Required Barrier to Air Leakage
(1) Wall, ceiling and floor assemblies that separate conditioned spaces from unconditioned spaces or from the ground shall be constructed so as to include an air barrier system that will provide a continuous barrier to air leakage,
(a) from the interior of the building into wall, floor, attic or roof spaces sufficient to prevent excessive moisture condensation in such spaces during the heating season, and
(b) from the exterior inward sufficient to prevent moisture condensation on the room side during the heating season.
(2) The continuity of the air barrier system shall extend throughout the basement.
Article 18.104.22.168. Ontario Regulation 332/12 Building Code, Information published by oncodes.ca for educational purposes only.
Air Leakage Control
The air barrier system must be continuous around the entire surface that separates the conditioned (heated or cooled) environment from the unconditioned (outdoor) environment. Therefore, the components that make up this “environmental separator” (such as walls, windows, doors and membranes) must be sealed to each other to make the air barrier system airtight. Consideration must be given to how the connections are made, for example, around rim joists between floors during the design. The air barrier system must be capable of resisting wind loads.
For house construction in Canada, the primary material used in the air barrier system is most often 0.15 mm (6 mil) polyethylene installed on the interior (warm side) of the insulation with all joints and penetrations taped or sealed. In this case, the polyethylene also serves as the vapour barrier. It resists wind loads by being supported by the insulation and the drywall.
Wood-frame houses should be constructed as airtight as possible. In addition to protecting the building envelope, airtightness promotes energy efficiency, eliminates thermal comfort issues related to drafts, allows for better control of natural and mechanical ventilation, reduces the transmission of outdoor noise and controls odours.
Air leakage through the building envelope can also result in water vapour condensing on cold surfaces, and this moisture can lead to deterioration of materials within the building envelope if allowed to accumulate for sufficient time. Airtightness is cumulative—good workmanship in the construction of each element of the entire building envelope is essential to its proper performance.
Air leakage through the building envelope occurs as a result of differences in air pressure from inside to outside the building caused by the operation of fans or the action of wind. When the air pressure is greater inside than outside, air will flow outwards through any holes or cracks in the building envelope, carrying with it any water vapour it contains. This is called “exfiltration.” The reverse, called “infiltration,” is true when the air pressure outside is greater than inside, and air will flow in through holes and cracks.
Source : Canada Mortgage and Housing Corporation (CMHC)