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Community and Q&A

Plywood on walls and ceiling in bathroom

Louie Rodriguez | Posted in Energy Efficiency and Durability on

The bathroom has 1/4″ plywood on walls and ceiling. It seems to be acting as a vapor retarder and steam is condensing on ceiling and walls. The room has a bath fan and window, no heat supply and smells very musty. I am wondering what to do to make this room a safer environment, thanks


  1. John Klingel | | #1

    What can be done to insulate outside the plywood? Is the bath fan actually drawing out much air? Any obstructions? Plugged up squirrel cage or pipe? Are you planning on pulling the plywood to look for mold and insulation, and can you replace it and air seal real well?

  2. Louie Rodriguez | | #2

    mold is already visible, I just pulled out the tub and tile surround. The back of the tub is against the garage wall, and I will insulate using 2" rigid foam and foam edges. When I set the tub I will block in the drain line, and foam all plumbing penetrations.

    The fan seems to be working Ok, I will inspect the attic tomarrow. I have never seen so much water on the ceiling and especially around the fan, I suspect that tomarrow inspection will reveal much more details, thanks

  3. Mike Duhacek | | #3

    If there's mold in one area, there will definitely be mold elsewhere ... if not everywhere. It sounds like a ventilation problem. Mold is toxic, remember, so if there is a lot of it, you got to get rid of it ALL. There most likely isn't anything wrong with the wood if it's treated properly. (i.e. primer and paint or some other type of sealer.) The only reason water is condensing is because there is not enough draw from the fan to pull out the humidity from the hot showers. If there was no mold, I'd just suggest replacing the fan with a stronger one, but since mold already exists, I suggest inspecting behind every wall or just ripping it all out to be on the safe side. If it's just a little bit of mold, you can clean it with regular household cleaners. Hope this helps.

  4. GBA Editor
    Martin Holladay | | #4

    Mike told you that "The only reason water is condensing is because there is not enough draw from the fan to pull out the humidity," but there is another possible reason he didn't mention: it's possible that the plywood is cold. Condensation occurs on cold surfaces. Air-sealing work and added insulation could raise the temperature of the plywood, reducing the chance of condensation.

    And it's not quite accurate to say that "Mold is toxic." I often eat mold, especially on French cheeses, and it tastes delicious. Some people can be sensitized to some types of mold that grow on damp building components, and this sensitization can sometimes lead to adverse health effects. Caution is advised, but not hysteria.

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