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What is the code requirement for cathedral ceilings in zone 5A?

CLAG | Posted in Building Code Questions on

My roof is a mess. I get huge ice dams . I have a wood stove pipe going through the cathedral ceiling.

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  1. GBA Editor
    Martin Holladay | | #1

    The short answer is R-49.

    The longer answer is that some jurisdictions allow cathedral ceilings to have a lower R-value of R-30 if (a) the depth of the space between the rafters isn't enough for R-49, and (b) if the cathedral ceiling represents no more than 500 sq. ft. or no more than 20% of the total insulated ceiling area. See here:

    The code also has a U-factor alternative which usually allows for cathedral ceilings to be insulated to a lower lever than R-49.

    Note that local jurisdictions often have code anomalies, so check with your local building department to determine which code applies.

    For more information, see these three articles:

    How to Build an Insulated Cathedral Ceiling

    Ice Dam Basics

    Prevent Ice Dams With Air Sealing and Insulation

  2. Expert Member
    Dana Dorsett | | #2

    Another short answer (with a longer explanation):

    Code minimum performance is U0.026 maximum in zone 5, per Table N1102.1.4.

    U0.026 works out to a "whole-assembly R" of about (1/U=) R38.5, after adding in the R-values of the various material layers, and factoring-in the thermal bridging of the rafters. With credit for ceiling gypsum, air films roof decking and potential nailer-deck on top you can usually get there with empty rafter bays and 6.5 - 8" of rigid foam (depending on type) above the structural roof deck.

    With essentially no thermal bridging the ice dam potential becomes fairly low, but it fattens up the roof by quite a bit. Usually a hybrid foam/fiber solution is cheaper and easier to build:

    Say you have 2x10 rafters. You can't really get to compliance on an R-value basis with fiber insulation with a vented roof. If you gut the ceilings it would take 7-8" of closed cell foam to hit R49 which would fit, but that would leave a relatively short 7-8" thermally bridging path through the rafters, and in most instances would end up with a U-factor higher than U0.026, which doesn't help the ice damming problem.

    But if you dense packed the cavites with cellulose (compressing any pre-existing batts in place) you'd end up with a 9.25" thermal bridge, and R34-R35 at center cavity. In climate zone 5 with that much R in the cavities it would take about R24-R25 of continuous insulation above the roof deck to protect the roof deck from interior side moisture drives, and it would put an R24-R25 thermal break over the rafters, reducing the ice dam potential. With 5" of 2lb rigid roofing polyiso you'd be good to go from a moisture point of view. Your center-cavity R would be well north of R50, and your U-factor well under U0.026, beating code minimum performance by either method.

    If the rafters are deeper or shallower than 2x10s the solutions that will work for both moisture and thermal performance will also differ.

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