I'm not sure quite how, but you seem to be confusing your isolation with your treatment. I can't quite put my thumb on what it is exactly, but that seems to be the issue.
Just to clarify:
1) The vapor battier goes INSIDE the
wall cavity, between the inner and outer leaf, and as Brien has explained, it goes on the warmer side of that cavity. So in climates that are mostly hot year-round, it would probably go up against the outer leaf, whereas in climates that are mostly cold year-round, it would go up against the inner-leaf, but in both cases it still goes
inside the MSM wall cavity, between the inner-leaf and outer-leaf. So it will not be visible from inside the studio, once you have the walls up and are ready to start treating.
2) Isolation and treatment are two different things: isolation is the two-leaf wall around your room, with insulation in the gap, and the vapor barrier too, if you have one. Treatment comes afterwards: once the isolation shell is completely finished, THEN you install the acoustic treatment in the room, and that treatment might include slot walls, absorption, diffusion, etc.
3) Slot walls are treatment, not isolation. If you build your walls inside-out, then the drywall is on the "far" side of the studs (facing away from the room) which makes it easy to use the "near" side of the studs to attach the slats, leaving the stud bays as the cavity for the slot resonator, (do not confuse this with the cavity inside the wall! They are two different, unrelated cavities).
4) There cannot be any barrier right behind the slots. If you put plastic there, it will block the movement of air and the resonators will not work. And if you DID put plastic there, it would not be a vapor barrier anyway! It would be in totally the wrong place: vapor barriers go inside the isolation cavity, not inside the acoustic treatment.
The inner leaf is built inside-out, and my plan was to do my treatment (roxul) and then cover it with plastic for my vapor barrier, and then cloth and slats.
That would not be a vapor barrier. The vapor barrier goes on the OTHER side of the drywall, facing the MSM cavity, not the slot-resonator cavity.
But...unless I'm misunderstanding Brien's (Xspace's) multiple posts on this subject, the vapor barrier needs to be the inner-most layer of the construction, at least in my part of the world (Seattle).
Right. It needs to be the inner-most layer OF THE WALL CAVITY, just prior to the inner-leaf drywall.
If the vapor barrier is not the inner-most layer, the moisture from inside the room would be absorbed by my Roxul instead of hitting the vapor barrier first.
Correct. That's why you put that BETWEEN your drywall and the rockwool: to prevent the vapor that migrates through the drywall from getting to the insulation, where it could condense on the cold outer-leaf. There won't be any such condensation taking place inside the ROOM, if you have your insulation, vapor barrier and HVAC set up correctly, since all the air in the room, and the room surfaces, should be at similar temperatures and with good air-flow and de-humidification.
My plan with my inside-out walls was to cover the insulation with the vapor barrier,
Yes, but that is the insulation INSIDE THE MSM CAVITY, between the two leaves. That is NOT the insulation inside the slot-wall cavity, which is inside the room, not inside the wall.
Or perhaps I'm off-base on the placement of the vapor barrier(?)
Yup!
My main area of concern is the control room, where I'm planning to have Helmholtz resonators along both sides:
To me it looks like there is almost no air gap at all in the right hand control room wall (MSM isolation wall), to the live room. Both sides of that wall seem to be built inside-out, with just a tiny air gap (if any) between the actual drywall leaves, and no insulation in that tiny gap. That will not work very well for isolation. You should have at least 4" of air gap between the leaves in an MSM wall, and it must be filled with insulation. I'd suggest you should move the live room wall to the right a few inches, and add insulation between them.
Also, since the left CR wall and the right CR wall have very different cavity depths in the slot walls, they will be tuned very differently, so the room will not respond the same on the left and the right. To fix that, you could build that wall inside-out as well, with drywall directly on the other side of those slot-wall studs. That would save you money, too, as you would not need a separate isolation wall on that side of the room.
- Stuart -
Glad it is working! It's always nice to get direct feedback like this, showing that things actually do work in practice the way that theory predicts. Of course, building it properly, like you did, is a big part of why it works so well, too. Nice job!