Polyiso vs. XPS vs. EPS: Which Rigid-Foam Insulation Is Best?

When building or replacing a commercial roof, choosing the right rigid insulation board matters for performance and long-term energy costs. Here's a direct comparison.

Polyisocyanurate (Polyiso)

R-value: R-6.5/inch nominal — but degrades to R-4 or lower in cold (below 20°F). Most widely used, most cost-effective. Good performance in moderate climates. Susceptible to moisture absorption if exposed. Best for: warm climates or moderate climate zones where cold-temperature R-value degradation isn't a significant concern.

Extruded Polystyrene (XPS)

R-value: R-5/inch, consistent across temperatures. Excellent moisture resistance (closed-cell structure). More expensive than polyiso or EPS. Best for: cold climates where consistent R-value matters, or in moisture-prone environments. Commonly used in below-grade and inverted roof assemblies.

Expanded Polystyrene (EPS)

R-value: R-4/inch, consistent across temperatures. Good moisture resistance. Lower cost than XPS. Best for: cold climates where consistent R-value is needed but XPS budget premium isn't justified. Also used in below-grade applications.

The Spray Foam Alternative

For commercial roofing restoration projects, spray polyurethane foam (R-6.5/inch, consistent across temperatures) outperforms all three board insulation options: no seams at board joints eliminating thermal bridging, no separate cover board required, and it simultaneously provides both insulation and waterproofing. For most commercial restoration applications, foam delivers superior value to any board insulation system.