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Community and Q&A

Mineral Wool Insulation in Bathroom

Bob0824 | Posted in General Questions on


I installed mineral wool insulation on interior wall of bathroom mainly for sound reduction purposes. Should the wool insulation be covered with vapor barrier as well? The interior wall with insulation is opposite the wall of shower and tub if that makes a difference.

Thanks for the help,


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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Bob,

    I’m going to give your question a bump. In the meantime, the comments from Expert Member Dana Dorsett in this Q&A thread might be of interest.

    1. Bob0824 | | #4

      Hi Kiley,

      Thank you for the response and forwarding me to that information.


  2. Expert Member
    BILL WICHERS | | #2

    It sounds like this is an INTERIOR wall, correct? BOTH sides of that wall are conditioned space within your home? If that's the case, you don't need to worry about a vapor barrier because you won't have any condensing surfaces in an interior wall since both sides will be at essentially the same temperature all the time.

    Vapor control becomes an issue in EXTERIOR walls because the two sides of the wall will often be at very different temperatures, and as soon as one side or the other gets below the dew point of the air on the other side you can have problems with condensation within the wall.

    BTW, to keep things clear, "mineral wool" shouldn't be shortened to just "wool" since there are also natural (sheep's wool) wool insulating products out there and they're not the same thing.


    1. Bob0824 | | #3

      Hi Bill,

      Correct this is an interior wall and both sides of that wall are conditioned space within the home. Thank you for the explanation and response. Great information and confirmed what I was thinking as well.


      1. Expert Member
        Dana Dorsett | | #10

        >"Correct this is an interior wall and both sides of that wall are conditioned space within the home. "

        No vapor barriers are wanted here. The partition walls are best left somewhat vapor-open (latex paint on wallboard is fine) to allow the more humid bathroom side wall to dry more thoroughly.

        If a true vapor barrier were on the bathroom side of the insulation the average moisture content of the wallboard on that side would be higher, increasing the potential for mold growth on that wall. If a vapor barrier gets installed on the other side of the studs the average moisture content of the studs is potentially higher, making it higher risk for mold inside the walls.

        1. Bob0824 | | #11

          Thanks for that information Dana.

  3. Expert Member
    NICK KEENAN | | #5

    I'll add that sound travels primarily through air, and the most important step in soundproofing is airproofing, just like you would with an exterior wall. If the wall isn't airtight you're wasting your time adding additional soundproofing material.

    And just like with an exterior wall, fluffy insulation is not air tight.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      This Spring I removed part of my T&G dining room ceiling as part of a project opening up the room to the kitchen. I'm loath to confess that although we have been in the house a couple of decades I hadn't got around to trimming the edges (cobbler's children and all that...). While I had the ceiling open I filled the joist cavities with rock-wool batts under the bathroom above, before replacing the wood (still haven't trimmed it). The difference in sound transmission just doing that has been appreciable. Sound attenuation is complex.

    2. Expert Member
      BILL WICHERS | | #7

      An air seal isn't necassary for fluffy insulation when used for sound attenuation purposes. Small air leaks like between drywall and framing (the kind that a piece of paper can barely slip into) are relatively small issues for soundproofing, but big issues for moisture migration into wall assemblies which is why air sealing is more critical for moisture protection than it is for sound. That's not to say some level of air seal is a good thing for sound -- that is why they make acoustical sealant after all :-), but you don't need a full air barrier the way you want in the outer building envelope.

      A lot of sound transmission through walls is actually by conduction, which is why double stud walls make such a huge difference -- you're removing the rigid path that connects one side of the wall to the other. Resilient channel is the next best thing to compeletly separate studs for each side of the wall. What mineral wool does is to help limit sound transmission that couples from one side of the wall to the other through the volume of air enclosed in each stud cavity. This is pretty effective, but tends to get the higher frequencies best (and resonanaces that are particular to the size of the stud cavities). To block the lower frequenies, you start needing to decouple things, which is where you have to step up to resilient channel or double stud walls. I generally double up on drywall before making that jump to resilient channel or a double stud wall.

      Where air leaks come into play is if you build a great double stud wall, with batts and double drywall and green glue, but you leave that 2" gap under the door to the room... Lots of sound can sneak through that huge gap. Tiny little gaps here and there where the framing isn't quite perfectly flat and the drywall doesn't quite perfectly seal are much less of an issue.


  4. JC72 | | #8

    No and the insulation should not touch both walls when used for sound-proofing. This is why for example Roxul Safe-n-Sound doesn't fill the entire stud bay as it's only 3-inches thick and consequently leaves a 1/2 inch air gap in a stud bay.

    1. Expert Member
      BILL WICHERS | | #9

      When I "upgrade" a wall for sound attenuation purposes, I like to push the safe-n-sound batt against the far side of the wall. Since I always use 5/8" drywall when I put up new material, this means the safe-n-sound batt is pressed against the old, usually 1/2", drywall on the opposite side of the wall. My thinking is maximum damping of resonances, and the thinner drywall is more able to vibrate with sound than the thicker drywall is. I haven't made any measurements though.


      1. JC72 | | #12

        Ya.. IIRC there's also an acoustic drywall which can add additional sound-deadening.

        1. Expert Member
          BILL WICHERS | | #13

          Yep, it's called "QuietRock". I'm told it works pretty well, but I've never tried it myself since it is pretty effective at emptying your wallet. I've always just gone with a double layer of 5/8" drywall and green glue when needed.


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