Location of the Air Barrier System

Because air movement through holes and cracks in the building envelope plays a much greater role in transporting water vapour than vapour diffusion through materials (in the order of 30 times more), the air barrier system is more important than the vapour barrier in preventing moisture from moving through the building envelope.

The air barrier system can be located anywhere in the assembly. However, in Canadian wood-frame house construction, a combined air barrier and vapour barrier is often provided by 0.15 mm (6 mil) sheet polyethylene under the gypsum board wall and ceiling finish that is sealed at joints, penetrations and interruptions, such as windows and floor junctions. When combined in this manner, the air and vapour barrier must be located on the warm side of the insulation to minimize the possibility of condensation in a wall or roof cavity due to vapour diffusion.

There may be reason for separating the air barrier system from the vapour barrier. For example, it may be beneficial to locate an air barrier on the exterior side when insulation is added outside the wall framing.

However, when an air barrier membrane is placed outside the insulation (for example, housewrap), it must be permeable to vapour in order for the assembly to dry towards the exterior. This is explained in more detail in the “Vapour Diffusion Control” section.

Polyethylene is commonly used for the air barrier in roofs and attics. It is usually attached to the roof framing on the warm side of the attic insulation and to the wall air barrier. For the polyethylene to perform as an air barrier, it must be structurally supported. In typical house construction, this is accomplished by having the sealed polyethylene layer supported on one side by drywall and on the other by framing and insulation.

The concrete basement wall can form part of the air barrier system and should be sealed to the concrete floor and to the air barrier for the upper floors. If radon or soil gases are present, they can be controlled by sealing the concrete floor to the walls, and by sealing at drains, cracks and penetrations.

Source : Canada Mortgage and Housing Corporation (CMHC)