2x4 on their flat typically aren't as strong, I'm sure this building has been standing a while though. I'm using a remodeled wooden grain bin near Ituna sk. It's 12x14 but I insulated (150$ 2x4 insulation on 2 foot centre's cost is about 33 cents a square foot) I used a grain bag as vapor barrier which should do really well for keeping drafts out. It has an eight foot ceiling but I will be building it down to six and insulating it Making less area tar needs to stay warm. Ya birds can take some cold, but they can't take moisture a draft and -30 -40 with a wind. I know expensive but I suggest taking some measures to insulate your coop. Their combs and toes ect can't take -30 there. I'm not familiar with radiant heat so I'm not sure how well it works or how much area it's capable of warming. I use heat lamps.