The Facts About Cool Roofs
Imagine you're walking across a black asphalt parking lot on a hot summer day. Not very pleasant, is it? The asphalt spends the day absorbing heat from the sun, and that heat radiates upward, making a hot day even hotter. The same thing takes place on your roof throughout the summer, and with worse results. On a sunny day, black asphalt roofing shingles can reach temperatures more than 80 degrees higher than the air temperature, and this heat penetrates your living space. Switching to a "cool roof" with shingles that reflect rather than absorb the sun's energy can stop this heat transfer, resulting in dramatically lower energy bills--up to 40 percent lower.
The notion of using lighter colored shingles to reflect heat from rooftops isn't new, but until recently, the appeal of cool roofing materials was undermined by some inherent drawbacks. Chief among them is that dirt and grime is much more visible on white or light-colored shingles, so cool roofs often looked dingy. And because cool roofing shingles don't get nearly as hot as black shingles, they are more hospitable to mold and moss, which can lead to roofs that are dingy--and green. Finally, the benefits of early versions of cool roofing shingles were limited, because, while they did reflect some of the sun's heat, they still absorbed quite a bit. Fortunately, the latest cool shingles overcome these issues and perform much better to boot.
..But Recently Improved
The latest cool roofing shingles contain algecides and fungicides, which significantly reduces the grunge problem that doomed earlier iterations. Some types of shingles are much smoother than traditional roofing shingles and are advertised as being "self-cleaning." These advances have largely resolved the issue of light-colored roofs that appear dirty. Reflectivity has also dramatically improved in recent years. Cool roofing shingles offer a Solar Reflectance Index (SRI), which measures the ability of shingles to reject solar heat, of greater than 60, which is double that of white shingles manufactured in the early 1990s.
Is a Cool Roof Right for Your House?
There is no question that a cool roof results in lower--and in many cases dramatically lower--cooling costs, but that alone doesn't guarantee that it ends up saving you money. To operate at peak efficiency, cool roofs--even self-cleaning ones--need to be washed periodically, which adds to the operating expense. Because they aren't subjected to the extreme heat that traditional shingles are, cool roofing shingles last longer.
Some manufacturers recommend resurfacing your cool roof every 10 years to maintain performance, which can offset the advantage of increased longevity. Unfortunately, long-term data isn't available for the latest cool roofing materials, so savings over the life of a cool roof are impossible to predict. As a general rule of thumb, the warmer the climate where you live, the more you can save with a cool roof. And a cool roof in unlikely to cost more than a traditional roof, so if you need a new roof anyway, the environmental benefit makes it a worthy investment, even if it's just a wash financially.
Brett Freeman is a freelance journalist. He also owns a landscaping and irrigation company in North Carolina. Previously he has worked as a beat reporter, a teacher, and for a home improvement company, and he used to own a bar/live music venue.