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Community and Q&A

Plumbing on exterior double stud wall

arnoldk | Posted in General Questions on


Would there be any concern to have a few water line running up the interior wall of a double stud wall (R-60) with the air barrier being the exterior plywood sheathing?

Thank you,

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  1. Patrick_OSullivan | | #1

    It all depends where inside that wall you intend to place the pipes. Some modeling could probably project worst case temps for the area in the wall you'd like to put the pipes, but I've taken a simpler approach. In exterior walls with supply lines, you can insulate behind them, but not in front of them. You take a hit on overall R value, but buy some piece of mind.

    If you can avoid pipes in exterior walls altogether, awesome, but sometimes it's a constraint that's hard to work around.

    1. arnoldk | | #2

      For a code minimum home, I wouldn't do it but this house has a double wall assembly I am thinking it may be fine on the interior wall.

      Thank you,

  2. Expert Member
    Michael Maines | | #3

    The temperature within a wall that has homogeneous insulation, as double stud walls usually have, follows a fairly linear path from outdoor temperature to indoor temperature. So if it's 0°F outside and 70°F inside, the center of the wall will be about 35°. The quarter points of the wall will be around 17°F and 52°F.

    If your outdoor design temperature (essentially the lowest temperatures you normally see) is much lower than 0°F then I'd be cautious, and it's a good idea to include a margin of safety for things like potential insulation voids, air leaks and power outages that leave the inside lower than room temperature, but in most cases I am comfortable having pipes in the inner portion of thick exterior walls. It's better to avoid it, but it's ok in a pinch.

  3. buildzilla | | #4

    i've seen strategies to use inner walls as service-cavities
    with the envelope being on the outside of the inner-wall.
    this minimizes envelope penetrations. no risk for electrical,
    but good question for plumbing.

    r-60 is impressive. side question for the curious, where are you building and what does the rest of the assembly look like?

    1. arnoldk | | #5

      I am building in Ottawa, Canada and I attached the wall assembly.

      I personally would not have done the 2" exterior insulation but my wife insisted on doing it based on an energy model a colleague which experience did for her. The model indicated that with the exterior insulation the house would be about 30% more efficient but what that would translate in additional hydro cost per year, I cannot remember but I doubt it's enough for a reasonable pay back.
      The main benefit I can think of having exterior insulation is minimizing the cold/wet sheathing often discuss with double wall assembly.


      1. Expert Member
        MALCOLM TAYLOR | | #9


        A couple of questions looking at your wall section:

        - Is the poly air/vapour barrier sufficiently close to the warm side of your wall to avoid condensation?

        - I don't know much about FPSFs, but all the drawings I've seen have the wing insulation above the footing.

  4. user-723121 | | #6

    "The main benefit I can think of having exterior insulation is minimizing the cold/wet sheathing often discuss with double wall assembly." This is not factual, a properly designed double wall will perform as well or outperform any other wall assembly without the aid of exterior insulation.

  5. Expert Member
    BILL WICHERS | | #7

    Ideally you want all your pluming to run in interior walls where you won't have freeze concerns. If you have to run plumbing in an exterior wall, try to keep it as much towards the interior side of the wall as possible, which will ensure the pipe stays as warm as possible. Once advantage here with a double stud wall is that you have lots of room to put that pipe, so most of the insulation will be between the pipe and the exterior face of the double stud wall, which is the safest placement for the pipe if you have no choice and have to locate it in an exterior wall.

    I would put some of the purpose made pipe insulation on these pipes too for some extra insurance, and to help avoid any sweating of the pipe getting into your wall's primary insulation. Pipe insulation is cheap, so it's an easy way to add some extra insurance. I like to use the stuff that has an adhesive tape preapplied to the cut edge so that you can "glue" the slit together when you install the pipe insulation on the pipe.


    1. arnoldk | | #8

      Hi Bill,

      All of the plumbing are inside interior wall except for one water line to one toilet which is just behind where the drywall will be installed. I also installed the pipe insulation for extra insurance as you mention.


      1. Expert Member
        BILL WICHERS | | #10

        Note that sometimes you can feed a toilet with a valve coming up from the floor. That is sometimes another way to avoid pipes in the exterior wall, although it sounds like you already have your plumbing work completed.


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