Both polyiso and XPS are types of insulation material serving the same basic function: to provide a means to manage the passage of heat in a building system.
That’s where the similarities between these two types of product end, however.
Polyiso insulation, which is short for polyisocyanurate, falls into a category of insulating foam called thermoset plastics. Thermoset plastics, formed from cross-linked polymers, can withstand higher temperatures and will not melt or become pliable when faced with such heat. This attribute allows polyiso insulation to retain its durability and insulating strength.
On the other hand, XPS, short for extruded polystyrene, falls into the category of insulating foam called thermoplastics. Thermoplastics are formed from non-cross linked polymers and are able to be reheated and remolded. This means that XPS is less rigid and can become pliable when exposed to a temperature of about 165 degrees Fahrenheit. XPS insulation meets its melting point typically between 200 degrees and 210 degrees Fahrenheit.
Polyiso has a higher range of R-value per inch of thickness when compared to extruded polystyrene products.
“For example, when both products are used as wall sheathings, the ASHRAE Design R-value for 1” foil faced polyiso is higher per actual inch versus polystyrene, when tested at 75°F mean temperature,” as stated by the Polyisocyanurate Insulation Manufacturers Association. “In roof applications, permeable facers are preferred and the average thickness of foam used is 2 inches. At this thickness, the advantage in thermal performance of polyiso over polystyrene is at least 20%.”
Polyiso insulation boards are pressed between facing material. Polyiso boards for roofing typically use felted paper or coated glass facer, whereas wall polyiso boards typically use either a foil or coated glass facer.
These components offer myriad benefits — including the addition of meeting certain fire ratings — and increased material properties for the intended application. The various facers aggressively bond to flashing and joint tapes, reinforcing it as an integral component of a building air and moisture barrier assembly.
Manufacturers of polyiso insulation note their products perform quite well in fire tests: FM 4450, ANSI/UL 1256, NFPA 285, and CAN/ULC-S126. In the FM 4450 test, specifically, polystyrene is known to melt through steel deck seams and spread fire underneath.
Another difference between the polyiso and XPS insulation concerns ultraviolet light. XPS is degraded by UV light but the facers on polyiso insulation protect the core from such damage.
Polyiso is compatible with a large number of roof and wall systems, too.
When selecting insulation for a construction project, it is important to research and consider the details of each type. Given the differences between polyiso and XPS insulation in categories such as durability, fire, and R-value, it’s no surprise proponents of polyiso insulation stand so firmly behind the product.