I'm in North Alabama. I have 6 hens that just began to lay 2 weeks ago. To my surprise, they have not been affected by the cold nights at all. Their water has frozen in the coop 2 nights during the bitter cold last weekend. Check your water often or get a heated water container. I was worried about them, but they were just fine and were outside during the cold blast of air we had. Our coop does have insulation in the walls & floor (which is raised), but the ceiling is open and covered only with chicken wire under the shingled roof. I imagine the insulation we installed probably doesn't do too much for warmth. We never finished the insulated removable ceiling panels we had planned to put up there during the winter months. I wanted to keep the dust from being an issue for their respiratory systems and I felt that if I added the ceiling panels it would create a problem.
Being in the south, I was more concerned with the heat in the summer, so ventilation was very important to me. We have east & west facing windows that can be propped open with chicken wire to screen them in. I made sure that the hens I chose were hardy, there were plenty of them to huddle together to keep warm, and I could always add heat if needed. Seems to be working for me & the girls so far.