2021 IECC Insulation Requirements
The 2021 IECC increases ceiling insulation requirements for Zones 2 and 3 from R-38 to R-49. Based on the math below, this increase seems to make a meager difference in performance, even at a large Delta T. I can’t imagine it would save much energy. What reasons for this increase might I be overlooking? Or, more generally, what other considerations go into setting code insulation levels?
Math: 100F outside temp (so, average roof at ~160F), AC set at 78. Delta-T = 82. The air film on the ceiling (R-0.92) is radiating heat down on you, so you want insulation…
Under the 2018 IECC, R-38 of *continuous exterior foam* (no thermal bridging) yields a ceiling temp of ~79.94F. (Math: 160 – [38/38.92 x 82]).
Under the 2021 IECC, upping that to R-49 yields a ceiling temp of ~79.51F. (Math: 160 – [49/49.92 x 82]).
Any indoor air stratification would raise the ceiling’s base temperature, reduce Delta-T, and thus lead to *less* difference between R-38 and R-49. Am I missing something here, or putting too much emphasis on the difference between ceiling temps?
Bonus question: Any reason to think about this differently with an unvented cathedral ceiling, vs. a traditional vented attic approach?
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