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Multiple layers of Foil Faced Polyiso for Flash and Batt

kramttocs | Posted in Energy Efficiency and Durability on

Did the flash and batt with ccSPF and Fiberglass previously on an unvented cathedral. Seems to have worked well.
Looking to do another section of the same roof line (this part is currently unconditioned storage) but wanting to do foam board + fiberglass this time around.

Is there a problem with using 2 layers of foil faced polyiso? Foil on both sides. 1.5″ thickness per sheet so 3″ total. This would be RMax Thermasheath brand.
Alternatively I can do  XPS but the polyiso is a little cheaper and has a better r-value per inch.

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  1. kramttocs | | #1

    Update: I spoke to a nice gentleman at Rmax and he stated that it doesn't matter (doubling up) so long as there is full contact with the roof deck and each other (no air gaps). Which I know is the key thing to avoid when doing anything 'cut-n-cobble' like this so already planning attention to detail there.

    Interested in any other thoughts as well though.

    Different usage but he also confirmed that thermasheath polyiso is valid to use below grade for foundation insulation (was a concern of mine).

    1. Expert Member
      Akos | | #2

      No matter how you do it and what foam you use, cut and cobble is a very risky assembly, there is a good chance the roof will fail.

      For unvented roofs with insulation only bellow the roof deck, spray foam (spray foam +batts) is your only option.

      1. kramttocs | | #3

        Thanks Akos.
        I know that's the general consensus and I've read a lot of articles/posts but I don't really follow it. Not arguing or saying I've decided to do it X way and will do that no matter what.

        Here's where I am coming from - this current roof deck is unvented except for air leaks in the fascia. No soffit venting and no ridge venting. 2x8 rafters with r13 fiberglass batts. There is an air gap but I can't say why it was done that way since no flow. It's been like this since the 90s and the roof deck shows no problems where I've looked. Just had the roof replaced and deck was solid.
        I know introducing change is risky but I just can't see how even if I put up the foam board blindfolded it would be introducing more risk than it already has. I've insulated all my crawlspace rim joists and check them yearly so I feel pretty confident with cutting the edges at a slight angle and then filling with spray foam (door+window likely). This would be done at both layers with staggered seams.

        While I like the ease (but not cost) of someone coming in and spray foaming, this section of the roof is 1x12s vs plywood like the other I had sprayed. While it may not be a problem, I don't like the idea of that many gaps where the foam could work through and bulge the shingles.

        I am all for and welcome a discussion on this though I know you regulars are probably tired of doing so. With attention to detail, I just struggle to see the real-world difference between sprayfoam vs using 8' strips.
        If it's a matter of most installs being poorly done, I can get behind that.

        1. kramttocs | | #4

          Still beating this dead horse.
          I know the primary (only?) concern with cut-n-cobble is that framing will move and gaps will open. As has been touched on before, it's just hard to imagine this will be any more of a problem with rigid foam vs ccSPF.
          We had some major issues with shrinkage (yes, I do think installer error) of the ccSPF so maybe I am a bit negatively predisposed to that since I saw it firsthand. Just that after that debacle, the rigid foam actually seemed less risky in that regard.

          1. Expert Member
            MALCOLM TAYLOR | | #5


            I wouldn't call Martin's blog and the following comments an interdiction against using cut & cobble, more a warning it's risky.

            One difference between it and spray foam is that while both can separate from the rafters at the perimeter of the bays, with foam that gap is the extent of the area where moist air can reach. When the perimeter of foam board separates, the air can migrate behind the board and wet the entire sheathing above.

          2. Expert Member
            BILL WICHERS | | #7

            ccSPF is fully adhered -- it has NO gaps anywhere for moisture to get to. With any kind of rigid foam, you have little cracks and voids that moisture can get to, and that's where the problems come from. Rigid foam can shrink too, but I don't think it's anywhere near as severe as can happen with a bad SPF job.

            If you can leave a 1" or larger vent channel above your stack of polyiso, then you don't have to worry about moisture. An easy way to do this is to put 1x2 furring strips on either side of the rafters up tight to the roof sheathing. Put your polyiso over those, and foam it in place with canned foam. The furring strips will create a 1.5" vent channel that will carry away any moisture that makes its way into the assembly. You just have to make sure those vent channels tie into a ridge vent at the top and a soffit vent at the bottom so that you get airflow through the vent channel to allow it to function.

            I would not install polyiso tight to the underside of the roof sheathing. How many layers you use makes no difference here, the face that there is a gap -- albeit a small one -- between the polyiso and the roof sheathing is where the problem is.


  2. kramttocs | | #6

    Hey Malcolm,
    (For some reason I can't reply to you). That makes perfect sense. A crack along the edge with the center still being adhered is a lot different than the zero adhesion of the board.

    I absolutely get that the more steps to make something work is just more steps where something can go wrong but what about adding flashing tape along both edges? Hear me out :) Would be in contact with the foam, bridging the can foam edge, and contacting the rafter. I know the first thought would be 'yuck' but after using roll after roll of 3m 8067 to detail all of my exterior rigid foam, this really seems viable. Seems like a long term solution for the likelihood of cracking and the flat surface of the rigid + rafter makes it possible.
    Not even looking at it from time/cost, would something like this make up for the shortcomings of the cut & cobble? Also knowing that everything is only as good as the install.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      I just don't know.

      I'm really risk adverse. That comes from designing and building for other people, and probably my temperament, so it isn't something I'd do - but (and I think this is what Martin is getting at in the blog I linked to) that doesn't mean you can't make it work. It just means you should be prepared for the possibility it won't.

  3. thrifttrust | | #9

    I make channels similar to those Bill recommends. However protruding roofing nails often interfere with installing wood furring. I make strips from foam. Now nails just poke into the strips. Sometimes they even hold the strips in place.

    A particular advantage of poly-iso is that it cuts easily and cleanly with a box cutter. I mostly use 1 inch thick foam. The cutter reaches 3/4 or more of the way through, then the piece snaps clean. A drywall T square is helpful. Even for long channels it is usually easier to cut material to 4 foot lengths.

  4. kramttocs | | #10

    Thanks all. Really appreciate all the replies.

    Let me take a big step back to see if doing anything is even the right move.

    To explain it simply this is a 60' long 20' wide section. 12/12pitch.
    30' of it has living space beneath and living space above. This living space above is what has the ccSPF flash and batt.
    The other 30' has garage beneath and then storage above. The storage is accessed through the aforementioned upper living space. Garage is accessed through the downstairs living space (dividing wall and door is detailed as an exterior wall in both cases).
    It's the storage space cathedral ceiling I am looking to do something with. No plumbing or electrical. Single window on the gable end opposite the door (to the upper living space)
    The floor of the storage above the garage has r30 fg batts. Can lights for the garage are present.
    Again, no soffit or ridge and r-13 batts in the storage space rafters. The space is 'finished' out with osb under the rafters and on the kneewalls.

    Right now I basically have an unvented attic over unconditioned space.

    I'd say there is a very slim chance on conditioning this space. The only reason I am considering this now is because we got a new black asphalt shingled roof last fall and I am concerned about it since I read something somewhere about if insulation is on the floor than the deck needs to be vented or hot-roofed. Maybe that doesn't apply when neither the above or below are actively conditioned though?
    Of course both leach some heat from the living spaces.

    Maybe the solution is to just open the window unless raining? :)

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