I have my mice in an old RV with a propane heater during the winter. They produce a lot of moisture in the air by breathing (condensation) so I always have to have the vents open a tad. I keep the RV no colder than 45 F and no hotter than 65 F in the winter. They tolerate cold a lot better than hot. In the summer I keep my colony in a screen tent in the shade, since I have no air conditioning.