We close our coop doors and popdoor into the barn pen nightly. I know we cannot prevent predators 100%, but we do as much as we can to secure their sleeping quarters. I'd be very concerned about leaving doors open at night. Our metal barn is closed up nightly also so once the popdoor outside is closed, the indoor hens are safe. We open both areas first thing in the mornings so the hens have early access to their water and feed. Vents are always open in our 4X6 coop. We've had no problems after two years of chickens. ~G