The 2015 Washington State Energy Code (WSEC) requires R value identification on all batt insulation over 12” in width. There are several code sections that reference R value identification. The first is R303.1. which requires all equipment, materials and systems be identifiable as energy code compliant.
Section R303.1.1 goes further and requires the manufacturer to provide an R value mark on all pieces of insulation 12” in width or more. Since this mark may be installed on only one side of the insulation, section R303.1.2 requires the mark to be installed so that it is readily observable for inspection.
Similar energy code sections (N1101.10) (N1101.10.1) (N1101.10.2) can be found in chapter 11 of the 2015 IRC.
Here is some additional information regarding insulation, vapor barriers, etc. from the Department of Energy.
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