How would you attach the lath for the stucco without providing a path for sound transmission? Would you rely on the glue to hold the stucco system on the building? Do a sketch so we can understand the assembly.

There are other assemblies that seem to be better and cheaper. Why did you choose this one?

The Green Glue is being placed between the plywood and the DensGlass.

Because plywood is necessary to achieve the required sheer stength, I thought it may be ideal to place the GG between the plywood and the DensGlass sheet.

Basically, this is what the wall would consist of (from inside out)

1) 5/8" sheet QuietRock 525 or 527
2) Standard 2 x 4 studs, 16' OC filled with fiberglass insulation (R13)
3) 1/2" sheet of plywood screwed to the 2 x 4s
4) 1/2" sheet of DenGlass screwed to the plywood
5) Dryvit stucco system consisting of mesh, cementitious coat and stucco finish on surface

Total wall thickness is roughly 5 3/8" thick.

I know that many homes don't use plywood as an exterior sheething and simply screw the DensGlass directly to the studs so the option to sandwich the GG is not there. I've looked all over the net and have not found anyone to do this and I'm assuming there's a good reason. As mentioned, I would think GG would be much more effective if applied to a wall that doesn't have so many penetrations in it like interior drywall has. Ultimately, I would like to achieve a wall assembly with an STC of at least 55, ideally 60. Thanks for your help.

I have a question: Are your efforts lessened on an exterior wall that has windows? Seems like you will have sound transmission through those avenues. I know sound studios only opening is typically a door that has been sealed and treated for density, but typically no windows in the room. I have seen studios that have fixed glass panes that i am assuming are either multi paned or super thick.

It might be beneficial to decouple the exterior finish from the wall but I would not trust green glue to hold it on the wall.

Have you considered double studs and a good resilient channel system plus double drywall on the inside?

Green glue is not and cannot be used as an adhesive. Just call it green acoustical mayo. You would still have to screw the densglass to the underlying stud (through the plywood). From a distance, it looks like it would work as GG pretty much works between any two solid surfaces that it can be applied between. That being said, you're bigger issues are going to be windows, doors, and vibration - especially since you are using QR inside (essentially very close to what two layers of drywall/GG is).

GG/QR is especially effective at certain frequencies. It is not effective against very low frequencies (like train over tracks, etc). I think with highway noise, you have the airborne noise along with the wind and vibration noise. A lot of this could be mitigated with mass (think brick/masonry), but since you don't have this - then yes decoupling might work better.

Let me clarify a few things.

I am not using GG as an adhesive. The plywood is being screwed into the studs and the DensGlass will be screwed into the plywood. I would have my carpenter avoid screwing the DensGlass into the plywood where studs are located. I'm sure this won't be a problem. This should reduce the "short-circuiting" effect somewhat. I realize that screws can act as noise/vibration conduits but they are also used in drywall installs where GG is sandwiched between two layers of drywall. I don't see the major difference.

My reasoning as to why I think this could work is GG states that their product can increase STCs of wall assemblies when placed between "two rigid dense layers of building material". I would assume DensGlass and plywood qualify as "dense layers of building material". GG does not specify that the material be drywall. And as I mentioned earlier, some benefit must be realized because the external sheeting has NO penetrations except for windows (but these are also realized in the interior). The interior wall (drywall) is cut up like swiss cheese as I have tons of outlets and light switches. For example, one wall has a 7-gang light switch on it. Yes, 7 light switches. That's one big honking hole in my wall that GG and two layers of drywall do nothing to address. But on the exterior I still have the trusty plywood, DensGlass and stucco system protecting me from the freeway noise. So why not make this material-combo soundproofed with GG? I read that GG becomes ineffective at very low temperatures (assuming it hardens and loses its viscoelastic properties) and this could be why it's not recommended for use on exterior walls. However, I am located in Honolulu, Hawaii and the night time temp NEVER drops below 55 degrees. In fact it's pretty rare that it gets into the low-mid 60's in my area. When I left a message with a rep I didn't tell them I was in Hawaii. This could be why I didn't get a call back.

Double studs, resilient channel or concrete exterior wall construction were not options because of limited space. You feel like you're in Japan when you're in these units. Rooms are 10 x 10, hallways are 36" wide and kitchens and living rooms squeeze into the same room of about 325 sq ft. Here in Hawaii, the Land Use Ordinance requires that you stay under a maximum interior building area dedicated to each individual lot. This maximum building area INCLUDES any exterior and interior walls. So if your exterior wall is thick, you literally lose that space forever and it cannot be made up elsewhere, forever. So it's not a matter of trying to save money - it's literally trying to get enough space to make a bedroom, bathroom or kitchen appropriately livable. And because these are high-density 3 story high units, and very narrow, you can see how 2" on each exterior wall will quickly eat up into useable space. And trust me, it doesn't help when land goes for about $140 sq/ft in my area. And there is no land available to buy in the entire downtown Honolulu area right now - not one sq foot.

