Q.: What is the difference between expanded polystyrene (described in ""insulated concrete walls built without forms,"" June 1980, page 467) and cellular polystyrene (described in ""a simple, efficient insulation system for poured concrete walls,"" June 1980, page 482)? Can one be substituted for the other?

A.: The popular terminology for polystyrene insulation has become slightly confused. The kind of material described in both articles cited is the same. This material is sometimes also known by a third name--bead board--because in manufacture, this kind of insulating polystyrene is expanded by blowing polystyrene beads with heat. It is properly called expanded polystyrene or EPS.

There is another kind of polystyrene insulation--extruded polystyrene--which is expanded with a fluorocarbon gas. The board is extruded after the polystyrene is expanded.

Discussion of the properties of expanded polystyrene (EPS) and extruded polystyrene, which are somewhat controversial, is given in:

""choice and installation of insulation,"" Concrete Construction, September 1980 pages 665, 667.

"Comparisons of Polystyrene Insulations," Concrete Construction, January 1981, pages 62, 63.

"Insulation for Earth-sheltered Construction," Concrete Construction, June 1981, pages 536, 537.