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sound reduction for townhouse shared wall

ben87 | Posted in General Questions on

I live in a duplex with a cinderblock firewall dividing the two units (built in 1970’s). For the most part, there are just 2×2 furring strips nailed to the cinderblock with drywall over top (kitchen & bathroom have 2×6’s).

Parts of it will be exposed during renovations, so just curious if there’s any low hanging fruit from a sound insulating perspective that I should take the opportunity to improve? Currently, I’d say the sound barrier isn’t great, but not terrible either. So not something I want to invest heavily in, but I also don’t want to be kicking myself later if there were easy improvements I missed.

Current ideas:
– Would filling the voids with rockwool or fiberglass make a significant difference?
– Should I try to make the drywall layer as airtight as possible by caulking along the bottom, etc?

Also, I have 2 mechanical/utility closets along the shared wall where I am trying to squeeze out every inch of depth, and I may remove portions of the drywall and furring strips. Will that pose problems? And is there anything thin that could be added either to the cmu wall itself or perhaps to the inside of the closet doors to help reduce sound transfer?

Thanks!

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Replies

  1. walta100 | | #1

    I think you want to decouple the drywall from the block wall by mounting the drywall on resilient channels. The empty void allows it to be isolated from the block wall. Also get 5/8 tick drywall the added mass will help.

    Walta

  2. Malcolm_Taylor | | #2

    Ben,

    The problem with adding materials in isolation to walls as sound mitigation is that they can sometimes be ineffective or even make things worse.

    This link shows wall assemblies (including block ones) that have been tested, which allows some predictability in what improvement you will see if you mimic them.

    Even with the best assemblies, flanking sound can move thought small gaps. Try and think of the walls a dam against sound and seal any penetrations or voids - like caulking the drywall as you suggested.

    https://free.bcpublications.ca/civix/document/id/public/bcbc2018/bcbc_2018dbp9rt

  3. Expert Member
    BILL WICHERS | | #3

    A block wall is usually pretty good at sound deadening already. filling a 1.5" cavity with fiberglass is unlikely to make any noticeable difference. Caulking the edges MAY help, but the block is probably already doing most of the work. The easiest way to get a little more performance would be to put up a double layer of 5/8" drywall with green glue between layers. Better yet would be to hang the drywall on resilient channel instead of the rigid connection to the block, but don't expect that to work as well as it does on a regular wood framed wall -- the block is MUCH MUCH less conductive to sound compared to wood framing.

    Removing the drywall in those closets shouldn't cause a problem, but it will cut down a bit on the wall's sound blocking ability. If you want to cut down on sound through that wall, glue drywall to it (if it's absolutely dry), otherwise glue durock (concrete board) to it. The extra mass and thickness will help with sound.

    You could try a layer of MLV on the closet doors to help with sound, but I doubt it will do much. The closet doors probably leak so much around the edges that the MLV would offer only minimal improvements at best, probably not enough even to be noticeable, and MLV is expensive stuff. If you want to cut down on sound transfer, I'd focus on building up the block wall and not the closet doors.

    Bill

  4. gusfhb | | #4

    Air leaks, wires, plumbing anything breaching the wall will be huge sound pathways. As mentioned, the block should be pretty good, but even the smallest gap will be like a speaker.

  5. Malcolm_Taylor | | #5

    If you go to the link I posted you will see the problem with adding small elements.

    - Bare block is STC 50.
    - Add gypsum both sides, still STC 50
    - Add resilient channel under the gypsum on one side. STC 51
    - Add resilient channels both sides, and you are down to STC 47
    - It isn't until you add stud walls filled with absorptive material that you get any really improvement. Then you can get up into the STC 70s.

    1. ben87 | | #6

      yes, I was just looking at that. Very interesting. And somewhat counterintuitive. Thanks for sharing that link. I will need to interpolate some, since I can only affect the assembly on my side of the wall, but this really helpful.

      It's nice to know that the bare CMU wall should be pretty effective by itself, as well. So it has me wondering what could be telegraphing certain thuds & bumps quite loudly, though. Sometimes it is hard to tell if noise is coming from my side or theirs. Voices are almost never heard, so it seems to be more specific kinds of noises that come through. I also think I am noticing the bumps more since lots of ceiling drywall has been removed. Maybe sound is transferring through the plane where the two floor systems meet the firewall on either side? Come to think of it, there are steel straps (maybe 2" wide x 1/4" thick that protrude from the firewall about 2 ft and tie into the floor joists). I wonder if those could have anything to do with it.

      1. Malcolm_Taylor | | #7

        Ben,

        To me it's by far the most counterintuitive building science field. Add a layer of gypsum to the middle of double-stud party wall and the STC goes down. Take an existing wall and add resilient channels and another layer of drywall over what's there and the same thing happens. In new construction it's a bit easier using complete tested assemblies, but even there it's hard to predict the effect of flanking elements like the floor systems at your place.

        Also worth remembering that STC is a composite of the various types of sound (structure and air-born) so even well rated assemblies can still yield the annoying thumps you experience.

        If it were my place I'd probably do as gusfhb suggested and put my efforts into sealing.

      2. Expert Member
        BILL WICHERS | | #12

        The issues that come into play that make this stuff counterintuitive is that some assemblies can introduce new resonances. Depending on the assembly, the resonance can either act to "notch out" (reduce) certain frequency bands of sound, or REDUCE the amount of reduction for certain frequency bands.

