Air Barrier System

An air barrier is not a product but a line. It is the connected set of surfaces a designer nominates to resist bulk air movement between the occupied interior and outside, together with everything that joins those surfaces to one another. On a section it should be possible to trace that line around the enclosure without lifting the pen, and the same should hold on plan.

The nominated plane changes as it travels. It may be a taped sheathing board across the walls, a wet plaster coat behind a lining, a membrane in the roof and the structural slab at the base. Because each of those planes belongs to a different trade and often to a different work package, the barrier is defined less by what carries it than by whether the changeovers between them were designed at all.

The confusion this entry exists to remove is with the vapour diffusion control system. An air barrier resists air moving through gaps; a vapour control layer resists moisture diffusing through material. A fully taped breather membrane is an air barrier while remaining open to vapour, and a vapour control layer left loose-lapped and unsealed at its edges is not an air barrier at all, however high its resistance to diffusion.

Because performance belongs to the connections rather than to the components, the line is normally verified by pressurisation testing of the finished enclosure rather than by inspecting materials. Anything that will be buried behind finishes therefore has to be complete, checked and recorded before it is covered over.

The barrier plane and its joint treatment are not two components but one, since neither has meaning without the other. A board of high resistance to air movement contributes nothing across an open seam, and the most capable tape contributes nothing where it bridges a gap wide enough to leave it unsupported. What the pair achieves at any point is set by the weaker of the two there.

Changeovers govern the whole. Where the plane passes from a sheathing board into a roof membrane, the two materials move differently, present different surfaces and usually belong to different installers. If the detail does not say which laps onto which and what bonds them, the line simply stops at the changeover, and the enclosure behaves as though the barrier ended there even though both planes are individually complete.

Services and the barrier compete for the same plane. Every socket enclosure, waste pipe, extract duct and cable route that crosses the line turns a designed surface into a set of holes, and if the barrier is drawn only in section then the services layout decides where those holes fall. Putting a service void inboard of the line changes the interaction, because most terminations then stop in the void and only services that must pass outward reach the barrier.

Background and bonding decide whether any of this survives. Tapes and sealants transfer wind pressure into whatever they are stuck to, so the bond available at a seam is set by the condition of the background beneath it, and a friable, dusty or wet surface gives that seam little to develop against. The dependency runs the other way at movement joints, where the barrier has to be free to move with the structure instead of being stretched across a joint that will open.

Protection is a continuity matter rather than a housekeeping one. Once the line is buried, a puncture made by a following trade is indistinguishable from a design fault when a test finds leakage, and the search then runs over the entire enclosure rather than over the one area that was disturbed.