Too much insulation?
It seems to me that many energy-efficient houses have some misplaced insulation: too much of it in the roof and not enough in the walls. This approach may be based on the assumption that heat rises, when in fact hot air rises.
Richard Caron, Salem, NH
Andre O. Desjarlais, program leader for the Building Envelope and Materials Research Program at Oak Ridge National Laboratory, replies: You are absolutely correct that heat loss is not directional and that the driving factors are area, R-value, and temperature difference. However, you neglected one item in your calculation: Roof surfaces grow much hotter than walls and therefore require additional insulation to overcome the larger temperature difference.
Most roofs are dark, so they reflect little heat. A typical asphalt shingle reflects 5% of the solar radiation hitting it; 95% of the sun’s energy is absorbed by the shingle and is turned into heat. Conversely, walls tend to be lighter in color and therefore have higher levels of solar reflectance.
Also, the amount of heat striking a roof is much greater than a wall. The sun bombards the roof most of the day, whereas the walls are heated intermittently. A dark-colored roof can heat up to 200°F, but walls rarely reach 125°F. The hotter surface temperatures and longer exposure mean that the average temperature difference across the roof is much greater than across a wall.
The type of insulation that you’re using is also a factor, and one key component here is the cost of that insulation. Attic insulation is typically loose fill, and these products are less expensive to install than the batts typically used in walls. When deciding how much insulation to install, we look at the return on investment, which, to us, justifies more insulation in attics. See our Web site to determine the best levels for your area: www.ornl.gov/sci/roofs+walls/insulation/ins_16.html.