Think of being in a car with a bunch of people in cold weather without the heat on. Almost immediately the moisuture begins to build up on the windows. This warm air condenses because it is in contact with the cooler window or wall inside a coop. That is damp, and that is what happens in a too small, too tight of coop with chickens. Damp chickens are cold chickens.
You do not give the dimensions or the number of birds, but I suggest you go out and measure how far your birds are away from the wall or from the ceiling of the coop, they should be a foot away from the wall, and their heads need to be about 15-18 inches below the ceiling when roosting. This keeps the moisture away from them, and allows the ventilation to move out the wet, moist air, keeping them dry. Dry feathers are very warm, and they should get through well below zero (as in -25) with no problem with plenty of feed.
Most of us in the beginning, think we want to keep our chickens warm in the very cold winter weather. We have been taught since children to keep the openings sealed up tight to trap heat inside our homes. It seems so counter intuitive to open up the coop to keep it warm. Instead of thinking warm, think DRY. You need good bedding that can absorb moisture, and you need openings that allow the moist air to escape along with the ammonia. And you needs adequate space between the birds and the walls and the ceiling. Too small of coop will make for wet chickens.
What you need is protection from the wind, but still allow air currents, you want movement of air. Hence free of drafts but well ventilated, which took me forever to figure out, and I got a bit of frostbite till I did.
Edited by Mrs. K - 11/6/15 at 1:34pm