They have feathers to keep them warm. Fresh air is much more important.In the winter, we get well below freezing, so I wanted them to be able to stay warm,
You wouldn't sit next to your wood stove with your down coat and leggings on would you?
Think of being a closed not running car in winter, windows fog up pretty quick right?
Here's some insight....it's a hard one to get used to.
https://www.backyardchickens.com/threads/ventilated-but-free-of-drafts.1048597/
I like wide open soffits, best protected ventilation for winter.The soffits are blocked off but definitely not "sealed" So there is air in and out of them.
See my Coop page.