Polyisocyanurate (PIR) rigid insulation is actually a mixture of rigid PUR and PIR. By incorporating the chemical benefits of both products, PIR is sometimes referred to as PUR/PIR foam. Other monikers include iso board and polyiso. Most commonly sold with a shiny foil facer on one or both sides, PIR foam board rigid insu­lation has the highest R-value of any common insulation material. Although somewhat water resistant, PIR is not recommended for below-grade applications. PIR products must meet the require­ments of ASTM C1289, “Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.”