I read on this site that you need approximately 1sq foot of ventilation per bird. I live in north eastern Indiana. My peak is open with a ridge cap over it. My eaves are open, plus I have 4 windows. 2 are 18x18" and the other 2 are 12x12". Plus my pop door. I left them all open all winter long with no problems. Ive been told more then once that lack of ventilation will cause more problems then the cold. We had a fairly mild winter but we did have spells with temps well below zero and wind chills near -20. My coops stays "open" year around.