to keep my chickens warm for the winter

[Fairview01]

Hi my Coop is 4 foot by 4 foot and 5 foot it is open all around except for the top and bottom. During the winters at night we get some single digit so snow and ice

I'm confused. You have an open coop which to me means there are no solid walls all the way around and your worried about keeping them warm? Wouldn't the first step be to enclose the coop. Chickens have feathers. Cold doesn't kill chickens, drafts do.

Maybe I don't understand what an open coop is.