Rock wool or mineral wool would be my insulator of choice. Rock wool comes in several "grades" or densities. A medium density should be plenty for building an oven. The other potential insulator I would consider is rigid fiberboard. It's cheap easy to get and can be stacked to increase it's efficiency. it comes in 1" and 2" thickness. I'm not sure about the rgid foam board. Even with the foil backing I would think it would break down very quickly and emit some pretty toxic fumes. I'm not sure if the foam board is made for that type of insulating. Perhaps a little research is in order.
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