Save Energy with a Cool Roof
A cool roof can be a good energy-saving option. Reflective roofing improves comfort and reduces cooling costs in the Deep South and anywhere the sun shines.
Synopsis: For homeowners in heating climates, a cool roof can be a good energy-saving option. Architect Linda Reeder outlines the reasons to consider a cool roof — largely, to save on cooling costs — and describes the way cool roofs work. Tile, metal, and asphalt all can be good cool-roof materials. Cool roofs aren’t foolproof; they can be prone to mold growth in hot, humid climates. Also, they may cost a little more than a traditional roof, depending on the roofing material you choose.
High-reflectance roofing, often called “cool roofing,” started appearing on homes more than 10 years ago, and its use has climbed steadily ever since. The reasons are simple: Cool roofing can reduce cooling costs up to 15%, it can lower oppressive summertime temperatures in cities, and because it doesn’t get as hot, it often lasts longer than traditional roofing does.
Just because it reflects sun and heat, though, doesn’t mean that it will stand out in typical urban or suburban neighborhoods. In fact, cool roofing can look a lot like traditional roofing and is available in more colors and styles than ever before, often at little or no extra cost compared to traditional roofing products.
What makes a cool roof cool?
The U.S. Department of Energy (DOE) looks at several factors when deciding whether a product meets its definition of cool roofing.
High reflectance: At least 25% of solar energy must be reflected off the surface. Standard asphalt shingles have a reflectance around 10%.
Long-lasting reflectance: Dirt and weathering can decrease solar reflectance over time. Cool roofs should still be reflecting at least 15% of the sun’s energy after three years.
High thermal emissivity: This describes a material’s tendency to release heat rather than store it. The higher a material’s emissivity value, the more heat it releases. In hot, sunny climates, a highly emissive roof is desirable; in cold climates, low-emissivity roofing may help to reduce winter heating loads.
Another way to affect cooling loads and attic temperatures is by venting the underside of the roofing material. This technique usually involves placing metal or tile roofing on a series of furring strips, or battens. The battens provide an airspace between the roofing material and the sheathing, which then allows any heated air to exit the ridge. Architect Peter Pfeiffer in Austin, Texas, routinely specifies the use of Galvalume metal roofing placed on battens to reduce temperatures in attics.
A study sponsored by the DOE’s Building Technology Program and conducted by Oak Ridge National Laboratory confirmed what Pfeiffer and other architects have learned in the field. The 2006 study showed that elevating stone-coated metal-shingle panels on a series of battens reduced the amount of heat penetrating the ceiling by 70% and reduced cooling loads by 30% compared to a conventional asphalt-shingle roof.
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