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Unvented roof detail question

Joe Norm | Posted in General Questions on

At the moment I have access to 3.3″ polyiso boards for $20/sheet. There was a mishap during shipping to a large industrial project so some of the edges are slightly compressed or a little crumbled. 

Climate 4c, so I would only need one layer above the sheathing.

My question is about a single layer of these boards over the top of my sheathing, there would be no lapping of joints, is this OK? Doing two layers is way overkill and thickens the roof unnecessarily. 

Is lapping of joints an absolute must for moister control or will it most likely be perfectly functional? 

These would be the layers: Metal roof, 1×4 straps, 3.3″ Polyiso, Ice and water shield, plywood, rafters with rockwool batts or cellulose, sheetrock ceiling with minimal penetrations. 

thanks for the input

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  1. Expert Member
    Zephyr7 | | #1

    You don’t have to overlap the sheets, but you’ll have slightly lower performance if you don’t. Look up Joe Lstibureks article about “3 dimensional airflow pathways” and his “professional embarrassment” with his barn. He has some pictures of what happens when the rigid insulation doesn’t overlap.

    If you tape all the seams you’ll be ok, just a little less than optimum performance is all.


    1. Joe Norm | | #2

      I would have a 2x12 with cavity insulation as well, so I doubt there would ever be noticeable heat exchange through the seems like in his house.

      Rough calc says this roof would be close to R-60

      Do the layers of the proposed assemble look right? Seems weird to not have any sort of underlayment directly under the steel roof

  2. Expert Member
    Dana Dorsett | | #3

    With 3.3" roofing foam you're looking at about R19. Using the IRC prescriptive values for an R49 total roof as a guide that's sufficient dew point control in zone 4C for only R44 of cavity insulation. At the 11.25" depth of a 2x4 rafter dense packed cellulose would be about 43, which close to the margin, but fine, since the cellulose can wick, store, and redistribute quite a lot of moisture that might begin to accumulate he areas that got compressed.

    But with rock wool batts you'd be at R46 or higher in the cavities. A pair of R23 rock wool batts nominally designed for a compression fit into 5.5" cavities when lightly fluffed to a total 11.25" depth would deliver about R47 performance. That's still fine on a sun exposed roof but marginal for a north facing roof pitch in the woods. The risk would still be pretty low, but real. Installing another inch of foam (any type) would fix that.

    Alternatively, installing a broadsheet smart vapor retarder such as Intello Plus or Certainteed MemBrain under the ceiling gypsum, or simply painting the ceiling gypsum with half-perm "vapor barrier latex" primer would mitigate an moisture accumulation risk.

  3. Joe Norm | | #4

    Thank you Dana, very helpful.

    So if I paint with the right paint on the ceiling, the rock wool + foam will be a non risk?

    1. Expert Member
      Dana Dorsett | | #5

      >"So if I paint with the right paint on the ceiling, the rock wool + foam will be a non risk?"

      It has to be air-tight to work, but it takes an already low risk down to a VERY low risk.

      The smart vapor retarder approach is in some ways easier to make air tight (lowering the risk), and if any moisture gets in there (by any means) it allows the assembly to dry toward the interior at about 5x the rate of using the vapor barrier latex approach. MemBrain (2 mil nylon) is pretty cheap compared to the more tear-resistant and somewhat more vapor retardent Intello, but more expensive than vapor barrier latex.

      If you are at the warm edge of zone 4C and get at least some sun exposure you can in most cases safely skip the vapor retarders, but not at the cold edge. A competent user of the WUFI heat & moisture modeling tools could give you an idea of how close to the margin you really are, but in the hands of a hack it's just more noise, not real information. Paying somebody to run that model would likely cost more than just installing 2 mil nylon detailed as an air barrier.

      1. Joe Norm | | #6

        In a vented approach, is it less important to get the air barrier right, since any moister will migrate to the vent channel? I'm just trying to wrap my head around the science.

        Not sure if I am in the warmer end of 4c, but I know I am in the less moist part with about have the rainfall of Seattle. The roof would have open southern exposure.

        Is it important to tape the seams of the foam in this approach? The foam is a little distorted on the edges so this may be difficult.

  4. Expert Member
    Peter Yost | | #7

    Hi Joe -

    Having the vent space does make the entire assembly more forgiving from a moisture management perspective but I get nervous when anyone says that means less worry about air tightness. And this is in part because the actual location and size of air leaks does matter so telling someone SOME air leakage is ok is a recipe for misinterpretation and problems.

    Go for airtight; you won't get a 2nd chance on getting your roof right.


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