Should I put vapour barrier under a basement floor slab in NB?

Yes — a vapour barrier under a basement floor slab is strongly recommended in New Brunswick and should be considered standard practice, not an optional upgrade. NB's variable water table, spring thaw conditions, and prevalence of older homes with subgrade moisture issues make below-slab vapour control an important step in any basement floor project.

The purpose of a vapour barrier is to prevent ground moisture from wicking up through the concrete slab by capillary action. Concrete is porous — even a solid, crack-free slab transmits moisture vapour from the soil below. Without a vapour barrier, this moisture can:

• Cause efflorescence (white mineral deposits) on the slab surface


• Prevent adhesive-applied flooring (vinyl, laminate, hardwood) from bonding properly


• Create conditions for mould growth under floor coverings


• Cause epoxy or polyaspartic coatings to delaminate


• Make the basement feel persistently damp

In New Brunswick's river valley communities (Fredericton, Miramichi) and coastal areas (Saint John, Bathurst coastline), water tables can be high, making below-slab vapour control especially important. Even in well-drained areas, spring thaw sends large volumes of water into the soil, temporarily raising the water table and increasing vapour drive through basement slabs.

What Vapour Barrier to Use

The standard product is 10-mil polyethylene sheeting ($0.15–$0.30 per sq ft) laid directly on the compacted gravel base, with seams overlapped by 12 inches and taped with vapour barrier tape. The poly goes under the concrete — not on top.

For situations with known moisture issues or below-grade slabs with high vapour emission rates, a 20-mil reinforced polyethylene or a purpose-built vapour retarder membrane (like Stego Wrap) provides better protection. These products cost more ($0.50–$1.00 per sq ft) but are worth it for finished basement spaces.

Under gravel, then vapour barrier, then concrete is the correct sequence — the gravel provides drainage and capillary break, the vapour barrier stops vapour transmission, and the concrete goes on top. Some older NB construction reversed this (poly over compacted soil, gravel on top), which can trap moisture under the poly — the current best practice is poly directly under the slab on top of gravel.

If you're not sure whether vapour is an issue in your specific basement, do the plastic sheet test: tape a 2-foot square of poly to the floor, seal all edges with tape, and check after 48–72 hours. Condensation on the underside of the plastic means moisture is coming up through the slab — a vapour barrier under a new slab is essential in that scenario.

Adding a vapour barrier is inexpensive insurance for any basement floor project in NB. New Brunswick Concrete can connect you with experienced concrete contractors who know NB's ground conditions and will spec the right moisture protection for your situation.