Photo Source: Summerhill

Implications of improper installation or failure due to damage:

If a vapour retarder is unsealed, or has any tears or protrusions then moisture can enter the building envelope, resulting in moisture damage to the insulation, wood, or other building materials.

If the vapour retarder is also acting as the air barrier, then the airtightness of the house will be greatly affected. This will cause an increase in air changes per hour of the home. It is important to keep this in mind, especially when working on an energy efficiency program such as EnerGuide, R-2000, ENERGY STAR®, NetZero, or Novoclimat 2.0 as they require an airtightness test and some have airtightness requirements.

Vapour retarders that also act as the air barrier are required to be sealed as per the requirements for an air barrier. Remember that air movement carries 100 times more moisture than vapour movement does.

Seams in the vapour retarder must overlap and be sandwiched together, ideally between framing members and wall finish (e.g. drywall). Use of tape and sealants alone where sandwiching does not occur tend to fail over time and the resulting leaks can cause major damage (e.g. acoustical sealant tends to crinkle and separate over time when not sandwiched).

The Simple Truth: The vapour retarder protects the wall cavity from moisture damage due to vapour diffusion.