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.
