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Inset FB Board in Cement

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  • Inset FB Board in Cement

    I just received my FB Girando70 and had a question about the ceramic board. I have the stand already build and the oven is going to be supported by 3 inches of structural cement. I planned on then placing the 2 inches of ceramic board directly on top of the 3 inch cement, and then pouring additional cement around the fiberboard so the cement is level with the fiberboard. Is there anything wrong with that ? Hope that makes sense thank you

  • #2
    Welcome to the Forum community. We do not recommend placing ceramic board directly on concrete. Most types of insulating boards and batting for a wfo readily absorb water. The current best practice (least expensive) is to drill or use tubing to form some 1/2" drain/weep holes through the concrete hearth and lay some sheets of porcelain/mosaic file on top, so the spaces between tiles "lead" toward the weep holes. The ceramic board is then set on top of this water barrier. Concrete can wick moisture to the board without this barrier/drain system. Without some sort of drain/weep system, the "embedding" of the board actually creates a trap for any water or moisture that finds its way in (and water is VERY good at that ).

    There is really no need to go to this embedding step. Your goal is to isolate the oven from potential moisture intrusion (while providing an escape route) and completely isolate the wfo with insulation. Lots of well documented builds on the Forum that illustrate and discuss the "why". Hope this helps a bit...but keep posting your questions...the more research and time you spend planning before the build, the happier you will be down the road.
    Mike Stansbury - The Traveling Loafer
    Roseburg, Oregon

    FB Forum: The Dragonfly Den build thread
    Available only if you're logged in = FB Photo Albums-Select media tab on profile
    Blog: http://thetravelingloafer.blogspot.com/

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    • #3
      Some coarse aggregate that has some decent air spaces throughout the layer would also work equally well but if a vermicrete slab is cast over it some shade cloth or weed may should be placed over it to prevent those spaces being filled. Same goes if tiles are used.
      Kindled with zeal and fired with passion.

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      • #4
        Thank you for the info. Another question, I was debating on whether to have one or two layers of fiberboard (I have the 2 inch thick board). Under the board is 3.5 inches of regular concrete. Also, I can place the bricks directly into of the FB Board, right ? I plan to add a very light layer of the mortar provided (dry) to ensure a perfectly flat surface.

        I'm not looking to have massive heat retention though
        Last edited by mandrews0809; 11-06-2020, 03:33 PM.

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        • #5
          Two inches of ceramic board underneath your oven is enough. Many people who want much longer heat retention may choose to add more than 2", but for normal use, one layer of the two inch stuff is enough. That said, the mantra here has always been, more insulation is better...you can't add it underneath once the oven is built. I have an equivalent of 2" of FB board and am very happy with the amount of baking I can do from a single firing. (Also, don't forget the water barrier/escape system beneath the board...it's very important.)

          Most people simply use a thin layer of sand with a little clay mixed in for the leveling on top of the board. Use a grooved tile trowel on your sand/clay layer and then lay the bricks directly on top, tapping them in level. You don't need to use dry mortar and in fact it will also pull moisture from the air and "clump & stick". Not that it's a problem, but if you ever do want to replace one of the bricks if it is damaged...the mortar may make it more difficult to replace than it needs to be. If your boards are level, there is no need to lay sand/clay on top...simply lay your bricks down on the board.
          Mike Stansbury - The Traveling Loafer
          Roseburg, Oregon

          FB Forum: The Dragonfly Den build thread
          Available only if you're logged in = FB Photo Albums-Select media tab on profile
          Blog: http://thetravelingloafer.blogspot.com/

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          • #6
            I agree. For your use, one layer is fine and I wouldn't embed it at all. It needs to stay dry, both during and after the build.
            My Build:
            http://www.fornobravo.com/forum/f8/s...ina-20363.html

            "Believe that you can and you're halfway there".

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            • #7
              Thanks, only used one layer. However I did inset it kind of out of necessity/aesthetics but I took some moisture precautions.. painted the edges w high heat paint , put weep holes under the board, raised the board on ceramic tiles (backsplash type) so water could channel to the weep holes, and put foil on the edge so the board didn't wick away cement moisture.

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              • #8
                I was interested in SableSprings's comment about placing a waterproof barrier on top of concrete then adding insulation board. I have a 6" reinforced concrete hearth and I'm about to start cutting bricks. I intent placing 50mm approx 2" of CaSi board on top of the concrete with weep holes. I intend laying the oven floor on the brick edge for additional insulation ie 4.5" will be the brick height.

                Given that this build is in Ireland I am concerned about having sufficient insulation.

                Questions
                1. Is this sufficient insulation?
                2. Should I have 2" of cement/vermiculite and place the insulation board on top of that?
                3. Does anyone have photos of the tile layer suggested to go between the concrete and insulation as a water barrier. I have some unused quarry stone tiles that were originally intended to cover an external step would they be suitable?

                Thanks in anticipation of much needed advice!

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                • #9
                  I've attached a link to a build that documents the use of a "waterproof" layer between the concrete and the oven insulation board quite nicely. Posts 8-18 discuss the options & post 19 has a great illustration of use of mosaic tiles as a moisture barrier & how the gaps provide a gateway to the weep holes. Fifty mm (two inches) of CaSi board is considered minimum (and completely adequate ) base insulation. Many builders have chosen to add an additional layer of insulation below the CaSi board and often the 5:1 perlcrete or vermicrete is used because it's very economical. The insulation value of 5:1 insulating concrete calls for about double the thickness of CaSi board...so 100 mm of insulating concrete is approximately equal to 50 mm of CaSi. Since it's relatively inexpensive, choosing to have 100 mm of insulating concrete on top of your moisture barrier (separating the concrete & the oven insulation), then adding your CaSi board which is just underneath the cooking floor bricks makes for an excellently insulated oven. (Don't forget that you want your entire oven insulated from the concrete hearth foundation...so the insulation extends underneath the dome bricks as well...).

                  I don't know if your quarry stone will wick moisture or not...if you decide they will work, (and you decide to pour insulating concrete over them) you'll want to put a layer of garden cloth over the quarry tiles to prevent the mix from clogging the channels to your weep holes. That's why I like to see the mosaic tile sheets laid upside down on the hearth...keeps the channels clear.

                  https://community.fornobravo.com/for...andpoint-idaho

                  Hope this helps...and remember that you should be able to find discontinued mosaic tile sheet patterns or left-over sheet squares at bargain prices.
                  Mike Stansbury - The Traveling Loafer
                  Roseburg, Oregon

                  FB Forum: The Dragonfly Den build thread
                  Available only if you're logged in = FB Photo Albums-Select media tab on profile
                  Blog: http://thetravelingloafer.blogspot.com/

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