How deep should exterior foundation insulation go below grade in NB? | Insulation IQ?

Exterior foundation insulation depth is one of those details that separates a properly insulated New Brunswick home from one that loses heat steadily through the slab and lower walls all winter long. The short answer is that insulation should extend down to the bottom of the footing — but the reasoning behind that target, and the practical trade-offs involved, are worth understanding before you start digging.

The NBC 2015 (National Building Code as adopted in New Brunswick) and provincial energy requirements under the NB Building Act require foundation walls to be insulated to a minimum of RSI 1.96 (approximately R-11) in Climate Zone 6, which covers virtually the entire province from Fredericton to Campbellton to Saint John. That R-value must be continuous — meaning thermal bridges through fasteners or gaps at the footing are not acceptable if you're trying to hit code performance.

As for depth, most building science guidance and NB practice targets 600 mm (24 inches) below finished grade as a minimum for the insulated portion, with the full wall down to the top of the footing strongly recommended for new construction. The reason is frost depth. In central and northern New Brunswick — places like Edmundston, Bathurst, and Miramichi — design frost depth can reach 1.5 to 1.8 metres. If exterior insulation terminates partway down the wall, the uninsulated section below acts as a thermal conduit, allowing frost to penetrate toward the footing and allowing heat to escape through the very bottom of the foundation where interior insulation offers no protection.

For existing homes doing a retrofit, excavating the full depth of the foundation is expensive — typically $8,000 to $15,000 or more for a standard bungalow foundation in Moncton or Fredericton when labour and backfill are included. In these cases, a practical compromise is excavating to at least 900 mm to 1.2 metres below grade and installing rigid extruded polystyrene (XPS) or expanded polystyrene (EPS) board, both of which maintain their R-value when wet. EPS is generally preferred from a lifecycle and environmental standpoint; XPS uses HFC blowing agents with high global warming potential, though it performs slightly better in wet soils.

The insulation board should be 2 to 3 inches thick (roughly R-10 to R-15 depending on product) for a retrofit, and 3 to 4 inches (R-15 to R-20) for new construction. Above grade, the exposed insulation must be protected from UV and physical damage with a durable cladding — parging, fibre cement board, or a metal Z-flashing and protective panel system are all common in New Brunswick.

One detail many homeowners miss is the footing-to-wall transition. Heat escapes readily at this corner because the concrete footing has direct soil contact and acts like a fin radiating thermal energy. Wrapping rigid insulation around the outside corner of the footing, even with just a short horizontal section, meaningfully reduces this loss. Some builders in the Moncton area taper the insulation outward at the base to direct water away from the footing as an added benefit.

From a NB Power energy efficiency standpoint, deeper and thicker exterior foundation insulation can contribute to eligibility for rebates under the Home Energy Efficiency Upgrade program. Pairing foundation work with air sealing and attic upgrades gives you the best chance at a meaningful rebate cheque and a noticeable reduction in your heating bill — critical in a province where most homes still rely on oil or electric baseboard heat.

For a comprehensive assessment of how deep to go on your specific home — especially on older poured-concrete or block foundations common in Saint John's South End or Fredericton's older neighbourhoods — an energy audit from a registered energy advisor, followed by work from an experienced New Brunswick insulation contractor, is the right path. The New Brunswick Construction Network lists vetted insulation professionals across the province who can advise on your foundation's specific conditions.

---

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

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