Understanding Vapor Barriers

Vapor barriers, which are more precisely known as vapor diffusion retarders, or just vapor retarders, are essential to moisture control for any home. This material functions by slowing down or completely impeding the movement of water vapor through a material. While the common term vapor barrier is still widely used, the term vapor retarder is more specific, and besides, it sounds more scientific so it makes you sound smart.

A material’s ability to impede the movement of water vapor is measured in a specific unit called a “perm,” basically just short for permeability. There are three classes of water vapor retarder according to the international residential code.

Vapor retarders in the first class include materials which function at a rate of 0.1 perms or less. These include:

  • Rubber
  • Polyurethane plastic 
  • Metal
  • Glass

Class 2 vapor retarders include materials rated between 0.1 perms up to 1 perm:

  • Polystyrene
  • Paper faced with asphalt
  • Plywood

Class 3 includes materials rated between 1 and 10 perms:

  • Brick
  • House wrap
  • Concrete Blocks
  • Cellulose insulation
  • Fiberglass insulation
  • Gypsum board

Vapor barriers are useful for moisture control in the following areas:

  • Crawlspaces
  • Basements
  • Walls
  • Ceiling
  • Foundations

The use of a vapor retarder in these areas of the home is only effective in controlling moisture when used with a comprehensive air sealing technique. The use of a vapor barrier on top of a surface without sealing off gaps will not be effective. The climate you live in and the construction of your home will determine whether and where you may choose to use a vapor retarder.

Different Types of Vapor Barriers

Vapor barriers can usually be found either as a coating or in the form of a membrane. While membranes are usually thin and flexible, they can also sometimes be found in thicker varieties. 

These thicker vapor barriers are often referred to as structural vapor retarders, and include construction materials such as stainless steel, aluminum, plastics, or rigid foam insulation. These types of vapor barriers are usually fastened to the structure using hardware and sealed around all the gaps.

The thinner membrane type of vapor barriers most often are available in rolls that can be installed in the area of interest or are already included as part of the building material. These are often found as the facing on fiberglass insulation or in rolls of plastic. There are also paints and coatings that can also function as a vapor barrier.

Installing Vapor Barriers in New Buildings

If you live in an area with a more moderate climate, you may be able to use materials such as a plastic coating for your wall, or wallboard made of gypsum. However, if you live in a more extreme climate, you may need the use of a more heavy-duty vapor barrier. 

Vapor barriers should be installed as close as possible to the warm side of a structure. For example in cold climates, the vapor barrier should be installed on the interior of the building, while in warmer and wetter climates, the vapor barrier should be installed towards the exterior of the building.

The installation of your vapor barrier should be continuous and without flaws. This is of even more importance in climates with more extreme hot or cold weather. Be sure that your installation is free of any gaps, tears, openings, or punctures. Make sure to cover any and all surfaces that may be vulnerable to accumulating condensation. If your insulation becomes damp, it will be much less effective, and also expose you to the risk of developing mold or wood rot.

Installing Vapor Barriers in Existing Buildings

Installing a vapor barrier can be an expensive project, making it difficult to do in a home which has already completed the construction process.  You might consider hiring an inspector to reveal any air leaks in your home and having them filled, rather than undertaking a huge project by yourself.