I always assumed it was no different that a potholder. My old cover was just a potholder folded in half and sewn together. The insulbrite keeps the heat from going to the top, but so far there doesn't seem to be any problem with the inside material getting hot. I will put my iron in when it's really hot and test it. Good question though.
The ones I've seen sold by a purse manufacturer (don't think I am can mention name brand here?), are just quilted cotton.