Use four strategies to keep the structure dry:
Provide exterior weather and moisture protection. Use building paper, siding, flashing, gutters and other construction techniques to shed water and repel wind-driven rain. Pay attention as well to below-grade measures. Proper drainage, grade slope and damp-proofing can protect the foundation from ground-water leaks or from moisture movement by capillary action.
Reduce moisture at the source. This means producing less moisture in the first place and exhausting moist air and bringing in drier air.
Prevent moist indoor air from getting into the envelope. A vapour barrier will reduce moisture movement by diffusion, and an air barrier can prevent moisture movement by air leakage. Although less moisture can be moved into the envelope by vapour diffusion than by air leakage, it is still important to provide a vapour barrier. An effective vapour barrier must be the following:
resistant to vapour diffusion
durable
installed on the warm side of the insulation
A number of building materials resist vapour diffusion well enough to be used as vapour barriers. These include polyethylene, oil-based paints and special vapour-barrier paints, some insulation materials and exterior-grade plywood. Different materials may act as the vapour barrier in different parts of the house. The same material may work as both an air barrier and a vapour barrier, provided it meets both requirements and is properly installed. Polyethylene sheets and foil-backed gypsum drywall can both combine these functions. To avoid confusion of terms, we refer to a material doing both jobs as an air and vapour barrier. As a general rule, the vapour barrier should be on the warm side of the insulation. In some cases, however, the vapour barrier can be located within the wall or ceiling assembly, provided that at least two thirds of the insulation value of the wall is on the cold side of the vapour barrier. Because this ratio should be adjusted for houses with high interior humidity or for homes in extremely cold climates, it is recommended that you consult a professional builder-renovator, who will apply the specifications outlined in the National Building Code of Canada.
Let the envelope “breathe” to the outside. This will allow the house to deal with seasonal fluctuations in humidity and to release any moisture that does penetrate the envelope from the interior or exterior. The materials of the envelope are layered, with those most resistant to vapour diffusion located on the warm side of the envelope and the least resistant (such as building paper) located on the outside. In this way, any vapour that penetrates the envelope can escape to the outside. Some wall systems work well with a relatively impermeable insulated sheathing because the interior wall-cavity temperatures are kept high. As a precaution, when retrofitting a wall, always ensure that the interior surfaces are vapour-resistant. Some siding applications have an air space immediately behind the exterior finish to promote drying out of materials that have been soaked by rain or dampness. This air space also provides an escape route for any moisture that has penetrated the wall cavity from the indoors. This type of installation should not be used with insulated siding, as convection in the air space will negate the effect of the insulated backer board on the siding.