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Thread: Drywall / ceiling choice for stacked bathrooms with plumbing in between

  1. #1
    DIY Junior Member Scott99999's Avatar
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    Default Drywall / ceiling choice for stacked bathrooms with plumbing in between

    Hello,

    We've got two bathrooms stacked - one small one (5x5) on the first floor adjacent to a laundry room, and a second one on the 2nd floor with a small shower. There's space between them for the plumbing (waste and water lines), but the area is generally closed off by joists.

    I was going to buy some drywall as a replacement ceiling, and I like the mold-resistance and fire rating of XP.

    QUESTION: Is the moisture resistance a plus or a minus for a ceiling between bathrooms like this?

    The bathrooms have small windows, so there are no fan(s). I'm wondering if the 2nd floor plumbing will generate some condensation that will get "trapped" due to the moisture resistance of the XP below. Should I put a small vent (no fan) in the 1st floor ceiling to get air into the space between the bathrooms?

    THANKS!

  2. #2
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    I would use the XP and insulate any water pipes you can get at.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Between floors, there shouldn't be much fresh air migration, so there wouldn't be an excess of moisture to condense. If you insulate the pipes as indicated, they shouldn't reach the dew point, so you shouldn't get any condensation at all. As to the type of drywall, I don't think it really makes much of any difference. Thicker stuff (fire rated) will also block sounds better, though. Before you close it up, would it be possible to add a vent fan for the first floor bath? It's a nicer way to ventilate the room than opening the window, especially in the middle of the winter.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member Scott99999's Avatar
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    Hello,

    Thanks. Is there a preferred insulation for that use? I know there's the armacell foam type and rubber insulation, but I'm a DIYer, so I'm not always clear which gets used in different situations.

    I've actually got this same question from another thread of mine (same bathroom) in which I've got to patch 2-3 inches of concrete around a copper baseboard heater pipe. I was going to put insulation around it first to allow expansion/contraction, and assumed the rubber type because it's similar to what's there.

    RE: FAN - the fan would be pretty tough in this scenario, mostly because it's a small room and other than venting into the laundry room, I think I'd have to go through a joist. There's a concrete slab underneath, a fireplace chimney behind one wall, two exterior walls, and the laundry room. So, looking up to the ceiling - all you see is joists and plumbing.

    Thanks again.

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