it's so cold

I'm worried about my chickie girls. It is in the teens and they stay huddled together most of the time. I wouldn't want to live like that. I've been reading and trying to get advice on free-range chickens in cold weather. Seems like half the people say they'll do fine in cold weather and the other half suggest a heating lamp of some sort. I've got a bit of a shelter set up against the house, so they can still be outside, but they seem to prefer crowding against the back door most of the time. It is pretty pathetic. Only got two eggs yesterday.