How does polyisocyanurate rigid foam compare to XPS board insulation for R-value retention in cold New Brunswick winters?

Polyisocyanurate (polyiso) loses significant R-value in NB's cold winters, while XPS maintains consistent performance — making XPS the better choice for exterior applications in our Climate Zone 6 conditions.

Polyiso's Cold Weather Performance Problem

Polyiso rigid foam is rated at R-6 to R-6.5 per inch at laboratory test temperatures (around 75°F/24°C), but its R-value drops dramatically as temperatures fall. At 20°F (-7°C) — a typical winter day in New Brunswick — polyiso delivers only about R-4.5 per inch. When temperatures hit -10°F (-23°C), which happens regularly during NB winters, polyiso's effective R-value can drop to R-3.5 per inch or lower. This represents a 40-45% reduction in thermal performance exactly when you need insulation most.

The problem occurs because polyiso's blowing agent (pentane) condenses at cold temperatures, reducing the insulating properties of the foam cells. In practical terms, that expensive 2-inch polyiso board you installed for R-12 might only be delivering R-7 to R-8 during January and February — the months when your heating bill is highest.

XPS Maintains Consistent Performance

XPS (extruded polystyrene) provides R-5 per inch and maintains this rating across the full temperature range experienced in New Brunswick. Whether it's +30°C in July or -25°C in January, XPS delivers the same thermal resistance. This consistency makes XPS far more predictable for energy modeling and heating cost calculations.

Best Applications for Each Material in NB

Use XPS for: Below-grade foundation insulation, exterior continuous insulation over sheathing, under-slab applications, and any location where the foam will experience outdoor winter temperatures. XPS also offers superior moisture resistance — critical for NB's wet soils and high humidity conditions.

Use polyiso for: Interior applications where it stays warm (interior basement walls, interior side of cathedral ceilings), flat roof assemblies where it's protected above the roof deck, and commercial applications with hybrid insulation systems. Polyiso works well when it remains above 40°F (4°C) consistently.

NB-Specific Recommendations

For exterior wall continuous insulation — increasingly popular in comprehensive energy retrofits — XPS is the clear choice in New Brunswick. A 2-inch XPS layer provides consistent R-10 thermal bridging protection through our 4,800-5,200 heating degree day winters. Polyiso in the same application might only deliver R-7 to R-8 when you need it most.

For basement wall insulation, XPS rigid foam boards are ideal because they handle soil moisture contact and maintain full R-value against cold foundation walls. Many NB contractors install 2 inches of XPS (R-10) against basement walls, then frame with 2x4 studs and add R-12 batts for a total of R-22 — excellent performance for our climate.

Cost Considerations

Polyiso typically costs $2.50-$5.00 per square foot installed in NB, while XPS runs $2.00-$4.00 per square foot. When you factor in polyiso's reduced winter performance, XPS often delivers better value per dollar of actual thermal resistance during heating season. For the modest price difference, XPS provides peace of mind and predictable performance year-round.

Professional Installation Notes

Both materials require proper installation with sealed joints and continuous coverage to prevent thermal bridging. However, XPS is more forgiving of installation imperfections because its performance doesn't vary with temperature. This consistency makes it easier for contractors to predict and guarantee energy savings.

Need help finding a professional insulation contractor experienced with rigid foam installation? New Brunswick Insulation can match you with local experts who understand the performance differences between these materials in our Maritime climate.