Although a crawl space foundation is not intended as living space, it is highly desirable to keep it dry. Good surface drainage is always recommended and, in many cases, subsurface drainage systems may be desirable. The goal of surface drainage is to keep water away from the foundation by sloping the ground surface and using gutters and downspouts for roof drainage.
Figure 3-3, 3-4, and 3-5 describe three different drainage techniques for crawl spaces. Figure 3-3 applies when the crawl space floor is flush with (or above) the surrounding grade. In most cases, this type of crawl space will not require perimeter drainage. On especially wet sites, or on sloping sites where part of the crawl space floor is below-grade, it may still be wise to install a perimeter drain system, described below. If the crawl space floor is above the top of the footing as shown, apply waterproofing on the interior face of the buried foundation wall to avoid capillary suction of water into the concrete.
Figure 3-4 and 3-5 describe foundation drain systems, which are recommended for all crawl spaces where the floor is below the level of the surrounding grade. On especially dry sites, it may be possible to eliminate the drainage system and not experience moisture problems. In most cases, a subsurface perimeter drainage system similar to that used for a basement is recommended (see Figures 3-4 and 3-5). Figure 3-5 describes the recommended best practice. It consists of two independent loops of perforated foundation drain, one inside the footing and one outside. These drain independently, either to daylight or to an internal sump. Figure 3-4 shows another option that is appropriate when site drainage conditions are good. There is no provision for drainage of the space inside the footings. Its single loop of foundation drain is on the outside of the footing, and drains to daylight or to an internal sump. It should be noted that the duct connection to the exterior of the footing can reduce the effectiveness of subslab depressurization radon mitigation systems by reducing the ability of the system to maintain sufficiently low pressures beneath the slab.
The final line of defense— waterproofing—is intended to keep out water that finds its way to the wall of the structure. First, it is important to distinguish between the need for dampproofing versus waterproofing. In most cases a dampproof coating covered by a 4-mil layer of polyethylene is recommended to reduce vapor and capillary draw transmission from the soil through the basement wall. A dampproof coating, however, is not effective in preventing water under hydrostatic pressure from entering through the wall. Waterproofing is recommended (1) on sites with anticipated water problems or poor drainage, (2) when the crawl space is intended for use as storage or houses mechanical equipment or (3) on any foundation built where intermittent hydrostatic pressure occurs against the basement wall due to rainfall, irrigation, or snow melt. Except on very dry sites, it is generally recommended to use waterproofing as a best practice. On sites where the crawl space floor could be below the water table, a slab-on-grade foundation is recommended.