White roofs, aka cool roofs, are a “hot” topic these days as a means to help stem global warming. Most of us know better than to wear black on a hot day, but when it comes to the roofs on our houses, temperature often takes a back seat to aesthetics . Dark roofs mean higher air conditioning bills, and higher carbon dioxide emissions as a result. The good news is you can save money and help protect the planet by lightening the color (and therefore the temperature) of your roof, or by switching to a white roof the next time your house needs a new hat.
Switching to a white roof can actually reduce energy use by about 20 percent in hot, sunny weather, according to the Lawrence Berkeley National Laboratory Heat Island Group in Berkeley, Calif. Hashem Akbari, the Heat Island Group’s lead scientist and a professor of Building, Civil and Environmental Engineering at Concordia University in Montreal, says that transitioning to reflective roofing and pavements in the world’s urban areas would offset the equivalent of emissions from the planet’s 600 million cars for the next 18 years. A 1,000-square-foot cool roof saves air conditioning use that otherwise would emit about half a ton of carbon dioxide per year. (The average total square footage of a home in the United States is more than 2,000.)
In urban areas, white roofs also help lower smog levels by lowering local temperatures, which tend to be higher due to the large proportion of paved surfaces.
Critics have suggested white roofs do more harm than good in colder climates. But research shows that the heating benefits of a dark roof in the winter are negligible because days are shorter, skies are cloudier, the angle of the sun is low, and sometimes roofs are covered in snow. “The amount of heat savings you may lose in the winter would be, at the maximum, 30 percent of the summertime savings,” Akbari says. “If you need cooling in the summer and heating in the winter, no matter where you are, a white roof will most likely save you money.”
“Cool” is the buzzword that encompasses a range of roofing materials that can reduce carbon footprints. And cool roofs come in a variety of light colors.
The best time to install a cool roof is when a new roof is needed or major roof maintenance is in order. According to researchers, there’s little difference between the cost of roof colors.
Not ready for a new roof but still want to cool down? Install a white roof coating instead. There are a number of coatings that will substantially improve an existing roof’s reflectance/emittance. Plus, coatings protect and seal, potentially extending the life of your roof. The installed cost of coating a roof is about $0.50 to $1 per square foot, according to Akbari, which will be paid back by lower energy bills and the extended life of your roof.
A cool roof is measured by two properties: reflectance (the ability of the product to reflect energy away from the roof) and thermal emittance (the roof’s ability to radiate absorbed heat). Ideally, you want your roof to be high in both.
The Heat Island Group has developed a Solar Reflectance Index (SRI) to rate cool roofs. You can browse the Lawrence Berkeley National Laboratory's Cool Roofing Materials Database for information about a product you’re considering. The Heat Island Group says an SRI rating of 30 percent or higher for sloped roofs (70 percent or higher for flat roofs) can reduce utility bills. The Cool Roof Rating Council also offers a product directory; just click on “Rated Products Directory.”
Federal and state rebates are available; check the Database of State Incentives for Renewables & Efficiency for details specific to your project. Also call your utility company to check for local programs. “This will not be the silver bullet that solves the global warming problem,” Akbari says. “But it cools your home, cools your community and helps cool the globe while putting money in your pocket.”
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