Living in Canada, I built my coop well insulated. However, ventilation is more important to a chicken's health than being a bit cold once in a while. Chickens are tough but I wanted them to be comfortable in the -30 to -40 weather. Multiple slider windows in my coop offer the light and ventilation they need, even in the dead of winter. I have 30 hens in a 10x15 coop and I rarely smell ammonia when the windows are cracked.
