Also referred to as PIR, polyiso, or polyisocyanurate, it is essentially a modified polyurethane (PUR) foam. The proportion of methylene diphenyl diisocyanate (MDI) is higher than for PUR and instead of a polyether polyol resin, a polyester derived polyol is used in the reaction. Catalysts, blowing agents and additives used in PIR foam formulations also differ from those used in PUR. Isocyanurates are generally manufactured in a factory into boardstock form and are used in exterior roofing and sheathing applications. Thermal performance of isocyanurate or PIR boardstock foams are comparable to medium-density closed-cell SPF.