I don't heat my coop at all. We lose power too frequently, so it would leave them too vulnerable to the cold. Chickens can handle the cold as long as their coop is dry and draft-free. Remember that "draft free" does not mean "wrapped in plastic without a vent." Moisture is what really causes frostbite and closing up completely allows condensation, which leaves warm combs with little pieces of frozen ice on the tips. Voila! Frostbite! If you keep the ventilation above their heads, not at or below roost level, they should be just fine.
Nankins do well, here. I have to watch my two single comb boys for signs of frostbite, but they generally fare pretty well in winter.
My JG is massive, and at only six months, she's not done growing, yet (they're late bloomers.) According to the judge at the MD State Fair, she's going to be huge. She lays a BIG, beautiful, brown egg ... but she takes a lot of coop space and feed to do it.
Still, I wouldn't trade her for the world. She's the sweetest bird in the bunch! They're really endangered, so they would be a really neat bird to concentrate on, if you intend to breed.
Check out The Livestock Conservancy's website to learn more about them - and lots of other endangered livestock. Its's pretty cool!
https://livestockconservancy.org/index.php/heritage/internal/conservation-priority-list
Oh - and is your coop a prefab or a custom build/conversion? The prefab ones sold commercially are woefully inadequate for the number of birds they claim to hold. If you haven't started yet, I highly recommend starting with a good shed kit or a used child's wooden playset. Both are sturdy and easy to convert. You may even be able to get "You Haul It" for free off of FB or Craigs List.
Good Luck!