WI chickens
Songster
I thought I had enough ventilation, but now I believe I don't. They all prefer to roost outside if it's above 35F, so I have always left my coop door propped open at night until we had this cold spell. Closed it up for night and all my single comb chickens got some frostbite, although nothing like poor Oscar got. I am preparing a new, larger built coop this spring and will be adding quite a bit more ventilation to it. For now I'm just cutting out the great stuff foam I had sealing 80% of the gap between the roof and sides.
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 enough. I'll never have another style coop again, I love it that much. I'm already planning my next one. My only debate is whether to build another 8'x16' (or maybe 10'x16') one, or go all the way to the 20'x20' version.
If you add more chickens, breeds with tiny combs are a great idea in our temps too.