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How to Build A Double Wall Around Tub and Shower

rockies63 | Posted in General Questions on

So I’ve heard that placing a combined tub and shower unit against an exterior wall is a bad idea. Of course, mine is on an outside corner and to make matters worse the toilet and vanity also back up onto the north exterior wall.
Therefore, I’ve decided to put a second interior 2×4 wall behind the long side of the tub (against the west wall) and a second 2×6 interior wall against the north wall to run the toilet vent pipe and water supply pipes.

Should I put a layer of water resistant drywall (or cement board or Schluter board) on the inside of the exterior wall before I build the second interior walls as an air and moisture barrier or just leave a gap between the exterior and interior wall studs and stuff the whole thing full of Roxul? Or should I use a smart membrane between the two walls?

I’ve also heard that as you thicken an exterior wall and add more and more insulation you risk condensation on the exterior wall’s sheathing since the indoor heat can’t reach that surface as easily and therefore the sheathing will be colder.

Any advice or links to articles would be appreciated. Thanks!

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  1. rockies63 | | #1

    So a follow up concern would be trapping moisture within the wall cavities. If I put an air and moisture barrier like a sheet of drywall between the exterior wall (2x6 with Roxul Comfortbatt insulation and ZIP sheathing) and the interior plumbing wall (also 2x6 with Roxul) I've essentially created two separate walls.
    Have I created two walls that can't dry to either side should moisture get into either one of them or should I eliminate any air and moisture barrier between the two walls and have them act like one thick wall?

  2. 5Stud | | #2

    I will start with this disclaimer.
    I do this all the time when developing basements in new homes.
    Your thermal and moisture assemblies should be consistent on your exterior walls.
    You need mechanical protection (ie drywall) on the wall before the plumbing wall is erected.
    The plumbing wall is just that, it is for plumbing only. No insulation.
    In most basements the bathroom is adjacent to the mechanical room so I don't make a wall. I use horizontal strapping open to the mech room and I make sure it is not tight (8 inches shy) to the corners so I have airflow.
    Again this works fine in basements where the plumbing wall is connected to mech room. My gut feeling is that oversized holes for plumbing will provide enough air circulation on upper levels but I have not done it so...

    EDIT! I want to know who is responsible for toilet on exterior wall!

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