I have 4 Leghorns, they are about 15 months old, survived the last bitter winter beautifully with no frost bite. We had a couple -10 F nights and many 0 F days last winter.
Even though our coop is insulated, we provided no extra heat. The pop door and windows are opened 24/7, so the outside and inside temperature and humidity are identical.
What we did for the winter (and year-round) is to make sure that the coop is well ventilated at all time, but with no wind blowing up the chicken feathers, no humidity or ammonia build-up, which will lead to frost bite or respiratory problems. The girls have a big round roost to sit on at night, so their feather can cover their toes. When it is really cold, they'd fluffed up their feather into a big ball which traps warm air between the feather to keep themselves warm.
Outside in the secured run, the top is always covered. We added additional clear polycarbonate roofing panels along the windward sides to block the wind and the snow drift. Our girls are outside in the run all day long, they also like to play in the snow sometime. The only time they are inside the coop is to lay eggs and sleep.
The biggest hurdle in the winter was to keep their water and eggs from frozen. Thankfully, our nest box is insulated as well, so the eggs don't freeze as fast. For the water, I had to built an insulated water bucket with horizontal nipple feeders, and put an aquarium heater inside, then run a heavy-duty extension cord from the house. This worked well over the winter when their regular water feeder is frozen by mid day.
However, I do spoil my girls with hot oatmeal mixed with their layer feed and hot fresh water every morning during the winter.
After going through a lot of trial and error, I feel much more prepared for their 2nd winter. The only thing I would need to do this time is to zip-tie the clear panels to the run when it gets colder, and active the aquarium heater for the water bucket.