Air Tightness

Air tightness refers to a building envelope's resistance to uncontrolled air leakage. It is measured by a blower door test, which pressurizes or depressurizes the building and measures the airflow needed to maintain a specific pressure difference (typically 50 pascals). The result is expressed as an air change rate (n50) or as air permeability (q50). A lower number indicates a tighter building.

Good air tightness is essential for energy efficiency, indoor comfort, and moisture management. In a leaky building, warm indoor air escapes through cracks and gaps in the envelope, carrying moisture with it. When this moist air reaches cold surfaces within the wall or roof structure, it condenses and can cause hidden moisture damage and mold growth. Air leaks also create uncomfortable drafts and increase heating costs.

Modern Finnish building codes require an air tightness of n50 less than or equal to 4.0 air changes per hour, but well-built passive houses achieve values below 0.6. Key areas where air leakage occurs include the joint between the foundation and wall, window and door frames, electrical outlets on exterior walls, penetrations for pipes and cables, and the junction between the wall and roof structure.

Improving air tightness in an existing building requires identifying and sealing leakage paths. A blower door test combined with thermographic imaging can pinpoint the exact locations where air is leaking. Common sealing methods include caulking, gaskets, spray foam, and vapor barrier tape. However, it is critically important that ventilation is adequate after tightening the envelope — a tight building requires a well-functioning mechanical ventilation system to maintain good indoor air quality.