Warm roof, insulation, vapour barrier

Hi,
I have a warm flat roof (with a vapour barrier and insulation above that) on my new extension. I am just getting round to plasterboard the room and I am wondering what to do about adding additional insulation underneath the joists. I understand that normally it isn't a good idea to insulate here because moisture may condense and potentially cause the roof joists to rot, but I really want to have some sound insulation in here because my extension is for music and it is very bad acoustically to have an undamped resonant chamber at the top of the room. With this in mind I am wondering whether I can have a secondary vapour barrier (say some heavy duty polythene stapled underneath the joists directly above the plasterboard I am about to put up) and some additional rock wool insulation underneath the joists or if that is a really bad idea?

 

Thanks for your reply. I think it was 140mm of celotex on the top.
But it can't do any harm to have an additional vapour layer underneath the joists can it?
50mm of mineral should be enough to damp some resonances, the joists are 8 inch.

 

Thanks, it sounds like I don't want to cause any interstitial condensation so I will give up the idea of a 2nd vapour barrier. But will 50mm of mineral wool cause significant condensation?

 

Got it, thanks.