@Candiru09
Did you say you’re climate is zone 6? If So you have a wee bit milder than mine, but the big thing is being wet and cold. Not sure how much rain/snow you get in the winter but wet is your biggest thing.
This is why you need to have really good ventilation as bgmathteach states. There is a fine line between ventilation and drafts so be careful not to have drafty windy cold getting into your coop.
What I have found with my gang is that they can handle the cold to about -5c or -10c (23F to 14F), lower than that and I find they start huddling around not being active or eating and drinking as much.
Colder than -20c and they become quite stressed. I have a couple of Sweeter Heaters that they like to hang out under, and last winter was very miserable here, I bought two IR heat lamps that were well used by my gang (and me hahaha). I am extremely leery of heat lamps and researched many before I bought them, and made sure they had all the proper safety approval ratings.
I am very fortunate in that I have the barn they stay in all winter, it stays about 10degrees warmer than outside (if it’s -20 outside it will be -10 inside). These days I only have 2 horses so they don’t keep the barn warm when they are in. This is why I have the heat sources. You want to make sure they are getting adequate food and water when it’s really cold, if they are cold they won’t feel like drinking or eating.
@rural mouse has a heater in her coop which I think is a great idea. I don’t have one in my hen houses but might put one in this year as I have many elderly ladies now. Just enough to keep it at 5c (41F).
An electric waterer will be your best friend, it will keep the water from freezing and warm enough that the chooks will drink it. It’s vital that water is changed everyday, bacteria can grow in the water very fast and cause sickness in your kiddos.
The take away is keep it well ventilated, prevent drafts, and make sure they are not cold stressed.
Extremes of heat and cold are equally dangerous for any animal.