Some of the ovens in our Photo Gallery show Brick-on-Firebrick ovens but we highly recommend building your oven with (1) layer of firebrick then using a few layers of Ceramic Fiber Blanket to insulate the oven.
If your oven is Brick-on-Firebrick, the thermal mass of the oven is going to take a long time to heat-up and it’s going to require more firewood than an insulated oven.
Brick-on-Firebrick ovens almost always show hairline cracking on the outside of the oven. Since ovens heat-up and expand, the inside of the oven may get a small hairline crack, but on the outside of the oven, that hairline crack is much larger.
Simple example… Put two-fingers together - now spread those two fingers out. See how the inside got a little larger while the ends of your fingers got much wider? Same principal.