There are two important reasons to do a careful job of air-sealing the floor of a bonus room over a garage: to improve the home’s energy performance
and to prevent carbon monoxide from entering the room. Even after air-sealing, it’s essential that you install a CO detector.
When insulating a bonus-room floor, you’ll usually be working from below. If the area of the heated bonus room above the garage is smaller than the area of the garage ceiling, you’ll need to install blocking between the joists to provide an air barrier where the floor insulation stops. Locate the blocking under the kneewalls.
Blocking can be made from 2x lumber or rigid foam. If it is being installed between I-joists or floor trusses, rigid foam will be easier to cut into odd shapes than lumber. Whether you use 2x or rigid foam, it’s important to seal the perimeter of each piece of blocking with caulk or canned spray foam.
If the floor is the same size as the garage, it’s essential to air-seal the rim-joist area carefully. If ceiling joists extend from the house into the garage, you need to install blocking between the joists to maintain the home’s air barrier. Once the blocking has been installed, seal air leaks at the blocking and the rim-joist area on all four sides of the garage.Air-seal, and hold insulation tight to subfloors
The first step in energy-smart floor assemblies is air-sealing, which is
particularly important in garages, where carbon monoxide and other
toxic chemicals can leak into the living space above. Another
significant detail is to make sure the insulation is held tight to the
bottom of the subfloor; gaps negate the R-value of the insulation.Floor above a garage
This detail shows a garage bonus-room floor framed with open-web floor trusses. To prevent air leaks, it’s important to install caulk at all of the indicated locations. Ideally, the floor assembly won’t include any ducts. If there’s no way to avoid installing ductwork in the floor, specify deep floor joists, such as trusses, that provide plenty of room under the ducts for insulation, and be sure to include a continuous layer of rigid foam under the joists.Smaller room over a large garage
It is important to maintain a continuous thermal boundary between conditioned and unconditioned spaces. The weak link in this situation is the kneewalls. Install blocking (either wood or rigid foam) under the kneewalls, and insulate the walls, floor, and ceiling as shown.
Floor above a garage Smaller room over a large garage