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DAVID2022 | Posted in GBA Pro Help on

My contractor wanted to know if he can flip the location of the R-5 c.i with the location of the plywood/OSB sheathing so that sheathing is nailed to the stud instead of the R-5 c.i.? I am specifically looking at detail 2-00020 in the detail library.

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  1. Expert Member
    ARMANDO COBO | | #1

    Unfortunately, searching for Detail 2-00020 does not bring out that specific detail, but a whole bunch of details in different categories, all without numbers. Maybe the GBA can fix that.
    However, I always design all my wall assemblies with taped and sealed PLWD/OSB nailed to the framing members and then taped rigid foam (Installing WRBs depend on type and purpose) . Most building officials will require an engineer's report with rigid foams over 2" thick for cladding attachment.
    You should NEVER install non-structural rigid foams against the framing members and then PLWD/OSB without and engineer's report. All insulated sheathings, like Zip-R, C-SIS and OX Strong-R, have their own ICC-ES reports.
    You could also install rigid foam boards to framing members IF you meet the requirements for structural bracing, see 2021 IRC R602.10 Wall bracing.
    Here is a good BSC and APA reports to study:
    APA also has more videos and papers on framing you can view and download for free.

  2. Expert Member
    BILL WICHERS | | #2

    If you have an assembly that goes sheating/ci/studs, from outside to inside, then you'll have a problem with shear since the nails through the CI will have a tendency to bend and "squish" their way throug through the foam. Visualize this as the studwall trying to slide over sideways (racking), and the plywood twisting off the wall as the nails bend in the foam of the CI. The reason Zip-R is OK is because if you follow their nailing schedule EXACTLY, they have done all the testing to know the shear strength of the assembly. If you do this in the field with your own materials, there is no testing and thus no way to know the resulting shear strength of the assembly.

    If you nail the sheathing directly to the studs, you have a known assembly that will hold up for shear, which is often important (but you need an engineer to tell you for sure for your specific project). You can then put your CI over the sheathing and not worry about structural issues. I personally would recommend CI/sheathing/studs and not sheathing/CI/studs, again from outside to inside, for this reason.

    You'll want to detail that sheathing as your air barrier, which means taped seams and a bead of sealant (I like polyurethane caulk) around the perimeter prior to putting the plywood/OSB up.


  3. plumb_bob | | #3

    The basic size and type of building are also factors. For example, a large 3 storey house with long open spans will require more shear strength than a 500sf carriage house. Rigid insulation fastened between the studs and framing may be ok with one but not the other.

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