Concrete Slab Vapor Barrier Placement

In general place the concrete slab directly on top of the vapor barrier with the subbase below.

Concrete slab vapor barrier placement.

Will ensure that the vapor barrier not only performs as an effective barrier to moisture and other vapors but will also maintain its physical integrity during the placement of the concrete slab. 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. If the subbase is sharp angular gravel a thin layer of sand can be placed on top of the gravel subbase next to the vapor barrier. For a vapor barrier below a concrete floor slab most choose 6 to 10 mil polyethylene.

Aci 302 1 which is an american concrete institute standard for slab construction was modified in the early 2000 s to reflect the change in the placement of the vapor barrier directly below the concrete if a low permeable membrane flooring was to be installed. In nonproblem areas we have traditionally used 6 mil polyethylene film directly under concrete slabs and over a granular fill on the subgrade. 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. Details that thermally isolate the slab edge from the foundation wall and outdoors appear to work the best.

Because extruded polystyrene insulating board will absorb little moisture the vapor barrier can be placed above or below the slab insulation. See below for more discussion on this.