Depending on your sealer but with some, I.E. chamber sealers, you can purchase heavier mil bagging material. I have in my arsenal of bagging material 3, 4 & 5 mil bags to choose from depending on what the use is for. If I recall correctly what I read was that there are times the regular zip lock bags fail on hotter and longer cooks. The weight of the product being cooked is a factor as well. The heavier the more stress on the seals. If using zip lock bags just for insurance I would use double bags without question. They are cheap so why not?