Ah, fall, when new livestock owners panic at the thought of cold weather and break out the heavy duty plastic. I was there a few years back, so I have some sympathy for you, but my learnings, let me show you them.
Chickens are not that fragile, y’all. Provided they can stay dry, they will do just fine roosting in the open air. Most of ours sleep in the barn rafters, a few go sleep in the trees. Before that they roosted in the run the geese currently occupy, which is open air except for the roof (check out HVAC Direct). We have never yet lost a chicken to winter cold, not even during the periodic polar vortices that swoop down from the north.
If you’re living way up north, windbreaks become necessary, but you still shouldn’t hermetically seal your barn or chicken house. Damp conditions arising from their own respiration will give chickens (and goats and sheep) pneumonia. If they’ve been living outside, your livestock are well-adapted to handling the weather. Make sure they have food, water, and a way to stay dry and they’ll be just fine. Honestly.
Remember that up until very, very recently nobody considered a chicken a pet. Their bodies are well adapted to outside conditions, and honestly if they can’t handle living like actual chickens you’re better off without them. Certainly don’t breed any bird that can’t deal with the very basics of chicken life. You aren’t doing its offspring or their future owners any favors.
But above all, remember: ventilation is more important than a totally sealed, draft-free chicken coop. Don’t make your comfort more important than the actual needs of your chickens. Sealing up their house is about what makes you feel good, not what keeps a chicken healthy and happy.
A good roof, lots of food, and available water are all you need to see chickens through the winter. They’ll be fine, I promise.