Vapor Barriers Explained

by Doug Berger

Most homeowners will never worry about their vapor barrier, unless of course, they discover a problem like mold, rot or mildew. A vapor barrier, is a material used to prevent the flow of water vapor from one area to another.  Water gets into a home as either a liquid or a water vapor.  When it gets trapped inside walls it could contribute to mold and mildew growth as well as rot.  Modern homes are designed to keep water out of walls, but in the event that water gets in, they need to allow it to escape.

When your favorite meteorologist talks about the relative humidity, he is refering to the amount of water vapor in the air.  In a cold winter, your skin dries out and you get chapped lips because of the low relative humidity.  You run humidifiers in your house so it is more comfortable.  If the high humidity air inside your house permeates through your walls and insulation and touches the cold outside wall, it will condense and form liquid water.  A low permeable material on the inside of the wall helps prevent this.

If there is water inside your wall and you have a high permeability barrier on the outside of the wall, it allows the water vapor in the wall to evaporate and move out of the wall.

The materials used vary, from polyethylene plastic sheet, to vapor retarder paints and coatings (commonly used to “paint” the exterior of a basement to prevent moisture from entering).

Some materials allow less vapor to travel through than others. The ability of vapor to move from one side of a vapor barrier to another is called “Permeability”. The 2015 IRC places Vapor Barriers in one of three “Classes” of permeability:

• Class 1: Impermeable
• Class 2: Semi-permeable
• Class 3: Permeable

Which ‘class’ the material belongs to is based on its ‘perm’, or the amount of vapor that passes through the material. In order to fall into the Class 1 category, the material has a rating of 0.1 perm or less. A Class 2 rating falls between 0.1 and 1.0 perm, while a Class 3 rating is 1.0 to 10 perm. The higher the perm rating, the more vapor will flow through from one side of the material to the other. This is an important number to keep in mind, especially when managing moisture movement in a home design.

While you may think that having an impermeable vapor barrier would be an obvious choice, this is not always the case, due to geographical climate conditions. The map below, as provided in the 2015 IRC, shows the United States broken down into hygrothermal regions.  Each region has specific allowable vapor barriers that are listed in the International Residential Code (IRC).

How can you know whether a vapor barrier is impermeable or permeable? The 2015 IRC references ASTM E96 to measure the rate that water vapor passes through a barrier, then reports that in units of perm The NTA Video, ASTM E96, explains the test method very well.

If you have questions about vapor barrier testing, please contact NTA. NTA is an IAS Accredited laboratory capable of performing testing per ASTM E96, ASTM D226, ASTM D4869, ICC-ES AC38 and ICC-ES AC48.