Air barriers control air leakage into and out of the building envelope. An air barrier does not necessarily have an effect on the movement of moisture. Air barrier products may take several forms:
- Mechanically-attached membranes, also known as housewraps, usually a polyethylene-fiber or spun-bonded polyolefin , such as Tyvek is a generally accepted moisture barrier and an air barrier (ASTM E2178).
- Self-adhered membranes, which are typically also a water resistant barrier and a vapor barrier
- Fluid-applied membranes, such as heavy-bodied paints or coatings including polymeric based and asphaltic based materials
- Closed-cell medium density spray-applied polyurethane foam, which typically provides insulation as well
- Some open-cell spray-applied polyurethane foam that are of high density
- Boardstock, which includes 12 mm plywood or OSB, 25 mm extruded polystyrene, etc.
Some air barriers may be water vapor permeable, while others perform the function of a vapour barrier. This is because water fits through narrower pores than the other main constituents of air, oxygen(O2) and nitrogen.
