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  • planning questions

    i'm planning a build in CA. probably a 36". i've worked a little w/ stone and metal before. i've been lurking and reading, but i still have a couple details i'm unsure of, if you don't mind.

    i live near enough FB HQ that i can drive there and pick up supplies to save shipping. i'm planning to get the mortar, insulation board and blanket, chimney, from them and source the bricks locally.

    is the hearth insulation set on a skim coat of mortar, or sand, or caulked down in some way, or is it just pressed down from the weight of the bricks? seems like it could move over a month+ build if not anchored.

    i see folks adding a bottle/can to the top of the vermiculite insulation, presumably as some kind of vent. what are the details on this? i don't really understand it.

    i see some folks separated the chimney from the dome. i don't really see why this is necessary. shouldn't both heat up, and both have insulation? the dome will heat cycle far more than the chimney, so why so much concern?

    if i stucco over the igloo, i'm assuming i still want the entire stucco igloo to "float" above the hearth for expansion. the FB instructions talk about rebar into the hearth and a mesh on that for stucco, but that doesn't seem right to me.

    thanks for any help you can provide a newbie.

    -SM-

  • #2
    Re: planning questions

    Originally posted by tikidollracer View Post

    is the hearth insulation set on a skim coat of mortar, or sand, or caulked down in some way, or is it just pressed down from the weight of the bricks? seems like it could move over a month+ build if not anchored.
    Just place the board on the hearth - it won't go anywhere

    i see folks adding a bottle/can to the top of the vermiculite insulation, presumably as some kind of vent. what are the details on this? i don't really understand it.
    Not sure I have seen this, your assumptions of a vent is probably correct. If you cure slowly it would not be needed. If you can afford it, I would purchase the blanket then it's a non issue.

    i see some folks separated the chimney from the dome. i don't really see why this is necessary. shouldn't both heat up, and both have insulation? the dome will heat cycle far more than the chimney, so why so much concern?
    Some have done this to act as a heat break. I personally did not do this and have zero regrets.

    if i stucco over the igloo, i'm assuming i still want the entire stucco igloo to "float" above the hearth for expansion. the FB instructions talk about rebar into the hearth and a mesh on that for stucco, but that doesn't seem right to me.
    This was my approach. I bent the rebar into a radius and dropped the ends into holes I drilled in the hearth
    Check out my pictures here:
    http://www.fornobravo.com/forum/f8/les-build-4207.html

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    • #3
      Re: planning questions

      "if i stucco over the igloo, i'm assuming i still want the entire stucco igloo to "float" above the hearth for expansion. the FB instructions talk about rebar into the hearth and a mesh on that for stucco, but that doesn't seem right to me."

      I went down the standard road with the igloo and put ceramic fibre insulation over the brick dome, wire mesh over that, vermicrete and then rendered over the top of that. The CF allows the vermicrete and render to 'float'. I have been using mine for some time and the dome is still like the day I finished it.

      Good luck with your build. There are some good ones to emulate on here. Mine is functional and I am happy with it. The pics in the link below might help you clarify some of the issues you are contemplating.
      Cheers ......... Steve

      Build Thread http://www.fornobravo.com/forum/f3/n...erg-19151.html

      Build Pics http://www.facebook.com/media/set/?s...1&l=1626b3f4f4

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      • #4
        Re: planning questions

        Originally posted by Greenman View Post

        I went down the standard road with the igloo and put ceramic fibre insulation over the brick dome, wire mesh over that, vermicrete and then rendered over the top of that.
        Agreed - I took the step of adding the rebar. It just seemed that I would be able to control the shape a little better
        Attached Files
        Check out my pictures here:
        http://www.fornobravo.com/forum/f8/les-build-4207.html

        If at first you don't succeed... Skydiving isn't for you.

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        • #5
          Re: planning questions

          thanks, just to be clear i plan on doing the blanket and a vermiculite layer over the blanket. i'm after heavy insulation for multiple day cooking. i haven't decided on final form. either stucco igloo or square form filled w/ vermiculite.

          -SM-

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          • #6
            Re: planning questions

            Originally posted by tikidollracer View Post
            thanks, just to be clear i plan on doing the blanket and a vermiculite layer over the blanket. i'm after heavy insulation for multiple day cooking. i haven't decided on final form. either stucco igloo or square form filled w/ vermiculite.

            -SM-
            Thats what is in the picture. I have a minimum of three inches of blanket under 2-3 inches of vermiculite ( if I remember the numbers right)
            Check out my pictures here:
            http://www.fornobravo.com/forum/f8/les-build-4207.html

            If at first you don't succeed... Skydiving isn't for you.

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            • #7
              Re: planning questions

              Originally posted by tikidollracer View Post

              ............i see folks adding a bottle/can to the top of the vermiculite insulation, presumably as some kind of vent. what are the details on this? i don't really understand it..............


              -SM-
              The beer bottle/can is kind of a joke. But, it does serve the purpose of forming a hole in the vcrete to allow trapped gasses (steam between the firebrick and vcrete) to escape at the apex of the dome on igloo styles. The hole adds another problem, though. It must be capped with a venting rain proof cover.

              I don't care what folks say behind my back........They are either braggin' or.......lyin'


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              • #8
                Re: planning questions

                Originally posted by Gulf View Post
                forming a hole in the vcrete to allow trapped gasses (steam between the firebrick and vcrete) to escape at the apex of the dome on igloo styles.
                you see steam coming out your vent cap when you heat the oven? photo?

                how is water getting trapped in there to form steam? all the gases in the oven should be convected out the chimney. the water in the mortar should have reacted. the outside shell should be impermeable to H2O.

