What moisture barrier should I use on a concrete slab in my NB basement?

For NB basements, a 6-mil polyethylene vapour barrier is the minimum requirement under any flooring, but the best choice depends on your flooring type, moisture test results, and how your basement will be used. Maritime moisture conditions make this decision more critical in New Brunswick than in most other provinces.

Concrete basement slabs in NB continuously release moisture vapour upward — our water table sits close to the surface in many communities, and the concrete never fully stops transmitting moisture regardless of the slab's age. This moisture migration intensifies during spring snowmelt (April through June) when ground water levels peak across the province. Without a proper barrier between your concrete and flooring, you are inviting cupping in wood products, mould growth under carpet, delamination of adhesives, and musty odours that become a permanent feature of your basement.

Here are your main moisture barrier options for NB basement slabs, from basic to premium:

A 6-mil polyethylene sheeting ($0.15-$0.30/sq ft) is the basic standard. Lay it directly on the concrete with seams overlapped 6 inches and taped with poly tape. This works adequately under floating laminate or LVP where a separate underlayment with built-in vapour barrier sits on top. However, on its own, polyethylene does nothing for flatness, thermal comfort, or sound — and it can trap moisture between the poly and the concrete if drainage is poor.

A foam underlayment with integrated vapour barrier ($0.50-$1.00/sq ft) combines moisture protection with cushioning and thermal insulation in one layer. This is the most popular choice for floating LVP and laminate installations in NB basements. Look for products specifically rated for below-grade concrete application with a moisture vapour transmission rate that meets your flooring manufacturer's requirements.

Cork underlayment with vapour barrier ($1.00-$2.00/sq ft) provides superior thermal insulation and sound dampening — your feet will thank you on cold NB winter mornings. Cork also has natural antimicrobial properties that resist mould growth, a meaningful advantage in Maritime basement conditions.

A dimpled drainage membrane (such as Delta-FL or DMX One-Step, $1.50-$3.00/sq ft) is the premium choice for NB basements and the one I recommend most strongly for homes in flood-prone river valleys, coastal areas, or any basement with a history of dampness. The raised dimples create a continuous air gap between the concrete and your flooring, allowing moisture vapour to dissipate harmlessly rather than being trapped against the flooring. Some products combine the drainage membrane with a built-in foam top layer, eliminating the need for separate underlayment.

Before choosing any barrier, test your slab's moisture levels. A calcium chloride test or relative humidity probe test ($50-$150) tells you exactly what you are dealing with. If results show excessive moisture, no barrier alone will solve the problem — you may need exterior drainage improvements, a sump pump, or interior waterproofing before installing any flooring.

For most NB basement flooring projects, a dimpled drainage membrane paired with waterproof LVP is the safest, most durable combination. A flooring professional familiar with Maritime basement conditions can assess your specific slab and recommend the appropriate moisture management approach for your situation.

---

Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• Gionetterenovations
• Forever Epoxy Inc
• Thirty Four Renovations
• Arctic Fox Construction Inc.
• First united design & construction inc.