Building half inside out ceiling for room in room

I'm building a room within a room using a 2x4 wooden framework with 2x6 for the ceiling joists, like a lot of people, I'm having trouble keeping enough ceiling height, the 2x6 joists will be placed between the existing ones, I was thinking of putting my 2 layers of plasterboard in between the joist half way up attached to some 1x2 on the side of the joists, then filling the rest of the cavity with RW3 rockwool, but I'm worried that having the joists exposed on the inside and not covered by plasterboard will affect the sound isolation. This design will give me a built in sound absorber and keep the ceiling at around 7' 3" which isn't too bad. The other option is to put the plasterboard underneath the joists and add absoption after, this will probably lower the ceiling to around 7' any thoughts?

The alternative option, which will give a 7 foot ceiling

Have you considered building the ceiling completely inside out? - Stuart -

Cheers for the reply Stuart, It's not something I'd thought of as it'd bring the ceiling fairly low, if I did this, how much gap would be required between the old joists and the inside out ceiling's plasterboard to avoid too much resonance? if it could be 1 inch I'd consider it a possibility as it would make a great absorber and I'm thinking of making the walls fully inside out too, they would also have between 1 and 4 inches insulation between the outer walls and plasterboard of the inner room. Pete

if I did this, how much gap would be required between the old joists and the inside out ceiling's plasterboard to avoid too much resonance?

The gap there doesn't really matter: as long as there's enough clearance to prevent mechanical contact, that's fine. The resonance occurs mostly between the leaves, very little happens between a leaf and the joist in front of it. As long as you can get some insulation in there (but without jamming it in!) then that's all you need. Maybe an inch or so is fine. I suggested the inside out more for the sake of simplifying construction than for acoustics: either of your first two methods will work, but having the drywall all on top or all below the joists makes it easier to build, and easier to seal. If you don't mind the extra work, then probably your first idea is the best overall. On those joists: do they have to be 2x6? Any chance you can use a tougher type of wood and maybe bring them down to 2x4? Just a suggestion. By the way, please check the forum rules for posting: there's something that you seem to be missing.... :) - Stuart -

Thanks again for your comments Stuart, I've read many of your posts and highly value your input. I'm spanning 3 meters with the joists, I could rip the beams down to 5 inches or use 2x4 if you think it'll span it, the original joists above are actually 5inch with a 1 inch taper on top of them, not sure of the wood type. My new joists are C16 rated which meets UK build regs quality but fairly weak I think I should point out I've already built the framework and bought the 6 inch joists Building inside out will be the trickiest way for me, the first plan I had is probably the easiest for me to build so I'll probably go with that, it may be time to put it off and have a think for a few more days whilst I'm at work I have started a thread for my build, this was more of thread for a quick answer, I need to fill in my build thread and get it up to date at some point, I'll read the rules again and see what I've missed Pete