WATER / MOISTURE MANAGEMENT

In general, moisture management schemes must control water in two states. First, since the soil in contact with the foundation wall is always at 100% relative humidity, foundation walls must deal with water vapor that will tend to migrate toward the interior under most conditions. Second, liquid water entry must be prevented. Liquid water can enter from sources such as:

• Uncontrolled flows of surface water
• High water table
• Capillary flow through subsurface foundation assemblies

Techniques for controlling the build-up of moisture in the basement wall assemblies are an essential component of the overall construction. Improper moisture management can lead to structural damage, damage to finishes or basement contents, and mold growth, which can be very costly to repair and hazardous to one's health.

The following construction practices will prevent excess water in the form of liquid water and vapor from infiltrating the basement. This is done by using adequate drainage and by the use of vapor retarders as shown in Figures 2-3F and 2-3S.

• Manage exterior ground and rain water by using gutters and downspouts and by grading the ground around the perimeter at least six inches of fall over ten feet of run. Install a foundation drain surrounded by gravel and enclosed by filter fabric. Apply either damp-proofing or water-proofing to the foundation walls (Dastur et al. 2005).
• Add back fill material or drainboard around the foundation that is free draining to allow ground or rain water to drain down to the perimeter drain installed at the base of the footing. There are a variety of approaches to footing drain design that are discussed in the next section.
• Add a capillary break (a closed cell foam sill sealer or gasket) between the top of the concrete and the sill plate to prevent moisture migration between the concrete foundation and the floor structure above. Similarly, to limit the amount of ground water absorbed through the footing, install a capillary break between the footing and the foundation wall (BSC 2006).
• Prevent moisture diffusion from the ground into the slab by covering the entire ground with a vapor retarder. It is recommended that the vapor retarder be in direct contact with the concrete slab and that no sand or gravel be placed in between (Lstiburek 2008).
• Include a four-inch deep, 3/4-inch diameter (no fines) stone bed above the ground and right below the vapor retarder. This functions as a granular capillary break below the vapor retarder, a drainage pad, and an air pressure field extender for the soil gas ventilation system.

Concrete foundation walls contain water from when they were poured which needs to be dissipated by allowing them to dry. In cases where the majority of the wall is below grade, it can only dry to the interior. The insulation material and the wall covers placed on the walls during the construction of the crawl space act as vapor retarders, not allowing the walls to dry to the interior. For this reason, it is recommended that these wall coverings be installed near the end of construction to allow for as much drying of the concrete as possible (BSC 2006).

In basement spaces it is important not only to have an effective vapor retarder, but also to have a complete air barrier. For this reason, all gaps between the foundation wall and sill plate, sill plate and band joist, and band joist and subfloor should be sealed. All gaps and penetrations in the foundation wall also need to be adequately sealed.