We both had similar temps overnight. I did buy an acurite wireless thermometer, the sensor hangs up near where they roost. The overnight temps got down to -5°F, with wicked winds giving us a wind chill of -20°F. The coop stayed around 20°F with no heat- just their body heat. The coop is vented well, and slightly insulated. Once their auto door opened this morning, it dropped to 14°F. I haven't turned on any heat in the coop, but once we get any colder, I may. My friend not far got through -27°F for 3 weeks last year no heat..... chickens did ok.
They were a little slow this morning, and I served them warm oatmeal which really helps. Biggest thing is frost bite on their combs & waddles and their feet. Your roost should be wide enough to allow them to be able to cover their feet with their bodies.
Keep us posted