How to Insulate Floors Over Garages

Insulation Options

• Insulation batts and rolls come in stock widths that are 1/2 inch wider than standard joist spaces. If the joists sit on 16-inch centers, meaning the distance from the center of one joist to the center of the next is 16 inches, the joist space will be 14.5 inches wide. In this case, 15-inch batts would be the appropriate size. Rigid foam board insulation is available for attaching to the underside of joists. Spray-in, closed cell foam is an alternative option for open joists, but installation requires a licensed professional. If the garage ceiling has drywall, you have the option of drilling holes and blowing in cellulose insulation. Because the joist spaces run horizontally, however, uneven coverage is possible.

Insulating Open Joists from Beneath

• Open joists are the simplest to insulate with fiberglass batts. Because the batts are lightweight, and slightly wider than the joist space, friction temporarily holds them in place during installation. For the long haul, however, use some type of support on the underside of the batts to keep them from falling or blowing out. You can install drywall, add sheets of insulating foam board or attach thin strips of wood, perpendicular to the joists, to hold the batts in place.

Batt Insulation Considerations

• Insulation depends on loft so don’t overly compress a too-thick batt into a too-shallow joist space. For example, 2-by-10 joists, which are actually 9 3/8 inches wide, hold 9.5-inch batts, giving the floor R-30 insulating value. Squeezing 12-inch batts in the spaces, however, would cause too much compression. If you need more R-value than the joist depth permits, you can use high-density batts or add rigid foam insulation panels beneath the joists.

Considerations

• Rooms over garages often have sloped walls, due to sloped garage rooflines. Additional insulation in the space behind a kneewall will reduce temperature fluctuations caused by air that enters the soffit vents under the roof eaves.