If you're using a basking lamp, thats part of the problem. Try getting a mister of some sort and using some kind of bark for a substrate. Like mentioned, if you are using a screen top, you may have to tape off part of it to reduce the amount of air exchange. Some people may disagree with my logic here but I think 40-50% humidity is fine unless in shed. My female has had successful sheds in 30% humidity before.