I like the door

.
How high is your roost and is the floor resilient? What breed of chickens?
Winter is hard on the elderly of many species. Cold is one part of that but less so for chickens if their feathers are in good shape. Since they so rarely need heat and I'm very risk averse to fire hazards in such a dusty place (and I bed mostly with dried maple leaves), I don't have much of a direct answer to your question. If I were to add heat, it would be some sort of thermal mass put in each night, probably helped with insulation.
Indirect answer but maybe helpful - another reason winter is hard on chickens is less time to eat because days ate shorter. Your light might be off setting that if the food is in the light. Having several food dishes can help if the other chickens aren't letting her eat enough. Also, having food out all day, well balanced food, and fresh clean water always available.
You might try the light in the morning rather than evening. The gradually dimming light of dusk is more important than the gradually lightening of dawn.
[/QUOTEi have 5 roost
They are right above her .Now noticing my rooster and her sister isa are staying in the coop with her this am. I'm so confused but all the pullets are free ranging .