Vapor Barriers under Concrete Floor Slabs: Friend or Foe?

by Steve Ragan, Director, Research and Technical Services

The answer to this controversial question, from both angles, is a qualified “yes.” Vapor barriers have been associated with certain concrete slab-on-grade problems including curling and cracking. But they have also proven to be beneficial in reducing failures of moisture-sensitive flooring materials, adhesives, and coatings caused by excessive emission of moisture vapor from concrete slabs. Therefore, it is understandable that designers and contractors on both sides of the question have justifiable arguments that cannot be easily dismissed.

Before one can make the best decision regarding whether or not to place a vapor barrier under a concrete floor slab-on-grade, it is important to understand the sources of moisture, how it moves through the slab, and how it adversely affects certain floor coverings, adhesives, and coatings.

The excess or free water within the concrete is the source of some of the moisture in a slab. Concrete requires sufficient water both to hydrate the cement in the mixture and to create a workable consistency. Cement hydration is simply the chemical reaction that occurs between water and cement, which causes the concrete to attain a set and to gain strength and durability. Water in excess of that needed for hydration (sometimes called water of convenience), is only required to make the concrete easier to place, consolidate, and finish. Once the concrete has been placed, finished, and cured, some of the excess water of convenience must escape in order for the slab to meet the manufacturer’s moisture emission requirements for the floor covering, adhesive, or coating. Many manufactures require that the moisture emission rate from concrete not exceed 3.0 pounds of water per 1000 square feet in 24 hours. This requirement is very challenging when one considers that a 4-in. thick slab, constructed with concrete having a 0.50 water-cement ratio (w/c) and a 4-in. slump, can contain between 1600 and 1700 pounds of free water in a 1000-square-foot area!

The rate that moisture escapes from a slab depends on the concrete w/c, the density of the finish, the ambient conditions above the slab, and the moisture below the slab. One laboratory drying study indicated that 4-in. thick concrete samples, having a w/c=0.50 and sealed from below, required 82 days for the vapor emission rate to reach 3.0 pounds per 1000 square feet per 24 hours. The ambient air above the sample was 73°F and 50% relative humidity. The same concrete not sealed from below required 144 days to reach the same vapor emission rate. Concrete having a w/c=0.40 and sealed from below required 46 days to reach the 3 pound per 1000 square feet per 24 hour vapor emission rate, while the same concrete not sealed from below required 52 days to achieve this rate. This study reinforces the benefit of using concrete having a w/c less than 0.50, and the need to reduce the slab moisture entering from below when moisture-sensitive flooring materials will be installed.

A natural source of water can be found at some depth below most building sites. This water can generally move upward through soil and contact the bottom of a slab-on-grade in one of two ways. The first way is through capillary action. Capillary action uses the forces of adhesion, surface tension, and cohesion to cause the water to be drawn upward above the water table through the very narrow passageways found in many soils. An example of capillary action is water rising to a higher elevation inside a narrow straw that is placed into a glass of water. Capillary action in soils can be interrupted by a capillary break, such as a layer of crushed stone between the slab and the subgrade.

Although a capillary break may stop the rise of water in a liquid state, it does not eliminate the potential for moisture vapor to reach the slab. Water changes from a liquid to a vapor as it evaporates, and water vapor will move from areas of high to low vapor pressure. This process of vapor movement is termed “diffusion” and occurs in both soil and concrete. Several investigations have shown that the relative humidity in the base and subgrade just beneath the slab is near 100%, regardless of the depth of the water table. Although capillary action can cause liquid water to rise, diffusion is how water vapor distributes itself above the water table. Unless this diffusion is restricted, water vapor will contact and enter the slab. In the absence of effective moisture protection directly beneath the slab, the high humidity environment beneath the slab can contribute to an increase in moisture within the concrete over time.

Moisture-induced failure of flooring materials may appear in the form of cupping, bulging, or swelling. Failure of floor coatings typically manifests as blistering. As moisture moves toward the top of a slab, soluble alkalies are frequently carried with it. This causes the pH level at the surface to increase above the 9 to 10 pH limit of most modern flooring adhesives, which in turn leads to a breakdown or re-emulsification of the adhesive. The use of a vapor barrier (more accurately termed a vapor retarder), having a water vapor permeance of less than 0.3 perm is frequently used to retard the flow of moisture through the slab. These vapor retarders are typically polyethylene or polyolefin sheeting materials. While the permeance of the vapor retarder is important, its ability to withstand construction activity is also important. A vapor retarder that is torn or punctured provides a pathway for moisture to enter the slab from below. The American Concrete Institute’s Guide for Concrete Floor and Slab Construction recommends that the thickness of the vapor retarder be at least 10 mils. Puncture studies of 6-, 8-, 10-, and 20-mil vapor retarder materials have shown that 10 mils is the minimum thickness that should be considered, and thicker material may be necessary over angular base materials.

Because the permeability of concrete increases with an increase in its w/c, a low w/c concrete mixture should be considered in floor slabs where moisture-induced failure of flooring materials is a concern. Watertight concrete is often considered to have a w/c less than 0.50. However, using a concrete with a low w/c alone is often insufficient to satisfy the floor covering industry’s moisture emission requirements. Even after what appears to be sufficient drying of the surface, moisture will redistribute itself once the floor is covered. Without sufficient subslab moisture protection the total moisture in the slab will increase over time, and sawed contraction joints and random cracks will provide passageways for moisture to migrate through the slab. Omitting a vapor retarder may also result in liability for a flooring failure, since its use is often published in guidelines from many flooring manufacturers.

Placement location of vapor retarders is often a source of confusion for designers and contractors. Until 2001, the American Concrete Institute (ACI) recommended 4″ of granular fill be placed atop vapor retarders. However the ACI has since revised this recommendation because of their recognition that fill courses above the vapor retarder may sometimes take on water from rainfall, curing, or sawcutting. As a result, the ACI Committee 302 now recommends that floor slabs-on-grade being covered with moisture-sensitive coverings have the vapor retarder placed on top of dry granular fill and directly beneath the slab.

Confusion and debate over the use of vapor retarders will likely continue into the future. However, the fact that the cost of floor coverings over concrete floor slabs in the U.S. is now estimated at over one billion dollars a year requires that greater attention be given to concerns about moisture within and below these slabs. Each project should be considered individually; however, the following general recommendations are useful in mitigating moisture-related problems in concrete floor slabs-on-grade:

  1. Use a low permeance vapor retarder to protect floor slabs that will be covered with moisture-sensitive floor coverings, adhesives, and coatings. Floor covering manufacturer’s published literature should be consulted.
  2. Include a capillary break of granular material below the slab. However, recognize that the capillary break will not prevent moisture from reaching the slab in vapor form.
  3. Consider using concrete having a w/c not greater than 0.45 to hasten slab drying time. Use of fly ash may help reduce soluble alkali content in the slab.
  4. Place the vapor retarder on top of the granular material and directly beneath the slab for moisture-sensitive floor covering and coating applications. However, in doing so, recognize that additional attention should be given to the design of reinforcement so that potential curling stresses within the slab are addressed.
  5. Use a 15 mil or thicker vapor retarder material, particularly if it will be subjected to traffic from ready-mixed concrete trucks, concrete buggies, or laser screeds.
  6. Cover slabs for 7-days with sheet material rather than using membrane curing compounds in order to minimize drying time and surface preparation costs.