Air Barrier
An air barrier is the continuous element of a building envelope that stops air leakage — and it has to be continuous on all six sides of the conditioned space (floor, walls, ceiling) for the air-sealing to work. Air barriers are often a layer of taped sheathing (ZIP system, plywood with sheathing tape), a polyethylene or smart vapor membrane, or a self-adhering peel-and-stick sheet. The air barrier is what a blower-door test actually measures.