Vapor Barrier Under Concrete Slab On Grade

Vapor retarders are generally not necessary when placing exterior slabs on grade.

Vapor barrier under concrete slab on grade.

We have a system that has worked well for us in the past. Aci 302 1 guide for concrete floor and slab construction states that the minimum thickness of an effective vapor barrier is 10 mils. This was verfied by some field studies conducted by concrete constructionmagazine. However concrete slabs are vulnerable to moisture from the ground.

Materials with higher perm numbers more permeable are classified as vapor retarders. This plastic sheet is in direct contact with the concrete as it should be in a rational world. Perm rating of less than 0 1 perm. However the far greater risk to a slab is long term curling over the lifetime of the concrete foundation which a high performance vapor barrier helps prevent.

Vapor retarders do not prevent migration of residual moisture from within the concrete slab to the surface. 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. In nonproblem areas we have traditionally used 6 mil polyethylene film directly under concrete slabs and over a granular fill on the subgrade. That s why in most cases a vapor barrier should be installed under the slab.

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. It is important to use a concrete mixture with the lowest water content that will afford adequate workability. Vapor barriers for slabs on grade. Chemical and mineral admixtures are generally used to minimize the water content in a concrete mixture and provide adequate workability for placement.

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. 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. Pouring a slab on grade is a common way to lay a sturdy floor or foundation where a basement or crawlspace is unnecessary. A vapor barrier is any material with a u s.