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Vermiculite depth - sturdy??? - Forno Bravo Forum: The Wood-Fired Oven Community



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Vermiculite depth - sturdy???

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  • Vermiculite depth - sturdy???

    Hi All

    I unfortunatley started my oven before I found this forum and have evolved it to suit a pompeii oven..

    I have got the bricks, vermiculite etc to start the build and am finding this forum a great help..

    My Main base is very sturdy and is filled with breeze blocks to support / spread weight of the oven. I have very deep foundations so am not worried about overall weight of the oven

    Main Base

    MY QUESTION - Photo Attatched.
    Ive been out tonight laying my paving slabs ontop of the base to create a level for my vermiculite insulation slab and also increase the area that the oven sits on..this vermiculite base will be as deep / to the level of the bricks that will edge it...

    1. Is the vermiculite sturdy to lay as one piece with no steel / reinforcement ???
    2. would it crack / move in winter with frost ( I live in England )

    I see from other posts people have raised concerns about how brittle the slab is???


    Attached Files

  • #2
    Re: Vermiculite depth - sturdy???

    Hi Ross,
    The vermiculite under my Pompeii is around the same depth as you are planning, ie around the 3" mark. As mine is built into an excavated foundation on top of a retaining wall:


    I cannot check for heat loss due to it's location but it holds the heat extremely well which would indicate good thermal insulation.
    My dome has a 1" superwool layer and 3 x 1" layers of vermiculite cement topped with a 1/2" stucco layer (a base for the final tile layer when done) and no heat at all comes through even with 500°C oven temperatures.
    I mixed the vermiculite as my posting above ie.1 part portland cement:5 parts vermiculite. You might consider putting your brick edging on edge to increase the insulation layer to around 4 1/2". Are you planning on running your hearth bricks out and under yourfirst course of dome bricks, or running a soldier course (first course of bricks standing on their ends), and then cutting your hearth bricks to fit?
    This would determine how strong you would need your insulation layer to be.
    As far as being brittle, I would make sure that the insulation layer was not affected by rain or moisture especially if the weather is cold or especially freezing, as it would be damaged with the freezing expansion factors that exist.
    Hope this helps you decide.


    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


    • #3
      Re: Vermiculite depth - sturdy???

      Your pavers should be enough support for the vermiculite layer if they are supported from underneath. You don't want to put re-bar or steel bars in the vermiculite layer because they would conduct heat out rather than hold it in.
      My geodesic oven project: part 1, part 2


      • #4
        Re: Vermiculite depth - sturdy???

        Thanks Guys

        Again sound advice - That makes sence about the rebars.

        So as a rule the vemiculite slab is layed as one piece and is strong enough to take the oven weight and outdoor temp ? - I guess at the end of the build this would be finally covered with somthing more decrotive..Ill make sure I lay it on a nice fine weekend and the mix s well done!

        Thanks for the info about insulation - Ive been thinking about the depth of the layers and how much it will add onto the overall dome size..Ive seen to many posts on here that havent allowed for enough.

        Im not sure about the floor yet - Just for ease I think the oven floor will run out under the first course which will be bricks stood on end. Once the therma blanket and insulation layers are over that it can be covered to be a smooth continuous dome...

        Any more advice is most welcome

        Thanks again