turning all of the world’s roofs “light” over the next 20 years could save the equivalent of 24 billion metric tons in carbon dioxide emissions. “That is what the whole world emitted last year,” Mr. Rosenfeld said. “So, in a sense, it’s like turning off the world for a year.”

This month the Waldreps’ three-bedroom house is consuming 10% less electricity than it did a year ago (the savings would be greater if the family ran its central air during the workday).

From Dubai to New Delhi to Osaka, Japan, reflective roofs have been embraced by local officials seeking to rein in energy costs. In the United States, they have been standard equipment for a decade at new Wal-Mart stores. More than 75% of the chain’s 4,268 outlets in the United States have them.

California, Florida and Georgia have adopted building codes that encourage white roof installations for commercial buildings. Drawing on federal stimulus dollars earmarked for energy-efficiency projects, state energy offices and local utilities often offer financing for cool roofs. The roofs can qualify for tax credits if the roofing materials pass muster with the Environmental Protection Agency’s Energy Star program. Still, the ardor of the cool-roof advocates has prompted a bit of a backlash.

For most types of construction, light roofs yield significant net benefits as far north as New York or Chicago. Although those cities have cold winters, they are heat islands in the summer with hundreds of thousands of square feet of roof surface absorbing energy.

The physics behind cool roofs is simple. Solar energy delivers both light and heat, and the heat from sunlight is readily absorbed by dark colors (an asphalt roof in New York can rise to 180 degrees on a hot summer day). Lighter colors, however, reflect back a sizable fraction of the radiation, helping to keep a building — and, more broadly, the city and Earth — cooler. They also re-emit some of the heat they absorb.


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