If you are converting your garage to living space and plan to install a new floor, it is usually smart to begin by insulating the concrete floor. You might also choose to insulate a garage floor if you plan to use the garage as a workshop, even if you are still storing vehicles in it. An insulated floor can make a garage much more comfortable to work in during cold weather.
There are two ways to handle insulating concrete floors. You can attach wood sleepers to the floor, fill the gaps with rigid-foam insulation, and then apply your finish flooring. Or, as demonstrated here, you can cover the slab with rigid-foam insulation, add two layers of plywood, and then add the finish flooring.
Either approach requires that you adequately prepare the slab before you begin. It should be in good condition, free of excess moisture, and as flat as possible. Any problems should be addressed before insulating the slab.
Any time a flooring material is laid over concrete, it is important to control moisture bleeding through the concrete. Although it looks like solid rock, concrete is actually fairly porous, and ground moisture easily migrates through the slab to affect any surface material laid over the concrete. It's especially important to control moisture in indoor locations to avoid damage to surface flooring materials, but it's important to control moisture even if the garage will continue to be a vehicle-storage space.
Controlling moisture in a garage is done by using exterior-grade plywood sheets laid over the foam insulating panels, and also by laying down a sheet of polyethylene plastic as a vapor barrier to block moisture. When a slab is being insulated in new construction, the vapor barrier is usually laid down first, before the slab is poured. But when adding insulation to an existing slab, the vapor barrier is laid over the top of the slab before the foam insulating panels are installed.
Dealing with the Edges
The foam insulation and two layers of plywood can add as much as 2 inches of height to the garage floor. This can be a problem if the garage will continue to be used to store vehicles, since it leaves you with a substantially raised lip at the doorway to the garage. A variety of edging strips or thresholds are available to help smooth this transition. Check with manufacturers of garage flooring tiles for transition thresholds that fit the height of your insulated garage floor.
Tools and Supplies You Will Need
Lay Vapor Barrier
Insulating concrete garage floors begins with covering the entire floor with 6-mil polyethylene, which will serve as a vapor barrier. Overlap the seams about 6 to 8 inches and seal the seams with waterproof construction tape. Run the polyethylene up the sides of the walls 3 to 4 inches, and secure to the walls with construction tape.
Lay Rigid Foam
Lay sheets of high-density rigid foam over the polyethylene. Use a utility knife to cut the foam to size (a hacksaw or drywall saw also work well for this). Leave a 1/4-inch gap around the perimeter. Tape the seams of the foam panels with heavy-duty construction tape.
Be sure to use compression-grade rigid foam panels, which are designed for installation beneath concrete slabs. Check with your supplier or the manufacturer for advice on the best thickness and R-value to use.
Some insulating panels come with attached vapor barriers. With this type of panel, no plastic sheeting is required since the panels itself provide a sufficient vapor barrier once the seams are sealed with an impermeable tape.
Lay Plywood Panels
Lay 1/2-inch-this exterior-grade plywood sheets over the foam, running the sheets perpendicular to the long dimension of the foam, so that the plywood seams do not align with the foam seams. Space the panels 1/4 inch apart, with a 1/2-inch gap around the perimeter. This perimeter gap allows for seasonal expansion and contraction of the plywood.
Add a second layer of 1/2-inch plywood over the first layer, running this layer perpendicular to the first, with the same spacing. Fasten the two layers of plywood together with 7/8-inch outdoor utility screws.
After insulating the concrete floor, you can install whatever residential finish flooring surface you choose, since our method seals the floor against moisture penetration. If the garage will remain a utility storage space for vehicles, a variety of garage tiles or mats are available for this purpose. Or, the plywood can be painted or coated with epoxy.
If the garage will still be used to store vehicles, smooth the transition between the raised flooring surface and the outside driveway by installing threshold strips of the appropriate size. Follow the manufacturer's instructions for attaching the strips; many come with construction adhesives designed for this purpose.