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?
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Re: FB Board
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.
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Re: FB Board
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!
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Re: FB Board
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.
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Re: FB Board
Well, concrete is pretty cheap, but you have to form it, mix it up, etc., etc.. Especially in this situation, you'd have to be extra careful to not splash stuff around. I had some old pavers with chips, cutoffs, etc., and it worked out ok.
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