I never use polyester in pot holders as it can burn you really bad if it melts.
I use insul brite between 2 pieces of 100% cotton. I quilt the top and use plain matching fabric on the bottom. I even use 100% cotton thread. Once you have been burned you will never use polyester again. Please play it safe.
Hobby Lobby has it for about 1/2 the price of JoAnn's and with the 40% off coupon it's less than $2. yd.