A great deal of moisture can move from the ground into and through a foundation and floor system, which can lead to decay, mold under flooring and high indoor humidity. To prevent these problems, keep rainwater away from the foundation with a 5% slope (6 inches/10 feet) of the grade around the house, prevent materials from absorbing water, drain groundwater with underground drainage before it gets to the foundation, and allow the moisture in a foundation to escape.

Slab:

Durable plastic sheeting is needed under the slab — including its grade beams — to prevent moisture wicking into the slab from the ground beneath. Sheeting should line the grade beams up to the soil level. Paint the exposed slab with latex paint to protect it from rainwater. Coarse gravel (no fines) under the slab and plastic provide a drainage pad.

Crawl spaces:

Typical enclosed, but vented, crawl spaces are notorious for moisture problems due to moisture buildup from wet soil and condensation on cool surfaces. Humid air cannot dry out a humid space. When humid air contacts cool floor framing, condensation occurs (“sweating”). Cooling and insulating the subfloor without air/vapor barriers keeps framing cold, increasing condensation, fungal growth and decay. To prevent this, the entire underside of a raised floor system should use a warm‑vapor retarder and air barrier — essentially designing the subfloor like a wall. This can be accomplished with:

• Foil-faced rigid foam insulation installed under the joists with cap nails and foil-taped joints. The sealed foam blocks water vapor and air entry and stays warmer, reducing condensation. Check local fire codes for required fire‑rated materials.

If the crawl space under the house is not completely open (house on piers), covering the ground with plastic sheeting helps block moisture evaporation. The ground level inside should be higher than outside level to prevent rainwater entry and ponding. Crawl space openings are needed for access, flood insurance requirements, and/or code compliance, but they do not adequately control moisture. Research is underway on alternative floor insulation systems.

• Low-permeable spray foam insulation (or painted foam) applied across the underside of floor framing provides an air barrier and warm‑vapor retarder. Combined with ground plastic, this system can be effective in tight crawl spaces and still allow drying outward if water spills from above.
• Unvented crawl spaces built like mini‑basements (ground cover, insulated walls, air grilles) are increasingly accepted, but not recommended in flood zones or high water table areas. Insulation in ground contact is not recommended in termite‑risk areas. Even inside crawl spaces, insulation can conceal termites. Though there are treatments and inspection strategies, the high termite‑damage risk makes this approach vulnerable.

Related article:

https://lsuagcenter.com/en/family_home/home/design_construction/Safer+Stronger+Smarter/Durability+Hazards/Moisture+Decay/Decay+Resistance.htm