Figure 1, Plastic Lining Slab Grade Beam

Figure 2, Vented Crawl Space

Water-managed foundation: 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 in./10 ft.) 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. Painting the exposed slab with latex paint will reduce absorption of rainwater. Coarse gravel (no fines) under the slab and plastic provide a drainage pad.  (Figure 1, Plastic Lining Slab Grade Beam)

Crawl spaces: Typical enclosed, but vented, crawl spaces are notorious for moisture problems - due to moisture build up from wet soil and condensation on cool surfaces. First, humid air cannot dry out a humid space. In addition, when humid air comes in contact with cool floor framing, condensation occurs (the surfaces “sweat”). The cooler the floor framing, the more it sweats – so air conditioning and insulation under the subfloor without air and vapor barriers results in floor framing that gets cold and wet, grows fungi and decays in summer.

To prevent this, the entire underside of a raised floor system is most effectively protected with an insulating vapor retarder and air barrier to keep humid air from reaching cool surfaces – in other words, design the subfloor like a wall. This can be accomplished with:

  • Foil-faced rigid foam insulation
(installed like sheathing under the joists with cap nails and the joints sealed with foil tape) with sealed insulated rim boards (with spray foam or rigid foam and caulk). The sealed foam board blocks water vapor and air entry, and stays warmer so is less likely to sweat. Check local fire codes to see if the foam should be a fire-rated type or covered with a fire-rated material.

If the crawl space under the house is not completely open (house on piers), cover the ground with plastic sheeting to block moisture evaporation from the ground into the enclosed crawl space. The ground level inside the crawl space should be higher than the outside ground level to prevent rainwater entry and ponding. Crawl space openings are needed for access, flood insurance requirements and/or to satisfy existing code requirements, but they are not an adequate means of moisture control.

Be sure to include termite shields on top of foundation walls or piers to prevent hidden pathways and provide a means of inspection.   (Firgure 2, Vented Crawl Space)


Research is underway to assess the moisture performance of alternative floor insulation systems.

  • Low-permeable spray foam insulation (or painted foam) applied to coat the underside of all floor framing provides an air barrier and insulating, vapor retarder. In combination with a plastic ground cover, this system may offer easier installation in tight crawlspaces, sufficient moisture protection, and the capacity to dry to the outside if spilled water enters the subfloor from above.
  • Unvented crawl spaces designed like a minibasement (with ground cover, insulated walls and conditioned air grilles) are gaining widespread acceptance in the U.S., but are not recommended in flood zones and may be risky where the ground water table is high.

Note that insulation in ground contact is not recommended in termite risk areas because it provides a hidden pathway. Likewise, insulation on foundation walls inside a crawl space is vulnerable and difficult to inspect. Although termite barriers, inspection points and termite-resistant insulation treatments are strategies to overcome that problem, the high termite damage risk in this area makes it a risky choice.

6/21/2008 1:22:58 AM
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