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Community and Q&A

The perfect bathroom wall?

Jason Crawford | Posted in Energy Efficiency and Durability on

Hi everyone,
Although I’ve not tried it, what I’ve read about exterior rigid foam sounds very winning. I’m now about to renovate for the first time and am starting with the bathroom. I’m also likely to reside the house at about the same time which seems to afford me a great opportunity to switch to exterior rigid foam. I’m also thinking of doing this because currently the framing of that part of my house doesn’t fully cover the foundation cinder blocks leaving a point of entry for rain in to my foundation wall. 2″ (or 3″) or rigid foam would allow the exterior edge house to be flush with the exterior edge of the foundation there. I’m currently thinking of making that half foil faced polyiso and half XPS.

Anyway, as I begin the planning of this, I recall that thick rigid foam can act a a vapor barrier… especially if foil-faced. At the same time the bathroom behind this wall, and three sides of the tiled bath/shower immediately behind this wall, is likely to be relatively humid at times and result in moisture entering the wall space. Certainly we’ll have some sort of barrier on the inside to prevent this, right? But then it sounds like we’ll be sandwiching the wall between vapor barriers which I understand to have very poor drying behavior.

I’m sure many of you have already solved this many times. How do I need to adjust my thinking?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I'm going to assume that you know the basics -- how to install exterior rigid foam, and how to calculate the minimum thickness of exterior foam. If you don't know the basics, you should read these two articles:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Assuming that the humidity levels in the bathroom are controlled by the usual means -- regular use of an exhaust fan -- and that the tub surround or shower surround is properly waterproofed, there is no reason to believe that installing exterior rigid foam will cause any problems. While it is true that foam-sheathed walls should dry inward, the area in a bathroom where inward drying can't occur (namely, at the shower or tub) usually represents only about 40 square feet or less. The area is small enough that the waterproof section of the wall won't cause the wall to fail. Small quantities of moisture will be able to dry by diffusion laterally in the wall assembly.

    If you are the kind of person who will lie awake at night thinking about this problem, you can always use mineral wool insulation instead of rigid foam to insulate the exterior side of your walls. That way, the wall will be able to dry outward.

  2. Jason Crawford | | #2

    Thanks Martin. I will keep those wall cavities open (to facilitate lateral diffusion and drying)... and insure that I use enough exterior insulation.

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