This is a quick summary of the results from the paper "Comparative Summer Attic Thermal Performance of Six Roof Constructions", Danny Parker and John Sherwin, FSEC, 1998.
I highly recommend that you read the full paper, which is very well written and quite readable, but for those who just want the bottom line, here are some of the conclusions (as I see them):
They tested 6 roof configurations with an eye to determining which result in the lowest attic temperatures -- the table below shows the results.
Typical mid June attic temperatures with various roof constructions
|Roof Material||Ventilation Area||Radiant Barrier||Peak Attic Temperature (F)||% reduction in attic to living space temperature difference|
|Black shingles||1 in 300||No||134.9||Base|
|Black shingles||1 in 300||Yes||128.1||12%|
|Black shingles||1 in 150||Yes||116.1||33%|
|White metal (68% reflectance)||1 in 300||No||102.2||57%|
|White tile (72% reflectance)||No||91.5||76%|
For this typical day, peak ambient temperature was 92F.
The radiant barrier alone results in a modest improvement
The radiant barrier plus more ventilation area results in somewhat more improvement
A dark colored tile roof is about as good as radiant barrier plus more ventilation
White tile or white metal roofs result in a marked improvement over all the above.
The authors also point out that lowering attic temperature not only reduces heat gain through the ceiling to the living area, but also reduces heat transfer into AC ducts and air handling equipment in the attic. Surprisingly, this heat gain to insulated AC ducts typically exceeds the heat gain through the ceiling!
They also point out that locating air handling equipment in the attic results in a large (30%) increase in AC bills, and that lowering attic temperatures helps reduce this hit.
So, attic temperatures are even more important than you (or at least I) thought they were.
If you are doing a new home, and want to minimize cooling bills and related CO2 emissions: 1) use a white metal or tile roof, 2) keep the cooling/heating ducts inside the conditioned space, 3) don't locate air handling equipment in the attic.
If you are trying to control cooling costs on your existing home, consider changing to a reflective white roof. Think about one of the white coatings that can be applied to existing roofs. Check all your ducts and air handling equipment located in the attic for sealing and insulation -- if the ducts are already insulated think about how you might insulate them better.
Again, I would strongly recommend reading the full paper -- lots of very good material that does not come through in this quick summary. The FSEC is an exceptional source of good solid material on minimizing energy use for your home and pool in cooling climates.
Gary June 17, 2008