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I have an exhaust fan in my master bathroom and also a wall heater. I like to run the heater without the fan running during my shower so that the room is nice and warm when towel drying off. I then run the fan for 15 minutes on a timer. This does a good job of removing the moisture and heat from the air, but condensation still remains on the drywall above the shower enclosure. Even with the fan running, so steam ends up forming condensation on those walls close to the shower.

Is it alright to have some condensation forming on these walls as long as it evaporates in a hour or so? Do I need to worry about moisture making it way through the paint and turning into condensation when it hits the vapor barrier behind the drywall? To date, I have never seen any spots of mold form on the painted drywall.

  • This is really hard to answer. Condensation is fine until it sits long enough to penetrate. – DMoore Apr 20 '15 at 15:21
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If you have a vapor barrier (e.g. sheet polyethylene) behind the drywall, then you have a major problem. Painted drywall is airtight but not vapor-tight; moisture can migrate through it. And when moisture migrates through the drywall and hits the polyethylene sheeting, it has nowhere else to go. The fact that it currently condenses on the walls tells me that the walls are cold. That means the polyethylene behind it is cold, too. That means water vapor that gets through the drywall will inevitably condense on it and drop to the bottom of the wall, eventually rotting the whole thing out from the inside. Not good. Mold will probably not develop on the drywall at the top of the wall because that's not where the water will be collecting; it's the bottom of the wall you should inspect.

Vapor barriers in walls are generally a bad idea, but if there is any place you could safely have one in a bathroom like this, it would be on the inside, in the form of a water- and vapor-right waterproofing membrane, like RedGard.

If you are unwilling to rip out this assembly and redo it properly (with Redgard-coated cementboard instead of drywall, and no plastic vapor barrier inside the wall), then I would highly recommend preventing moisture build-up in the bathroom in order to preserve the integrity of the structure.

  • Aren't most suburban homes built with drywall around the shower enclosures? – Evil Elf Apr 21 '15 at 14:33
  • Yes, and they're all going to need to be replaced. By coincidence, mine was built this way and and as I'm currently replacing it, I'm getting to see firsthand how disastrously wrong is it to build a shower this way. Drywall is NOT waterproof or vapor-tight. Rotted wood, mold and termite damage everywhere. Just a disaster. – iLikeDirt Apr 21 '15 at 15:01
  • Your advice seems good, but is the opposite of what I received as an answer to my question here: diy.stackexchange.com/questions/63777/… Would you advise against the vapor barrier in that situation too? – mankoff Apr 22 '15 at 11:24
  • Isn't it a good thing to see the condensation forming on the paint as opposed to getting through the drywall and condensating on the vapor barrier? – Evil Elf Apr 22 '15 at 12:07
  • Paint isn't vapor tight. If there's condensation on painted drywall, and the space behind the drywall has less humidity, there is a natural vapor drive through the paint and drywall into the space behind. – iLikeDirt Apr 22 '15 at 14:07

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