A.Bruce Harley, technical
director of Conservation Services Group in
Westboro, Mass., and author of Insulate and
Weatherize, responds: Unless the homeowners
park in the driveway, there's no way to provide a
guarantee against fumes, and unfortunately building
codes don't address pressure boundaries and
potentially unhealthy airflow at all. But there are
two strategies to help reduce risk: sealing leaks
between the house and garage and establishing a
known direction of airflow. Once you have ensured
that the garage is at a lower pressure than the
house, any leaks you haven't sealed won't pull air
and fumes from the garage into the house.
Your first priority should be to eliminate
ductwork from the garage. Regardless of what else
you do, leaks in return ducts or air handlers with
leaky filter racks could draw in fumes and send
them straight into the house. In an existing home,
if ducts can't be eliminated, seal them with duct
Next, caulk or seal any obvious gaps in the
garage walls or ceiling. Trouble spots include
where the drywall meets the foundation at the
bottom of the wall separating the garage from the
house; electrical penetrations and other holes
(such as garage-door-hanger hardware); and weather
stripping on the door between the house and garage.
Don't, however, seal leaks between the garage and
outdoors — those are helpful.
Another sealing strategy, if the home has little
or no insulation in the ceiling of the garage or in
the wall between, is to blow high-density (3 to 3.5
pcf) cellulose insulation into the cavities.
Sprayed insulating foam seals leaks, too, but is
easier and more cost-effective to use in new
construction than in existing buildings. In new
construction, be sure to install blocking between
joists where they cross partitions that separate
the garage from the home.
The easiest way to establish an airflow pattern
from the house to the garage is with a local
exhaust fan that vents garage air to the outdoors.
Depending on the size and leakiness of the garage,
it may take between 100 and 250 cfm of air to
depressurize the garage relative to the house with
confidence. The fan could be wired to run full
time, but that would use a fair amount of
electricity. A better approach might be to use a
time-delay switch that activates the fan each time
the garage door is opened (if it's an automatic
door) and runs it for 20 or 30 minutes after the
door is closed.
The most important precaution, of course, is for
the homeowners to avoid idling the car in the
garage with the garage door closed. And it's always
a good idea to install CO detectors in any rooms
that are next to or above the garage.
A building performance consultant may be able to
help you with all of these issues by using a blower
door, pressure-differential tests, and duct testing
or sealing. There are many Energy Star-approved
HERS raters in Texas with the tools to conduct
diagnostic tests and recommend a specific strategy
for your home. They should also be able to test the
effectiveness of the work once it's complete.