Warm Roof Poor Acoustic Performance

Hi,

I've got a Warm Roof 100m2.

8mm Slate
Batten
Breathable Mem
Batten (holding PIR down)
60mm PIR
145mm Joists 60mm PIR between, held in with 50x25mm batten
Vapour Barrier
Plasterboard
Skim

Due to reg's, I am not allowed to put any additional insulation material between the joists. In hindsight, I should not have used 60mm PIR between the joists, but 140mm RockWool or similar. However, I'm not about to remove essentially 30 sheets of PIR from between the joists.

So, my question is... How would you improve the acoustic performance of this pitched roof ?

I was looking at either

2 x Sheets of Plasterboard

OR

Some of this product after the vapour barrier.

http://www.customaudiodesigns.co.uk/tecsound-acoustic-membrane.htm

I've used it before. It's as heavy as hell, but really deadens airborne sounds - I've used it to build bass traps.

Many Thanks.

ps. I've asked BC if I can squeeze like 50mm rockwool into the remaining 85mm gap... Not heard back yet.

Picture of underside of roof prior to PIR being installed between rafters.

It seems you already know the various options. If you try to "squeeze in" some rockwool, consider a heavier variant i.e. acoustic mineral wool or maybe wood fibre batts or board. Regarding which will be most effective, it may depend on what sort of noise you're trying to limit.

Thanks. I'd go for the heaviest density kg/m2 I can get (within limits as you can get some 90kg stuff). I'll wait and see if BC will allow a deviation. I'll keep you informed.

Double boarding with staggered layers of dbcheck board will have some beneficial effect - more if fixed to resilient bar. But check the loading on that. In acoustic panels the thickness of a mineral layer is less important than the presence of it and filling a void with mineral can reduce sound attenuation rather than improve it over, say, a 25 or 50mm layer. Optimum panels are always a combination of high and low density material and voids. My advice is a 25mm layer of dense acoustic mineral. If you can suspend that within the void all the better.