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Community and Q&A

Window in a tub/shower unit

cgmorgan | Posted in Energy Efficiency and Durability on

Hi all,

We’re redoing our second floor and have now come to the bathroom. The house is a Cape Cod style and there was a small addition added to the second floor to allow for a bathroom likely in the 80s. Suffice it to say it is cramped, so there’s not a lot of wiggle room for design. Previously it had a shower unit in one corner, and a window overlooking the back across. We’re looking to add a 54″ tub/shower unit and unfortunately the only space is underneath the window. I know this is not an ideal location. 

I’m down to the studs and just about to start installing the insulation, and I was wondering if anyone had any advice on how to keep this absolutely water tight. The plan is 4″ of dense blown cellulose, a vapor barrier, and Prova board for the tile. Beyond the standard sealing I was wondering if there’s any additional products/ techniques I should look into before I install everything and realise I made a big mistake.

I’m happy to provide pictures if it helps.

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  1. nynick | | #1

    I did this fairly recently in a cottage we owned in Canada. There are all sorts of videos and instructions to accomplish a water tight surround around the window on the web. Google is your friend. Just don't take any short cuts.

  2. Expert Member


    Treat the window surround as you would a shower alcove for shampoo etc. using the same membranes and layering.

    Remember that many codes require that windows in shower/baths have safety glass.

  3. cgmorgan | | #3

    Hi all,

    Thanks for the replies. I found some helpful videos and a good walk-through on Fine Homebuilding.

    One question/ conundrum I'm wondering if you could advise on:

    As you can see in the attached photos, the window is certainly not ideal for having a tub-shower unit around it (don't worry, electrical is getting moved). It's impossible to get a membrane over the jamb joint, so I fear when I install the backer and add the membrane I won't be able to properly wrap it around where my board meets the jamb for solid seal. The insulation is going up in the next month or so, followed by backer and membrane, but the window isn't getting replaced until next Spring or Summer.

    Would you recommend I install my Prova/ Kerdi board, thin set the membrane all around edges of the board and tile over it (leaving 4 inches around the window unfinished), but leave the portion that goes over the joint "loose" and simply put plastic wrap over the unfinished portion until the window is removed? Once it's taken out in the Spring I can pull the membrane over the joint to complete the seal and install the new window.

    Am I overcomplicating it and there's a better approach I'm missing?

    Thank you.

    1. Expert Member
      Akos | | #4

      That is not a window you can have in a shower. It needs to be pulled completely and replace with something that will holdup to water (vinyl, fiberglass or aluminum). Once the window comes out you can detail the rough opening properly to tie it in with shower waterproofing.

      I hope that is board sheathing with a WRB on the outside of the wall not just siding directly on the studs. Insulating an exterior wall in a shower without a proper WRB and good window flashing details is asking for trouble.

      1. cgmorgan | | #5

        Hi Akos,

        Yes, the window is definitely not suitable. We've contacted a few window suppliers as we're redoing the whole house, but the wait time for install is not until spring at the earliest. Currently I'm looking to finish as much of the project as possible while we wait for that to happen, and was hoping there was a way to get it partially usable, even if just as a tub, over the winter. It sounds like you would strongly recommend against it until the window is fully replaced?

        The reason I'm anxious to get it partially usable is because the other bathroom, main floor, currently has a shower-tub unit with the exact same window in it (except the window opens into an extension ...). That one has been like that for over 20 years and I don't want to start ripping it apart to inspect any possible damage until we have another washing solution available. Currently I've taped plastic over that window to try to prevent any extra damage.

        As for the exterior, it is vinyl siding overtop of what I can only hope is a correct sheathing. I guess it's time to take some of it off and see what's underneath. If there isn't an adequate WRB, is there anything I can do to the interior of the wall before adding insulation, or is the only proper approach to remove the exterior vinyl and redo that part of the extension?

        First time home buyer without a lot of construction experience so I appreciate the input.

        1. Expert Member
          Akos | | #6

          You can partially finish the space provide you don't do the tiling around the window.

          Run the Kerdi up the wall up to the rough opening of the window and stop there. When the window is pulled, make sure you get a new build window not a repalcement inserts, you can do the proper flashing and run the water proofing to the Kerdi. Finish the tiling once it is nicely sealed up.

          As for the WRB, you need to find out if there is one. If there isn't you can use the guidelines here to insulate:

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