How many chickens?
I went out to double check. There is no frost on the inside of my coops. My birds don't have any on them either. They are even staying outside under the coop for most of the day. I have that area closed off with plastic sheeting to keep it dry.
Are you cleaning out the poos...
Sounds as if you need better ventilation.
My coop is a prefab (Not great) and we are in negative temps as well, I've seen no issues of frost on the birds. I did have an issue of a frostbitten comb on the one who has the largest comb. That happened when I got scared it was too cold and closed...