It was -10° F at my house this morning. I had left two brooder lamps running all night in the coop, but it still managed to get down to 4° inside there. The coldest I had exposed them to in the past was 10°, which they seemed to survive. It's a large coop. They roost near the lamps, so it may have been a bit warmer where they were roosting last night. The thermometer is on the opposite end of the coop in a nest-box. The chickens all looked fine this morning. I'll be looking for signs of frostbite, though.