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                • #9
                  Re: planning questions

                  Gudday
                  Sounds good in theory.... But in the real world high humidity will be drawn into the bricks naturally. The Hearth has brick / tile has with have cracks that capillary action will take the water straight to your under insulation. Chimneys brick, ceramic,or metal expand and contract and cause cracks.
                  A damp oven is going to happen sometime you just fire it till it dries and cook in it as you do . The vent helps when it does happen.
                  Regards dave
                  Measure twice
                  Cut once
                  Fit in position with largest hammer

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                  • #10
                    Re: planning questions

                    It is about both getting rid of any moisture that is left in the mortar/firebricks/stucco from the build. Some people are inclined to moisten their bricks to different degrees when building the dome. (best we don't start that conversation again). It also provides a vent for moisture that finds its way into the oven after it is completed and cured.

                    I used an irrigation fitting with a threaded cap and then put a small chinamans hat over that. I was going to just use the little cowl but it was pointed out that the aluminium would react with the mortar over time so it now just sits over the PVC threaded cap.

                    How you form the cavity through the vcrete is up to you.

                    I might be considered to be one of those things that it is better to have and not need rather than need it and not have it.

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                    Cheers ......... Steve

                    Build Thread http://www.fornobravo.com/forum/f3/n...erg-19151.html

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                    Forno Food Pics https://www.facebook.com/media/set/?...1&l=1d5ce2a275

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                    • #11
                      Re: planning questions

                      sorry, i'm new here, and i'm sure this is boring for old hands, but i don't understand. if you don't have a vent cap in the top of your structure, and it was a humid night so the bricks absorbed some moisture. then you heat your bricks to 900F. where do you think the steam is going to go? presumably it doesn't build up pressure and blow your igloo to pieces. vermiculite and stucco are not vapor barriers.

                      -SM-

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                      • #12
                        Re: planning questions

                        About half the water in a concrete brew, (assume the same for mortar) is taken up in the hydration process as the concrete cures. So there is still plenty to remove. Many builders also wet their firebricks, either prior to, or as they are laying them. Vermicrete takes about four times the amount of water that is taken up in the hydration process. So there is usually a large amount of water to be removed. As the water travels away from the fire in an effort to escape it gets trapped in the insulation layer. Water expands some 1600 times it's volume when turned to steam and can easily crack the outer shell of an igloo oven through the increase of pressure.if you see steam then you are pushing the process too fast.
                        Last edited by david s; 10-20-2013, 11:16 PM.
                        Kindled with zeal and fired with passion.

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                        • #13
                          Re: planning questions

                          Hi David

                          So wouldn't a valve as in cobblerdave build and others I presently cannot recall alleviate that somewhat.

                          Until the stucco is sealed, one sealed I could see a problem.
                          Cheers Colin

                          My Build - Index to Major Build Stages

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                          • #14
                            Re: planning questions

                            If doing an igloo I think it is a good idea to drive out the water before doing the render/ stucco outer shell to reduce the possibility of cracking it. A vent will help reduce the steam pressure should the oven get wet after it is finished.if an igloo oven is exposed to the elements, even if the outer shell has been made waterproof, water will still get in, it needs an exit.i generally give a new oven about 10 decent cooking fires, after it has been cured, before making the outer shell waterproof. If your outer shell keeps water out, then it will also keep water in. A vent will allow that insulation layer to breathe and the waterproof coating will keep out most of the water.
                            Last edited by david s; 10-21-2013, 01:07 AM.
                            Kindled with zeal and fired with passion.

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                            • #15
                              Re: planning questions

                              Originally posted by david s View Post
                              About half the water in a concrete brew, (assume the same for mortar) is taken up in the hydration process as the concrete cures. So there is still plenty to remove. Many builders also wet their firebricks, either prior to, or as they are laying them. Vermicrete takes about four times the amount of water that is taken up in the hydration process. So there is usually a large amount of water to be removed. As the water travels away from the fire in an effort to escape it gets trapped in the insulation layer. Water expands some 1600 times it's volume when turned to steam and can easily crack the outer shell of an igloo oven through the increase of pressure.if you see steam then you are pushing the process too fast.
                              sorry, still not making sense to me. is there some place this vent is documented, or is this just local list practice? it seems to me that it's simply placing a 3 sq. in. hole in the insulation layer for little or no practical purpose.

                              the concrete and bricks are wet. then they cure. during this curing the concrete shrinks in volume, and it is then that cracks commonly form. when you start heat cycling it is these fractures you're exposing. vapor forced through the concrete passes through, and does not increase the volume of the concrete by 1600x.

                              the concrete is cured, then additionally it is forced dry by firing. at this point there is no appreciable water left. if this forced drying cracks the concrete then you've developed the vents you seek. afterwards a stucco barrier is applied which is also not a 100% vapor barrier, but should repel water. a liquid barrier and a vapor barrier are not the same thing. at this point the oven will maintain an equilibrium w/ the environment until it is fired, when it will again be forced dry depending upon the length of the firing. but we're talking small amounts of moisture here. these domes are not large enough to develop their own climate. and even if they do have a small amount of moisture and steam migrating through the insulation w/ every firing cycle, so what? you can't stop it from happening, and any venting is at best just providing a preferred direction for the micro-fractures to form, while allowing the heat to conduct right out the top.

                              -SM-

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