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What goes between concrete hearth and CaSi insulation

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  • What goes between concrete hearth and CaSi insulation

    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!

  • #2
    From my personal experience, 2” of ceramic board will still allow the base to be a heat sink, 3” works better but will still allow the concrete to warm up after a couple of hours but is fine generally speaking.
    4” of ceramic board (or calsi board) is what I normally use.
    Vermiculite will work but is time consuming to make and painfully slow to dry out but it does work ... 4” would be a bare minimum.
    On the last oven I built, I tried something different, I use 3” of ceramic board and laid two pieces of 50mm galvanised steel mesh under the board on top of each other so I had in effect a 20mm air gap.
    Of course it is impossible to know what is going on underneath but I think it is working well..

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    • #3
      Hello dogshark58, I've seen the pros on the Forum cite this build for pictures of tile between the hearth and the insulation board: https://community.fornobravo.com/for...andpoint-idaho

      My Build: 42" Corner Build in the Shadow of Mount Nittany

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      • #4
        Thanks for that. I was considering using 6" square quarry tiles with a gap between them. My weep holes would be in the gaps between the tiles.

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        • #5
          Dogshark58, my only concern is if the quarry tiles will wick water as does concrete. The reason I like to see ceramic/mosaic tiles used is that they are impervious to water transfer. If you put a quarry tile in a bucket of water for an hour...does it absorb water (gain weight)? If it does, then it would not be suitable to create the waterproofing layer on top of the concrete hearth.
          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 use an additive in the concrete mix which makes it waterproof when I cast the supporting slab. Another alternative is to coat the top of the slab with a sealer. These treatments should prevent or at least reduce moisture wicking up from the stand, but will not prevent refractory material from absorbing moisture from the air in periods of very high humidity. In that case neither will a roof. We live in the tropics and during the wet season I’ve experienced times when the oven has picked up moisture from very high humidity even though it hasn’t actually rained. Knowing how wet Ireland is, you may have similar problems with high humidity. A long gentle fire or two does the trick of restoring normal function.

            Also make sure no sprinklers wet the stand on a regular basis, this has proved to be a problem with water wicking up the stand.
            Last edited by david s; 05-24-2021, 12:19 PM.
            Kindled with zeal and fired with passion.

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