If you can move the coop closer to the house in the winter, my husband drug ours with our tractor, or put it in the midst of trees to protect the coop from wind and additional snow and ice that might help. We moved the coop close to our house and garage last year and added several tarps over the already shingled coop to protect it even more from wind. I didn't add any heat source at all. I did the deep litter method and made sure they had WARM water to drink instead of cold, ( which takes a lot of effort but worth it). I seemed to worry more that they would peck and hurt each other out of boredom and darkness more than being cold. I live in Michigan and they did very well, just a little frostbite on the roosters comb!
I'm finding it's all trial and error, Good Luck!!
I'm finding it's all trial and error, Good Luck!!