Insulating foundation

[Bartondad]

Before I build I am looking at numerous designs. I have recently seen a one piece insulating base under the oven floor and on top of the concrete foundation. It seems nice to be able to cut this stuff to exactly the matching size of your first course of fire brick. Does anyone know what this board is called and how easily is it to cut to shape? Where can I get it and is it an economical choice? Thanks.

[splatgirl]

It sounds like you're talking about the ceramic fiber insulation board. You can get it here on the site or from any refractories supplier. It's more expensive than using vermicrete or perlcrete, but it's fairly quick and easy.
Cuts about like any other rigid insulation board. I used a jab saw.

It doesn't necessarily have to be cut to fit the footprint of your oven exactly,if you're going with an enclosure for your dome vs. keeping the igloo shape with stucco. Lots of guys cut it to fit the footprint exactly, but what matters is that your hearth and dome bricks are all sitting on it.

[Neil2]

Do you mean on top of the structural suspended slab ?

4 inches of vermacrete will do the job nicely.

[Bartondad]

Originally Posted by splatgirl

It sounds like you're talking about the ceramic fiber insulation board.

Is there a standard thickness I need and do I fire mortar it to the structural slab? I'm sure vermacrete is cheaper but I like the clean look of the board (ironic since it will probably not be seen by anyone but me.) Is it also called cal sil?

[splatgirl]

I think cal sil, yes. The Forno Bravo stuff is 2" thick, but if you're sourcing elsewhere, you'll find every thickness and density under the sun, IME. Likewise the insulating blanket.

It is not mortared to the slab, just set on top on top, or at least that seems to be what most folks here do, myself included. The weight of the dome and hearth hold it in place.

In my case I bought the board thinking it would save me some time and hasssle vs. messing around trying to get a vermicrete base level (one is pretty sick of mixing concrete by this point!). NOT the case. Even though my hearth slab was perfectly flat and level, the boards I received were not at all of even thickness, so I still had to spend quite a bit of time trying to mitigate that unevenness with a topping of refractory sand. Very annoying. It seems like I've been one of the few here to have that experience with the board insulation so perhaps I just got some bad ones.

[dmun]

Is there a standard thickness I need and do I fire mortar it to the structural slab? I'm sure vermacrete is cheaper but I like the clean look of the board (ironic since it will probably not be seen by anyone but me.) Is it also called cal sil?

There are two kinds of refractory insulation boards. There are the mineral fiber boards like the insblock19 that I used, and I think FB is selling now, and there is the more high tech calcium silicate (cal-sil) boards. This may be overkill for our thousand degree f. working temperatures.

You really don't need to mortar the boards to the slab, or the bricks to the board, although you may need some leveling as splatgirl notes. Gravity always wins. In my case, I put my boards down in the wet concrete after I poured my slab, and that worked fine.

The standard thickness for either type of board in two inches.

[Fairview WFO]

Bartondad, I just used the perlitecrete, or was it vermicrete. Anyway, it's easy to mix and place, especially if you have a small mixer. I have had no problem with heat retention with 4" over 4" of structural. If you do want to price the board, or ceramic insulation I would suggest looking at McGillis Warehouse. Best price I found on 2" blanket. Just figure it out before you get started as they will make the floor height different.

good luck

[HENARRY JAYA]

It will cure FASTER because the heat of hydration cannot escape into the soil. In fact, you have to be careful if the walls are thicker than say 6 inches (it could crack from getting too hot).

How much faster will it cure? It depends on what type of concrete you use...but i'd say you could expect 20% faster cure if it's insulated completely on one side.

[samw334]

Thanks for the advice on getting to hot. Did not know that.