Should I Put a Vapor Barrier on an Inside Wall?

Vapor Barriers on Inside Walls

• In general, it is never a good idea to install a vapor barrier on a wall on the inside of a home. Condensation forms when warm air -- such as the air inside your home -- makes physical contact with a colder surface. The moisture content of the air condenses and forms visible droplets of water. Therefore, a vapor barrier will only be effective if it is installed on the warm side of a wall that contacts colder air outside. To install a vapor barrier on an inside wall would simply be a waste of money and construction materials since condensation is unlikely to collect on an inside wall.

Proper Location

• For your vapor barrier to actually work, you need to install it on the warm side -- the inside of the house -- of an exterior wall. For most residential installations, the ideal location for a vapor barrier is the area between the framing and the interior sheathing or wall finish. However, the Environmental Protection Agency warns that vapor barriers should not be installed on interior walls with "high condensation potential," such as an interior stud wall erected next to a below-grade basement wall and insulated with mineral wool, fiberglass or cellulose insulation. In these cases, EPA recommends omitting the vapor barrier altogether.

Other Areas

• Of course, exterior walls are not the only areas where vapor barriers can be useful. Vapor barriers are commonly installed underneath concrete slab foundations to prevent the concrete's making direct physical contact with moisture in the soil -- this moisture can seep into the home through cracks and imperfections in the concrete. Vapor barriers in basements and crawl spaces are used to the same effect.

Suggested Materials

• Any construction materials have some degree of resistance to vapor movement, but materials such as polyethylene film, asphalt-coated or laminated paper, kraft-backed aluminum foil, oil base or aluminum paints and vinyl wallpaper are able to resist vapor movement much more strongly than are other materials. Resistance to vapor flow is measured in perms, and the lower the perm value, the better suited it is for use as a vapor barrier. So, when shopping for materials to use for your vapor barrier, look for those with a perm rating of 1.0 or lower and avoid those with a perm rating higher than 1.0.