According to the U.S. product standard for polystyrene
insulation, ASTM C578, "Standard Specification for Rigid, Cellular
Polystyrene Thermal Insulation," 15 types of polystyrene insulation
are manufactured in the U.S. ASTM C578, Type VIII, designates 1.25
pounds per cubic foot nominal density expanded polystyrene (EPS)
insulation typically used in roof system installation. ASTM C578,
Type IV, designates 25 pounds per square inch compressive strength
extruded polystyrene (XPS) insulation typically used in
below-membrane roofing applications.
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