If you read the Doonesbury comic strip on Sunday, August 7, you might have seen Michael Doonesbury and Bernie discussing cool white roofs. Cool roofs are an energy-efficient technology developed here in the Environmental Energy Technologies Division of Lawrence Berkeley National Laboratory (Berkeley Lab). The characters also mention in passing a global initiative to help cool the earth by increasing its solar-reflective roofed and paved surface area. ( See "Global Cool Cities Alliance Q&A") The strip suggested that the wintertime heating loss from cool roofs is greater than is actually the case in real life, as has been demonstrated by more than two decades of scientific research.
Ronnen Levinson, Staff Scientist in the Heat Island Group, sent Garry Trudeau an erratum, which was published on the comic strip's website Doonesbury.com.
In case you missed it, we are re-publishing here Levinson's comments in their entirety.
Dear Mr. Trudeau and fellow Doonesbury fans,
This happens to be my field (cool roofs), so it's fun to see it in Doonesbury. The short answer to Mike's question is that the summer cooling savings associated with substituting a white roof for a black roof are much greater than the winter heating penalty, primarily because in cold U.S. climates a roof receives about three to five times more daily sunlight in summer than in winter. (In winter the sun is low, the days are short, and the sky tends to be cloudy.) Hence the annual net energy cost savings (annual cooling energy cost savings - annual heating energy cost penalty) is positive nearly everywhere in the U.S.
There are three further issues with switchable roofs.
- The small magnitude of a white roof's annual heating energy cost penalty limits the potential benefit of switchable roofing. That is, a switchable roof would save more energy than an always-white roof, but not a lot more.
- The added benefit of a switchable roof is nil when the roof is under snow, because all snow-covered roofs are white.
- The switching method would have to be electrochromic, rather than thermochromic, because a thermochromic roof will tend to suffer from a negative feedback that drives it toward gray. That is, a thermochromic switchable roof will darken as it cools, but getting darker will make it warmer, which will make it lighter, which will make it cooler, and so on.
Hope this helps!
—Ronnen Levinson, Staff Scientist, Heat Island Group, Lawrence Berkeley National Laboratory, RMLevinson@LBL.gov
P.S. For more about cool roofs, including maps of energy savings and penalties, please see R. Levinson and H. Akbari. 2010. Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants. Energy Efficiency, 3 (1), 53-109.
Map of the ratio of mean global horizontal solar irradiance in winter to that in summer, illustrating how the northern mainland U.S. (latitude >= 40°N) receives 3 to 5 times as much daily sunlight in summer as in winter. [PDF]