There is none stated that I could find in the T20 book.
Under MT, it was Armor x10 meters, but that was a different armor scale. For T20, I'd probably use (Armor +10) x40 meters of *water* or similar, on the basis that MT starships had a vehicle-scale armor of 40 just for being starships.
The life support necessary for a starship should be able to regulate internal air pressure, though I'd consider dropping effective armor from the rating calc above if this were done to extremes.
I'll leave it to someone else to figure out what those depths mean in terms of actual pressure (which could thus be translated to things like Gas Giants)...