Polyisocyanurate is a type of plastic made with closed-cell foam. Inside its cells is a low-conductivity gas, which gives the insulation an R-value ranging from R-5.6 to R-8. Gas can leak out of the cells over time and let air in, which can lower the R-value.
Polyisocyanurate insulation comes in several forms: liquid, sprayed foam and rigid foam board. It also can be made into laminated insulation panels with an array of different facings. Foam-in-place applications are often less expensive than installing foam boards, and they tend to perform better because the liquid foam molds to all surfaces.