Polyisocyanurate (also known as polyiso or PIR) is essentially an improvement on polyurethane (PUR).
The proportion of methylene diphenyl diisocyanate (MDI) is higher than for PUR and instead of a polyether polyol, a polyester derived polyol is used in the reaction. Catalysts and additives used in PIR formulations also differ from those used in PUR. Advantages of polyisocyanurate over polyurethane are...
Higher R-value per inch of thickness
Greater stiffness and dimensional stability
Better compressive strength
Superior performance in fire tests
PIR insulation is typically produced as a foam and used as rigid thermal insulation. Its thermal conductivity has a typical value of 0.16 BTU*in/hr*ft2*°F (0.023 W/mK).
The widely used "R-value" measurement simply refers to thermal "resistance value" - that is, a measure of insulation's ability to impede heat flow, thereby keeping hot or cold air outside. A higher R-value means that insulation works more efficiently. Increasing the thickness of an insulation layer increases R-value.
R-value Comparison of Common Insulation Materials
Polyiso is the most thermally efficient rigid insulation available in the marketplace; it has the highest R-value per inch of thickness. Because of its superior R-value, buildings that use Polyiso have lower heating and cooling costs than buildings insulated with other materials of the same thickness. For every three and a half inches of polyiso foam (R27), this much of another insulating material would be needed for equivalent insulatin performance:
Thickness to Match 3.5" of PIR (R-value:27)
R 7.6 - 8.0/inch
R 5.6 - 6.0/inch
R 3.8 - 5.0/inch
R 3.6 - 3.8/inch
R 2.9 - 3.8/inch
Polyiso meets the strict standard of both FM Class 1 Approvals (FM 4450/4470), UL 1256, and CAN/ULC S126M