If you go with a very small 600 watt oil rad heater, cage it in and keep the t/stat low, it will cycle on and off, and keep the deep chill out of the coop. This far more effective thana brooder heat bulb, even though I do have both, but it gets extreme up here. I have only had the little heater turned up 1/2 way in the coldest weather, it is usually on low....
Your coop should be very comfortable with that many in that size coop alot of heat given off by them,I have a 10'x13' coop with about 35 chickens and it's not too bad in there but I do have plenty of shavings and nest boxes and several perches for them too huddle.I never had problems too much with frostbite. My rule of thumb was temperatures around the freezing point door and ventilation windows open. Temperatures around 15º F or -10ºC and ventilation windows closed. Since I added 7 more birds to my flock (17 birds housed in coop) I find my ventilation windows have to be left open 24/7 to avoid frostbite. My coop is a salvaged 4x8 footprint shed.
Here is my answer for this, but I am going make a "Blanket Statement", to all of those in the South or northern areas to, I do sympathize with this cold, it will be brief for most of yall, in my case it will be 75
% of the winter with nightly temps below 0f.
This is my little insulated coop with heat sources, works for me. We are blessed with the lowest priced electricity in North America, and power outages are very rare.....
I honestly thought I would have more trouble with the pecking order in that close of quarters but I have been very lucky so far. The three levels see to be the secret. They ventured out side a bit today we had temperatures around the freezing mark (that is a heat wave for us as of late). My shavings are frozen harder than concrete and I have folded feed bags in my nest boxes which are a snap to maintain. I have a few chickens that sleep in the nest boxes. The majority sleep on their roost and clean up is a Easy Peasy in my set up.
