forgot to add that this wouldn't be safe if you are using higher powered incandescent lighting for heat.
also, how much coco is in the bottom of the tank. and when you touch it, is it dry to the touch? or moist. you can get away without water in the tank, but you need to make sure the coco fiber is dampish. with your set up, THAT is your humidity retention. kind of like a pacman viv. if your substrate feels dry, mist it heavily. how often do you need to drain the tank? and btw, a pic is worth a thousand words here. can you post one of your viv so we can maybe get a better grasp of this?