If you don’t plan to convert your garage to living space, look carefully at the boundaries between house and attached garage, and between garage and outdoors. Good air sealing and insulation between your garage and living space will minimize unwanted heat transfer and keep vehicle exhaust out of your home, as well as fumes from stored household chemicals.
Regular maintenance will improve your garage’s durability and air quality while minimizing the chance of moisture problems.
Converting a Garage to Living Space
Garage conversion can be relatively inexpensive; the foundation, walls, and roof are already in place. And garages don’t have the headroom limitations of attics. The key issue with a garage is the floor; the concrete slab is usually sloped and lower than the main living space, and soil moisture may penetrate the slab and enter the garage.
Improve the Floor
You can create a finished floor in your garage either by building up the floor or by finishing the existing concrete floor. Either way, you should first seal the concrete floor with a moisture barrier to keep any dampness in the slab from reaching the room.
If you plan to raise the garage floor, you can use a continuous layer of 6-mil (or heavier) polyethylene over the slab as a moisture barrier, with seams and perimeter taped and sealed. This seamless barrier can also keep soil gases from entering the space. Your contractor can then set 2 x 6s on top of the sealed slab, shimmed or trimmed to level the floor, with moisture-impermeable insulation between them.
If you don’t plan to raise the floor, have compression-grade rigid foam insulation installed on the slab, sealing seams with heavy-duty construction tape and leaving a ¼-inch gap around the perimeter for expansion.
A subfloor, usually plywood, goes on top of the insulation (and framing, if any). Make sure your contractor is familiar with the recommended procedures established by the building codes in your area for constructing a subfloor over a garage slab.
Seal and Insulate
If the walls and ceiling of your garage have enough insulation for your climate zone and are covered with drywall, make sure all penetrations in the drywall are sealed with caulk or foam. Seams at the ceiling, walls, and floor should likewise be sealed.
If the garage walls and ceiling have drywall but are not insulated, cellulose can be blown in at little expense. If the garage framing is exposed, you have more insulation options. Make sure your contractor is familiar with the airtight drywall approach.
If the walls between the garage and the outdoors are framed with 2 x 4s, consider adding furring strips to the interior of the studs to accommodate additional insulation. In some areas, building codes mandate this.
The existing garage door opening is a good place to put a new door and windows. This avoids the expense of cutting new openings in existing walls, which disturbs interior and exterior surfaces. You’ll need to have a new curb installed at the base of the former garage door, built up with concrete or masonry blocks to the same height as the adjacent foundation stem walls. This curb will raise the infill framing above ground moisture, keep water from running in at floor level, and allow the new siding to blend in with the rest of the home.