Control Attic Temperature: 3 Tips 1

How to Control Attic Temperature

by Mike McClintock

When attics heat up under blistering summer sun, AC costs go up as well- and without enough insulation and venting they can skyrocket. When it’s 90 degrees outside, for example, the AC system has to create only a 20-degree drop on a downstairs wall. But on a ceiling where temps can reach 140 degrees in the attic above, the AC has to create a 70-degree drop- and run longer to do it. Prevent that and lower your AC bill using these guidelines.

*More insulation. The old rule of thumb was you don’t need more once the cavities between ceiling joists are filled- a plan that usually falls short of modern energy codes. You can check requirements with your local building department, and use Web sites like the DOE’s that allow you to zero in by zip code. The recommended R-number is often more than you have. For example, in cold zones requiring R-60, using fiberglass batts rated at R-3.5 per inch would require a pile 17 inches high.

*More venting. The goal is for attic air to be only a few degrees hotter than outside air. To do that you need at least ten complete air exchanges an hour, and with almost all roof designs that’s possible with passive venting. In a classic Cape, for instance, vents along the roof overhang take in air, and gable-end vents or ridge vents exhaust it. How much venting do you need? For a general estimate in homes with ceiling insulation and vapor barriers, figure about one square foot of vent per 300 square feet of attic floor. Low-slope roofs need less and high-slope roofs (over 45-degrees) need another 20 percent or so because there’s more air to move.

*Equal venting. The same area for inlets and outlets is key, because before more cool air can come in along the eaves an equal amount of warm air has to go out. But with two overhangs and only one ridge, it’s often easier to reach the input than the output requirement. Large, gable-end vents can make up the difference on the exhaust side. But by themselves they tend to leave hot spots in the corners while continuous ridge vents draw air up and out more evenly. If you don’t have one, removing some shingles and cutting a ridge vent is easy, for instance, using one of Rockwell’s compact circ saws with the depth of cut set to reach just through the plywood.

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