Will the bottom of the hearth slab get hot?
If I build a 5 inch thick hearth slab on top of the stand, and then put 5 inches of perlcrete on top of that and then build my hearth bricks / oven floor on top of the perlcrete...will the botton of the 5 inch thick hearth slab get hot? At all?
What If I left a piece of plywood between the stand walls and this hearthslab as a slip plane? Would it burn up? Smolder?
the purpose of the insulation layer on top tof the hearth concrete slab is insulation. The idea is to keep just the floor of the oven hot and not lose the heat, through conduction, to the surrounding surfaces. A well insulated harth will yield very little heat transfer through to the concrete layer. It is probably prudent to remove the plywood support from the bottom of the structure. As you can see from the construction photos posted on this web site that there are temporary support pillars that are removed. Some folks have had problems of the concrete not releasing from the wood. There are ways around this, some are better than others.
I have seen ovens, and we have built ovens, where you use concrete board to form the bottom of the hearth, and you don't have to worry about dropping the plywood form. Concrete board is saggy, so you have to brace it more, but you don't have to remove the plywood. It might be six of one and half a dozen.
I would drop the plywood personally, even though it will not get hot undernearth the concrete bottom layer. The 5" vermiculite/vermiculite layer you are building will stop a majority of the heat.
I talked with one builder who did not use the vermiculite layer, and his 6" concrete slab got very hot on the bottom, and lost of lot of heat out of th oven that way. I experimented with a thin sand only layer above a concrete slab once, and the bottom of the concrete got really hot, really fast. Scary.
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