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I live in the Pacific NorthWest in what I believe is a zone 4 marine environment. Our house was built in 1979 and is in decent but outdated shape. I'm remodeling a 5'x7'bathroom and have removed the sheetrock on the one exterior wall to reroute some plumbing. When I removed the original sheetrock I also removed the insulation which had a soft almost flour like texture that would pretty much crumble in my hands.There was no vapor barrier on the interior, and as far as I can tell there is no vapor barrier on the exterior.

I'm now getting ready to close up the wall and have two questions.

  1. When I insulate this exterior wall using mineral wool, will I need to put up some kind of vapor barrier? if so what do you recommend?
  2. Part of this exterior wall will be used by the shower/tub. The shower surround I'm using is Flexstone over cement board, will I need to use a vapor barrier on that area as well? If so where would it go, over the insulation before I put the cement board on?
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  • I take it you're using the mineral wool as cavity insulation, no?
    – ThreePhaseEel
    Commented Jan 15, 2021 at 1:22
  • Yes, between the studs, although I'm open to options on the type of insulation.
    – EdColeman
    Commented Jan 15, 2021 at 3:46
  • Refer to this PDF finehomebuilding.com/pdf/021151052.pdf specifically page 2
    – TylerH
    Commented Jun 14, 2021 at 19:29

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In the Pacific Northwest you do want an exterior vapor barrier we get enough wind driven rain that this is a big deal. The original insulation sounds like a cellulose type, I have never used on new construction, by 79 fiberglass was the common type for new construction in this area. I have always used heavy plastic behind my backer so I don’t have any experience with flexstone. as I have always used a membrane behind backer because I don’t want the constant moisture that can penetrate grout and backer or even tile depending on the type a non permeable surface there but still a vapor barrier on the exterior wall. Try to overlap and make sure to go up to the eves so water can not easily get behind it and damage the sheathing.

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  • Can you clarify: If putting a vapor barrier on the exterior, are you recommending that the OP pull siding off the outside to get the barrier in the right place? If not, how would you suggest getting a good vapor barrier sealed up on the "exterior" but done from the interior?
    – FreeMan
    Commented Jan 15, 2021 at 15:57
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    And are you really advocating for a true, impermeable vapor barrier on both the inside and outside of the wall cavity? I though that was frowned upon because of the possibility of trapping moisture in the wall cavity?
    – SteveSh
    Commented Jan 15, 2021 at 16:56
  • INo a vapor barrier like house wrap outside it breathes but blocks water behind the backer board impermeable I thought the op removed some of the exterior wall. It should have had some kind of wrap in 79 especially in the costal regions I was using tyvek back then prior to that 30 lb felt.
    – Ed Beal
    Commented Jan 15, 2021 at 17:10

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