As a lightweight closed-cell foam, expanded polystyrene, or EPS, is popular in a multitude of applications, including crafts, insulation and construction projects that call for firm closed-cell foam. The unique structure of polystyrene provides almost total resistance to moisture and water vapor. Polystyrene foam has a superior R-value, so polystyrene sheets will insulate to keep heating or cooling inside of any room or commercial space and will not rot, mold or attract fungi or mildew.