Air Leaks: Why Every House Needs a Good Air Barrier

Air leaking through a house's walls, roof, and floors does more than waste energy. It carries moisture into framing, where it can cause mold and rot, and it can drag in pollutants and soil gases that hurt indoor air quality. Three forces push and pull that air: wind, the "stack effect" (warm air rising and escaping through the top of a house while cold air gets pulled in at the bottom, like a chimney), and fans from range hoods, bath fans, dryers, and HVAC equipment. A powerful kitchen exhaust fan without a makeup-air supply can depressurize a whole house, pulling air backward through a water heater vent, a fireplace, or the soil around the foundation, bringing carbon monoxide or radon with it.

A tight house isn't automatically good. Sealing up air leaks (making a home more airtight, something installers measure as ACH50) without adding proper ventilation just traps moisture and pollutants inside instead of keeping them out. The fix is pairing a continuous air barrier with a mechanical ventilation system that brings in a known amount of fresh air from a known source, rather than relying on random gaps and cracks.

Moisture problems tied to air leaks show up differently by season: in winter, warm humid indoor air escaping through gaps can condense on cold roof sheathing, sometimes showing up as frost in the attic or water stains mistaken for a roof leak. In summer, humid outdoor air leaking in can condense on cool surfaces like air-conditioning ducts or cold plastic vapor barriers. Homes built before mechanical ventilation were often too leaky to have these problems, but as houses got tighter, ventilation became necessary to keep indoor humidity in check.