To reduce air leakage in the basement, you sometimes have to seal the attic. Dealing with air pressures in buffer zones such as attics and basements, is an important part of energy efficiency and durability since moisture and air leakage problems are often related. Jim Fitzgerald, a Minneapolis insulation contractor who invented a high-density cellulose blowing technique, demonstrated the importance of buffer zone pressures at a recent blower door training in Portland for the staff of Oregon low-income weatherization agencies. His ideas apply to both new construction and existing buildings.

Fitzgerald gave a common example of a house where the air pressure gauge indicates the basement is not completely inside house. Since it's considered good weatherization practice to bring a basement inside the conditioned space, air sealing the basement appears to be a reasonable decision. However, to fully seal the basement, you may have to climb into the attic. Fitzgerald showed how basement air infiltration can be driven by air leaks in the attic that draw basement air up through the interior framing.

With the blower door depressurizing the house to -50 Pa., Fitzgerald showed that if the attic and basement are interconnected, the basement pressures can change when an attic access is opened or closed. The amount of change indicates how interconnected the spaces are. If a basement and attic are found to be interconnected, the basement can often be brought inside by sealing attic bypasses, which would likely be recommended anyway.

In this case, spending more time on house pressure diagnostics can lower weatherization costs and improve cost effectiveness. This interconnection test is very useful for houses with basements, vented attics and two-story houses with ducts between floors.

Contact: Oregon State University Energy Extension Program at 503-731-4570