I live in southern VA, near the border with NC. Apparently the "Midgard" confuses computers, it sent me to Columbus, OH.
I've only been here for one winter, but in my experience(and according to the internet), we almost never get below the late teens. Last winter I think the lowest temps were like 17 degrees Fahrenheit. We did have one snow in February that had about 2-3 inches of accumulation, and a couple little snows where there wasn't any accumulation.
My coop is not really good for anything, because non-chicken-owning relatives gave it to my family as a gift. The predator-proof-ness is probably bad, but the main thing I don't like about it for winter is the ventilation. Here are pics of some of the issues:
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^so this is one of the worst parts. This door has about 1/4 inch cracks around it, and a window in the middle which is right at head level with the hens. Not ideal. I don't really know what I'm going to do with the crack around it, but I'm definitely going to have to cover that window.
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^this is the sliding door from the coop into the run. Dirt gets in the track that it slides on and makes it so there is a small crack where it can't close all the way. This shouldn't be too hard to fix. Ignore the mess that someone made by backing up and pooping directly onto the wall. Ughhh.
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I know this is hard to see, but these are the small cracks where the nest box lid hinges from. Mildly irritating, but not unfixable.
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And this is the whole coop and run. Not the best, I know.
Oh, and I also have to get some better perches in there.
Any ideas for closing cracks?
Also, when I cover the window, I will get a hole drilling thing and add a bunch of ventilation holes up near the roof of the coop.
You don't need to cover it up, you need to open it up.
Dry chickens are healthy chickens, even in the winter. Allowing warm, moist air to escape from vents over the chickens' heads when they're sitting on their roost prevents frostbite -- which is caused by moisture in the air freezing on the chickens' combs.

Those little cracks are not a problem unless rain is coming in through them. What you need to avoid is actual wind -- wind that's capable of ruffling their feathers. Gentle air movement is good.
Have a look at my Outdoor Brooder to see how I have generous ventilation arranged with no drafts blowing directly on my chicks.