If you live in a cold climate, i.e. snowy, then you want to make sure the wind does not blow on the chickens. If it never gets below freezing much, then its not as much of an issue. Working windows are the best, so you can control the amount of ventilation based on the weather. I have two 30x50 windows on opposite sides of the coop, but away from where the chickens nest. In the summer, I keep the windows mostly open all the way. In the winter, they are 1/4 to 1/2 open.