Cold is not the enemy of chickens, heat is. They wear a down coat year round. They can handle the cold, but they do need a little help, mainly protection against frostbite. They need good ventilation to keep the humidity down and they need to be out of direct drafts.
To me, the best way to get this in winter is to have a lot of ventilation over their heads when they are roosting. Cut out under the overhang along at least two sides and maybe all four sides and cover that with hardware cloth to keep predators out. I don't know what your prevalent winds are, but you might take that into consideration. As long as the roosts are low enough, the cross breeze won't hit them. This should stay open year round.
Because each location is different, it is hard to say exactly how much ventilation you need. It's really hard to get too much as long as it is over their heads in the winter.
In the summer when the heat is more dangerous, I suggest you have additional ventilation at or below roost level. A breeze won't hurt them in the heat of summer, but I'd have this on the side away from the prevalent wind to stop rainwater from blowing in. There are many different ways you can achieve this, usually involving windows or hardware cloth covered openings yoou can block off in the winter. Maybe have a shutter that is hinged at the top so you can block it open and it will shed rain, then just lock it closed in the winter. You need some type if window for light anyway. Maybe the shutter could be some type of clear plexiglas or such. Or have a sheet of plexiglas you bolt or screw on in the winter. But if you can find a regular window, that works really well. You might find some cheap on Craigslist or at one of those recycle stores.
Good luck!
