FWIW, good momma hens know what to do, even in the cold. I live in NH and I have a broody who incubated and hatched a clutch of eggs when the nightime temps regularly went down below 0F. She did this outside in a coop. It kept the wind out, and we did give her lots of bedding, but no supplemental heat. The chicks hatched fine, she raised them fine, she knows exactly how to keep them safe and warm. Trust your broodies, they don't need much more than a safe, quiet place that is sheltered from the weather to succeed.