Vapor retarders can be an intrinsic and crucial part of the building envelope and help manage moisture issues to keep the home dry. However, there are some instances when vapor retarders (materials that slow or stop the flow of water vapor) should not be installed because they can increase the likelihood of condensation issues.
Condensation problems can occur in below-grade walls when vapor retarders are installed on the interior side of air permeable insulation. A concrete foundation below grade can allow water to pass through the concrete from the damp earth outside. If the insulation material is air permeable (such as mineral wool, fiberglass, or cellulose insulation) water vapor will pass through and condense on the vapor retarder. This retarder can include material attached to the insulation, such as foil-faced paper, traditional vapor retarders like polyethylene film, or vinyl wallpaper on the interior surface of the wall. When this condensation occurs, the liquid water has no way to dry to the interior of the home and will accumulate. Over time this dampness can lead to ruined insulation, mold, and structural rot of framing members.
To reduce the risk of condensation in below-grade walls (i.e., basement walls), you must:
- Understand perm ratings and how to select the right wall assembly to reduce condensation risks.
- Construct an appropriate assembly for the inside of a below-grade wall.
Understanding Permeability Ratings on Vapor Retarders
If water vapor is allowed to pass through the below-grade wall and insulation, it must be allowed to dry inward in order to avoid condensation. To do this, the wall covering on the interior of the framing must have a relatively high vapor permeability rating.
The vapor permeability (commonly referred to as breathability) is a material’s ability to allow water vapor to pass through it. The moisture vapor transmission rate (MVTR) is the measurement referenced in building codes. The MVTR is measured in a lab using the American Society for Testing and Materials (ASTM) procedure E-96. The test method measures how much moisture vapor is allowed to pass through a material in a 24-hour period (adjusted for vapor pressure across the sample). The resulting number is the moisture vapor permeance (MVP). The unit of measurement for MVP is perms. The higher the perm number, the more moisture vapor the material will allow to pass, and the better drying the material allows. The water vapor permeability of a material is roughly inversely proportional to its thickness (i.e., doubling the thickness halves the permeability).
Vapor Retarder Code Classifications
According to the IRC 2009 and 2012 definitions, vapor retarder classes are defined as follows:
- Class I: 0.1 perm or less
- Class II: 1.0 perm or less and greater than 0.1 perm
- Class III: 10 perm or less and greater than 1.0 perm.
The following materials are typically rated as Class I and therefore should not be used on the interior side of air permeable insulation in below-grade exterior walls in any climate:
- rubber membranes
- polyethylene film
- aluminum foil
- sheet metal
- foil-faced insulating sheathings
- foil-faced non-insulating sheathings.
Note that this list is not comprehensive, and other materials with a perm rating of 0.1 or less should not be used. Also, if manufacturer specifications for a specific product indicate a perm rating above 0.1, then the material may be used even if it is on this list.
Figure 1 - Mold growth. The image above shows mold growth on the interior of the vapor retarder. Water passing through the foundation wall is unable to dry to the interior resulting in compromised indoor air quality.
Selecting the appropriate vapor retarder
The main point to remember when using air permeable insulation in below-grade walls is that moisture must be allowed to dry to the inside of the building or it will become trapped and condense on the vapor barrier. Specifically remember the following:
Select a vapor retarder that is Class II or higher (greater than 0.1 perm) for the interior of a below-grade wall with air permeable insulation.
Install a Class III vapor retarder in areas of excessive soil moisture which will allow for the greatest amount of drying and is often the best choice.
Foam insulation can be used if the manufacturer specifications indicate a perm rating greater than 0.1 and if the wall finishes are vapor permeable, such as latex paint.
Figure 2 - Below-grade wall assembly. This spray foam insulation (with a perm rating greater than 0.1) is covered with gypsum board, which is a Class III vapor retarder. The gypsum should be covered with permeable latex paint to allow for drying to the interior.
A few exemptions to these requirements apply:
In some situations there are exemptions to the perm rating rule when it comes to vapor retarders used in below-grade walls. These exemptions include:
Showers and tubs on exterior walls: Class I vapor retarders, such as ceramic tile, may be used at shower and tub walls. The insulation behind the tub or shower should be equivalent to the insulation in the rest of the exterior walls and should be covered with an air barrier of cement backer board, rigid foam insulation, or non-paper-faced drywall that is sealed at the edges and seams to provide a continuous air seal. The recommended backer for exterior walls behind showers and tubs (in both above- and below-grade situations) is cement board. Note that cement board is not waterproof and must be coated with a fluid-applied waterproofing, or a water-resistive barrier must be applied behind it that allows drainage (BSC 2009
). The low-perm vapor retarder is allowed due to the high moisture content created by the shower or tub and is used to protect framing and insulation from the excessive moisture.
Class I vapor retarders, such as mirrors, may be used if they are mounted with clips or other spacers that allow air to circulate behind them. Unlike other wall coverings like ceramic tile or vinyl wall paper, mirrors can be installed with enough space behind them to allow for moisture to migrate through the air permeable insulation and dry to the interior. If the mirror is directly fastened to the wall, it will act as a vapor barrier, and condensation will form on the foundation side of the mirror.
Special note about existing homes
In retrofit situations, accessing the exterior of a below-grade wall may be cost prohibitive or impossible. When this happens, there are several steps to follow to create an appropriate assembly for the inside of a basement wall.
Clean the interior of the basement wall and install 1-inch or greater of plastic foam board or closed-cell polyurethane spray foam followed by a stud wall. The foam board serves two purposes: providing a capillary break between the foundation wall and the stud wall, and keeping warm, humid basement air away from the earth-chilled foundation.
Install fiberglass insulation into the framing cavities, making sure to fill all spaces but being careful not to compress the insulation.
Cover the wall assembly with moisture- and mold-resistant gypsum board.
Paint the gypsum board with two coats of latex paint.
Special note: It is good practice to put 1 inch of foam board beneath the sole plate of the stud wall and the basement floor and to leave a ½-inch gap between the basement floor and the bottom of the gypsum board. Doing so provides protection from minor floods and damp concrete floors (EPA 2012).
Figure 3 - Rigid foam against the foundation. Rigid foam insulation is installed directly in contact with the below-grade basement wall. This stops water vapor from passing into the home and also keeps warm and moist interior air from condensing on the cold walls.