Vapor Barrier

An under slab vapor barrier/retarder is employed to reduce the water vapor transmission through a concrete slab into the building envelope. The vapor barrier is installed prior to pouring the slab. Preventing excess vapor transmission through the slab is important to avoid damage to floor finishes or condensation that can cause mold and mildew. The barrier also helps reduce infiltration of soil gases like radon and methane. The terms vapor barrier and vapor retarder are often used interchageably, but a material is generally considered a true vapor barrier if the permeance is less than 0.01 perms as tested per ASTM E96. Besides permeance, other important properties of vapor barriers are puncture resistance and strength - the barrier materials are often subjected to foot traffic and sometimes machine traffic on a jobsite and must be able to resist damage in order to function properly once the slab is installed. ASTM E1745 requires a maximum permeance of 0.1 perms and describes the three classes of vapor retarders with Class A being the strongest material. Under slab vapor barriers meeting Class A requirements are generally available in thicknesses of approximately 10 mils or 15 mils. Often the 15 mil thickness will also meet the American Concrete Institute standard (ACI 302.2R) requirement of 0.01 perms or less ( a true vapor barrier), required for very sensitive flooring. The vapor barriers are typically made of polyethylene, containing low, medium or high density polyethylene or copolymers of ethylene and α-olefins. The sheets may be comprised of multiple layers, coextruded or crosslaminated. Most typically, virgin polyethylene resins are used. During installation, seams in the film must be overlapped by at least 6 inches and taped to ensure a consistent barrier.