Damping sound in rehearsal/studio

Hi. We are on a very tight budget here. Our room is 3,5m x 5,5m and 3 of our walls are faced out and towards neighbors. We play Metal style music with 5 members and it can get pretty loud. This house if from the ww2 era and there is no isolation what so ever, except for the wall-paper and wooden walls. How can we best/cheap isolate our room? My thought whereas follow: Use 10cm Rockwool with 2-3cm space between the Rockwool and the wall. This is about the budget we can afford at this time. Does this have any effect for our neighbors? Are we wasting our mony here? Any suggestions appreciated.

Please read the forum rules first and we'll gladly help you. Javier