John Sayers' Recording Studio Design Forum Archive

Greetings!

My question is regarding weather or not I am building a true triple leaf ceiling assembly for a room-within a -room ISO booth I am building in my apartment
If you look at the photos above, you'll see I had an existing 9' ceiling (just added a layer of 5/8" purple board and green glue over the existing 1 1/4" plaster ( there is a hardwood floor in neighbor's apartment above... )

There is about a 6" gap between the purple board on the main ceiling and the top layer of sheet rock I am using for the ceiling of the ISO booth. i am planning to install a ceiling assembly of 3 layers of sheet rock (2 on the top side), green glue and insulation) I have already installed the 2 layers of 5/8" rock on the top side of the ceiling..

The reason for the drop ceiling and losing valuable ceiling height is because this is an old partment building with a wood floor above, and a child that is known to jump and cause painful structural borne transmissions through the existing top ceiling. I am striving to create as quiet a space as possible inside that booth for recording vocalists.

My question is: it appears that I am indeed building a sort of triple leaf ceiling assembly due to introducing adjacent 6" cavity and a ceiling stud cavity which are adjacent to cavity between the neighbor's wood floor and my plaster/sheet rock ceiling. Is this going to be a major problem for my Isolation goals? Now that I think about it, this seems to be a quadruple leaf system

I asked a professional who had been consulting me and who seemed well aware of the dangers of the triple leaf effect ( originally id wanted to build new studded wall off of the existing plaster wall to form the back corner of the ISO booth, but under his recommendation opted to simply add a layer of Sheetrock and green glue directly onto the existing plaster wall to maintain the 2 - leaf system)

For this ceiling assembly, my understanding was that because this is a room-within-a room assembly , not a "closed" wall or ceiling assembly, the 6" gap would be a sufficient gap to minimize any major STC loss...am I correct?

If this detail is not suggested, how can I create the most isolation given the circumstances?

Some more details and photos are below for reference. Any other observations, advice, recommendations based on what you can see would be of course always appreciated. Thank you!


----

The booth is approximately 8.5' long by 5.5' wide, and 7'11" high.
It is built on top an existing wood floor over which I have placed of 1/2" granulated rubber, 2 layers f 3/4" plywood, and another layer of 1/2" granulated rubber

Hi there " johnnyboy86", and Welcome!

I'm not sure if I understand your explanation completely, but you are building either a three-leaf or a four-leaf ceiling....

There are already two leaves up there: the floor of the apartment above you ("there is a hardwood floor in neighbor's apartment above...") and the ceiling visible in the photos (""just added a layer of 5/8" purple board and green glue over the existing 1 1/4" plaster"). So of you then add drywall to the metal framing visible in the photos, that will either make it third leaves (if you only put it on one side of the framing) or four leaves (if you put drywall on both sides of the metal framing).

i am planning to install a ceiling assembly of 3 layers of sheet rock (2 on the top side),

That's what confuses me! If you are planning to add 3 layers, two of which will go on the top of the metal framing, that implies the third layer will go below the metal framing, thus making four leaves.... Why do you want to do it that way?

I have already installed the 2 layers of 5/8" rock on the top side of the ceiling..

Do you mean that you have attached two layers of drywall onto the top edges of the metal joists visible in the photos? If so, then you have already built a three-leaf....

The reason for the drop ceiling and losing valuable ceiling height is because this is an old partment building with a wood floor above, and a child that is known to jump and cause painful structural borne transmissions through the existing top ceiling. I am striving to create as quiet a space as possible inside that booth for recording vocalists

From what I can see in that photo, you will not be achieving what you are hoping to achieve! You did not mention any inner-leaf walls, and none are visible in the photo... in fact, it looks like you have attached that new metal framing to the existing walls, which are firmly attached to the existing ceiling, which is firmly attached to the floor above, on top of which the kid is jumping.... In other words, there's no isolation. The structure-borne impact noise from the jumping kid will simply flank directly into the new ceiling, which will act like a loudspeaker, and transmit that same sound into the room.

