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Insulation between floors

DavidLLarsen | Posted in General Questions on

I’m building a 3 story house. the first floor is stick framed but the top two floors are 20″ open web trusses on 19.2″ centers. In the past, we’ve used regular unfaced r30 batts which had very little effect on sound control. I’m looking for a better option. I’d love to use Rockwool Safe n Sound but it doesn’t come in the right widths. My insulation guy suggested we staple some fiber mesh to the bottom of the trusses and blow in cellulose to R38 rating. Any other suggestions? I suppose we could turn the rockwool perpendicular to the trusses and cut 19.2″ or slightly bigger sections. Would just be more labor. 

In any case, if batts are used on such large trusses, is tightly against the underside of the floor sheathing the best spot or flush with the bottom of the trusses the best spot?

My plan is also to use rolls of thin cork membrane under the hardwoods on the floors above

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Replies

  1. Expert Member
    Akos | | #1

    There are a lot of myths out there for sound control.

    The type of fluffy insulation really doesn't matter much, it also doesn't make as much difference as people think (or hope). Typically any type of fluffy insulation only decreases sound transmission by 3dB (people only notice a change when it is around 6dB, so not much).

    There are also two types of noise you try to isolate for. Air borne sound (people talking) and structure borne (heel strikes of people waking).

    Air borne is relatively simple. The key is:

    -ISOLATE, that is air seal the floor and ceiling, small gaps can transmit huge amounts of sound, gaps around the perimeter should be caulked

    -DECOUPLE, have something compliant for hanging the ceiling, resilient channel or decoupling clips with hat channel

    -MASS, use heavy materials, 5/8" type X drywall, two layers if you want more sound control

    You get the above right, any type of insulation between the joists works. You want the insulation sitting on the resilient channel not touching the ceiling bellow. You definitely don't want the cavity full as this effectively couples the drywall to the subfloor.

    Heel strikes are very very hard to deal with in light weight wood construction. Underlayments (be it cork or synthetic) only help with impact noise (think the sound of of dog's nails on the floor as they walk). The do absolutely ZERO for heel strike. I've tried a number of ways of reducing it, it is not easy.

    Things that work are increasing the stiffness and mass of your floor by a LOT. Like changing your 19.2OC to 6"OC and using two layers of subfloor with a 4" concrete topping. Clear spanning with structural steel and using shorter dimensional lumber joists between the steel beams also works but gets expensive. Again you want the stiff but heavy.

    Another option is putting a layer of 1.5 rigid mineral wool ontop and floating a 2nd subfloor. This helps, but heel strikes still come through.

    The reality is, if you go with light weight wood construction, you just have to live with it.

    For wood construction, eventually we'll have more CLT options in North America, which is also works.

    1. DavidLLarsen | | #3

      Very informative! thanks!

      fortunately we don't have a rental or something below the living space so it really doesn't matter too much, we'd just like to make a bit of a difference. We are already pretty strict about shoes etc in the house so that helps a wee bit with heal strike (i hope at least)

      What would be the best bang for your buck if you had to choose?? The cork underlayment (about 50 cents a sqft), resilience channel and single layer of 5/8, or double layer of 5/8 (no channel) ?

      1. Expert Member
        Akos | | #4

        The cheapest compliant underlayment works almost as well as expensive. Anything around 3mm (1/8") gets you most of the benefit it will ever give with wood construction.

        I would be more worried about VOC, some are pretty nasty on that front, good to do a quick smell test before committing to it.

        Best bang for your buck is fiberglass in the floor bays and 5/8 drywall without RC. It takes the edge off on sound but cost very little extra to build. Anything more than that you have to start watching the installation details, quickly gets labour intensive.

        For partition walls between bedrooms/bedrooms and bedroom/bathroom go with 5/8 on both side with batts (quietzone or safensound) in the wall. Make sure to stagger outlets, you don't want outlets in the same stud bay on both sides.

        For something like a media room, 5/8 on resilient channel with caulked perimeter is not too much extra cost and makes a big difference. It does require thicker door jamb, so make sure to order accordingly.

  2. Deleted | | #2

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  3. AlexPoi | | #5

    Without decoupling your ceiling from the floor above there is no way to block impact noise. Cork will help a bit but it won't be perfect.

    You have two options. Build a ceiling independant from the subfloor above by installing new ceiling joists under the floor joists or use some resilient channels. The independant ceiling is more effective and probably cheaper to build but you will loose more headroom.

    1. Expert Member
      Akos | | #6

      I've tried a lot of these options. Prettymuch anything that is slightly compliant works for impact noise (not real impact, but stuff like utensil falling the hardwood). There is nothing special about cork, I've tried 1/8 1/4 and 1/2, all work about the same, which is they don't do much. Going over 6" structural concrete slab is a different story, but that isn't the case here.

      Decoupling is good for air born sound but does very little for impact. It is also something that can be easily screwed up by trades, pretty much undoing any benefit. I would only recommend this if you have somebody that knows what they are doing otherwise you are wasting your money.

  4. Matt F | | #7

    This is a decent resource for soundproofing:

    https://www.soundproofingcompany.com/soundproofing-101

    Akos has given good guidance. A bit of the cheapest insulation available, heavy drywall and a basic underlayment are the low hanging fruit. From there it can get complicated quickly.

    Carpet with a thick pad is the ultimate footfall solution, but often not an acceptable one.

    1. Expert Member
      Malcolm Taylor | | #8

      Matt,

      There are some interesting debates going on between senior's advocates and condo boards about soft floor coverings. Some buildings require them to alleviate sound transmission, but they are the leading cause of falls among seniors.

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