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That's certainly true for standard size chickens, but Seramas are are tiny bantams not known to be cold hardy. Some are so small they weigh less than a pound.
I have Serama crossbreeds and live in North Texas where we also sometimes get freezing weather. Last winter I brought my bantams into our attached garage on freezing nights and bedded them down in pine shavings in several big dog crates. The temperature in the garage stayed just above freezing. On the coldest nights (teens) I ran an oil filled radiator in the garage, too.
We had several days that stayed below freezing, and I cautiously let my flock out in their run and watched them carefully for distress. Nope. They all acted normally even in weather that cold. The run was tarped on three sides so they had no wind chill to contend with.
Be careful with a heat lamp in a small coop. We know someone with Seramas here last winter who had one of her chickens burn its comb on a heat lamp.
Are your birds the very tiny (A class) or the bigger ones?