Why Is a Vapor Barrier Used Under Concrete Slabs?

Written by anne rose
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Why Is a Vapor Barrier Used Under Concrete Slabs?
Moisture collects under concrete, necessitating a vapour barrier. (hoses on concrete image by pearlguy from Fotolia.com)

A concrete slab is the supporting base poured as a single, solid piece before building construction. A concrete slab can sit directly on the ground, but installation of a vapour barrier between the slab and the ground prevents moisture damage to the concrete.


The soil or fill under a concrete slab retains moisture. This can be caused by a high water table or groundwater. It can also be caused by climate changes; frozen ground under a concrete slab produces moisture when it thaws. Such conditions make a vapour barrier necessary.

Properties of Concrete

Concrete is porous, so moisture under a concrete slab will deteriorate the concrete, causing it to crack and crumble, unless a vapour barrier is installed.


Without a vapour barrier, concrete that is subject to moisture deterioration will affect any surface installed over the concrete. Carpet, vinyl and hardwood flooring can be damaged by a concrete slab with high moisture content.


A vapour barrier acts like a "raincoat," keeping moisture away from the underside of a concrete slab. Vapour barriers are made from visqueen plastic, also called poly sheeting.


Properly installed during construction, a vapour barrier prevents moisture under the concrete from damaging the inside of a building.


A concrete slab poured without installation of a vapour barrier cannot be remediated after construction.

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