I have been shutting my two windows at night for the past week. Like you, it has been getting into the 40's at night. Not tight, so there is still ventalation. My windows also hinge at the top so that when they are open they are a cover from the rain and snow. I would say that if you are more comfortable closing one at night and opening it back up in the day, you will be fine. And of course when you get your new coop done, you will surely be all set!