Regarding the effectiveness of a thicker wall, going from a 2 x 4 assembly to a 2 x 6 (16 OC to 24 OC) only increases the STC by about 3 points. Nothing to sneeze at but I'd rather spend the money on other bldg techniques and material to make up the 3 points than lose 2" around the entire perimeter of each unit. I just wish someone could tell me what kind of difference

I'm focusing on soundproofing windows also... looking at aluminum frame fixed and operable casement style windows with 1/4" laminate (exterior), 1" air space, and 7/32" plate glass (interior). Another option (which is a bit nutty) is to go with two separate windows - one on top each other. Similar to what the company Soundproof Windows does except it's new construction. My understanding is that STC is measured by averaging... meaning the window STC is measured and the wall STC is measured. The STC of windows and wall surface is then taken and calculated into an average. So even if my windows are 40-45 STC, and my wall is 55-60 STC, I could still get somewhere in the 50-55 range as an average. I would be content with that. So I'm going to concentrate on tackling both independently and achieving the highest STC reasonable for walls and windows.

I also want to add a little bit about the freeway located next to my property. It runs on the fringe of downtown towards the east and east Oahu consists almost entirely of homes and condos - no industry. So big trucks don't traverse this section of freeway. Only during the day do you get the occasional delivery truck, concrete truck or 18-wheeler. But in the evenings it's almost exclusively cars and light trucks.... and sirens (home is about 1/4 mile from the island's largest hospital). So while lower frequency noises are an issue, it's not so much an issue as your typical freeway. The freeway is much louder in west Oahu where it connects the island's largest industrial park and shipping dock. All day and night large commercial trucks run through that section of freeway. The freeway actually ends about 5 miles east of where my house is. But it goes about 45 miles in the other direction.

I forgot one last thing... would it make any sense to install a 1/2" deep batton or furring strip to create an air gap between the DensGlass and plywood (instead of using GG)? I've heard of the triple-leaf effect and how this could actually REDUCE my STC. Would this make any sense?

I forgot one last thing... would it make any sense to install a 1/2" deep batton or furring strip to create an air gap between the DensGlass and plywood (instead of using GG)? I've heard of the triple-leaf effect and how this could actually REDUCE my STC. Would this make any sense?

The decoupling systems advertised on the internet are all intended to reduce the transfer of sound levels and frequencies from voices and office equipment and their claims are usually exaggerated even for these narrowly defined conditions.

The double layer sheathing you describe, even if it were possible for the green glue to function as intended (and I don't see how it could) will probably result in an STC below 40 which is not going to make you happy.

There are exterior wall systems only an inch or so thicker than yours that are rated at an STC over 55 adjusted for use in a traffic condition so why try to cobble together an assembly from internet sales pitches?

If you really want to maximize the bang for the buck you should hire an acoustic consultant.

The plywood is being screwed into the studs and the DensGlass will be screwed into the plywood.

Not acceptable. DensGlass requires fasteners at least 1" long.

Ditto the furring. The DensGlass will bend in if it's not flat on the supporting plywood.

Yes, 7 light switches. That's one big honking hole in my wall that GG and two layers of drywall do nothing to address.

That's why I have used the QR system, which includes a GG like caulking and an acoustical sealant that you use on the intrusive service boxes.

Rather than dreaming up new systems with unapproved methods, invest a few hundred dollars and consult a specialist or stick with the tried and proven methods.

Will respond in more detail shortly. I contacted two acoustics professionals here on island and their quote for a SFR was $2,200 and $2,700. They only do large commercial projects so I'm sure they spit those numbers out just to get rid of me.

So if you know anyone in the "couple hundred" range, please let me know. I will hire them in a heartbeat.

if you know anyone in the "couple hundred" range

Not in Hawaii. We're having trouble enough finding two independent commercial appraisers at the moment!

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I would have my carpenter avoid screwing the DensGlass into the plywood where studs are located. I'm sure this won't be a problem.

Except that DensGold installation instructions call for the product to be fastened to the framing. Now, you may find a "carpenter" to do what you tell him and a building inspector who doesn't notice. But that doesn't mean it's not a problem.

OK, I finally got a call back from a Green Glue engineer. I spoke to Brian Ramnes.

This is what he told me:

Utilizing GG in the assembly I suggested would LIKELY yield a HIGHER STC rating than if used between two sheets of 1/2" drywall. His reasoning was inline with what I stated - that having less penetrations in the wall will increase the overall STC. Even if you take all precautions and use special utility boxes, the STC will be compromised to some extent if these boxes are present. In addition, he said utilizing GG on an EXTERIOR wall partition vs interior is yields a higher STC because you are focusing on the side where the noise originates. Doing it the other way around is less effective as it allows the noise to pass through deeper into the entire wall system.

