Insulation and acoustics in flat roofs

[NickB]

We are finishing the design work on our timber frame and finalising insulation and i wanted to get thoughts on soundproofing. All our walls are full filled with 140mm of rockwool with a pir liner. Wit the roof, we wanted to do a warm deck roof, with hardrock multifix and PIR combined to reach our .11 target. The problem is Rockwool and Kingspan are not comfortable with a hybrid system, and we don't have enough room for rockwool (355mm). We have about 210 mm to play with. Kingspan did the calcs and it all looks ok but then they emailed with concerns with regards to fire and condensation etc.

 

We are near a road that can be noisy at times, and also concerned about rain noise etc. Does anyone have experience with 210mm of PIR and noise levels on a flat roof. I should mention, we have no plasterboard as the joists are exposed douglas fir.

 

Also, is building regs .11 or .13 for flat roofs, i'm finding some conflicting info.

 

Thanks!