Does the NB Building Code specify vapour barrier requirements for insulation? | Insulation IQ?

Yes, the New Brunswick Building Code does specify vapour barrier (more precisely, vapour retarder) requirements for insulated assemblies, and these requirements are critically important to understand -- because a misplaced or inadequate vapour control layer is one of the leading causes of moisture damage, mould growth, and premature structural decay in Maritime homes.

Under the NBC as adopted in New Brunswick, a Class II vapour retarder (with a permeance of 60 ng/Pasm2 or less) must be installed on the warm-in-winter side of all insulated wall, ceiling, and floor assemblies that separate conditioned space from unconditioned or exterior environments. In practical terms, this means the vapour retarder goes on the interior side -- behind the drywall, facing the living space -- in New Brunswick's heating-dominated climate. The standard product used to meet this requirement is 6-mil polyethylene sheeting, which has very low vapour permeance and is familiar to anyone who has watched a New Brunswick house being framed and insulated.

The physics behind this requirement are straightforward: during winter, warm moist interior air contains more moisture than cold exterior air. If that moisture-laden air migrates into the wall cavity and reaches the cold outer sheathing, it condenses. Over time, that condensation leads to wood rot, mould, and degraded insulation performance. The poly barrier, installed as a continuous and well-sealed membrane, keeps interior moisture from entering the wall cavity in the first place.

Continuity and sealing are as important as the barrier material itself. The NBC requires that penetrations through the vapour retarder -- for electrical boxes, plumbing, and other services -- be sealed with acoustical sealant, tape, or gaskets. Junction boxes that penetrate the poly should be sealed at both the box perimeter and where wiring enters. Poor sealing is endemic in older NB homes and is often the primary moisture pathway even when the poly is nominally present. During a renovation or new build in Fredericton or Dieppe, having the poly installed by an experienced crew and inspected before drywall is worth the extra attention.

There is an important nuance in the code around vapour barrier placement when using exterior continuous insulation. When rigid foam insulation (such as EPS or polyisocyanurate) is applied to the exterior of a wall, it raises the temperature of the interior wall cavity, shifting the dew point outward. In these assemblies, a vapour retarder may not be required on the interior, or a variable-permeance smart vapour retarder (such as MemBrain or similar) may be preferred over 6-mil poly. Using a perfectly vapour-impermeable poly on the interior of a wall that also has a vapour-impermeable exterior foam layer can create a double-barrier assembly -- trapping any construction moisture with nowhere to dry -- which is a code deficiency and a moisture risk. Builders and insulation contractors in New Brunswick working with hybrid wall assemblies (cavity insulation plus exterior rigid) need to understand which side of the assembly controls vapour drive.

Spray foam considerations add another layer of complexity. Closed-cell spray polyurethane foam (ccSPF) applied to the interior of a wall cavity or as part of a hybrid assembly is itself a Class II or Class III vapour retarder depending on thickness. If sufficient thickness of ccSPF is applied (typically 50 mm or more in climate zone 6), the poly vapour barrier requirement may be satisfied by the foam itself, eliminating the need for a separate poly layer. Open-cell SPF (ocSPF), however, is highly vapour permeable and does not function as a vapour retarder -- poly or another vapour control layer is still required with ocSPF assemblies.

In basement assemblies, the code approach shifts somewhat. Basement walls typically receive rigid foam on the interior or exterior, and because below-grade moisture drive can come from both directions (exterior soil moisture and interior conditioned air), the vapour management strategy must account for the specific construction. Dimple mat drainage and drainage tile address bulk water; the vapour retarder or control layer must address vapour.

For homeowners in Moncton, Saint John, or anywhere in New Brunswick undertaking new construction or significant renovations, understanding vapour barrier requirements and ensuring they're correctly executed is non-negotiable from both a code compliance and a long-term building health standpoint. Find qualified professionals through New Brunswick Insulation at newbrunswickinsulation.com.

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