Many people who are buying added insulation for their homes may be doing little or nothing to save energy costs, according to Consumers Union.

In documents filed by the group with the Federal Trade Commission, CU says an investment in more insulation "will often yield a surprisingly small savings of energy and money." The filing was based on a CU study to be published in the February issue of Consumer Reports.

Ira J. Furman, a spokesman for the consumer organization, said in an interview that "insulation is being oversold as a conservation tool."

He pointed to the results of one CU test which showed that six inches of added fiber glass insulation in an attic that already has 3 1/2 inches of the material would save less than $10 a year.

"If the hypothetical house we are using in this example, which is in Columbus, Ohio, has no attic insulation, then the added six inches would save $117," Furman said.

But, he pointed out, many insulation companies are selling added insulation to people whose homes already have some insulation in place.

"Actually," he said, "the first 3 1/2 inches take you about 90 percent of the way. And when people buy insulation they don't need, it prevents people who need it from getting it."

There are critical shortages of materials in the fiber glass insulation industry, because few manufacturers have enough capacity to handle rapidly increasing demand.

CU's filing to the FTC asked that proposed labeling on home insulation indicate that while a higher "R-value" (a rating used to evaluate the ability of insulation to retard to flow of heat) means more effective insulation, consumers may by over-buying if they purchase the highest "R-value" insulation available.

"You can insulate a house to a point where the money spent on insulation far exceeds the money saved on fuel," Furman said.

Furman said much of the money spent on the ineffective, added insulation could be better spent on other energy-saving devices, such as storm windows, exterior caulking, weather stripping or energy-saving thermostats.