I also have an 18x18x24 set up with a digital hygrometer (zoomeds) and I have the same problems. It's bizarre because there can be water drops glistening on the plants and the digital readout shows about 41 percent humidity. There's dirt substrate and plants and a huge water bowl, but I can't seem to keep the humidity up when the heater's running. Even if the tank temperature is only 73 F.

I put a damp cloth over half of the top, still having problems. Maybe it has to do with the placement or accuracy of the hygrometer sensor (halfway up one wall).