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Insulating Hearth Questions - Forno Bravo Forum: The Wood-Fired Oven Community


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Insulating Hearth Questions

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  • Insulating Hearth Questions

    I finally began construction of my oven over the weekend, completing the block stand with little difficulty. I have a couple of questions as I move on to the next phase, which I intend to frame this week and pour over the weekend. The 2x6 frame is meant to be cut 1/2 inch shorter than the outside dimensions of the stand. Does this imply that only 1/2 inch of the 2x6 actually rests on the stand, leaving a small lip when the forms are removed? Also, the outside dimensions of my stand are 64x72 inches - does anyony have a rough idea of how much vermiculite, cement and concrete I will need to pour the hearth? Thanks in advance for any replies and thanks to all for the vast amount of information I have accessed over the past months.

  • #2
    Plans change

    There is a change in the plans that has been updated today.

    As far as 1/2" overlap, that is the way I took it and did it this weekend. I have seen some with no overlap, I don't think it matters much, if you are going to "dress" your oven up.

    To make it easier, I used Durock cement board instead of plywood, and just layed it down (rough side up) overlapping the sides of the opening 3", so I could leave it in. I braced it with 3 2x4's equidistant, running long ways and 9 feet (3 each). I had no problems or sag whatsoever and am very glad I did it that way. Much more simple.

    My stand is more or less the same size. I saw the posts about thickness changes and added a 1x2 on top for a 7" thick form. I used the Quickrete calculator and then padded 2 bags. It turned out I had 2 left over. I used 24 bags for 4" crack resistant bottom. The insulating was another story, what a pain. I bought 8 cu. ft. (2 bags) of vermiclulite and then 2 small .75 bags for insurance. I used all of it (and 1 90lb bag of Portland cement) and was 1/2" short. DOHHH! Anyway, if I were you, or doing it over again, I would go with the latest change and pour the 4-5" layer of crack resist cement and forgo the vermiculite in favor of the super isol board. It works out to be close in $$ and would be much less hassel. If you go ahead with the vermiculite, mix the cement and water first, very wet, then add vermiculite. I also found it to be much better consistency to add just a little sand.

    Good luck,
    Wade Lively


    • #3

      In the new hearth plans, the extra thermal mass isn't recommended for regular backyard cooking. Does it diminish the oven's ability to perform when cooking mass quantities of pizza at high temperatures? What are the downfalls of this design? Slower heat up time? Anyone wanting to mail me their offcuts of isol board will make it into our documentary.


      • #4
        Super Iso on a wet hearth

        Any issues with laying the superIso on the hearth after the pour?

        Would that be an acceptable alternative to the adhesive step?

        I'm wondering if one could mist the back of the super iso and lay it right on the freshly poured concrete, put a level on it - tap down and around until perfect.

        Any concerns with this plan?

        Thanks (as always)

        My oven progress -


        • #5
          extra thermal mass

          Originally posted by redbricknick
          In the new hearth plans, the extra thermal mass isn't recommended for regular backyard cooking. Does it diminish the oven's ability to perform when cooking mass quantities of pizza at high temperatures? What are the downfalls of this design? Slower heat up time? Anyone wanting to mail me their offcuts of isol board will make it into our documentary.
          Extra thermal mass makes it harder to achieve and maintain high temperatures. It's good for bread baking, where you need to hold heat enough for multiple loads of bread, and for commercial pizza operations where the oven is fired every day, and the heat retained from the previous day makes it easier to fire.

          For home operation, where periodic operation for pizza followed by maybe one batch of bread baking, speed and economy of operation are served by having no more than the thermal mass of the commercial modular ovens.
          My geodesic oven project: part 1, part 2


          • #6
            Cost of Super Isol

            Thanks for the replies, I am feeling confident about pouring the hearth and like the new plans. I am very willing to use the super isol, but am confused regarding the cost. Members have stated that the cost is roughly equivalent to using vericulite/perlite, but if my calculations are correct I will need 5 or 6 panels of the super isol. This would make it far and away the most expensive single component of the oven, and roughly ten times more expensive that using perlite (in my case i have found a supplier with 4 cu ft bags for about $8 each). Lastly, if I use perlite, the supplier is asking what grade I need. This refers to the coarseness and he claims that the very finest grade is usually used in making concrete, but I think I need something more coarse. Any physical comparisons on what the perlite should look like?


