Moisture problems in walls have been attributed in large measure to two mechanism: vapour diffusion, and now more importantly, air leakage, specifically the deposition of moisture by moist air exfiltrating through the building envelope. While significant technology dated to vapour diffusion control (the vapour barrier) exists (i.e. calculation procedures, regulatory requirements, material standards and performance testing procedures), there is minimal technology available related to air leakage control (the air barrier) for the repair of existing building envelopes and virtually none for the design and construction of the envelope of new buildings. This paper presents a definition of the air barrier, outlines the performance requirements, and examines the application of the theory .to several typical commercial constructions. This application could provide a functional and durable air barrier within the building envelope.