airtightness

Airtightness is the degree to which a structure’s envelope resists the passage of air in and out. It depends on the performance of materials and construction details, including joints, penetrations, and openings around doors, windows, ducts, and fixtures. Affected factors include weather, wind, temperature differences, and the quality of workmanship during construction and testing.

A common way to quantify airtightness in buildings is through a blower door test, which measures the leakage rate at a defined pressure difference. Results are often expressed as ACH50 (air changes per hour at 50 pascals) or as a leakage rate Q50. Standards for testing and interpretation include ISO 9972 and EN 13829, with similar methods in ASTM E779/E783 in the United States. The test can be conducted in either pressurization or depressurization mode to determine how much air must be moved to achieve the target pressure.

Airtightness influences energy efficiency, thermal comfort, and moisture and indoor air quality. Reducing uncontrolled air leakage lowers heating and cooling loads, but excessive airtightness without adequate ventilation can lead to poor indoor air quality, condensation, or mold. Therefore, airtightness is typically addressed together with a deliberate ventilation strategy, such as mechanical ventilation with heat recovery.

Construction practices to improve airtightness include using continuous air barriers, properly sealed joints and penetrations, high-quality windows and doors, and careful insulation detailing. Post-construction testing verifies performance and helps ensure that the envelope meets design goals. While most discussions focus on buildings, airtightness concepts also apply to packaging and other systems where preserving contents and preventing contamination are important.