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  • FB Board Thickness

    Anyone know if there are any benefits with laying 4 inch thick FB board beneath the oven floor (2-2" layers) or just one 2 inch layer? Goint to be hopefully starting the floor by the end of the week and I have enough FB board to do two layers but wasn't sure if there was any benefit to it??????

  • #2
    Re: FB Board Thickness

    2 inches should be enough. I have 2 inches on top of several inches of vermiculite concrete- so it's overkill. If you want a little more insulation without having to buy more board, use both.
    Elizabeth

    http://www.fornobravo.com/forum/f8/e...html#post41545

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    • #3
      Re: FB Board Thickness

      I only used 2" of FB board on top of my regular concrete hearth slab (no vermiculite layer). When my cooking floor temp is >1000 F, the bottom of my hearth slab is around 110F. I think 2" is more than sufficient.
      My WFO project: http://picasaweb.google.com/stevprin/WFOSmallPhotos#

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      • #4
        Re: FB Board Thickness

        Elizabeth and Steve, Thanks for the responses.

        I, like Steve, haven't used vermiculite concrete anywhere. It sounds like the 2 inches of FB board is going to be plenty of insulation based on the #'s given by Steve (great information BTW). Sounds like the 2 inch of FB board is keeping the heat in the bricks and oven.

        I'm also going to be installing 2 inches of FB blanket and a couple inches of loose vermiculite over the dome, so it sounds like there should be no problems with insulation.

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        • #5
          Re: FB Board Thickness

          Does the insulation board have any structural value? Is structure the sole purpose of the thick concrete hearths when ample insulation has been provided by the board?

          arri

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          • #6
            Re: FB Board Thickness

            Does the insulation board have any structural value?
            No. It has a consistency slightly denser than the old acoustic ceiling tiles.
            Is structure the sole purpose of the thick concrete hearths when ample insulation has been provided by the board?
            Yes. Brick ovens are heavy and fragile and need support. The support slab has no insulation value.
            My geodesic oven project: part 1, part 2

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            • #7
              Re: FB Board Thickness

              So, theoretically, I could construct a structural base out of steel to support the insulating boards upon which the firebrick floor of the oven could rest?
              How much heat would be transferring to the base with a single thickness board?

              In perusing the technical characteristics, one might expect the conductivity to be around 0.1 at 1000 deg F, meaning to answer my second question I could expect the bottom of the board to be at approximately 100 deg?

              In SteveP's example above where he suggests the bottom of his concrete hearth slab is right around that number, I am surprised his slab is offering basically 100% thermal conductivity. Unless his thermometer doesn't read higher than 1000 def F and the actual floor temp is much higher?

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              • #8
                Re: FB Board Thickness

                thought about it a little bit. i'm going with 2 layers of 2" fb board. my wife's cousin had an wfo installed but seems to loose way too much heat in the floor. It goes good for about 5-6 pizzas then it really looses heat in the floor. i don't want that to happen to me. the seams will not overlap other than where they cross at 90 degree intersections. i have enough fb board to do it, so what the heck.

                i have my base complete (finished in fall), I've cut a bunch of brick with my new 10" wet saw and it looks like the weather will cooperate for the next 4 days in the pittsburgh area.

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                • #9
                  Re: FB Board Thickness

                  It would seem as though the issue there is more likely the retention value of the floor rather than the energy loss through it... Remember that the Pompeii design would have you place the floor bricks flat (2.5" thick), thereby leaving nearly half the 'thickness' of the walls that are half bricks at 4.5".
                  From an insulation standpoint, remember that a single 2" board only conducts one tenth the heat put into it, which means normally your hearth isn't likely to be more than a few degrees above ambient unless it is below freezing!

                  I'd save the board for another oven or use it around the oven. If you want more energy to be held in the floor, you'll be better served by increasing the thermal mass there.
                  Last edited by arriflex; 04-22-2009, 05:03 PM. Reason: spelling

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                  • #10
                    Re: FB Board Thickness

                    I'd be careful about adding mass to the floor. I've investigated this extensively on the forum, and the experts here have tried the variations you contemplate. If I remember right, you would be altering the heat exchange balance between the dome and the floor. The way the Pompeii is designed the dome and the floor gain temp at the same rate. I can't say I understand the whole dynamic but this design is fairly proven.

                    Mark

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                    • #11
                      Re: FB Board Thickness

                      Originally posted by MK1 View Post
                      The way the Pompeii is designed the dome and the floor gain temp at the same rate.
                      This issue of retained heat comes up a lot. The dome will get hotter faster and retain the heat longer (heat rises). The only way to recharged the hearth is to A: spread coals back across the floor, B: let the heat of the dome reflect back down. I have 4 inches of board under my brick and I can see the heat drop after a few pizza's. After a point, insulation doesn't help. It comes down to mass. If I could do it over again, I WOULD sit the bricks on the side. It will take more fuel, longer heat up time, but at the end of the day, we should be able to cook more pies (within reason) without the delay of thermal drop. This is just my opinion (and I respect it)

                      This whole forum has been a study of evolution - please, someone go to the edge and report.
                      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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                      • #12
                        Re: FB Board Thickness

                        I certainly agree that adding mass to the floor has downsides as well. It's better to defer to those who actually have experience with these ovens, as I do not

                        My suggestion was addressing the problem mentioned above through design modification. Likely it would be more practical to simply take advantage of Pompeii's presumably quick recycle time with the given floor.

                        Putting another two inches of non-masonry product under all that fancy brick work with so little to gain energy wise just makes me nervous!

                        If you do it, please report back. I'm extremely curious how close to the starting temp of your hearth you can stay, and whether or not you feel a difference in cooking.

                        Like many lurkers here, my engagement (in conversation) is the first step to building. While I don't have experience with WFO's, thermal transfer is my business (albeit in the opposite direction, making and keeping produce cold). I'm looking forward to building my own someday and apologize for any naivete I might bring to the conversation.

                        Comment


                        • #13
                          Re: FB Board Thickness

                          Arriflex
                          Welcome to the discussion. Experimentation is half the fun. Several of the WFOs out there that are focused on baking have a much larger thermal mass/thicker hearth, thicker walls. It all comes down to how much energy you want to burn to get the mass up to temp. It's a balancing test. The Pompeii design is balanced for a relatively quick fire-up with good heat retention. Good for home Pizza. If you want to go commercial, you would probably be willing to add fire-up time for better retention.
                          I depends on what you want. All I want is pizza and wine.
                          Greg Geisen
                          Chula Vista, CA

                          Click to see my Thread:
                          http://www.fornobravo.com/forum/f8/g...iego-6169.html

                          Click to see Google web album:
                          http://picasaweb.google.com/gpgeisen...eat=directlink

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                          • #14
                            Re: FB Board Thickness

                            Les,
                            If I had my oven finished I would perhaps, experiment with this by laying a layer of splits on top of my existing floor. I just cut my floor today and started halving the rest of the firebrick. Support slab will be a week old Saturday. Now I know why those clever dogs sized the floor at 42" The compass just nicks the edges at 41.5"

                            Mark

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                            • #15
                              Re: FB Board Thickness

                              Originally posted by arriflex View Post
                              In SteveP's example above where he suggests the bottom of his concrete hearth slab is right around that number, I am surprised his slab is offering basically 100% thermal conductivity. Unless his thermometer doesn't read higher than 1000 def F and the actual floor temp is much higher?
                              My IR thermometer only goes to 999F. Not sure how much above 999F the floor actually is.
                              My WFO project: http://picasaweb.google.com/stevprin/WFOSmallPhotos#

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