Is this going to be a major problem for my Isolation goals? Now that I think about it, this seems to be a quadruple leaf system

Yes. And yes. But that's the least of your worries if you don't also isolate the walls! The concept of "room in a room" construction is exactly that: you have an existing room, and you build a totally separate new room inside it, not touching it at all, fully decoupled from it. You cannot isolate a room by just putting a new ceiling across it.

but under his recommendation opted to simply add a layer of Sheetrock and green glue directly onto the existing plaster wall to maintain the 2 - leaf system)

Sorry, but you have been mislead: There is no "2 leaf system"! All you did was add mass to the existing wall, which is NOT decoupled from the existing ceiling, which is NOT decoupled from the exciting floor above!

The concept of isolation is simple: there can be no mechanical connection between the existing structure of the building, and the new inner-leaf that you create around you. You CANNOT use the existing walls as the inner-leaf, because they already have the structure-borne noise running in them. You MUST have a new wall all around you, on all four sides, and build your new ceiling on top of that, to create your iso booth.

In other words, your iso booth will be a free-standing structure that consists of four walls, a ceiling, a door, a ventilation system, an and electrical system, all of which sits inside the "shell" that is the original room, and the two rooms cannot touch at any point. Even a single nail that joins the two would destroy your isolation.

For this ceiling assembly, my understanding was that because this is a room-within-a room assembly

I do not see any evidence of a "room in a room" assembly. The photos are rather small and low resolution, so maybe there's something that isn't visible, but to me it looks like the metal framing is very much attached to the existing walls and ceiling.

not a "closed" wall or ceiling assembly,

I'm not sure what you mean by a "closed wall".

would be a sufficient gap to minimize any major STC loss...am I correct?

If your "professional" advisor is talking in terms of STC, then you have an even bigger problem... STC is not a useful system for measuring isolation in studios. It is not applicable. It was never meant for that, and should not be used for that.

It is built on top an existing wood floor over which I have placed of 1/2" granulated rubber, 2 layers f 3/4" plywood, and another layer of 1/2" granulated rubber

Did you check with a structural engineer that your existing wood floor is able to handle that large additional load, safely? You added well over 500 pounds of mass to your floor, increasing the dead load by nearly 12 PSF... I assume that you did hire s structural engineer, who checked your existing floor and OK'd that rather large increase? It would be rather sad if you built that, and the the floor collapsed under you...

- Stuart -

5.4 plywood (560) + 6.2 rubber (1200)

Thank you Stuart for that prompt, frank and detailed response. I really do appreciate it.

You did not mention any inner-leaf walls, and none are visible in the photo...

Sorry this is not a great photo, but the inner leafs of the walls which form the existing corner of the apartment (back and to the right) are existing plaster walls -1 1/4" plaster over wood stripping and studs, plus a layer of green glue and 5/8 rock. The wall you are facing (directly through and opposite the door opening) is a demising wall into a quiet neighboring unit, the right adjacent wall covers a closet within my unit's foyer.

in fact, it looks like you have attached that new metal framing to the existing walls, which are firmly attached to the existing ceiling, which is firmly attached to the floor above, on top of which the kid is jumping.... In other words, there's no isolation. The structure-borne impact noise from the jumping kid will simply flank directly into the new ceiling, which will act like a loudspeaker, and transmit that same sound into the room.

I have one strip of metal ceiling track that is attached to the exiting demising wall, and one strip of metal wall track, but they are both decoupled via a new layer of 5/8" rock with green glue over the plaster. The new later of rock, however, does not make contact with the existing ceiling. It stops roughly 6 to 8" inches under neath the existing plaster wall - ceiling joint. Does the added rock and GG not provide any decoupling from the existing wall and ceiling? I was under the impression that while it does not create ideal decoupling, it does decouple to some degree.

"I do not see any evidence of a "room in a room" assembly. The photos are rather small and low resolution, so maybe there's something that isn't visible, but to me it looks like the metal framing is very much attached to the existing walls and ceiling."

There is no part of the new structure that is in any contact with the existing ceiling. (Again, the roughly 6 to 8" inches underneath the existing plaster wall - ceiling joint) The existing ceiling also has an added layer of 5/8 rock and green glue.

Sorry, but you have been mislead: There is no "2 leaf system"! All you did was add mass to the existing wall,

Isn't this still a 2 leaf system being that there are cavities and additional mass on the other side of these walls?

