Friday, July 31, 2009

Using White to Decrease the Red

As anyone who has stepped barefoot on a sunny day from a concrete surface to a slate one knows, dark colors get really hot. Since white surfaces reflect more of the sunlight's energy, using light-colored roofs can reduce air conditioning needs and expenses. How much? 20% in hot, sunny weather, says an article in the New York Times. Cumulatively, this could have a big effect.
Turning all of the world’s roofs “light” over the next 20 years could save the equivalent of 24 billion metric tons in carbon dioxide emissions. “That is what the whole world emitted last year,” Mr. Rosenfeld said. “So, in a sense, it’s like turning off the world for a year.”
What about locations where there's little AC load, and you want the sun's heat in the winter? For one, there's far less sunlight in winter (that's why it's winter), so the unhelpful 'loss of solar gain' in the winter is less than the beneficial loss in the summer. Even so, having light roofs in Sweden or Alaska is probably not a good idea.
The extra heating costs may outweigh the air-conditioning savings in cities like Detroit or Minneapolis. But for most types of construction, they say, light roofs yield significant net benefits as far north as New York or Chicago. Although those cities have cold winters, they are heat islands in the summer, with hundreds of thousands of square feet of roof surface absorbing energy.
By the bye, I will be on vacation next week, so a little break from posts.

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