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Floor thickness and insulation

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  • Floor thickness and insulation

    I have moved on from the basic dome design of my 42" project to the floor and hearth design.

    I have read numerous threads about floor insulation and floor construction but have a few direct questions for those who cook both pizza and use retained heat for a day or three after a firing.


    1) Does a 2.5" floor thickness have enough thermal mass, or would more mass be better to keep from having to recharge the floor during a pizza party, etc.

    2) Will 4" of FB board placed directly on the concrete slab be enough insulation, or do I need to also add a layer of insulated concrete below the 4" of FB board?

  • #2
    Re: Floor thickness and insulation

    Four inches of refractory insulation board is more than enough. I got two and half inch boards, and my support slab stays cool during cooking.

    If your principal interest is cooking pizza, there in no need for thermal mass greater than 2.5 inches. The only reason for thicker mass is if you are baking multiple batches of bread, as in a commercial bakery.
    My geodesic oven project: part 1, part 2

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    • #3
      Re: Floor thickness and insulation

      Sounds good then. I will go with 4" of FB board directly on the slab.

      From what I can gather from numerous threads then the standard 2.5" of floor and a well insulated 4.5" thick dome (with or without cladding) can keep heat and still be 200 or more degrees 2 or 3 days later. I think that is what we are aiming for (cooking with residual heat a day or two later).

      Foundation questions are next, but I will reserve those for a new thread

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      • #4
        Re: Floor thickness and insulation

        I have 4 inches of cement under my refractory hearth and about eight inches of insulated cement/vermiculite under that. That's probably overkill but if you really want to bake (or slow bake) days later or bake multiple batches of bread a bit of extra refractory both in the dome and floor would not hurt and will not drastically affect your heat up time. My other suggestion would be to be sure to have a heat break at the outside of the hearth slab for heat will migrate out if you don't insulate around the hearth.

        Good luck!
        Jay

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        • #5
          Re: Floor thickness and insulation

          I had intended on pouring a 6" thick slab and then laying 4" of FB board on top of that and building the overn ON the board with the floor inside of the oven. The insulating blanket and vermiculite would cover the dome and fill the 4-6" gap between the exterior wall and the dome/exposed floor FB. Does that sound like enough of a thermal break?

          Increasing the floor mass to 3.5" thick using standard bricks and splits may be an option. It appears that most of the floors are 2.5" thick and folks have good heat a few days later. You mention it should not drastically change the heatup time, but I am still a bit concerned to deviate from the common path with regard to the floor.
          Last edited by BeanAnimal; 03-26-2011, 10:26 AM.

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          • #6
            Re: Floor thickness and insulation

            Hello,

            My oven is a barrel type, modeled after Tscarborough's. I have 4" slab, 4"perlcrete, 4 1/2" heavy duty firebrick, the type used in steel mills, and I cook pizza on a sunday afternoon and then cook a turkey on tuesday. It holds the heat well. If I leave the coals in to completely go to ash then the bottom of the slab heats through and gets rather warm. A termocoupIe in between the firebrick and the perlite on top of the oven in the cladding, read into the high 400's the next day. I imagine if you cleaned all of the ash/coals out immediatly afer pizza the heat would be a little less.

            No matter what your design, WFO's are pretty neat things.

            Derk

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            • #7
              Re: Floor thickness and insulation

              "200 or more degrees 2 or 3 days later. "

              Maybe not 3 days with that combination (2 1/2 inch on bottom, 4 1/2 inch dome, 4 inches insulation), but certainly in the order of 250 to 350 F up to 12 hours later. It will hold to 180 to 200 F at the 24 hour point.

              We generally do pizza one day, some bread in the evening, and a slow roast the next day on the one firing.

              There is always a trade off. More thermal mass allows you to cook longer but you will use more wood, take longer to heat up and, maybe, as a result use your oven less often. As Dmun noted, you have to think about how you plan to use your oven.
              Last edited by Neil2; 03-26-2011, 05:28 PM.

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              • #8
                Re: Floor thickness and insulation

                To be honest, I am not sure how I plan to use the oven. I think part (most?) of the alure here is building it. Don't get me wrong, I really look forward to pizza and other food, but I like building things as much as using them.

