If you have plenty of holes for ventilation and the humidity is still too high, try getting a ceramic heat emitter and turn it on close to the rack. I don't know for sure that this will work, but I used to use one for heat when I had my boa, and my humidity was almost impossible to keep up because of the CHE (I didn't know it at the time). When I switched to a normal heat lamp the humidity went up. Don't know for sure that it will work, but they really seem to suck up extra humidity.