Insulation above hearth slab

[masuzzu]

I have just completed the hearth slab and the vermiculite/cement layer with one minor problem:I was unable to fully complete the vermiculite/cement layer (instead of the five inches , I only have one inch).
I cannot continue with the insulation because I am unable to find the vermiculite.
Question1 : Can I use the fornobravo ceramic blanket on top of the one inch inslation layer I already have ?
Question2 : If I understand correctly, the ceramic blanket is Alumina and silica
which is chemically similar to bricks. Will this blanket function as an insulator or as added mass?
Thanks!

[dmun]

Question 1: for the insulating slab the blanket is the wrong stuff. You need an insulating board, like insblock 19 or cal-sil boards. There should be no trouble getting vermiculite or perlite if that's what you want to use: check with landscaping suppliers or pool suppliers, they should have them in the big bags. If you go with the refractory boards, you can use them right on top of what you have if it's level, and if it isn't, a little layer of dry fireclay or refractory mortar mix will level it out.

Question 2: The insulation blanket is mineral wool, as you say, mostly silica. It is an insulator because it traps air spaces. If you use it under the mass of your oven, it will squeeze all the air out and become ineffective. Keep your blanket for on top of your oven.

[DrakeRemoray]

Dmun, as usual, is correct. Don't skimp on this step!!

Drake

[masuzzu]

Sorry , I am referring to the fornobravo ceramic board, not the blanket.
Thanks!

[dbhansen]

Yes, you can use the Forno Bravo board on top of the 1" vermiculite/concrete layer. That will provide even more insulation than many folks are using (you can use the 2" insulation board without any additional insulation under the oven floor).