I like Insulbrite with a layer of all cotton batting next to it. And, as others have mentioned, I think that wool would also be a great choice.
When I first moved to my present home, I went to the Christmas Cafts Fair and wondered over to a booth that was selling pretty potholders, hoping to strike up a conversation and possibly make a new quilting friend. We were chatting happily and I mentioned to the woman in the booth that I had planned to make potholders for Christmas gifts, but didn't know where to find Insulbrite. A dark, sheepish look came over her face...for just a moment. Anyway, I bought a few potholders from her and left with a smile. Sadly, those potholders were terrible. The heat went right through them. She obviously didn't use the proper batting in them. I finally had to throw them away and so did the people that I had gifted them to. I should have torn one open to see what she put in it, but I didn't. Too bad, because the designs were beautiful.
~ C