Talking about isolation; what's the best material for combined thermal and noise isolation?
kg/dB(a)/m^2?
I've been told Cork as a core material is pretty good at both?
With no material science topic I’ll answer under general questions.
Yes, cork has been used both as a thermal insulator and for sound deadening (insulation).
For sound deadening you usually look for a material with high mechanical damping. Then depending on the application you look at the acoustic impedance which is the product of density and speed of sound in the material.
What is best is then usually unobtanium. Cork might be substituted with good results in many cases.