Now that there is very little ventilation we have to pay extra attention to the coop litter. Too much moisture and the ammonia content builds up which of course, is a bad thing! I add a mix of barn lime and DE with extra bedding and it's just fine. We also open the pop door every day so they can go out if they want, which lets in fresh air. We keep the big doors closed when it's this cold.
Hope your chickens are staying warm and dry this winter!