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Hi Finb,
If you can get some specificaly soundproof rockwool or similar, that'll be the best stuff to use. I used Wickes 50mm batts, and this was easy to use and fit because you can cut it to (over) size and snug fit it between joists or studds. Wickes also make a thinner but denser stuff (30mm I think) that is specifically designed for soundproofing and you glue it to the party wall using tile adhesive. If you have the room, you could use this first, and then make the new wall just in front ( say 5mm). You'll get better soundproofing but only lose a further 10mm from the room.
The best method for soundproofing a party wall (from what I've read) is to do exactly what you're doing though - 4x2 wood for the wall (rigid walls is good) built one inch from the existing wall, rockwool or similar to fill, then two layers of platerboard to finish. Use half inch or bigger, and seal all gaps between the joins in the first layer, and floor and ceiling using silicon or similar. Fit the second layer making sure the joins don't line up.
You can lay the first layer horizontal, and the second layer vertical if you wish. This does introduce a crossover point where joins can meet though, but if your sealing is good, it won't be an issue.
Use no more nails to glue the second layer to the first (adds rigidity). You can use further soundproofing by fitting some of the soundproofing matting between the layers (see the link Johnson gave).
Make sure you seal all joins at floor and ceiling level and at the sides. Where air can go, so can sound, so this will make a big difference.
I've done a similar thing in my loft, but not as effective due to using just 2x1 studding and the 30mm batts. It's very effective, so what you are doing should be better still.
HTH
Gary.
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