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How do you insulate an acoustic double stud wall with a deliberate air gap between the two walls?

mateohao | Posted in General Questions on

Hello,

I live on a noisy, traffic-heavy street that is also popular to use by fire engines and ambulances. I am remodeling my house and I intend to build a second 2×3 wall inside of my exterior 2×4 wall in order to dampen street noise. Both stud assemblies will have wool insulation, with a 1 inch air gap in between.

How should I secure the wool bats inside the studs to prevent them from drooping into the air space? I cannot put any solid panel material in this air gap as it would defeat the sound-deadening properties of the wall.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Mateo,

    The gap is there to stop the transmission of structure-borne sound. Adding rigid board in the gap will reduce the effectiveness of the sound attenuation, but allowing the insulation in the two walls to touch won't. If you use the same 3 1/2" batts in both of the cavities, they will fill the whole thickness of the wall.

    There are a number of other assemblies you might consider as alternatives to a second wall. Adding res-bar to the existing studs, and two layers of drywall, may yield similar or better results. Then again, they wouldn't improve the energy efficiency of your walls the way your method would.

    1. mateohao | | #3

      Hi Malcolm,

      Thanks for allaying my concern about the batts touching.

      I like your suggestion about double layer drywall. That would be a good deal of mass to absorb sound. I will look into those bars.

  2. onslow | | #2

    You might also want to look into Green Glue and Liquid Nails acoustic sealant. They are used slightly differently, the Green Glue is more for sealing edges and penetrations of the drywall. I have heard of people use it on the studs like the Liquid Nails products. If you know any musicians or sound people, ask them about recording studios they might visit and ask who built them. There are a number of methods to muffle noise. The biggest sound transmitters in many houses are the windows and doors. Sealing them tightly with good gaskets or even sealant if the window is not needed for fire exit. Double hung windows are the noisiest in my experience, with hairline gaps that may not seem like much, but transmit sound very well. An expensive loaded vinyl sheeting is also made for extreme cases. You might also google floor noise barrier materials that are sometimes spec'ed for use in modern concrete buildings to use under flooring. Keeps the tap dancing dog nails from being heard below.

    One trick I heard about for dampening sound between adjacent condo units was adding two layers of drywall to the entire shared wall. I believe the Green Glue product was used like an adhesive as each layer was added. A bit pricey for the amount of sealant used, but client was happy. I think this was chosen over the bar strip method because it was a vertical installation rather than a ceiling.

    The walls I have done for isolating bedrooms were set up with offset stud spacing so weaving batts horizontally worked well as a starting point. I added vertical batts between studs on both sides and used 5/8 drywall to keep it smooth. A bit of a push getting the batts settled in, but it works well.

    1. mateohao | | #4

      Hi Roger,

      I've never heard of Green Glue before. I will look into that.

      I checked out the vinyl, and for the cost of those rolls I think I can get a better deal with buying extra drywall to make the sandwich you mention.

      I agree about the doors and windows. I notice most of the noise enters through my ancient windows. I have access to a CNC machine and I intend on building my own casement windows and exterior doors with some good gaskets and insulation. I have also seen "sound proof window" products that are just a second set of windows inside the window well like an old-school European window.

      I wonder how an assembly of two spaced single pane windows performs against a double pane/single pane spaced window assembly in the energy and sound department?

  3. Expert Member
    Dana Dorsett | | #5

    Air sealing the exterior sheathing is of primary importance for both sound abatement and thermal performance.

    A 1' gap would't be more effective than a 1/4" gap, but ANY gap is a potential thermal bypass allowing convection currents. It also is a path for spreading fire, and thus a bad idea in general.

  4. Expert Member
    MALCOLM TAYLOR | | #6

    Dana,
    The 1" gap came from party-wall construction to ensure it's continuity over several stories. All the STC rated double-wall assemblies incorporate it. The also include fire-stopping at each floor. I think some jurisdictions down your way now also include vertical fire-stop requirements as well. If the gap is filled with insulation it wouldn't need the fire-stopping up here.

  5. Jon_R | | #7

    Two single pane windows can be further apart, providing better sound attenuation. If possible, use laminated glass.

    You probably want to use steel studs.

    Sound attenuation is a complicated subject and it's easy to waste money with little effect. For example, a lower STC rating can perform much better with traffic noises than a high STC rating (normally beneficial).

  6. Stockwell | | #8

    I built a theater and did everything to mitigate sound transmission.I used double 5/8" drywall with GreenGlue between them, and hung the drywall on metal channel that was attached with clips with rubber isolators(look at QuietClip as an example). The densest Rockwool is best for insulation/sound control, although there are rigid fiberglass boards that are even denser(OC703, for example). I have not seen those used as insulation. As others mention, any place that air can get through is a path for sound--bad windows and doors, leaks around outlets, etc. Caulk everything.

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