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Insulating Hearth Question

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  • Insulating Hearth Question

    Sorry I don't trust myself in pouring the insulating Hearth. One Idea I had was to buy th concrete backer board and have it sit on top of the stand. Then just leave that there (removing the under supporting structure)

    Does anyone see a problem with that?

  • #2
    insulating hearth

    I'm not going to pour an insulating hearth, either.

    I plan to use the ISO board sold by Fornobravo. I think it will save me time and give a nice level surface to lay my hearth bricks upon.

    My oven progress -


    • #3
      Originally posted by mrpbjnance
      Sorry I don't trust myself in pouring the insulating Hearth. One Idea I had was to buy th concrete backer board and have it sit on top of the stand. Then just leave that there (removing the under supporting structure)

      Does anyone see a problem with that?
      The assumption I am going to make is that on top of the block hearth stand you will first pour your reinforced concrete layer. On top of that is the insulation layer. If you have doubts about your ability to mix 6 parts vermiculite to 1 part cement (you mix the water and the cement first and get properly mixed then you add the vermiculite) then the best bet is to go with the ISO panel that Christo suggested.

      I am confused about using the concrete backer board (sold at Home Deport/Lowes for use as backing for bathroom walls). Backer Board does not have the structural strength to support a brick oven by itself. The assumption would be that you will use this as the very bottom of the hearth and pour your reinforced concrete on top of it. Remember that when you do this you will need extra support underneath the backer board to support the weight of the concrete until the concrete sets, at least a week. Unlike ? plywood which can carry a large load, backer board will flex and fail. Make sure that you have a method to get the concrete to bond with the backer board. On top of this you will put the ISO board.

      If you want to get totally away from pouring a concrete hearth and you know how to weld you could make a tubular frame and put in a 1/8 thick mild steel tray and then top that with the ISO board. major weight reduction. Of course you will need to do some engineering calcs to determine minimum wall thickness and determine the load bearing capacity.


      • #4
        insulating hearth

        Yes, i still plan to pour the cement insulating hearth I just thought If i used the cement backer board i could leave it ther and it would last a little beter than plywood (if I left lywood there)

        A second question but in the sam line is..i have not been able to find vermiculite.
        What if I just do a standard concrete hearth. What will I loose?



        • #5
          You need an insulating layer

          You can definitely use concrete board to "form" the bottom of the hearth, and then just leave it in place. Still, you need to support it underneath with some sort of form to keep it from sagging, or even breaking through. If you go that way, you can leave the concrete hearth and concrete board in place permanently, and don't have to worry about dropping the form down and removing it.

          On the second point, we highly recommend using an insulating layer under the oven. While it is true you can put an oven directly on a concrete slab, and it will function, for the small amount of energy and investment it takes, your oven will work much better with an under oven insulating layer.

          This has been mentioned in a number of postings, including today at:


          The basic problem is that heat basically pours through the concrete layer and vents out the bottom. I have test it personally, and have talked with different builders who did it wrong, and had to fix it. We are working with a pizzeria right now to stop heat escaping through their hearth floor.

          I would never discourage anyone from building a brick oven -- and any brick oven is better than no brick oven. But having said that, for a little more effort, you can make your oven a lot better.
          Pizza Ovens
          Outdoor Fireplaces


          • #6
            Insulating Hearth

            Thanks James...

            2 questions to follow up...

            If I pour the insulating hearth but use plywood underneath can I just lay the plywood on the bricks...with proper support underneath so it won't collapse then leave the plywood there? Will the heat be to much and possibly cause it to catch fire? ...your comment mentioning the heat pours through...

            If I poir a 6" hearth then on top of that do a mixture of fireclay and cement under the floor would that be ok... My problem is finding the vermiculite..
            I have checked home depot and lowes and only found small bags.


            • #7
              mrpbjnance, Where are you located?
              My Oven Thread:


              • #8
                Southern Calif about 45 East of Los Angeles


                • #9
                  Vermiculite is available in large bags from garden wholesalers and pool supply places, and of course from Forno Bravo
                  My geodesic oven project: part 1, part 2


                  • #10
                    The place I bought mine from was listed in Yahoo yellow pages under:

                    Food and Agriculture > Supplies and

                    Greenhouse Equipment & Supplies (whol) and

                    Nurseries - Plants Trees & Etc - Wholesale

                    I ended up using perlite, not vermiculte, and I found these listings for that near LA (here is the yahoo search link)

                    Paramount Perlite Co
                    (562) 633-1291 16233 Illinois Ave
                    Paramount, CA Map 12.2 mile from LA

                    Redco II
                    (818) 759-2255 Web Site 11831 Vose St
                    North Hollywood, CA Map 13.1 miles from LA

                    Hope that helps!
                    My Oven Thread:


                    • #11
                      Is Fireclay Good for hearth slab insulation

                      I have seen in other places using fireclay under the cooking surface.
                      Will that provide good insulation under the oven?

                      Thx and sorry to keep going at this.



                      • #12
                        No. Fireclay is not an insulator. You need vermiculite concrete or equivalent. If you skip this step your oven just won't heat up enough to make pizza, or stay hot enough for retained heat baking.
                        My geodesic oven project: part 1, part 2


                        • #13
                          Expanded clay

                          I think this might be one of those "lost in translation" moments. There is an Italian product called Expanded Clay (Argilla Espansa) that shows up in some of the oven installation guides. It's an expanded clay product, where the "expanded" part serves the same purpose as the "popping" that happens to vermiculite. It's the air holes that provide the insulating value.

                          I have never seen expanded clay here, haven't worked with it, and I don't know how efficient it is. I have to put that on my to-do list. I put my hands in a bag of it when I was meeting the Artigiano builder. He uses it in his preassembled ovens.

                          Pizza Ovens
                          Outdoor Fireplaces


                          • #14
                            Thanks To all

                            I decided and have ordered the Super Isol from James...
                            It will be a few weeks before I get it installed and the oven up and running but if anyone is interested I will let them know how it goes.


                            • #15
                              Cement board OK

                              I just poured my 2" oven hearth today. What a pain that vermiculite concrete is! Anyway, I used Durock cement board and with 3 2x4's running equidistant I had no problems at all with the weight.
                              Wade Lively