Design Foundations for Moisture Protection

Foundation options generally include basement, slab-on-grade, crawl space, or a mix of these foundation types (e.g., split level construction). One thing is common in all foundation construction: ground moisture will find its way “in” unless appropriate measures are taken.

An important measure to include is a ground vapor barrier under all basement, slab-on-grade, or crawl space construction. This will eliminate (or suitably minimize) a large potential moisture source to a house that can create or aggravate above-ground moisture problems (see Chapter 4).

The ground vapor barrier should be placed directly below the concrete slab. There are still some reservations regarding curling of the slab with this technique, which can be addressed with a low water-to-cement ratio (less than 0.5). Curling of the slab describes when the concrete slab distorts by either upward or downward bending, typically at the edges of the slab. This is typically the result of moisture differentials that occur while the slab is curing. If curling still seems to be an issue with this ratio, continue to use it but wet cure the top concrete using burlap or some type of fabric that will enable wet curing (see Section 3.3 Additional Resources from ACI and Building Science Corp).

For slab-on-grade and crawl space foundations (which also require a vapor barrier), moisture protection usually involves placing the building on a slight “mound” relative to the surrounding site. If the site is properly graded, a perimeter drain system is unnecessary in mounded foundation systems. If the site has potential for wetting, a perimeter drain system should be incorporated.

Typical basement construction practice for waterproofing, which is recommended for all sites unless they are extremely dry, is illustrated in Figure 3–4. However, “waterproofing” is not meant to resist water from flooding or a high water table; it is merely able to resist water and vapor movement more so than damp proofing measures. It should be noted that concrete has a considerably lower vapor permeability (i.e., can stop water vapor better) than masonry. However, available data seems to suggest no significant difference between concrete and masonry relative to the potential for basement water problems in actual practice when proper waterproofing measures are taken.

Backfill and grading specifications should be shown on the construction documents, and also clearly stated in the foundation contractor’s agreement. The backfill and grading should be inspected for compliance with these specifications. Important foundation measures and best practices are listed below:

• For backfill avoid silt, heavy clay, or expansive clay, particularly for basement walls. Granular soils are preferable that allow water to penetrate to the foundation drainage system and move the water away from the building.
• Backfill should be compacted to avoid flat or negative grades from developing around the foundation. Alternatives to compaction include providing extra soil to offset settlement, using self-compacting backfill material such as pea gravel, or adding more soil after 6–12 months.
• Use minimum 3,000 psi concrete in slabs with welded wire fabric, and foundation walls with reinforcement per code (at a minimum) to control cracking. In both cases it’s critical to ensure proper positioning of the welded wire fabric and the reinforcement. If CMU foundation walls are used, provide proper code minimum reinforcing and specify coatings to create a monolithic barrier to water penetration.
• Vibrate poured concrete walls for good consolidation in forms.
• Use high quality urethane caulks to seal all penetrations through the foundation wall prior to applying waterproofing measures
• Six-mil polyethylene sheeting as a waterproofing layer provides the added benefit of being a Class I vapor retarder. This greatly limits water vapor diffusion from the soil into the foundation wall. Alternative waterproofing treatments should likewise be evaluated for their ability to limit water vapor migration into the wall.