Often the problem is sloppy framing. "If the studs are 16.5 inches on center and you're using batts made for 16 inches, you'll have a crack on each side that air can move through," says Tom. In this case, it's best to use cavity-filling foam or blown-in insulation.
Compressed Fiberglass Batts
Fiberglass insulation gets its R-value from the amount of air it traps between its fibers. If it's jammed too tightly into a cavity, it can't trap as much air and won't be as effective.
Doubled Vapor Barriers in Attic
Laying a second layer of fiberglass batts in the attic is an easy way to boost R-values. But if the new layer has a kraft-paper backing, it can trap moisture and turn the layers underneath into a soggy mess. "I see this mistake all the time," Tom says.