In general, moisture management schemes must control water in two states. First, since the soil in contact with the foundation and floor slab is always at 100% relative humidity, foundations must deal with water vapor that will tend to migrate toward the interior under most conditions. Second, liquid water must be kept from accumulating around and under the foundation. Liquid water comes from sources such as:

figure 4-2

Figure 4-2. Structural System Components of Slab-on-Grade Foundation with Grade Beam

figure 4-2

Figure 4-3. Drainage Techniques for Slab-on-Grade Foundations

Techniques for controlling the build-up and movement of moisture in the foundation are an essential component of the overall construction. Improper moisture management can lead to structural damage, damage to floor finishes, 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 creating problems. This is done by using adequate drainage and by the use of vapor retarders. These guidelines and recommendations apply to thickened edge/monolithic slabs and stem wall foundations with independent above grade slab configurations (PATH 2006). These two slab-on-grade configurations are illustrated in Figures 4-2 and 4-3.

For more information visit Water Managed Foundations within the Building America Solution Center.