When I make potholders I use two layers of thick old terry bath or hand towels and quilt them with a #18 needle. This is my excuse to freshen up with new towels and a great use for the old or spotted ones. When you are using insulbrite you want to use at least a #16 needle, if your outside fabric is heavy go up to #18. I prefer the #18 because I know I will not break a needle and it will stitch through the thick areas no problem.