Hello all, I have a 2 x 4 wall on my side of a semidetached party wall. I am planning to put 3" Roxul safe and sound between the cavities and board with 5/8" drywall. While I'm at it do you think it would be beneficial to put a 12mm poly between the drywall and the insulation? My side of the semi will be much more energy Efficent than my neighbours as I am gutting and renovating. I figure the air/vapour barrier will keep my heat in and also help with smells from cooking etc. Does anyone see any negatives this may cause? Much appreciated

Yes, air seal it well. Consider a double layer of drywall with green glue for sound. Maybe with Whisper Clips.