I think raber hit it on the head. A heat lamp (especially as a primary source of heat) is going to just zap humidity. I use a glass cage, with the screen 75-90% covered. I have it so I can fold the foil back if i need to, this allows me to make minor adjustments to keep my humidity right around 48-50% without moss or misting the newspaper. For heat use a UTH on a thermostat or at the least a dimmer. I have had good luck with ExoTerra or you could even probably get away with flexwatt from RBI.
Doing this will not zap your humidity and will provide belly heat, which snakes will like.