Specifying and purchasing polystyrene thermal insulation can be complicated because there are multiple product types with distinctive physical properties. Using the ASTM International standard applicable to polystyrene can provide useful guidance.
The U.S. product standard for polystyrene thermal insulation is ASTM C578, "Standard Specification for Rigid, Cellular Polystyrene Insulation." It applies to expanded polystyrene (EPS) and extruded polystyrene (XPS) products.
Within ASTM C578, there are Types I through XV classification designations. Type III was deleted because the product no longer is available. Seven type classifications describe EPS products, and seven describe XPS products.