Cool It and Warm It With a Chameleon Roof

by Bill Chameides | February 12th, 2010
posted by Erica Rowell (Editor)

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Temperature-sensitive roof tiles, like the Thermeleon pictured here (which is black when temperatures are cold and white when temps are warm), could eventually make “cool” roofs a winner in places that experience cold winters. (Patrick Gillooly/MIT)

Will white roofs really slow global warming?

I’m feeling a little bit dated. No, it’s not because I go around singing tunes from the ‘60s. It’s my roof. I have one of those dark, shingled affairs. Energy Secretary Steven Chu advises to “make it white.”

What’s wrong with dark roofs? Simple: they absorb most of the rays from the sun that happen to hit them each day, and those rays ultimately show up as heat that brings warmer temperatures for your home, your neighborhood, and your city. Not good for urban heat islands — and if you run your air-conditioner a lot, not good for global warming.

White roofs, on the other hand, reflect rather than absorb most of the sunlight, and that means less heat, lower temperatures, and less energy needed for air-conditioning.

So white roofs are a great idea, right? Well, depending on your regional climate, there might be a catch, so maybe hold off calling your local roofer for some brighter shingles.

Cool in Summer Is Good, Cold in Winter Is Not

Sure, having a white roof in the summer is great because it cools when we all want some cooling. But what about the winter, when we actually want the heat from the sun? Is it really better to lose the winter heat in order to lose the summer heat?

Keith Oleson of the National Center for Atmospheric Research in Boulder and colleagues attempted to answer that very question. They coupled a global climate model to a so-called urban canyon model (to simulate the effects of changing roof surfaces on near-surface air temperatures), and reported their results last week in Geophysical Research Letters.

As expected, Oleson et al found that white roofs definitely provided cooler temperatures during the summer months but also caused cooler temperatures in the winter. And there’s the rub. The cooler it is in winter, the more energy (and presumably fossil fuels) will be needed to heat homes and buildings. Given that emissions from energy consumption contribute far more to global warming than urban heat island effects, this is a problem.

The question then becomes which is more important: the extra heating in winter or the cooling savings in summer?

Study Finds Extra Heat Needed in Winter Would Wipe Out Summer Energy Savings

Oleson’s calculations indicate the former: overall, if white roofs became ubiquitous, the extra energy needed for heating in the winter would exceed the energy savings in the summer. And, assuming that most heating and cooling comes from burning fossil fuels, that would mean an overall increase in global carbon dioxide (CO2) emissions. Clearly not an ideal recipe for addressing global warming.

(One caveat to Oleson’s calculations: his team’s model adopts the current global distribution of heating and cooling, and, in case you’re wondering, most of the current cooling occurs in the United States. The researchers note that the benefits of white roofs will grow as the use of air-conditioning around the world grows. Of course, rising A/C use, unless it’s from carbon-free energy sources, which is a tough bet for now, is probably not a great recipe for addressing global warming either.)

So is the white roof craze just plain crazy? Well, no. It makes a lot of sense in the summer, just not so much in the winter.

Solving the White Roof in Winter Problem

One solution would be to have the roofers out to your house twice a year to change your roof from white to black and back to white, and so forth, to get both energy savings and temperature benefits. (See NYT graphic [pdf] showing top five states where CO2 emission cuts would be greatest.)

If you don’t care for that solution, some students from the Massachusetts Institute of Technology have struck upon a better idea. A group of recent M.I.T. grads, as reported on the school’s web site, has developed a temperature-sensitive tile — it’s black when temperatures are cold and white when temps are warm. It’s a chameleon roof tile — so adding the Greek word for heat to the English word for the colorful lizard, they named their invention Thermeleon.

Cool, huh? And warm! But don’t get too excited. The Thermeleon still needs some work so you won’t be able to find it at the local Home Depot or Lowe’s.

But there are similar products on the market already heating up competition, such as these temperature-sensitive glass tiles, though they haven’t quite made it onto a roof yet. White or “cool” roofs, however, encouraged in commercial building codes in California, Florida, and Georgia, are available and are taking off in some areas here in the states and abroad. (More info on cool roofs from Energy Star here and here.)

If I lived in California or Florida, where winter heating is negligible, I’d definitely “make it white” with one of those cool roofs. But here in North Carolina, where we experience chilly winters, I will remain dated with my dark, shingled roof, awaiting technology and the marketplace to catch up with the idea.

filed under: carbon dioxide emissions, climate change, faculty, global warming
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