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


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Blanket Insulation

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  • Blanket Insulation

    I am planning on building an oven in my backyard, hoping to break ground this April 2009 for the pad.

    I am a Ceramic Engineer with over 15 years of field experience with refractories (the majority spent with A.P. Green refractories ... prior to the US closing its manufacturing base and A.P. Green closing its doors).

    With that said, I believe that this site offers some of the best information and content. I have been reviewing literature and web sites for the past several years and wonder if a thread dedicated towards insulation might be a good idea. And two words should be added to the available literature:

    Mineral Wool.

    This is available in two basic forms - as loose "wool" shipped in bags and as boards in a variety of thicknesses (1" and 1 1/2" being the most common). The boards typically are sold (in the United States) in 3 ft. lengths and sold by the "board foot". Note: Mineral wool blanket is - or was - made but not readily available and was usually a special-order product.

    The insulating properties of mineral wool are far superior to ceramic fiber at the temperatures encountered with wood-fired ovens. Mineral Wool products typically are rated to up to 1800 F depending on manufacturer and product.

    Among furnace engineers, talk about insulation was often discussed in terms of 'equivalent inches of firebrick'. Mineral wool board would have well over 18 equiv. inches firebrick around 1000 F as compared to @ 8" for ceramic fiber blanket (uncompressed), making it a superior insulation and resulting in a cooler surface on the outside of a furnace or oven. And more heat being kept inside where we want it.

    And mineral wool board should cost less than half the cost of ceramic fiber.

    Availability: I would call up any company that supplies refractories (fire brick,castable & plastic refractories, mortars, ...) and ask them if they sell it or could recommend any boiler repair contractor or other supplier that might have some.

    I don't work in the refractory field anymore and do not have the literature at hand any more. But this material has been the much more commonly used insulation on industrial furnaces ranging from small heat-treat ovens on up to the largest molten-metal reverboratory furnaces. Ceramic fiber was not used as insulation behind firebrick due to cost and insulative properties. Ceramic fiber was better suited as a substitute to firebrick - as the actual hot-face lining where gas velocity, chemical attack, dusting, and other factors were not an issue. As an insulation, ceramic fiber is not subjected to these factors and can certainly be used there.

    Note: I do not promote ceramic fiber exposure to foods as it will gradually develop (in the presence of sufficient temperatures) a form of silica that is harmful to humans (crystobolite). Wood fired ovens would not ordinarily experience these temperatures, especially being used as an insulation layer behind firebrick. Additionally, the fibers are encapsulated between the firebrick and any cladding so they can't go anywhere. Ceramic fiber is a good product but more expensive for my pocket with a great alternative being available.

    The designs I have found here are fantastic and I am looking forward to building my own oven (finally). My only planned change will be substituting mineral wool in place of ceramic fiber as the insulation layer based on my furnace experience and background. Vermiculite will then be the next layer (closest to the outside cold face).

  • #2
    Re: Blanket Insulation

    Welcome aboard! Intersesting first post, we can always use some experts on refractory materials!
    My Oven Thread:


    • #3
      Re: Blanket Insulation

      Welcome aboard Altamont,

      As Drew says, we can always use a few more experts here. :-)

      Your comment on ceramic fibers is well noted, but I have a question. The Pompeii oven design calls for a layer of ceramic insulation full enclosed outside of the oven chamber and held in by the oven enclosure itself. I definitely would not want extended exposure to ceramic fiber, but isn't it OK as it is enclosed?

      Have fun with your project -- we're looking forward to seeing you progress.
      Pizza Ovens
      Outdoor Fireplaces


      • #4
        Re: Blanket Insulation

        Wow Thanks Altamont!
        I have also heard that one wants to encapsulate ceramic fiber to prevent exposure.
        Will the board hold up under the hearth?
        I am very excited to have a "next generation" Pompeii oven ASAP


        • #5
          Re: Blanket Insulation

          Minral wool and rock wool - is that the same kind of product?

          In any case, this post makes me feel a lot better about the insulation I used. Although my oven works beautifully, I've still always had this sneaky feeling that it might have been even better with the FB blanket... but maybe not after all.

          The guy who sold me the stuff said much the same as you have here, but its nice to have it confirmed (by someone who isn't trying to sell me something ).
          "Building a Brick oven is the most fun anyone can have by themselves." (Terry Pratchett... slightly amended)



          • #6
            Re: Blanket Insulation

            Regarding exposure to ceramic fiber - relax. "Don't Panic" (as you can tell, I really enjoyed Douglas Adam's 'Hitchiker's Guide to the Galaxy'). Because the fiber is totally enclosed there is no exposure so it is safe.

            This material (ceramic fiber) is not a hazard like asbestos - it is very safe. The major health concerns arise after extended periods of time at 'elevated temperatures'. When ceramic fiber is behind the firebrick it never really gets a chance to even be exposed to the temperatures required. Furthermore, the dangerous change (the conversion to the crystobalite form of silica) at ELEVATED temperatures is very gradual and 'slight' - just a small portion gets converted. Seeing how the insulation in wood-fired ovens never get that hot ... well, it wouldn't be an issue.

