The Winter Penalty
The same steps that make buildings easier to cool in the summer also can make them more difficult (and expensive) to heat in winter. It turns out, however, that in hot climates the summertime benefit greatly outweighs the wintertime penalty. That’s because in summer the sun is high overhead, and shines mainly on the roof of a home; in winter the low sun shines on the walls and through the windows. So if we want a home to stay cool in the summer, we want it to have a light-colored roof. But to capture solar heat in the winter, the roof plays less of a role; it is more important to have large south-facing windows.
For example, in a climate like that of the inland parts of Los Angeles (say, the San Fernando Valley), a homeowner will save about $40 less for a season’s worth of air conditioning if the roof is white rather than green. But the winter heat bill for the white-roofed home will be only $10 more than the green-roofed home, for a net savings of $30.
White roofs retain their energy advantage surprisingly far north. Let’s compare the solar intensity on a flat surface in June and in December at the latitude of New York City. By December, the length of the day has halved, and the sun is so low that it “sees” only half the roof area that it saw from on high in June. Moreover, New York is about three times cloudier in winter than summer. The three factors multiply: 1/2 x 1/2 x 1/3 = 1/12, so potential solar absorption on a roof is only 1/12 as great in December as in June. The bottom line: because so little winter sunlight ever makes it to the roof in the first place, it doesn’t much matter what color it is. White singles therefore allow buildings to be much cooler in summer and yet be only slightly colder in winter (because only a relatively small amount of absorbed sunlight is foregone).
Arthur H. Rosenfeld is senior adviser, and Joseph J. Romm is principal deputy assistant secretary for energy efficiency and renewable energy, at the U.S. Department of Energy. Alan C. Lloyd, former chief scientist for California’s South Coast Air Quality Management District, is director of the Energy and Environmental Engineering Center at the Desert Research Institute in Reno, Nevada. Hashem Akbari is principle investigator of the Heat Island Project at DOE’s Lawrence Berkeley National Laboratory.