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Foil-faced polyiso moldy roof sheathing Q

Kevin McGuire | Posted in General Questions on

I live in zone 4c, and have had trouble with moldy or wet roof and wall sheathing in the past. After research on this website and the 2018 IRC, I think i have it figured out for the most part, but have never used this foil faced rigid before (class I). I was planning to use 3 layers of the thermasheath rmax polyisocyanurate 2″ thick r-13.1 sheets.

This will be a cathedral style ceiling. I currently have soffit intake, w/ ridge exhaust and I plan to maintain a 1.5″ air gap between sheathing and insulation. the 3 layers of insulation will also have spray foam insulation in the gaps and sealed joints etc. Then sheetrock, with two layers of latex paint.

Are there any potential problems with this install? I am not confident moving forward with this application because my usual way of doing things around this house is to have a class III vapor retarder system in the walls and ceilings. But this is different, given the cathedral, and class I foil-faced… Maybe im answering my own question here but in the 2018 IRC it says class I or II required in 4c on interior side of framed walls, and class III permitted if you have rainscreen… but that doesnt tell me about my ceiling necessarily.

Thanks in advance.

One more question i had to add: if everything above sounds good and will work fine, if instead of sheetrock, something like shiplap or beaboard gets installed, does this change anything?

And is it a good idea to air seal the board joints? or is the spray foam insulation applied in the gaps of the rigid good enough for that.

Thanks.

 

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Is there any reason you can’t put the polyiso above the sheathing? It sounds like you’re trying to do a “cut’n’cobble” installation. This can work in a vented roof assembly, but it’s a lot of labor. You’ll find it easier to use two layers of 3” insulation too.

    Bill

    1. Kevin McGuire | | #2

      If i can find 3" pieces i'd use them, sure. Didnt know they made then.
      second, this is a renovation, roof is already on. inside of room is currently demolished waiting for insulation wiring etc. and yes, this is a vented assembly, soffit to ridge with 1.5" air gap between dwelling side of sheathing and insulation.

  2. Expert Member
    Peter Engle | | #3

    Kevin,

    While a cut 'n cobble approach can work, it is very labor intensive and prone to error. All those joints are difficult to seal properly. The multiple layers improve your odds of getting adequate air sealing, but again, at a high labor cost. T&G ceiling panels would make air sealing the foam strips that much more important.

    There are better (and cheaper) methods of insulating cathedral ceilings. Fluffy insulation in the cavities is usually the best. Install an inch or two of foam on the inside of the rafters if you need additional R-value. Taping the seams of a continuous sheet of interior foam is easy.

    1. Kevin McGuire | | #7

      Peter, how about cut n cobble inbetween rafters, and then an additional layer or two of polyiso on dwelling side of rafters? Yes, labor intensive, but i wont have to deal with insulation dust overhead and second, there's higher r value/in.. Plus, this is for only one bedroom, so i dont mind the extra cost and labor for improved r-value. That should be ok right?

  3. Jon R | | #4

    > if instead of sheetrock, something like shiplap or beaboard gets installed, does this change anything?

    Check what is allowed as a thermal barrier over foam.

    I wouldn't build with any Class I vapor retarders (except in very cold climates like Alaska). Not that it can't work - it's just an unnecessary impediment to more drying and less risk.

    1. Expert Member
      Dana Dorsett | | #5

      No ship lap or bead board qualifies ad a thermal barrier over the foam. They would also be pretty air-leaky too.

      It sounds like these are milled 2x8 rafters(?). Installing high R/inch foam between thermally bridging framing is a waste. Do the math:

      https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

      Installing 3/4" of foam board as the exterior batt baffle (R5, if polyiso, R6 if foil faced polyiso with the foil facing the air gap) for R21 cathedral ceiling batts or R23 rock wool is a good starting point. That comes to 6.25" of thickness leaving the code-prescribed 1" air gap.

      Then, a continuous layer of 2" polyiso (R13) on the underside of the rafters (seams taped) would beat the performance of the R39-ish cut'n'cobble, despite the lower center-R value, due to the R13 thermal break over the R7-ish length of rafter. If one used fire-rated Dow Thermax for that 2" layer, cap-nailed or caps-screwed & glued to the rafters you could probably get away with just gluing the bead board or ship lap to the polyiso, but run that by the local code-enforcement first.

      The 3/4" path through the rafters around the edges of the exterior baffle is an adequate drying path into the vent channel- no need to worry about creating a moisture trap between the interior side foam and the baffle foam.

      1. Kevin McGuire | | #6

        Dana thank you so much for sharing that I think it saved me a lot of money. my rafters are 2x6 but I understand you in principal i think

        If you wouldnt mind clarifying something and answering one more question:

        you said, "The 3/4" path through the rafters around the edges of the exterior baffle is an adequate drying path into the vent channel- no need to worry about creating a moisture trap between the interior side foam and the baffle foam."

        Im not sure what you mean by this. But the way I'm understanding you, minus your last statement, is to do a site built baffle out of 3/4 foam, gapped 1" away from sheathing, then install the rockwool, then polyiso over everything including rafters. that's the gist of it right?

        What if I did my baffle, then cut and cobble inbetween rafters with the polyiso, and covered it all with another layer of polyiso... that'd be fine too right? (giving me more r value). Im just trying to conserve space in my room, as i have 7 and a half foot ceilings. the more furring i have to do on my 2x6 rafters, the worse it'll look.

        Thanks.

        1. Expert Member
          Zephyr7 | | #9

          That "3/4" path is a way for any moisture to escape into the vent cavity so that you don't create a moisture trap -- the "double vapor barrier" problem. You'd think that if you put a vapor barrier on both sides, moisture wouldn't get in. The reality is that moisture seems to ALWAYS find a way in, so you need to make sure it can get out and not get "stuck". Dana is telling you that you won't have that problem here, the permeability of the rafters in that littel 3/4" space is enough to make your assembly safe.

          BTW, earlier you mentioned that you didn't know about polyiso in thicker than 2" panels. The box stores usually only carry up to 2" thickness, bu the manufacturers make much thicker material. Up to 4" thickness is commonly available, and even thicker material exists. This is the same for polyiso, EPS, and XPS.

          Bill

  4. Expert Member
    Peter Engle | | #8

    The 3/4" path is half of the rafter width. Moisture can soak/wick/diffuse through the rafters around the edges of the foam baffles. There is always good airflow in the vent channels and that dries any moisture out of the rafters that might accumulate under the foam baffles.

    You can still install foam cut 'n cobble between the rafters if you want. Dana has showed you why it's a bit of a waste of money. It WILL improve the R-value of the assembly, but your $/R ratio is not very good with high-R insulation and low-R wood framing.

  5. Kevin McGuire | | #10

    Dana said, "Then, a continuous layer of 2" polyiso (R13) on the underside of the rafters (seams taped) would beat the performance of the R39-ish cut'n'cobble, despite the lower center-R value, due to the R13 thermal break over the R7-ish length of rafter."

    im wondering, what, if any difference is there in using 1" vs 2" foil-faced poly over the underside of the rafters - if in the end the assembly as a whole comes out to the same r-value? If thats confusing... for example, say there is r27 in between the rafters, and 2" r13 poly is used (r40 total), vs. the assembly with r34 in between the rafters, with 1" r6 poly is used (r40 total)

    using 1" over the underside of the rafters might make things easier for me in the end, wondering how much it'll have an effect in the end, thanks.

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