Well.. it seems like many people in this forum think that chickens are just fine with temperatures below freezing, but there is a difference between surviving and thriving.
Personally I would be very stressed out about keeping my chickens in that coop when the temperature drops below freezing.
I live far north in Norway where temperatures now are around freezing, and it will be around -10 and -20 degrees celcius (I believe that converts to between 14 and -4 fahrenheit) for most of the winter. I have an insulated coop with IP55 tube heaters and hope to keep the temperatures inside the coop for over 5 degrees celcius (41 fahrenheit). Right now with freezing temps outside it's 10 degrees celcius over the tube heater on top of their roost, and experienced norwegian chicken owners mostly believe that the chickens should have temperatures like that for the chickens to thrive.
For you I would at least install IP55 tube heaters that are much more safe than heating lamps.
This is pictures of my coop during and after insulating it. You can see the tube heater under their roost.
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