Screen tops are a nightmare for humidity. The easiest way to keep humidity up in a glass aquarium is to seal it all but a small opening for air. So if you have a screen top you can just get a piece of plexiglas cut to the size of the screen top minus about a 1/2" for air exchange. Once you do this you'll never have humidity issues again, just keep a good size water bowl in there and a good substrate for humidity like reptichip. You may have to rewet the substrate occasionally but most likely less than weekly.