Did you check with a structural engineer

Regretfully I have not yet done this, although based on what you have said I will to do this immediately, before continuing on with any further building.

I do realize the existing circumstances and conditions were a far cry from ideal and I knew that from the beginning and am working to move forward in the best way and maximize my results with what I am working with. You're advice and guidance is deeply appreciated

but the inner leafs of the walls which form the existing corner of the apartment (back and to the right) are existing plaster walls

From the point of view of your iso booth, those are not the inner-leaf walls: they are the outer leaf walls. They are the walls of the apartment, therefore they cannot be the inner-leaf of the booth.

The wall you are facing (directly through and opposite the door opening) is a demising wall into a quiet neighboring unit,

Ditto

the right adjacent wall covers a closet within my unit's foyer.

Ditto

... and one strip of metal wall track, but they are both decoupled via a new layer of 5/8" rock

That's not decoupling! "Decoupling" means that there is no mechanical connection at all. Either empty air, or some form of resilient mount that has been specifically designed and calculated to NOT be a solid path down to an octave below the lowest frequency you need to isolate.

I was under the impression that while it does not create ideal decoupling, it does decouple to some degree.

Not really. It's a solid, hard, rigid, massive link, therefore it does not decouple. The only things that decouple are "springs", in the acoustic sense of that word, not necessarily the sense that most people think of when you say the word "spring". In acoustics, air is a spring, and so are some types of soft rubber, and some types of metallic device that have been specifically designed to be "springy" at the frequencies where isolation is needed. Not all rubber is a "spring", and even the ones that are acoustically useful are still not a "spring" at all frequencies and for all loads: the frequency and load have to be calculated, and the right type of "spring" chosen.

Think of it this way: The spring in the suspension system of a Mini-Minor works just fine to decouple the car from the wheels, because that's what it is designed to do. But if you take that spring out of the car and put it in a Mack truck, then it is no longer a spring: It is now squashed flat, and is not in the least bit springy. By the same token the correct spring for a Mack truck works just fine in the truck, with many tons of load on it, but if you take it out of the truck and put it on your motor bike, there isn't anywhere near enough load to make it "springy": to the bike, it is a dead solid chunk of metal, with zero spring it in it.

So for any give piece of your studio, you need to choose the right "spring", based on the load that it has to support, and the frequency at which it needs to isolate. Anything that is either too springy or not springy enough for that specific load, will not be decoupled.

That's why we use air so much to decouple studios: It is "springy" over a very wide range. As long as you can "trap" it inside properly, then it's a pretty good spring.

Drywall (sheet rock) is not a spring at all.

There is no part of the new structure that is in any contact with the existing ceiling.

Yes there is: the existing ceiling is attached to the existing walls, and the new structure is attached to the existing walls. If "A" is connected to "B", and "B" is connected to "C", then "A" is also connected to "C". If there's a train in the station with three wagons in it, and the locomotive starts pulling wagon "A", then that one pulls "B", which in turn pulls "C". So "C" will go along for the ride just as much as A and B...

Flanking paths can be long and convoluted, but as long as there's a physical connection path between them, then there will be flanking going on. Structure-borne sound is a real hard beast to deal with; Once it is in the structure, you can't get it out.

The existing ceiling also has an added layer of 5/8 rock and green glue.

That's great, but that's solid mass, not decoupling. They are actually the opposite of each other!!!

Isn't this still a 2 leaf system being that there are cavities and additional mass on the other side of these walls?

... and studs join the mass on one side to the mass on the other side, hence no decoupling.

A typical stud-framed house wall that consists of a wood or metal frame with a sheet of drywall screwed to each side, acts mostly like a single-leaf wall at some frequencies, while acting more like a coupled two-leaf wall at other frequencies. But it does not act like a decoupled wall at any frequencies! The studs provide a direct flanking path across the cavity.

Regretfully I have not yet done this, although based on what you have said I will to do this immediately, before continuing on with any further building.

