Let me preface this by saying I know this isn't the best hygrometer I could have but it's what I have to work with until I can get another one. Last night I noticed my daughter's tank was running lower in humidity than usual, sitting at about 70°. It's been holding steady at 75-76° and then suddenly dropped. I turned up the temp on the heat mat slightly thinking that may help. I checked this morning and it's at 66% humidity but almost 90°. There are no openings where air is getting in/out. The "lid" we're using right now is just a heavy duty piece of cardboard that we secure to the front and back of tank with tape, we've been using that for a few weeks since our glass lid had an unfortunate meeting with the floor. I plan on replacing our glass lid very soon. Any recommendations? Could the cardboard be absorbing moisture? Should I just use plastic wrap or tin foil to secure instead, or possibly wrap the cardboard in that for the time being? Any advice is appreciated!