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Not very well, if by 'on concrete' you mean the structural slab. For a pizza oven, firebrick directly on the structural slab is *the* formula for insufficient floor heat and/or excessive fuel consumption. You don't have to use vermicrete, but please put some form of high-temperature insulation (cal-sil, foamglas, Insblok 19) under the floor. If you're looking to build a high-mass floor for bread baking, you'll still want to isolate the heated mass from the structure of the base.
Since this is your second thread on this, I am guessing you are still getting pushback from your contractor on insulation?
You'd be best off with a 2 or more inch layer of Ceramic Fiber insulation and one layer of firebrick. But if you already have the 2 layers of FB in and are committed on height to something that comes in at 2.5 inches - you might try one layer of Insulated Firebrick (IFB) and one of firebrick. I think I have seen ovens built this way (Karangi Dude?).
IFB won't work as well as other insulation - but it is probably the most efficient solution if you have to swap something out and at least it give you some insulation. Not sure if it is better or worse than Vermicrete. Probably better, as I don't think 2.5 inches of Vemicrete is enough to be very useful but that is a guess.
but I've read enough stories here to confidently state that you absolutely don't want an oven floor with no insulation - no matter how thick the firebrick.
It will not work, period. You want an envelope of thermal mass surrounded by insulation. Anything else is a waste of time and wood. Fire your mason. A good mason does not equal a good oven builder. I spend more time than I should telling consumers that the very expensive oven they had built by the low bidder is worthless (can you imagine spending 5 grand on a worthless oven?). Use the FB plans. Again, use the FB plans. One more time: Use the FB plans.
It's really not rocket science. Masonry is a great heat sink. Said another way, it has the ability to absorb very significant amounts of energy.
People confuse the ability of materials to act as a heatsink as being the same as being a good insulator. That is not the case. In some ways the final results are similar, but how you get there is very different.
For example, if you build a home with concrete walls that are four feet thick, that home will generally stay cool during the heat of the day, and will stay relatively warm during the cold of night. But that does not mean that the concrete (or masonry) is acting as an insulator. What is happening is that all that masonry has a great deal of thermal mass. So at night (when it is cold outside) the masonry is radiating heat collected during the day into the structure and keeping it relatively warm...and during the day (when it is hot outside) the masonry is able to absorb a great deal of heat before the inside of the structure starts to get real warm. And before the masonry gets warm enough to start radiating a lot of heat into the interior, the sun goes down and it gets cold outside again...which tends to establish a relative equilibrium and the temps of the interior don't fluctuate all that much. As a result, the inside temp of the structure remains relatively cool (compared to the outside) during the day...and relatively warm (compared to the outside) during the night.
But that doesn't mean the structure is well insulated. The reality is that an all- masonry structure (be it a home or oven) is not well insulated at all. It just happens to have lots of ability to soak up energy...and then radiate it back later...unless that energy has flowed somewhere else.
So if you think of this heatsink property in the context of an oven with no insulation between the hearth and the masonry stand, the result becomes obvious. The heat in the oven is "soaked up" by the bricks of the hearth floor...and then that heat is soaked up by the masonry stand...and then that heat is soaked up by the concrete slab the stand is sitting on...etc, etc. You are left with a situation where the heat from the wood you are burning in your oven is constantly migrating away from the oven...because all of the materials surrounding your oven are really good at absorbing heat. In other words, the material is a heat sink.
In order to have an efficient oven, you must encapsulate the oven in a layer of material which is no good at absorbing and transfering heat energy....i.e. INSULATION. You need to stop that heat from migrating anywhere so that the bricks in your oven get hotter and hotter and hotter. Materials which are good insulators are the exact opposite of masonry such as firebricks. By defintion, they are terrible at absorbing energy. They act as a wall preventing the transfer of heat energy....Just like the rubber insulation around an electrical wire acts as a wall preventing electrical energy from being transferred from that wire.
So to sum up...it makes no difference whether you have one layer of firebricks in your floor...or ten layers. Because firebricks are not insulation. Firebricks are like batteries. They are designed to store...and then release...energy. But if there is no insulation around the firebricks, the energy just flows right out of them to the nearest area which is cooler than the hearth. In other words, the energy flows away from you hearth into your stand...and then to the earth.
I hope that clears it up for you on a conceptual level.