Wow, now I want to know how many BTUs I give off!
OP, the only thing I see to add to the great advice already here, is to be wary of drafts on the same level as the chickens when they're sitting on the roost.
Doesn't matter here in Florida, but in Alaska you would want the ventilation well over their heads. You don't want their feathers getting ruffled in a storm, as that allows their body heat to escape from the only location it matters - under their fluffy parkas, not the coop. Seal well around windows and such. A candle test is advised to check for breezes.
If you're still concerned about temperatures, perhaps consider heated roost bars you can turn on in the worst weather, since they directly contact the chicken.