So, as my inner leaf consists of a layer of osb on the warm side of the cavity insulation I may find I do not need to use a separate vapour barrier. If I find out through building regs that I do need a separate one then I will just install it over the osb before putting up my outer leaf.
That would, indeed, be the correct way to proceed, but it would not go over the OSB; It would go over the drywall. If you do use it, then it goes in the wall CAVITY. Not between layers of the leaf. If you put it over the OSB then it would be between the OSB and the drywall, so it would not be protecting the drywall the way it is supposed to. It would have to go on the cavity-facing face of the drywall.
But that's not what you are showing on your diagram on the previous page of the thread! You are showing an inside-out wall design, with something labeled "vapor barrier"
inside the room, (NOT inside the wall), as the final layer, on top of your acoustic treatment. That would not be a vapor barrier, and would in fact be an acoustic device: as Greg mentioned, it would have an effect on the frequency response of whatever treatment is behind it, and it would also change the sound of the room. But it isn't a vapor barrier, and would not work as a vapor barrier. That's the point I was trying to get across, but I guess I didn't explain it very well...
If you were to install the type of plastic that is typically used for vapor barrier, all around the room, over your treatment (or just over the insulation in the stud bays), that would at as an acoustic foil, reflecting back some frequencies while allowing others through, and following the equation for such foils. There is a reflection curve for foils (plastic is a foil too, acoustically) that defines the point where foil of any given surface density will be 20% reflective (80% transparent) to sound (allowing 80% of that frequency through to the other side, reflecting back 20%, or coefficient of reflection = 0.2). The curve rises to about 99% reflective above that (1% transparent), at about 6 dB/octave, and falls off to practically 0% reflective (99% transparent) below that, at the same rate:
F = 90 / m
F = The frequency at which the foil transmits 80% of the sound
m = The surface Mass of the foil in kg/m2
So that would be the effect of putting plastic around your room, over the insulation in the stud bays: acoustic foil. It might also trap moisture in the air between the foil and the leaf itself, and depending on whether or not the rest of the wall was constructed with vapor retardation in mind, that trapped moisture might have no means of escape, possibly leading to the growth of fungus or mold.
There's also the issue that if you stretched the plastic tight across the studs, it would no longer act as just a foil, but also as a resonant membrane, tuned to a certain frequency, or set of frequencies: that's probably not a good thing! So if you do go down this path, then don't stretch it tight!
But all of this begs the question: Why do you want it in that location anyway?
- Stuart -