A.John Evans, codes and standards manager with Icynene, responds: Most building codes require that foam plastic insulation — including both rigid and open-cell low-density foam products — be separated from occupied living space (including basements) by an approved 15-minute thermal barrier.

While 1/2-inch gypsum board is recognized for this purpose by most codes, other less-common materials that can be used are sprayed-on cementitious coatings (A/D Fire Protection Systems, www.adfire.com ) and mineral-fiber coatings (American Sprayed Fibers, www.asfiusa.com). Before using a system not specifically mentioned by code, of course, it's always a good idea to consult first with your local building inspector.

In concealed spaces — like crawlspaces or unvented, conditioned attics — where there's no occupancy, an ignition barrier is usually required. Most building codes specifically approve 3/8-inch drywall for this purpose, but other options include spray-on intumescent paints — such as FF 88 (International Fire Resistant Systems, www.firefree.com ) — 1 1/2-inch mineral fiber, 1/4-inch wood structural panels, and .016-inch-thick corrosion-resistant metal.

In some unvented, conditioned attic applications, Icynene has been approved for use without an ignition barrier; for details, see Icynene's ICC-ES NER-420 report. Again, consult with your local code officials before choosing this approach or deciding on a specific ignition-barrier product or material.