...and that'll be after the couple-few of days above freezing.
Should be 'interesting'.
That's good!
Most their body heat stays under the feathers,
it will by no means 'heat the coop' no matter how low the ceiling.
Block any winds, keep the water liquid.
I like to give a dose of electrolytes during extreme cold,
it definitely helps with cold stress.
Soak it up with rolled oats(not cooked, room temperature EL solution) and they'll gobble it down.
https://www.backyardchickens.com/articles/aarts-extreme-weather-spiel.75893/