The vapour barrier is an important component of the house envelope because it provides the structure and insulation some protection from moisture damage. The vapour barrier must resist the flow of water vapour from the interior, be durable and be located on the warm side of the insulation. In some cases it may be part of the insulation or air barrier. It should be easy to install and appropriate for other work being done on the home.

Install the vapour barrier to protect the entire surface areas of insulated walls, ceilings and floors. Give special attention to all penetrations, joints and junctions (e.g. light fixtures, wall plugs, switches and window frames) and areas that receive high humidity such as bathrooms and kitchens.

Usually located on the warm side of the insulation, the vapour barrier can also be installed part way into the wall, provided that no more than one third of the insulating value of the wall is on the warm side of the vapour barrier. (For information about the 1/3 - 2/3 rule, see Control of moisture flow.)

This amount of insulating value should be reduced to one quarter or less in very cold climates or in buildings with high moisture sources such as swimming pools. Like an air barrier, the vapour barrier can be made of various materials, including existing building components such as plywood, OSB, paint or vinyl wallpaper. In most older homes, the layers of oil-based primer and paint can function as an adequate vapour barrier for walls and ceilings.

Vapour barrier performance

The effectiveness of a vapour barrier (also known as a vapour diffusion retarder) is measured in terms of its permeability (perm) rating. The lower the perm rating, the more effective the moisture barrier will be. Materials considered to be effective vapour barriers include polyethylene, aluminum foil, polamide film (smart barrier), oil-based and latex vapour barrier paints (varying by type and thickness, some types and thickness of insulation and even some vinyl wallpapers).