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July 26, 2011

Cool Roofs, Cooler Planet

NYC cool roofs

Temperatures soar to 104o F here in downtown DC, but the humidity makes it feel like 120. City-dwellers squint and grimace. Tourists shield their bodies with light-colored fabric, like nomads trekking across the Sahara. We swelter.

Last Thursday, I listened to Dr. Art Rosenfeld of the Lawrence Berkeley National Laboratory explain why cities - and their visitors - roast during the sweatiest days of summer. Basically, dark surfaces like concrete and asphalt absorb a lot of solar radiation. Without the cooling balance of shade and transpiration from trees, asphalt-covered urban areas become giant reservoirs of heat energy. At midday, a city can be up to 10 degrees hotter than the surrounding region, described as the "heat island effect."

The solution, according to Dr. Rosenfeld, starts on the roof. If you paint a flat roof white instead of black, sunlight is reflected back into space rather than retained by the building. That's because white surfaces have a higher albedo, or reflecting power, than black surfaces.

People in the Mediterranean have been taking advantage of this for centuries – just think about the ancient hilltop towns of Greece. In the average city today, rooftops represent 20% of total surface area. If all of these roofs were white, city temperatures could be reduced by 1-1.5 degrees.

For the modern metropolis, whitening rooftops has a number of powerful perks. First, lower surface temperatures make a city safer and more comfortable – inside and outside. In the Chicago heat wave of 1995, 739 people perished. The people most at risk lived on the top floors of buildings with black roofs.

Second, cooler roofs reduce the amount of energy needed for air-conditioning. With a white roof, building owners and occupants can save 15% on their utility bills.

Lastly, cool roofs help mitigate climate change. Besides directly reducing carbon emissions through energy conservation, the albedo of white roofs prevents more heat from being trapped by the escalating greenhouse effect and warming our planet.

Dr. Rosenfeld's research suggests that painting all flat urban rooftops white would offset carbon emissions by 24 billion tons. Seriously, that's the equivalent of taking 300 million cars off the road every year for the next 20 years! For an added carbon smackdown, cool roofs can be combined with photovoltaics or solar thermal technology.

Take the High Desert Government Center in California, where a sparkling white roof also sports a solar array to provide 70% of the building's power. Or, put your green thumbs to use on a vegetated roof. They don't have quite as much reflecting power as white roofs, but they do reduce the heat island effect and can be a sustainable local food source.

Cool roofs are a low-cost solution with wide-ranging benefits. California lawmakers have taken notice, and their building code now stipulates that "all flat roofs shall be white." In New York City, a volunteer outreach corps (photographer above) has taken to the streets armed with pamphlets and paintbrushes.

Beat the heat, everyone. Join a cool roof campaign in your city today!

-- Bari Greenfeld, intern for the Sierra Club Labor Program

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