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Polystyrene Insulation
Polystyrene is an excellent insulator. However it is somewhat expensive and flammable. It can be manufactured in two ways: extrusion (which results in fine, closed cells, containing a mixture of air and refrigerant gas) or 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, and high structural strength compared to other rigid insulation materials. They are easy to work with and can be used as a sheathing. The disadvantages are that they can be expensive, flammable (requiring a fire-protective covering), and can degrade when exposed to sunlight or temperatures over 165°F.
