Dangers of Attached Garages
Attached garages are one of the most questionable practices of common construction and deserves a close look at pollution management. Garages carry one of the biggest risks of Carbon Monoxide CO poisoning. Most poisoning cases are small and go undiagnosed. Cumulative health effects are best avoided.
How Most Poisoning Occurs
Garage doors closed before the garage-air clears, traps combustion exhaust inside the garage. Older engines or those with leaking fluids, continually contaminate the air along with gas powered lawn tools, garden products and other garage related storage items. The garage-to-house door is an easy conduit but even when closed, various pressures work to deliver the garage's polluted air into the main home's air.
Even a slight breeze will blow attached garage-air into a home. Dryer vents, kitchen vent hoods and bath exhaust vents put the building envelope under negative pressure sucking garage-air inside. For spaces built above the garage, the stack effect is a strong force to be aware of. The more attached the garage, the greater the risk.
Designing homes for cars
Eliminate the garage
Many situations are fine without a garage. The WNC climate is favorable for this strategy. Does your car need it's own house?
Consider a carport
Most value the extra storage and workspace of a garage. Carports with built-in rooms and storage areas can often serve these needs in more interesting, useful and healthy ways. Garden sheds make good alternative storage areas. Carports allow more light, views of the landscape and almost eliminate CO risks.
Find room for a detached garage
For many situations, garages are mandatory. Beware of the risks associated with combustion exhaust and enclosed spaces. Risk is greatly reduced by detaching the garage from the home's building envelope. Covered breezeways are good connections for car-home to human-home. Security is often cited as a reason to attach, but designers have many ways to build more safety into this aspect of design if its a goal.
Dealing with Attached Garage Risks
For existing homes, or for those building new without the space or budget, there are some ways to address the risk.
Airseal the shared building components
Existing homes should increase airtightness in these areas when possible. Builders need to treat this boundary seriously. The mudsill is a particularly important location. Existing homes may benefit from a little demo, to better seal this leaky location. Pick a good garage-to-house door, ensure framing is airsealed to the jambs, and install it tight. Homeowners need to maintain the weatherstripping and keep the door shut as much as possible.
Garage Exhaust Ventilation
EPA's best indoor air standard requires proving airtightness between the garage and home or installing a garage exhaust vent if the home's outdoor-air ventilation system is an Exhaust-Only Type. We recommend garage exhaust vent fans for attached garages (sharing a door with the house), wired to the garage door opener, with a timer set for a certain amount of time.
This air-cycler garage ventilation product is a good option for attached garage exhaust ventilation.
Avoid floorspace above the garage
The most tempting space to exploit in a house and also the most dangerous. The stack effect is a powerful force, and conventional framing makes this ceiling to floor location very difficult to air-seal.
Never put HVAC ductwork in a garage
This is not common in our climate but is important to point out. It applies to all climates. The only acceptable ducting or ventilation in a garage are exhaust-only vents. Heating, cooling, make-up air and outdoor-air ventilation supply ducts, need to stay out of the garage. Existing homes should consider hiring a home performance professional to improve exisiting ductwork integrity in attached garages.
Park with tailpipe facing opening and never run car without door open
Be mindful about running a car in an enclosed space. Open garage doors is not always enough to keep garage-air at safe levels and even with good air-sealing techniques, garage-air will likely find its way into an attached home.
Attached garages are becoming better recognized for their dangerous effects on indoor-air. Perhaps electric vehicles will become prevalent enough, to alleviate most concerns. Vented crawlspaces and combustion appliances inside the living spaces are two other common building practices needing attention for those with goals of healthy indoor-air quality.
Vented-Crawlspaces Should Be Illegal. 
Combustion Appliances Inside
Springtime Builders is a custom home builder based in Asheville, specializing in cost-effective energy efficiency and building science craftsmanship.
Posted in Indoor Air Quality