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

A vapour retarder is a material that reduces vapour diffusion through the building envelope. This prevents building components from getting wet and being damaged.

The vapour control layer is engineered to only allow a specific amount of vapour to pass through it, depending on the air pressure, amount of time, and surface area of the control layer.

Common practice is placing up to one third of the insulating value can be installed on the warm side of the vapour retarder, but this is not consistent with newer versions of the building codes. Depending on the assembly components, the vapour retarder may be placed closer or further from the interior of the home. Calculating the required inboard and outboard ratios for placement are covered in the advanced course.

Criteria:

• Installed to the warm side of the insulation
• Impermeable to vapour diffusion
• Must cover as much surface area as possible
• Not necessarily resistant to air flow
• Durable during construction and over expected building life
• A vapour retarder material must have a permeance of 60 ng/(Pa· s · m2) or less

Materials:

• Polyethylene
• Aluminum foil
• Some oil-based paints
• Special vapour retarder paints
• Some insulation adhesives (mastics)
• Some insulation materials
• Metal and glass
• Some exterior-grade plywood's

Placement/Location:

• Roof or ceiling
• Exterior walls above and below grade (interior or exterior side)
• Can be installed part way throughout the wall assembly
• Slab/foundation

NOTE: In the photo above you will notice the lack of acoustical sealant. The sealant is added after the poly is hung. This makes it easier and far less messy.

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).