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Underlayment under exterior foam on new roof

CcsIA | Posted in General Questions on

Building new home unvented cathedral ceilings Zone 6
Wondering if I need any type of underlayment on my first layer of plywood or if 2 layers of 2.5″ polyiso insulation  1/2 sheathing with peel and stick is fine?

From inside to out  Drywall,  11 7/8 I joist with fiberglass batts, 1/2 ” OSB, 2 layers of 2.5” polyiso insulation, 1/2 sheathing with peel n stick membrane, metal roof.

Thanks for any help

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  1. creativedestruction | | #1

    You need an air barrier underneath the polyiso. The peel and stick would work great there. You may be able to attach the metal roofing to 2x4's laid on the flat as opposed to a second sheathing layer. Check with the roofing manufacturer and product data.


    1. CcsIA | | #2

      The roofing needs a flat surface.
      If the peel and stick is on the first layer of sheathing would a less expensive sheathing work on the outer sheathing directly under the metal roofing?


  2. Expert Member
    Akos | | #3

    5" of polyiso is not enough for condensation control of a full fill of the 11 8/7 rafters. In your area you want 50% of the assembly R value to be the rigid insulation. This ratio can be pushed if you install a smart vapor retarder and are careful with air sealing the ceiling but R40 of fluffy is on the high side.

    You don't have to fully insulated the I-joist, you can use thinner batts as long as they get pushed up to the roof deck with insulation wire.

    P.S. You want the peel and stick bellow the foam. I prefer to work with plywood as it holds screws much better but OSB is good enough for the top layer.

  3. onslow | | #4


    For what it worth, I put the Grace Water shield under the metal roof because local code requires it. The first layer on the sheathing was a synthetic that was walkable and very quick to put down. Weather demands warranted getting dried in promptly, so it was nice wide and fast product to use. I can't attest to the air sealing quality as everything is closed to inspection, but there should be little to no path ways for air to flow outward once detailed. My dry to interior plan is abetted by living in a very, very, dry climate.

    Do be sure to get high temperature rated materials, whichever way you elect to put the two choices on. Personally, I would put the Grace on top because it can seal the numerous little punctures you are putting into the assembly as you apply the clips for standing seam. If you are going with a face screwed metal roof, all the more reason to have the additional security if a fastener gasket fails. The metal roof certainly does not telegraph the presence of overlaps of the Grace material, which is pretty thick compared to many other roof papers.

    My outer insulation layer is 7.75" nailbase so with EPS my ratio of exterior to interior R value is about 2 to 1 since I used R-19 batts. Wire batt holders come in different pre-cut lengths for 16" and 24" framing. Unfortunately, they are meant to work with solid framing so they are 15 1/2 or 23 1/2" long. Jamming them between solid wood forces them to bow, which if aimed upward will hold the insulation in place. Some will insist on bowing down or horizontally which means you will have to pay attention to placement to ensure the batt is being held up in contact with the sheathing.

    I joist web material doesn't offer the same grip as soft hem/fir and the 15 1/2 wire won't span the distance web to web on I joists set on 16" centers. Cutting 24" wires down would work but I suggest looking to see if suppliers of the I joists have compatible wire choices. What the crew used to anchor the batts between truss elements might be a better option.

    The roof truss top members were 2x6 material so the R-19 batts matched the depth . The crew glued metal posts to the underside of the sheathing with construction adhesive, then impaled the batts onto them finishing with metal washers that push on one way to stay.

    Being the paranoid type, I followed up the crew with strapping tape stapled to the bottom edge of the truss elements all the way across the field, about every three feet of run. The strapping tape was harvested from the many bundles of material being delivered. And yes I am insane, but I now sleep with assurance (real or imagined) that the batts are staying put, all sealed away behind the drywall. Don't have access to monitor and I am too old to be crawling around truss framing.

    I would second Akos' preference for plywood as final sheathing. The nailbase is flake board and seems to hold up on the scraps I have around the shed. Just the same I have more faith in the plywood. I would suggest 5/8 if you can swing it. The clips for the standing seam would benefit from the extra bite and likely provide a smoother overall surface for the metal roof.

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