It seems with Avics that air is more important than humidity. In fact humidity is usually what kills them from what I am learning. If the enclosure has high humidity but does not move the air, tiny bacteria develops which get caught in the book lung and the T dies.
@ Gemini, yes putting holes on the soil line is a great idea as this helps with natural induction. Heat rises naturally so air exits the holes at the top, sucking in fresh air at the bottom.
I keep my Avic with two water dishes to keep humidity high but not too high, and the normal glass tanks with the round plastic ventilation grid on top, and it's doing just fine. We call it MERRELL like the shoes with those nifty pink sneakers it has on!
HS