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.
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