Q. Am I the only one confused by the claims of manufacturers of foil-covered bubble wrap and foil-covered flexible thin plastic foam? Apparently, if one is to believe the advertising, a 3/8-inch-thick roll of foil-faced bubble wrap can give you the same thermal performance as 2 inches of rigid foam insulation. How is this possible?

A.Martin Holladay, editor of Energy Design Update, responds: The R-value of 3/8-inch-thick foil-faced bubble wrap is about 1.3. The R-value of 3/8-inch-thick foil-faced expanded polystyrene foam is about 1.6.

By contrast, 2 inches of extruded polystyrene insulation has an R-value of 10.

Bubble-wrap manufacturers who claim their product matches the performance of 2 inches of rigid foam are breaking the law.

In 2004 and 2005, the Federal Trade Commission sent out a series of letters to manufacturers of foil-faced bubble pack, warning, "The FTC staff is aware that certain claims have been made in the marketplace for foil-faced bubble pack products (or similar reflective or radiant barrier products) installed under concrete slabs. In the staff's view, it may be misleading for industry members to suggest that such foil products will reflect radiant heat when installed under concrete.

"It is well accepted that reflective insulations and radiant barrier products must have an air space adjacent to the reflective material to be effective. Such air spaces are unlikely to exist under concrete slabs.

"Accordingly, it is unlikely that the reflective qualities of these products will yield any significant benefits when they are installed under slabs. …

"In the staff's view, advertising that suggests otherwise could harm the ability of builders and other consumers to make appropriate insulation choices."

Foil-faced bubble wrap is a product in search of an application. The product has too low an R-value to provide much benefit under a concrete slab; moreover, the foil facing provides no benefit under a slab beyond that of a vapor barrier.

Although some manufacturers of foil-faced bubble pack recommend installing the product under roof sheathing to reduce summer heat gain in an attic, there are cheaper, foil-only products that serve that purpose just as well.

In other applications — crawlspaces, above-grade walls, and attic floors — the R-value of conventional insulation (for example, rigid foam, fiberglass batts, and cellulose) per dollar invested will be considerably higher than that of foil-faced bubble pack.