Effectively decoupled or not?

Hullo Everyone, great forum! Great books too, thanks for the recommendations. I run a music store and school in Almonte, Ontario - just outside the greater Ottawa area. We've expanded and I'm taking this opportunity to relocate our drum instructor's room, isolate him from the rest of the school and give him some more elbow room. :shot: The building was built a year and a half ago, and let me tell you. It's awfully painful watching contractors who say they've done it a thousand times install RC upside down! And that ain't all, either, but we'll move on... We're graduating from less than 6' x 8' to a little over 11' x 3', so space is still a concern, and I have an idea that might buy me another 6" that I think will count. I propose to build a room within a room - sort of... The south wall is exterior and not an issue. The west wall is a staggered stud 2 x 8 common top and bottom plate with a bathroom for a neighbour - not a problem. The north and east walls divide the room from our piano showroom, and our accountants office, so we don't want him counting triplets when he should be counting profits :D . I'll reinforce these with another layer of drywall (from the inside using GG), caulk, and then pink. So. What if I build a double wall on the north and east sides, and where these meet the east and south, the only thing connecting them is two layers of drywall - the south and west with RC. I'll stop the (new interior wall) studs short by 1/2", and I'll tear the existing drywall off the south and westerly, and install RC (correctly!). The ceiling joists will float on the two new walls and a 4 x 4 in the other corner... please see the drawings with my sketchy Sketchup skills... (I forgot the pink and the doors in the drawings, but I won't when I build it!) I realize I'll only gain about 6" by not building four interior walls, but that's half a whole foot, and we need to get as much as we can. Plus a double wall on the south would need another door (not drawn)... Do you think this plan, done with great care, will effectively destroy flanking paths?

Whoops. 11' x 3'. That's pretty small. I meant 11' x 7'. I should also clarify the drywall on the interior of the existing north and east walls will be removed and look like this...

I guess you are going to want acoustic treatment in there too, right? so have you considered building those walls "inside out"? It might gain you another couple of inches, and this forum IS the home of the inside-out wall! - Stuart -

Acoustic treatment is in the plan for sure yes, but space and isolation are my primary concerns, in that order. Can you point me to a post that outlines what you mean by "inside out"? I tried the search engine but the engine replies "too general". Much obliged.

viewtopic.php?f=2&t=9797&start=30 Googled it, pretty good picture about 2/3's of the way down. Basically, it's as described. You "flip" a standard wall around so the drywall is not inside the room, but outside. So, from inside out you have: Fabric covering studs/insulation 5/8" Drywall Possibly green glue 5/8" drywall Hope this helps, Andy

After a bit of thought, I think I know what you mean. That the existing wall should be my interior wall, and I should build the new wall as my exterior... I thought of that - it does gain space, yes. But first, that'll really make a mess of my beautiful (and operational) piano showroom. But more importantly, that wall is already locked to the south and west walls, so existing flanking paths negate the room within a room effect. No?

I'm going to make some assumptions based on my interpretation of the drawings you attached. Please correct me if I'm wrong. Hopefully I'll answer your questions at the same time, and be correct with the information. Again, this is just my understanding of the inside out wall. 1) The new wall you are building will be the interior wall, not the existing. Take a look at the close up detail pic you posted- "colourcorner.png." Now, assuming that the left side wall is the new interior wall.... Working left to right, you would have fabric, studs/drywall/drywall/air space/studs/drywall/drywall. Insulation between the studs in both cases. 2) This type of construction would turn your inner leaf into a large bass trap (effectively). Look at Lou's studio build where they actually start the construction of the control room. His is built using the inside-out method with fabric, and he didn't have to treat most of the control room with bass trapping etc. I'll do you one better- here's the link. viewtopic.php?f=10&t=9156&start=270 If you read from that point on you'll see basically step by step what it takes to build an inside out wall. So, you wouldn't be touching the existing structure really. However, you should do what you have to in order to remove what flanking paths may exist (if any). My 2 cents, Andy

"Inside out" construction just means that on the inner leaf of your MSM wall, instead of putting the sheetrock on the side of the studs that faces the room, you put the sheetrock on the other side of the studs, the side that directly faces the outer leaf. The advantage of this is that you now have the stud space facing your room, and you can use that space for your acoustic treatment. It gains you over three inches of space on each side of the room, plus extra headroom in the ceiling, since you can now put a lot of your treatment in the gap between the joists. The down side to "inside out" construction is that you do lose a little isolation, since your air gap in the MSM system is smaller than if you build the wall normally. But of course you could nudge your wall an extra inch inwards, so that you gain an inch of air gap, and lose an inch of room space. Or you could put an extra layer of sheetrock in there, so that your inner leaf is three layers instead of just two. Or both. It's a balancing act! Add layers or nudge walls, or whatever else you need to do, to get to the isolation level you need. So if space is really tight in your room, an inside-out wall might be the answer. - Stuart -

