We have one silkie in with a flock of standards. It has been down to the negatives at night here at times, and she's been doing fine. In fact, she's been better than some of the larger, more cold-hardy girls.
I'm not really sure how cold they can get. However, I live in North Western PA and have never heated my Silkies. They handle the weather wonderfully. At night, I have to put them in their box, as they want to sleep outside.
However, they don't handle the wet/mud/snow well. I rabbit hutch mine.
We've had a bitterly cold winter up here in NY. Physically, my Silkies have been thriving. Just keep them in a tight, dry coop. But I've been keeping them in so much, I worry about their emotional state.
Kidding, of course. Apples and salad will keep anyone occupied. Don't worry about it.
I live in Michigan, in the upper part of the thumb area. Its very cold and also we have the wind off of Lake Huron. My coop is not heated and my hens have done fine in our winters. My silkie does especially well. She has gone broody twice this winter and so I think that means she thinks she is in a habitat that would allow her to raise chicks. I am not sure I think she could right now but she does.