What is the difference between XPS and EPS rigid foam for New Brunswick homes? | Insulation IQ?

XPS and EPS are both rigid foam board insulation products used extensively in New Brunswick homes, but they differ in manufacturing process, performance characteristics, environmental impact, and cost. Choosing between them is not simply a matter of picking the cheaper option — the application location and moisture conditions in NB's Climate Zone 6 environment make certain properties much more important depending on where in the house you are insulating.

XPS (Extruded Polystyrene) is manufactured by melting polystyrene resin with a blowing agent and extruding it through a die, which creates a uniform closed-cell structure with a smooth skin on both faces. In Canada it is most commonly sold under the trade name Styrofoam (blue, manufactured by Dow) or as pink board (Owens Corning). XPS has an R-value of approximately R-5 per inch and maintains that value reliably across a wide range of temperatures and moisture conditions. Its closed-cell structure makes it inherently resistant to water absorption — XPS absorbs less than 0.3% of its volume in water over a 28-day immersion test. This moisture resistance is why XPS is the default choice for below-grade applications, under-slab insulation, and exterior foundation walls in NB.

EPS (Expanded Polystyrene) is manufactured differently — polystyrene beads are expanded with steam in a mould, fusing the beads together. The result is the familiar white beadboard foam, visible at the cut edges as small spheres. EPS has an R-value of R-3.8 to R-4 per inch at standard Type II density, though higher-density grades (Type IX) achieve closer to R-4.2. EPS is also a closed-cell foam but has a slightly more permeable structure than XPS — it absorbs somewhat more moisture over long periods, though it also dries out more readily than XPS when conditions allow. The cells in EPS contain trapped air rather than a blowing agent, which means EPS does not degrade in R-value over time — a meaningful advantage over XPS, which loses some R-value as the blowing agent slowly escapes over the first decade of service.

Where Each Product Makes Sense in New Brunswick

For below-grade foundation walls and under-slab applications — common in Fredericton, Moncton, and Saint John basements — XPS is historically the dominant choice because of its superior moisture resistance and higher R-per-inch value. When soil moisture is in constant contact with insulation, EPS can absorb slightly more water and experience R-value degradation in persistently wet conditions.

However, for above-grade wall assemblies, attic hatches, and interior basement applications where the insulation is protected from bulk water, EPS at high enough thickness is equally effective and typically costs 20 to 40% less per R-value than XPS in New Brunswick markets. Many energy-efficient builders in NB use EPS on the interior of basement walls (2 to 4 inches) where the foam is not in direct contact with soil and moisture management is handled by other means.

Environmental impact has become a significant factor in product selection. XPS uses HFC blowing agents (primarily HFC-134a in North American products) with a very high global warming potential — over 1,400 times that of CO2. EPS uses pentane as its blowing agent, which has a global warming potential roughly 7 times that of CO2. From a lifecycle environmental perspective, EPS is substantially less harmful. The federal Greener Homes Grant, while not specifying product type, reflects a broader push toward environmentally responsible retrofits — and EPS aligns better with that intent.

Cost comparison in NB (approximate, 2025 market): 2-inch XPS runs $1.00 to $1.40 per square foot; 2-inch EPS Type II runs $0.70 to $1.00 per square foot. To achieve equivalent R-value with EPS, you need slightly more thickness — about 2.5 inches of EPS to match 2 inches of XPS at R-10 — so the cost difference narrows but EPS still typically comes out cheaper.

Vapour permeability differs slightly: XPS at 2 inches has a permeance of roughly 1 perm, while EPS at 2 inches is approximately 2 to 5 perms depending on density. In most NB applications, both products function as adequate vapour retarders at the thicknesses used for code-compliant assemblies.

For insulating between rafters or in cathedral ceilings, neither XPS nor EPS is ideal alone — spray foam or a combination assembly is usually preferred. But for sheathing over exterior walls (continuous exterior insulation), both products work well, with EPS increasingly favoured for its lower environmental footprint and lower cost at the required thickness.

When selecting between the two for your NB home, consider the application location, long-term moisture exposure, your environmental priorities, and the installed cost at the required R-value. An insulation professional familiar with NB conditions can help you make the right call. New Brunswick Insulation and the New Brunswick Construction Network directory list qualified local contractors across the province.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

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• 3Tone Construction Ltd
• Thirty Four Renovations
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• Arctic Fox Construction Inc.