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Sound insulation for existing apartment interior wall

rainair | Posted in Green Products and Materials on

My apartment has a common wall with a neighbor that has poor sound insulation consisting of a thin fiberglass batting. I can hear conversations taking place on the other side of the wall. It’s got to be reciprocal, right? When this room was built above the garage, we had cellulose insulation blown into the exterior walls and ceiling. At that time I had no idea how bad the sound insulation was between the two apartments. The contractor said that they couldn’t blow the insulation into the common wall because of existing fiberglass batting insulation in that wall. This became a detail that fell into the preverbal crack.

If cellulose insulation is not viable because of the existing fiberglass batting, is there a way of injecting under pressure a thick slurry consisting of some kind of natural fibrous material and perhaps lyme? This might be a very good sound attenuation material once it dries. My intuition tells me that a thick material injected under pressure from the top of the wall between the studs might compress the fiberglass batting enough so that the space between the studs becomes filled with this material.

Is there anything like this out there? Are there any other solutions that would address the interior wall space? Putting additional layers of sound attenuating sheetrock on the existing wall is not really viable because of an existing door and moulding that would have to be relocated.

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  1. user941025 | | #1

    Daniel wrote:
    "I can hear conversations taking place on the other side of the wall. It's got to be reciprocal, right?"

    Yes and no. A few points, which may be obvious already to you, but here goes:

    1) sound is directional and follows the inverse square law as waves do, and the dynamic range of the human voice is great; if they're talking loudly and facing directly at the wall, or directly at an uninsulated point perhaps in the common ceiling (sound flanking problem), and are standing very near that common wall/ceiling, that may come through more loudly than you, on your side, facing away from the wall so the sound bounces and diffuses back towards them from a faraway wall, plus if you're talking more quietly at frequencies more readily absorbed.

    2) from the source, you can also mitigate your sound bouncing toward your neighbors by adding diffusion and absorption within your space. 2'x4' 8lb mineral wool panels wrapped in fabric, at points on the walls around the room, placed properly (use your ears) can be very effective.

    3) density and decoupling, for that common wall, help--if you've taken care of flanking. Decoupling is apparently impossible in your situation, so, sure, pack it tight. The best bang for your buck that will probably never be allowed due to a whole host of issues: a wall packed with sand.

    Good luck!

  2. rainair | | #2

    Thanks for your informative reply. Density was exactly what I was thinking about with my idea of pumping a concrete-like slurry between the studs. Your sand solution intrigues me. What host of issues are there with the sand or slurry as described in my earlier post?

  3. user941025 | | #3

    Are you able to make these kinds of modifications to an apartment wall? (Do you own the building?)

    Hate to say it, but it seems if you want to do something this involved, you'd need to tear into the wall anyway to remove the fiberglass and actually see what you're accomplishing, in which case, you may as well rebuild the wall as a double-stud cellulose dense-packed wall. Or, if you need to keep it nearly the same thickness, something similar and thinner, with extra drywall, etc. (as you've said would be impossible).

    A wall filled with sand is a *lot* of weight.

    But again, the wall is only one part of your trouble. The paths for that sound could include the ceiling and the floor.

  4. user941025 | | #4

    and the weight of that sand not only wants to push down (how beefy are the floor joists?) but it wants to push out. Through your neighbor's side, and through your own. Drywall alone wouldn't cut it, nor would probably plaster and lathe.

  5. themask | | #5

    I get the same situation. Being certain that the noise comes only through the wall, would it be effective to glue cork plates all over that wall? I don't care about the space (they are at most 3 cm thick) nor the looks. If that part of the house has to look like a recording studio, so be it. I just want to be in silence.

  6. rainair | | #6

    The apartment is a condo. Though I don't "own" it, we need no permission from the condo board to modify unless it breaks a threshold to the outside. I would need to get permission from my neighbor, which would be easy, as we discussed this being a joint project.

    I am concerned with weight issues. That's why I conceptualized a slurry that would harden made from a natural fibrous material mixed with lyme. I imagine this would be considerably lighter than sand or concrete yet have substantial density and strength, though strength is not really needed.

    I do have considerable concerns about the fiberglass mat insulation. There's no way to guarantee that injecting this material will compress it enough to consider the project a success.

    Sound attenuating material applied to the wall may make some difference, and it might be worthwhile. I will continue to look into that.

    Thanks again for your thoughts!

  7. user941025 | | #7

    The Mask wrote:
    "Being certain that the noise comes only through the wall, would it be effective to glue cork plates all over that wall?"

    Not really. There are two things you're hoping to accomplish, probably: (1) absorb sound as it's bouncing around the original space, which, if you're talking about your own side, this won't accomplish; (2) absorb sound as it passes through the wall, which cork doesn't really have enough mass to accomplish.

    Depending upon the frequencies you're talking about, some materials will be sufficient if they're employed in ways to maximize their efficacy. For absorption within a room, rather than through a wall, panels of 8lb mineral wool wrapped in fabric mounted on frames for dead air space against the walls can help. Treating a floor (tougher) and a ceiling can help.

    As for the transmission through the wall, though: people who know waaaaay more than I do and are paid accordingly to dispense their advice can give you far better information on the detailing than I ever could, but here's something, below, that came up in a Google search. I haven't looked closely at it, but I'm linking it anyway.

    About sound problems: you can put a lot of money and energy into accomplishing very little.

  8. themask | | #8

    As a curiosity, since this is about green building, I found some interesting info about this ultra green-renewable material that cork is.
    Actually, cork is also used to absorb sound as it is transmited through walls. It's not so much the mass that counts with this material, but it's structure full of cavities that break down, decelerate and even neutralize sound waves.
    I had no idea cork was so good in civil construction, but if anyone understands portuguese, here goes a biblic reference about this material's properties and applications:
    This country is market leader when it comes to several cork products (cork is even used in space!), and if anyone has the know-how and knowledge on how to extract it's value it's them.
    Check out the data on sound absorption through walls for higher frequencies, it's just amazing. All natural, and so versatile and green.

  9. Expert Member

    Daniel, a lot depends on the composition of the existing wall. Your best bet is to remove the drywall and insulation on your side and then replicate a proven sound attenuation assembly that has tested predictable performance. In Canada our building code contains a large number of such assemblies. A Google search will yield lots of similar ones.
    One important thing to remember before starting any work is that the wall between the units is also a rated fire assembly and you must be careful not to compromise that.

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