16 CFR 460.20: R-value per inch claims.

In labels, fact sheets, ads, or other promotional materials, do not give the R-value for one inch or the “R-value per inch” of your product. There are two exceptions:

(a) If an outstanding FTC Cease and Desist Order applies to you but differs from the rules given here, you can petition to amend the order.

(b) You can do this if actual test results prove that the R-values per inch of your product does not drop as it gets thicker.

You can list a range of R-value per inch. If you do, you must say exactly how much the R-value drops with greater thickness. You must also add this statement: “The R-value per inch of this insulation varies with thickness. The thicker the insulation, the lower the R-value per inch.”