|antique French oven|
The simplest of these is vermiculite, or expanded mica. These particles, easily found at greenhouse and nursery supply stores, are fireproof and full of air. Mixing them with a small amount of Portland cement allows you to cast shapes that will provide rigid insulating barriers. I have used loose vermiculite as a final filler in ovens of mine that have a ''house" type enclosure. 'Insulcast' a Mt. Savage product, duplicates the vermiculite/cement capabilities with a controlled albeit more expensive product.
|Foamglass, Fiberfax board and brick|
Foamglass, a refractory insulator, comes 4" thick in slabs that can be easily cut with a handsaw. It is expensive but very effective.
For the outer part of the oven, ceramic fiber blanket can be layered to any depth to provide a high-temperature barrier.
The long and short of it is: more insulation holds heat longer. If that is your goal (a 30 hour cooking window per firing) then more is better. I've "over-insulated" the house I designed and built for myself. The result is one-fifth need for whatever fuel source heats it. Kind of a no-brainer.