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I have a brick house built in the 1920s. I'm installing a bathroom basement based on an existing layout, but I'm trying to keep the "wet" side (mechanical room with hot water heater, etc.) physically separate from the "dry" side (scrapbook room, home office). The new bathroom will have a shower, whereas the old bathroom did not.

I'm installing a 2x4 stud wall between the bathroom and home office. The home office side will have mold resistant 1/2" drywall on the walls. The bathroom side will have concrete board covered with a water barrier (Kerdi, more than likely) and tile in the shower and FRP in the non-shower bathroom area. Of course, I'm also installing a fan and venting to remove moisture from the air after a shower.

My question, and I'm planning on installing bat insulation in the wall, is:

Should I put a vapor barrier on the home office side, or will this eventually trap moisture/condensation in the wall?

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    Why are you installing fiberglass insulation on inside walls? If you are using something like Safe-n-sound, it is not fiberglass and you shouldn't need a vapor barrier
    – Steven
    Mar 22, 2013 at 1:13
  • I was planning on using fiberglass batt, but I'll take a look at Safe-n-sound.
    – cigarman
    Mar 22, 2013 at 13:57
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    There is no need to insulate the inside of a conditioned space - just wasting money. unless you want it for fire/sound protection but then fiberglass is not the right choice
    – Steven
    Mar 22, 2013 at 14:40
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    The plan was that it would be more for sound than anything, not really a thermal purpose. I wasn't aware of the Safe-n-Sound product, but it's what I'm looking for. Should I add plastic to each side of the wall after the insulation is in, though? Since it's water proof, I don't think I need to, but is there a benefit to doing it?
    – cigarman
    Mar 22, 2013 at 14:50
  • No definitely not. It is mineral wool so it is not affected ny moisture like fiberglass. You want the moisture to be able to escape
    – Steven
    Mar 22, 2013 at 18:54

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No, there's no need to install a vapor barrier between two conditioned spaces. The purpose of a vapor barrier is to prevent warm moist air from traveling through the insulation and condensing on the cooler side, which can easily happen in cold climates. With a conditioned space on both sides, you can allow any moisture in the air to pass through.

There's also no need for insulation except for sound proofing (fires can pass through doors, as can any heat). For sound, make sure your insulation is designed for that, and also consider a resilient channel or similar product that will keep the drywall physically separate from the studs, which is how many sounds travel through walls.

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