On a new build, the first floor is suspended timber 225x50 joists @350-400 with 18-22mm boards over. I would like to sound proof the upstairs/downstairs as much as possible, I presume simply stuffing some rockwool in is the way to go? Or a more expensive material eg celotex 50mm perhaps? On the house I am currently in, there is carpet upstairs and you can hear everything downstairs, phone calls even, I presume the floors are hollow.