I don't know if your ventilation is bad, I've seen lots of different coops and it's hard to visualize what you have, but chickens are adaptable and I wouldn't worry about that too much. don't use a heat light overnight, they don't really need it and it is a fire hazard. Idk, just kind of keep an eye on her if you are worried about the breathing. I sometimes hear them breathing when I catch ours (and we do have some tame ones) and sometimes not. She might have been breathing harder just because it's really cold out. Some of ours suffer from frostbite, and some don't, so like I said, it's really a matter of how they position themselves when they sleep, and probably there are other physical factors involved. If I were you I'd just casually observe her for a few days if you are still worried (don't stare or fuss over her, they conceal symptoms of illness anyway and they definitely will if you do that)...and try to keep the coop as sheltered as you can at night, til it warms up again. When it's bitter cold like it has been, we do leave a couple of heatlights on for our flock during the day, so they have a little heat if they need it...but we always turn the heat lights off when we go to bed. Our barn where they roost is not very insulated, either, but they do keep it warmer, with the combined heat of all their bodies, plus the barn cat, too! If she does start showing other symptoms that worry you, don't hesitate to address them quickly though. You can also give them poultry vitamins in winter, in their water, like VitaproB or AviaCharge, or the feed stores even sell vitamins now. Vitamins and also organic apple cider vinegar (the kind with the mother in it)..1 TBSP per gallon..are good to help chickens stay healthy during the extreme cold. We alternate them in the drinking water, over the winter.