Air Barriers

Air barriers are materials or assemblies installed to control the movement of air through the building envelope. Their primary purpose is to limit uncontrolled air leakage driven by wind, stack effect, and mechanical pressure differences, thereby improving energy efficiency, durability, and indoor environmental quality. Without an effective air barrier system, buildings may experience drafts, increased heating and cooling loads, reduced comfort, and moisture accumulation within assemblies.

Uncontrolled air leakage can transport significant quantities of water vapor into wall and roof cavities, where condensation may occur. In fact, air leakage typically moves an order of magnitude more moisture than vapor diffusion, making air control the most critical component of enclosure moisture management.

Air barriers function by creating a continuous plane of airtightness across walls, roofs, and floors so that conditioned air remains inside and unconditioned air remains outside. Although some air barrier materials may also resist vapor diffusion, their primary function is air control rather than vapor control. Vapor retarders, by contrast, are intended to restrict the diffusion of water vapor through materials. They address moisture transport by diffusion rather than by air movement.

While both systems influence moisture behavior, they serve distinct functions and should not be considered interchangeable. Some materials may perform both roles—for example, fully adhered membranes or certain sheathing systems—but not all air barriers are vapor retarders, and not all vapor retarders provide adequate air control. Proper placement and detailing must therefore respond to climate, assembly design, and drying potential.

The building envelope must incorporate a continuous air barrier system that is durable, continuous, and fully sealed at transitions. Per the IECC, approved air barrier materials must demonstrate an air permeance not exceeding 0.004 cfm/ft² at 0.3 in. water gauge, when tested in accordance with ASTM E2178.

Assemblies may also be evaluated using ASTM E2357, while whole-building airtightness may be verified through blower door testing. Continuity is essential. The air barrier must be fully connected to adjacent components—including foundations, roofs, windows, doors, and penetrations—with all joints, seams, and transitions sealed in accordance with manufacturer instructions. Because air leakage is the dominant mechanism for moisture transport, air barrier continuity is often more important to enclosure durability than the selection of a specific vapor retarder class.