Still in the Design Phase, (going on 4 years now)

[Gytaryst]

I'm curious if anyone has done something similar to this and if it's a good idea. I'm planning to do the first row with fire brick on end, cut 8.25" at the back and 7.5" at the front.
Then I will set up the indispensable tool for a 20" radius, 2.5" off the concrete slab.
Finally I will put 4" of insulation board in beneath 2.5" fire brick floor.
It gives me an oven floor with a 38.75" diameter and only 16" to the top of the ceiling.
That's the plan... so far... anyway

[UtahBeehiver]

There is a fatal flaw in the design, the dome walls sit directing the on concrete hearth. This will cause a heat sink on the oven, drawing away precious BTUs from the oven. The dome wall and floor must be isolated from the concrete hearth. Why do you want to raise the floor? Make the IT adjustable.

[Gytaryst]

I want the 40" floor space, but doing a dome gives me a 20" ceiling in the center. That seems like unnecessary space to heat up. (in my mind), making the ceiling 16" is less to heat up and puts more heat closer to the floor - which is where I want it.

Not sure how to insulate under the wall. It I extend 2" of the insulation board, is it strong enough to support that weight?

[Gytaryst]

I was also thinking I could pour the 4" slab at 36" height which is standard counter height, then the 6.5" rise inside puts the oven floor at 42.5". When/if I add counters on the sides I thought it would be easier to blend them in without having to deal with 3 different heights.

[Gytaryst]

And last. The fire brick themselves provide (some) insulation. Even if the bottom row touches the concrete, the fire is 6.5" above the concrete. The heat would have to radiate thru 6.5" of fire brick to reach the concrete slab. How much heat would actually be lost?

[david s]

Heat travels by three methods, conduction, convection and radiation. Firebrick floors are dense and conductive. Heat travels readily by conduction through them. The omission of underfloor insulation will lead to disappointing performance. Even a small break in dense material by a low density one will reduce markedly the heat transfer. The lower the density and thicker the material, the more effective the prevention of heat transfer.

[UtahBeehiver]

Ceramic fiber board insulation can easily support the weight of these ovens as long as the board specs show a min of 75 pounds per square inch compression strength or a 5 to 1 p/vcrete. So you just make the insulation foot print big enough and slightly larger the the OD of the oven. I am not sure lowering the ceiling height is worth the effort or gain efficiency. If you have not read the eplans from Forno Bravo, download them and study them(free). they are a good base line for WFO design. Also many documented builds in the Newbie Section under treasure archives.

[rsandler]

Just for further color on Utah's point about the ability of insulation to hold weight, a dome with a 4.5" thick walls and a 20" internal radius will cover about 600 square inches (the math is ((inner radius+4.5")^2 - (inner radius)^2)*pi). Even 2000 pounds of bricks, more than you'd need for a 40" dome, would only put about 5 psi on the insulation.

[Gytaryst]

Perfect. That's what I wasn't sure about. Heat radiating down thru the brick to the concrete makes sense. I just wasn't sure if insulation board under the brick would support the weight. Easy fix. Thanks guys.