Just sharing as someone who lives in a region where it Sub-Zero for 2 months of the year and below freezing for the rest of the 4 and a half months a year of winter, what I do to help the chickens out on these especially cold snaps when it gets into the negatives!
I bake 4 bricks (you could do as many as you want I'm just not willing to carry more than four out to the coop
) at 400° on an old sheet tray for 40 to 45 minutes and then I cover them with a towel ***this is important they will absolutely burn their feet on the bricks if you do not cover the bricks with a towel*** sheet pan and all and just put it on the coop floor. It stays hot for HOURS, the chickens sit on it, huddle around it etc and according to the thermometer in my coop it raises the temp almost 5 degrees (my coop is 8 x16 feet it will heat a small coop much more, also this is not "adding heat" to my coop I don't want to hear it... it going from -6° up to -1° is not going to kill or "spoil" my chickens lol)
I just tuck the towel ends under the sheet tray so that their scratching with their feet at the towel doesn't pull the towel off the bricks. Today it was -6° highs all day, I went out 3 hours after I put the bricks out there and they were still warm to the touch.
I bake 4 bricks (you could do as many as you want I'm just not willing to carry more than four out to the coop
I just tuck the towel ends under the sheet tray so that their scratching with their feet at the towel doesn't pull the towel off the bricks. Today it was -6° highs all day, I went out 3 hours after I put the bricks out there and they were still warm to the touch.
