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Advice re the insulating hearth - Forno Bravo Forum: The Wood-Fired Oven Community



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Advice re the insulating hearth

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  • Advice re the insulating hearth

    Hello I have a question about the Insulating Hearth: does it matter if the vermiculite insulating concrete layer is poured onto the structural concrete once it is dry? I notice that the guidelines here suggest doing it at the same time so it all bonds together, but unfortunately I discovered this forum too late and have already done the structural layer!

    If it is too late for me to add the vermiculite layer I was thinking of using the Superisol block instead. Does anyone know if this is available in the UK?

    Any help would be much appreciated! Thanks.

  • #2
    Re: Advice re the insulating hearth

    There is no problem laying the vermiculate concrete after the support slab is dried. Even if it's completely detached (which it won't be - the water and concrete settle to the bottom) it doesn't matter. All the force is down.

    The main difference between calcium silicate block or equivilant, and vermiculite/perlite concrete, is that you can get away with two inches of insulation rather than four.
    My geodesic oven project: part 1, part 2


    • #3
      Re: Advice re the insulating hearth

      David has it right.

      We are looking for a source of Cal Sil (or Alu Sil) boards in the UK and will let you know when we find something. For now, you can definitely pour a 4" vermiculite layer on top of a curing slab. Gravity is on your side. The comments about same day pour go back to the very, very early days, when some builders were putting the vermiculite layer under the slab, and hanging the whole thing with rebar.

      If anyone finds "pour the same day" mentions on FB.com, let me know, and I will delete them.

      Last edited by james; 06-13-2007, 12:45 PM.
      Pizza Ovens
      Outdoor Fireplaces


      • #4
        Re: Advice re the insulating hearth

        Thats great, thanks for getting back to me so quickly, very reassuring.

        FYI: I got the "same day pour advice" from:

        where it states "It is important to pour both the insulating and thermal layers in the same days, as you want them to bond and cure together".

        Thanks again for the information. Really helpful.



        • #5
          Re: Advice re the insulating hearth

          Excellent! I was planning on pouring the same day, but a staged approach will let me get them both done sooner (I have to squeeze construction hours in whenever I can).
          "You better cut the pizza in four pieces because I'm not hungry enough to eat six."

          -- Yogi Berra

          Forno Tito


          • #6
            Re: Advice re the insulating hearth

            WOW! Wish this had come up 3 months ago. I followed the instructions to the letter. Made sense at the time, so I didn't ask. I then proceeded to mix ALL of my concrete by hand (yes, pretty stupid, I'm not 1/2 the man I thought I was) and do BOTH pours the same day. I can't believe I didn't have a stroke and a heart attack.....the 1 week wait for curing allowed my back and shoulders time to recover.
            I'm not an advocate of hand mixing, but it sure would have been easier knowing I could have taken a break between the 2 pours.


            • #7
              Re: Advice re the insulating hearth


              I feel terrible. But look on the bright side -- your hearth did fuse, and it will be that way forever. Hope your back has recovered.

              If it helps (which I know it doesn't), I have changed that page on the web-site plans. Sorry about that again.

              Pizza Ovens
              Outdoor Fireplaces


              • #8
                Re: Advice re the insulating hearth


                no apology needed. My ego (I can do anything) got the better of me and I proceeded to mix 60 bags of concrete by hand. I struggled with the rental/buying options of a mixer, then said screw it and just started mixing.

                Having the option to pour them separately would have been nice, but the more I think about it, the more satisfied I am that my oven is as structurally sound as it can possibly be.


                • #9
                  Re: Advice re the insulating hearth

                  I did the same thing (pour both on the same day). My next door neighbor pulled me out of the fire on that day. He and his son came over and really really helped. I am sure I would not have been able to do it without them.

                  So pretty much every time I make pizza I either invite them over or at least hand one over the fence!

                  My Oven Thread:


                  • #10
                    Re: Advice re the insulating hearth

                    I poured both today. How long do I have to wait to start laying the cooking floor?
                    Also, What is the best way to cut your forms using foam and how to you get them to stay in place?


                    • #11
                      Re: Advice re the insulating hearth

                      You only need to wait until the vermiculite cement is hard, which is about 3-7 days. I didn't use forms, unsure what your question about cutting it is (a jigsaw should work well, but I may be misunderstanding your question). I'm sure there are lots of clever forum members who have better ideas, but if you just support the base of the forms with bricks (just a few stacked against the foam) until the first few rings are up you ought to be able to remove the support bricks after the first few rings.


                      • #12
                        Re: Advice re the insulating hearth

                        Yea!! Another Alabamian!!!
                        "He is no fool who gives what he cannot keep to gain what he cannot lose." - Jim Elliot

                        "Success isn't permanent and failure isn't fatal." -Mike Ditka


                        • #13
                          Re: Advice re the insulating hearth

                          Thanks for the quick reply. Today the hearth feels hard but I am going to give it a few days. I can't decide whether to use forms or not. It almost seems like it would more trouble than it is worth. I am sure I will have many more questions ( probably some stupid ones ). I just dont want to spend this much time and work in 102 degree weather and mess up.
                          Thanks again for your help.
                          Gardendale, Alabama


                          • #14
                            Re: Advice re the insulating hearth

                            I planned on using forms with my oven but I found it easier to work from inside my oven so that I could attempt to fit my joints as tight as possible. I made a template of the dome shape I had in mind and cut that in half leaving me with just a quarter piece which I used to check my angle every 4-5 brick or so. I think you will find that once you set the angle of the first brick in each row (especially the first 4-5 rows) that you can set all of the rest in that row on the same angle just using your eye if you are careful. Nice to have that quarter form though just to make sure you aren't getting too far off course.

                            Cheers, John
                            "Opportunity is missed by most people because it is dressed in overalls and looks like work." Thomas A. Edison


                            • #15
                              Re: Advice re the insulating hearth

                              I think the half dome form idea is a good one. It may be easier to make than the shims I used. With shims, you have to cut a different angle for each course. The quarter form could perhaps also be supported by stacked bricks on the oven floor to support the ends of the upper rings which can be a bit difficult to hold in place until each ring is finished as you are fighting gravity.

                              If the form had some heft to it you might even get away without having to reinforce it with bricks. Maybe you could make a plywood cradle for the lower part, that accepts a few bricks or even a half concrete block at the base, then sandwiches foam up top. After the build the foam culd be cut out or pulled out of the cradle, the wood could be burned if it did not fit out the oven opening, and the block could be pulled out the opening.