Thanks Stuart. I guess I should clarify: space is my primary concern after I've built the north and east walls, a double wall construction I plan to do like so: gypsum / GG / gypsum / studs & pink / airspace / studs & pink / gypsum / GG / gypsum The south and west walls are exterior or thick and unimportant for transfer, so in order to achieve a true room within a room without building double walls as above all round, is to disconnect the S & W with RC. Take a look at this new drawing: So you see, the inside room is sealed (tight as a drum) :? , and all flanking paths eliminated. From what I've learned (and much of what I've forgotten), I think this is an excellent compromise for maximum isolation with minimum spatial loss, and really bad acoustics. Whaddyer think?

The thing is, my goal is soundproofing over acoustics. We're running a school here, not a studio. The ceiling of this room will be a great help toward controlling rambunctious reflections, and I will treat the corners some, possibly put an area rug or two on the walls, but I do recognize it's still a box. But the question still remains - though allow me to rephrase it: Is this plan defeating the purpose of the double wall, or is it a good plan? I had hoped to proceed today...

Looking at the left side of your sketch, it seems to me that you have your inner leaf coupled to your outer leaf through the RC! That doesn't seem like a good idea. - Stuart -

It is an odd place but looks like it is right. Doesn't the RC decouple?

Exactly my thinking. Doesn't the RC decouple...

My point is, if you are only going to have RC on the one wall, then why go to the trouble of building a full separate stud wall for the other wall? The weakest link there is the RC wall, so that's the level of isolation you are going to get for the entire structure. So it seems pointless to go to the expense and trouble of building a double-stud wall for the other side. Why not just go with RC all around, if that level of isolation is all you need? Or, on the other hand, if you really do need the better isolation that you get from a full double-stud wall, then why not do a double-stud for that wall at the bottom of the diagram too, where the RC is at present? Even an inside-out double stud wall there would be better than doing it with RC. You would have the exact same air gap, and zero connection through the RC, so it would HAVE to give you better isolation. I just don't see the point of having a section of wall with good isolation that is connected to a section with not-so-good isolation. - Stuart -

So RC, you're saying, then is a connection of a sort. Yes? Makes some sense... However, let's just take one more look at this. I will build a double wall all round if it is the final recommendation of experience on this here excellent forum. It's just gonna get tight in there, but it'll still be a step up in size from the room we're in now. And there's more, but I'll get to that in a minute. Take a look at this drawing. Bleed through the south wall, is represented in green (green means go) - sound transfer through this wall is unimportant - goes outside. The red arrow is where transfers gotta stop, thereby the red light. :cop: In my mind, the sounds gotta travel through exactly the same amount of msm no matter what path it takes - to get to where we don't want it to. Now given that you've elucidated that RC is indeed a mechanical connection and not a 100% decoupler (am I right in saying that??) that might not be totally accurate (?). Which if so, answers my question, "effectively decoupled or not". If this is so, would you agree then that my approach would at least be 50% better than single wall construction with RC, and say, 75% (area) efficient of full double wall? I'm not trying to :horse: I just think that this is a good compromise (?). Building a double wall also means I've got to build a double door (I didn't illustrate it, but there's a door in the south wall), and that's adding up to a lot of effort... not that I'm not up for it... Anyways, sorry if this seems confusing, which it really isn't. I do appreciate your input!

"Now given that you've elucidated that RC is indeed a mechanical connection and not a 100% decoupler (am I right in saying that??) that might not be totally accurate (?). Which if so, answers my question, "effectively decoupled or not"." The difference is the air depth of the two walls and that the RC is part of a wall. It isn't RC specifically, it is the way you are mixing and matching the two. The fully disconnected wall-air-wall is going to produce better. An RC wall has a reduced ability due to this connection and the reduced air depth. Will it work? Yes it will work. Is it the weakest link in your wall? I don't know, not having seen the rest of it, but it is the front runner. But if it is >all< you can do, then it is what it is.

I will build a double wall all round if it is the final recommendation of experience on this here excellent forum. It's just gonna get tight in there....

