It is a myth that installing vapor barriers is the most important step for controlling moisture in walls. Vapor barriers only retard moisture due to diffusion, while most moisture enters walls either through fluid capillary action or as water vapor through air leaks. All climates require these additional steps:

* Install a polyethylene ground cover on the earth floor of houses with crawlspaces, and slope the ground away from the foundations of all houses.

* Install a continuous vapor barrier, if your climate needs one (see the map below) that has a perm rating of less than 1.

* Place a termite shield, sill gaskets, or other vapor-impermeable membrane on the top of the foundation wall. This will prevent moisture from wicking into the framed wall from the concrete foundation wall by capillary action.

Preventing Rain Penetration

Causes of rain leaks through exterior walls include improper installation of the following:

* Siding materials

* Poor-quality flashing

* Weatherstripping or caulking around joints in the structure’s exterior (such as windows, doors, and bottom plates).

Wind-driven rain can also penetrate the exterior finish. To enhance protection against rain penetration, create a drainage plane within the wall system of the home.