Pretty much the line is drawn seasonally. A "draft" in the summer is good, that means a cool (not so much recently) breeze blowing on the birds through a window or a wall vent, either high or low. A "draft" in winter is bad, that means icy air blowing on the birds who are already using energy to stay warm as it is. But in winter, you still need a way for 'warm', moist air (from respiration, droppings, etc.) to escape the coop, so vents up high (above the heads of the roosting birds) are best, or vents far away from where the birds are roosting. Hope that helps a little.