That was one thing I was worried about too; ventilation. I have 10.25 SqFt of hardware cloth covered windows in my coop. Add the two chicken doors and that's almost 12 feet squared of ventilation arranged near the floor and at the top of the A-frame providing ventilation with their their roosts half way between to prevent drafts in the winter. In the hot Southern summers because the run is secure I can also open the 70" x 24' people door as well.I mix it a little. For one thing our summer has been really hot so leaving the door open means the coop doesn't get quite so hot & stuffy. If I am away it is open as it's 1 less thing for DH to have to remember. Normally I do shut it as a precaution against a determined predator. I figure if I'm not predictable the occasional open door is not going to bring trouble.