Airtightness & Air Sealing
Compiled by the Passive House Accelerator editorial team – Editorial review by Jay Fox, August 6, 2026
A tight envelope shuts down the uncontrolled airflow that drains conventional buildings of energy and stops air from carrying moisture into wall and roof assemblies where it can condense and feed rot or mold. Achieving Passive House levels of airtightness requires precise design in the studio and discipline on the job site to ensure the air sealing is done correctly. What the discipline produces goes beyond a number on a calibrated fan. It results in a building that holds 70°F when the power's out for three days in February; an apartment where outdoor smoke and pollution stay outside because every cubic foot of fresh air comes through a filter, not a wall crack; a retrofit where the masonry behind sheathing finally stops icing up because warm humid interior air no longer leaks into a cavity that sits at 20°F.
While exceptional airtightness is universal across all Passive House projects, not all Passive House projects measure airtightness the same way. PHI requires ≤0.6 ACH50 (volume-based); Phius CORE requires ≤0.060 CFM50/sf (envelope-area-based), tightened to ≤0.040 CFM50/sf for their prescriptive path. Phius's metric scales consistently regardless of building geometry, a useful property when the same standard has to govern a single-family home and a high-rise. Retrofits get slightly more leeway. EnerPHit, PHI's retrofit standard, relaxes the threshold to 1.0 ACH50, while the retrofit standard for Phius (REVIVE) has its own retrofit-specific targets.
The metrics differ; the discipline doesn't, and air sealing specialists from across the world stress the importance of testing the air barrier as you build. Their reasoning is simple: every leak found at rough-in is one that doesn't have to be chased after the wall is closed.
People assume Passive House airtightness is some huge feat. Really, it's about the right materials and attention to detail. Want proof? A 264,000-square-foot high school in Bellingham, Washington built to standard energy code (not Passive House) tested at 0.39 ACH50 (0.095 CFM per square foot of envelope at 75 Pa). That's well below PHI's airtightness threshold of 0.6 ACH50. The team was just trying to meet code by applying a fluid-applied air barrier system to a conventional building. With the right materials and diligence, they cleared the Passive House airtightness bar without aiming for it.
The concepts in this hub trace what steps teams need to take to make an airtight building from both the design side and the construction side.
Deciding on the right building material given specific conditions is crucial, and different kinds of barriers—peel-and-stick, mechanically fastened, fluid-applied, sheet goods—are suited to specific construction contexts.
Continuity at junctions is where most buildings fail, so certain areas need special attention, including slab-to-wall, wall-to-roof, floor-plate-through-masonry, and the trade boundaries where one scope's tape has to find the next scope's membrane.
Penetration sealing without sloppy spray foam, with service cavities that move wires and pipes inboard of the air barrier.
Blower door testing is the construction-management spine—a four-stage protocol that gates each phase from air-barrier commissioning through final compliance, with smoke pencils, fog machines, and infrared cameras finding what the fan only counts.
Multifamily compartmentalization isolates each unit on its own pressure plane.
Retrofit airtightness where the existing building has its own opinion about where the air is.
Prefab and factory air-sealing where panels arrive pre-tested and assembly is the variable.
Workforce training programs build the hands-on competence required to seal at scale.
And vapor control—because the membrane that handles air also has to handle moisture.
Get this layer right and everything above it works. Get it wrong and the rest of the building can't compensate.