Should I install a vapour barrier under my garage slab in Bathurst New Brunswick?

Absolutely yes — a vapour barrier under your garage slab in Bathurst is essential, not optional. Every garage slab poured in New Brunswick should have a minimum 6-mil polyethylene vapour barrier installed between the crushed stone base and the concrete. In Bathurst's coastal Maritime climate, skipping this step virtually guarantees a perpetually damp garage floor, peeling coatings, and mould and mildew problems that will plague you for as long as the garage stands.

Here is why this is so critical in Bathurst specifically. The town sits on the Chaleur Bay coast in northern NB, where relative humidity is high year-round — summer moisture from the bay and winter condensation from extreme temperature differentials between the cold ground and the garage interior. The ground beneath your slab contains moisture at all times, and concrete is not waterproof — it is porous. Without a vapour barrier, ground moisture wicks up through the concrete by capillary action, a process that never stops. The moisture migrates to the surface of the slab where it evaporates into the garage air, raising indoor humidity, causing condensation on cold surfaces, and creating conditions where rust, mould, and deterioration thrive.

The effects of a missing vapour barrier are immediately visible and progressively worsen. In the first year, you may notice that the garage floor always looks damp or has a darker appearance in patches. Cardboard boxes stored on the floor develop mould. Metal tools begin rusting faster than they should. By the second or third year, if you have applied a floor coating — epoxy or polyaspartic — it begins to bubble, peel, and delaminate as moisture pressure from below pushes it off the concrete surface. This is the number one cause of garage floor coating failure in NB, and it is entirely preventable with a $200-$400 vapour barrier.

Proper installation matters as much as the barrier itself. The vapour barrier should be laid over a minimum 4-6 inch layer of compacted 3/4-inch clear crushed stone that serves as a capillary break and drainage layer. The poly sheets should overlap by a minimum of 12 inches at all seams, and many contractors in northern NB tape the seams with poly tape or acoustic sealant for an even more effective seal. The edges of the vapour barrier should extend up the inside face of the frost walls and be trimmed flush with the top of the slab after the pour. Take care during rebar and mesh placement not to puncture the barrier — use rebar chairs or supports with flat bases rather than pointed legs that can tear through the poly.

Some contractors argue that a vapour barrier causes the concrete to cure too slowly because moisture cannot escape downward. While there is a kernel of truth to this — concrete over poly does cure slightly differently — decades of research and field experience have conclusively shown that the benefits far outweigh any curing concerns. The minor difference in curing rate is easily managed by proper finishing technique and appropriate curing compound application on the surface. The alternative — a slab with no vapour barrier that becomes a permanent moisture wick — is vastly worse.

In Bathurst's climate, consider upgrading to a 10-mil or 15-mil vapour barrier rather than the minimum 6-mil. The heavier material is more resistant to punctures during construction, lies flatter, and provides a more reliable long-term seal. The cost difference is minimal — approximately $50-$100 more for a two-car garage slab — and the peace of mind is worth every penny.

One additional consideration for Bathurst: if your building site has a high water table or sits in a low-lying area near the bay or the Nepisiguit River, the vapour barrier alone may not be sufficient. You may also need a perimeter drainage system (weeping tile) and a sump pit to manage groundwater pressure against the underside of the slab. Your contractor should assess the site conditions during excavation and recommend additional measures if needed.

This is a detail that should be included in every garage contractor's scope of work, and any reputable builder in the Bathurst area will include it as a matter of course. If a contractor proposes pouring your slab without a vapour barrier, consider it a significant warning sign about their understanding of NB construction requirements. Find experienced garage contractors who understand Maritime climate construction through the New Brunswick Construction Network.