            Regarding if Mineral Wool and Ceramic Fiber are the same thing: They are if you generalize similarities such as "fiberglass insulation is the same as ceramic fiber" - they perform what we want the same way (insulate by reducing the conduction of heat from a hot face to a cold face by reducing the density of the layer, lowering the thermal conductivity and thermal radiation, etc...). The primary difference is the raw materials (resulting in a different composition, yada, yada, yada), cost of forming, binders used, ....).

            I guess we could look at this differently:
            - I need a car to get me from point A to point B.
            - I could use a good old 1972 VW bug ... or a nice Mercedes (any model & year).
            - I have to pay for it out of my own pocket.
            Which one is a personal decision. However, both will accomplish what I want. And my experience indicates that the mineral wool would be not only cheaper but provide a little more insulation per inch thickness.
            The one drawback is I will probably be using mineral wool board. It is stiff (but cuts very easily with a knife or drywall saw blade (hand powered course saw). It does not conform to a curve. So that means that I will be cutting the board up and/or 'curfing' it to fit. Ceramic fiber blanket is easy and faster to work with.
            My labor costs me ... time. If I were paying someone, I would have to sit down and figure out what I would be willing to pay extra for a skilled worker to hassle with the board versus throwing a more expensive blanket over the top.


            I want to point out here that manufacturers of furnaces take these things into account as well as having to make assumptions about the skill levels of the people that install or repair their products.
            I have a skill level and understanding based on years of experience.
            But how about some person purchasing an oven kit that has never been around a furnace before? For these reasons companies, like FB, will go the extra mile and provide easier to work with materials that might cost more but are easier to work in their kits and maintain a very high level of safety.

            I don't have anything to gain or loose here but thought that I should point out that I only initiated this conversation to point out an alternate insulating material that might make it easier to source than other materials mentioned.


            • #7
              Re: Blanket Insulation

              Heat can only be transferred several ways: Conduction, convection, radiation

              If any one (or more) of those are reduced in any way there is less heat loss. Insulating layers do this in a variety of ways.

              Take an old-fashioned vacuum thermos bottle for example. These are nothing more than a container welded inside another one at the rim. The secret to their keeping things hot or cold is that almost all of the air between the two containers is removed (hence the 'vacuum' in the name). That reduces the convection part of moving heat (gas transferring heat).

              Conduction of heat can be thought of as a function of the density of a material. A block of metal will conduct heat much faster than a wad of feathers (why goose-down feather quilts are so popular during cold and snowy nights). Porosity, air-entrapment, interrupting radiation, etc. all come into play here.

              But heat transfer in all of its complexity boils down to conduction, convection and radiation. Interrupt those and you burn your fingers less and less.


              • #8
                Re: Blanket Insulation

                Quick question about Mineral Wool. I have found a supplier of Roxul AFB Mineral wool which will supply batting material for $40 for 6 sheets of 24" x 48" X 2". Although they specify the material as Acoustic damping and fire resistant, they do not mention insulative properties. Is all Mineral wool the same or can there be huge fluctuations in insulative properties?


                • #9
                  Re: Blanket Insulation

                  Acoustical batts are basically domestic insulation without the vapor barrier. It's not nearly dense enough to provide proper refractory insulation.

                  If you don't want to spring for the refractory blanket insulation, just use lots and lots of vermiculite or perlite concrete.
                  My geodesic oven project: part 1, part 2


                  • #10
                    Re: Blanket Insulation

                    Insulation not rated to pizza oven temperatures can also scorch. It puts out a black sooty smoke and smell that's pretty awful.
                    Pizza Ovens
                    Outdoor Fireplaces


                    • #11
                      Re: Blanket Insulation

                      I am still struggling with how much insulation is required. I have purchases 8# kaowool 1" blanket. I am planning an igloo style oven and don't want spend more on insulation than necessary. Opinions seem to be all over the board. I want to have a fully functioning (not only for pizza) oven. Is 2" enough or do I need more? I have a lot invested already and i certainly don't want to skimp here but I also don't want to spend more than I need to. Is there an official FB opinion?


                      • #12
                        Re: Blanket Insulation

                        I am also building an igloo style oven. I am planning on 3 inches of blanket insulation, with no additional insulation. On to p of the insulation will go my stucco. I have researched what you are asking on the site. I do remember that the consensus was 3 inches of blanket insulation was more then sufficient.



                        • #13
                          Re: Blanket Insulation

                          I used 12" of Roxul blanket, which is a mineral or rock wool product.
                          It is working great. I also put down 3 layers of aluminum foil first as a radiant heat barrier. I did not use a Portland product for mortar which some have said would corrode the foil. I suspect though that even for those who do multiple layers would work fine.