Temps are 32 to 36. It can get as low as 18 or 20, but that's in Dec and Jan and rare. It's typically 28 on average in winter at night and early morning. I shut the door tonight to let the coop warm up with their body heat. I am not sure if leaving it open will make it too cold for them? I read they can handle cold weather.
You don't have to worry, that isn't cold. They will handle that easily.