16 CFR 460.6: “Representative thickness” testing.

All tests except reflective insulation tests must be done at a representative thickness for every thickness shown in a label, fact sheet, ad, or other promotional material. “Representative thickness” means a thickness at which the R-value per unit will vary no more than plus or minus 2% with increases in thickness. However, if the thickness shown in your label, fact sheet, ad, or promotional material is less than the representative thickness, then you can test the insulation at the thickness shown.