I live in Vermont and it gets like northern MN in the winter. A radiant heater is too much. An ordinary light bulb (on a timer so they'll lay through the winter) is all you need. You could use a mild heat bulb if you really want to, but not too warm. My husband put a little tiny heater block under the water to keep it thawed but it does not add any heat to the house itself.
My chicken house has only a hardware cloth floor with straw over top and hardware cloth windows with loosely tacked plastic over them to keep cold winter winds out and my chickens stay happy and healthy through the most brutal of winters. When their light goes out at night they sit on their own feet and huddle together and they stay nice and toasty. During the day I open their door, throw straw over the snow around the entrance to their house, and they come out and stand on it. I'll throw down some cracked corn to give them something to do. Also, if you give them cracked corn at bedtime, the digestion of it produces heat in their bodies that helps to keep them warm.
