If this were my coop, I would prefer some screened holes in the board at the top where the rafters come through. Those would allow the warm moist air to escape. If cut with a hole saw, and the cut outs saved, you could put the cut outs in or out to adjust the amount of ventilation. Or you could simply have doors to cover individual holes. Generally, you will leave them open all year.
We have 2 doors that can cover a 2 inch opening across the front of our coop on the high side (and East) under the roof that are the length of the coop. Nine months of the year they are wide open, and in the winter we throttle them a bit. When it is really cold, (below 10F is my rule of thumb), I close one of them, and the other is dropped down, but not closed. The three windows are always open at least a crack and the crack is about 6 inches below the perch. (In the summer all the windows are wide open.) It seems to be working, and as I have learned, it is better to give them more air, even when really cold (0F or below).
This will be our 3rd winter in Ohio with chickens. Last year we had 2 straight weeks below zero, often with highs below -7F. Since I have some glass windows, I watch them for Jack Frost, and open more ventilation panels if I see any! It has worked. The first year we had a little frostbite on their combs, but they recovered, and being more worried about cold that first winter then moisture is definitely why we had it. This past winter (which was far more severe in the temperature department), they had more ventilation and less comb damage. We didn't loose any chickens either winter. However, being concerned with the cold that first year, we had a lot of frost inside the windows. That concerned me enough to open the vents more, and it probably saved some lives!
Other things that need to be considered are how many birds to how much space. More birds will keep it warmer, but also wetter, so you need more ventilation. You really have to watch. If you see frost on the inside of the wooden walls, it is WAY TOO WET in there!! It is much harder to create venting in the winter, so do it now, and cover it if you feel you have to, just be ready to open it up if there are signs of frost in the coop. Water containers also add moisture to the air. You might consider removing them at night if you can't get rid of frost formation inside the coop. We use the inverted flower pot and light bulb trick, so that releases considerable moisture. I am aware of that, and keep the vents open!
It is very hard, especially when it is less than 10F outside, to resist closing up the coop, but experience has taught me that it is NOT the right thing to do. You have enough wall ventilation, but I would add some holes higher to create some convection to move out the moisture. As the birds breathe, they let out a lot of damp air, their poop adds even more dampness, and moving that wet air out is the key. The colder the air gets, the less moisture it can hold, and if condensation starts to form on the birds, the frostbite and chill set in. You really just have to believe that, until you've seen it. I have, so I know.
If you live where it is rarely below 20F, you can pretty much leave everything open, so long as it isn't windy in the coop. I designed mine to replace air from the windows below the perch, and exhaust it at the top. Again, it seems to be working, as long as I leave some open.
-fab