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. Airflow 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. 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, spray foam, or foam gaskets. Seams between the bottom plate and the slab of the adjoining wall should also be caulked, foamed, or gasketed. 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 plenum so that it is not drawing garage air to circulate through the house. Do not have supply or return registers in the garage. Determine whether a garage exhaust fan is advisable.

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).

   

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 (Figure 2), make a backstop for the foam board by tacking furring strips to the framing in line with the foundation or house wall.

   

3. Insert rigid foam pieces into each joist bay (Figure 3).

   

4. Use caulk or spray foam to air-seal all four edges in each bay (Figure 4). Make sure to completely air-seal around the rigid foam to prevent moist air from reaching and condensing on the rim joist.

   

5. Add an additional layer of 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 and seam with the top plate or subfloor (Figure 5). Spray foams that are high density (closed cell, 2 pounds/cubic foot) or low density (open cell, 0.5 pounds/cubic foot) can 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 trades using 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 (2009 IRC and 2012 IRC R316.5.11) allows the spray foam at rim joists to be exposed (i.e., without a 15-minute thermal barrier such as drywall) as long as the thickness is less than 3-¼ inch, the foam has a density between 0.5 and 2.0 pounds per cubic foot (PCF), and the foam has a flame spread index of ≤ 25 and a smoke index of ≤ 450 when tested in accordance with ASTM E 84. 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).
   
   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).

   

2. For additional protection when living space is located above the garage, consider a “flash” seal approach - spray the entire ceiling of the garage with a thin layer of foam to air-seal any cracks, holes, or seams in the subfloor. Then add batt insulation to meet the insulation R-value requirement (Figure 6). This reduces costs compared to filling joists to the required thickness with spray foam alone. Cover the ceiling insulation with taped and mudded drywall.

   

Other Considerations

1. 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 the garage slab-foundation wall junction.
2. Install a self-closing, insulated, metal, fire-rated door with a good weather seal between the living space and the garage.
3. 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.
4. If the central HVAC system 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 the unit in an air-sealed closet. Mastic seal any ducts located in the garage. Do not install any return registers in the garage.