Where do your prevailing winter winds come from?
I saw a mention of north but can’t find where that was posted.
Basically:
Make your coop tight along the three feet or so where they roost, above and below. No breezes blowing directly on them.
A good foot or more above their heads are when they roost, provide openings in the coop wall screened with hardware cloth. This prevents buildups of moisture and ammonia by allowing warm rising air to escape to the outside.
Maybe have an additional air gap near the ground for fresh air to come in.
Unless you’re up around Barrow Alaska or so, you shouldn’t need auxiliary heat. Your chickens are wearing their own down jackets 24/7. But you can’t let drafts blow directly on them when they’re roosting, and you ABSOLUTELY can’t allow moisture/humidity and accompanying ammonia to build-up in a too-tight coop. <- That’s when frostbite etc occurs.
If you simply can’t stand the idea of not having an extra heat source (again, chickens generate a lot of heat all by themselves), look into heat plates, I think they’re called. They are apparently less of a fire hazard.
(Please let me know about this last paragraph. This was a summary of what I’ve learned here from experienced posters. I’ll update with edits if I’m wrong.)
