ASTM C591

Covers — unfaced preformed rigid cellular polyisocyanurate (PIR) insulation for cold and cryogenic surfaces, classified by compressive-resistance Type (I–VI) and service-temperature Grade, with thermal and physical properties.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Non-combustible cold insulation (cellular glass) → ASTM C552.
• Hot-service mineral fiber → ASTM C547.
• Chloride limits on stainless → ASTM C795; calculation → ISO 12241.

PIR is combustible — where fire performance is critical, cellular glass may be preferred despite higher cost.

C591 sets minimum requirements for PIR insulation:

• Type & Grade — compressive-resistance Types (I–VI) and service-temperature Grades.
• Thermal Conductivity — k-value at cold mean temperatures (accounting for aging).
• Physical Properties — density, compressive strength, closed-cell content, water vapor permeability.
• Marking — Type/Grade and product identification.

ASTM C591 specifies rigid cellular polyisocyanurate (PIR) foam insulation for cold and cryogenic piping. Lightweight with good thermal performance, it is classified by compressive-strength Type — higher types serve as load-bearing supports — and by service-temperature Grade.

Unlike cellular glass and mineral fiber, PIR is organic and combustible, so fire requirements must be evaluated. As with all cold insulation, its performance depends on an intact vapor barrier to keep moisture out and prevent corrosion under insulation. Thickness follows ISO 12241.

Engineering Note: Select the PIR compressive Type to match support loads — using a low-strength grade at a pipe shoe or support crushes the insulation, creating a thermal short and a CUI initiation point.