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Do I need to use R10 minimum on the roof deck in climate zone 4C in all cases?

ansalk | Posted in Building Code Questions on

I am detailing a low slope unvented roof. I will be using spray foam & then batt insulation to total R38 under the roof deck in Climate zone 4C. I could meet code just with that but would like to use rigid EPS board, R6, on top of the roof deck to prevent thermal bridging. This introduces a third insulation that is not covered in the code
– Do I need to use R10 to meet the minimum above deck value? Does this rule still apply since I’m using spray foam directly below the deck?
Thanks, Andy

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  1. Expert Member
    Dana Dorsett | | #1

    What type of spray foam?

    As long as ~20% of the total R is outside the first condensing surface there isn't going to be significant moisture accumulation. If it's open cell foam the first condensing surface is really the roof deck, but if it's closed cell foam it's the conditioned-space side of the closed cell foam.

    At R38 total (= less than IRC code minimum) R6 on the exterior wouldn't cut it for open cell, but it would be fine with as little as 1" of closed cell foam. For the same 1.5" of rigid foam depth you could use fiber faced roofing polyiso (about R8.5) instead of EPS and the type of foam under the roof deck wouldn't matter.

  2. GBA Editor
    Martin Holladay | | #2

    Here is a link to an article with answers to your questions: "Combining Exterior Rigid Foam With Fluffy Insulation."

    You may also want to read this article: "Flash-and-Batt Insulation."

    Dana is correct: the answer to your question depends on whether you are installing closed-cell spray foam or open-cell spray foam.

    In Climate Zone 4C, at least 20% of the total R-value of the roof assembly must come from the closed-cell foam layer.

    If you have R-38 total, you'll need at least R-8 of the insulation to come from closed-cell spray foam or closed-cell spray foam plus rigid foam.

    If you have R-44 total (R-38 + R-6), you'll need at least R-9 of the insulation to come from closed-cell spray foam plus rigid foam.

  3. ansalk | | #3

    Thanks for the responses.
    I am using 4" closed cell foam (R26) under the roof deck plywood + 3" batt (R15) below that for R41 within the truss cavity.
    If I want to use rigid board on top of the roof deck, does the R value really matter since the closed cell below the deck is keeping the plywood warm.? In other words the 20% can be below or above the roof deck, correct?

  4. GBA Editor
    Martin Holladay | | #4

    With 4 inches of closed-cell spray foam, you're good to go.

  5. Expert Member
    Dana Dorsett | | #5

    I'm wondering why the 4" of closed cell foam? It sounds like you have 7.0" deep trusses (not rafters) and were perhaps looking to get close to the IRC code minimum R49?

    An R15 batt compressed to 3.0" won't be more than ~R13 - R13.5, and unless it's a 1.5" thick 2x4 truss chord on either side of the batt the fit isn't going to be very good or requires quite a lot of meticulous batt-sculpting.

    Seven inches of an HFO-blown closed cell foam would hit the R49 on it's own, if that's the goal. It would also be greener than 4" of HFC blown foam due to the comparatively very low impact of HFO1234ze compared to the current industry standard HFC245fa used for most closed cell foam.

  6. ansalk | | #6

    Thank you for your material observations Dana.
    I do not have a truss depth or a R value problem. There is plenty of depth in my 16" roof trusses.
    I only have the question of the thickness of the exterior rigid insulation. I would like to keep it to 1 1/2" EPS. Having met the code R-value that I need with Flash and Batt (or another closed cell option) within the truss cavity, can I simply use the EPS on the deck for thermal bridging?

  7. Expert Member
    Dana Dorsett | | #7

    >Having met the code R-value that I need with Flash and Batt (or another closed cell option) within the truss cavity, can I simply use the EPS on the deck for thermal bridging?

    Yes, the R-value of the EPS can be anything you like.

    But open web trusses don't have much thermal bridging to speak of, and even I-joist trusses have less than half the thermal bridging of milled lumber rafters. Are you sure the additional complication of 1.5" of exterior EPS is really "worth it?".

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