I am patching horizontal lapped cedar siding on a 1950s house in Minnesota with fiberboard sheathing. I think it’s Nu-Wood. There’s no vapor barrier. I will have to put a vapor barrier per code where I patch (thinking roofing paper). Will I need to take off all siding and install a vapor barrier?

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    Vapor barrier does NOT go on the outside of the wall in Minnesota. Air barrier (breathable, "housewrap") can, vapor barrier in that position in that climate will soak your walls. You must be misinterpreting code to think that.
    – Ecnerwal
    Oct 10 '20 at 14:16
  • Maybe I am ... the way the building inspector talked about it, I thought he said sheathing then vapor barrier then siding. I read elsewhere that tyvek housewrap over nu-wood Is a bad idea so I wasn’t sure where to go from there except roofing paper. I can only see the fiberboard over studs then siding over the fiberboard right now. Thanks!
    – Flipper
    Oct 12 '20 at 0:51

To me, moisture barrier goes on the exterior side of exterior walls and vapor barrier goes on the interior side.

We install moisture barriers because we know siding leaks...especially your type of horizontal lapped siding.

If you are patching a hole (or small area) in the siding, there’s no reason to remove all the siding to install a uniform moisture barrier throughout the wall. I’d just make sure the patched area has an adequate moisture barrier and it’s lapped properly onto the existing moisture barrier before installing the new patched-in siding.

  • I agree, but it's confusing to use "moisture" to mean "liquid water". Moisture also includes vapor in common parlance. I often refer to it as house wrap or a "drain plane".
    – isherwood
    Mar 9 '21 at 21:38
  • Also, it may be necessary to lap new house wrap on to existing siding below where that siding won't be removed.
    – isherwood
    Mar 9 '21 at 21:40
  • @isherwood Maybe the term “house wrap” is more appropriate for exterior applications to prevent rain driven moisture (water). But I do like the term “vapor barrier” for inside applications. (We don’t use a poly sheet for vapor barrier where I live. We rely on coats of paint.)
    – Lee Sam
    Mar 9 '21 at 23:30

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