I use 3 layers all the time without problems with the sewing machine. Only 1 layer of insulbrite, if that. I don't always use insulbrite. I use whatever is left over that is cotton - old towels, cotton batting, worn out mattress pad, ... You don't want to leave insulbrite in the microwave while it's being used.

I use one layer of insult brite for pot holders.
DO NOT use insul-brite for micro bowls or potato bags, only the 100percent cotton potato bag batting should be used. Also, even using cotton batting can catch fire if there is food items dried on the bags or not cleaned after use, very I portait to make sure is clean before using, and absolutely NO metallic fabrics should be used.

When doing shiny side out toward the "hot" side for pot holders - if only using one piece of insul-bright plus a piece of regular cotton batting, you should tell whoever you are giving them to...i make one side a single piece of plain fabric - that is the "to the heat" side. If i make 2 sided items - i put a piece of heavier muslin between the outward facing pieces of insul-brite.

The first insulbrite batting I bought had the paper with it that said to use the shiny side toward the heat. The next time I bought it, the paper said nothing about a preferable side. It looked the same on both sides. The manufacturers must have changed their product.
The product for microwave bowls is called Wrap n Zap and it's made by Pellon.

Thank you all. I had planned on using one quilt square on the front and a coordinating flannel for the back of my pot holders. Think I will use one insul bright and one warm and natural and then try them this weekend before I go to far.

When I use insulbrite I use one in the middle of two warm and natural and then I put two layers of fabric I don't like or denim on each side, I also quilt it. Not too thick and doesn't let the heat in. So many ways, make them and try them out for what will work for you.

I also use one layer of insulbrite with one layer of regular batting - and put the shiny side out. That plus the fabric is four layers. That's at the recomendation of the quilt shop lady, and I don't get burned when using them.

I use 2 layers of cotton batting and then back the pot holders with ironing board fabric instead of regular cotton fabric.

Pennyhal, the cotton batting that can go in a microwave has no scrim in it. Your normal 100% cotton batting has scrim to hold it together and that is what is subject to bursting into flame. Any bag/pad you make for the microwave must be all cotton fabric and batting with no scrim (specialized product that will note it is safe for microwave use).

I don't know if this is what happened in your case, but:
Make sure to wash your microwave bowl or potato bag if there is a spill on it. A friend also caught her microwave on fire using a 100% pad that she had accidentally sloshed some soup in. The burned on food is what actually caught fire.