FB Board vs vermiculite mix

[Gulf]

Michael,

Inch for inch, FB board is a better insulator than vermicrete. Only 2" of FB board (ceramic fiber board) is required for about the same insulation factor as 4" of 5-1 (portland-vermiculite). 4" is the minimum recommended thickness for vermicrete and is considered enough for most home WFOs. Vermicrete can be poured on top of a cured hearth slab. Many have poured it the next day or the next weekend. In either case, it is a good idea to place a moisture barrier between the hearth slab and the insulation layer. There are other materials that can be used for insulation under and over the oven. It sometimes boils down to availability.

[mleuck]

Does anyone have an opinion regarding the hearth insulation choices? Is the FB board better than the vermiculite mix? If using the vermiculite mix is 4" enough? Finally, if using the vermiculite mix, can it be put down on a cured hearth or must it me laid on wet cement?

Thank you for the help.

Michael