Airtightness & Air Sealing
Compiled by the Passive House Accelerator editorial team – Editorial review by Jay Fox, October 28, 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.
Explore the concepts
Air Barrier Material Selection: Peel-and-stick, mechanically fastened, fluid-applied or sheet goods: which barrier suits which construction context.
Air Barrier Continuity at Critical Junctions: Where most buildings fail: slab-to-wall, wall-to-roof, floor plates through masonry, and the handoffs between trades.
Penetration Sealing: Sealing penetrations without sloppy spray foam, with service cavities that keep wires and pipes inboard of the air barrier.
Blower Door Testing Methodology: A four-stage testing protocol that gates each construction phase, from air-barrier commissioning to final compliance.
Multifamily Compartmentalization: Isolating each apartment on its own pressure plane.
Retrofit Airtightness Strategies: Air sealing a building that already has its own opinion about where the air goes.
Airtightness in Prefab and Factory Construction: Panels arrive pre-tested, so assembly on site becomes the variable.
Airtightness Training and Workforce Development: Training programs that build the hands-on skill to seal buildings at scale.
Vapor Control and Smart Membranes: The membrane that handles air also has to handle moisture.
Why it matters
Airtightness is one of the core design principles of Passive House, alongside continuous insulation, construction free of thermal bridges, high-performance windows and doors, and fresh air with heat recovery. The airtight layer works like a windbreaker, stopping outdoor air from pushing through the envelope. Start with energy. Air leakage is wasted energy: heated air slips out in winter, cooled air slips out in summer, and the outdoor air that replaces it has to be conditioned all over again. Stopping that flow is central to Passive House's ultra-low energy use, and it lets heat-recovery ventilation do its job, because air that leaks in through the walls never passes through the heat exchanger.
Airtightness is also what keeps the envelope sound. The stakes rise in retrofits that add insulation to the inside of old walls, because insulating against the indoor heat leaves the existing wall colder through the winter. Warm, humid indoor air that leaks into the assembly can condense and freeze on those cold surfaces, and over years that is how a well-insulated wall rots. An unbroken air barrier keeps that moisture out of the assembly in the first place.
Then there's how the building performs for the people inside. Paired with filtered, balanced ventilation, an airtight envelope means nearly all incoming air passes through the filters instead of seeping through the walls, which matters most in cities and in regions prone to smog or wildfire smoke. A tight, well-insulated building also holds its heat when the grid fails. When the February 2021 freeze cut power in Austin for three days, a passive house retrofit read 62°F inside on the first morning, with 9°F outside, while a neighboring house that had been identical before the retrofit sat at 36°F. That is passive survivability: a home people can stay in through an outage.
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. In design, teams run the "red pencil test," checking that an air barrier line can be drawn around each cross-section of the building without the pencil ever leaving the paper. In the field, sheet membranes, fluid-applied membranes, tapes, and sealants have to transition without interruption between the components of the envelope, and a blower door test verifies the result. Air sealing specialists from around the world stress the importance of testing the air barrier as you build, for a simple reason: 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.
Get this layer right and everything above it works. Get it wrong and the rest of the building can't compensate.