Smart move! ! You might be OK (and hopefully you are), but it always pays to play it safe and get an expert to check, especially when lives are in danger. There's a principle carefully hidden in the fine print of most home insurance policies, rental agreements, and building code regulations, that basically goes like this: If you modify your building in any way at all, without having all the right paperwork signed, stamped, sealed and approved, then anything that goes wrong is your responsibility: no insurance payout. All damages become your responsibility entirely, in both the civil and criminal sense. So if your floor is overloaded and collapses down into the room below, injuring or killing someone, then your insurance will throw you to the wolves. You will be responsible for all repairs to the building, as well as the medical and funeral expenses of the victim(s), and any legal costs, and any law suits they happen to bring against you, not to mention the criminal charges of manslaughter, homicide, injury, or whatever. Not a nice thing to have to face, I would imagine! It's well worth the expense to make sure you meet all the legal requirements, pass all your inspections, and get all the right signatures on all the right dotted lines. That way, in the worst case, at least you get to share the blame among everybody who signed, not just you.

I do realize the existing circumstances and conditions were a far cry from ideal and I knew that from the beginning and am working to move forward in the best way and maximize my results with what I am working with. You're advice and guidance is deeply appreciated

I would suggest taking down that framing you have up there, taking off the drywall on those outer-leaf wall that are facing the location where you want the iso booth, so that just the studs are facing your booth, then "beefing up" the mass on the other side of those studs, sealing it well, and building a new stud frame next to that one, with a small gap between the old frame and the new one, then putting drywall on only ONE side of those new studs. It could be either the side facing the booth, which would be "conventional construction", or it could be the side facing the wall cavity, in which case it would be "inside-out construction", but it would ONLY be on one side of the studs. Then building your new inner-leaf ceiling frame ONLY on top of the new inner-leaf walls, not touching the outer leaf in any way, and put drywall on only ONE side of that too.

That would be a fully-decoupled 2-leaf MSM isolation system for your booth. That's the most effective, least expensive way of getting decent isolation, easily.

- Stuart -

Thank you Stuart.

I was aware that ripping down the existing man plaster walls and ceiling (to avoid the triple leaf) and building the stud wall was the ideal method. Unfortunately I was not really able to do that (nor would I be able to add mass on the surfaces of the walls in the neighbors units. I kind of justified going thru with this way by saving floor and ceiling space in an already small ISO booth and of course time ) I've know for years that the existing space was less than an ideal but had access to a bunch of free excess building materials and wanted to take the opportunity with what I had, realizing that in the worst case I'll have a great learning experience for my first studio, and a noticibly quieter room for recording vocals, and a cooler more confortable studio expeience for my clients. That is assuming the floor doesn't go down and i get the sign off from the structural engineer.

That being said, if I go forward with this design at this time due to time and budget constraints, perhaps I am able to get permission to do that work on the outer leaf walls as ceiling At a later time.. would there be the possibility of doing that revision later by temp bracing the current ceiling, taking the existing sheet rock off the outer leaf walls, building a new stud wall underneath the ceiling and then trimming the excess outer ceiling off to finally decouple it? This would also give me the experience of getting to A B the before and after (and I suppose learn the hard way.)

My basic approach is to add mass and just make the thing as airtight as possible which I feel confident I will be able to do.

Any other construction tips or guidance you might offer if I am to go forward with my original plan? Or Do you think I will be gaining here ? Not with it? Etc? Your honest opinion is valued.

nor would I be able to add mass on the surfaces of the walls in the neighbors units.

Why not? They would never even know, apart from the sound of you hammering a bit on the wall (which you must have been doing anyway!). Why would you not be able to beef up those walls?

if I go forward with this design at this time due to time and budget constraints, perhaps I am able to get permission to do that work on the outer leaf walls as ceiling At a later time..

As long as you understand that in order to do that, you'd have to rip down the entire iso booth again. If you are OK with doing that, then fine! But if you are OK with doing that then, after it is already completed, then why are you not OK with doing it now, when you've only just started work on it? Here's a thread that might interest you, from a guy in Greece who got into a situation that is rather similar to where you are right now. It would be worth your while reading through that, to get a better grasp of the issues. Like you, the poster of that thread started out with a half-built room that he thought would be fine... http://www.johnlsayers.com/phpBB2/viewt ... =2&t=17363

would there be the possibility of doing that revision later by temp bracing the current ceiling, taking the existing sheet rock off the outer leaf walls, building a new stud wall underneath the ceiling and then trimming the excess outer ceiling off to finally decouple it?

