An uninsulated concrete floor in a finished basement or garage lets valuable heat escape from your house. The technique described here for insulating such a floor sacrifices the finish flooring (you can roll up wall-to-wall carpeting and re-lay it later) to create a plywood subfloor you can then cover as you choose.

Because the materials are rigid, it’s best to insulate a basement or garage floor on a clean, relatively flat slab, although it’s possible to grind minor high spots flat with an abrasive block.

Do not insulate a basement floor that is often wet; tend to the moisture problem first. If the floor is sloped toward a drain, talk with a contractor before proceeding.

Keep in mind, too, that building codes often require the distance between finished floor and finished ceiling to be at least 7 feet, 6 inches. An insulated subfloor will reduce headroom in the garage or basement by at least 2 inches.

Heat also escapes through the edges of a concrete slab foundation, so insulate them as well. Dig a trench around the perimeter at least 6 inches deep. Clean the sides of the slab and attach rigid foam insulation. Cover the insulation with pressure-treated plywood, a cement coating, or another protective layer before filling in the trench.

Preparing the Floor Vacuum the floor thoroughly. If the insulation you have chosen requires a vapor retardant, cover the floor with 6-mil polyethylene sheeting. Overlap seams at least 6 inches, and tape them. On a basement floor, run the vapor retardant up the walls several inches. To keep the polyethylene from slipping on the floor, stick it to the concrete with dabs of caulk.
Adding Sleepers & Foam Arrange pressure-treated 2-by-4 sleepers end to end around the perimeter of the floor. Fasten the sleepers to the floor with a powder-actuated nailer or 2 1/4-inch masonry nails. Then install a gridiron of sleepers across the floor, centered every 16 inches. Cut panels of 4-feet-wide and 1 1/2-inch-thick rigid foam insulation into three 12 1/2-inch-wide strips. Lay the strips between the sleepers.
Attaching the Subfloor Lay 3/4-inch plywood perpendicular to the sleepers. To stagger the seams between panels, begin every other row with a half sheet of plywood. Fasten the plywood to the 2 by 4s with 2-inch ringshank nails or 2-inch galvanized screws.

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