Reprinted from the Portland Cement Association Engineering Bulletin

Design & Control of Concrete Mixture

Thirteenth Edition

by Steven H. Kosmatka and William C. Panarese

© Portland Cement Association

Vapor Barriers and Moisture-Problem Prevention (Pages 107-108)

Many of the moisture problems associated with enclosed slabs on ground [floors] can be minimized or eliminated by (1) sloping landscaping away from buildings, (2) using a 4-in, granular subbase to form a capillary break between the soil and the slab, (3) providing drainage for the granular subbase to prevent water from collecting under the slab, (4) installing foundation drain tile, and (5) installing a vapor barrier, often polyethylene sheeting.

A vapor barrier should be placed under all concrete floors on ground that are likely to receive an impermeable floor finish or be used for any purpose where the passage of water vapor through the floor is undesirable.

Good quality, well-consolidated concrete at least 4 in. thick is practically impermeable to the passage of liquid water unless the water is under considerable pressure; however, such concrete (even concrete several times as thick) is not impermeable to the passage of water vapor.

Water vapor that passes through concrete slabs evaporates at thesurface if it is not sealed. Floor coverings such as linoleum, vinyl tile, carpeting, wood, and synthetic surfacing effectively seal the moisture within the slab where it eventually may loosen, buckle, or blister the floor covering.

In addition, to prevent problems with floor covering materials causedby moisture from the concrete itself, the following steps should be taken: (1) use a low water-cement ratio concrete, (2) moist-cure the slab for 5 to 7 days, (3) allow the slab a 2-or-more-month drying period, and (4) test the slab moisture condition before installing the floor covering. In one commonly used test, a four-foot square polyethylene sheet is taped to the floor. If after 24 hours (or before the flooring material or its adhesive cures) no moisture accumulates on the underside of the plastic film, the slab is considered dry enough for placing the flooring material. Flooring-material manufacturers often have a recommended test to use before installing their product.

Insulation is sometimes installed over the vapor barrier to assist in keeping the temperature of a concrete floor above the dew point; this helps prevent moisture in the air from condensing on the slab surface. This practice also creates a warm floor for thermal comfort. Codes and specifications often require insulation at the perimeter of a floor slab. Placing insulation under the entire slab on ground for energy conservation alone usually cannot be justified.

Vapor barriers directly under concrete may increase the time delay before final finishing due to slow water evaporation, particularly in cold weather. They may also increase warping or curling and aggravate shrinkage crack development in the slab surface. To minimize these effects, a 3-in.-thick sand layer may be placed over the vapor barrier (or insulation if present). If concrete is placed directly on a vapor barrier, the water-cement ratio should be low (0.45 or less).