sub-floor oven slab
So I'm working from the Tom Jaine book Building a Wood-Fired Oven and the plan calls simply for poured concrete, a layer of sand, and then the firebrick. I'm using 3" thick firebrick. I've seen some people recommend additives to the concrete (such as perlite) to give it some additional inulating quality. Is this necessary or overkill? It seems that between the sand and the firebrick, not much heat will permeate down through the slab as lost heat. Isn't the bigger concern to insulate the roof? The plans show 18" thickness from the interior wall surface to the exterior.
Incidentally, this oven is being build inside my house, which is a hundred years old and rather cool in the winter. A little heat loss to aid the house might not be a bad thing?