Air movement is the major factor in transporting moisture through building envelope assemblies. Many building envelope problems can be attributed to inadequate or failed air barriers. The National Building code of Canada has required air barrier systems since 1960. Unfortunately, there is considerable confusion between what is an air barrier and what is a vapour barrier. The two are distinct functions that, in some situations, may be satisfied by the same material components. In 1990, the wording of Part 9 of the Code was modified to clarify and separate the functional requirements for air barrier systems and vapour barriers. The Code now requires that the air barrier system provide an effective barrier to air exfiltration under differential air pressure due to stack effect, mechanical systems and wind. Discusses essential features of an air barrier, tightness, and air barrier joint materials. States that silicone based sealants and adhesive tape achieved the best performance under all conditions. Spun bonded olefin paper and acrylic base sealants should not be used at connections where the temperature may be hot. Staples to attach the spun bonded olefin should be avoided. Because of their high air permeability, open cell gaskets, mineral wool and perforated polyethylene membranes should not be used. The use of closed cell gaskets is not recommended because of problems with long-term performance.