Supply (& DWV) lines in exterior walls

Perhaps Terry is best one to answer this one as I'm in Seattle so he knows the weather.... I'm wondering if it is acceptable to run supply lines in an exterior wall? (We'd be able to insulate.) As none of the bath's interior walls are shared w/ the downstairs (large open kitchen area) my other choice would seem to be to run up to attic and back down. What's best?


(For sure I'd have to run the drains down the exterior... I assume that is fine?)

DavidTu said:

Perhaps Terry is best one to answer this one as I'm in Seattle so he knows the weather.... I'm wondering if it is acceptable to run supply lines in an exterior wall? (We'd be able to insulate.) As none of the bath's interior walls are shared w/ the downstairs (large open kitchen area) my other choice would seem to be to run up to attic and back down. What's best?


(For sure I'd have to run the drains down the exterior... I assume that is fine?)

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The trouble with questions like this is all your answers have to be comfirmed by your local jurisdiction.

A lot of areas will allow supply lines on exterior walls, but they require insulation on the outer wall and none on the pipe, the pipe has to be run as close a possible to the inner wall.

Most attics are cold areas, not a good idea to run water pipe in there.

Drains run on the exterior are usually not permitted, like I said you have to ask locally, sorry.

shacko said:

Drains run on the exterior are usually not permitted, like I said you have to ask locally, sorry.

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Does that mean you cannot have a fixture on the exterior wall (i.e. drop down wall to floor level) or that you cannot have a stack on the exterior wall? (or both!?)

(No insulation on pipe so that inside air can warm it, right?)

In Seattle you can do it.

Just don't insulate the pipe in the wall, make sure the blanket of insulation is on the outside.
If you put pipe insulation around the pipe in an outside wall, it will freeze quicker.
It needs to get warmth from the inside of the home.

Running pipes in an attic
Don't put any insulation around the pipes.
Remove all insulation below the pipe, and lay fiberglass batts over them.
Between the ceiling joists on either side, lay two more batts, keeping space open below the pipes.
The heat from the home must be able to get to the pipes, and then you trap it.
A pipe in an uninsulated attic will freeze otherwise.

If you change the pipes, then PEX would be good.

Example of pipes in an attic.

Terry said:

In Seattle you can do it.

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Double check: You mean you can do both, right? (i.e. DWV and/or supply lines in outside walls)

In Seattle it never gets that cold.
Surrounded by water on both sides, the Salish Sea (Puget Sound) to the West and Lake Washington to the East.
Seattle plumbing code allows waste, vents and water in exterior walls.

As always, during a cold snap you should open doors on exterior cabinets to allow warm air to the exterior walls that have plumbing ( kitchen sinks are a biggie)