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Community and Q&A

Details of Double Wall for Exterior Wall Plumbing

brentwilson | Posted in General Questions on

Climate Zone 6/7

I am wondering about some details for a potential floorplan that would involve some exterior wall plumbing (bathroom, washer/dryer). Heated slab foundation. 2×6 exterior walls. If I were to build an additional 2×4 wall inside of the exterior 2×6 wall for the purpose of running the plumbing, does that effectively mean that the plumbing is no longer in an exterior wall? What would the details of this look like? Would the exterior 2×6 wall need taped drywall to create the proper barrier, then build the 2×4 wall inside of that for the plumbing?

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  1. Expert Member
    BILL WICHERS | | #1

    If the plumbing is entirely inside the building envelope, then it's not really in an "exterior" wall anymore. This would mean you'd frame and insulate your exterior wall normally, but you'd have a bump out inside where the plumbing would reside. You wouldn't necassarily need drywall between the plumbing cavity and the exterior wall, but you'd at least need an air barrier. You could use plywood here, or even just a vapor retarder if you're careful (note that if you are going to sweat copper fittings in there, this option is out since the vapor retarder won't be able to handle the heat). The easiest option is probably just to use plywood as the "back" of the plumbing cavity.

    There wouldn't be much else in the way of special detailing, since your exterior wall would still be framed and air sealed in the normal way, you're just adding an extra, empty, cavity on the interior side of the completed wall.


    1. brentwilson | | #2

      Thanks, Bill! Your response is helpful!

  2. Expert Member


    As with all service cavities, it's worth actually thinking through exactly what will be run in them before deciding if the extra work and loss of space is necessary.

    Baths have no drains in the wall, sink drains can often be routed through adjacent interior walls, and clothes washers have a 2" drain. Meaning in most situations you only have water supplies, which can be located in the exterior wall if the outside of that stud bay is lined with foam to make up the required R-value. Even the 2" washer drain can be run that way if you fur out the wall by 1".

    I'm not saying service cavities aren't sometimes a good idea, just that in my experience they often get built, and then end accommodating next to nothing.

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