I'd be cautious about closing it because the venting is already minimal. :(
Better to follow @aart's advice about the furnace filter material. (I'm thinking of trying that in my brooder's vents if I do any winter hatches because while they're well-sheltered from rain, snow is lighter and can be...
I'm afraid that's one of the known problems with A-frame designs. :(
It might be possible to extend the roof to shelter the vents. Here are two things that might help:
https://strousehomeinspections.com/blog/structural-roof-extensions.html...
I hope that works for you.
Once you've been through the winter you need to write up your arrangement with lessons learned, things that worked, and things you'll do differently so that people can learn from it. :)
@Ridgerunner has given EXCELLENT advice.
Don't close up the structure too much. They need to be able to get out of the wind and to roost out of the wind, but ventilation is just as critical in the winter as the summer. Think square feet, not square inches. :)
Chickens are wearing built-in down parkas. :)
They readily tolerate cold down to 0F and even below when acclimated. Here are some useful articles for you:
https://www.backyardchickens.com/articles/aarts-extreme-weather-spiel.75893/...
Welcome to BYC. Where, in general, are you? Climate matters, especially when it comes to housing.
As others have noted, that coop is WAY too small. Unfortunately, coop makers often claim numbers based on legal minimums for commercial chickens. :(
And yes, being overcrowded can cause social...