                My current line of reasoning:
                I think if I were just going to cook pizza and leave it at that, I would just purchase a cast dome and be done with with. So I think I certainly like the idea of residual heat at least on the following day. That said, I don't want to burn a forest and wait 6 hours just to get to pizza temperature. So maybe the happy medium is a 3.5" floor and 4.5" dome with a bit of cladding and heaps of insulation?
                Last edited by BeanAnimal; 03-27-2011, 06:47 AM.

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                • #9
                  Re: Floor thickness and insulation

                  Hi BeanAnimal...

                  Based on comments from those with well made ovens using the FB plans they can hold heat well. There is, however a difference between holding temperature and holding enough heat to cook with. Example: imagine an oven with a 1/4 inch refractory lining and perfect insulation. You could probably heat the ceramic to 800 degrees in 15 minutes. And it would hold the temp. But put a cold turkey in it to roast and the temp would quickly fall for there simply wouldn't be enough heat held in the refractory to cook the bird.

                  That is an extreme example, but...highlights the reason I suggested extra refractory - so you have more mass at 300 degrees (and therefore more stored heat) when you put the roast in.

                  All that said, unless you plan to use it for roasting and baking a lot it isn't a very big deal. My response was based on your original emphasis on holding heat and cooking for up to two days,

                  Part of the good news is that almost every WFO works reasonably well (unless it is wet) once you learn your oven so... build one!
                  Jay

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                  • #10
                    Re: Floor thickness and insulation

                    So given the extreme example and ignoring heat loss due to opening the door to put the bird in (and through the insulation), the only heat loss would be to the mass of the bird. Assuming it is "mostly" water, then 1 BTU per degree per pound

                    So unless I plan on baking loaf after loaf, or bird after bird, and keep the door closed after firing, then the FB plans should provide enough residual heat to slow cook something the next day. If not, a small fire will recharge enough for the baking temp. I suppose that is what I am looking for.

                    Sorry to make such a big deal about a topic that gets attention at least every other day around here, but I don't want to have buyers remorse after I settle on a plan.

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                    • #11
                      Re: Floor thickness and insulation

                      but I don't want to have buyers remorse after I settle on a plan.
                      Unfortunately, there is no chart that correlates x dimensions (thermal mass) to residual temperature. All ovens and environments are different, YMMV.

                      There was a thread last year that asked owners to report their oven's performance over the two or three days following initial firing. My takeaway was that while the best-insulated ovens (regardless of dome mass) retained heat the longest, the biggest contributor to heat loss was conduction through the integrated entryway and chimney.
                      John

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                      • #12
                        Re: Floor thickness and insulation

                        Hi BA!

                        Your interpretation is I think pretty much what I was trying to convey. A lighter oven will clear just a little faster. A heavier oven will (assuming equal or better insulation) be warmer longer. And as you suggest, a warm oven can be "recharged" with a smallish fire but it still takes a while for heat migration through refractory is not exactly "fast". (The Pompeii or refractory ovens should be at pizza temp in 45 min to an hour, but the refractory won't really be "loaded" for about an hour and a half to two hours. An Alan Scott/barrel vault oven which is about twice as thick needs about three to four hours to fully heat load. You will probably still need about a half hour to 45 min to recharge the oven and another hour or so to equalize, i.e. have the temperature stabilize in the oven and in the refractory.)

                        There is no ideal, universal oven for all purposes. Most of the Italian style ovens seem reasonably close in performance and behaviour and are versatile. I think you are pretty safe going either way.

                        Despite the stories of hot ovens after several days, I doubt you should plan to bake/roast on day two unless you add mass. And...part of why I used the example I did...if you roast on day one you will have a lot less heat left to roast with on day two - assuming your oven has that capability.

                        As your response suggests you understand...it's all about BTUs!
                        Jay

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                        • #13
                          Re: Floor thickness and insulation

                          Thanks for all of the input folks. I will look for that thread with data from other builders.

                          I have contempalted a small thermal break between the inner and outer arch bit have not quite wrapped my head around exactly what I want to do. Even a mortar joint thickness sealed with RTV instead of mortar may provide a fairly significant thermal break and prevent the insulation from being exposed. Another idea would be to use a 1" thick refractory insulation brick layer between the inner and outer arch. I don't think I would go as far as isolating the floor bricks between inner and outer, as they are not in great thermal contact to begin with.

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