Installing a vapour barrier

Moisture problems such as rot and mould can be avoided with the proper installation of a vapour barrier.

What happens in the winter, the air in a house has a higher rate of moisture than the outside air and the moisture inside the building seeks to escape. To prevent this water vapour from creating condensation in the envelope of the building, promoting rot and mould problems and eventually structural damage.
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Types of vapour barriers
1. Cover the back of all switches and outlets
2. Apply the sealant
3. Staple the sheet
4. Press the sealant
5. Make seams
6. Make openings for electral boxes
7. Prepare for doors and windows
8. Cover the insulation
9. Install strips astride the partitions

Types of vapour barriers

The vapour barrier is not to be mistaken for air barrier material that covers the outside of the intermediate envelope of the building (fibreboard). Its function is to stop much of the air movement through the building walls while still allowing what water vapour that does get in to diffuse back out again.

The vapour barrier ensures a waterproof barrier between the exterior and interior of the house. It is installed over the insulation. This 6 mil (0.15 mm) thick polyethylene CGSB membrane is sold in rolls 8 or 12 feet wide to match wall height.
There are also "reflecting" vapour barriers available on the market. These may be foil-backed kraft paper in which the aluminum reflects heat back inside the house. These types of vapour barriers have been losing popularity to polyethylene sheets. Being less resistant, they are offered in narrower rolls, making installation more complicated. Also, the greater number of seams required increases the risk of moisture infiltrating the walls.

Whatever type of vapour barrier you use, remember that its function is to block water vapour from reaching the insulation. Keep the number of seams to a minimum and make sure that all seams are perfectly sealed. Also check with your local building department and see what is required by code.

1. Cover the back of all switches and outlets

Start by covering the back of all switches and outlets (on the outside walls). Cut a piece of polyethylene and make a hole in it to run the wires. Apply sealant to the back of the electrical boxes and press on the polyethylene sheet to seal completely. You will come back to these components after installing the vapour barrier on the wall. There are specially designed electrical boxes for outside walls, the back of which is pre-sealed. A great time saver!

2. Apply the sealant

Apply a bead of acoustical sealant along the cap and sill plate to which you will later apply the vapour barrier.

3. Staple the sheet

Stand the roll near the wall in such a way as to be able to unroll it easily along the length of the wall. As you unroll, staple the sheet to the studs and/or sill or cap plate as required (and as infrequently as possible to reduce the number of perforations in the vapour barrier).

4. Press the sealant

Press along the previously applied sealant bead to seal the sheet to the frame.
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