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Insulation Efficiency - Forno Bravo Forum: The Wood-Fired Oven Community

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Insulation Efficiency

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  • Insulation Efficiency

    I know that some folks are talking about getting the insulation right to keep heat inside the oven, and wood consumption down. This is true for both site built and FB precast ovens.

    We recommend a 1" Insulfrax insulating blanket (6#) and 4"-6" vermiculite or perlite as a general rule. Here are some simulated insulation numbers we had an engineer do a while back.

    James

    *******************

    I had an insulation engineer run a test, and conclude that the blanket replaces 2" of loose vermiculite. We had them run a simulation where they added 1" of insulfrax, and reduced 1" of vermiculite. 1":4", 2":3", etc. over a 24 hour 1000F exposure. The outer face tested consistently dropped by adding 1" more insulfrax and 1" less vermiculite.



    1" Insulfrax Blanket 6#, 4" Vermiculite 1000F** 172F

    2" Insulfrax Blanket 6#, 3" Vermiculite 1000F** 161F

    3" Insulfrax Blanket 6#, 2" Vermiculite 1000F** 151F

    4" Insulfrax Blanket 6#, 1" Vermiculite 1000F** 142F

    5" Insulfrax Blanket 6# ******* 1000F** 135F



    Thinking about it, I should ask him to add 1# insulfrax, and drop 3" of vermiculite and re-run the test. I would note that after 24 hours of 1000F, (which you will not approach), the outer face is barely warm.
    Last edited by james; 06-27-2006, 03:17 PM.
    Pizza Ovens
    Outdoor Fireplaces

  • #2
    james,
    ask him if it is possible to compare these figures to castable refractory insulation (in place of either the refrax blanket OR the vermiculite insulation. i would think the castable might perform similar to the blankets.

    the closest figures i can contribute are:

    roughly 1.5-2 inches of 2500deg F rated castable insulation reaches 120-140 deg.F when the interior is 700-950 for 2 -3 hours. (i know those are a lot of variables, but it at least gives us a ballpark idea of how well it functions).
    Last edited by paulages; 08-22-2005, 03:22 PM.
    -paul
    overdo it or don't do it at all!

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    • #3
      I'll see if I can do that and get back.

      The figures above are aftter 24 hours -- which is a more extreme test, so the insulation is more efficient at those thicknesses. Paul, are you going to add loose vermiculite around the dome still?

      James
      Pizza Ovens
      Outdoor Fireplaces

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      • #4
        yes---whenever i get around to finishing the walls. i have a million projects going on right now...but i've found plenty of time to cook in it!
        -paul
        overdo it or don't do it at all!

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        • #5
          Re: Insulation Efficiency

          Just wondering if anyone has used fiberglass insulation around their dome? I wouldn't use it directly on the dome but if you had a layer of insulating blanket couldn't you use batts around your dome. The r-value would be much higher than perlite or vermiculite. I don't think temperature would be much of an issue since fiberglass is made at a higher temperature than we could fire the oven
          "Opportunity is missed by most people because it is dressed in overalls and looks like work." Thomas A. Edison

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          • #6
            Re: Insulation Efficiency

            Versachi:

            Yes, you could use fiberglass to isolation your dome.
            Depending on cloth type, the blanket has different temperature capability. As lower as 150°C to higher 500°C.
            You could use an inch of vermiculite/cement mixture and cover it with fiberglass blanket.
            Search by Pizzabrasil and see oven pictures and better explanations about this matter.

            Good luck

            Luis

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            • #7
              Re: Insulation Efficiency

              If you use domestic fibreglass batts, be sure to strip off the paper vapor barrier, which is combustible.
              My geodesic oven project: part 1, part 2

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              • #8
                Re: Insulation Efficiency

                I'm shopping for this in Dallas and I'm getting quotes on the blanket for 1" and 2" but also for 8 pound and 6 pound. I'm assuming the heavier is better but does anyone know by how much (i.e. 8/6 better
                RCLake

                "It's time to go Vertical"
                Oven Thread

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                • #9
                  Re: Insulation Efficiency

                  Since this thread was dredged up, I'll ask a question.

                  I have been taking in all the insulation conversations & I'm wondering about the FB blankets. I haven't actually seen it so I'm wondering how compressible it is, seeing how people are adding a hardcoat of vermiculite concrete/stucco to their oven over the blanket.

                  Wouldn't heavy-weight coating like that tend to compress the blanket, reducing it's efficiency?

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                  • #10
                    Re: Insulation Efficiency

                    TD, et al,

                    The vermic/cement mixture is quite light, really, so the blanket(s) don't compress significantly. I'd recommend 8 lb over 6 anytime.

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

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                    • #11
                      Re: Insulation Efficiency

                      Thanks Jim. Not having seen the product, I was thinking it was maybe less dense than it actually is.
                      My inclination is to use a box enclosure around the oven. Have you ever used unfaced fiberglass at the point you would use vermiculite fill (after a hi-temp blanket)?
                      I think it has better insulating properties & is certainly easy to get.

                      Ted

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                      • #12
                        Re: Insulation Efficiency

                        Ted,

                        I have no experience with using fiberglass insulation around an oven. What I normally do is use three inches (five at the apex) of insulating blanket, then a further three (and five) of castable refractory insulator. The product I use for this last is called Matrilite 18, although you could use vermiculite/cement at a ratio of 5:1. After that, you could fill with dry vermic, but I really don't think it's necessary, even in my climate.

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

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                        • #13
                          Re: Insulation Efficiency

                          I used fibreglass insulation on my first oven. It works ok but compresses easily which made it really hard to get the outer spherical form right. I ended up with only half the intended insulation thickness. I would only use vermiculite again. There is a cloud over the safety of ceramic fibre (carcinogen)
                          Kindled with zeal and fired with passion.

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                          • #14
                            Re: Insulation Efficiency

                            1" insulfrax, 2-4" vermiculite then 9" fiberglas seems to be working real well for me-- no temps taken on exterior of insulation, but if I put the oven "to sleep" at 400, it's 300+ in the morning. under hearth slab is 4" of vermiculite concrete (slab per Alan Scott method, pompeii dome oven per Forno). I may add some fiberglas underneath this winter. Seems to me the key is not to have 1,000 degree brick against the inexpensive fiberglas. But fiberglas is sure a great value compared to alternatives.
                            sigpic
                            Lovin my Oven

                            Ben Ciliberto, Doylestown Pa

                            Oven Pics Link- http://picasaweb.google.com/bciliber...OfTheTwoHearts

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                            • #15
                              Re: Insulation Efficiency

                              Is it possible to over insulate ?

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