What is a vapor barrier under-slab?
A membrane layer placed below a concrete slab to prevent ground moisture from wicking up through the slab and compromising epoxy and other floor coatings.
A vapor barrier under-slab is a moisture control membrane installed on soil or gravel before concrete is poured. It stops water vapor rising from the ground from entering the concrete itself. When moisture moves upward through the slab (called "wicking"), it can cause epoxy and polyurethane floor coatings to fail, blister, or peel, particularly in Ontario's freeze-thaw cycles and humid climates.
Standard under-slab vapor barriers include polyethylene sheeting, specialized membranes, or tar-based products laid directly on the subgrade. A proper installation overlaps seams, extends to the slab edges, and sits under any reinforcement mesh. The barrier creates a physical boundary between ground water and the concrete body.
This barrier is critical for commercial and industrial floors where epoxy coating must remain intact. Without it, moisture vapor pressure builds inside the slab, separating the coating from the concrete surface. Flooring contractors often require proof of vapor barrier installation before applying coatings, and some specify maximum moisture vapor transmission rates (MVTR) the concrete must meet before coating begins.
For facilities planning new construction or replacement slabs, a proper under-slab vapor barrier is one of the most cost-effective ways to extend coating life and avoid costly repairs. Concrete preparation specialists in Ontario typically address or replace vapor barriers when diagnosing coating adhesion problems.