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This is a sticky posting that describes the logic behind the Forno Bravo hearth design.
I think of the hearth and cooking floor assembly as serving three functions:
1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.
2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.
3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).
That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.
I think of the hearth and cooking floor assembly as serving three functions:
1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.
2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.
3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).
That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.
, but for sake of discussion this Easter morning...would you then have a full layer of the Super Isol beneath the slug and surrounding insulation?
and I would recommend it. What I suggest is for those of you that get the "spin buggy" the little tow-able spinning mixer, the day before get a 94# bag of Portland and have it ready, after you get the mixer home spin it up (make it go faster) and add the whole bag a shovel full at a time (adjust water content if necessary). 
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