I’m in Canada so sometimes it’s pretty cold. But I can open the whole door if it’s okayIt doesn't sound like enough ventilation to me. Drilled holes and a cracked door isn't ventilation. I wager most respiratory problems this time of year are due to lack of fresh air with poultry keepers who are afraid their birds are cold.
Where are you located? Is it double digits below zero F?
Open the door all the way if it is predator proof.
Oxygen is the most beneficial thing you can give your chickens.