The first few cold nights (in relative terms), I picked my near-naked Chipotle, who also did a December molt, out of the nestbox and tucked her in between two of her fluffiest sisters for good snuggling. The third night I was going to do it again and found her already there.
Have you read this article? https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/
Fully-feathered chickens in a dry, draft-free but well-ventilated coop should be good down to at least 0F.