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How much wider should the hearth insulation and oven floor be in relation to the dome

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  • How much wider should the hearth insulation and oven floor be in relation to the dome

    I have spent the summer cutting my stock of fire brick and I am now turning my attention to placing my oven floor on the hearth.


    Having had previous useful comments and suggestions I have considered my next steps and have settled of the the following - all comments and suggestions welcome
    1. I intend placing a layer of glazed cheap ceramic tiles covering the footprint of my oven floor. These will have a drainage gap between them and weep holes through to the wood store below.
    2. I intend placing 100mm (approx 4”) of Ceramic fibre board on top of the ceramic tiles. The weep holes to match those below.
    My first question is in relation to the footprint of the ceramic tiles / insulation on which the oven floor will sit. Should this be...
    1. Trimmed to the size of the exterior walls of the dome?
    2. Extended to cover an area beyond the width of the exterior walls of the dome plus the width of dome insulation and final rendering
    What would the best practice be to enable the dome / floor to expand At this time I plan to go with option b leaving 2-3” of the extended oven floor beyond the finished rendering.

    I intend placing fire brick in herring bone pattern but placed on their edge which will give a fire brick height of 4.5” for the oven floor on top of the 100mm ceramic fibre board. Although using more fire brick my thinking is that this will provide further insulation and this build is in Ireland.

    Final question, I haven’t decided but I will either tile the remaining hearth or lay polished granite to butt against the protruding oven floor. I would see the granite being about 1-2” below the level of the oven floor. Do I need to leave a gap for floor expansion against the granite?

    Any comments of approval or suggested improvements are welcome.

  • #2
    I may be reading this incorrectly, but you don't need drainage holes in your 4" thickness of under-floor board insulation. Your under-floor insulation should be continuous in my opinion. No drainage holes in the insulation itself, and with any insulation board seams offset from one layer to another if indeed you are doing multiple layers of board insulation to get the 4" total thickness. If one of the pro's has another idea on that, I'd be happy hear what they have to say.

    I'd venture that the drainage tiles should have the same footprint as your insulation footprint. While I did not use tiles under the insulation, my understanding is that the goal is to isolate any and all insulation from the slab to prevent slab moisture from wicking up in to the insulation. And to allow any moisture that does get in to the insulation a drainage gap between the insulation and the slab.

    With my dome, I extended the board insulation out past the footprint of the brick dome. I felt that was a good choice for two reasons:
    1) The dome bricks were not right on the edge of the 4" thickness of insulation. I felt that would minimize the possibility of the weight of the dome bricks compressing the 4" thickness of board insulation right at the edge of the board.
    2) When I insulated my dome, the first 1" thickness of blanket insulation landed on top of the floor board insulation. The following three layers of 1" blanket insulation covered the sides of the floor insulation, extending down to the slab. With the first layer landing on top and the following layers extending all the way to the slab, the seam between the blanket and board was not a linear path.

    I'll try to add a couple of photos. They help more than words.

    Best, Mongo




    Mongo

    My Build: Mongo's 42" CT Stone Dome Build

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    • #3
      Mongo

      Thanks for your prompt reply.

      I understand about no holes in the insulation, I expect this is because most moisture finds its way from outside the dome. Given the size of Ceramic board I will be using it will be two layers and I understand about staggering the seams,

      Regarding the footprint of the insulation / oven floor, I too am concerned about building the dome sitting right on the edge of the insulation. If I extend the insulation and oven brick floor I can square the edges of the floor more easily.

      Also if I understand correctly the width of insulation around the dome enables the oven to expand and contract without cracking the external render.

      Could I ask what the red material is around the edges on your photo, is this a waterproof compound?

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      • #4
        dogshark58, Read through Mongo's build. The answer is there and the overall value is worth every minute it will take you to review. Giovanni

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        • #5
          Thanks Giovanni, will do

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          • #6
            Originally posted by dogshark58 View Post
            Mongo

            Also if I understand correctly the width of insulation around the dome enables the oven to expand and contract without cracking the external render.

            Could I ask what the red material is around the edges on your photo, is this a waterproof compound?
            1) Correct. My dome for example sits on top of 4" of board insulation. The completed brick dome was then covered with 4" of blanket insulation, and the blanket insulation was covered with expanded lathe and a hard render shell. I can fire my oven and because the 4 inch thickness of cushy blanket insulation isolates the brick dome from the hard render shell, the brick dome can move as needed and not stress the hard shell.

            2) The red stuff is RedGard. A liquid waterproofing membrane. I put it on my slab to prevent moisture in my slab from wicking up in to my board and blanket insulation. I had it on hand, so I used it. RedGard is NOT vapor permeable, so I would NOT use it over the render shell that covers the insulated dome. I DID waterproof my render shell but with Thoroseal. Thoroseal is a cement-based waterproofing that is waterproof (liquid waterproof) but it is also water vapor permeable. Water vapor can pass through it, but not liquid water.

            In an idle oven, one sitting over a wet winter for example, the oven structure may absorb moisture simply due to latent moisture in the atmosphere. When I fire my oven after a period of non-use, the fire will try to drive that moisture out of the oven. While the moisture can NOT pass through the RedGard (vapor impermeable) it CAN pass through the Thoroseal (vapor permeable) and then in to free air. I also have a vent at the apex of my dome. Moisture vapor driven out of the dome structure by the heat from a fire does exit the oven structure through that vent as well.

            Try to keep water out with proper detailing of your build. But plan on it getting in, thus provide an appropriate escape path. Just in case!
            Mongo

            My Build: Mongo's 42" CT Stone Dome Build

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