Questions about hearth insulation

[chickade]

I am a bit confused about how to use vermiculite or perlite in conjunction with concrete for the hearth. In the Alan Scott plans, the vermiculite/perlite is BELOW the concrete in the floating hearth design and the entire hearth sits in a space within the base.

In the Pompeii oven, does the concrete go in a 4 inch space within the concrete block, and the vermiculite ABOVE the concrete, filling in the entire area above the base? Is the rebar used for the concrete or the vermiculite or both?

I'm wondering whether there are considerable advantages to the Alan Scott floating hearth, as it would be more difficult for me to put together with my limited skill set.

[Laku]

Re: Questions about hearth insulation

Above is the right way.

Alan Scott plans aren't that efficient unless you do huge batches of bread. And even then the concrete is getting heated more than it should since it's right below the bricks.

Vermiculite doesn't need any rebar, in fact it would make it less efficient and possibly crack it.

Vermiculite doesn't need to cover the whole concrete slab. Just enough to extend about 4" outside the ovens outer surface, and as far as any possible heat break you're going to build.

Just stick to the pompeii plan.

[Ken524]

Re: Questions about hearth insulation

Chickade,

Just to clarify...
The easiest way: pour a flat concrete slab for your hearth. Then form a 4" slab of vermiculite/portland cement on top of the concrete slab.

As Laku said, the 4" layer of vermiculite/cement should extend out a few inches from under the oven.

[chickade]

Re: Questions about hearth insulation

Thanks for your suggestions.
Here is my plan for the hearth for your comments.

Hardibacker for the bottom layer, supported by a frame that will be removed.

A 4 inch layer of rebar-reinforced concrete (I plan to use a 1 x 4 frame to make sure it is smooth and level, suspended around the hardibacker by 2 x 4's on the outside of the base.

A 2.5 inch layer of Foamglas, topped by a 3 inch layer of cal-sil board. I'm not sure of the best way to frame those layers together. Any thoughts?

Then the firebrick hearth.

Right now, I'm leaning toward a barrel vault oven, that is 36" x 48".

My base came out to be 62.5" wide and 86" long.

Thanks in advance for your wise advice. (Chris, my mason consultant is in the picture of the base).

[david s]

Re: Questions about hearth insulation

If you have not laid your slab yet, then when you do, it is a good plan to mound it up a little, say 10mm, in the middle. This prevents any water that may intrude, from pooling under the spongey vermicrete which sucks and stores moisture.Try to slope the slab slightly all the way to the perimeter so water does not sit at the base of the outer shell. (I'm assuming you are doing an igloo style, if you are doing an enclosure this design aspect is less important)

[brickie in oz]

Re: Questions about hearth insulation

A 2.5 inch layer of Foamglas, topped by a 3 inch layer of cal-sil board. I'm not sure of the best way to frame those layers together. Any thoughts?

Overkill, 2" of calsil by itself is ample.