16 C.F.R. § 460.20
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:
(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.”
[44 FR 50242, Aug. 27, 1979, as amended at 70 FR 31276, May 31, 2005]