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Community and Q&A

Drying Potential of this Roof Assembly

Daniel Herskowitz | Posted in Green Building Techniques on

We are renovating an  cabin in zone 5 with 8″ of exterior GPS insulation. In order to avoid super long fasteners (hard to get, expensive) we are installing the first later of 3.5″ GPS sandwiched between on-end 2x4s running parallet to the ridge. The second layer of 4.5″ GPS will be continuous without 2×4’s. This allows us to screw the cladding into the 2x4s and avoid 10″ fasteners. My question is, is there a problem sandwiching 2×4’s between these vapor barriers? Does the moisture in the wood need a place to dry to?

2/12 roof
Standing seam metal roof
Ice shield barrier
3/4″ plywood
8″ GPS
Ice shield barrier
1″plywood ceiling

5/12 roof
Standing seam metal roof
Ice shield barrier
3/4″ plywood
8″ GPS
Ice shield barrier
1″ tongue and groove cladding.

 

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Replies

  1. Expert Member
    Zephyr7 | | #1

    I think this could be a problem. You would need to seal off that 1.5" air space created by the 2x4s. If you want to build this way, I would rip pieces of 1.5" GPS to put between the 2x4s (BTW, 2x3s would take up less space in your insulation sandwich). Another option would be to just put a layer of 3/4" plywood/OSB between the layers of GPS to avoid the air space.

    I think the air space is asking for trouble, and not just due to moisture -- critters are going to think you're building high rise housing for all of them!

    Bill

    1. Kyle Bentley | | #2

      Bill,

      I think the question was along those lines, IE 1 layer of GPS, 2 x 4's on edge, with GPS cut to fit between them, with no airspace above it, creating an un-vented assembly.

      Unless I'm reading it wrong!

      1. Expert Member
        Zephyr7 | | #3

        I could see how it could be the way you're thinking to. Edgewise 2x4's, 3.5" GPS between them, then a second layer of 4.5" GPS over the top of everything. This is why we engineers do like our drawings :-)

        If things are the edgewise way, then I don't see any problem with the assembly. What needs to be avoided is any large air spaces between layers. Everything in the assembly needs to stack together tightly with no gaps.

        Bill

      2. Daniel Herskowitz | | #4

        Yes, that is correct. 2 x 4s on edge = 3.5” and would use 3.5” gps for first layer so no air gaps

  2. Daniel Herskowitz | | #5

    Currently we have Polystick TU plus (omogeneous rubberized asphalt waterproofing underlayment) covering the sheathing (part of which is plywood and part of which is tongue and groove). I am concerned its hard to make an air tight connection between the interior wall vapor barrier (Intello) and the Polystick. I am afraid there that might be air gaps between the polystick and the sheathing. Any thoughts on this?

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