        Low frequencies, which include bass for things like music and movies, but also the low/dull "thud" sound (not the sharper "knocking" sound) are the hardest to block. Think about entering a movie theater: when you're about to walk through the padded hallway entrance, notice that the corridor is lined with "soft stuff", and makes a bend, usually a sharp corner, part way through. As you walk in, you gradually start hearing the low "bump, boom" of bass from the movie, but no treble -- no high pitched sounds. When you go around the corner, you start hearing more of the higher frequencies, which gradually build up to full volume as you enter the seating area. The bass is more of a low pressure wave, carried by air movement through the hallway, and it isn't effected as much by reflections. The higher frequencies get more absorbed by the soft lining of the corridor, and can't make it around the corner. If you put a hard material like a piece of plywood diagonally across the corner, you'd hear a lot more of the higher frequencies before going around that corner due to reflection of the sound waves.

        In your case, the block wall is about as good as you're going to get. You could try painting with block fill paint, or putting more drywall, but even the resilient channel, using Malcolm's numbers, probably won't make enough of a noticeable difference to be worth the effort. I would look for "less than concrete" materials that bridge the space, such as wood blocking between joists if the ends of the joists are set on top of the block wall. If you see wood framing ABOVE the block wall, try gluing or screwing cut pieces of 5/8" drywall to the framing there to beef up the mass of the wood part of the partition between spaces. Seal any gaps you find with acoustical caulk, or a good polyurethane sealant. Air leaks can carry sound. The drywall panels attached to the framing will help to damp resonances and cut down on sound transmission.

        Bill

  6. Expert Member
    Akos | | #8

    I can't find the link but distinctly remember that painting the CMU improves STC values, so does pargeing.

    From the link that Malcom sent, look at the assembly with the 1 5/8" steel studs on both sides. It does significantly improve the STC values without taking up more space than what you have now. Even if you only do it on one side, it will still make a big difference. You can get 1.5" Rockwool AFB (this is like Safe N sound for steel studs available from drywall suppliers) to fill the stud space. For something like a bedroom, I would definitely go that route. Important detail is to caulk the perimeter of the drywall, you want this layer air tight to get the most out of the assembly.

    If you have access to the floor joists either from bellow or by pulling up the subfloor, check how they installed. If on a ledger you are good but if they are pocketed, you want to do some air sealing around the joists as this tends to be a big flanking path. In either case, stuff a bunch of insulation in there before closing it up.

  7. Malcolm_Taylor | | #9

    Akos,

    Unfortunately, I think those studs are 2 5/8" deep (65mm). All the solutions shallower than that don't seem to improve the STC rating much.

    The table comes from our building Code. Pretty useful.

    1. Expert Member
      Akos | | #10

      Good catch. I'm pretty sure 1 5/8 works just not as well as the 2.5". I would expect a couple dB loss based on some other assemblies.

      EDIT:

      Found it:

      1 5/8" with batts on one side, 6.4db improvement.

      https://www.masonryandhardscapes.org/?create-tek-pdf=1&post_id=521758&filename=TEK%2013-01D

      6dB is a value that most people would classify as significant improvement. If you do have the space, going up to more might be worth it. The 12dB gain for a 2x4 wall if there is no flanking will mean you will almost never hear anything.

      1. Malcolm_Taylor | | #11

        Akos,

        "I'm pretty sure 1 5/8 works just not as well as the 2.5" "

        They probably do. The 2"x2" s work pretty well.

  8. DennisWood | | #13

    There are two things that come to mind with your project. I've done a few projects where sound attenuation is important, from a full on commercial film studio with double walls and isolated HVAC etc, to my own home theatre. I've used quite a few approaches that were particular to the build.

    1. Resilient channel has many paths to failure, via unintentional acoustic coupling, however is effective if you are very careful on installation.

    2. Green Glue works, and is an easier path (less failure prone). You can pick it up at HD. Use 2 or 3 layers of 5/8 Type X, in layers directly over your existing drywall or furring. Stagger seams. Leave a small gap on the first layer of TypeX at the room edges for air sealing and transmission isolation. Air seal the first layer then move on to layer 2 and/or 3, air sealing those as well at the periphery. I've used Green glue/drywall with comparative back to back testing with a db meter in various test locations. It works quite well, very comparable or better then resilient channel.

    3. Air seal the periphery and any utility boxes etc. Green Glue also has a decent product for this, again available via HD. Leave a small gap on the first layer of TypeX at the room edges for air sealing and transmission isolation.

    4. Check for any flanking noise paths via HVAC etc. which will absolutely obviate any gains made in steps 1-3. My guess is that due to fire code, there are no penetrations between units...and this is a good thing. If there are any penetrations, they need to be dealt with using dead vents, duct mufflers and/or acoustic duct lining.

    1. Expert Member
      BILL WICHERS | | #14

      I would add to your #3: use a putty pad to seal the back of the box to the drywall. Putty pads are moldable clay made for fire stop applications, but they also work well adhered to electrical boxes to cover them with sound deadening materials. Many of the putty pads even indicate they can be used for this application.

      Bill

  9. ben87 | | #15

    Just wanted to say thanks to everyone for the great info. I think I'll steer away from the resilient channel b/c I'm not confident I'll get the details right, and won't have access to the whole wall to ensure a consistent assembly. Sounds like all are agreed that air sealing is really important, so I'll focus my efforts there, including around any outlets or pipe penetrations (joists do not pass through the firewall). And I think I'll also add batts to any accessible cavity spaces since that can only help and could cut down on any higher pitched or echo-y sound transfer.

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