What R-value does NB code require for floors over unheated spaces? | Insulation IQ?

Under the New Brunswick Building Code, floors over unheated spaces — such as vented crawlspaces, unheated garages, and unconditioned storage areas — must meet a minimum effective thermal resistance of RSI 5.02, which converts to approximately R-28.6, for Climate Zone 6. This is the prescriptive requirement that applies to the vast majority of residential construction in New Brunswick, including homes in Fredericton, Moncton, Saint John, Miramichi, Bathurst, and surrounding communities.

This R-value represents the total effective thermal resistance of the floor assembly, not just the insulation product's nominal rating. That distinction matters because the structural framing, air films, subfloor sheathing, and finished flooring all contribute something to the total, but they also create thermal bridging through the framing members. In a standard 2x10 joist bay, the usable cavity depth is about 235mm (9.25 inches). Filling that cavity with fibreglass batts rated at RSI 3.35 (R-19) gets you partway there, but it falls well short of the R-28.6 target. That's why many NB contractors use a combination of R-20 or R-22 high-density fibreglass batts plus an additional layer of rigid foam board applied to the underside of the joists, or alternatively use spray foam to achieve both air sealing and thermal performance in a single product.

Open-cell spray foam at around R-3.5 per inch and closed-cell spray foam at approximately R-6.5 per inch are both used for floor applications in New Brunswick, though closed-cell is generally favoured for floors over crawlspaces because it is also a vapour retarder, which is critical in NB's cold climate. The National Energy Code for Buildings (NECB) and the Part 9 prescriptive path in the NB Building Code both recognize that vapour control is inseparable from thermal performance in Climate Zone 6 — moisture moving through an insufficiently controlled floor assembly can cause joist rot, mould growth, and structural damage over time.

For floors over unheated attached garages, the same RSI 5.02 minimum applies, but the details differ slightly. Garage environments are more variable than crawlspaces — they experience vehicle exhaust, temperature cycling, and potential fuel spills — so the air barrier and vapour control layer on the garage side of the floor assembly must be robust. Many NB building inspectors pay close attention to continuity of the air barrier at the rim joist where the floor assembly meets the foundation wall, as this junction is a common weak point.

It's worth noting that RSI 5.02 (R-28.6) is the minimum — it is not the recommended target for energy-efficient construction. The NB Power Home Energy Savings Program and the Canada Greener Homes Grant both push for higher performance. Greener Homes, for instance, encourages homeowners to aim for R-35 or higher in floor assemblies over unheated spaces to maximize energy savings and rebate eligibility. NB Power's energy advisors will model your home and calculate the incremental savings from going beyond code minimum, and in many cases the payback period for the upgrade from R-28.6 to R-35+ is under ten years given New Brunswick's heating costs.

The air sealing requirement that accompanies floor insulation in the NB code is often overlooked by homeowners. Section 9.25 of the NB Building Code requires that the air barrier system be continuous — gaps at the rim joist, around plumbing penetrations, and at partition walls that intersect the floor assembly must all be sealed before or alongside insulation installation. An uninsulated but unsealed floor can lose far more heat through air leakage than through conduction, so product R-value alone doesn't tell the full story.

For floors over crawlspaces specifically, NB code also addresses whether the crawlspace is vented or unvented. In a vented crawlspace, the floor above is the thermal boundary and must hit RSI 5.02. In an unvented conditioned crawlspace, the thermal boundary moves to the crawlspace walls instead, which changes the required R-values entirely and introduces different vapour management requirements.

If you're planning a floor insulation project and want to confirm your assembly will pass inspection and qualify for available rebates, connecting with a qualified insulation contractor through New Brunswick Insulation is a good starting point.

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