Where Polyiso Foam Board Is Used in North American Manufacturing

Polyiso has the highest R per inch of any board, takes a glass facer that passes the FM and UL roof fire tests, and is made on continuous laminators by the same companies that sell the membrane over it, so it is the default under every commercial low-slope roof in North America

Polyiso wins continuous insulation on commercial walls because it gives the most R in the least thickness, which shortens fastener lengths and cladding attachments, and because the board makers have paid for NFPA 285 tested assemblies with the major cladding, WRB, and window systems

Foil-faced polyiso is the volume insulated sheathing on residential walls because it delivers R-5 to R-6.5 per inch, the foil facer doubles as the water-resistive barrier when the seams are taped, and it is the cheapest way for a production builder to hit the continuous insulation column of the IECC table

Polyiso is not used below grade because the foam and the facer take on water; it is listed here so the matrix shows where the polyiso market stops

Highest R per inch of the rigid boards and a Class A fire rating with facers, so it owns commercial roof decks and most continuous wall insulation

Polyisocyanurate poured in place between the skins is the volume core, and it does four jobs at once: insulation at roughly R-7 to R-8 per inch, structural shear transfer between the skins, adhesion to both skins without a separate adhesive, and air and vapor barrier continuity. Nothing else does all four, which is why the product exists in this form. The live issue is combustibility. NFPA 285 assembly testing and IBC height and area limits push mineral wool cores into taller and institutional buildings, at a real penalty in R-value per inch and a large penalty in weight and handling.

Polyiso board carries the highest R-value per inch and a foil or fiberglass facer that bonds well under a TPO membrane, so OEMs marketing a four-season roof spec it; it costs more and its facer can wick water at the seams if the membrane fails

Polyiso board in heated-underbelly and four-season floors for the R-value, usually with a fiberglass facer so it takes the lamination adhesive