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Flat Roof EPDM – limited height – zone 4a – rigid foam then closed cell?

jroz | Posted in General Questions on

Hi All, 

zone 4a, doing a 170 sqft mudroom roof that is pitched but essentially “Flat” no attic space, just 2×12 rafters, plywood down.  I was planning to do 3″ of polyiso then 1/2 fiberboard, then EPDM, it was rated to R19.1, then would do R30 in the rafters below to get R49 and the ideal 40/60 split per Martin and team’s articles/suggestions.  I’ve hit a snag, I don’t have the additional 3″ above the roof deck because of a very odd configuration with where another over lapping mansard roof is, some additional trim that needs to go on top, and gutters, so essentially aesthetically but also potentially functionally I don’t have that head room.

What is my ideal scenario at this point?  I was thinking I should use 1″ polyiso against roof deck then fiberboard , as that I still have room for, but anything more is pushing it.  Is it worth at least doing 1″ for some added R and some additional vapor/air retarder qualities? Below the roof deck I was going to have them do 6 or 7″ of closed cell foam to get me my R49, so the question is, should I do the 1″ above, or just go with the 7″ below and hope for the best.  I understand it doesn’t get me the 40/60 split, but I thought that was just rigid to fluffy.  If i did something like closed cell underneath I don’t know if that 40/60 and dewpoint issue is a concern.  I also understand that i’m creating a sandwich in-between now with rigid foam on top, so i’m at the mercy of the epdm being done right and moisture not getting in between.  So is it add the 1″ on top just for good measure, and closed cell underneath?  or skip the 1″ above the roof deck and just do the closed cell below?   Thanks guys!!

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    jroz,

    With a flat roof you are always at the mercy of the EPDM membrane. Venting or being vapour-open won't save the roof if it leaks.

    I don't think the 1" foam layer over the sheathing does anything. I'd build it like Assembly #4 in this link:
    https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work

    Another overview:
    https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs

    1. jroz | | #2

      Thanks for responding, so the 1” poly iso doesn’t do anything, or it’s bad to have it up there? So you’re saying based on 4 I just have him do the fiberboard and epdm above and spray foam below, no added benefit at all for the 1” polyiso above? I already have the material so I can just return the 10 boards for 350 or so, or have him use it above?

      1. Expert Member
        MALCOLM TAYLOR | | #3

        jroz,

        When I said the 1" foam doesn't do anything I should have been more specific. It doesn't help one way or another to make the roof any more resistant to moisture problems, but yes a layer of continuous insulation will up the R-value a bit, and reduce thermal bridging.

  2. Expert Member
    Akos | | #4

    Zone 4A means 30% rigid/spf for condensation control. When you are using exterior rigid, your assembly R value is higher, so most codes allow for U factor based compliance. This might be only R38 or R42 of insulation.

    Assuming R38 that means R11 of exterior rigid. That is about 2" of polyiso. If you can stretch your height a bit, you can do the roof without any spray foam.

    EPDM can be installed directly over the rigid, or if your roofer is not comfortable with that you can get a foam cover board. In either case this would only mean an extra 1/2" from your starting point, so it should be doable.

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