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Misreading of the instructions about the insultation layer :/ - Forno Bravo Forum: The Wood-Fired Oven Community

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  • Misreading of the instructions about the insultation layer :/

    Hi guys,

    I have started to build my oven few months ago (slowly but truly), following the instructions until i realized that i might have made a mistake.
    I have made two layers on top of my stand, one structural layer of 4", and an insulating layer (vemriculite+concrete) of 3,75".

    I wanted to add more insulation, si i bought some high density bricks (1,2" thickness), and made an island that i sank into the insulating layer.
    In the instruction i then read the following: "You may “sink” your island into the insulating layer, but take care to not reduce your insulating layer to less than 3 1/3 inches."

    When the instruction "dont reduce your insulating layer to less than 3 1/3", is it talking of the vermiculite+cement layer (that became 2,5" under my island in my case), or is it talking about the vermiculite+cement and the brikcs together (that should be of 3 1/3")???

    Thank you guys, and have a great Sunday

    I attach some pictures so you can see what i have done so far

  • #2
    I think the instructions are to insure you have at least 3.5" (87mm) of insulation under the floor. Depending on how you intend to use the oven, this is a minimum and you could certainly add another 2" (50mm) for better performance.
    By adding high density splits (apparently), you have reduced the insulation layer by about 1.25" (31mm) because high density brick have very little insulating qualities. Personally I would remove the high density brick, and put back in the insulation at least to the original height if not more. Originally you were intending to add more, it will never be a better time. (actually it will never be possible to add insulation again without disassembling the oven, so now is the time to add insulation. ) If you can find them, insulating brick in the same size as fire brick are available that would be easy to replace the high density brick with. Check with the pottery supply locations and see if they sell insulating brick. They are the same size but weigh about 10% of the high density. Very light and easy to work with.
    The cost of living continues to skyrocket, and yet it remains a popular choice.

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    • #3
      Thank you for your reply.
      As i was not able to find insulating bricks, i will add 2" of vermiculite+concrete this weekend and keep the high density tiles for the barbecue that i am building (that you can see on the second picture).

      I have another question regarding the vermiculite mix. In the instructions it says 5 volume of vermiculite for 1 volume of concrete, my mix was more like 4:2, otherwise i had no oatmeal consistency but just a slightly grey wet vermiculite. Is it a problem?

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      • #4
        The only problem is it will have less insulation value. The 5:1 ratio is a starting point, unfortunately 4:2 is 2:1 which is not just a slight variation, it would be considered a deviation. It could be a bit brittle, but since it is under the floor, no real problem. The slightly grey wet verm. is about what your looking for, if you continue to mix it for an extended time, it eventually becomes consistently coated with portland and add a bit of water to get to oatmeal. Some like the oatmeal sloppy and some like it thick... Mixing smaller batches like half a wheelbarrow at a time helps to learn what your looking for. An overfull container just encourages a short mixing period and then you get inconsistencies. I like to mix the ingredients dry until they are consistent and then add water so you don't get clumps of verm with no portland and other areas with all the portland. I also use a blend of Verm and Perlite which seems to work for me.


        This is a little more of an experience thing, after making a variety of blends and watching how they work, you get an idea what works for each situation. Unfortunately for a one time builder, you don't have any reference point and the new information is pointless since you will never use it again. I guess that is why we discuss the same thing over and over again, hopefully to help the next builder out who is at this point and doesn't know if they are doing it right or not.
        The cost of living continues to skyrocket, and yet it remains a popular choice.

        Comment


        • #5
          I have no idea what an oatmeal consistency is but I've found that adding the correct amount of water is vital. Too much water and it pools in the bottom of the barrow washing the cement off the grains of vermiculite or perlite, if this occurs just add a little more dry material. Too little water and the mix is too dry resulting in a loose structure lacking strength. Depending on the grade of vermiculite or perlite also has an effect on the required amount of water. The finer the grade the more water is required. Generally 3 parts water for every 10 parts vermiculite or perlite (by volume) works well. Another trick I've found when making a very lean mix is to add a little powdered clay which improves the mix. Also breaking up the lumps created with the back of the shovel during mixing helps. Using a mixer is not particularly successful as it sticks to the paddles and sides a lot.
          Last edited by david s; 05-29-2017, 12:32 PM.
          Kindled with zeal and fired with passion.

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          • #6
            I almost always use my mixer, too lazy to hoe all that up. You are correct about grades and size, seems like you can have batches that don't come out very well at all and others that are perfect. I haven't found the exact combination since I seem to get a lot of different stuff.
            The cost of living continues to skyrocket, and yet it remains a popular choice.

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            • #7
              David's you described exactly my situation. When i added water, it washed out all the cement :/ this is the reason why i had to add cement.
              I am going to do my additional 2" layer now, i'll try to follow your advices and will let you know about the results.
              Thank you for your answers


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              • #8

                here some pictures to show you the mix that i get. This is why i added more cement last time, it seems like its not linked.

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                • #9
                  When I said "add more dry material" I should have added "in the same proportions". Adding only significant extra cement will raise the density of the mix and reduce its insulation value.
                  Kindled with zeal and fired with passion.

                  Comment


                  • #10
                    Done, i've added my extra layer with the right proportions.
                    I removed the tiles, so i have 3" under my ovenfloor, and 2" on the other parts of the oven.
                    Under it, i have my 4" (2.7" under the ovenfloor) layer of (4:2) cement vermiculite mixture.
                    Hope it will do the job

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