Summer heat can turn your attic into an oven. (May 29, 2013)

Attics and crawl spaces can bake under summer sun that's fiery enough to make asphalt shingles gooey and protruding vent pipes too hot to touch. Insulation helps. But without adequate venting your air conditiong costs can skyrocket.

On a 90-degree day, the AC has to produce a 20-degree drop from the outside to the inside of walls. But factor in the attic where temps can reach 140 degrees, and the AC has to run more to produce a much larger 70-degree drop. To prevent that, check these guidelines for attic insulation and venting.

More insulation. The old rule was fill the cavities between ceiling joists, but that won't meet modern energy codes, and most houses don't have ceiling joists anymore; they have trusses. The local building department can tell you what minimum code is, or check websites such as the Department of Energy's, and insulation calculators on the Home Depot and Lowe's sites. Values range from a low of R-30 to a high of R-60, R-value being the standard measure of insulation effectiveness, the higher the better. Even several Deep South states have the same 30-60 range because of the attic-oven effect and trying to keep heat out during the summer. Your recommended R-value may be higher than expected, and more than you have now. With fiberglass batts rated at R-3.5 per inch, for example, to reach R-60 you need a pile over 17 inches high.

More venting. The goal is for attic air to be only a few degrees hotter than air outside. To do that the Home Ventilation Institute recommends venting capable of at least 10 air exchanges an hour. It may seem hard to do without a fan assist. But in almost all houses, passive venting without fans does it. In the classic Cape, for instance, vents along the roof overhang take in air and gable-end vents or ridge vents near the roof peak exhaust it. Contractors can calculate how much vent area you need based on the attic design and volume. For a general estimate in typical 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. Contractors' proposals should express the airflow in cubic feet per minute, and the vents in net free air ventilation area — the actual vented space not including the frame.

Equal venting. The same area of inlets and outlets is key. Making up the total with 75 percent in and 25 percent out, for instance, doesn't work. Before more cool air can come in along the eaves an equal amount of warm air has to go out along the ridge. But with two overhangs and one ridge, it's often easier to reach the input than the output requirement, especially with continuously perforated fiberglass or vinyl soffits. Gable-end vents can be large enough to 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. Insulation, the other part of moderating attic temps, is also the part that can make the system fail. It has to be kept back from the eaves, which is easy to do cutting batts to length. But when extra loose fill is blown in you need baffles to keep it from sifting over and blocking the vents.