Hello Folks
This is my first post/question
I believe I'm in the right section though I'm not sure if I follow correct protocol - so, sorry in advance : )
Do direct me to correct forum if necessary.

I build a barrel style oven to which I'm committed on account of space parameters that preclude dome style (which I prefer).
The barrel will be wrapped in ceramic blanket and then a cement-based casing.
The barrel will be housed in a brick box.

What is best insulation material between the barrel and the interior walls of the brick box?
I consider perlite only.
But the ease makes me think that I'm missing something.

By the way, the brick housing will be topped with a concrete roof.
I suppose I must be sure to seal it properly should water/humidity make its way into the housing and generate deterioration within.

Thank you kindly for your answers.