I guess you could do that, but it would be far easier to take down the ceiling too! It's not such a big ceiling, and you could take it down in a couple of hours. No big deal.

There's another option here, depending on how much isolation you need and how bad the flanking noise is in the structure: You MIGHT be able to get away with using RSIC clips and hat channel, instead of building that new frame. RSIC clips are small decoupling devices that disconnect the hat channel from the studs behind them. They have been carefully designed to provide just the right amount of "spring" when loaded with two or three layers of drywall. It MIGHT, possibly, maybe, be an option for you to take off all the existing drywall, beef up the other side of the wall, put RSIC clips on the studs, put hat channel in the RSIC clips, the re-attach the drywall to the hat channel, with screws. Do that on all four walls and the ceiling, and you can get a decent increase in isolation. However, you would first need to determine what your real needs are for isolation, in terms of "How many decibels of isolation do you need?".

My basic approach is to add mass and just make the thing as airtight as possible which I feel confident I will be able to do.

Adding mass is good, but only if the mass is decoupled.

Here's the thing: There's a principle of physics known as "Mass Law". It tells you how much extra isolation you get from adding more mass to a single-leaf wall. Let's assume that your wall initially had a sheet of drywall on each side, and it was acting mostly like a single-leaf wall. Let's also assume that you need about 50 dB of isolation for your booth, which is typically what most home studios need.

The equation for empirical mass law goes like this:

TL = 14.5 log (M * 0.205) + 23 dB

Where: M = the surface density of the wall (mass) in kg/m2.

So lets assume that the original drywall on your walls was typical 16mm fire-rated drywall, which has a surface density of roughly 11 kg/m2. You have one layer on each side, so a total of 22 kg/m2. According to the Mass Law equation, with that setup you would be getting:

TL = 14.5 log (22 * 0.205) + 23 dB
= 32.5 dB

That is typical for a decently built house wall.

So let's say that you added another two layers of drywall on your side, and now you have a total of four layers. That must have gotten a HUGE improvement, right? That must have got AT LEAST 50 dB. right?

Do the math. It's sad:

Your total mass has increased from 22 kg/m2 to 44 kg/m2:

TL = 14.5 log (44 * 0.205) + 23 dB
= 36.8 dB

Yup, there's your answer. By doing all that work on a single leaf wall, you have increased the isolation by roughly 4 dB.

To put that in perspective, 3 dB is about the smallest change in intensity that most people can hear. So you have made just enough difference to be noticeable.

If you wanted to cut the noise level in half, you would need a change of 10 dB. That's what most people would subjectively call "half as loud".

So how much mass would you need to add to get to 50 dB?

You really don't want to see this, but I'll do it anyway:

TL = 14.5 log (360 * 0.205) + 23 dB
= 50.0 dB

Yup. You'd need a surface density of around 360 kg/m2. That's roughly 32 layers of drywall..... Or roughly 15 cm of solid concrete. So if you really do want to have just a single leaf wall, then I'd suggest that a 6" concrete wall would be the best way to do it...

On the other hand, by building a proper fully-decoupled 2-leaf MSM wall, you are no longer subject to Mass Law. The equations are rather more complex, but it turns out that you can get about 50 dB of isolation from a wall that uses exactly what you have already: a total of four layers of drywall. Two on one side, and two on the other side, with an air gap between, filled with insulation.

The only difference between Mass Law and MSM, is that the two sides MUST be fully decoupled.

It's the decoupling that does the trick. Without that, any vibration that hits the drywall on one side of the wall is transmitted straight through the studs to the drywall on the other side, which then acts as a speaker cone, and transmits the vibration into the room. If you put that second drywall on a DIFFERENT stud frame that is NOT connected to the first one at all, then there is much, much less transmission, since the only thing connecting the two sides is air. And air is rather "springy"...

So that's the situation: All your work will likely increase the isolation by around 4 or 5 dB, which is only just noticeable to most people. It will be a bit quieter inside, yes, but not very much.

Any other construction tips or guidance you might offer if I am to go forward with my original plan? Or Do you think I will be gaining here ? Not with it? Etc? Your honest opinion is valued.

