White roofs save energy
By Staff and wire reports
© St. Petersburg Times,
That's what a new study commissioned by Florida Power & Light Co. recommends for homeowners.
The four-month study by the Florida Solar Energy Center, a state-funded research lab at the University of Central Florida, found that the whiter and more reflective a house's roof, the lower the electric bill.
"Whatever type of roof you're going to be putting on, picking the lightest color you can is going to save you energy on your monthly bill and reduce the demand for electricity at peak times by a large amount," said Craig Muccio, FPL's research coordinator.
Researchers evaluated six types of roof: dark gray shingle, white shingle, white flat tile, white barrel tile, terra-cotta barrel tile and white galvanized metal.
The top rating went to the white metal roof, which reflected 77 percent of the sun's energy.
Such a roof should save customers who live in a 1,770-square-foot home about $128 a year in cooling costs, compared with a dark gray shingle roof, which was ranked at the bottom.
Researchers were surprised to find that one of the lowest ranked roofs was the red barrel "Spanish" tile so common in Florida, which reflected only 34 percent of the heat associated with sunlight.
Dark-gray shingles reflected only 8 percent of the heat, white shingles 25 percent. White flat tile and white S-shaped cement tile each reflected 66 percent.
White metal did best not only because of its reflecting abilities, but because it cools down quickly once the sun goes down. That "causes the attic temperature to cool down more rapidly than other types as the roof radiates to the night sky," researcher Danny Parker said in an e-mail. Even so, white tile, either barrel or flat, provides "very similar reduction to cooling of that of white metal. I believe these are all good choices," Parker said.
His own home in Cocoa Beach is roofed with white-painted metal.
"Unsurfaced galvanized metal gets very hot -- it's Tennessee Williams' Cat on a Hot Tin Roof,' " Parker explained. "The reason is not the reflectivity of unsurfaced metal, which is fairly high, but is due to a lesser-known surface property: low-infrared emissivity. This is the same property that makes space blankets work. Whatever heat low-emissive surfaces pick up, they do not readily re-emit as radiation, making them very hot." The white-painted metal, by contrast, re-emits its heat as radiation, cooling quickly.
There are some light-colored mottled tiles on the market now (such as Monier's Colonial Amber Sand) "that have high levels of reflectance and would exhibit most of the described advantages while still possessing some visible color," Parker said.
In the future, consumers can look forward to help from the military-industrial complex in selecting "cool" roofs for their home. Drawing on research used to develop paint colors to "hide" battleships from heat-seeking missiles, the Ferro Corp. has created pigments that will soon be available to the roofing industry. Those will allow homeowners to choose roofing materials suited to Florida's hot climate while maintaining the color palettes they like.
The FPL study, which examined six new houses in Fort Myers built by Habitat for Humanity, is the first to examine cooling performance on identical homes during realistic weather conditions, says FPL's Muccio.
Each house was built with a different type of roof but operated identically. Temperature controls on air conditioning thermostats were set at 77 degrees. The houses were studied both occupied and unoccupied.
Metal roofs are more expensive than traditional cement tile, said Gary Waldrep, president of Olen Waldrep & Sons Roofing in Davie.
Waldrep said the average metal roof costs about $5.50 a square foot versus $3.50 for a tile roof, including materials and labor for new construction. Re-roofing costs about $7 a square foot for metal versus $4.25 for tile.
"It's a good investment in the long run, though. It's more energy efficient. The longevity and wind resistance make it pay for itself," said Waldrep, who added that metal roofs hold up well under hurricane conditions.
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