Draft free and dry, but well ventilated. As far as moisture level goes, it should be no more humid than the outside air....so 100% humid some days, only when it is 100% outside. In the winter it is usually much drier in the air, so humidity will only get raised above the outside humidity due to breathing and pooping birds, heated and open waterers and lack of or inadequate ventilation.
We experienced the polar vortex this past winter, with wind chills to -36F, but the actual temp around -18F, with fierce winds. No losses of our various breeds of birds (although single combed varieties did experience some frostbite due to the extreme cold). We did add a doubly secured heat lamp in our coop, connected to to a thermostat so it would not turn on unless colder than 15F. When it turned on, it would basically keep the coop a little bit warmer than the outside combined with the heat of the birds inside.
Here is our coop during construction. It sits fully behind the barn, and is protected from the prevailing west winds in winter. Lots of ventilation in the warm months as the triangle areas are fully open, along with a low window you cannot see on the east side (not where the roosts are located). We have since added 6ft of metal roofing on each side of the coop to protect the openings from rain, snow, and this was a completely worthwhile endeavor since we were able to get into the coop all winter without ice or snow sealing or blocking the door!
Due to mice chewing on some wires in the barn, we had to get some electrical re-done in the barn. As the coop is right behind the barn, we had the electrician add basic electric to the coop. It has been a great thing to have. There is another outlet that cannot be seen on the east wall in case we decide to add an electric fence.