Not if you build an "inside-out" wall! I just noticed your previous comment on that:

rhythmrhymer wrote:

After a bit of thought, I think I know what you mean. That the existing wall should be my interior wall, and I should build the new wall as my exterior... I thought of that - it does gain space, yes. But first, that'll really make a mess of my beautiful (and operational) piano showroom. But more importantly, that wall is already locked to the south and west walls, so existing flanking paths negate the room within a room effect. No?

It looks like I confused you with the issue of "inside out walls", so I just prepared a SketchUp file showing what the term means, and why it would be useful for you. Look at the images below, and it should be clear. So, what you gain by doing that is that the space between the studs is now exposed to the inside of the room, and you can use that space for your acoustic treatment, such as filling it with rock wool and covering it with acoustic cloth, or building broadband slot resonators on the front of it, or packing it even deeper with rock wool for bass traps, etc. If you build the wall normally, then you STILL have to add the exact same acoustic treatment anyway, except that it now starts about 13 cm further into the room. So in effect you GAIN about 13 cm on each wall, which is roughly 5 inches. If you do that on walls on opposite sides of the room, effectively your room is now ten inches BIGGER than it would have been. You have an extra ten inches of room width that you would NOT have had if you build the wall normally (drywall on the "room" side of the studs). So a room that would have been, for example, 7'4 is wide is now 8'2 wide. Magic! What you LOSE, or course, is 5 inches of depth in your air cavity, which is the "spring" part of your MSM equation. So you lose some isolation. But you would lose even MORE isolation if you were to just put RC on the outer studs, put the drywall on that, and forget about the second stud frame. Now, getting back to your drawing: You did not show any drywall on the "outside" studs, but I'm sure there must be some, right? It's just that you forgot to put it in the diagram? It would be good if you could take the time to build a proper SketchUp model of your room(s) and surrounding areas, so that folks here don't wast time assuming things that just aren't so. It would also make it easier for the experts here to suggest changes to your design that could help you. You still need two doors in your wall, by the way, regardless of how you build it: if you only plan to have one door in your wall, then it seems rather pointless to bother building an MSM wall in the first place, since your door will be your weakest link. Unless, of course, you plan to build one of Rod's "Superdoors"! That will work.

If this is so, would you agree then that my approach would at least be 50% better than single wall construction with RC,

Putting that in perspective, if it really is 50% better in terms of total power reduction, then that means it would be just 3 dB better.... hardly worth worrying about! :) It would have to be maybe 10 dB better before I'd think it was worthwhile, and 10 dB is an awful lot more than 50%, if we are talking power! :) (Yeah, OK, so that's nit picking, I guess, but the numbers don't lie!) - Stuart -

Using your own diagram, I just modified it a bit to show what you need to do, to maximize isolation WITHOUT changing the position of the drywall. For both walls, the drywall stays in the exact same location that you show it in your diagram, but I took the liberty of changing the structure, making it into an inside-out wall, to show how you would build that to get the best possible isolation without changing the room size. While I was doing that, I noticed that you have a line on the lower wall, right behind the RC, labeled "ND Drywall". Is that REALLY a sheet of drywall there? If so, then not only is it pointless to put RC right on top of an existing layer like that (it won't work), but you also have a three-leaf wall, which means that your isolation in the low end of the spectrum would be even worse than for even a standard single-stud wall WITHOUT RC. It would really help if you would build an accurate and detailed SketchUp model of your rooms, to show exactly what you have. - Stuart -

Awesome. Thanks xSpace and Soundman, I'm (slowly) getting the hang of this... To clarify one or two things before I nod off, no, there is drywall there now, but we're gonna rip the drywall off that exterior wall first thing tomorrow. That was definitely the idea before installing RC - which we likely won't do; I'm gonna have to process this new info in the morning with fresh caffeine. My Sketchup skills aren't up to par with you cats. It reads "no drywall", not "nd drywall". I can't seem to tell Sketchup how to make text bigger and that's been bugging me. :roll: (maybe I oughtta bite the bullet and buy the pro version)... And I did sort of half misunderstand the inside out thing for sure, but I've got it now. Lemme review this business in the morning, and thanks for the great advice so far. GT

there is drywall there now, but we're gonna rip the drywall off that exterior wall first thing tomorrow.

