To get back to the Humidity issue for a moment, Humidity days and cool damp nights have little effect on the birds other than trouble cooling off on really Hot, really Humidity days like in Southern Coastal States. Where high Humidity is DISASTROUS in the Coop is during the Winter, when temps drop below 30°F. High Humidity and Cold equals Frost and Frostbite on Combs, Waddles and Feet. Frozen appendages Die. Small areas of necrotic tissue can lead to infection in attached healthy tissue. Infection can spread and lead to Sepsis and death.
During Cold weather, removing moister from respiration, waste matter and Damp bedding is important. Birds generate heat, heat rises. Good ventilation at or near the Peak or high point of the coop is much more important than that on the sides. Side vents can merely be Cracked with the highest vents open to allow Moisture to escape. Except in extremely cold climates, temps consistently well below Zero, No heat source in the coop is needed. The Down and Feathers on the birds will provide more than enough insulation to maintain body temp. So, as long as there are No Drafts at the Floor and Roost levels, vents can be open to keep Cool in the Summer and Dry in the Winter...JJ