White Roofs Best For Slowing Global Warming: Scientists Say Get Rid Of Black Roofs

By Rhonda J. Miller on February 2, 2014 11:24 PM EST

White roofs
A study found that white roofs make the most economic sense when compared to 'green' vegetative or black roofs. In warm climates white roofs are common, like this community pictured in St. Petersburg, Fla. (Photo: Courtesy of Shutterstock / Rhonda J. Miller)

White roofs have long been a tradition in tropical climates to minimize the impact of the sun's heat. In the U.S., white roofs got a big push from former Energy Secretary Steven Chu. Now, as concern about global warming intensifies, scientists at Lawrence Berkeley National Laboratory in California have put an economic comparison in the mix when it comes to considerations about topping off structures.

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In their new report titled "Economic Comparison of White, Green, and Black Flat Roofs in the United States" by Julian Sproul, Benjamin Mandel and Arthur Rosenfeld of Berkeley Lab, in collaboration with Man Pun Wan of Nanyang Technological University in Singapore, white roofs take the top spot in terms of cost over a 50-year span. The study will appear in the March issue of the journal Energy and Buildings.

"White roofs win based on the purely economic factors we included, and black roofs should be phased out," said study co-author Rosenfeld, former commissioner of the California Energy Commission, according to a statement.

Rooftop gardeners don't win when it comes to global warming. Unlike white roofs, green roofs do not offset climate change. White roofs reflect more sunlight back into the atmosphere, so they absorb less sunlight at the earth's surface, offsetting a portion of the warming effect from greenhouse gas emissions.

"Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe," the abstract of the study said. Green roofs with gardens or plantings of any type do have benefits, even though they don't win in the cost-savings category.

"Owners concerned with local environmental benefits should choose green roofs, which offer built-in stormwater management and a 'natural' urban landscape esthetic," the scientists said in the study. Green roofs last twice as long as white or back roofs, but green roofs don't compensate for the cost of their installation, according to the findings.

 "We leave open the possibility that other factors may make green roofs more attractive or more beneficial options in certain scenarios," said Mandel, a graduate student who was one of the researchers on the study. One benefit that can't be measured by cost is the opportunity for urban dwellers to make use of green roof space where they work and live, the scientists acknowledged. As far as black roofs, the researchers said in the study, "We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities." 

The study analyzed 22 commercial flat roof projects in the U.S. The researchers conducted a 50-year life-cycle cost analysis, based on a 20-year service life for white and black roofs and a 40-year service life for green roofs. White roofs cost $9 per square foot less than green roofs over 50 years, according to the cost analysis. And even though green roofs cost $7 per square foot more than black roofs over 50 years, the researchers said the negative impact of black roofs make the dark roof worthy of being phased out.

A roof covered with solar-reflective white paint reflects up to 90 percent of sunlight, in contrast to the 20 percent reflected by a traditional black roof, according to The White Roof Project, a grassroots volunteer group working on roof-whitening projects in New York City.

(Photo courtesy of Shutterstock)

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