Attached garages can be a significant source of indoor air pollutants. Fumes from cleaning chemicals, garden fertilizers, pesticides, and vehicle exhaust are among the many contaminants that can be drawn into homes from attached garages if the walls and doors between the home and the garage aren’t adequately air sealed. Open joist bays above the garage that extend into living spaces are one air pathway. Air flow through cracks between and around the boards of the rim joist, the top plate, and the sill plate-foundation wall intersections are other areas where air can flow through if seams aren’t adequately sealed. Certain conditions in the home can cause the home to become depressurized, making it even more likely for garage air to be drawn into the home through leaks in and around the rim joists. Depressurization can occur when the house is airtight and an exhaust fan, range hood, clothes dryer, or combustion appliance is operated, if adequate makeup air is not provided to the house through a fresh air intake (a duct that brings outside air to the return side of the air handler).
For occupant health and safety, consider designing homes with detached garages. If attached, the garage should be completely air sealed from the living areas of the house with rigid foam, spray foam, and/or caulk. When garage ceiling joists span both the living space and the garage, the joist bay cavities must be blocked off and sealed. It is preferable to design the garage so that rim joists run parallel with the adjoining wall to act as a natural air barrier. The rim joists should be insulated and all seams where components (including the rim joist, top plate, and subfloor) come together should be sealed with caulk or spray foam. Seams between the bottom plate and the slab of the adjoining wall should also be caulked or foamed. This air sealing could be done by the framer or the insulation installer. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.
If the air handler for a central furnace must be located in the garage, it should be in an air-sealed closet with a dedicated air intake for combustion, a flue that exhausts to the outside, and a fresh-air intake ducted from the outside to the return side of the plenum so that it is not drawing garage air to circulate through the house. Determine whether a garage exhaust fan is advisable. Do not have supply or return registers in the garage.
Install carbon monoxide detectors inside the home.
How to Air Seal the Rim Joist between the garage and living space using rigid foam
1. Design walls adjoining garages so that the rim joist board runs parallel to the wall, providing a continuous natural air break. Where ceiling joists run perpendicular to the adjoining wall, one option is to make the rim joist continuous and have separate but aligned ceiling joists on each side of the rim joist.
Figure 1 - A continuous rim joist separates the garage and living space
2. Cut a rectangle of rigid foam (polyisocyanurate or extruded polystyrene) to fit into each floor joist bay cavity. If the joist bay is open, make a backstop for the foam by tacking furring strips to the joists in line with the foundation or house wall. If the joist area is closed, caulk all seams.
Figure 2 - Garage wall rim joist open to area between or under floors of living space
3. Insert rigid foam pieces into each joist bays and fasten with caulk or nails.
Figure 3 - Fitting rigid foam in garage wall rim joist cavities
4. Use caulk or spray foam to air seal all four edges in each bay. Make sure to completely air seal around the rigid foam to prevent moist air from reaching and condensing on the rim joist.
Figure 4 - Caulk around each piece of rigid foam in garage wall rim joist cavities
5. Add additional layer or rigid foam or batt insulation to meet or exceed the code-required insulation level for an exterior wall.
How to Air Seal the Rim Joist between the garage and living space using spray foam
1. Use urethane spray foam insulation to cover the rim joist, top plate, and subfloor above. High density (closed cell, 2 pounds/cubic foot) or low-density (open cell, 0.5 pounds/cubic foot) foams provide acceptable results; open-cell foams might require additional vapor and condensation control measures in IECC Climate Zone 6 and higher. Foam can be applied by a spray foam subcontractor, or by the use of two-part spray foam kits.
Spray foam in band joists is typically concealed between floors, so no other thermal barrier is required; however, the International Residential Code (IRC) allows the spray foam at rim joists to be exposed in basement and crawlspace applications (i.e., without a 15-minute thermal barrier such as drywall) as long as the thickness is less than 3-¼ inch (see 2009 IRC R314.5.11). High-density (closed-cell, 2-PCF) spray foams were approved in the 2003 IRC, and low-density (open-cell, 0.5-PCF) foams were approved in the 2009 IRC, as well as any intermediate densities (BSC 2009).
Climate Note: Although open-cell spray foam is acceptable in this application, closed-cell spray foam is preferred in hot-humid or extreme cold climates (IECC Climate Zones 1A, 2A, 7, and 8) (CARB 2009).
Figure 5 - Spray foam insulates the rim joist and air seals the subfloor-rim joist and rim joist-top plate connections
2. For additional protection when living space is located above the garage, consider a “flash” seal approach - spray foam the entire ceiling of the garage to air seal any cracks, holes, or seams in the subfloor. Then add batt insulation to meet the insulation R-value requirement and save cost compared to filling joists to the required thickness with spray foam alone. Cover the ceiling insulation with taped and mudded drywall.
Figure 6 - Garage ceiling with spray foam flash air seal plus batt insulation
Seal all penetrations through the common wall and ceiling. Use gaskets, airtight drywall technique, etc., to make the common wall and ceiling airtight. Caulk or spray foam garage slab-foundation wall junction.
Install a self-closing, insulated, metal, fire-rated door with a good weather seal between the living space and the garage.
Install a passive roof vent to keep the garage at a negative pressure in relationship to the house or consider installing a timer-operated exhaust fan that vents to the outside.
If central HVAV is installed in the garage, install a closed-combustion unit that draws intake combustion air from outside, vents exhaust air to outside, and has a fresh air intake ducted to the outside. Install unit in an air-sealed closet. Mastic seal any ducts located in the garage. Do not install any return registers in the garage.