Building Science
Air Barrier
The layer or system that stops air moving through the enclosure. It only works where it is continuous, so its detailing at joints, transitions and penetrations matters more than the material chosen.
In more detail
An air barrier is the layer, or set of layers, that stops air moving through the enclosure. It can be sheet membrane, fluid-applied coating, taped sheathing, sealed drywall or spray foam — the material is a means, and the requirement is continuity.
Four properties define one. It must be air-impermeable, continuous across the entire enclosure, structurally supported against the pressures it sees, and durable for the life of the assembly. Continuity is where real buildings fail: the field of a wall is straightforward, the transitions are not — sheathing to foundation, wall to roof, around windows, at ledgers and balconies, and at every plumbing, electrical and mechanical penetration.
An air barrier is not a vapor barrier, though one material can serve both roles. Air movement carries far more moisture into an assembly than diffusion does, which usually makes the air barrier the more consequential of the two for durability. Verification is a whole-building measurement — a blower door result in CFM50 or ACH50 — rather than an inspection of the material.
Why it matters
Airtightness sets the ceiling on everything else in a house's air strategy. Ventilation rates cannot be controlled in a leaky enclosure, filtration is undermined by unfiltered infiltration, and pressure imbalances pull air from crawlspaces, attics and attached garages. It sets comfort too: drafts and cold interior surfaces are frequently leakage rather than insufficient insulation. Air barrier detailing is drawn and inspected during framing, and it is the item on which a blower door test most often disappoints.
The common misunderstanding
That insulation stops air. Most cavity insulation — fiberglass, mineral wool, cellulose — filters air rather than stopping it, and air moving through insulation carries heat and moisture past it. A well-insulated wall with no continuous air barrier can perform far below its rated R-value and accumulate moisture wherever the leaking air condenses.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.