People forget, warmth in the coop isn't the enemy, moisture is. The key is to build a bit of a heat bank while not retaining all the moisture from their respiration.I would agree with new chick about insulating I think the advantages are worth it especially when we have those winter winds kickin,as long as you have proper ventilation and it's also a good idea to put a few windows in it if building yourself,this allows alot of cross ventilation which keeps the chickens cooler during the summer.
Yes, a window is great for instant ventilation in Summer but also, keep in mind how much time they spend inside in Winter. A light source will make it nicer inside and help them lay. Egg production is stimulated by the amount of light that comes though their eyes.