Your original plan will work, provided that you don't mind adding another 28 layers of drywall... (The room might be a bit small inside by then, though! )

Mass Law is not your friend. For a single-leaf wall (or fully coupled two-leaf wall) you need huge amounts of mass to get decent isolation, because Mass Law is a real b**ch. You can get the same isolation with much, MUCH less mass if you switch to fully-decoupled 2-leaf MSM.

That's why studios are almost always built with 2-leaf walls these days: It's the cheapest, most effective way to get good isolation.

I'd suggest that you read through the thread I linked you to above (its long!) to see what happened to a guy who found himself in your situation, and what he did to resolve it. Your situation is not really as serious as his, but the principles are still the same...

- Stuart -

Thanks Stuart. Looking at the numbers you gave me as far as what I would be gaining in terms of DBs of isolation was helpful, though painful.

I think you have convinced me to de couple. I have a few more questions.

I guess you could do that, but it would be far easier to take down the ceiling too! It's not such a big ceiling, and you could take it down in a couple of hours. No big deal.

I have two layers of drywall with green glue on the top side of that ceiling frame. I should be able to unscrew the double rock layer, the studs and re-use these materials , right?

put the hat channel in the RSIC clips, the re-attach the drywall to the hat channel, with screws. Do that on all four walls and the ceiling, and you can get a decent increase in isolation.

Remember only two of the walls are actually "coupled" to the existing walls....the other two are built off of an acoustical rubber flooring, Am I able to keep these 2 walls ? Do I have to tear these down as well?

I am still confused about what to do with the ceiling. If I DO go with the clip system, Do I need to rip the sheet rock and plaster of off the existing apartment ceiling too, in order to attach the clips, and then build the wall up to the new (higher ceiling)?

And, if I dont use the clips, but build new stud walls, on top of which I would attach the ceiling, and I maintain that 6" gap between the Iso booth ceiling and the apartment ceiling, am I going to be ok with this quadruple leaf ceiling system?

However, you would first need to determine what your real needs are for isolation, in terms of "How many decibels of isolation do you need?"

I am looking for 45 to 50 Db of isolation inside the Iso booth. How can I calculate if the clip system will work?

On the other hand, by building a proper fully-decoupled 2-leaf MSM wall

Random question but what does MSM stand for?

I'd suggest that you read through the thread I linked you to above (its long!) to see what happened to a guy who found himself in your situation, and what he did to resolve it. Your situation is not really as serious as his, but the principles are still the same...

I will definitely do that.

Thank you Stuart!

*Update -- i decided to begin breaking into this "coupled wall" in order to demo and begin the decoupling process when I found something interesting but terrifying -- the wall appears to be an inch of plaster directly over BRICK. HOLLOW BRICK (see photos) The reason this terrifies me is because isnt a hollow brick in itself a kind of 2 leaf system, having mass on both sides, with air cavities built into the brick cells? Please tell me this is not the case as by the time the 3rd decoupled leaf is installed we automatically have 3 leafs, right? The wall adjacent ( the other one I will be demoing) is plaster, lather stripping, and wood studs, which Is what I thought this wall was....

I have two layers of drywall with green glue on the top side of that ceiling frame. I should be able to unscrew the double rock layer, the studs and re-use these materials , right?

Right! And with a bit of luck, you might be able to get the whole thing down in once piece. It will be rather heavy, though, so take care, and have a helper or two around.

the other two are built off of an acoustical rubber flooring,

What brand? How thick? Are you sure that your wall load is within the specs for that rubber, to make sure that it floats? And are you certain that those walls do not touch any other walls?

This is what you are looking to do:
ALL of your inner leaf walls must be fully decoupled from EVERYTHING else. They can only be attached to each other, but nothing else.

I am still confused about what to do with the ceiling. If I DO go with the clip system, Do I need to rip the sheet rock and plaster of off the existing apartment ceiling too, in order to attach the clips, and then build the wall up to the new (higher ceiling)?

