Air Barrier
CSI Division 07, Thermal & Moisture Protection
A continuous material or system installed across a building's envelope specifically to stop uncontrolled air leakage into and out of the building, a distinct function from a [[concept-vapor-barrier|vapor barrier]], which blocks moisture vapor diffusion; an assembly can control air leakage without controlling vapor diffusion, or vice versa, and a well-designed envelope addresses both.
Why it matters
Air leakage wastes far more energy than vapor diffusion alone and can also carry moisture along with the air, so a continuous air barrier, sealed at every seam, penetration, and transition, is now a core energy code requirement on most commercial projects, not just a nice-to-have.
On a real project
A building's air barrier is installed as a continuous membrane across the wall sheathing, with every seam taped, every window and door opening sealed, and every penetration for pipes or conduits carefully sealed back to the membrane, so the whole envelope acts as one continuous air-tight boundary.
Who this matters most to
A Carpenter or a specialty air-barrier crew installs and seals the membrane, a task that requires real attention to continuity, since a single unsealed gap can undermine the whole system's performance. A Sustainability/LEED Consultant may require a blower-door test to verify the building's actual measured air leakage rate against the design target.
Where this goes wrong
A crew installs the air barrier membrane carefully across the wall field but doesn't properly seal it where it transitions around a rooftop mechanical penetration, assuming that detail is someone else's scope. That one gap becomes a significant point of uncontrolled air leakage, and a blower-door test after completion reveals the building falls short of its target air-tightness, with the failure traced back to that single unsealed transition.