Tuesday, August 4, 2009

White roof, green roof, red roof, blue roof

Secretary of Energy Steven Chu appeared on the Daily Show a few weeks ago to plug for the Markey Waxman bill and brought up the subject of weatherization. The Secretary mentioned some research from LBNL (his old stomping ground) that touts the benefits of using white materials on roofs. The reasoning makes sense: the white roofs reflect more sunlight than dark ones, keeping homes cooler in the hot summer months, so that they use less air conditioning and consequently less energy = less carbon = less climate change. There is the added benefit that white roofs increase the amount of energy from the sun that is reflected back into space, thereby decreasing the amount of heat trapped in the atmosphere. [An aside: Could this be the administration's proverbial "toe in the water" for testing geoengineering concepts on public opinion? I'll come back to this when they start floating some of the more extreme geoengineering ideas.]
This sounds like a simple, inexpensive way to cut down on energy bills and do something good for the environment. But wasn't it just last year (or last month for that matter) that green roofs were all the rage? Green roofs help the environment by improving runoff problems and mitigating heat island effects in cities, and help would-be LEED certified buildings acheive that silver, gold or platinum star. So what is the confused homeowner, businessowner, or developer to do? Should she buy some grass seed and fertilizer or a bucket or ten of white paint?

It depends. A New York Times article from July 30 cedes that white roofs may make energy bills higher during cold weather in northern climates [Math police: In the same NYT article, Hashem Akbari asserts that since roofs have a lifespan of 20-25 years, if 5% of roofs replaced every year were turned white, the country's transformation (presumably to all white roofs) would be complete in two decades. I'm no mathmatician, but wouldn't fully 5% of all roofs (not just those being replaced anyway) need to be converted each year to get there in 20 years? I welcome corrections.]. Green roofs are likely more expensive ($8-25/sqft according to this source) than white roofs, which range in cost from just buying the paint to replacing the shingles ($2000-3000 for medium sized homes according to this). White roofs are more accessible to a wider range of consumers because green roofs might have weight and drainage restrictions, and might not work on very slanted surfaces (I'm sure there are some green roofies out there who could prove me wrong on that). But more to the point, green roofs and white roofs stemmed from different environmental signals: green roofs were envisaged to help with water issues in cities, white roofs for energy conservation anywhere (there is an oversimplification there - green roofs also mitigate heating and cooling needs, and white roofs could also help mitigate heat island effects). This seems like a situation that calls for using common sense in figuring out which type of roof is best for you. For the indecisive like me, you might want to try combining the two methods.


  1. What is the net benefit in power savings of the white versus solar panels? Obviously they'd be lot more expensive initially, but does the return in power generation outweigh AC savings over 20 years?

  2. Still, you'd want to keep your solar heated water tank on the roof painted black for it do its job properly. Dalmatian rooftops are clearly the way forward.

    I worry about the people who talk about this like its a newfangled idea. It's not like they haven't been doing this in the Mediterranean for, I don't know, 4000 years...

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