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What is a Vapor Barrier: Vapor Barrier for Homes

All you need to know about vapor barriers.


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What is a Vapor Barrier: Vapor Barrier for Homes

Vapor barriers or vapor diffusion retarders is a material that reduces 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. If a vapor barrier is not installed correctly, vapor will be trapped in the wall producing moisture and related problems. The type, amount and place where you will need a vapor barrier are related directly to the climate and type of building materials used in your construction.

Three Different Vapor Barriers

Vapor barriers are measured in perms or permeability and the International Residential Code describe three different classes of vapor barriers. The three major groups are:

  1. Class I vapor retarders (less than 1 perm):
  • Glass
  • Sheet metal
  • Polyethylene sheet
  • Rubber membrane
  • Class II vapor retarders (between 1 and 10 perms):
    • Unfaced expanded or extruded polystyrene
    • 30 pound asphalt coated paper
    • Plywood
    • Bitumen coated kraft paper
  • Class III vapor retarders (greater than 10 perms):
    • Gypsum board
    • Fiberglass insulation (unfaced)
    • Cellulose insulation
    • Board lumber
    • Concrete block
    • Brick
    • 15-pound asphalt coated paper
    • House wrap

    Vapor Barrier Installation: New Construction

    The type of vapor barrier being used depends on the type of weather where the construction will be made. In moderate weather, around 60 to 90 degrees it is possible to use gypsum wallboard and plastering as vapor barrier elements. However, in extreme climates, vapor barriers related to the third group are recommended for new buildings. The closer they are placed towards the warmer side of the wall, the better they will perform. For example, in cold climates vapor barriers should be installed towards the interior of the structure, while in hotter climates vapor barriers should be installed toward the exterior side of the wall. Vapor barriers should form continuous layer of protection, and every opening must be sealed accordingly to reduce leakage. 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.

    Vapor Barrier Installation: Existing Buildings

    Vapor barriers should be installed at the moment the building is being constructed or when it will undergo extensive remodeling. You might choose to seal leaks instead of installing new vapor barrier to minimize moisture movement in your building. In colder weather it might be necessary to use vapor barrier paints, paints that have high percent of solids and thickness in application. Normally, paints will indicate their perm rating, but remember that glossy paints perform better than flat paints, and so acrylic paints too. If you are not thinking about a major remodeling project, you should try to apply additional coats of paint that will act as vapor barrier. Vapor diffusion retarders will control moisture in basements, ceilings, crawlspaces, walls, and floors.

    Vapor Barrier Installation Tips

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

    • Vapor barrier must be completely sealed
    • Housewrap are not a substitute to Vapor Barriers
    • Tape all seams with Tuck Tape, expensive, but there is no substitute
    • Tape all the seams with special tape
    • 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
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