Never ever think you have too much ventilation....even here when it's in the range of 20 below zero and even colder I have vents, windows, and their pop door to the run open 24/7. Our run is a hoop run and we partially cover it with greenhouse plastic that is mesh reinforced, but the south side is a separate piece that can be rolled up or down like a window shade, depending on which side the bad weather is coming in, and the north side has a big gap in the plastic above the people door. I didn't do that the first year - I sealed that puppy up tight! Then we had condensation dripping from the "ceiling" onto our heads and down our necks, and running down the walls. The birds were damp, the litter was wet, and every surface in there had some kind of moisture on it. Not good! Opened it up and that problem was finished....even though it took time for it to dry out!
There is a huge difference between drafts and ventilation, and it took me forever to find that. Think of the difference as like a great down coat you bought. Now, if you are wearing that jacket outside, your pretty comfy. But if that zipper breaks, all that warm air that was trapped between your body and the outside is gone! It's the same with chickens. Their down and feather insulation does a pretty doggone good job of keeping them comfortable. But if they are in a feather ruffling draft, where is all that trapped warm air going? So you want good air flow without it being so drafty that it moves their feathers.
As for moisture, that needs to get out of the coop. It settles on combs and wattles and can be a major contributor to frostbite on those areas. So ventilation up above their heads moves that out. It comes from everywhere...from their waterer if you keep it in the coop, their respiration, and their droppings, not to mention what's in the air when it's raining or snowing. Ever put 4 people in a cold car and drive off? What happens to the windows? Yep, condensation from those warm bodies in a cold enclosed space and from their breath as well.
So put as much ventilation as you can in the coop. If you use operable vents and/or a window, you can always have something open above their heads so stale, moist, ammonia laden air can escape. Good luck!