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Perlite hearth insulation thickness

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  • Perlite hearth insulation thickness


    I am in the middle of an oven build and I am about to lay some firebricks onto 4cm of perlite concrete insulation with about an inch of sand in between. Below the percrete is a concrete table top. Is the percrete too thin? Could the concrete table top crack under the heat?

    Thanks guys

  • #2
    What is the ratio of the pcrete? Typically 5 to 1 under the floor, 8 to 10 -1 on the dome. 4" of 5 to 1 is abt equal to 2" of CaSi board. Make sure the pcrete is really dry, several weeks, Because one you cap with the fire brick floor it takes a lot longer for moisture to evaporate, How thick is the concrete hearth and is it steel reinforced, these ovens can get really heavy really fast. Do you need an 1 inch of sand, typically only a need a thin layer mix of 50/50 sand/fireclay to level the floor.
    Last edited by UtahBeehiver; 06-24-2020, 05:23 PM. Reason: typo on thickness 5 to 1
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    • #3
      UtahBeehiver Hi,

      thanks for your response and advice.The percrete floor is 5:1 and I’m gathering from what you say is an adequate thickness. The concrete table top is reinforced, but it is quite thin - only 30mm thick. Most of the surface area of the concrete table top is supported by dense hollow blocks though so I think it should hold up fine. The one inch of sand was suggested by another source which said that you need a decent amount of it to allow levelling of fire bricks. What do you think? Shall I cut down on the amount of sand? Also what does the addition of fireclay do and is it absolutely necessary for creating a flat surface for the bricks?

      Thanks again!


      • #4
        I'm new to oven building but from my research I would have said 4cm is too thin for the Pcrete floor insulation? Unless you meant 4"?

        From what I've read 10cm is the normal recommended thickness. UtahBeehiver has vastly more knowledge than I so maybe he can confirm.

        Again from what I've read a 50/50 mix of sand and powdered clay is best to level the floor bricks and you should only use a thin covering just enough to get the floor bricks level.
        I'm not 100% sure as to why that mix but sand is neither a good insulator or conductor so maybe it's to mitigate that, it would be good if someone could explain the reasoning.

        If you increase the Pcrete layer I would say cut down on the amount of sand between the bricks and Pcrete.


        • #5
          I put 12cm of 5:1 perlcrete insulation on the hearth, that's more than 4 inches, and as Hattori-Hanzo commented 4cms would not be enough. Generally on the forum a thickness of 10cm/4" is recommended. Thickness of concrete slab under the insulation should equally also be around 4" or 10cm, reinforced with rebar. Putting a center pillar under the hearth is also a way to strengthen the structure for the oven, which as UtahBeehiver said can get real heavy real quick.
          My build:


          • #6
            Yes 4cm of vermicrete insulation is inadequate, 4” ok.
            The reason a 50/50 mix of sand clay is used as a leveller is that applied wet the bricks can be wriggled into it. The problem of wet application is that the dry insulation, particularly cal sil board, is that it sucks the moisture out so fast that the stuff drie becomes too stiff to sink the brick into it. Also the sand/clay dries really hard reducing the bricks ability to move under expansion and contraction. It also makes replacement very difficult if that’s needed down the track. A dry mix allows movement, easier placement and replacement. The clay fills tha spaces between sand grains and makes a better levelling medium than sand alone, although that does work too.
            Kindled with zeal and fired with passion.


            • #7
              My original response is incorrect that 2" pcrete equal 2" CaSi, it should of said 4" pcrete equals abt 2" CaSI, sorry for the confusion. Fat Fingers
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              • #8
                Hi guys,

                thank you all for your responses! You’ve all convinced me to fix the insulation problem. What I did was put a layer of ceramic fibre blanket that I had left over from the dome underneath the perlite floor. The perlite floor was made in a mold before putting onto the concrete base in case anyone wonders how I moved it to put the blanket underneath. That should fix the problem of heat affecting the concrete... right? I have pictures but am unable to upload as too big. Anyone know how I can upload from my iphone?


                • #9
                  I'm afraid this may not have been the best approach.
                  From what I've read ceramic fibre blanket is too weak to use under the hearth and will be compressed by the weight of the oven. I believe it reduces the insulating effect and can lead to problems with the build of the oven as the blanket starts to compress under the weight.

                  One of the pro's will be able to explain in more depth.

                  Try uploading your photos to a site like imgur and post the links provided in one of your posts.

                  Also, you say you have blanket left over from insulating your dome but the dome is not yet on the base?

                  What have you made the dome from, and how are you going to move it onto the base?
                  Last edited by Hattori-Hanzo; 06-25-2020, 12:33 PM.


                  • #10
                    Pics are limited to about 1.25 mg and about 5 per post. You will need to resize them to this size to post.

                    Ceramic Blanket is too soft to be under the floor. You need CaSi board that has min 75 psi compression strength at 5% compression.
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