What batt thickness suits New Brunswick walls?

For most New Brunswick homes, the right batt thickness depends on your wall framing — R-22 batts in 2x6 walls are the current code target, but the full story involves thermal bridging, vapour management, and whether you're building new or retrofitting an existing home.

Understanding Wall Framing and Batt Fit

The vast majority of NB homes fall into two categories. Homes built before roughly 1985 typically have 2x4 framing, which gives you a 3.5-inch cavity. The best batt you can fit in that space is R-12 or R-14 (high-density fibreglass or mineral wool). Homes built after the mid-1980s are more likely to have 2x6 framing, giving you a 5.5-inch cavity that accepts R-20 or R-22 batts.

The problem is that even a perfectly installed R-22 batt in a 2x6 wall only delivers a whole-wall effective R-value of around R-16 to R-17. The wood studs — which run every 16 or 24 inches and are only about R-6 — act as thermal bridges, bypassing the insulation entirely. In NB's Climate Zone 6 with 4,800 to 5,200 heating degree days, that thermal bridging is a significant and ongoing source of heat loss.

What NB Building Code Actually Requires

The current NB Building Code targets R-22 to R-28 effective whole-wall performance for above-grade exterior walls in new construction. Hitting that number with batts alone is impossible — you need continuous exterior insulation (rigid foam or mineral wool board) over the sheathing to eliminate thermal bridging. A common code-compliant assembly is R-22 batts in a 2x6 cavity plus R-5 to R-6 of XPS or mineral wool board on the exterior, bringing the whole-wall effective value up to the required range.

For existing homes being retrofitted, the code is more flexible, but the principle is the same: batts fill the cavity, and if you want to meaningfully improve performance, you need to address the studs.

Choosing Between Fibreglass and Mineral Wool

Both come in the right thicknesses for NB walls, but they behave differently. Standard fibreglass batts (R-12 for 2x4, R-20 for 2x6) are the most affordable option and widely available at NB building supply stores. They work well when installed carefully — no gaps, no compression, no voids behind electrical boxes.

Mineral wool batts (Rockwool/Roxul) cost 30-50% more but offer real advantages in NB's climate. They're dimensionally rigid, so they hold their shape in the cavity without sagging. They're hydrophobic, meaning they shed water rather than absorbing it — a meaningful benefit in a Maritime climate where wall assemblies occasionally see moisture. They also provide better fire resistance and soundproofing. For basement walls or any assembly where moisture is a concern, mineral wool is worth the premium.

The Vapour Barrier — Non-Negotiable in NB

Regardless of which batt you choose, 6-mil polyethylene vapour barrier must go on the warm (interior) side of the insulation — between the batts and the drywall. NB's cold winters create strong vapour drive from inside to outside, and without a proper vapour barrier, warm moist interior air migrates into the wall cavity, hits the cold sheathing, and condenses. That hidden moisture causes mould and rot that can go undetected for years. Every seam must be lapped and sealed with acoustical sealant, and every penetration (electrical boxes, pipes) must be sealed carefully.

Practical Tips

For a 2x4 wall retrofit, use R-14 high-density fibreglass or R-15 mineral wool — these are "friction-fit" batts designed to fill a 2x4 cavity without compression. For a 2x6 wall, use R-22 mineral wool or R-20/R-22 fibreglass. If you're doing a full exterior renovation and replacing siding anyway, add 2 inches of XPS rigid foam (R-10) over the sheathing before re-siding — this is the single best upgrade you can make to wall performance in NB and eliminates thermal bridging entirely.

If your existing walls have no insulation at all (common in pre-1970s NB homes), dense-pack cellulose blown into the closed cavities through small holes is usually more cost-effective than opening walls for batts — typically $2.50 to $4.50 per square foot installed.

Need help finding a professional insulation contractor to assess your walls and provide a quote? New Brunswick Insulation can match you with local contractors for free through the New Brunswick Construction Network.