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Thickness of floor?

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  • Thickness of floor?

    I have the posts, and I am still a little confused.
    I built an insulated floor with 2" of perlite concrete and 2" of CalSil.
    is this enough insulation?
    I wanted to be careful before going to next step.

  • #2
    I would also like to know if using 2 insulaton materials for the floor is ok, bad or even better than only 2" calsil/FB boards or 4" vermiculite/perlite concrete.

    are you gonna cover the calsil with aluminum foil before pouring the concrete on it?
    Last edited by MakaDemian; 02-12-2024, 06:56 AM.


    • #3
      Others can weigh in to confirm, but here’s my understanding. What you describe has been done by a number of folks. Cal Sil on top of 5:1 Perlcrete mix. The 2” of Perlcrete is like adding another inch of CalSil so you’ll have a total of 3” which from what I’ve read is plenty. More may be better if your objective is to preserve saturated heat for cooking at lower temps after you’ve fired (bread, etc.). That said, it is a matter of diminishing returns so adding another 25% of insulation won’t equate to additional 25% of heat retention.

      For my build I’m planning on 4” of Perlcrete as an insulator which many people have done and my objective isn’t prioritizing heat retention for days.

      MakaDemian Again, others can confirm, but no foil over the CalSil. You can build on top of the CalSil but most place their floor on the CalSil first and build on top of that.

      Also, since the Perlcrete holds a lot of water and can attract water that may enter from the outside it’s important to have weep holes underneath in your counter so this moisture has a way of escaping. Moisture is the enemy. Less is better. Higher temps and less potential for cracks.

      Hope this helps.


      • #4
        Originally posted by MakaDemian View Post
        I would also like to know if using 2 insulaton materials for the floor is ok, bad or even better than only 2" calsil/FB boards or 4" vermiculite/perlite concrete.

        are you gonna cover the calsil with aluminum foil before pouring the concrete on it?
        Waterdog hit the important issue of keeping moisture away from insulation. Also want to stress the recommended layers: hearth (concrete supporting slab) with 4-5 weep holes, porcelain/glass (mosaic tile square sheets) laid backing side up, garden cloth (if not using tile squares), perl/vermicrete, thin layer of sand/clay for leveling cooking floor bricks, and finally your cooking floor & dome. Note this allows any moisture an alternate route to escape out through the slab & provides an effective barrier to any moisture wicking up from the concrete slab. You can place your dome base either around the cooking floor perimeter or on top of long as it is isolated from the slab by your insulation layer(s)..

        Aluminum foil used to be used to provide slip during firing/cool down brick movement and act as a moisture barrier. This method is no longer used or recommended as it can keep water from escaping the insulation and is just a waste of effort...underscored by thousands of successful modern WFO builds without foil.

        Hope this clarifies & explains the stratification layers of the "modern" WFO build.
        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


        • #5
          Wait, wait wait... too much new information for me in a couple of posts

          First of all i wanted to follow the line of the original question from Travelinman about using more than one insulation material under the floor. I think WaterDog tried to answer the question saying that more than a mix of 2" calsil and 2" perlcrete insulation would help if you only wanna keep heat during for a long period of time (for instance 2" FB board with 4" perl/vercrete or 4" calsil with 2" perl/vercrete).

          y objective isn’t prioritizing heat retention for days either so i assume that choosing from 2" calsil/FB Board or 4" perl/vermcrete is enough for floor insulation.

          Now, WaterDog said: "Cal Sil on top of 5:1 Perlcrete mix" and all the constructions i saw about it they made it the other way, i mean, first they put the calsil loose over the slab (i think you call it "hearth"), and then they framed the calsil to pour the perl/vermcrete on top of it. Thats why i asked about aluminum foil coz they covered the calsil with it to avoid moisture coming down from the vermcrete affecting the calsil.

          So the first question is: if you are gonna use both materials (calsil/FB board and perl/vermcrete together, one on top of the other. What would be the correct order ? FB board first loose on the hearth and then the perl/vermcrete on it or vice versa ?.

          WaterDog said: "You can build on top of the CalSil but most place their floor on the CalSil first and build on top of that."
          Sorry i couldnt understand this. You mean that the cooking refractory floor is supported on the calsil and not on the perl/vermcrete? can you mortar the fire bricks of the floor on the calsil ?

          The weep holes on the slab is new for me and i didnt know that. How can you drill 4 holes in a whole concrete slab avoiding drilling on the rebars that are inside of it ? Is this only necesary if you use perl/vermcrete ? what about if i only use 2" FB boards for floor insulation, is it necessary to do it too?

          SableSprings the "garden cloth" material is new for me, im very interested in these material, do you mean something like this:

          What would this garden cloth would do ? absorb moisture ? so you should put it on top of the slab before anything else ?

          SableSprings said: "thin layer of sand/clay for leveling cooking floor bricks, and finally your cooking floor & dome." the sand/clay mixture should be dry of mixed with water ? this option means that you are gonna keep the refractory brick floor loose, isnt it ? i was thinkng about mortaring (refractory mortar as glue) the floor bricks/tiles.

          Sorry for the extension of my post


          • #6
            Generally the higher quality insulation is best placed closest to the heat source which means cal sil on top of the vermicrete.
            To achieve the equivalent strength the vermicrete needs to be, at 5:1 ratio, and to achieve similar insulation value, needs to be double the thickness of the cal sil.
            So really the only reason to use vermicrete rather than cal sil is to save $. A second layer of calsil would be preferable, but cost you way more. Many commercial builds have only one layer of cal sil, probably for the cost reason. The stuff available to me is 40mm and I think two layers is easily enough. The downside is that the vermicrete requires drying to get rid of the free water, before building over it, or wait months of cooking fires to eventually get it dry.
            The idea of a weed mat over tiles is to keep the channels between the tiles from being blocked by the vermicrete sitting over them.

            The floor bricks don’t have to be laid loose, but doing so gives two advantages,
            1. It allows free expansion of individual bricks, because they will see a range of different temperatures and therefore thermal expansion.
            2. It makes removal and replacement easy if ever needed down the track.
            The dry 50/50 sand, powdered clay mix just allows levelling because sometimes the bricks are not of an even thickness.
            Last edited by david s; 02-12-2024, 09:38 PM.
            Kindled with zeal and fired with passion.


            • #7
              Thanks David, the idea of the mosaic tile square sheets upside down on the slab is too much for me, maybe too much bizarre for me (i saw a video of a guy doing it and was too much), i dont think im gonna do that but Sable Springs says that the weed math replace that idea, thats why i asked about it.
              On the other way, unfortunately i cannot find calcium silicate boards in my country (they only sell them to industries), but i can get ceramic fiber boards with the same specification of the forno bravo's spec sheet (depth 50mm density 300kg/19lbs, same Al2O3, Al2O3+SiO2, etc) from CCEWOOL, LUYANG or UNIFRAX brands.

              Anyways, I dont want to abuse talking about other insulation materials in this thread coz its not mine, so i'm gonna open a new thread about this issue because i have a lot to ask about it, specially after reading forno bravos's brick primer "The Right Firebrick for Your Brick Pizza Oven".



              • #8
                Placing the tiles upside down simply puts the net that they’re glued onto on the upper side, so that if you are placing a vermicrete mix onto it then it prevents the mix blocking the gaps between the tiles, allowing a free passage for moisture to escape towards the drain hole(s)
                If placing cal sil board over the tiles the gaps between them can be much wider.
                Kindled with zeal and fired with passion.