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Vapour barrier. Vapour barrier paint. Polyethylene vapour barrier.
The vapour barrier is one of the most important components of the house. Without a properly installed vapour barrier, condensation will form within the exterior wall structure and, over time, lead to structural damage (wood rot) and unhealthy (mold) living conditions.
It is an absolute must that the vapour barrier is installed on the warm side of the insulation. It has to be strong and be able to stand up to abuse during construction. Seams should be sealed with a special tape, see red tape in photo.
Electrical boxes and other outlet boxes should be covered with a piece of vapour barrier before installation. Later, when the barrier is installed on the entire wall, the loose ends will be sealed to form a continues vapour barrier, see photos.
There are also airtight outlet boxes that come with a rubber seal (the black gasket in the photo). They are more expensive than the standard outlet box but are great time savers as they eliminate the need of wrapping each box in a piece of vapour barrier.
A bead of acoustical sealant should be applied along the top and sill plates to provide a good seal. Caulking is also required around electrical wires that are routed through the top plate and attic, see photo.
Perm rating. The effectiveness of vapour barrier material is measured in terms of its "perm" rating. The lower the perm rating, the more effective the vapour barrier. For new construction and renovations the material most used is 6 mill (0.15 mm) polyethylene. It is sold in rolls 8' 6" (2.59 m) and 10' (3.05 m) wide. Rolls of polyethylene should not be stored in direct sunlight over a long period of time.