phonic and thermic insulation

Cork. It has excellent sound absorbing and thermal insulation properties. It is also cheap and fire-resistant. Also, it is available as an off-the-shelf material, unlike the do-it-yourself syntactic composition suggested by that other poster.

If you are interested in materials selection you might want to try something like this software.

You could produce a materials selection chart: Probably you need something with low stiffness, and low thermal conductivity, but maybe you want to design a merit index depending on what other material properties are important. You really have to chose what parameters are important yourself unless you want to describe your problem more.

Mathew