I was just logging in to ask the same question when I saw your thread. I have a very similar set up. I have been able to keep the temps right by covering a good 80% of the tank lid with plastic wrap to keep humidity and heat in the tank. However I did notice that the tank had dropped this morning from the usual 80 degrees the cool side runs at to 75. I live in Arizona and we had our first chilly night last night so I could easily see this being the reason it dropped. Wish I had an answer to your question, but I am in the same boat. I would like to avoid putting an overhead light on the tank since they tend to dry it out so much, but I just don't see that UTH generating enough heat to keep the cool side as high as it needs to be. Hopefully someone will come along soon who has encountered and corrected the same issue. Is there a such thing as an overhead/ambient heat source that doesn't eat up humidity?