Expanded polystyrene and extruded polystyrene are both made from polystyrene, but EPS is composed of small plastic beads that are fused together and XPS begins as a molten material that is pressed out of a form into sheets. XPS is most commonly used as foam board insulation. Its density range is about 28–45 kg/m3.
Similarly as EPS, XPS has a wide variety of applications. It can be used for the insulation of buildings, roofs and concrete floors. Extruded polystyrene material can be also used in crafts and model building, in particular architectural models.
Although both expanded and extruded polystyrene have a closed-cell structure, they are permeable by water molecules and can not be considered a vapor barrier. In expanded polystyrene there are interstitial gaps between the expanded closed-cell pellets that form an open network of channels between the bonded pellets. If the water freezes into ice, it expands and can cause polystyrene pellets to break off from the foam.