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I have a really weird problem. I have a bedroom wall where the paint kept bubbling and peeling off. I have re-textured and painted it several times. I finally ripped all the drywall off and discovered that there is a large space behind the wall that has all the plumbing for two adjacent showers. The problem is that those pipes changing temperatures are causing enough moisture to build up in the space that the adjacent drywall gets wet and will not hold texture and paint. I checked for any leaks because it seemed weird to have that much moisture from the existence of pipes.

The question is: Is there anything that I can put in there when I close it back up to reduce the moisture? Would a vent work? A piano dehumidifier? A combination of the two? I would rather not have a random thing plugged into the wall.

EDIT: There are some really good answers so far, but I think I need to better explain. Here is the set up: enter image description here

I tried spraying down the roof and looking for leaks. I tried turning all showers and flushing all toilets. There is no water coming from anywhere. The vents go straight to the roof. There is some insulation in there so I hate to just put an HVAC return register on it, but that and insulating the pipes looks like the only option.

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I'm guessing the insulation in the wall is for noise. If that's the case, you can put a vent in the bathroom next to the toilet and the exhaust fan in the bathroom should pull air out the vent instead of letting humid air going in (or you can install an exhaust vent in reverse). –  BMitch May 16 '11 at 16:16
    
What do you mean by "an exhaust fan in reverse"? You mean a fan next to the commode that would pull air from the dead space into the bathroom? –  Tatton Chantry May 16 '11 at 16:48
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That's what I'm thinking. You could install it with the same wiring that's going to the existing exhaust fan. It's probably overkill, but hard to tell without seeing your specific situation. –  BMitch May 16 '11 at 17:25
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Possible temporary solution: damprid.com –  nibot May 17 '11 at 4:27
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3 Answers

up vote 9 down vote accepted

I'd be surprised that you'd have this much moisture from condensation, especially since condensation doesn't change the net moisture level, it's actually removing moisture from the air. Double check for leaks, signs of corrosion on pipes, particularly at the joints, for caulk that is cracking in the showers, and the drain pipes.

Edit: One more thought, check where the bathroom vents are blowing. It should be outside, but a lazy installer may have them venting into this space.

That said, without knowing more about the floor plan and what's above and below this space, it's hard to give a good suggestion, so I'll give several:

  • While you've got the opening in the wall, consider turning it into an access panel to more easily get to the plumbing in the future. This can just be a few pieces of trim that are attached over the joint between the wall and cutout drywall.

  • If you want a vent in the wall, you can put in a simple hvac return grill. That could also double as an access panel.

  • Insulate the pipes to prevent the air from interacting with the cold pipe and to reduce loss from the hot pipe.

  • If there's an exterior wall involved, check the insulation and make sure the vapor barrier (paper on fiberglass) is intact.

  • If you redo the entire wall, use moisture resistant wall board (green board may be more than enough, but they make stuff that's even more resistant). You can also install an extra vapor barrier behind this.

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I was going to suggest insulating the pipes. Seems like it can't be a bad idea, no matter what… –  blalor May 16 '11 at 13:05
    
I really like the check the exhaust of the bathroom fan part of this answer. –  Keith Hoffman Mar 5 at 6:57
    
But I think your physics comment on condensation is flawed. In most older homes, the drywall and the polyethylene vapor barrier behind it are the only air barriers. Inside the exterior walls (and we don't know how this interior wall is tied to the exterior), you can have plenty of air moving. So the cold pipe would continuously pull humidity from warm passing air. –  Keith Hoffman Mar 5 at 6:59
    
Because we don't know where the air barrier plane is in his house, I'd be reluctant to put an air grille into the wall space. The wall could connect to a furnace room with exhaust byproducts, it could connect to an unconditioned basement or crawl space vertically (another source of moisture). it could connect to exterior air, it could even allow insects in. Yes, in his drawing it looks like it is an interior partition wall but I'd want to know more. –  Keith Hoffman Mar 5 at 7:01
    
Sorry about all the criticisms. Not trying to be a jerk. But ... you only want one vapor barrier. So an 'extra' vapor barrier is probably a bad idea. –  Keith Hoffman Mar 5 at 7:07
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You could try lagging the pipes - both hot and cold. This should keep the temperature in the space more constant and therefore reduce the build up of condensation.

Another thing to try is to put a second wall between your bedroom wall and the shower pipes. Again the goal here is to keep the temperature behind your bedroom wall more constant and less humid. This would mean that your access to the pipes would be more difficult.

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I'd do the following, which is similar to BMitch's generally excellent answer:

  1. Make an airtight access panel for future access while you are there. Make sure it is airtight unless you know that no air from anywhere can get into that stud bay.
  2. Foam or tape off the entry points of the pipes into this stud bay. You might be getting unconditioned air in via these holes. Seal any other holes while you are at it such as around the edge of the vent pipe or drain pipe.
  3. Get the good R-6 thick rubbery (not foamy) pipe insulation of the right diameter and seal it on the pipes in an airtight fashion on the cold pipes. You can seal the hot pipes too for energy efficiency but I don't think it will help with the condensation problem.
  4. Don't forget to look for roof leaks while you are in there.
  5. Check the back of the shower valves and shower gooseneck.
  6. Look for a vapor barrier behind the shower wall (drywall isn't a vapor barrier without a polyethylene layer). Because of the high bathroom humidity, you probably want a vapor barrier. If you don't have one, don't rip out your drywall. Instead look for a high gloss paint. Latex paint, especially high gloss, acts as a vapor retarder and will limit how much shower humidity gets into the walls. It's also easier to clean than matte finish paint.
  7. Test the output of your bathroom fan. It's not unusual for older ones to run but produce no flow outward. Sometimes they are wired backwards too and do some old stuff like push instead of pull. I'd go outside to your vent and make sure it is pushing exhaust out. 7.

Good luck.

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