The news came as a surprise even to those well-versed in metal roofing.
Late last year the Florida Solar Energy Center released the results of a real-world study of the heat gains in houses with different metal roofing materials. The experiment monitored indoor cooling energy usage for seven side-by-side homes in Ft. Myers, during the summer of 2000. Each home was virtually identical except for roofing material (one had a sealed attic and roof deck insulation).
The study showed that white S-tile produced the lowest attic heat gain.
But the home with the white metal roof posted the lowest overall cooling cost.
Compared to a dark gray shingle roof, the study reported, "a white, galvanized metal roof should save a customer who lives in an average-sized 1,770 square foot home approximately $128 or 23 percent annually in cooling costs." Flat white tile offered a savings of 17 percent. Terra cotta roofing, the most popular roofing material in Florida, netted a modest $15, or 3 percent savings over dark shingle.
The thrust of the study, of course, was that reflective colors have significant bottom-line benefits even on pitched roofs with attics.
"The difference seems to be metal's higher emissivity," says Danny Parker, one of the study's authors. "During the night, the metal radiated heat to the night sky better, and so fell below ambient temperatures." This effect, he says, would be even greater in more arid regions such as the Southwest, where clouds would seldom interfere with the radiant transfer.
Metal's emissivity is usually a source of concern. Unpainted metal has emissivities as low as 20 percent, even when it has reflectivities in the 60-70 percent range. This means that it dissipates less of the heat it does absorb and sends it into the building instead.
Painted metal, on the other hand, has an emissivity of about 80 percent -- not as high as some surfaces, but high enough to make for a respectably cool roof.
Metal has other
strong suits relative to other roofing materials, especially for steep slopes,
including weatherability and resistance to mold and fungus, which can darken
The chief weakness of cool metal, in the eyes of most homeowners, is simply aesthetics: most prefer dark-colored roofs, energy costs be damned.
The new reflective pigments made by Ferro and being put into the Ultra-Cool coatings by BASF offer a new angle on the problem. These paints manage to reflect high percentages of solar radiation outside the visible spectrum, allowing darker colors to remain much cooler.
Classic Products has used the coating, which they call Hi-R, for its aluminum shingles. Since the Metalcon show last fall, several other manufacturers have picked up the product, including Architectural Metal Systems, Custom-Bilt, and Englert.
The BASF coating doesn't achieve the high reflectances of plain white, but it may be the best way yet to cool off residential metal roofs.
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