Should I insulate my basement walls from the inside or outside in NB? | Insulation IQ?

This is one of the most common questions New Brunswick homeowners face when planning a basement upgrade, and the honest answer depends on your specific situation — but for most existing homes in NB, interior insulation is the practical choice, while exterior insulation offers superior performance when it's feasible.

Interior basement wall insulation is by far the more common approach in New Brunswick because it doesn't require excavating around your foundation. It's typically done using either rigid foam board (extruded polystyrene or polyisocyanurate) fastened directly to the foundation wall, followed by framing and batt insulation in the stud cavity, or with closed-cell spray polyurethane foam applied directly to the concrete. The rigid foam method is preferred by many building scientists because the foam acts as both insulation and a vapour retarder, keeping the cold concrete thermally isolated from the warm interior air. In Climate Zone 6 conditions — which covers all of New Brunswick — you need enough continuous rigid foam on the cold side of the stud wall to keep the dew point from falling within the stud cavity. A minimum of R-7.5 of rigid foam (typically 1.5" of XPS or about 2" of EPS) against the foundation wall before framing is the generally accepted threshold, though many contractors use R-10 to R-15 to be safe. The remaining R-value can be filled with batt insulation in the framing.

Exterior foundation insulation is the thermally superior option. By wrapping the foundation on the outside with rigid foam, you keep the entire foundation wall mass within the conditioned envelope, which dramatically reduces thermal bridging and moisture movement through the concrete. You also avoid losing any interior floor space. The challenge is cost and disruption — excavating to the footing around a full foundation in Moncton or Fredericton can easily run $15,000–$30,000 or more depending on foundation depth, landscaping, and drainage work required. Exterior insulation also requires careful waterproofing, proper drainage board, and protection of the exposed foam above grade. For most homeowners doing a retrofit, this simply isn't economical unless the foundation is already being excavated for waterproofing reasons.

There is a third hybrid scenario worth knowing: interior spray foam only, with no additional stud framing. Closed-cell spray foam applied at 2"–3" directly to the foundation wall gives you R-12 to R-21 in a very thin profile, acts as its own air and vapour barrier, and is moisture-resistant. It doesn't give you stud bays for wiring or the higher R-values of a full assembly, but it's clean and fast.

For a typical New Brunswick home doing an interior renovation, the best practice is rigid foam continuous against the foundation wall (at least R-10), then a 2x4 stud wall framed 1/2" away from the foam, filled with R-12 or R-14 mineral wool or fibreglass batts, for a total assembly of roughly R-22 to R-24 below grade. This meets and exceeds the NB Building Code requirement of R-17 for basement walls in Climate Zone 6 under the 2020 NBC as adopted in New Brunswick.

NB Power's Home Energy Efficiency Upgrade program and the Canada Greener Homes Grant both include basement wall insulation as an eligible upgrade — the Greener Homes Grant offered up to $5,000 for basement walls, depending on the R-value improvement achieved, when accompanied by a pre- and post-retrofit EnerGuide evaluation. This can significantly offset the cost of a quality interior insulation system.

If you're unsure which approach makes sense for your home — especially if you have an older foundation, drainage concerns, or limited interior headroom — a qualified insulation professional registered with New Brunswick Insulation can walk you through the options specific to your property.

---

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

• Gionetterenovations
• Thirty Four Renovations
• Brunswick insulation & roofing
• moose luxury painting
• 3Tone Construction Ltd