I hope you see this post before you start ripping it off! If you take it off CAREFULLY, then you can save money by re-using it to beef up the inside of your outer leaf. Use some kind of drywall saw, and saw down the edges of the studs, so that you end up with pieces of drywall that are the exact right size to fit in BETWEEN your studs.... That way, you won't have to buy a whole bunch of new drywall, or pay someone to cart away stacks of drywall that could have been re-used, instead of thrown away. - Stuart -

Be mindful of electrical wire, plumbing and things that can be behind the wall sheathing. Best to treat it like it is there, rather then it may not be there. Cut shallow, not deep.

Uh oh ... why didn't I think of that? Wait - I did!! :D Observe! Nice clean cuts, that'll fit like a glove. Hell of a lot less mess, and I'll be counting a couple of brownbacks in savings.

Heck, even the wires are intact... I've decided also to bite the space bullet and build it right. We're going to run with a double wall construct all round. That back door's bugging me though... bit of a chore but oh well. I'll do my best to get some better drawings posted soon. Meantime, I'm not in possession of any Green Glue just yet, but I've got lots of acoustical sealant. Is it necessary to secure the two layers with anything more than caulking and a few cleats? (I'll use nails driven in sideways). GT

rhythmrhymer wrote:

I've decided also to bite the space bullet and build it right. We're going to run with a double wall construct all round.

Smart move! I think you won't be sorry about the results! (You might be sorry about the blood, sweat, tears and money, but not about the final outcome.)

That back door's bugging me though... bit of a chore but oh well.

It shouldn't be too bad: Two nice thick solid-core doors, back to back, swinging in opposite directions on hefty hinges, with really good seals all the way around each door, and no handles piercing the wood.... Piece of cake! :)

Meantime, I'm not in possession of any Green Glue just yet, but I've got lots of acoustical sealant. Is it necessary to secure the two layers with anything more than caulking and a few cleats? (I'll use nails driven in sideways).

I don't think you need Green Glue for "beefing up" the outer leaf. Cleats and generous caulking around the edges should be plenty good enough. I'd save the GG for the inner leaf, where it will be more effective. I think I'd go with something a bit more than just nails for your cleats. Maybe some wooden strips or something. You don't want anything coming loose and spoiling your isolation a couple of years down the line. Or even worse, rattling and vibrating inside your wall... Don't forget that you need your studs for the inner leaf on 24" centers, not 16". It will drive down the resonant frequency a bit, which is where you want it: as low as possible. IIRC the GG tests were done on 24" centers. The outer leaf already is what it is, but 24" OC for the inner will help. Glad to see that you rescued your old drywall, too! You'll have more money to spend on GG now! :) - Stuart -

All righty... right after dropping a couple of queries here, I'm going to pick up the Sketchup ball and get at least closer to delivering some real plans, which I promise are coming... One of my questions - actually they're all kind of bound together like layers of drywall stuck together with GG... is ... Q1? We took full care to cut the inner layer of drywall (existing wall, to become outer leaf) flush with the studs so that the chunks would simply slip into place to reinforce the outer leaf. Wow. The friggin' studs are so warped and twisted, very few of them fit! What a load of work. Here's the scary part. Once we've fought to get the best fit, some of the gaps are too wide for caulking. It's fairly simple (and much faster) to mud these wider spaces and caulk the rest - but. I've seen mud crack where there's no tape. I'll provide a photo to illustrate, but is this a big no-no? It certainly seems too easy to be true. Q2? [ a very 'grey area' question ] For new walls (the inner leaf) - wood studs vs. steel studs. Rod prefers to work with wood (I second that) but concedes that light guage (Canadian spelling) studs deliver slightly better results... In Canada, our wood studs are stored outside and subject to rather severe weather changes. I've seen a perfect wood stud transform itself into a banana literally overnight after bringing it indoors. Add to that being rather badly allergic to dust makes me favour (Cdn. spelling again) steel studs - cleanup is also such a breeze. However, the ceilings got to fly, so... does anyone know where there's a thread where there are some stats posted as to what or how to calc the load bearing ability of a given wall? In laymen's terms??? I don't have an engineer in my back pocket, and am a bit afraid of their invoice book for all of 5 minutes of "work". I do promise to get some fairly complete Sketchup drawings up, but I'm awful slow at that. Now, Stu mentioned going for 24" centers to lower the walls fundamental (excellent advice) - but doesn't that also lower it's ability to carry a load? This particular room only needs to float just over 11' of gypsum / GG / gypsum. I had hoped and thought to run with steel studs, and 6" wooden ceiling joists. I'm gonna post this, and go grab my camera for some photos, and jump on the curveball of Sketchup learning. :D