It was 4 degrees here this am. I do not use heat but do provide a good building for them. It has ventilation to let out the moisture and prevent frostbite. That is nec. even in the coldest of weather due to ther resp. containing water vapor. Too tight and they will have frost bite. All hens doing well. Their water on the other hand, in spite of the cookie tin heater on ,did have ice on it. extra scratch or feed at bedtime helps with the ability to stay warm. warm oatmeal cooked does too. It beautiful at 14 now. The sun is out and so are the chickens.