Air Barrier Fundamentals and Building Science
An air barrier is a continuous, structurally supported, airtight system separating conditioned from unconditioned space. Candidates must distinguish three control layers that exam questions routinely conflate: the air barrier (controls bulk air movement), the vapor retarder (controls diffusion), and thermal insulation. One material may serve several functions, but the performance criteria and continuity requirements are separate, and a compliant air barrier is not automatically a vapor retarder, or the reverse.
Air moves only when a pressure difference exists. The drivers are wind, stack effect (buoyancy from indoor-outdoor density differences), and mechanical system imbalance. Flow goes from high pressure to low pressure; in winter the typical pattern is outward at the top of a building and inward near the bottom, with a neutral pressure plane between. Air leakage carries far more moisture than diffusion in most enclosures, which is why one unsealed penetration can wet a cavity that a well-detailed vapor retarder never could.
Continuity is the concept examiners test hardest. The barrier must be continuous across roof, walls, and foundation, and at every penetration and transition, and must be supported against design pressure without displacement. A frequent failure is a boundary that stops at a top plate, a dropped ceiling, or a party wall.
Trap: treating "I sealed the inside" or "the sheathing is taped" as a complete strategy without confirming that the specified boundary is the one actually closed.
Rule of thumb: a 20 m building with roughly a 25 K indoor-outdoor difference generates about 20 Pa of total stack pressure, or approximately 10 Pa from the neutral plane to the top. That is enough to drive real leakage through gaps a hand dismisses as insignificant.