Exterior Wall Moisture Barrier

In designing exterior wall systems moisture infiltration is controlled following two basic design concepts or strategies.

Exterior wall moisture barrier.

This wall assembly will dry from the vapor barrier inwards and will dry from the vapor barrier outwards. The first design concept is the barrier system where all moisture is resisted by the exterior surface material also often. Not every wall does. The exterior face of the wall or floor cavity should remain permeable in order to allow dissipation of any moisture that does enter the wall cavity.

Dupont tyvek drainwrap is a moisture barrier designed to provide enhanced drainage in areas subject to extreme wind driven rain. A vertically grooved surface provides added protection against water by enhancing drainage away from the wall assembly. The standard installation of a plastic vapor barrier is between the studs and the drywall but there are some exceptions to this. In hot climates any moisture that condenses on the exterior side of the vapor barrier will be drained to the exterior since the vapor barrier is also a drainage plane.

Finished basements often receive a vapor barrier layer between the concrete and the floor treatment to prevent damage from heavy rains or other moisture seepage. Some moisture barriers for this application combine a foam strip with a waterproofing membrane that covers the bottom of the plate as well as the joint between the foundation and the exterior of. Very hot and humid climates may benefit from an exterior vapor barrier that keeps outside humidity from penetrating into walls. Of course you can have moisure problems here even without the exterior vapor barrier because of what bill rose calls the rule of material wetting.

If water vapor diffuses or infiltrates into the wall cavity and finds the cool surface moisture problems can occur. Either design strategy can be applied to load bearing or non load bearing walls. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter.

The cool surface is the sheathing assuming no exterior insulation. Vapor barriers are applied to the interior walls in climates which are predominantly cold while they are best applied to exterior wells in predominantly hot climates. Whether or not you need a vapor retarder hinges on three main factors. In any case the vapor barrier must point to the warm side.