"I think he's talking about the noise floor in the studio" - Right, Michael - otherwise known as NC, or Noise Criteria. See Everest's Master Hanbook of Acoustics, page 386-7, complete with chart. (These criteria are NOT flat)
You're also right about the difficulty of achieving NC-20, ESPECIALLY with everything running. Not to mention, that an SPL meter that will tell you if your room is that good will cost more than Charoo's entire budget, from the sound of his posts... Steve
PS - of course, if you could record in the middle of an Iowa cornfield at midnight on a calm night right after you sprayed for crickets, you could get NC-15 with no walls at all... :=)