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Community and Q&A

Installing Polyiso Insulation on Roof

tyleralex1 | Posted in General Questions on

in a pickle…again
we start roofing tomorrow and i think i’ve put the framing in a pickle.
i have advised them to lay down 1/2 plywood then go over that with 1 3/4″ polyiso, then ice & water shield, then standing seam metal roof.
am i screwing up not adding a 2nd layer of sheathing on top of polyiso also?

or can i even flip it and lay the poyiso directly onto the rafters, then 1/2″ plywood, then ice & water, then metal roof?

much thx!

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  1. user-2310254 | | #1


    You should check with the manufacturer of your metal roofing. They have different requirements and will tell you if you are going to void your warranty or create performance issues.

  2. creativedestruction | | #2

    Make sure to choose an appropriate location for your primary air barrier. I would recommend at the structural sheathing underneath the polyiso. Taped plywood works and should connect with your wall air barrier. 2x4 flat furring strips above the polyiso or even nailbase with integrated sheathing/polyiso may work for metal roof attachment depending on the product, as Steve noted.

    I wouldn't recommend smushing polyiso against rafters. This would compromise the structure, air and weather tightness of the assembly.


  3. Expert Member
    Dana Dorsett | | #3

    >"...can i even flip it and lay the poyiso directly onto the rafters, then 1/2″ plywood, then ice & water, then metal roof?"

    Nope. The polyiso will compress against the much narrower rafter edge area over time, and with 1.75" of polyiso between the roof deck and rafters the fasteners would have to be timber-screws on some pre-engineered spacing, not nails.

    Also, half inch plywood doesn't quite meet the structural requirements for roofs in many areas (but 5/8" usually does.)

    Preferred stackup, from the bottom up:

    -Structural roof deck nailed directly to the rafters per code prescriptive fastener schedule.

    -Ice & Water Shield goes directly on the structural roof deck

    -1.75" polyiso, cap-nailed in place, seams taped

    - nailer deck or 2x4 purlins, timber-screwed to the structural roof deck (not to the rafters) with pancake head timber screws.

    - if nailer deck (not purlins), weather resistant semi-permeable underlayments prescribed by the steel roofing vendor. If purlins (not nailer deck) use the roofing vendors preferred fastening method.

    1. dustin_7022224 | | #9

      I too am curious about the comment on attaching 2x4 purlins to the roof deck (not to the rafters). We are planning to run 2x4s diagonally to create a 'rain screen' vent space just below the standing seam metal roof.

    2. merlinm | | #11

      Dana, thanks for this as we are planning a similar roof except with 2" of foil-faced polyiso over structural sheathing. There will be 2 layers of polyiso foam with staggered seams. On the underside of the roof deck, we plan 2" of closed cell spray foam and then 6 or 8" of open cell spray foam. Are the 2x4 purlins on top of the polyiso adequate without a nailer deck on top of the polyiso? Should the purlins be oriented at 45° or vertically?

      1. pakrat1 | | #12

        FYI foil faced poly is vapor closed and 2" of ccspf has very low permiability IE vapor barrier on both sides of deck "risky"

        Dana needs to chime in on this

        1. merlinm | | #13

          Good point, Michael. Thanks.

          Are ice & water shields low permiability, which Dana recommends on the structural deck, as well?

          1. maine_tyler | | #14

            John, Ice and Water is low permeability. But the typical stack-up would not include a 'sandwiching' of sheathing between two low permeability layers.*

            This is accomplished by forgoing ccspf under the rafters and using vapor (and usually air) permeable insulation instead. (Why use spray foam of any kind in the rafter/truss bays if using exterior foam anyways?)

            But do make sure to hit minimum R-value ratios for the above-deck rigid foam, based on climate.

            *That said, the 'sandwich' assembly is occasionally used and considered not a concern by some despite others' nervousness about it. I personally don't see why one would design that from the start, when other options exist.



    3. dustin_7022224 | | #15

      Anyone seen Dana? I'm still hoping he can answer my question (post #9) and a few others have asked for his advice/clarification as well.

      1. b_coplin | | #16

        This question was answered in another thread: "Instructions from some of the nailbase panel manufacturers explicitly state it should not be screwed into the rafters."

        There is a lengthier hypothesis on the rationale, if you're curious:

        Asking a new question will often yield more answers than reviving an old thread, especially if commenters have not uniformly signed up for email notifications.

        1. dustin_7022224 | | #17

          Thanks! I'll look there.

  4. pakrat1 | | #4

    Q for Dana

    why screw into the roof deck, not to the rafters

    Mike Lind

  5. tyleralex1 | | #5

    dana, yes its 5/8" plywood.

    you have some what answered this, but it seems to me its best to tape the seams of the polyiso, not the plywood seams underneath as I will be coming from the inside and spray foaming open cell foam in all the corners of the underside of the plywood where they meet the rafters.

    MBCI, the stand seam roof manufacturer, recommended using a plate (top hat as he called it) to nail thru the polyiso and into the plywood sheathing as this would eliminate overdriving nails into the polyiso. we also would apply a synthetic underlayment directly over the polyiso.

    so outside to inside:
    standing seam roof
    plates nailed through synthetic underlayment, polyiso and into plywood.
    open cell spray foam from underside of plywood in corners of rafters and plywood
    batt insulation

    any and all advise is greatly appreciated...thx so much!

    1. Jon_R | | #6

      So no wood above the foam. Do you have a link to such a plate?

    2. maine_tyler | | #10

      Any particular reason for the open cell spray foam, rather than just taping (or ice and water) the sheathing for air barrier?

  6. tyleralex1 | | #7

    This is from MBCI:

    Please see pic in the middle marked "bearing plate". Please let me know if you have any questions.


    Quote. Order. Build. | Order Metal Roof and Wall Products Online at
    P: 770-948-7568 |
    Direct/Mobile: 812-322-4283| F: 770-739-6228 |

  7. pakrat1 | | #8


    Not trying to butt in to Tyler's post, Just wondering why you say not to screw nailboard into rafters, only primary deck. comment # 3

    Sorry for intruding, but this applies to my project also

    Thanks Mike

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