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Quick Between Floors Insulation Material Question

93tilInfinity | Posted in General Questions on

New construction, ADU over the garage and a small part of the main home so trying to minimize sound transfer between 1st and 2nd floors.  2×14 (a couple 2×12) floor joist bay.  Note this is just about the insulation material choice, not the other (more important) floor assembly choices for sound deadening. Thoughts:

– The part over the main house has a ton of plumbing runs in the ceiling.  Are regular fiberglass batts actually better in spots like that as it’s easier to stuff in the joist bay versus RW being more stiff?
– Rockwool rep says anything over 5.5 inches thick has seriously diminishing returns for sound deadening.  That said I have to think that having 2x the material thickness (5.5” vs 10.25”) matters.  Still enough room to have an air gap even at 10.25”.
– I chose RW over fiberglass in the interior walls because I reasoned those were spots where the fiberglass batts might sag over time.  I’m thinking that’s less of a concern in the floor?

Options are: (material only 744 sqft)

Johns Manville Fiberglass Batts 10.25” R30 – $600
Rockwool Comfortbatt 5.5” R23 – $950
Rockwool Comfortbatt 7.25” R30 – $1700

Price matters but if there was going to be a noticeable difference in using either RW option I’d do it.  Just don’t want to spend more for a worse or equivalent solution.


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  1. Expert Member


    You will get a much better result by putting the savings between the fiberglass and rock-wool into other sound attenuation products, like resilient channels or two layers of drywall on the ceiling.

  2. Expert Member
    BILL WICHERS | | #2

    I'm a big fan of mineral wool, but in your application, if sound control is your primary concern, I'd go with the cheaper fiberglass option and put the savings towards a double layer of 5/8" drywall on the first floor ceiling. You'll end up with a better performing assembly for sound control this way.

    Insulation helps to block sound transfer between surfaces through the air space between studs or joists, but it does close to nothing for sound that is conducted THROUGH the studs or joists. To deal with that conducted sound transfer, you need to do one or both of two things: isolate the drywall with something like hat channel, or add mass with a double layer of drywall. Ideally you'd do both, since mass and isolation both help.

    If you want a quiet home, at minimum use 5/8" drywall everywhere, instead of 1/2". There are other advantages to that besides just sound control, and it's a cheap upgrade -- even in today's world of crazy building material prices. Next step is insulation in the walls. I usually use the sound control mineral wool in the walls, but if you have 10+ inches to work with, just use fiberglass and save money -- there will be little difference. Using hat channel or resilient channel in the walls and ceilings adds significant complexity to the build, so that's a more expensive option, but it performs very well. Adding a second layer of drywall is relatively easy, so it doesn't cost as much but does help to reduce sound transfer.


    1. Expert Member
      MALCOLM TAYLOR | | #3


      1. Expert Member
        BILL WICHERS | | #4

        Great minds think alike and all that? :-D


  3. 93tilInfinity | | #5

    Thanks for the responses :)

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