This molded synthetic is an excellent insulator, but is somewhat expensive and flammable. It is manufactured in two ways: One is extrusion, which results in fine, closed cells, containing a mixture of air and refrigerant gas. The other is molded or expanded, which produces coarse, closed cells containing air.
Molded or expanded polystyrene is commonly called beadboard and has a lower R-value than extruded polystyrene because of its lower density and because it does not contain refrigerant gas. It is also less expensive than the extruded form.
Both types of polystyrene insulation have the advantages of high R-value, good moisture resistance, high structural strength compared to other rigid insulation materials. They are easy to work with and can be used a sheathing. The disadvantages are they can be expensive, flammable, meaning they require a fire-protective covering, and can degrade when exposed to sunlight or temperatures over 165°F.