What R-value does 2-inch rigid foam provide and is it enough for NB code? | Insulation IQ?

A 2-inch sheet of rigid foam insulation provides varying thermal resistance depending on which type you choose, and understanding these differences is essential before deciding whether it meets New Brunswick's building code requirements. Expanded polystyrene (EPS) at 2 inches delivers approximately R-8, extruded polystyrene (XPS) provides roughly R-10, and polyisocyanurate (polyiso) achieves approximately R-12 to R-13 at the same thickness. These are nominal values — polyiso in particular loses some performance in cold conditions, which matters significantly in New Brunswick's Climate Zone 6 winters.

New Brunswick follows the National Building Code of Canada, with the 2015 NBC adopted provincially and updated provisions carrying forward. For most residential applications, code-prescribed minimum insulation levels are substantially higher than what 2 inches of rigid foam alone can deliver. Basement walls require a minimum of RSI 1.96 (approximately R-11) for the above-grade portion, with the full wall assembly typically targeting R-15 to R-20 to be competitive with energy standards and to qualify for incentive programs. Above-grade walls have a prescriptive requirement under the NBC of RSI 3.08 (roughly R-17.5) in Climate Zone 6, meaning 2 inches of rigid foam alone is insufficient as a standalone wall insulation solution.

That said, rigid foam is very rarely used as a sole insulation source in a wall or basement assembly — it functions most powerfully as a thermal break layered over or inside other insulation. A common and code-compliant approach in New Brunswick basements is to combine 2 inches of XPS against the foundation wall (R-10) with a 2x4 stud wall filled with R-14 or R-15 mineral wool or batts, achieving a total assembly value in the R-24 to R-25 range. This significantly exceeds code and positions the homeowner well for NB Power efficiency rebates and the Canada Greener Homes Grant.

For attics and roofs, 2 inches of rigid foam in isolation is never sufficient — the NBC and energy programs targeting Climate Zone 6 call for attic insulation in the range of R-50 to R-60 for high-performance buildings, and even the minimum prescriptive levels are around R-40 for ceilings. Rigid foam in attic or cathedral ceiling assemblies is used strategically, for example as a vent baffle or as part of a hybrid assembly, not as the primary thermal layer.

Where 2 inches of rigid foam is effectively used as a standalone solution is in specific retrofit scenarios — adding continuous exterior insulation over an existing wall assembly, for instance, where the existing wall cavity already contains batt insulation. Adding 2 inches of polyiso (R-12.5) to an existing R-12 wall brings the total closer to R-24, meaningfully exceeds code for the above-grade wall, and importantly moves the dew point outward, reducing condensation risk inside the wall cavity during Fredericton or Moncton winters.

One important detail for New Brunswick homeowners: when adding exterior rigid foam, the NBC requires that the ratio of exterior to interior insulation is sufficient to keep the sheathing warm enough to avoid condensation. In Climate Zone 6, the exterior insulation should represent at least 40–50% of the total wall R-value for safe sheathing temperatures. Two inches of polyiso over a standard 2x6 wall (R-20 cavity) meets this threshold reasonably well.

In summary, 2-inch rigid foam provides R-8 to R-13 depending on type, and is not sufficient on its own to meet NBC prescriptive requirements for most applications in New Brunswick. It performs best as part of a layered assembly. For project-specific guidance on your home's wall, basement, or ceiling assembly, the professionals listed on New Brunswick Insulation can assess your existing assembly and recommend an approach that meets or exceeds code while maximizing available rebates.

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