Thanks jhbrandt,
No one has mentioned it, but be sure not to 'short' out the 2 walls by allowing the figid insulation to be smashed between the two, coupling the walls - or with wiring or any other such thing. The frame would be 5cm away from existing structue, 10cm thick filled with insulation and double gypsium board on the inner side of the frame.
This way there would be 5 cm between wall frame and insulation and 15cm between outer mass - wall and inner gypsium wall. If this still would not be enough I could still use third layer GB with GG.
For the asphalt sheets I still did not get any information is it of any benefit?
Why so much isolation? Do you have traffic or neighbor problems? Isolation for keeping out the outside noise: There is the main road 15m from the house with medium traffic, most problematic would be trucks I think.
Isolation for keeping sound inside: I'm a drummer and if there would be possibility I'd use this room for practicing drums also.
Nearest neighbours are 10m away. They say they can live with some noise, but will cost me a bottle of wine from time to time.
Bye