hearth slab

[David Ryle]

Hopefully someone can answer this, I'm building a bread baking oven that will be in daily use(Sunday excepted), a cross-section of the hearth will be as follows: 4" firebrick.4" thermal concrete reinforced 8" vermiculite mortar. Is this enough insulation and secondly should the vermiculite mortar layer given its thickness also be reinforced with a rebar matrix.

David

[Blunt Tool]

Re: hearth slab

Hi David,

I can answer this question! Yes I think it is enough insulation under the hearth.

My build is a bit of a hybrid, but my hearth is similar:
115 mm of firebrick (about 4 1/2")
100mm (4") reinforced concrete (as a "floating" slab, so separated from the surrounding base structure)
150mm (6") Vermicrete insulation underneath.

A link to my build: http://www.fornobravo.com/forum/f21/...ula-20323.html

Admittedly I have only cooked in my oven twice, but on both occasions I had the internal temp up around 500C at the dome and under the Vermicrete it was mildly warm at best (OK to leave my hand there as long as I liked). My dome has no extra thermal mass but is well insulated, and the next day after firing the internal temp is still around 250C.

I didn't put any reinforcing through the Vermicrete layer, and it didn't need it. I put a few large screws between the vermicrete and concrete layers, and I also put a few short pieces of rebar (3-4") from the sides of the Vermicrete into the blocks of the stand before I core filled them. I was a bit worried about the weight of the Vermicrete layer being held by those crews only. I'm still not sure if this was overkill but it didn't hurt. I think to put rebar all the way through the Vermicrete might detract from its insulating properties?

Having said all that, I think there are probably better ways to insulate under the hearth slab. On my next build () I think I will investigate polystyrene blocks (eg Foamex) for under the concrete hearth slab. Surely these would be easier to fit and better insulators?

Hope that helps.

Andrew