Is foil-faced rigid foam better than unfaced for New Brunswick basements? | Insulation IQ?

For New Brunswick basements, the choice between foil-faced and unfaced rigid foam board matters more than most homeowners realize — and in most situations, foil-faced polyisocyanurate (polyiso) or foil-faced XPS offers distinct advantages that make it the preferred choice along basement walls.

Foil-faced rigid foam uses a reflective aluminium facing laminated to one or both sides of the foam core. That facing serves two functions simultaneously: it acts as a radiant barrier, reflecting heat back toward the conditioned space, and — critically for NB basements — it functions as an integrated vapour barrier. In Climate Zone 6, the NB Building Code (Part 9, Section 9.25) requires vapour control on the warm side of insulation assemblies. Foil-faced foam, when installed with taped seams and lapped against the slab, satisfies that vapour retarder requirement without adding a separate layer of 6-mil poly. That's one fewer step in the installation and one fewer potential weak point in the assembly.

Unfaced rigid foam — whether EPS, XPS, or polyiso — is a capable insulator, but it has a vapour permeance rating that varies by product and thickness. At common thicknesses (1–2 inches), most unfaced EPS is semi-permeable, meaning it slows but does not stop moisture-vapour transmission. In a New Brunswick basement where the concrete wall is in contact with soil that can reach near-saturation during spring thaw or heavy coastal rainfall events like those around Shediac or the Miramichi valley, that difference matters. A semi-permeable assembly requires careful detailing to ensure the dew point never falls within or behind the foam layer. Foil-faced products eliminate that uncertainty.

From an R-value perspective, foil-faced polyiso carries the highest R-value per inch of any common rigid foam — approximately R-6 to R-6.5 per inch — compared to XPS at around R-5 per inch and EPS at roughly R-3.8 to R-4 per inch. The NB Building Code minimum effective thermal resistance for basement walls in Zone 6 is approximately R-14.8 (RSI 2.6) for the above-grade portion and varies for below-grade, but most energy consultants and blower-door-conscious builders aim for R-20 effective on a full basement wall assembly. Two inches of foil-faced polyiso at the rim joist plus two inches down the wall, combined with interior framing and batt insulation, gets you there comfortably.

There are a few nuances where unfaced foam makes sense. If you're installing between studs in a wall cavity where the facing would be hidden and serves no reflective function, or if you're layering multiple sheets and need the layers to adhere or be cut to irregular shapes easily, unfaced EPS is more forgiving and slightly less expensive — typically $0.65–$0.95 per square foot for 2-inch unfaced EPS compared to $1.10–$1.60 per square foot for 2-inch foil-faced polyiso at NB building supply yards.

Installation details that matter regardless of facing: all seams should be taped with foil tape or a compatible sealant, the bottom edge should sit on (or be sealed to) the concrete slab, and the top should be sealed to the sill plate. In finished basements where the foam will be covered with framing and drywall, the foam must be protected by a 15-minute thermal barrier — typically 12.7 mm (½-inch) drywall — per the NB Building Code fire protection requirements.

For most homeowners in Fredericton, Moncton, or Saint John retrofitting an existing basement or finishing a new one, foil-faced rigid foam is the cleaner, lower-risk choice. You get a higher R-value per inch, built-in vapour control, and fewer details to coordinate. The modest cost premium over unfaced EPS is well justified given NB's cold winters and the importance of keeping basement assemblies dry for the long term.

If you're planning a basement insulation project and want guidance on which product and thickness suits your specific wall assembly, the professionals listed on New Brunswick Insulation can help you evaluate your options.

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