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Hearth Slab Insulation Question - Forno Bravo Forum: The Wood-Fired Oven Community



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Hearth Slab Insulation Question

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  • Hearth Slab Insulation Question

    A couple of other sites I have been to have the insulating section of the hearth as the bottom layer and not the top layer, as I see on Forno. What are the pros/cons of doing it either way. I would think it would not be very smooth if you have it as the top layer and as such, would be harder to lay the firebricks on level. What have you guys done?

    Thanks, Scott...
    Smuth's Build

  • #2
    Re: Hearth Slab Insulation Question

    I have not done anything but study so far. But in general, for any style oven, you need a heart slab to support the weight of the dome or barrel, which ever you decide you want. I am going dome. The slab is usually just concrete with rebar in it, to give it strenght. Between the supporting slab and the floor of the oven, you need some kind of insulation. This serves two purposes. It keeps the concrete hearth slab from getting real hot, which would weaken and possible damage the slab, and it keeps the heat that you build up in the floor of the oven from draining out into the slab, and keeps the heat where you want it, in the fire brick floor so you can cook with it.

    The dome or barrel is build with brick, fire brick is preffered, and that is covered on the outside with either more insulation, or possibly with cladding (more refractory mortar) and then insulation. You only add cladding, if you are doing a half brick dome or barrel, if you want more mass there to hold more heat, with the tradeoff that it will take longer to heat up and more fuel to heat up the more mass you have.

    In effect the final dome or barrel is insulated on the bottom and the top and the sides, everywhere except to the entrance to the oven, and most people build insulating doors for the entrance, so that you can keep the heat in for bread and other cooking.

    If you are seeing insulation under a slab, then it is an attempt to put more mass into the part that will be heated, which is basically everything that is inside the insulation layer.

    It will all depend on what you want to do with the oven, but unless you plan on running a bread business, I am convinced that a well insulated oven with the mass described in the Forno Bravo Pompeii oven plans, is just about perfect for home use, and will allow you to do pizza's obviously, but also do a load or two of bread, and still have enough heat stored to do some slow cook roasts and veggies, dry wood for future use, etc.



    • #3
      Re: Hearth Slab Insulation Question

      And just now I see that was your first post

      Welcome to the forum! Everything you need to know, and all the help you most likely need can be found here. And there are a ton of people here that have actually built ovens and can offer far better advice than I. Glad you found it



      • #4
        Re: Hearth Slab Insulation Question

        A couple of other sites I have been to have the insulating section of the hearth as the bottom layer and not the top layer, as I see on Forno.
        This was the earliest version of the Pompeii oven when I first got here in 2005: The insulating slab was below the support slab. It was thought that extra thermal mass was needed below the brick floor. It was learned through experience that the oven took forever to heat up built this way, consumed far too much wood, and never really got hot enough to get the proper char on the bottom of the crust. This was first modified to an "island" hearth where there was some additional thermal mass added below the floor surrounded by insulation, and then the floor went directly on the insulation as is done with modular ovens. This was the method that worked best, for pizza, from actual ovens in use.

        If you are insulating with vermiculite/perlite concrete, you don't get a very smooth surface. You can level the rough area under the floor with a dry mix of sand and fireclay, or mortar mix, lay your floor, and when it's as flat as you want it, you dampen the floor to firm up the powder layer underneath.

        What a lot of builders, including myself, did was to lay refractory board insulation down on the concrete slab, and lay the brick floor directly on this smooth surface. Not only is it smoother, it's thinner and a better insulator.

        Good luck with your project. Keep us posted.
        My geodesic oven project: part 1, part 2


        • #5
          Re: Hearth Slab Insulation Question

          Hi smuth10 and welcome aboard.
          Travis and Dmun have said it all, nothing else to add.
          The Forno Bravo Pompeii oven is very practical, easy to build, cheap to fire and works a treat.
          Don't skimp on your insulation, but also don't waste it either. No sense in spending money on something that is not serving any useful purpose and it isn't really chap for 'good insulation'.
          Download the plans, read them a couple of times and decide what you really want, then start collecting your materials. Any other queries, just throw them up and someone will give you good advice.

          Prevention is better than cure, - do it right the first time!

          The more I learn, the more I realise how little I know

          Neill’s Pompeiii #1
          Neill’s kitchen underway


          • #6
            Re: Hearth Slab Insulation Question

            I think Dmun had the answer I was looking for. When I started doing research for this project several months ago I saw the traditionalovens.com site and Rado puts a piece of cement board down first, then a 1 1\2" layer of vermiculite/perlite concrete where the firebrick is going to be and then he fills in the rest of the 6 1\2" deep form with a special heat resistant concrete. When I downloaded the Forno instructions I saw they do the bottom half regular concrete and then the vermiculite/perlite concrete on the top half. This made more sense to me considering the heat would be coming from the top and not the bottom. I figured the heat would pass through the top layer before it got to the vermiculite/perlite concrete mix, and weaken the slab.

            I like the idea of putting the vermiculite/perlite concrete mix as the top layer and then just filling in any small gaps with the fireclay mixture, as it needs to go on there before you lay the firebrick down anyway.

            Thanks for everyone's input. This site is awesome!

            Smuth's Build