Clips can only be attached to joists (ceilings) or studs (walls), so if you wanted to go with clips, you'd have to take off everything until you reach joists. But you'd only need to do that if you also use clips in your walls, and it seems you wont be able to do that. So your best bet is to build your inner-leaf walls as stud frames, and put joists across the top of that for your inner-leaf ceiling. Just like the image above (it does not show the ceiling, though).

And, if I dont use the clips, but build new stud walls, on top of which I would attach the ceiling, and I maintain that 6" gap between the Iso booth ceiling and the apartment ceiling, am I going to be ok with this quadruple leaf ceiling system?

If you have decently large air gaps, and very high mass on at least a couple of the "leaves", and completely fill the cavities with suitable insulation, then you should be OK.

I am looking for 45 to 50 Db of isolation inside the Iso booth. How can I calculate if the clip system will work?

Clips might get you 45, but I doubt that they'd get you 50. And with your "brick" problem, you won't be able to use clips anyway: they can only be used on studs or joists.

Random question but what does MSM stand for?

Mass-Spring-Mass. Also sometimes called MAM for Mass-Air-Mass. It's the principle of physics that makes the system work. Your outer-leaf is one "mass", your inner-leaf is the other "mass", and the air between them is the "Spring". It's a resonant system, very much like a heavy object bouncing up and down on a spring:

I found something interesting but terrifying -- the wall appears to be an inch of plaster directly over BRICK. HOLLOW BRICK

Well that's unexpected! I was assuming you were talking about stud-framed party wall!

An inch of plaster over hollow brick is actually a mixed blessing. Good, because there's LOTS of mass in there! Bad, because of the "hollow" part.

So it seems that you have few choices left here...

Please tell me this is not the case as by the time the 3rd decoupled leaf is installed we automatically have 3 leafs, right?

I ain't gonna lie to you, so I wont say that... It is, in fact, going to be a 3-leaf system, and you have no choice there. However, it's not as bad as it sounds: It will be 3-leaf, yes, but brick is massive: nearly four times the density of drywall. So one inch of brick is roughly equivalent to 4 inches of drywall, which is equivalent to about 6 layers of drywall. You also have the inch of plaster, which is equivalent to another 5 layers of drywall. So you have a LOT of mass in there, and that's a good thing!

Also, for a 3-leaf system, you get maximum efficiency when the middle leaf has the most mass, which is true in your case.

So I'd suggest that you should do your best to seal up that wall again: fill and restore the holes as well as possible with mortar, then re-plaster the bits you broke, and you could even make the plaster thicker if you wanted: more mass. Just make sure that the wall is sealed very well. Air-tight seals are important for isolation.

the other one I will be demoing) is plaster, lather stripping, and wood studs,

OK, that's fine. That one is also easy to deal with. Once you get down to the studs, beef up the other side of that from INSIDE the wall, between the studs, by cutting strips of drywall to fit in each stud bay. Cut it a fraction smaller than the bay, so you can fit it in easily, then seal that gap and all around the edges with good quality caulk that never hardens, and hold the drywall in place with cleats nailed sideways into the studs. Do not nail the drywall! Just put cleats in there to hold it.

If you want high isolation, and if your underlying floor structure can handle it safely, then you could út two layers of drywall in like that.

Once you have "beefed up" that other side, and made sure that it is all sealed fully air-tight, then install suitable insulation in the remainder of the stud bays.

Next: build the other two sides of your OUTER leaf, so you have a complete shell that encloses the space where you'll be building the inner-leaf. See that diagram above once again-

NOW you can build your inner-leaf: stud framing bolted/screwed to the subfloor, a small distance away from the outer leaf wall, and drywall on only ONE side of that frame, with the cavity being filled with suitable insulation (either fiberglass or mineral wool).

And your new inner-leaf ceiling goes on top of the new inner-leaf walls. The entire inner-leaf room stands all by itself, all alone, not touching anything else.

Instead of doing two layers of drywall on the inner-leaf, I would do one layer of 16mm OSB on the studs, then a second layer of 16mm drywall over the OSB. That gives you much better sheer strength in the walls and ceiling, from the OBS, and also gives you nailing surfaces all around the entire room.

So that's what I would do, if I were in your situation.

The news is not all bad! Bricks are good, actually. It's a pity they are hollow, but I'm assuming that there's still a good thickness in the brick wall.


- Stuart -