Concrete Slab Vapor Barrier Placement

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.

Concrete slab vapor barrier placement.

See below for more discussion on this. Because extruded polystyrene insulating board will absorb little moisture the vapor barrier can be placed above or below the slab insulation. In general place the concrete slab directly on top of the vapor barrier with the subbase below. We have a system that has worked well for us in the past.

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. Details that thermally isolate the slab edge from the foundation wall and outdoors appear to work the best. For a vapor barrier below a concrete floor slab most choose 6 to 10 mil polyethylene. 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. We would like to resolve the question of proper placement of vapor barriers under concrete slabs on grade.