Insulating Castable Refractory is a scientifically formulated mix of refractory aggregates that provides excellent heat retention properties. When used under the cooking surface firebrick (hearth), the Insulating Castable absorbs and harnesses the radiating heat and prevents the heat from spreading to your concrete slab and base. It’s designed to keep the heat where it belongs - in your oven and under the cooking surface.
Vermiculite and Perlite are “Nature’s Insulation” as they are natural earth elements - not man made. It wasn’t too long ago that houses used Vermiculite and Perlite in attics and crawlspaces as insulation. It was inexpensive and it worked.
The same can be said for Vermiculite / Perlite and Insulating Castable. While Vermiculite and Perlite are excellent at insulating your pizza oven naturally, there is a slightly better insulator on the market - Insulating Castable. Insulating Castable is designed to be used under the firebrick as an insulator that keeps your oven super-hot with minimal firewood usage. It also dries much faster than Vermiculite / Perlite, so you can save a couple of days of drying time by using Insulating Castable.
As you’re budgeting the costs of your oven, keep in mind that Insulating Castable costs about 3x - 4x the price of Vermiculite / Perlite. It’s pricey… but good stuff.