Author Topic: Soundproofing plan for demising wall in new-build condo  (Read 8219 times)


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Soundproofing plan for demising wall in new-build condo
« on: June 20, 2013, 07:07:16 PM »
Hi all,

I am buying a new-build condo before it's been built, which gives me the exciting opportunity to get the noiseproofing right ahead of time. The building is two stories, and it is split into two condos which share the long central wall right down the building.

I am going to be putting a home theater and music room alongside this wall, and for that reason I'm trying to do the soundproofing right. My builder has given me some great flexibility in terms of picking the soundproofing setup, given certain constraints on space, studs, and materials. I am already paying for additional inches of wall space and foundation to widen the wall, and paying extra for foam in addition to the blown fiber, and likely green glue too.

Could you please take a look at the diagram I've created as a plan of the wall (below), and let me know what you think? My side of the wall is the right-hand side, and I'm mainly trying to ensure I'll be able to play movies or loud party music from my floorstanding speakers and surround sound system whenever I want without waking or annoying my new neighbor.

If anyone has an insight into the type of STC they think this would provide, that would be great. Thank you!


Randy S

  • Guest
Re: Soundproofing plan for demising wall in new-build condo
« Reply #1 on: June 20, 2013, 07:20:45 PM »
This system will work as long as the subfloor does not share both units.
Because you are already doing a complex structure your STC will be around 55-60.
If the current soundproofing design doe not yield enough reduction you simply can just add mass to each wall to increase the sound reduction.
Feel free to give me a call direct if you want to know more of the science behind soundproofing.

Randy S.
760-752-3030 ext 104


  • Guest
Re: Soundproofing plan for demising wall in new-build condo
« Reply #2 on: June 22, 2013, 02:49:16 PM »

The sprayed on closed cell foam does not help the STL (sound transmission loss) and could, in-fact make matters worse by making the wall more rigid that it would be with a regular construction technique. - sort-of defeating the effects of the CLDM (Green Glue). Closed cell foam and any sort of rigid THERMAL insulation is useless acoustically. This stuff is not normally used between building/apartment units but on the exterior wall where it will provide an air-lock and seal for moisture ingress plus the thermal barrier.

IF this type of wall system is used and the wall is decoupled from the structure, you CAN get ratings approaching STC 69. In 'real-world' applications, this is reduced 10 to 15 points due to 'flanking paths'; where the sound will couple into the structure and 'flank' around your sound-proofing measures. ;) So there you have what Randy already stated. Adding TWO more layers of 5/8" drywall will increase your STL by about 5dB. This follows the rule of doubling the mass to gain an increase in STL by 5 points... calculations say 6 but, again other factors come into play and the isolation is reduced.

More information is available free on my publications page.



  • Guest
Re: Soundproofing plan for demising wall in new-build condo
« Reply #3 on: October 02, 2013, 04:37:03 PM »
Guys, many thanks for your replies.

We decided not to go with the foam for several reasons, including your points above.

In the end, I have opted for wet-blown borate-treated cellulose, which I hear is a good sound blocker. The entire cavity will be filled with the cellulose.

The subfloor is not exactly 'shared' (there will be the same insulation space between subfloors as there is between the walls), but it's not sheetrocked. However, I got the builder to put a 1/2" of ply all the way along the outside of the neighbor's subfloor in the demising wall. Hopefully this will assist as a sound block? I didn't want to put another against my studs on the demising wall too, otherwise there would only be 4" of insulation between the sheets of ply, with no insulation on the other sides. Instead on my side the cellulose will go back between the joists up to the inside edge of my 2x6 studs, which will mean an additional 5.5" of cellulose there. Hopefully this will be of equivalent effect to another 1/2" of ply without the 5.5" extra cellulose.

If, on the other hand, the insulation contractors end up putting in ply on my side too (perhaps to hold the cellulose into the wall so there's no chance of it gradually pushing out into the floor space), maybe I will get some open-cell foam and foam up around the other side of the ply to provide a good air barrier and help insulate that weak spot there too.