I'd like to give you an idea of how much ambient temps could impact stabilizing your scale kids home and how a good thermostat helps. During the summer months I keep the room temps around 76 degrees, this gives me tub temps of 90/80. The thermostat is set to its summer setting of 88.1. During the cold months I keep the same room heated to 73 degrees (air is drier during this time), this gives me tub temps of 89/80. The thermostat is set to its winter setting of 89.9. That is a difference of almost 2 degrees to stabilize my scale kids year round.

Many moons ago (when I picked up my first ball) I kept him in a 40L tank with a screen top. I also had a humidity challenge and looked to solve it by covering up portions of the screen with an adhesive material (cloth type) found at a local hobby store. It took several attempts until I reached my goal. It seemed like an obvious solution considering the heat was at belly level. I hope this helps the effort.