[chairman]

The screen top will allow humidity to escape and the heat bulb will dry out the air. I'm not sure that I could predict how much the substrate heater would raise humidity in your situation. In a past setup I used a heat cable to raise the humidity in an enclosure by about 20% or so.

The nice thing about under tank heaters is that they do very little to raise cage temperatures. Combined with having a male gecko that is highly unlikely to burrow, a heat pad should be a fairly low risk addition, even if controlled by a rheostat.