Probably not a FAK item, but I have carried an extra Navy type knitted watch cap to help retain body heat. In the correct situation, it could be used as a FAK item to help someone suffering from hypothermia.
Question - Would it make sense to put a 8x8 inch (or so) piece of the mylar type emergency blanket inside the cap to help retain body heat, or would this have a negative effect on a person suffering from hypothermia?

Any thoughts?