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Sheathing Replacement?

brokemedic | Posted in Green Building Techniques on

I am residing and improving my insulation currently. I discovered rot in my sheathing an am replacing it on my walk out basement with treated plywood. 

My upstairs has osb still. I live in northern Wisconsin and it gets to -30 in the winter. My walls are currently siding, osb with no house wrap, 2×6 with fiberglass batt, plastic sheet, and drywall from the outside in. 

My plan is siding, rain screen, liquid housewrap and joint sealer, sheathing, 2×6 with R21 mineral wool, smart vapor barrier, 2×4 with R15 mineral wool, drywall. 

My question is will leaving the good osb be a concern for condensation with the added insulation or should I go with plywood. I’m ok replacing it, but obviously it’ll be way cheaper and less effort to leave it. I will not use exterior insulation as my eaves do not provide enough overhang for comfort. 

Thanks for the help!

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Replies

  1. insaneirish | | #1

    > I will not use exterior insulation as my eaves do not provide enough overhang for comfort.

    I don't understand the connection between these two things.

    1. Expert Member
      PETER G ENGLE PE | | #2

      Adding exterior insulation would decrease the effective overhang and allow the walls to be wetted more frequently by rain. It also messes with the exterior architecture. So the OP doesn't want to do that.
      In response to broke, Your plan will work fine so long as the OSB is undamaged. With the current conditions, you will most likely find some rot in the above grade walls. Just go ahead and replace it with either OSb or plywood. Make sure the liquid WRB you use is vapor permeable. Above 5 perms would be great. There is little risk of condensation because you have the smart VR in the middle of the wall. This acts as a "valve", allowing moisture into the space behind it more slowly than it can dry through the OSB to the exterior. So long as the permeability of the VR membrane is lower than the OSB+WRB, everything is OK.

  2. walta100 | | #3

    Before we design a fix lets take a moment and diagnose why the sheeting rotted.

    Did the sheeting rot because it got wet from condensation from interior moisture reaching the sheeting or because of the lack of a water barrier is allowing the sheeting to get wet and rot.

    What is your indoor relative humidity in the winter?

    Walta

  3. Expert Member
    PETER G ENGLE PE | | #4

    Walkout basement, OSB sheathing, no WRB. Do you really need to ask?

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