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Community and Q&A

Proper assembly for below grade exterior shower walls

kyleaustin | Posted in GBA Pro Help on

I’m wrapping a largely-DIY basement finishing project in Michigan (Zone 5A) that includes a full bath w/ stall shower and am in need of guidance re: wall assembly and waterproofing in the shower area, as constraints necessitated locating the 34×36 shower stall in a corner abutting two exterior walls.

House is single story, cinder block construction, built in 1938. To the best of my knowledge, there’s no exterior insulation or waterproofing of the foundation. 

Relying heavily on the expertise in this forum, I employed the #3 approach to wall assembly detailed in Martin’s “3 Ways to Insulate a Basement Wall” guide ( for my exterior walls. 

All exterior walls, including those in the bathroom, are 2″ EPS (R8.8) followed by a w00den 2×4 wall with R15 mineral wool in the cavities. 

We have installed a fiberglass shower pan and are planning to tile the walls. The ceiling is already finished with purple drywall. 

Given the wall assembly that will be behind the tile, am I good to proceed with the standard approach of affixing cement board to the studs, sealing all seams/screws, and then using a liquid waterproofing membrane over top before laying tile? 

Or does my situation necessitate changes to the wall assembly in the shower area? To the waterproofing approach? Both? 

Any help and guidance you folks could offer would be greatly appreciated!


Kyle from MI

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  1. canada_deck | | #1

    I'm ashamed to admit that I'm not a prime member right now so I don't have access to that article. That said, it's an interesting question and I'm keen to see what others will say. The thing that strikes me is the potential for you to sandwich a wood wall in between two vapor barriers (again - I don't know exactly what your wall assembly is so I don't know if that would be the case.

    If that is a concern, the thoughts would then be:
    1) What can you do to allow that section of wall to dry out if needed?
    For example, given that this is just two short sections of wall, do you get enough breathability through the top plate and sides? Would it be wild to drill a number of holes in the studs so that the cavities can exchange air with the stud bays beyond the section of shower? That seems no different than the types of holes that are regularly drilled for pipes and wires.

    2) What can you do to prevent moisture from ever being an issue in that wall?
    Are there additional steps you can take to ensure that section of wooden wall will not be exposed to higher levels of moisture? What are you doing against the floor for example which would probably be the main source if you have a great vapor barrier against the cement wall and you waterproof your shower correctly.

    3) Can you change the wall assembly?
    Perhaps you could change the assembly in this corner so that you still retain some or all of the insulation value but you aren't sandwiching wood.

    As an aside, you might also want to look into different options for backing of the tiles. I know there are a ton of options these days. I've only done one shower but I don't like the idea of a liquid applied membrane because if you ever need to scrape out the grout to re-grout the shower or make a repair then you will scratch that membrane. I used the Wedi Board system. My understanding is that the foam board is waterproof through and through so even if you scratch it, it won't leak unless you put a hole through the entire sheet.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      Providing holes to vent cavities or create vent spaces only works if there is a mechanism to cause the air to move. On roofs it is either the stack affect or wind (which is why low slope roofs don't vent well). In a stud space filled with batt insulation, there would be virtually no air movement through the drilled holes.

  2. kyleaustin | | #2

    Thanks for the reply, canada_deck.

    All exterior walls, including those in the bathroom, are 2″ EPS (R8.8) attached to the block foundation wall, followed by a w00den 2×4 wall with R15 mineral wool in the cavities. I've attached a diagram here.

    Re: drilling holes in the shower area studs to enable air exchange between cavities, that's an intriguing idea I've seen proposed on similar threads. I'm certainly in a position to do that, and am curious to hear others' thoughts about that approach.

    Re: #2, I'm open to taking additional moisture mitigation steps, but aren't sure what those might be. We had the fiberglass shower pan installed by a local licensed plumber. It is set on a mortar bed on top of the concrete floor—he did not recommend or install any additional moisture of vapor barriers in that assembly. As for the wall assembly in that area, I have sill gasket under PT bottom plates throughout.

    Re: #3, I'm open to changing the wall assembly, but again aren't sure what options are available. I would only consider ripping out the existing framing/insulation (which would also entail removing the shower pan) as ab absolute last resort.

    Re: alternatives to liquid membrane, I'm leaning toward the cement board/liquid membrane approach for cost effectiveness (compared to foam board) and ease of installation (compared to sheet membranes, which research leads me to believe can be tricky for a first-time DIYer).

    1. Expert Member
      MALCOLM TAYLOR | | #3


      The wall will be fine.

      Any moisture in the wall can dry to the adjacent cavities and up through the top plates. That will occur by adsorbtion and diffusion, so there is no point in drilling holes, there will be no air movement anyway. If you want to take a belt and suspenders approach, strap the wall horizontally with 2"x2"s and leave the ends open to the walls on each side.

      1. kyleaustin | | #5

        HI Malcom,

        I really appreciate you sharing your time and expertise with me. Your reasoning makes perfect sense. Thank you for helping set my mind at ease.

        1. Expert Member
          MALCOLM TAYLOR | | #6



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