While a building can get away with one vapor barrier, experts recommend more than one air barrier system. Why more than one air barrier? It's what the specialists call the 'belt and suspenders' approach. The typical Canadian climate puts specific requirements on air barriers. As buildingscience.com explains,

"Air barrier systems can be located anywhere in the building enclosure - at the exterior surface, the interior surface, or at any location in between. In cold climates, interior air barrier systems control the exfiltration of interior, often moisture-laden air. Whereas exterior air barrier systems control the infiltration of exterior air and prevent wind-washing through cavity insulation systems."

As stated earlier, air barriers must be continuous. That means no leaks, cracks or penetrations. Therefore attention to detail during installation is important:

• Crews must be vigilant.
• The components themselves must be impervious to air penetration and durable.
• If they are damaged, they must be capable of being repaired.