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Community and Q&A

Soundproofing Assembly Above Round Timbers

Corkydork | Posted in General Questions on

I am working on the second floor loft of our home. We would like to soundproof this as much as possible as this room will be enclosed into a bed, but I’m not finding any tried and true assemblies for this because of the unique joists.  We would prefer not to hire an acoustical engineer. I have a few ideas which I’ll detail but I’m wondering if anyone here has some advice. We are musicians and my husband is a luthier so we are somewhat familiar with sound.
The essence of the issue: See image. The log joists are flat on top and round below, so there is not layer of drywall on the ceiling below that we can use in combination with a hat channel. We can certainly insulate from below (in between the logs) and put some sort of finished ceiling in between logs such as shiplap. Due to the low loft ceiling, we would like to avoid raising loft floor up too much.
My idea: In between ledger/cross beam and floor joists (anywhere there is wood to wood contact) put in felt or cork or some sort of damper material. On top of logs, before subfloor, also lay damper material. I haven’t decided yet but I’m thinking 1/2″ rigid cork board. (Not sure how much better that is than fiber board.) These come in 2’x3′ sheets and spacing of logs is 16” center to cetner so there will probably be gaps in this layer that we have to fill in from below. And we will have to make sure there are no cracks is my understanding. Then plywood, then either cork flooring or another layer of soundproof underlayment then wood floor. One drawback I’m seeing of cork floor is that it’s easily stained with spills and has to continually be refinished for waterproofing. We aren’t fastidious with liquids. From below, in between logs, we can insulate before adding finished ceiling material such as shiplap, but not sure if that is any better than adding an air space. Especially since most sound will be transferred through logs.

Any advice on this system would be great!

-Courtney

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Replies

  1. Expert Member
    Akos | | #1

    There is no easy answer for sound problems that don't take up space. You can read through this for some ideas:

    https://www.woodworks.org/wp-content/uploads/presentation_slides-BLOUNT-Knock-on-Wood-Acoustics-for-Mass-Timber-WDS-180719.pdf

    I'm guessing that you are looking for more for minimizing ambient sound transmission while keeping the visible logs.

    The big problem that you have is that most acoustic assemblies rely on two air tight layers that are separated. Your situation will not allow for this, trying to do some version of it between the logs will not work (or not well at least). An acoustic assembly that is 95% air sealed might only deliver 50% of the sound isolation. Your idea of drywall between the logs+batts will help a little but won't be quiet by any means.

    I think the simplest assembly in your case would be to go for 2x6 T&G over the logs (this can also be your ceiling bellow), 1.5" of rigid mineral wool over it covered by 3/4" OSB floating subfloor.

    Cork will do very little for STC, it is mostly about impact noise, even than it only works well over something heavy like concrete/CLT not light wood construction.

    Be careful with the clear coat you use of the 2x6 T&G floor. Lot of the water based finishes tend to "stick" to nearby surfaces with can cause the floor to make cracking noises when you walk on it. Some of the hybrids are a better options.

  2. Steve Knapp CZ 3A Georgia | | #2

    You might be able to do something with an acoustic isolation clip or a noise control system. (See https://kineticsnoise.com/rim/wood/rollout-isolation-material.html for one example.) But I'll bet that none of these approaches will be cheap. And even if you decouple the floor, you probably will have to give the walls and ceiling a similar treatment (and install an airlock-type door while still providing heat and ventilation).

  3. Corkydork | | #3

    Thanks so much for the replies. I really like the idea of a floating floor or subfloor over some acoustic barrier such as rockwool. I'm wondering if a floating cork floor over the rockwool could take the place of the floating subfloor+finished floor material? It would be thinner (better for the low ceiling height) and cheaper, and I feel like it may serve the same purpose as the floating subloor.
    I like the idea of the stickers in the link you sent Steve but 2 layers of 3/4" plywood seems like overkill. I'm trying to figure out how stickers which create an air layer could work in this assembly.

    1. Expert Member
      Akos | | #4

      Most, if not all, flooring needs to be installed over a solid substrate. This generally means plywood/osb/concrete. Now way around this.

      Even floating a single layer of OSB is slightly sketchy, you really should be using two layers of T&G subfloor screwed together with staggered seams. I have done single floating 3/4" OSB and it seems to hold up well under engineered click.

      I've tried installing engineered click floor directly over foam and it failed at the click joint. There is just too much deflection.

    2. Expert Member
      Zephyr7 | | #5

      They are probably using a double layer of 3/4" plywood so that they can spread out the supports more. Fewer rigid supports = less transmitted sound. The extra mass of the extra layer, as well as the additional rigidity, will also help to limit sound transmission.

      Soundproofing is all about two things: mass (heavy stuff that doesn't vibrate much), and squishy stuff (stuff that tends to absorb the sound and not pass it through). Combinations of these provide the best results. Ideally you want to limit coupling by having as few rigid connections as possible, and have a heavy mass too. Exactly how you achieve this depends on what you're doing, and how much money you want to spend.

      Bill

  4. Corkydork | | #6

    Thanks ya'll!

    Akos- I've read the floating floors like subfloors can be installed over a bamboo underlayment, but that was my question when you suggested the 1.5" rockwool layer+floating subfloor. I did some research and couldn't find any info about installing a floor right over this rockwool layer.

    The link that Steve sent -to Kinetics Noise Control- has a bunch of different underlayment options https://kineticsnoise.com/arch/floating_floors.html
    They also have their engineers work with you to build a whole assembly, which is a huge bonus. So I'll probably use T&G subfloor + underlayment or RIM options from Kinetics Noise Control + floating floor *or* T&G OSB + finished floor.

    Mass is the biggest issue, although we could put some mass in between the logs from below...Nailers along logs with drop ceiling. It may help to put some insulation or mass material in between the logs.

  5. Corkydork | | #7

    Also, we're willing to spend a little more on these materials because we only need 240 sq ft.

  6. Steve Knapp CZ 3A Georgia | | #8

    Courtney,

    Since you are so limited on headspace, you might want to remove the logs and use that space for your sound-suppression floor.

  7. Expert Member
    Akos | | #9

    There are no magical soundproofing products that will work in a tight space. Anybody that says otherwise is selling snake oil. A virtually soundproof stick framed floor assembly is about 1' thick minimum.

    The closest you can come, if your structure can hold it, is 3" to 5" of concrete ontop of the T&G subfloor. Won't take up much space and not much sound will make it through (~STC 50 at 4"). You can leave the concrete exposed as your finished floor or install flooring directly over it.

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