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Typically when you say slab you referring to the bottom-most portion.... do you mean hearth? Perhaps some pics to help understand the problem.
Did you start the dome already and that's why you don't think you can? Too many variables, not enough info.
Foam board is no good (it will melt), but the fancy iso board sold by fornobravo is great. What I did was pour 3" of concrete followed by 3" of vermiculite concrete. You can do this as an overpour on your existing slab- it's not as good an insulator as the hi tech stuff but it's way cheaper. It's also soft and crumbly, so don't leave any exposed in the finished structure! It is strong/dense enough to support the hearth, I just wouldn't leave any exposed to the elements.
thanks for info sorry i wasnt more specific when i first posted building 36 inch dome i ended up putting 3/8 hi denesity insulation with 1/2 inch cement board on top and then laid the hearth in fire clay high heat refactory mortor hope this works because i laid my first course and two rows above on my way im sure ill be back for help again talk to ya soon
3/8" high density insulation? First and foremost, 3/8" is just about worthless, not quite, but just about. Second, you say high density, don't all manufacturers call all of the different types of insulation "high density"? What is the composition? You need a REFRACTORY product such as ceramic fiber board (minimum of 2 ") or vermiculite concrete or perlite concrete (minimum of 4" of either of these). Most of the products calling themselves "high density" are either styrofoam or fiberglass based and are not rated for temps anywhere near those achieved in our ovens.
As for the cement board on top...????????????
Please, go to the Forno Bravo store and purchase (FREE!!!) the Pompeii oven plans, they will set you straight and get you to building an oven that will actually perform, not just lead to massive deforestation due to the huge amounts of wood you will be burning trying to achieve and maintain pizza temps.
I would also recommend spending AT LEAST a few hours checking out the many incredible oven builds on this forum. A lot of members have gone to great lengths to document their builds for others to learn from and drool over.
You should STOP whatever it is you're doing because it's all new ground. This insulation you're using... foil-face? This sounds more like home-center house insulation board than high-heat. It's only two rows; you should take it apart, clean off the bricks so you can re-use them, then re-do the hearth deck with the proper insulation. It does seem from your comments you are not following the proven, time-tested version from the plans.
There are a lot of folks on this board willing to assist you. As mentioned by several - FREEZE. Do not do more work. If you continue down this course, it will be questionable if your oven will ever work properly. You have too many hours invested at this point to waste it.
The good thing is that you recognized it. Feel free to message anyone and we will happily help you work through the error. We all had issues going along, and I can definitely say that several folks messaged me back to questions.
What is the temperature rating? YOU CANNOT use any household insulation, nothing that you purchased at a local big box or hardware store.
We are talking about temps at or above 1000F for an extended period of time. IT WILL MELT AND OR BURN.
You need refractory (extreme high heat materials).