Quick, answer this question without getting on a ladder: what color is your roof? If you're like 90 percent of Californians, your answer will be "dark." The most common color for roofing materials in the U.S. is black. And that's a problem. Summer sunshine can raise the roof's temperature by a startling amount, and that heats the interior of the building, leading to more energy use for climate control.
Fortunately there's a relatively straightforward fix, and it's called a "Cool Roof."
Here's the problem. Because common roofing materials like asphalt shingles are made of tar, those materials start out being dark. Changing their color requires adding pigment, which increases costs to the roof-owner. When people considering replacing a roof shop for the best value in that replacement, basic economics tends to push them in the direction of dark roofs.
But dark materials have a very low albedo: they absorb sunlight more efficiently than do light or reflective materials, and that absorbed light turns into heat. (A low albedo means less light is reflected, and a high albedo means more light is.) In plain English, dark roofs get really hot on summer days, often reaching temperatures between 160° and 200° F. Unless your attic is supremely well-insulated, some of that heat will penetrate into the building's interior, making your air conditioning work harder to keep you somewhere below 88° or so.
And that means more electricity consumption to keep those air conditioners working: up to 15 percent more, according to one estimate by the Lawrence Berkeley Lab, than if the roof didn't absorb so much energy. It also means that dark roofs contribute greatly to the Urban Heat Island effect:
Meanwhile, a roof that has been slathered with a reflective white roof paint might heat up only a fifth as much compared to ambient air temperature. White reflects more sunlight than black does, which means that that energy doesn't get turned into heat. In fact, some of the light reflecting off light roofs will bounce back out of the Earth's atmosphere, helping ease the burden of global warming.
In 2004, according to the California Energy Commission, the typical California household used about 600 kilowatt hours of power to run air conditioning. (That figure has almost certainly gone up in the decade since.) Using the 2004 figures, that amounts to around 7,800 megawatt-hours used to air-condition California's 13 million households in 2004. Conserving 15 percent of that by making roofs cooler would have made a significant dent in peak power consumption, which in summer in California is mainly driven by AC.
In most American urban areas, up to a quarter of the land area is made up of roofs. Roads and other asphalt surfaces make up as much as another 50 percent. That's a lot of absorption, and it doesn't have to be that way. According to one recent study, replacing dark roofs and pavements with lighter ones worldwide could lower global temperatures by more than a degree Fahrenheit. At least a third of that contribution would come from roofs rather than roads and parking lots, and that estimate doesn't account for lower fossil fuel use due to decreased demand for AC.
For a sense of perspective, that would be cooling equivalent to what we'd achieve by taking every single car on the planet off the road for 50 years.
Cool roofs are not just about painting your roof white, though that's an accessible entry-level solution. In addition to reflecting a lot of light, a truly cool roof will allow the little bit of heat that it creates to radiate back out rather than be absorbed into the building. This means that in addition to considering the albedo of your new roof, you also need to keep its thermal emittance in mind. Different substances will emit different amounts of heat, and the choice of roofing material thus makes a difference.
Pretty straightforward stuff for those of us with degrees in engineering. For anyone else considering a new roof? We may need a little handholding. Fortunately, we're not on our own: organizations and trade groups like the Cool Roof Rating Council are good sources of helpful, accessible information for roof replacers.
Cool roofing materials include a whole lot more than that coat of white paint. The category includes everything from white vinyl sheeting that must be installed with epoxy and heat -- and which can reflect more than 90 percent of the light that hits it -- to light colored gravel for flat roofs, more conventional shingling, metal and bitumen roofing, and wooden shakes. If for some reason you have to go with black shingling -- say, a pesky HOA or a historic building -- you can find products that reflect as much as a quarter of the light that hits them. That's no 95 percent reflectivity, but it''s better than nothing.
You can even go crazy and install a green roof, if your building can take the weight. The San Francisco-based California Academy of Sciences did that when it renovated its old building in Golden Gate Park in 2008: its roof is now a prairie of California native wildflowers, a thick coat of soil providing insulation from the sun. It doesn't reflect as much sunlight as a white roof would, but the plants cool the roof when they water they transpire evaporates.
We called cool roofs a "straightforward fix" to overheated rooftops, and they are. But in a world as complex as this one, there usually aren't completely uncomplicated solutions to our problems. The problem of current roof design increasing our energy use is no exception. One common objection to cool roofs is that energy saved during the summer months must be measured against increases in energy use for heating those same buildings during winter. After all, if you cool the house by reflecting sunlight, that's going to work year-round. Fortunately, rooftop heating doesn't account for much in winter: the sun's lower in the sky, meaning most roofs won't have nearly as intense sunlight hitting them. In really cold climates, those roofs may well be covered in white stuff all winter anyway.
The issue of shiny roofs' effect on the neighbors is more problematic. If your roof reflects sunlight right into your neighbors' windows, you may be saving on cooling bills at their expense. One recent study suggested that by reducing the urban heat island effect, white roofs may actually reduce the amount of water vapor evaporated into the air, reducing cloud formation and increasing global temperatures. That last study has had its critics, and research continues.
One undeniable downside of cool roofs is the carbon footprint of their manufacture and transportation: tearing down a perfectly serviceable non-cool roof to upgrade it might mean a few years of service before the new roof pays off the carbon emitted in its creation.
Of course if your roof needs replacing in the next year or three you're spending that carbon anyway. And it turns out that cool roofs tend to last longer: overheating is a huge contributor to roof decay. Which means your cool roof won't just save on your electric bill: it could keep you from having to set buckets in your living room during the rainy season in 2023.
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