            • #7
              You seem to have it all in hand. About the cost, you will need 3 panels to fit under your oven (two if your oven is smaller). That puts the Super Isol cost at between $130-$195 (plus glue or mortar).

              You will need about 3-4 4 cu ft bags of vermiculite and one page of Portland cement to pour that layer. Your price on Perlite is great, so you are in good shape if you want to mix and pour. Other folks have to pay a lot more than that, which puts the cost difference closer.

              I think Super Isol and Insulfrax are similar in that they are very efficient, which gives you good oven performance, and they are easy to install, which saves time and mess. And they do cost a little more, but not a lot more.

              Armed with good information, you get to choose.

              Pizza Ovens
              Outdoor Fireplaces


              • #8
                Quantity of super isol

                I was basing the 5-6 panels of super isol on covering the entire hearth. Is it only necessary to insulate with the super isol under the actual firebrick cooking surface? If that is the case, then 2 panels would suffice.


                • #9
                  That's right.

                  You should put it under the brick cooking floor (which includes the oven walls above) and probably under the floor under the vent. If your oven is small enough, the 36" panel will work. Otherwise, you need the 3rd panel.

                  Pizza Ovens
                  Outdoor Fireplaces


                  • #10
                    hearth insulation timing issue.

                    If I decide to go with perlite for my hearth insulation, how important is it to pour it on the same day as the reinforced cement slab? We're going to the perlite factory to shoot footage of the perlite process next week, but we want to pour our slab this weekend. My line of thinking tells me that If I let the top of the reinforced slab cure with ridges on it, the perlite mud will bond to it without an issue.. But I've been wrong before.


                    • #11

                      Hmmm... I don't think it will slip off. I imagine the same day pour would be more important if you used the traditional perlite as the lower layer approach. Leaving ridges should allow plenty of grip.


                      • #12
                        Pour the same day?

                        I think some of us suffer from the mis-guided (but well meaning) instructions in the Bread Builders book, which recommends a hearth, with the insulating layer below, hanging from a rebar grib resting on the block stand. I did it twice, and cannot recommend it - either for structural, thermal or culinary reasons.

                        Because the insulation layer hangs below the concrete layer, it is important in that design that the two layers be poured together to bond. In fact, I have done crazy things with nails and wire mesh to hold the two layers together. With the structural layer resting on the stand, and the insulating layer resting on the structural layer, it just doesn't matter.

                        As an interesting side note, I have never seen instructions for an Italian oven that recommends that you pour the same day. In fact, our producer recommends discrete insulating blocks, such as Super Isol.

                        Nick - you will be fine.
                        Pizza Ovens
                        Outdoor Fireplaces


                        • #13
                          Originally posted by james
                          As an interesting side note, I have never seen instructions for an Italian oven that recommends that you pour the same day. In fact, our producer recommends discrete insulating blocks, such as Super Isol.
                          The Pompeii oven plans on the fornobravo website still indicate it is important to pour the same day to allow the layers to bond - maybe this can go away in the next edit


                          • #14

                            I don't think it's important or necessary to pour both on the same day. The insulating mixture isn't all that smooth, and the slab will bond to it quite well. I used brick ties in the vermic layer to help with the bond, but mainly because I'm paranoid about such things.

                            "Made are tools, and born are hands"--William Blake, 1757-1827


                            • #15
                              Question in regards to concrete for the hearth. I will probably pour the structural concrete part of the hearth this weekend and was planning on going to one of these rent-all places and purchasing pre-mixed concrete. Does anyone see a problem with this? I know I have seen some posts where people are recommending a fiber reinforced type of concrete. I am planning on buying the "strongest" concrete the place has to offer.

                              Otherwise, looking forward to this part of the construction to being completed.