Growing up we always used heat lamps in our coops but I did some googling and came across this article, they seem to know what they are talking about but idk, I am not a professional in any way.

https://the-chicken-chick.com/whether-to-heat-chicken-coop-in-cold/#:~:text=Not%20only%20do%20they%20have,a%20heat%20lamp%20with%20chickens!