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Q.I'm building a house that has an attached 24-by-24-foot garage with a master suite above. The owner is concerned about fumes getting into the living spaces next to and above the garage. What's the best way to seal out the fumes?

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 mastic.

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.