Chickens are very resistant to cold--they are basically covered in down jackets but if moisture condenses on their combs, wattles, and feet (the bits not covered in feathers) it will result in frostbite. So, the key to keeping chickens in winter in cold climates is to keep them out of drafts (which can ruffle their insulating feathers and let cold into their "jacket") and to keep their coop as dry as possible.
Sources of moisture in a chicken coop assuming it's weather-tight:
(1) the waterer--move it outside if at all possible
(2) droppings--keep this in mind when choosing your bedding
(3) breathing chickens--this is the most significant source of humidification of the air and the only way to address it is to provide excellent ventilation so it is carried out of the coop
So. Ventilation. Heat rises and it takes moisture with it since warm air holds more more moisture than cold air. Your most significant ventilation should be at the roof level then. Popular methods are roof ridge vents, gable vents, clerestory windows (esp above the roosting area)... In addition, you need a place for drier cooler air to enter and create cross-ventilation. This can come in through the pop door during the day, but if you plan on closing it up at night, you probably want to put some adjustable vents in the walls. Leaving the walls a little less sealed would have actually worked in your favor for this.
