There are mainly three types of heat insulation panels: polyisocyanurate (ISO), extruded polystyrene (XPS), and expanded polystyrene (EPS) – and each has different performance attributes. They are all foam core panels.
ISO is primarily used in roofing applications. The panels provide up to R-6.5 per inch of thickness but degrade slightly over time. ISO panels are expensive, and cannot be recycled.
XPS is a recyclable material mostly used in walls and lower grade applications. XPS foam panels provide about R-5 per inch of thickness and fall between ISO and EPS in cost. However, XPS absorbs more moisture than other insulation panels over time.
EPS foam has the highest average R-value per dollar of the rigid foams (about R-4.6 per inch) without degrading. EPS is one of the most versatile candidates in heat insulation panels because you can use it anywhere in the building envelope.