That’s a good question and, as usual, there are different ways to answer. I’ll assume you know why you need ventilation and no breezes hitting them. I purposely said breezes, not drafts. A draft is when you hold a candle next to a window to see if there is a tiny bit of air movement. That’s not what we are talking about. We are talking about a breeze strong enough to ruffle their feathers. That kind of draft from a window is good. It moves bad air out and good air in.
I don’t know where you are so I don’t know what kind of temperatures you are dealing with so I can’t get real specific.
JackE likes to push the Woods Coop design. That’s a very specialized design where the front is pretty much open and there is another opening up high so you get any breezes in the lower opening and flowing out the upper opening while the back of the coop where the roosts are sees only gentle air movement. That works up in Canada if it’s built right.
In the winter if you really have cold temperatures to deal with, I really like openings up high over their heads when they are on the roosts. If you have more than one opening up high and a breeze is blowing, that goes over their heads yet creates enough gentle turbulence to stir up the air and replace bad air with good. On a calm day, whether you have one or more openings, you still get air movement. Warm air rises. Warmth is being generated inside the coop from their body heat, breathing, warm poop, thawed or even heated water, and if the coop is on the ground the ground is warmer than the air during a cold snap. That warmth is enough to create air movement with just openings up high.
Another reason for openings up high is that, if it is not frozen, their poop generates ammonia as it decomposes. Ammonia is lighter than air and is hard on their respiratory systems. Since it is lighter than air, gravity forces ammonia to rise and replaces it with heavier fresh air from outside, even just with openings up high. You do not need openings down low to get air movement.
Your biggest danger from the cold is not them freezing to death, it’s frostbite. Frostbite is possible anytime the air temperature is below freezing, but with good ventilation and no breezes hitting them chickens can handle temperatures below zero Fahrenheit without getting frostbite. Mine do. A big contributor to frostbite is dampness, excess moisture. Ventilation gets rid of excess moisture. People as far south as Georgia have caused their chickens to suffer from frostbite by closing the coop up too tightly while some chickens sleep in trees in upper Michigan or even in Canada during the winter as long as they can stay out of breezes. It’s hard to get too much ventilation as long as you keep breezes off of them.
I don’t know what your coop looks like or your climate. Openings at the top of walls under overhangs can move a lot of air. Gable vents, roof vents or cupolas are good. Ridge vents work great as long as you don’t get enough snow to block them.
Whenever you create a hole in your coop remember predator protection.
