I have nine hens and live in area where temps at night get to teens and next week the low's will get down to single digits!! The hens are 12-14 weeks and various breeds, Leghorn, RIR, Easter egger, Wyandotte, Ameraucana, Maran. Coop is fully inclosed with upper ventilation. I have read on some sites insulating the coop was not necessary. It is built sturdy with 3/4 inch plywood all around. Do I need to add a heat lamp or will they be fine?