My flock is in an unheated coop and they are doing just fine. Tips about keeping moisture down are spot on, but other than that be sure they have plenty of feed to keep their caloric intake up to keep them warm. Water access is important as they wont eat if they can't drink. I give extra whole corn right now as I have heard that helps them stay warm
I do the same, I just bought 100# of corn and 100# of BOSS to help keep calorie intake high, I do not worry about the protein as much this time of year or the "harmful" affects of corn.