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Sanity check: wall essembly for dormer reno

woobagoobaa | Posted in Energy Efficiency and Durability on

Hoping to have the GBA gurus check our plan for the wall assembly for a dormer renovation.  Pics attached.  Challenge is getting anywhere near the required R49 min required here in zone 5 roof.  We have 2″ depth on the sidewalls and 4″ on the window stud wall.  Cavity depth overhead the ceiling is a minimum 4″ at the outside edge.

Here is the assembly plan being discussed.

Walls (inside to out)
– 1/2 wall board / skim coat
– closed cell foam in the wall cavities (2″ to 4″ depth)
– existing 1×12 board sheathing.
– WRB.  Tyvek or Typar is being proposed.
– maybe an inch or two of rigid foam.
– rain screen / air gap
– cedar shingles.

Roof (inside to out)
– 1/2 wall board and skim coat.
– closed cell spray foam (4″-6″) at the outside edge of the roof
– existing 1×12 board sheathing.
– ice/water shield
– roof membrane (maybe metal) … very low pitch.

– Tyvek (perm 56) or Typar (perm 12).   Which perm rating is preferred?
– WRB under or over the rigid foam?
– Air gap / rain screen.  I don’t see how the synthetic options (Slicker, Drain Vent) are not compressed when the shingles are applied?

Much appreciate your advice.  Thank you

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Wooba Goobaa.

    I interviewed one of the principals at RDH building Science when I was researching WRBs and he told me that he doesn't think that perm ratings matter too much once you are out of the class III vapor retarder zone (over 10 perms). All will allow sufficient drying. That said, the perm rating of the WRB will be insignificant on your walls because the rigid foam is likely to have a much lower rating anyway.

    The WRB can go in either location. If you are keeping the windows at the sheathing plane, putting the WRB between the sheathing and foam makes it easy to integrate flashings. If you are furring the windows out to the plane of the rigid foam, it might make sense to bring the WRB out as well, or use the rigid foam as the WRB (make sure to choose a product approved for use as a WRB). You may find these articles helpful:

    Where does the Housewrap Go?
    Wrinkled Housewrap Behind Exterior Rigid Foam

    3D rainscreen products are pretty rigid, at least those that I have seen. Try to get your hands on a sample and you will see why they do not compress.

  2. woobagoobaa | | #2

    OK thank you. The Typar / Tyvek decision will be made for reasons other than the perm rating.

    BTW the perm ratings on the Typar/Tyvek WRBs. That is a bi-directional rating, correct? Unlike some interior smart vapor retarders, which quote an "inward" and "outward" perm?

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