Insulating the Attic
Laying new insulation on top of old
I'm planning to put more fiberglass insulation on top of the existing layer in my attic. Should the batts be laid perpendicular to the existing insulation or parallel to it?
-John Brigham, Vineland, N.J.
Tom Silva replies: That depends on the thickness of the existing insulation compared with the height of the joists. If your existing insulation sits even with the top of the joists or is less than an inch or so above or below them, then lay the new batts perpendicular to the old ones and push them tightly together. Make sure the new batts have no paper facing on them, otherwise you risk trapping moisture in the insulation.
But if the top of the existing insulation is more than an inch above the joists, or more than an inch shy of their tops, then use blown-in insulation—either cellulose or fiberglass. It does a better job than batts at filling in the gaps.
Whatever approach you take, make sure to leave a space at the eaves between the top of the insulation and the underside of the roof. That way, air can enter the attic through the soffits and exit through the ridge vent.
Before you start, find out how much insulation you really need. Visit the U.S. Department of Energy's Zip Code Insulation Program. By plugging in your zip code and some information about your house, the program will show you the recommended insulation levels for attics in your climate. There's no point in adding too much.