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almost perfect wall?

chemosabe | Posted in Pretty Good House on

hi- trying to plan (as it’s being built!) a workable solution for my exterior walls.  
I’m in climate zone 3c ( wet/moist marine on the northern California coast).   it’s a 2×6 framed wall with plywood sheer wall (7/16) on the exterior AND  interior.  I’m planning on rock wool bat insulation in the wall.  so this space will have plywood on both sides.  the exterior ply has a vapor open WRB with a rain screen under wood siding.  
since it will be moist both outside and inside most of the year, I’m thinking the siga majrex membrane might keep some of the vapor out of the wall cavity yet let it dry too.  would i put that directly on the studs before the plywood on the interior or covering the plywood on the interior?  thanks for any insight.

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  1. andy_ | | #1

    Is the plywood on both sides an engineering requirement? Or was this a choice?
    I've only ever seen interior plywood used here in the PNW as a seismic retrofit above stemwalls in basements.

    1. chemosabe | | #4

      no not a choice, an engineering requirement due to the large area of windows...

  2. Expert Member
    Michael Maines | | #2

    It could go on either side of the interior plywood. It would probably be easiest to install it on the interior side.

  3. matthew25 | | #3

    See Malcom's comment on this other thread, plywood is already a variable perm "smart" retarder:

    I don't see a need for a membrane on the interior side with this assembly, given that it can dry in both directions.

    1. chemosabe | | #5

      thanks, so just let it dry rather than try to prevent it from wetting in the first place, or the plywood itself will perform that function well enough? -the membrane is superfluous and will impede drying somewhat. i plan on applying Borc acid to the plywood before the insulation as a mold preventative. but I'm located in a forest where even covered interior dry wood molds quickly if unheated.

      1. Expert Member
        Akos | | #6

        Doesn't matter what outdoors conditions are like. If the interior is properly conditioned and RH controlled, mold won't grow.

        In your mild climate, that interior plywood will work just fine as it is. No need for any vapor control or coatings.

        Of course this assumes the house is conditioned. If you are planning to leave the place unheated and without any active humidity control, all bets are off. No coating will keep mold at bay if the interior RH is too high.

  4. chemosabe | | #7

    it will be heated (radiant slab) but the exterior ply behind all that insulation won't be heated. so i thought an advantage might be gained by the asymmetric siga membrane. but with the interior plywood in place, would it add anything?

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