Badly insulated bonus room

I have the same problem.

Only turn it on when I need to

Honestly, I find bonus rooms to be basically useless for this very reason. Mine is freezing cold during the winter and hot as hell during the summer. I basically don't use it.

I don't think it is good for a structure to have no conditioning of the air at all, as humidity builds up.

I don't have the answer to your question, but I would probably set the heat on 50 or 60 and let it roll.

I gotta say my bonus room came out great but we did somethings that most builders don’t do. We insulated the entire downstairs garage (walls and ceiling) plus installed insulated garage doors. Then we insulated the walls upstairs instead of relying on the attic blown in insulation. Easy to see if you have insulation on the walls of your bonus room if you’re in the attic. If not, that’s cheap and easy and would make a huge difference.

Insulated garage door and sealing up air leaks in it can really help out. It may be easier to insulate the garage ceiling than the bonus room floor.