Carbon Footprint: White Roofs

Now that the summer has had a couple of serious heat waves, it's a good time to talk about how we Americans need our cool refuges from the hot and humid air. One obvious strategy to reduce your carbon footprint is to turn down the A/C or keep it from running when you are not at home. This keeps carbon from the atmosphere and dollars in your pocket.

But here's another creative solution: The next time you re-shingle your home, consider light colored material; White tile, silver paint, or cedar shakes each have a high reflective index. Instead of absorbing all of that energy and transferring it into the buildings below, as much as 15% of total energy could be reflected back into space. White roofs do the same thing as clouds and snow, in that they reflect incident energy like a mirror. The quantity of total energy reflected is referred to as albedo. White surfaces have very high albedos for light in the visual range, but are less efficient at reflecting ultraviolet or infrared wavelengths (hence the ~15% value cited above).

Evidently, there is a movement afoot to design new roofing materials that reflect light not only in the visual range, but in the longer more energetic wavelengths. These could be used as tiles to keep homes and other buildings more energy efficient. California is also sponsoring research that could make pavement more reflective. Combined with lighter roofs, this could go far in reducing the heat islands that form in urban areas. It's hotter in cities because more energy is absorbed by man made structures! And in heat waves, we have to cool those structures.

So painting your roof white may not have an immediate impact on your carbon footprint, but it should keep your cooling bills down in the summer. Now whether any of you cold winter dwelling folk benefit from darker roofs in the winter is something I don't know. Having done my fair share of attic crawling as a boy, my experience with dark colored roofs is that attics always got really hot in the summer, yet retained frigidity in the winter.

I found some fuzzy math on this subject in an article in Business Week, and a lot of commentary on it at the Island of Doubt, so if you are interested in this issue or are considering a new roof, check those sources out first.