Perhaps the most overlooked and challenging technical issue for the builder is foundation moisture control. It is frequently overlooked because professionals and consumers alike fail to recognize the tremendous impact the below-grade moisture can have on overall building durability and indoor air quality.
While there may be no simple answers, it is useful to review the various below-grade moisture issues. First, it is important to recognize that many of the moisture problems we see in homes are a result of significant amounts of below-grade moisture entry, both liquid and vapor, into the home. Second, it is helpful to isolate each moisture movement mechanism that ultimately contributes to building durability and indoor quality.
Liquid flow by gravity is relatively simple and fairly easy to accommodate by properly draining rainwater away from the building. Control measures include overhangs, gutters with extended downspouts, positive slope away from the building, porous back fill (or drainage mat) with an impermeable cap, a carefully installed drainage system at the footing, and a high quality damp proofing system.
Hydrostatic flow results from water standing next to the foundation. This can be due to large quantities of surface water saturating the adjacent soil or a rise in the ground water next to the foundation. A properly installed foundation drainage system should be designed to quickly relieve this hydrostatic pressure. If longer-term hydrostatic pressure is anticipated, a high quality waterproofing system must be used to provide resistance against hydrostatic flow.
Capillary action or "wicking," is the flow of water through small pores due to surface tension and adhesion. For below-grade moisture, the key capillary concerns are through the floor slab and from the footing to the foundation wall. For the floor, use a layer of washed aggregate or impermeable membrane below the slab. For the foundation wall, install a capillary break such as a waterproof coating or durable membrane between the footing and foundation wall. For concrete masonry walls, it is important to keep the cores dry.
Water Vapor Flow
Diffusion of water vapor is a slow, but relentless, moisture movement mechanism. Water vapor will try to travel from more to less. In saturated soils, the water vapor moves from the ground into the foundation and home for most of the year. Fortunately, diffusion is easy to slow with a vapor retarder. A quality damp proofing or waterproofing coating or membrane on the foundation walls and a durable, impermeable membrane under the slab will generally control below-grade diffusion.
