I was having this problem too -- mostly in the RTB cage, but all have been a challange to keep the humidity in. I went out and bought a piece of plexiglass from the local hardware store -- 30" x 30" for only about $10.00. I have most critter cages with the sliding lids, but I also have some normals with a lift off top. I cut pieces of plexiglass to fit over the portions of the tops that did not have heat lamps. I drilled a few wholes in them just so they still will allow for the exchange of air. These things have worked wonders on the humidity. I still mist, but at one point with the boa, I was misting like 5 to 6 times a day, and I mean misting. He was getting ticked off with getting so wet, but to keep the temp up, it would evaporate quickly. The corns weren't as bad, but this has definetly helped.