Under the cooking floor bricks

[Virago]

I am building a 42" Pompeii dome oven using your plans.
I have built the hearth with concrete (3 1/2") and am at the point to prepare for the cooking floor.
My dilemma is: Do I use the Perlite and cement mix only under the Herringbone cooking floor or ...
Home Depot has rigid foam insulation with silver backing which I can get in 1 1/2", and then place the same cooking floor with fire bricks as I was planning.
Could I use the rigid foam insulation as a base under the cooking floor and then fill around the remainder with Perl-crete?
Would this be a fatal error? Would the rigid foam breakdown or, perhaps, melt and I lose my dome oven floor?
My thinking is that the rigid foam provides better insulation than the Perl-crete alone.
Thank uou, (virago)

[SableSprings]

Welcome Virago. The rigid foam available at Home Depot will melt at fairly low temps and is NOT suitable for WFO use. We recommend drilling 3-4 weep holes (1/2" diameter) through the concrete hearth slab to help moisture escape. On top of the slab, either mosaic tile sheets or other porcelain tile pieces to provide a water barrier & channel system leading to the weep holes. Garden cloth on top of the tiles or simply place mosaic shees upside down so backing mesh keeps the perlcrete from filling in the " channels ". Pour 4-5" of 5 :1 insulating concrete (using either perlite, vermiculite, or a mix with the 1 part cement) on top of the file layer. After this layer has cured & dried a bit, your cooking floor bricks can be laid directly on this insulating base (may use a thin layer of fine sand to level bricks). This 5:1 insulation concrete is used by many WFO builders who have trouble sourcing (or affording) ceramic board. Rule of thumb is 4" of perlcrete/vermicrete is equivalent to 2" of ceramic insulation board. I hope that info helps you. Looking forward to your build.

[Virago]

Thank you, Mike.
Using the rigid foam would have been a fatal mistake (fatal for the WFO!).
This is the first time I have heard of moisture concerns on the slab. I can do that. Weep holes in the center of the slab or where/what pattern?
I guess I am safest with the full 4" Perl-crete and a layer of sand/clay/water combo and then setting the cooking floor bricks.
- Virago

[UtahBeehiver]

Wet floor and dome insulation is one of the most common causes of poor oven performance. As Mike mention a 5 to 1 p or vcrete floor insulation is the bare minimum thickness, if space allows you would be better off with 6" or so. There are several threads on installing weep holes you can easily do a search for them.

[SableSprings]

Virago, below is a link to a nicely documented build with weep holes & tile spacers.

https://community.fornobravo.com/for...andpoint-idaho