My guys do well with an ambient temp of 80. I've got a space heater in their room. I don't have my UTH's on the bottom, but stuck on one side, with overhead heat lamps. Does a fine job of keeping the tanks warm. I know everyone says to put them on the bottom with thermostats, but I'm too paranoid about that sort of setup failing. I've got plastic wrap taped to the screen top to hold in humidity, it prob holds in some heat as well.

I live in Houston where it doesn't get *that* cold, but it's 37F outside atm.

You might try getting some styrofoam insulation and attaching it to 3 sides of your tank.