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Vermiculite on top or below?!!- HELP! - Forno Bravo Forum: The Wood-Fired Oven Community



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Vermiculite on top or below?!!- HELP!

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  • Vermiculite on top or below?!!- HELP!


    I have been researching how to build my brick oven and am getting seriously confused about the conflicting advice I find online.

    Should the vermiculite layer go on top of the concrete hearth or below it? I have seen both recommended and am wondering which makes more sense.

    If I have concrete directly under the firebrick (oven floor) will it not heat up to the point of risking cracking? (Since regular concrete is not rated for high heat) Also, if I have to heat up that much more mass, will it not take a long time and keep pulling heat away from the oven part?

    On the other hand, some think I need to rely on the mass of concrete as thermal mass for the oven to stay hot longer, and therefore the vermiculite should be below the concrete to insulate it against the outside.

    Is there one "correct" answer?

    Please help, I was hoping to pour next weekend and would like to start on building the form - but now I am stumped.

    Any expert advice is greatly appreciated!

    P.S. I am also wondering if a 3" hearth would not be strong enough instead of the 4" I often see?

    P.S. I am in a very cold climate, if that makes a difference...our winters get to 0 degrees Fahrenheit and below....

  • #2
    Re: Vermiculite on top or below?!!- HELP!

    There is a correct answer: The concrete support slab should go under the insulation. The insulate-under-slab method has been tried and rejected because of thermal bridging: the support slab doesn't just add to the thermal mass (which isn't needed anyway) it wicks away heat from your floor, and you will have extreme difficulty getting your oven floor to pizza heat, no matter how long you pre-heat or how much wood you burn.

    A three inch support slab? I wouldn't. What's the advantage? Concrete is cheap, and a thinner slab risks getting your rebar too close to the surface, and getting corrosion damage.
    My geodesic oven project: part 1, part 2


    • #3
      Re: Vermiculite on top or below?!!- HELP!

      To get the terminology straight, from top to bottom;

      - Hearth. This is the surface you cook on. Generally refractory brick or other refractory material (never Portland based concrete). Typically 2 1/2 inches thick for a pizza oven but may be made thicker if you want more thermal mass.

      - Insulation. Many use a 4 inch vermiculite cement mix (6:1). Other insulation material may be used.

      - Suspended Structural Slab. 4 inches of reinforced concrete (I agree with Dmun -3 inches is too thin)
      Last edited by Neil2; 08-09-2010, 03:05 PM.


      • #4
        Re: Vermiculite on top or below?!!- HELP!

        Hi dmun and Neil2!

        Thank you both for the straightforward and prompt information! I guess I had hoped for a thinner slab for a couple of reasons: aestetics, and total height (I'm short, and I fear that after adding the slab, then vermiculite, then firebrick I will have to raise those pizza peels to an uncomfortable height. But really, an inch won't make that much of a difference...)

        Also, I wasn't sure why that much thickness was structurally necessary since I thought most of the weight of the barrel vault would be transferred to the concrete block and concrete base below.(Is that not true?) I do want the oven to be solid but I didn't want to go way overboard if it wasn't necessary.

        Thank you also for the clarification of the terminology. It is only after reading these forum threads that I started calling the concrete slab the hearth, since that seems to follow the most common usage. (For instance most of the photos in the "hearth and stand" category do not include the actual oven floor and pictures of forms are described as "getting ready to pour the hearth". ) I am not trying to argue against your use of the terms, they make good sense to me, I just wish there was a bit more agreement on this...

        Anyways, thank you again for replying so quickly, now I can start building the form!



        • #5
          Re: Vermiculite on top or below?!!- HELP!

          "total height "


          The "best" height for the oven may be higher than you suppose. Remember that you want to see into the oven and work the peel without a lot of bending. My wife is 5'4" and says the hearth height of our oven at 44 inches is just right.

          "I wasn't sure why that much thickness was structurally necessary"

          Structurally, three inches will work, but as Dmun has noted it is difficult to get sufficient concrete cover around the reinforcing steel on such a thin slab.
          Last edited by Neil2; 08-10-2010, 08:42 AM.


          • #6
            Re: Vermiculite on top or below?!!- HELP!

            I thought most of the weight of the barrel vault would be transferred to the concrete block and concrete base below.(Is that not true?)
            That is true. Still, there are lots of issues with sub-4" slabs. If you want to knock off a couple of inches of height, substitute refractory board (2") for vermiculite concrete (4")

            And about that barrel vault thing? You may want to do some more reading before committing to that:


            The founder of this site built two barrel vault ovens before giving up on them in frustration. They're less than ideal for pizza.
            My geodesic oven project: part 1, part 2


            • #7
              Re: Vermiculite on top or below?!!- HELP!

              Thank you, once again to both Neil2 and Dmun!

              This is all great information, thanks! I will just stick with the 4" slab.

              As far as the barrel vault is concerned, I may still change that to a dome, I know there is a discussion about it and I haven't looked into it that much yet. (Won't get to the next part until late September). I should point out, however, that I am equally interested in pizza and baking bread (as a German I am dreaming of nice loaves of sourdough bread, baked traditionally in a brick oven), so it should function o.k. for both.

              Anyways, thank you two sooo much, this has really been a great help to me!