I'd like to have 10' inside the isolation shell, which us1ually means 12-13' clear below the lowest obstruction BEFORE we start the shell construction...
Hey, more is better.. With height typically being the limiting factor in modal calculations, the more we have, the more options we can explore. 10' + in a finished control rooms shell (9' finishes in middle of the room) works well.
I have recently done control rooms with 14' high Isolation shells!! Man, we had over 4' deep bass trapping in the rear ceiling soffits. Let me tell you, the bass sounds tight in those rooms!