Canada s best builders have learned that moisture movement in houses isn t a simple issue and that homes must be viewed as a system of interrelated components if they are to meet the expectations of today s consumers. Insulated houses require an effective air barrier and enough mechanical ventilation to maintain interior humidity at acceptable levels during cold weather. But even in houses with poor air barriers and marginal insulation, mould problems don t usually occur unless inadequate ventilation allows interior humidity levels to become excessive. In cold climates like ours, new buildings need both a vapour barrier and an air barrier. The air barrier s function, stopping air from passing from the inside to the outside of the building, is much more important than the vapour barrier s.
Plastic is an excellent vapour barrier.
It can also act as an air barrier when all joints and holes are sealed carefully. The 1995 National Building Code expressly permits alternative systems. The air tight drywall system, for example, has at least a fifteen year track record in North America. It creates an air barrier by using gaskets and sealants to seal up the joints between building materials that block air flow such as drywall, wood framing, plywood and some types of sheathing paper. Low permeability paints can provide enough vapour resistance to meet code requirements.