Now that it's going to start getting cold here I'm wondering about 'winterizing' the coop. We built the Purina hutch. It has a vent at the top under an overhang on both the front and back that is about 1 1/2" and the length of the coop like 4'. I think I probably need to close them up, but I know they do need some ventilation. How much should I leave open?