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Insulating the Hearth - Forno Bravo Forum: The Wood-Fired Oven Community


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Insulating the Hearth

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  • Insulating the Hearth

    Hi all,

    I live in the UK, and hope to build a wood-fired oven soon using the Ephrem Four a Bois 2061.

    Has anyone on here got any experience of this particular dome?

    My main concern is what type of insulation to use underneath the dome; it will probably be mounted on a concrete paving slab on a brick plinth. I've seen various things suggested, from sand, to mortar, to vermiculite board. I am worried that if I use mortar, then something might crack if the dome and slab expand at different rates. I appreciate the need to prevent heat-loss through the base, but I also read that the base of the dome should run a little cooler anyhow. What if I used nothing underneath the dome? Would the heat cause the slab to explode? I figure that if I use sand, the constant thermal cycling might cause the dome to sink downwards over time as it expands and contracts. Also, if I just rested the dome on insulating board, what would stop it moving around inside the outer enclosure over a period of time? To me, the best thing might be heat-proof silicone, which would seem to be good for holding the dome in place whilst allowing a certain amount of expansion/contraction.

    Any advice here would be gratefully received!


  • #2
    Re: Insulating the Hearth

    Down load the FB plans. They have a very detailed explanation of insulating the floor.


    • #3
      Re: Insulating the Hearth

      Hi, Rich,

      You MUST insulate under your floor. Sand is not an insulator. Silicone is not an insulator. Mortar is not an insulator. Vermiculite board is an insulator, but a very old-fashioned one. If you want to go the vermiculite route, you may want to mix your own vermiculite concrete. There are better refractory insulation boards, of Cal-sil, and mineral fiber. The advantage of these is that they are dead flat, and you can get away with only two inches of insulation, instead of four for vermiculite or perlite.

      Your floor will run a little cooler than the dome, simply because heat rises. You need insulation top and bottom, if you don't want to burn forests of wood without getting to pizza temperatures.

      You don't need to worry about your oven shifting on your insulation board. These things are VERY heavy and will stay put. You do need to worry about your support under the oven. If you're putting it on a concrete paver, look into if it's properly steel reinforced, and if it has the minimum three and a half inch thickness needed to support this much weight.

      As a side note, If your oven vendor isn't telling you these simple things, you have to wonder what else they aren't telling you.
      My geodesic oven project: part 1, part 2


      • #4
        Re: Insulating the Hearth

        I agree with Dmun's post above.

        Also, typically concrete pavers are not designed to support much of a load. I would cast your own structural slab - then you know what you have.