Garages

By: Rob Parker, RHI, CHI, CMI

Some indoor air-quality problems may begin in the most unlikely places, Canada’s nationals housing agency says.

According to Canada Mortgage and Housing Corp. (CMHC), an attached garage is a source of home air pollutants that we are only now discovering.

Starting a car in a garage, even with the garage door wide open, can produce elevated concentration of combustion pollutants that can find their way into your home as “respirable particles.”

Even driving a car into the garage, turning off the engine and closing the door, results in emissions of various chemicals that can linger for hours as the engine and its fluids cool down. Over time, pollutants in the garage air can be drawn into the house.

This is not just a theory — a Health Canada study involving more than 100 houses, revealed that houses with attached garages had measurable concentrations of benzene (a gasoline-related pollutant) in the indoor air, while houses with no garages or detached garages had little or no benzene.

There are also secondary sources of pollutants in garages:

• Gas-powered appliances, such as lawn mowers, chain saws and edging tools, have emission systems are not as good as those found in cars.
• Chemicals such as pesticides and herbicides are also sources of pollutants.

It can be difficult to understand how and why garage air moves into the house. After all, there is at least one layer of drywall between the house and the garage; a significant amount of insulation; and the door to the house typically has weatherstripping and a spring closer.

So how does air get in? It can move through small cracks in the walls between the house and the garage, and through the top floor ceiling. There are many tiny holes and cracks that permit this air exchange to take place and they exist in all houses. It takes a sophisticated test with specialized tools, such as a blower door and leakage detection equipment, to find infiltration and ex-filtration points.

The best way to prevent garage air getting into the house is to ensure that there are no leaks between the garage and the house.

This is easy to do in a house that is being built. The builder should make the interface walls and ceilings as airtight as possible. This is more readily done if the builder knows that reducing pollution transfer from the garage is a priority.

The builder should:

• Ensure the airtightness of the garage ceiling and walls that are next to the house, before the insulation is installed and before installing drywall on the garage side.
• Check all wall-to-wall junctions or wall-to-floor junctions and seal them. If the top of the basement wall is exposed in the garage, this header space can be notoriously leaky.
• Diligently seal all penetrations from the house to the garage, such as wiring or central vacuum exhaust.
• Keep mechanical systems, such as furnaces and water heaters, out of the garage. While most Canadian builders wouldn’t consider putting such systems in the garage, it’s common practice in parts of the U.S. The few Canadian houses CMHC has tested (in B.C.) with heating systems in the garage had high levels of garage pollutants in house air.

It is much harder to prevent air movement from garage to house in an existing home. Leakage areas are usually hidden, hard to find and are tough to seal; but air-sealing the garage-to-house walls and ceilings may still be worthwhile.

If the garage side has no drywall, sealing air leaks can be simple.

If the drywall is simply screwed on the wall and isn’t finished, removing it will give access to the interior spaces. Finishing the drywall itself with drywall compound and paint, and caulking all visible cracks and joints may improve airtightness.

Another option is to install an exhaust fan — a good bathroom fan will do — to vent garage air outside. Running the fan will lower air pressure in the garage relative to the house, and prevent air moving into the house. This won’t affect home heating costs much, but you will have to pay for electricity to run the fan, so choose one with low energy consumption. To further reduce fan use, only run it for a period (for example, one hour) after the garage door is used.

Continuous use of the exhaust fan is recommended if:

• There are a lot of noxious chemicals in the garage. Consider sending them to a hazardous waste disposal site.
• The garage is used to store or maintain older vehicles with higher emissions.
• There is a lot of coming and going via the main garage door.

If your attached garage is not used for vehicles and there are no other major chemical sources in that space, garage-to-house air movement should not significant problem.