My 8 chickens reside in a small 100 year old smokehouse. The walls are about a foot thick of solid stone, and I have a small window for them to come in and out of. I have also added a small outdoor thermometer and the past few days it has been around -5F to -10F outside, but has been 6F to 14F degrees inside the coop. They are clearly a bit chilly, refuse to come out of the coop, and seem a little disgruntled, but honestly I don't blame them! This weekend we are headed out of town, but the temperatures drop to be about -20F outside. So far I have been vigilant with not having any heaters or bulbs other than a simple heated dog dish to keep them with fresh water because you just hear of all the horrendous stories. Because of the coming dip in temps my Dad has been asking if he should set up a little space heater on a timer to go off every now and then to keep them comfortable. I am also aware of moisture build-up and to be completely honest I really don't have a lot of ventilation (especially with them being closed in because we'll be gone). We have people coming over to feed them, but someone won't be able to monitor the heater in case anything goes wrong. I am ok with them being cold, I am not ok with having frostbitten chickens. Is -20F outside too cold? Help! I need your opinions!
I live in the same area as you, presumably, and I've never used a space heater except for with my chicks in all my years of experience. Never had them die yet. More chickens die of fires per year than freezing, just so you know your risks. My birds never froze, and I've had worse. One of the best things around here is just to own birds who are cold tolerant. I've had frostbite, but my Buckeyes, Ameraucanas, and d'Anvers (especially the d'Anvers) never did. That's not always possible though. Some people do heat, but I prefer to think that in our darkest hour if the power is to go out the chickens won't become dependant upon heat, then freezing. My two cents worth. Others might have different opinions.
-8 here on Monday. They all stayed inside the coop but with no drafts they are plenty warm. My 10 week Silkie even was up moving around and they were not huddled together.
Birds are very cold resistant they will be fine if they are able to roost without draft they are fine ventilation and draft are not the same air can go above or below them both are good best to look in our coop section but your 100 year old building is pretty cool for a coop... I would vote no Heaters