Polyiso vs. XPS vs. EPS: Rigid-Foam Insulation for Commercial Roofing

Choosing the right rigid insulation board affects your roof's energy performance and long-term cost. Here's a complete comparison of the three main types.

Polyisocyanurate (Polyiso)

R-value: R-6.5/inch nominal, but performance drops significantly in cold (degrades to R-4 or lower below 20°F). Most widely used, lowest cost per R-value. Good in warm to moderate climates. Susceptible to moisture absorption if exposed. Best for: warm/moderate climates where cold-temperature R-value degradation is not a concern.

Extruded Polystyrene (XPS)

R-value: R-5/inch, consistent across temperature ranges. Excellent moisture resistance (closed-cell structure). More expensive than polyiso or EPS per board foot. Best for: cold climates, below-grade applications, inverted roof assemblies where consistent cold-weather R-value and moisture resistance are needed.

Expanded Polystyrene (EPS)

R-value: R-4/inch, consistent across temperature ranges. Good moisture resistance. Lower cost than XPS. Best for: cold climates needing consistent R-value without XPS premium, tapered insulation applications (custom fabrication).

The Spray Foam Advantage

For commercial roofing restoration, spray polyurethane foam (R-6.5/inch, consistent in cold) outperforms all three board options — it provides both waterproofing and insulation in a seamless layer, eliminating thermal bridging at board joints and the need for a separate cover board. Where insulation improvement is a priority, foam delivers more value than board insulation in most commercial applications.