I then asked why nobody installs GG in exterior wall systems and he said (again what I assumed) temperature will affect GG's performance. Very low temperatures render the insulating characteristics of GG useless. GG is only effective in the 40-90 degree range. He said that because I am in Hawaii and the temperature never dips below 50, the exterior application would work better.

He also said GG will work well between any two RIGID and flat surfaces and the materials do not need to be like materials. He said 1/2" plywood and 1/2" DensGlass should perform similarly to two sheets of 1/2" drywall.

Regarding wall thickness, he said that a 2x6 wall is NOT going to provide higher STC ratings than a 2x4 wall. However, placing the studs 24" OC vs 16" OC could yield an STC of 5-7 points, which is higher than what I thought. He did say that using GG on both the interior and exterior wall system would definitely yield a higher STC but could not speculate on what the increase would be. Being that I am using QR 525 or 527 on the interior wall, I should be at a 51 STC rating minimum, right? This is what QR claims they achieve with 2x4, 16"OC, at least to my knowledge. Utilizing insulation may add another point and utilizing GG between the plywood and DensGlass I'm assuming should at least add another couple points or so. So 55 STC for a 2x4" wall may be realized? Or maybe I'm just crazy.

Before you bite my head off and say all this is ludicruous, please contact GG and ask to speak to Brian Ramnes. I'm only the messenger...

All the recommended attachment methods for the DenGlass and EIFS systems will short-circuit most if not all of the soundproofing benefits of the exterior system.

Why not go with a traditional scratch/brown/top coat stucco over a double ply underlayment over the wall sheathing and quit worrying about it. You will have 3/4" to 1" product on the exterior wall. With dryvit, you wont have jack as far as thickness is concerned. Double quiet rock the interior with green glue.

I like that. But instead of double QR, go to a higher STC rated QR. The big thing that's missing though is the windows. If they're not close to the wall ratings, it's all for naught.

Worthy, why would attaching the DensGlass to the plywood/stud with screws short circuit GG and more than attaching drywall to the same studs?

Decoupling the DensGlass by screwing it into the ply, not the framing, was your suggestion: I would have my carpenter avoid screwing the DensGlass into the plywood where studs are located. I'm sure this won't be a problem. This should reduce the "short-circuiting" effect somewhat. It was pointed out that DensGlass must be attached to the framing.

Whether there is any more short-circuiting than using other materials and how effective the assembly performs is only speculation by GG in the absence of independent testing which they have provided for other assemblies.

Before building this way, be sure your design is spelled out in detail on your building plans. Personally, I'd leave the experimentation to others.

The suggestion of screwing the outer wall layer to the inner layer indicates that you do not understand how Green Glue works and how it must be installed. You should read the information on their website carefully.

Here is a link that might be useful: Green Glue

Worthy - I understand what you are saying but drywall also requires that it be attached to studs with screws.

So why would two sheets of drywall w/GG sandwich attached with screws through the 2x4 framing perform any differently than two sheets of plywood w/GG sandwich attached with screws (or nails) through the 2x4 framing? Wouldn't the screws in the drywall also "short circuit" any benefit?

GG techs have specifically told me that installing GG between two sheets of plywood located on the EXTERIOR of the framing wall will LIKELY perform better than two sheets of drywall in the INTERIOR of a framing wall because the noise is tackled at the source - the outside - before giving the noise a chance to vibrate the entire wall assembly and then trying to stop it in the interior, AFTER it has already traveled through most of the wall.

The other benefit is that there are less openings in the exterior wall so therefore less area for sound to transmit.

I realize that this is a bit experimental. While the GG techs don't have testing to ascertain its performance they have assured me that it should perform well.

mightyanvil - I have talked to two different techs at GG and have spent time perusing their website. The conclusion on all fronts is that GG installed on the exterior partition should perform as well as that in the interior partition.

When you install two sheets of drywall with GG sandwich you are in effect screwing the "outer" wall layer (outside sheet of drywall) to the "inner" wall layer (inside sheet of drywall). It's inevitable that they will be screwed together. So what difference does it make if the two materials are not specifically drywall? GG techs (and their website) specifically state that the materials need not be similar in composition or thickness... just that they are both RIGID materials.

Just passing along what the techs have been telling me.

Worthy - I also wanted to mention that (per your advice) I did hire the services of an acoustics engineer. It was reasonable - about $800. Unfortunately, he had no experience with GG but had recommended different window assemblies and floor/ceiling partitions that have been helpful.

Green Glue does not work through the process of decoupling therefore there is no concern regarding short-circuiting. Rather it works by damping, that being the case as long as the sheet is uniformly damped the screws attaching them to the studs do not matter that much. That is also the simple reason why the Green Glue will perform as well between the DenseGlass and the plywood as well as it would work between 2 layers of drywall. BTW the name is Brian Ravnaas

Here is a link that might be useful: Green Glue