White roofs: New solution for global warming

White roofs: New solution for global warming


The roof atop a store, shows the white covering, along with solar panels, now being touted as a low-tech solution to check global warming. NYT Photo

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 Waldrep, a regional manager for a national company. All that has changed. “Now we come home on days when it’s over 100 degrees outside, and the house is at 80 degrees,” Waldrep said.

Way to cool the planet

Their solution was a new roof: a shiny plasticised 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 per cent 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 per cent more than its dark counterpart, depending on the materials used, while slashing electricity bills.

Energy Secretary Steven Chu, a Nobel laureate in physics, has proselytised for cool roofs at home and abroad. “Make it white,” he advised a television audience recently.

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 per cent 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. Some roofing specialists and architects argue that supporters fail to account for climate differences or the complexities of roof construction.

In cooler climates, they say, reflective roofs can mean higher heating bills.

Scientists acknowledge that the extra heating costs may outweigh air-conditioning savings in cities like Detroit or Minneapolis. But for most types of construction, they say, 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 colours. (An asphalt roof in New York can rise to 180 degrees on a hot summer day.) Lighter colours, 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.

Long, humble history

Unlike high-technology solutions to reducing energy use, like light-emitting diodes in lamp fixtures, white roofs have a long and humble history. Houses in hot climates have been whitewashed for centuries. Before the advent of central air-conditioning in the mid-20th century, white and cream-coloured houses with reflective tin roofs were the norm in South Florida, for example. Then central air-conditioning arrived, along with dark roofs whose basic ingredients were often asphalt, tar and bitumen, or asphalt-based shingles.

These materials absorb as much as 90 per cent of the sun’s heat energy, often useful in New England, but less so in Texas. By contrast, a white roof can absorb as little as 10 or 15 per cent.

Light-reflection experiment

In an experiment, the National Laboratory in Oak Ridge, Tenn., had two different kinds of terra-cotta-coloured cement tiles installed on four new homes at the Fort Irwin Army base in California. One kind was covered with a special paint and reflected 45 per cent of the sun’s rays, nearly twice as much as the other kind.

The two homes with roofs of highly reflective paint used 35 per cent less electricity last summer than the two with less reflective paint. Still, William Miller of the Oak Ridge laboratory, who organised the experiment, says he distrusts the margin of difference; he wants to figure out whether some of it resulted from different family habits. Hashem Akbari, Rosenfeld’s colleague at the Lawrence Berkeley laboratory, says he is unsure how long it will take cool roofs to truly catch on. But he points out that most roofs, whether tile or asphalt-shingle, have a life span of 20 to 25 years.

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