What thickness of spray foam do I need for exterior walls in New Brunswick? | Insulation IQ?

Determining the correct thickness of spray foam for exterior walls in New Brunswick requires balancing the NB Building Code minimum requirements, climate zone performance targets, the type of foam being used, and the wall assembly you're working with. The answer varies depending on whether you're using closed-cell or open-cell foam, whether it's a new build or a retrofit, and how the foam fits into the overall wall system.

New Brunswick falls entirely within Climate Zone 6, which demands high-performance wall assemblies. The National Building Code of Canada, as adopted in New Brunswick, requires a minimum of RSI 3.52 (approximately R-20 effective) for above-grade exterior walls in new residential construction. However, "effective" R-value accounts for thermal bridging through studs, which means the insulation itself must exceed R-20 to achieve R-20 effective. Many energy-efficient builders in Moncton, Fredericton, and Saint John target R-24 or higher in wall assemblies to account for this and to meet energy model requirements under Part 9 of the Code.

Closed-cell spray foam delivers approximately R-6 to R-7 per inch. To reach R-20 effective from spray foam alone in a 2×6 stud wall, you need approximately three to three and a half inches. However, most spray foam wall applications use foam to handle the air/vapour barrier function and a portion of the thermal resistance, with the remainder provided by another product:

• 2×6 wall with 2" closed-cell foam (R-12 to R-14) + fibreglass batts (R-12 to R-14) is a very common NB hybrid assembly. The foam handles vapour control, air sealing, and thermal bridging reduction on the cold side; the batts fill the remaining cavity. This assembly comfortably exceeds R-20 effective.
• 2×4 wall with 2" closed-cell spray foam (R-12 to R-14) + R-12 batts hits approximately R-20 to R-22 total, though the shallower stud depth limits options. In a 2×4 retrofit, 1.5 to 2 inches of closed-cell foam against the sheathing is the sweet spot before adding batts.
• Open-cell spray foam at R-3.7 per inch requires roughly 5.5 to 6 inches to hit R-20 — which only fits in a deep 2×6 cavity, and requires a separate vapour retarder (6-mil poly or vapour barrier paint) on the warm side because open-cell foam is vapour-permeable and cannot act as the vapour barrier required by the NB Building Code in Climate Zone 6.

For continuous exterior insulation — a technique increasingly used in NB new builds to eliminate thermal bridging — closed-cell foam is sometimes sprayed onto the exterior face of sheathing before cladding. At 1.5 to 2 inches, this adds R-9 to R-14 to the wall assembly, pushes the dew point outside the stud cavity, and dramatically reduces air leakage. Combined with R-12 or R-20 batts in the stud bay, this creates high-performance walls well above code minimum.

For retrofit work in older Moncton or Saint John homes being opened up for renovation, 2 inches of closed-cell foam against the exterior sheathing is the most common recommendation — it provides the critical air and vapour barrier at the cold side, and the remaining cavity can be topped up with blown-in insulation for an economical hybrid assembly.

Vapour barrier compliance is an important consideration for New Brunswick. The Code requires a vapour barrier with a maximum permeance of 60 ng/(Pa·s·m²) on the warm side of walls in Climate Zone 6. Closed-cell spray foam at 1.5 inches or more typically achieves a permeance well below this threshold, meaning it can serve as the vapour barrier. Open-cell foam cannot — it is far too vapour-permeable and requires a separate 6-mil poly sheet on the interior side.

For basement walls in NB, which face both exterior cold and potential ground moisture, 2 to 3 inches of closed-cell spray foam directly on the concrete or block is generally the minimum recommended, delivering R-12 to R-21 while also acting as moisture control on the cold side of the assembly.

As a practical rule of thumb for New Brunswick exterior walls: 2 inches of closed-cell foam is the working minimum for most wall retrofits where batts will fill the remainder; 3 to 3.5 inches of closed-cell foam alone can get you to code in many cases; and open-cell foam should generally be reserved for areas where a separate vapour barrier will be installed.

For a precise specification suited to your specific wall assembly, connecting with a qualified insulation contractor through the New Brunswick Insulation directory is the best path to a code-compliant and energy-efficient result.

---

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

• Arctic Fox Construction Inc.
• Gionetterenovations
• Thirty Four Renovations
• Brunswick insulation & roofing
• 3Tone Construction Ltd

View all contractors →