for potholders i use three layers: 2 of batting and 1 of insulbrite in the center. for hot pads i use only two layers: 1 of batting and 1 of insulbrite. i usually make my potholders 9"-10" in size as i make them for gifts quite frequently. my family are getting potholders and cookbooks from me for christmas.
you can purchase insulbrite quite inexpensively at hobbly lobby for around $2.50/yard.