Are Vapor Barriers Required or Recommended?

All you need to know about vapor barriers.

Wall foundations in landscaped garden with plastic sheeting covering new brick walls
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What is a Vapor Barrier: Vapor Barrier for Homes

Vapor barriers are used in many construction projects as a method to reduce the rate at which vapor can move through a material. Vapor barriers, when installed properly, could reduce the condensation problems and will reduce air leakage on fiberglass insulated walls. Let's say you don't want to install the barrier, then water or moisture will be trapped in the wall producing moisture and other related problems.

The vapor barrier can be installed in many areas such as walls or slabs but ultimately the installation process will depend on the type of plastic being used and the climate on which the building will be constructed.

Three Different Vapor Barriers

Vapor barriers are measured in terms of much water or moisture will pass through the material. The International Residential Code defines three major groups of vapor barriers:

Class I vapor retarders provides less than 0.1 perm and the following materials can be considered as such.

  • Glass
  • Sheet metal
  • Polyethylene sheet

Class II vapor retarders controls the rate between 1 and 0.1 perm and is applicable to the following materials:

  • Asphalt coated paper
  • Plywood
  • Bitumen coated kraft paper

Lastly, class III vapor retarders (between 10 and 1 perm) can be applicable to:

  • Gypsum board
  • Fiberglass insulation (unfaced)
  • Cellulose insulation
  • Board lumber
  • Concrete block
  • Brick
  • House wrap

    Vapor Barrier Installation in New Construction

    As mentioned previously, the environment plays an important role on the vapor barriers are used. If you are working in temperatures that range between 60 to 90 degrees, you can opt to use gypsum board and plaster as substitutes for the vapor barrier. However, in extreme climates, vapor barriers such as concrete blocks and bricks shall be used or are recommended.

    These materials will perform better when they are installed towards the warmer side of the wall. In cold weather, vapor barriers should be installed towards the interior of the structure but always, independent of the weather, vapor barriers should form a continuous layer of protection. It is extremely important to seal and protect every opening reducing the probability of a leakage.

    Problems Affecting Vapor Barriers

    If vapor barriers are not sealed properly, the insulation will absorb vapor, reducing the capacity of thermal resistance, and will probably generate mold in the wall. Vapor barriers should not be installed on both sides of the wall as it will probably trap large amounts of moisture. Also be sure to control traffic or minimize it once the vapor barrier has been installed. Sometimes objects, rebars, and just people walking over the product might punctuate and damage the material. Another important area is that improper sealing might affect performance, as it happens normally along the edges as it is not sealed against floor to wall joints.

    Can Vapor Barriers Be Installed in Existing Buildings?

    Most of the time you will be called to remodel an existing building and you will note that there is no vapor barrier installed.

    At that point, you might choose to seal the existing leaks instead of installing a new vapor barrier to minimize moisture movement in your building. If you are working in colder climates it is preferred to use vapor barrier paints, those that contain a high percentage of solids and thickness when applied. Paint manufacturers will annotate their perm rating, but remember that glossy paints perform better than flat paints, and acrylic paints too. If you are not completing a major remodeling project, it would be better and more practical to apply additional coats of paint, performing the same function as the vapor barrier. Remember that this has the ultimate goal of controlling moisture in basements, ceilings, crawlspaces, walls, and floors.

    Vapor Barrier Installation Tips

    If you are planning to install vapor barrier follow these tips:

    • Vapor barriers must be completely sealed.
    • Tape all seams with tuck tape, expensive, but there is no substitute.
    • Use plastic boxes around electrical boxes.
    • Caulk all wire and plumbing pipes penetrations through the vapor barrier.
    • Use acoustic sealant in corners and uneven surfaces.
    • Attached vapor barriers to insulation, such as glass fiber batts or blankets.
    • Vapor barriers should be placed around the perimeter of the building just under the exterior finish.
    • Note that when pouring concrete over vapor barriers will take more time to cure.