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Q.My painting subcontractor is recommending Thermo-Tek, a paint the manufacturer says is a radiant barrier. What can you tell me about products like this one — or similar coatings made with something called “ceramic microspheres” — that claim to have thermal characteristics and promise significant energy savings?

A.Martin Holladay, editor of Energy Design Update, responds: Thirteen years ago, Energy Design Update referred to the marketing of “insulating” ceramic paints as “one of the most enduring frauds in the energy-efficient products industry.” Indeed, the insulating-paint scam lives on to this day, even though researchers at Oak Ridge National Laboratory and the Florida Solar Energy Center have shown that these so-called ceramic paints perform exactly the same as ordinary white paint.

According to its maker, Thermo-Tek contains “ceramic microspheres and microfibers that bind tightly together to form a barrier against radiant, convective, and conductive heat transfer.”

By definition, radiant barriers work by reducing heat transfer by radiation across the air space between two materials. The actual amount of thermal energy radiated by a material depends on its surface temperature and a property called “emissivity” (also called “emittance”). A material’s emittance is indicated by a number between zero (0) and one (1); the lower the emissivity, the lower the emitted radiation.

But according to Ben Feinsod, a technical representative from Thermo-Tek, a surface coated with two coats of Thermo-Tek paint has an emissivity of 0.83, far higher than the legal maximum for a true radiant barrier, which is 0.10. (As it turns out, no paint on the market can meet the radiant barrier standard.) Thermo-Tek doesn’t even qualify as a “radiation-control coating,” a less stringent category that requires an emissivity rating of 0.25 or less.

In any case, the emissivity of Thermo-Tek is a moot point, since the emissivity of interior or exterior paint is basically irrelevant to the thermal performance of a typical U.S. home. A low-e coating can be effective only if there is a big temperature difference between the surface being coated and its environment. As Vermont energy consultant Andy Shapiro explains, “If a wall is cold enough in winter or hot enough in summer that emissivity is going to make a difference, the wall must be uninsulated. In that case, you should insulate the wall, because that’s where you’ll achieve your savings.”

Claims that a Thermo-Tek coating offers the same insulation R-value as a 6-inch fiberglass batt and that it can deliver energy savings of up to 35 percent mirror those of another marketer of “ceramic microsphere” paint, Kryton Coatings International. This company was forced to cease its marketing efforts in 2002 when a Federal Trade Commission complaint cited its claims as “false or misleading.”