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.
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. 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 stays 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 a warm-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). The sealed foam board blocks water vapor and air entry and stays warmer so it 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. (See Figure 1: Vented Crawl Space.)
If the crawl space under the house is not completely open (house on piers), it’s helpful to 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. Research is underway to confirm 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 warm-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 mini-basement (with ground cover, insulated walls and air grilles) are gaining widespread acceptance in the United States but are not recommended in flood zones or where the groundwater 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.
Decay Resistance: Moisture Control