HCA: Hermit Crab Association

Hi all,
My family and I got 2 PP crabbies a week ago. We have them in a 10 gallon tank. I originally had a gifted UTH that I was told to put on the bottom of the tank… but the more I read the more I saw it should be on the side and it had no thermostat. So I unplugged that one and got a new 6x8 UTH and have it mounted on the back wall and have covered the UTH and back wall with foil. The temp won’t rise above 76 degrees Fahrenheit, it’s usually closer to 73-75 AND I have the heat on inside the house so I’m guessing the tank will only get colder as it gets warmer outside. Now the top of the tank is a screen, I have it covered with plastic wrap, I’m working on getting a plexiglass lid. Will the lid help hold the heat more? And does the lid need holes for air ventilation?
One crab was more active and climbing but has since gone underground, can’t see it at all. The other just hides day and night behind a piece of wood.
Any suggestions would be greatly appreciated!

Welcome to crabbing and to the HCA!
First, check to be sure that the brand of UTH can be insulated over top - some cannot, and are a fire hazard if you do. What brand is yours? You can also insulate the sides of the tank, try wrapping cardboard with tinfoil, or using a product like Reflectix, or one of those shades for windshields.
Where is your thermometer in relation to your UTH?
What is your humidity reading?
Plexi glass or glass lids won't necessarily hold in any more heat or humidity than plastic wrap. You can put a blanket over top or insulation to hold in more heat. Many of us do leave a corner open for ventilation.

https://a.co/d/1QIdmgE
That’s the UTH I purchased

That heat mat cannot be safely insulated over top. Also it is very small and won't heat up a tank. For crabbing, don't go by the number of gallons it says on the mat (those measurements are for reptile enclosures). For crabs, we go by the dimensions. Get a UTH that covers most of the back of your tank side to side. It should be positioned mostly above the substrate, across the whole back. Ultratherm and the mats from Bean Farm are really good, but the are expensive. They can be insulated over top, and are attached with tape, so you can re-position them if you need or want to. The sticky ones cannot be removed and stuck back on again.