Air Barrier Systems vs Vapor Barriers

The fact that many vapor barriers also retard or eliminate air flow sometimes causes confusion. In fact, much of the older literature confuse or combine the function of the ABS and vapor barriers, and the difference between the two is still one of the most common building science questions. Hence, the distinction will be presented here once again.

The function of a vapour barrier is simply the control of water vapor diffusion to reduce the occurrence or intensity of condensation. As such, it has one performance requirement: it must have the specified level of vapour permeance and be installed to cover most of the area of an enclosure.

Many codes require the use of a vapor barrier in all enclosures. Some codes wisely require that vapor diffusion be controlled when an assembly “would be adversely affected by condensation.”  The need for a specific vapor barrier layer can be assessed by simple calculations, and rarely is a layer with very low permeance like polyethylene sheet justified.

Air barrier systems control air flow and thereby control convective vapor flow. As can be seen from the previous sections, the control of air flow provides other benefits and has at least five performance requirements to meet.

Canadian building codes require an air barrier system in all enclosures that would be adversely affected by condensation. In practise, this means air barriers are required for almost all conceivable types of building enclosures, especially since they provide more than just control of condensation.

The vapor permeance of the air barrier must be considered in the same way as all other materials in an assembly should be. For example, in cold climates, a vapor barrier on the exterior is usually not acceptable (but can be designed for, as it is in an exposed membrane low-slope roof or a wall with metal cladding), whereas in hot humid climates, this location would be desirable. But the vapor permeance of the ABS is no more important than the vapor permeance of any other materials in an assembly, such as the cladding, sheathing, insulation, interior finish, etc. 

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