FB Board

[Dagored154]

Why do I need FB board? why not just use vermiculite or nothing at all and just put the fire brick on top of the concrete and vermiculite?

[azatty]

1. The structural slab isn't rated for 900+ degrees.
2. Even if the slab were rated for that kind of temperature, the slab would act as a massive heat sink. It would take days to get your oven to temperature.
3. Vermicrete is a good solution, and the early builders used it extensively. Lots of us still use it. As higher quality insulating materials became more widely available, recommendations changed.
4. FB board or Insblok or whatever other ceramic fiber board is used cuts diwn on construction time. Drop the board on the hearth slab and you can set hearth bricks immediately. Vermicrete has to set, and vermiculite can hold a bunch if water.
5. Insulating board does in two inches the work of five inches of vermicrete. Depending on your style, it could be aesthetically desirable to have a lower hearth.
6. Shaping the board to fit your hearth is easier than trimming excess vermicrete or forming vermicrete. I much preferred using a utility knife on the board to the Sawzall on the vermicrete.

[Dagored154]

thank you. When you say structural slab you are using that interchangeably with the vermicrete?

[azatty]

No, the structural slab is the base upon which you set either the vermicrete or board. Your post referred to using "nothing at all", so I thought that you might be thinking of not using insulation under the hearth.

[Dagored154]

got it. thank you. i understand you now. thanks
Gene

[Retodd87]

What about fire cast concrete? I was planing on using 3" of fire cast on top of a regular slab

[Faith In Virginia]

Fire cast concrete is not insulation. Fire cast concrete will just add to the thermal mass of the floor it is the same as using two layers of firebrick in the original post. You need an insulation layer between the oven floor and the slab.

[rjdealhome]

On the subject of FB board, which then will have two inches+ of brick oven floor on top of it, what is the best material to "fill" the space so that a stone countertop hearth will be a a same level as the mouth of the oven when the oven is finished?
Do you pour vermiculite concrete around the FB board and oven floor bricks to within 1 and 1/2" of the top of the floor (or whatever the thickness of the expected countertop is)?
Trying to get a leg up on future issues in my own build. Thanks!

[Mingy]

What I did for the countertop was I put a row of pavers on top of a thin layer of mortar. You could carefully pour some concrete, but the pavers were a bit thicker than the FB and then I added around 3/4" mortar to support the countertop. This amount of mortar gives you a fair bit of 'play' to make sure the countertop is the right height and level. If you push, wiggle, and pound (with a rubber hammer) it oozes out of the way.

[rjdealhome]

Mingy: Thanks for the suggestion. I like the KISS genius of it, and it has got to be more cost effective than using concrete.
Robert