Im in southern mi near Adrian. I dont heat my coop and dont worry about mature hens freezing or getting sick because i keep enough straw around for them to snuggle in and keep warm but most of the time they dont seem to need to. Some of my bantams lost a toe here and there i think due to using metel pipe for roosts. Ive since removed all those and installed flat 2x4's for roosts (thanks backyard chickens

). Interestingly some still roost on the 2x8 sticking out from the posts with barely enough room to sit.
I only heat the brood pens and after all this discussion wil be looking into some changes to my setup. Best brood setup ever is no heat and a good mom to protect them and keep them warm. I used that method with my bantams. Sadly that brood mom died one winter. She was on the bottom of the pecking order for some reason so probably got ganged up on and killed. She was the prettiest of the bunch too with feathered feet and gold coloring.