Best way to waterproof shower with one exterior wall?

We are in the midst of a master bath remodel and I chatted with my contractor this morning about the shower construction. It sounds as though he typically uses a sheet membrane behind hardi board, which is fine. However, one wall of this shower is an exterior wall (no window), and I seem to recall reading something about needing to slit the paper-backed insulation behind the membrane to ensure that you don't create a "mold sandwich." I am having trouble understanding why this is necessary, at least in my case - the insulation in my walls is cut into strips that lie between the studs (which are set more closely than 16" on center for some reason), and it looks like there should be plenty of places for moisture to escape. Am I totally misunderstanding this concept? Any advice you can offer would be great. I am not at home currently but can take photos later today.

Comments (2)

  • Tile Corrector

    There is a lot to your question, i suggest you refer to your local IRC inspector who is verifying your steps and signing off on your permit.

    Below grade? above grade? exterior wall makeup, cavity depth, layering all play into it.

    You cannot have FACED insulation PLUS a vapor barrier nor do you want faced and a moisture barrier.

    You need to look at the bigger picture and relevant to your climate zone. No one can answer without a full and accurate description of the above questions.

    This is why you are grateful to have permits and inspectors double checking your contractor if he is not able to convince you he knows how to do the job correctly to the point your on this forum looking in the wrong place for accurate answers.

    biondanonima (Zone 7a Hudson Valley) thanked Tile Corrector
  • biondanonima (Zone 7a Hudson Valley)

    Thank you so much for your response. I actually haven't discussed this with my GC nor the sub who will be doing the work yet, I was just trying to get some education here so I would at least know the basics when I do discuss it with them. Here are some photos of the space:

    The wall with insulation is obviously the exterior wall. The 2x4s that are visible between the insulation are not the actual studs of the exterior wall, though - presumably the previous owners placed them there during their renovation. I don't know if my contractor plans to leave them as is or remove them so that the new shower can be constructed directly on the exterior studs, but presumably there will be insulation there either way. When you say "FACED" insulation, you mean the paper-backed sort that is there, correct? I have seen instructions online to slit the paper backing on any insulation that will be next to a moisture barrier, presumably to give moisture some escape routes. I assume that when you say you can't have faced insulation plus a vapor or moisture barrier, you mean faced insulation with fully-intact paper backing, in a large sheet that would leave moisture nowhere to go, correct?

    Mine is obviously NOT in intact condition, so unless we decide to put new in, I don't think we'd have to worry about moisture getting caught between paper and barrier. I assume that since it is in strips between the studs, just those breaks in the paper would be enough for moisture to escape, but perhaps I'm wrong about that? Is there a better type of insulation to use behind a moisture barrier, or a better kind of barrier to use with paper backed insulation?

    ETA: This is a second floor bathroom, and we have stucco siding.

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