A variety of vapor retarders is available for use under concrete floor slabs. These materials offer a wide range of resistance to moisture movement and mechanical properties, such as puncture and tear resistance.

Vapor retarders are sheet materials used under concrete slabs on ground to restrict the flow of moisture vapor from the subgrade into and through the slab. Moisture migration through concrete slabs can lead to microbial growths (mold and mildew) and failures of adhesives, flooring coverings, and coatings. Therefore, all concrete slab-on-ground floors that will receive floor coverings or coatings must have a vapor retarder below the slab. Even floors that might not initially receive floor coverings should have subslab vapor retarders to reduce humidity in the conditioned space, to prevent mold and mildew, and to provide for future adaptive reuse. Installation of floor coverings during reuse or remodeling of industrial or warehouse space often leads to failures when the slab lacks a vapor retarder.

Vapor retarders must have a permeance rating less than 0.3 perms according to ASTM E1745. There is no standard specification for vapor barrier compared to vapor retarder. However, ACI 302.1R states, “True vapor barriers are products that have a permanence (water-vapor transmission rating) of 0.00 perms when tested in accordance with ASTM E96.” It is generally accepted in the construction industry that a material having a permeance rating less than 0.01 perms is considered a vapor barrier. Vapor barriers are commercially available with permeability ratings of less than 0.001 g/m2 hr. Even so, much published literature mistakenly refers to vapor retarders as vapor barriers.

Quality portland cement concrete has extremely low permeability to liquid water, but will permit passage of water vapor. It is essential to keep liquid water from contacting the underside of a concrete slab and at the same time minimize the exposure to water vapor. A vapor retarder installed directly under the slab fulfills this dual purpose. Materials with a variety of properties are available as vapor retarders.

Specifications for Vapor Retarders

Vapor retarders are produced to meet specifications such as ASTM E1745, Standard Specification for Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs, or ASTM D4397, Standard Specification for Polyethylene Sheeting for Construction, Industrial, and Agricultural Applications. ASTM E1745 defines three classes of membranes with a single moisture vapor permeability rating and three levels of physical strength, Class A has the most resistance to tearing and puncture, and Class C the least, as shown in Table 1.

Table 1 – Classification of Vapor Retarders

Construction methods may rule out less robust materials (such as simple polyethylene sheeting) if mechanized ride-on screeds are used to consolidate and strikeoff concrete or if many electrical or plumbing penetrations must be sealed.

Location of Vapor Retarder

Construction practice and placement of vapor retarders has been the subject of much debate for many years. Some experts believe that concrete placed directly on a vapor retarder will bleed excessively, warp and crack more frequently, and take longer to dry than a slab placed on a compacted granular subbase. Other experts believe that vapor retarders function best to exclude moisture when directly below the concrete with no intervening material that can act as plenum space for the passage of moisture. This debate has resulted in many articles and letters in construction trade journals.

ASTM E1643, Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs, includes an Appendix with a detailed discussion of materials that should or should not be used above and below the vapor retarder, along with arguments in favor and opposed to cushions, blotters, and protective courses.

The 1996 ACI Committee 302 Guide recognized the importance of maintaining the integrity of the vapor retarder and its resistance against moisture transmission by recommending a minimum vapor retarder thickness of 0.25 mm (10 mils). Additionally, a thin layer of fine material was recommended over any crushed stone subbase to protect the underside of the vapor retarder sheet. In April 2001, after much debate, ACI Committee 302 issued an update to its 1996 report defining recommendations for vapor retarders. The Committee now recommends that any floor that will receive a moisture-sensitive finish should have a vapor retarder directly under the concrete slab with no intervening blotter or cushion layer (Figure 2).

See the flowchart (Figure 2) to determine how vapor retarder should be installed. Note it permits omitting vapor retarder for floors without floor coverings where humidity will not be controlled, such as unconditioned warehouse space. However, adaptive reuse and installation of flooring in such spaces often leads to flooring problems due to subslab moisture. Therefore, vapor retarders should be considered for use under all floor slabs (adapted from Figure 3-1 of ACI 302.1R-04).

Figure 2 – Vapor covering or humidity controlled

See Concrete Floors and Moisture, EB119, and Concrete Slabs on Ground, EB075, for more information on vapor retarders.