I have 4 heat lamps in my coop this year. I only had one last year and only lost one chicken. I am surrounded by corn fields so with the wind it was getting below 0 in my coop a lot. My coop is 18x18. I also put in a heated waterer got sick of thawing it out last year. I also give them extra food. I added the extra heat also because I got 5 leghorns this year and I didn't know they don't like cold weather.. Just my 2 bits.
Brian
Brian