R-Value for 1″= 5.7 PIR SIPs are available in 4′ and 8′ widths with lengths up to 24′. PIR has the highest insulation value, but with limited structural properties it is better suited for applications that don’t require the SIP to handle longer spans and large loads, such as for Timber Frames and Truss Roofs with spans less than 4 feet. Structural Insulated Panel with Polyisocyanurate (PIR) core. Structural Insulated Panel with Polyisocyanurate (PIR) core. PIR is a great insulator, especially when a thinner profile is specified. PIR cores come in specific thicknesses which can be laminated together to create a variety of SIP Thicknesses. 3″, 4.5″, 6.5″, 8.25″, 10.25″, and 12.25″ are common PIR SIP thicknesses that we manufacture.