You really do need to let some air move through that coop. I'm in MN, and it's been double-digits below for several nights. There is a lot of air movement through my coops. The windows tip in to open, and are still open a couple of inches at the top for ventilation, along with "venting" around the top of the coop where the walls meet the ceiling. (In other words, gaps where the air can move through) As previously stated, they are more susceptible to frostbite when the coop has too much humidity.
