I have shipped Ayam Cemani to customers in very cold climates like Michigan and they say they have done fine. The Cemani surprisingly seem to be cold hardy here in America but I do not think that's the case for the actual real ones in Indonesia. Indonesian Ayam Cemani do not have the same feather type as the ones here. Our are soft feathers and short bodied. Their's are hard feathered and similar bodied to an oriental fowl like Thai or Shamo chickens from Thailand for example. Besides their all black look, Indonesian Cemani look more like oriental fowl than our so called Ayam Cemani here in America. They are totally different birds than what we have here.
For example, my first shipment of Ayam Cemani ever I bought directly from Greenfire Farms in 2014. Greenfire Farms has supposedly the best Ayam Cemani here in the US and are supposed to be pure but are definitely far from that. They sent me a young rooster of poor quality with mulbury face similar to Hybrid Smithsonian line. After 3 more months of owning him, he grew into a bright red face and neck, white ear lobes, bright white skin and white and pink inner mouth and tongue. If they so called have pures then they would never have had any birds in their line pop out with these characteristics that no pure Ayam Cemani in Indonesia should ever have. That's my point why I think that we do not have he true pure birds like they are supposed to be in Indonesia. But this is all we have to work with for now.
Greenfire eventually sent me another batch of Ayam Cemani in Spring 2015 and they were allot better quality with nice black skin pigment but still no black inner mouths or tongue. My black mouth birds came from chicks from Feather Lover Farms. Though their line is supposed to be mainly Greenfire Farms. Perhaps they dd a good job selecting for traits and breeding them back. I too am only hatching good quality black chicks with black mouths now after carefully selecting for only the best traits and culling the rest. This is the only way we can have quality Ayam Cemani with dark black features here in the US.
As for the cold temps, I have had some cold nights here in California as low as under freezing and haven't had a problem with my Cemani getting sick in a normal type chicken coop setup with no extra heat source. Just keep them in a dry pen and able to get into a barn with straw and they should be good. They are a great breed to own, seen if you'd just like a couple running around the barn. They are an eye catcher to anyone that has never seen them in person.