Vapor Barrier Under Concrete Slab On Grade

This was verfied by some field studies conducted by concrete constructionmagazine.

Vapor barrier under concrete slab on grade.

Vapor retarders do not prevent migration of residual moisture from within the concrete slab to the surface. Chemical and mineral admixtures are generally used to minimize the water content in a concrete mixture and provide adequate workability for placement. Aci 302 1 guide for concrete floor and slab construction states that the minimum thickness of an effective vapor barrier is 10 mils. However concrete slabs are vulnerable to moisture from the ground.

Thinner plastic can t stand up to the abuse of construction. Vapor retarders are generally not necessary when placing exterior slabs on grade. That s why in most cases a vapor barrier should be installed under the slab. A vapor barrier is any material with a u s.

Perm rating of less than 0 1 perm. In nonproblem areas we have traditionally used 6 mil polyethylene film directly under concrete slabs and over a granular fill on the subgrade. This plastic sheet is in direct contact with the concrete as it should be in a rational world. Almost everywhere except california that slabs are placed on the ground a plastic sheet a vapor barrier is located directly under the concrete slab and on top of the ground photograph 1.

Pouring a slab on grade is a common way to lay a sturdy floor or foundation where a basement or crawlspace is unnecessary. Without a vapor barrier moisture would continually wet the bottom of the slab as vapor will continuously drive from the high humidity below slab to the relatively low humidity above. Vapor barriers have been associated with certain concrete slab on grade problems including curling and cracking. We have a system that has worked well for us in the past.

We would like to resolve the question of proper placement of vapor barriers under concrete slabs on grade. 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. A concrete vapor barrier is simply a sheet of polyethylene plastic visqueen placed directly on top of the sub grade before the concrete floor or slab is poured to help keep moisture from the soil from passing up through the concrete. Materials with higher perm numbers more permeable are classified as vapor retarders.