Lamps and CHEs will suck humidity out of a cage. The most important heat source is the UTH because "belly temp" is what really effects your snake. If you cover your screen with a few sheets of aluminum foil, and leave a spot for a low wattage bulb (50w) this will help keep humidity. To generate humidity you can do simple things like placing a large water dish over the UTH or under the lamp. A misting or two a day shouldn't soak the substrate enough cause any problems, but with the tips I have suggested (which are things I personally do) you should only have to mist once every couple of days as a sort of booster.

P.S.
Do you have a thermostat or rheostat for your UTH? Do you have a thermometer with probes to see what your UTH is heating up to?