I live in Northern Wyoming, between 3 mountain ranges and not too far from Yellowstone Park. Here we KNOW winter. I don't heat my coop at all. I haven't insulated it either. Chickens do very well with cold as long as their coop is well ventilated with no drafts directly on them. That lovely down and feather coat they wear keeps them pretty comfortable. They suffer more from heat than they do from cold.
Last year mine went out at 5.5 weeks when daytime temps struggled to reach 40 and nighttime temps were in the 20s. I put a heat lamp out there - at first. But every time I checked on them, they weren't anywhere near it. It was in there for the 1st and 2nd day, and the morning of the third day I decided that if they weren't going to use it then the fire danger wasn't worth it. The day I took the heat lamp out, it snowed, and we got our last snowfall on June 6th. They did just fine.
Right now I am brooding this years batch of chicks outside in the run from the get-go. Actually one group went out a week old, because my husband was hospitalized and we didn't get the outside pen finished before the chicks got here. But the new chicks went outside at one day old. They are now three and two weeks, respectively. I am not using a heat lamp out there for them, either. They are under a heating pad and they are positively thriving.
So relax. Chickens are a lot tougher than most folks give them credit for, and they know what they need to do to survive - better than we do, in fact. Too often we project our human needs on them. I stressed out over my first batch that I couldn't even enjoy them. After having them thrive though their first cold temps as babies to doing great this winter, I'm very pleased with their ability to do well despite having no heat or insulation. And with this newest group of chicks, I am totally enjoying watching them grow and develop instead of looking for problems where there really weren't any.
