That's a guideline, not a hard-and-fast rule, but it's usually considered a minimum. As a rule of thumb, the inside of the coop and the outside should be approximately the same temperature (actual temperature, not wind chill). Dry chickens who are out of the wind easily tolerate down to 0F or...
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If your coop is adequately ventilated insulating it will make little difference in the interior temperature. The point isn't to keep chickens warm, but to keep them dry and out of the wind. :)
Generous ventilation will prevent moisture from building up to condensate:
The...