FB Board vs vermiculite mix

[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

[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.

[UtahBeehiver]

Also depends on the budget, v-crete or p-crete may be less expensive than CaSi board or alternative insulations, ie FoamGlas.

[SableSprings]

I had the opportunity to put together a modular oven with the CaSi board below the hearth and the experience of putting in perlcrete below my home built oven. I used the perlcrete based on my slim budget and have been happy with it. With the modular oven (Casa G90), I thought the placing of precast oven pieces on top of the board was a bit more difficult when leveling. We laid the pieces of insulation board down and then put the hearth pieces in position. As we placed the modular cast dome pieces on around the hearth and on the underlying board, the board had a tendency to lift up and move (consequently lifting and moving the hearth plates). I'm sure it was because we were physically struggling to lift & place the three pieces correctly, but ultimately the hearth floor ended up with several edges lifted rather than flush. Fortunately the peel slides in fine--the edges up are all on the back sides of the hearth sections, it's just when we rake out ash that the tool gets stuck on a couple lips of the hearth sections.

On my next oven I will save up my bottle money and buy/use the CaSi board because of its superior insulation properties per inch (and now that I've seen it in action and have a better idea of possible installation issues). Either work well...as Russell noted above, it's primarily about budget and as Joe pointed out, it's also often about availability.

[RandyJ]

I went with 4" of calsil type board from Mcgills warehouse. I don't have my door yet but still have 400F temps 24 hrs later. Whatever you do just remembler you can't go back and re insulate later.

Randy

[gastagg]

I would definitely follow gulf's advice on a moisture barrier between the insulation and the hearth slab. I hadn't read that until it was too late. I have a moisture barrier between my slab and the walls of the stand, but my insulation has soaked up a lot of moisture from the slab. it's taken a bit to get that moisture out.

[mleuck]

What would be the best choice for a moisture barrier between the hearth slab and the insulation?

[david s]

There are many products out there that you could paint on over the slab, like the membrane used between tiles and concrete in bathrooms. You could also use plastic, the heat below the insulation should not be high enough to worry it. I use a product that actually makes the whole supporting slab waterproof. It is called Xypex and also enhances the strength of the concrete.

[mnagy]

I thought the best thing was to drill holes in the hearth, lay mosaic tile scraps on top, and then pour the vermicrete over that… This thread has people talking about a moisture barrier? I’m confused. Should I have a moisture barrier and drill holes as well?

[Gulf]

mnagy,

This thread is from 2015. That was before elevating the insulation along with weep holes were combined as a better technique. The glazing on the ceramic tiles provide a moisture barrier to prevent wicking.

[mnagy]

Ah, ok. I bought some sealer already, should I still use it for extra protection? Or just go with the mosaic tile method? Thanks!

[Gulf]

It can’t hurt anything if you apply it to the hearth slab. May be just a little extra insurance. Your call.

[Garouda]

I see that some are putting the first row of soldiers outside the vermiculite base. I wonder if it's not a mistake because the soldiers are not isolated from the concrete slab, and it creates a thermal bridge with heat loss, am I right? I'm asking because I had planned to put the soldiers outside to gain a little height, but I think that is a mistake. Thank you for your insightful reviews.

[JRPizza]

You are correct that all your thermal mass should be sitting on insulation and not contacting the concrete slab. I have not seen very many people following this forum doing it wrong. We will occasionally get someone posting about why their oven will not heat properly, and later find it was built wrong.

[Squirt Reynolds]

mnagy,

This thread is from 2015. That was before elevating the insulation along with weep holes were combined as a better technique. The glazing on the ceramic tiles provide a moisture barrier to prevent wicking.

Weep holes? explain pls… I have hearth done…