We have 4 big hens in a small, insulated henhouse, which opens to a run below. We put some plastic around the run underneath the henhouse (that small run opens to a larger run) and installed a small mason jar light fixture in the run under the henhouse. We will only turn the light on when it gets really cold. The warm air generated by the bulb should rise to the upper level, the henhouse, and provide a bit of heat. We also have a heater under our galvanized water container, to keep the water from freezing, and we assume that could have some residual heat affect in an enclosed space, too. The coop and run are in an area protected on the north side by a hill, so they will be sheltered from the north winds. Also, our hens are winter hardy (red sexlinks and Buff Orpingtons), and that helps, too.