What is the minimum R-value required for basement floor insulation under a slab in New Brunswick when building a new home or addition?

The NB Building Code does not specify a minimum R-value for under-slab insulation in most residential basement applications, but current best practice and energy code requirements point to R-10 (roughly 2 inches of rigid foam) as the accepted standard for new construction in New Brunswick.

This is one of those areas where code minimums and practical recommendations diverge, and it's worth understanding both sides before you pour concrete.

What the Code Actually Says

The National Building Code of Canada (as adopted in NB) addresses below-grade thermal performance primarily through the building envelope requirements in Part 9 (houses and small buildings). For basement floors specifically, the prescriptive energy efficiency path requires R-10 under-slab insulation in NB's climate zone (Zone 6 for most of the province, Zone 7A for northern areas like Edmundston and Campbellton). In colder northern NB, some energy compliance paths push toward R-15 under the slab to meet overall envelope performance targets. Your local building inspector or an energy advisor can confirm which zone applies to your specific location and which compliance path your builder is using.

It's worth noting that if your builder is using the performance compliance path (an energy model rather than prescriptive tables), the under-slab R-value can vary — but the overall building envelope must still hit the same energy target.

Why R-10 Is the Practical Minimum in NB

NB's climate makes under-slab insulation more than a code checkbox. Concrete slabs sit in direct contact with soil that stays cold year-round — typically 8–10°C even in summer. Without insulation, that cold slab acts as a heat sink, driving up heating costs and creating a cold, uncomfortable floor surface. It also contributes to condensation on the slab surface during humid NB summers, which is a direct path to mold under any flooring you install. Two inches of rigid EPS (expanded polystyrene) foam delivers approximately R-10 and is the most common under-slab product used by NB contractors. XPS (extruded polystyrene, the blue or pink board) offers slightly higher R-value per inch at around R-5 per inch.

For a heated basement living space, many NB builders now recommend R-15 to R-20 under the slab — especially if in-floor radiant heating is planned, where the insulation prevents heat from being wasted downward into the soil.

Practical Tips

Before your slab is poured is the only practical time to install under-slab insulation — retrofitting it later means breaking up the concrete, which costs $15–$30 per square foot just for demolition and repour. If you're building new or adding an addition, don't skip this step or undersize it to save money. The material cost difference between R-10 and R-20 under a typical basement slab is often only $1,000–$2,000 — a fraction of what you'd spend heating a cold, uninsulated basement over 10 years.

Also confirm that your under-slab drainage (gravel bed and weeping tile) is properly installed before the foam goes down. In NB's high water table conditions, getting the drainage layer right is just as important as the insulation above it.

When to Hire a Pro

Under-slab insulation placement itself is straightforward, but the decisions around it — drainage layer depth, foam type and thickness, vapour barrier placement, and tie-in with the wall insulation assembly — should be coordinated by your contractor and confirmed with your building inspector before the pour. Getting the inspection sign-off on your sub-slab work before concrete is placed is mandatory, and skipping that inspection is a code violation that can require breaking up the slab.

If you're planning a new basement or addition and want to make sure the insulation and waterproofing are spec'd correctly from the start, New Brunswick Basements can connect you with experienced local contractors through the New Brunswick Construction Network.