I think you'll find that opening your coop up a lot will help. If you can notice any humidity, it's too closed up. As long as they have a draft free place to roost, the more ventilation, the better.
If you are going to build a new coop, I can't recommend the Woods Fresh Air coop strongly...
I've only had frostbite like that if the coop was too closed up and humid. Since switching to a much more open coop I haven't had frostbite issues with very cold temperatures, -40F for a couple nights. That wasn't the wind chill temperature, that was the actual temperature. Windchill temp...