Airtightness & Air Sealing
Compiled by the Passive House Accelerator editorial team – Editorial review by Jay Fox, May 12, 2026
Passive House airtightness shuts down the uncontrolled airflow that drains conventional buildings of energy and carries moisture into wall assemblies where it can condense and feed rot or mold. PHI certification asks for ≤0.6 air changes per hour at 50 pascals — about five times tighter than the 2021 International Residential Code allows. Phius CORE Prescriptive asks for ≤0.040 CFM50 per square foot of gross envelope area. Either threshold makes the envelope the boundary that lets the other four Passive House principles actually work.
What does Passive House airtightness actually do? In a word: it shuts down the uncontrolled airflow that drains conventional buildings of energy and carries moisture into wall assemblies where it can condense and feed rot or mold. A tight envelope—measured by a blower door test—is the foundation of every Passive House principle, because it's the boundary that lets the other four (continuous insulation, thermal-bridge-free detailing, balanced mechanical ventilation, high-performance windows) actually work.
How tight? PHI certification asks for ≤0.6 air changes per hour at 50 pascals (0.6 ACH50), about five times tighter than what the 2021 International Residential Code allows for residential construction in U.S. climate zones 3-8 (3 ACH50). Phius CORE Prescriptive asks for ≤0.040 CFM50 per square foot of gross envelope area. Either threshold means the building's air leakage is so low that conventional infiltration-driven heat loss collapses—heat exchange happens deliberately, through the ventilation system, not accidentally, through whatever gaps happen to be in the walls.
That control matters in every climate. In cold climates, airtightness prevents interior humidity from exfiltrating into cold cavities and condensing on cold sheathing. In hot-humid climates, it stops outdoor moisture from infiltrating and condensing on A/C-conditioned interior surfaces. Pro Clima's Jens Lüder Herms calls one experiment foundational: a controlled study at Germany's Fraunhofer Institute showed that a single 1 mm gap in an airtightness layer allows 1,600 times more moisture into a wall assembly than diffusion through an intact barrier. Airtightness and moisture management are not separable problems—the Vapor Control and Smart Membranes concept covers what that interdependence means for assembly design.
Airtightness isn't only theory or building physics. It's a daily job-site discipline. The wall-to-foundation transition, electrical penetrations, window rough openings, plumbing chases—these are where measured air leakage actually accumulates. Curt Rich of BPC Green Builders presented data across twelve certified projects showing that the wall-to-foundation transition alone accounts for roughly 30 percent of total measured leakage. Catching these failures requires blower door testing during construction, not just at final certification.
The regulatory landscape is catching up. Australia added airtightness terminology to its National Construction Code in 2019. The 2022 update added Class 4 vapor-permeable membrane requirements in climate zones four through eight, with WUFI hygrothermal modeling as the performance pathway. Jessica Allen of Climasure describes the practical impact for South Australian practitioners—a sevenfold increase in certified Passive House projects in the eighteen months following the code change.
The concepts below cover this ecosystem from every angle: testing methodology, material selection, junction detailing, vapor control, retrofit-specific challenges, and the quality-control disciplines that turn a designed envelope into a built one.