Quote: You'll have to assess how the air moves in your coop to decide what and how much to leave open all winter,
it can vary greatly depending on the weather patterns at the site of
your coop.
Sometimes you have to go in on a really windy day and observe...
....as well as long term observation over your first year to decide what works and what doesn't.
This is a pretty good discussion thread, might give you some ideas:
https://www.backyardchickens.com/t/1048597/ventilated-but-free-of-drafts
This is a great video showing an air movement experiment(tho that roost is way to high for reasons that have nothing to do with airflow)