point the fan outofthewindow, that way cool air has to flow in to replace it. wet the floor if things get bad as this will dramatically lower the temperature. Shade the roof and front from the sun. If you can do neither of these your mice are in the wrong place, and need moving quick. My mice have always ben kept in the coldest place in the garden or home, that way I can bring the temperature UP and be in control of it with thermostats, but bringing the temp down due to outside influences is very, very difficult.