Chickens are very well insulated and can tolerate temperatures well below freezing. The roosts should be at least 2 feet lower than the lowest vent so the chickens aren't caught in a draft. Moving air near the roof line is critical for effective ventilation. Ammonia from the their poop can build up and make them very sick is there isn't enough ventilation. Moisture is also an issue in the winter. If moist air can't escape, it can cause frostbite on the combs and feet. Chickens can handle cold, and they can handle dampness, but they can't be both cold and damp.