The difference between air barrier and vapour barrier systems is often misunderstood. These are two distinct components of a building’s assembly, and both have a different purpose. The function of an air barrier is to stop air leakage between a conditioned space and an unconditioned space, while the vapour barrier prevents the diffusion of vapour through an assembly. The National Building Code of Canada specifies that the principle air barrier material may have a maximum air permeance of 0.02L/s·m2 (liters per second per square meter) @ 75 Pa. The BC Building Code defines the acceptable materials suitable for a vapour barrier as a material with a vapour permeance of <60ng/Pa·s·m2 (<1.0 US perm).
We typically see polyethylene (poly) used as both the air and vapour barrier. However, we’re quickly leaving those days behind as Energy Codes continue to get more strict on airtightness. Builders are also using alternative, more advanced air and vapour barrier strategies. The air barrier must be continuous and sealed to cover the entire building enclosure. According to the Canadian Home Builders Association’s Builder Manual:
“The vapour barrier however needs only to be continuous, but the joints and penetrations do not need to be sealed. The vapour barrier must cover as much area as possible.”
In our cold, Canadian climate, the vapour barrier must be installed on the warm side of the assembly. This will help to prevent condensation from forming. The air barrier, on the other hand, can be installed anywhere in the assembly. Specific tables can be found in the building code. These define where the vapour barrier should be located.
“…at a location where the ratio between the total thermal resistance of all materials outboard of its innermost impermeable surface and the total thermal resistance of all materials inboard of that surface is not less than that required by table 188.8.131.52.”
How does water vapour (moisture) travel?
Water vapour is driven through the building envelope by air movement. This is controlled by the air barrier system. Moisture movement can be further prevented via vapour diffusion, which is controlled by the vapour barrier.
Why is airtightness so important?
Air leakage through a 2x2cm hole in 1m x 1m sheet of drywall will result in the accumulation of 30 litres of water over the course of one heating season. Vapour diffusion through the same sheet of drywall with no hole will result in the collection of only 0.3 litres of water over the course of the heating season. This is why air-sealing our homes is critical in the mitigation of moisture-related issues, especially in Canada’s colder climates. Building airtight is also the most cost-effective way of saving energy as it minimizes heat loss due to air transmission.