# How are insulation R-values calculated for wall cavities of inconsistent thickness?

I'm designing a house which will have unconventional wall construction that varies in thickness. The wall cavities will be filled with dense-packed cellulose, rated at R-3.7 per inch. I want to understand how to be sure that I'll meet IECC/IRC code requirements for minimum R value for the wall insulation.

I know that it's typical, for example, for the ubiquitous 2x4 framed exterior wall with studs 16" OC to meet the minimum of R-13 if the stud bays are filled with fiberglass batts. Clearly, though, the wall's actual average R-value will be less than R-13 due to the studs themselves, which have a much lower R value than the fiberglass.

So, what exactly is the method used to evaluate a wall's insulation design against code? Is it:

• the R-value of the insulation material at its minimum thickness, ignoring studs?
• the R-value of the insulation material at its average thickness, ignoring studs?
• something else?

As a more concrete example, the illustration below depicts top-down views of wall cross sections, where light gray represents 2x4 studs (vertical) and sheathing (horizontal) and dark gray represents insulation fill. My understanding is that examples A and B would be rated equally at R-13. At what R-value would a wall like example C be rated?

• When we built our house, the roof and wall insulation was 12" thick and that was not including the wall coverings. Makes the heating bill very low. Feb 1, 2021 at 6:52
• No matter how you calculate it, the left end of wall `C` will allow more heat transfer than the right end. If the left end meets your (code) requirements, then all is good, if not, then you likely have an issue. Feb 1, 2021 at 15:40
• @FreeMan My situation is that most of the wall will be significantly above code requirements, but some small portions would be slightly below. That's why I'm interested in knowing whether all parts of the wall must be at or above code requirements, or if it's the average that counts. Feb 1, 2021 at 19:48