Is polyiso rigid foam board effective in cold New Brunswick temperatures? | Insulation IQ?

Polyisocyanurate, commonly called polyiso, is one of the most efficient rigid foam board products available — but it has a well-documented performance characteristic that New Brunswick homeowners and builders need to understand before choosing it for cold-climate applications. The short answer is: polyiso is excellent in many NB applications but should be used carefully in locations where it will be continuously exposed to very cold temperatures.

The thermal drift issue with polyiso is real and supported by decades of building science research. At room temperature (around 20–24°C), polyiso delivers its rated R-value — typically R-6 to R-6.5 per inch, which is the highest of any common rigid foam product. However, at sustained temperatures below approximately -4°C, the blowing agents trapped in polyiso's closed-cell structure begin to condense, and the effective thermal resistance drops. In laboratory testing and real-world measurements, polyiso at -20°C (a temperature Fredericton, Moncton, and Saint John routinely see in January and February) can perform at only R-4 to R-5 per inch — a significant reduction from its rated value.

This means polyiso rated at R-20 (3+ inches) installed in a fully exposed exterior application — such as on the outside face of a wall or roof deck in New Brunswick — may only deliver the thermal equivalent of R-14 to R-16 on the coldest winter days. That is still a meaningful amount of insulation, but it means calculations based on the label R-value will overestimate performance in extreme cold. By contrast, XPS (extruded polystyrene) at R-5 per inch is more thermally stable across the temperature range, and EPS (expanded polystyrene) at R-3.8–4.2 per inch actually improves marginally at colder temperatures.

So when is polyiso the right choice in New Brunswick? The answer comes down to temperature exposure. In applications where the foam will be kept relatively warm — such as roofing systems (installed above the roof deck where daytime solar gain keeps temperatures elevated), interior wall applications, or above-grade walls where the foam is protected by cladding and only partially exposed to outdoor air — polyiso performs excellently and its high R-per-inch rating is a genuine advantage. On a flat commercial roof in Moncton, polyiso is the industry standard precisely because it delivers high R-values with less thickness and weight.

For NB basement walls, polyiso is generally not recommended. Basement applications keep the foam in direct contact with cold concrete that may be at or below 0°C through much of the winter, placing the foam in exactly the temperature range where polyiso underperforms. XPS or EPS are better choices for basement foundation walls across New Brunswick.

For above-grade exterior continuous insulation (the continuous foam layer added outside the sheathing on a wall), polyiso is a reasonable choice if properly designed. Some building scientists recommend applying a temperature-correction factor of 15–20% to polyiso's R-value when sizing it for Climate Zone 6 applications. Using a corrected R-value of approximately R-5 per inch (instead of R-6.5) for polyiso in NB exterior wall calculations will give more realistic performance estimates and help ensure the assembly behaves as designed in a Fredericton or Campbellton winter.

Cost-wise, polyiso boards typically run $0.80–$1.20 per square foot for 2-inch panels, which is comparable to XPS. The trade-off between polyiso and XPS for above-grade NB walls is essentially a wash once the temperature correction is applied — you may need slightly more polyiso thickness to achieve the same corrected R-value as XPS. For roofing and interior applications where full rated performance is realised, polyiso's density and R-per-inch advantage make it excellent value.

NB Power rebates and the Canada Greener Homes Grant both recognise insulation upgrades by R-value added, so choosing a product that delivers its rated R-value in NB conditions will maximise your rebate eligibility. When specifying polyiso for an exterior application and submitting for rebates, use the temperature-corrected R-value to avoid overstating performance in your EnerGuide audit documentation.

For guidance on selecting the right rigid foam type for your specific New Brunswick project — whether it is a foundation wall, an attic hatch, an exterior re-cladding, or a low-slope roof — consult with a qualified insulation contractor through New Brunswick Insulation or the New Brunswick Construction Network.

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