What is a Good Alternative to Polyiso Board Insulation?

Polyiso (polyisocyanurate) board insulation is commonly specified in commercial roofing — but it has known limitations. Here are the best alternatives and when to consider them.

Polyiso Limitations

Polyiso's R-value degrades significantly in cold temperatures. Its nominal R-6.5 per inch can drop to R-4.5 or lower at temperatures below 20°F — a significant performance gap in cold climates. Additionally, polyiso can absorb moisture if exposed, reducing performance over time.

EPS (Expanded Polystyrene)

EPS provides consistent R-4 per inch across temperature ranges, doesn't absorb moisture, and maintains performance in cold climates better than polyiso. It's commonly used as an alternative in cold-climate applications where polyiso's thermal drift is a concern.

XPS (Extruded Polystyrene)

XPS provides R-5 per inch with excellent moisture resistance and consistent performance across temperatures. It's a premium option that maintains performance better than polyiso in cold conditions.

Spray Polyurethane Foam: The Best Alternative

Spray foam provides R-6.5 per inch consistently across temperature ranges, doesn't require a separate cover board, bonds directly to the substrate without mechanical fasteners, and eliminates the seams and thermal bridging at board joints that rigid insulation systems introduce. For roofing applications where insulation performance and simplicity matter, spray foam outperforms board insulation on every dimension.