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Community and Q&A

Tub/Shower Surround on Exterior Wall

John Cuddy | Posted in General Questions on

We are about to start a new construction project.  For one of the planned bathrooms, we’re really stuck with the tub/shower surround against an exterior wall.  I’d like to know if this is considered inherently risky from the perspective of the wall being able to dry and, if so, what steps can I take to mitigate the risk.

On the interior, the wall in question is the long wall of a 3×5 tub/shower surround.  Plan is to use Schluter KERDI and porcelain tile to finish the surround.

On the exterior, wall will be from outside in: fiber cement siding; rainscreen gap; felt paper WRB; foil-faced Rmax polyiso (taped) for continuous insulation; sheathing nailed directly to 2×6 studs; blown-in fiberglass cavity insulation.  I know technically it would be possible to detail the foil-faced polyiso as a WRB, but my GC prefers to use felt paper for that and I kind of like the idea of the polyiso being a backup here.

Does the foil-faced polyiso on one side and the KERDI+tile on the other side create a dangerous vapor impermeable sandwich?  Should I be considering other details?

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    1. John Cuddy | | #2

      Thanks much. My searches didn't seem to pull up that thread--appreciate the link.

  1. DCContrarian | | #3

    In a very influential 2010 book Joe Lstiburek advocated that basement walls should dry to the inside. He has since reversed himself and now says that basement walls should be impermeable and impervious on both sides.

  2. AlexPoi | | #4

    One thing you can do if you are really concerned is to fur out the wall on the inside. This way air will be able to move and if you ever get too much moisture in your wall, it will be able to dry through a more vapor open part of the wall. Plus, you'll get a service cavity :-)

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