This article was originally published in the May/June 1994 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online May/June 1994

Every House Has a Roof


One of the feature articles in this issue describes the air conditioning savings that can be obtained by simply changing the color of a home's roof (see Saving Energy with Reflective Roof Coatings, p.15). To be sure, the savings from this measure appear to be greatest in Florida and other southern states, but the results shouldn't be ignored elsewhere.

Reducing cooling-energy use is a challenge because the heat comes via so many different paths. Unlike space heating, where most of the heat loss is caused by a difference in temperatures, the heat gains to be removed by the air conditioner are spread among several paths, all of which can be significant: outside temperature, solar gains, humidity, and internal heat gains.

An air conditioning efficiency improvement that works well in one house or climate might be much less effective elsewhere. That is why it should come as no surprise that changing the roof's color can save up to 40% in some uninsulated Florida test houses. But it is a surprise that similar savings have been found in California, which has a very different climate. With such compelling savings figures, it's time to reconsider the roof's role in energy use. The big peak power reductions should make every utility demand-side planner reconsider roofs.

If roof color is important, then the energy specialist must learn to measure a material's solar reflectivity (this is not something one can eyeball) and also needs to become knowledgeable about high-reflectivity building materials, some of which are not yet commercially available.

Finally, the energy specialist will need to enlist a new partner: the professional roofer. Changing a roof's color is not cost-effective when implemented as a straight retrofit, but it does pay when a roof needs replacing. Thus, the energy specialist must coordinate activities with a knowledgeable roofer, or at least ensure that the roofer uses materials with a high reflectivity.

The article also touches upon a second intriguing aspect of building color. That is, one building's color may affect the energy use of its neighbors. The colors currently used in buildings tend to absorb and re-radiate heat to nearby surfaces, leading to higher temperatures. This phenomenon is know as the urban heat island. Small-scale measurements and computer simulations suggest that city-wide changes in building color can have profound effects on temperature and air quality. And even if we can't change all of Los Angeles in one day, the same phenomenon can happen in a neighborhood. Suddenly the building energy specialist becomes a community energy specialist!

Alan Meier


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