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Trying to make sense of IRC2021 roof insulation requirements

jollygreenshortguy | Posted in Building Code Questions on

I’m hoping my friends and colleagues here can help me make sense of the IRC2021 roof insulation requirements.

“Assemblies shall have a U-value equal to or less than that specified in Table N1102.1.2…”
Table N1102.1.2 gives U values for roofs.  For example zone 6 = 0.024

The next paragraph says, “Assemblies with an R-value of insulation equal to or greater than that specified in Table N1102.1.3 shall be an alternative to the U-factor in Table N1102.1.2”

For zone 6 Table N1102.1.3 an R-value = 60.

As I read this I should be able to use either table.
By definition R-value = 1/U-value
If I go with the U-value table I need a U-value no greater than 0.024. This translates to an R-value = 1/0.024 = 42

So according to the U-value table I can meet the requirement with R42 insulation but according to the R-value table I need R60. That’s a big difference.

I understand that the U-value is referring to the entire roof section, including thermal bridging at trusses and rafters. But if, for example, I do R25 of continuous rigid insulation above the roof deck and r21 high density batt insulation between the top chord of my roof trusses I can easily meet the R42 criteria. Does that mean I don’t have to go to R60?

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Yes. If you use a hybrid system and the calculated whole-roof U value is less than .024 you are OK. The R-60 requirement is for the insulation only, when using cavity insulation with no continuous insulation.

    1. jollygreenshortguy | | #2

      I just did the math and it looks like I can make it work with 4.5" of rigid insulation on the roof deck and R21 batts underneath. It satisfies the condensation control provisions of IRC 2021 as well.

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