I don't test them either, like above poster said. Now, not trying to argue with you, but be warned that most coop fires start with heating devices acting up or heat lamps dropping. I'd much rather have a fluffy chicken then a roasted chicken.I see no reason to test how much a chicken can take of cold any more than I would do for a dog, child, etc. It's 22 below zero today and they have an Eco Heater on the wall with a thermostat which turns on at 35 and off around 40. Fourteen eggs from Fourteen young hens yesterday. The coop has ventilation, about five inches of opening at the top of two walls because it is humidity which is the problem in a winter coop. I change the straw on the floor almost weekly because it softens (getting moist). I could not live with myself if I thought that they were just surviving an ordeal I created for them. So rant away about how little combs freeze, how tough your birds are, how much electricity you save, but keeping a coop just above freezing is the way I do it - not every hen cuddles with another on the roost, not every hen is away from the vents all the time, not every hen likes to be at full fluff trying to survive.