Polyiso and XPS are each used at “lower” temperatures, generally below ambient. While polyiso is suitable for low-temperature steam, XPS is not. Typically, the lower the temperature (through cryogenic), the less often XPS is used and the more often polyiso is used. Where cycling temperatures may exceed 165°F (74°C), XPS cannot be used since it softens, whereas polyiso is suitable at 300°F (149°C) and beyond per some manufacturers (e.g. Dyplast up to 350°F). Physical properties of PIR and XPS are quite different, and indeed the properties vary among manufacturers of the same class of insulant. Clients should insist on full disclosure of properties.
If you would like to learn more about when and why polyiso is more suitable than XPS, click here.