Are Vapor Barriers Needed Under Siding?

Moisture in Home Construction

• Moisture is the enemy of home construction materials. To prevent the constant intrusion of moisture, vapor barriers are installed behind walls, under flooring and in attics. Vapor barriers, also known as vapor diffusion retarders, reduce the rate at which water vapor can flow through materials, according to the U.S. Department of Energy’s EnergySaver website. The ability of a material to retard water vapor is measured in “perm,” a measure of the number of grains of water vapor that can pass through a square foot of material each hour.

Types of Vapor Barriers

• Plastic and foil vapor barriers are the most common types used in home construction. Often vapor barriers are combined with other building materials in an integrated moisture-protection and insulation product, such as plastic-coating foam board or asphalt-coated paper. Plywood, glass and metal can also be considered vapor barriers in that they provide a barrier to moisture. The most effective vapor barriers prevent water vapor from passing into materials yet allow air to pass through for adequate ventilation.

How Vapor Barriers Preserve Siding

• Vapor barriers may be used on interior walls in northern areas to conserve indoor heat. In humid areas such as the Gulf Coast of the United States and Florida, vapor barriers are placed on exterior walls to keep humidity out of the interior. In some southern areas, the vapor barrier may be omitted entirely. Vapor barriers are intended to keep internal moisture from seeping into siding materials, which can cause deterioration and weakening of the siding itself, as well as of the supporting materials.

Installing Vapor Barriers Under Siding

• Vapor barriers generally go onto interior walls to prevent water vapor from reaching siding materials. This vapor barrier is often combined with a “housewrap” that is gas permeable and provides an additional layer of moisture control, according to the Vinyl Siding Source website. These barriers are also used under other types of siding material. Siding manufacturers generally provide recommendations on the type of vapor barrier for use with their products. Building codes regarding the use of vapor barriers vary from state to state, so homeowners should consult their local building department for specific requirements.