No announcement yet.

New Under Oven Insulation Board

  • Filter
  • Time
  • Show
Clear All
new posts

  • New Under Oven Insulation Board

    I have uncovered a whizzy new insulation product for under the Pizza Oven floor. It's an engineered Calcium Silicate insulating board made for industrial applications, such as aluminum melting and holding furnaces. It's about twice as efficient as pure vermiculite board, and more than twice as efficient as castable vermiculite and portland mixed together (which closes an indeterminate number of air holes in the vermilcute and reduces its efficiency).

    It comes in 24"x36"x2" sheets, and a single 2" layer takes an 800F face down to about 160F at equilibrium. Cool. It's easy to cut and clue, making it fast and easy to install. It doesn't even cost that much more than the cost of the vermiculite, and if you calculate in labor costs, it saves a lot of money. With way less hassle. I have a sample sitting on my desk, and it's great stuff.

    We've already added it to the Forno Bravo Store.

    Looking at the installation guide from our precast Forno Bravo oven producer, they recommend vermiculite insulating blocks (which you can buy at the building supply story) in their instructions. This is basically a high-tech version of that.

    Let me know if anyone has any questions or comments.
    Pizza Ovens
    Outdoor Fireplaces

  • #2

    This product would be able to sit on top of the foundation with no other insulation involved? If the above assumption is correct, this would work for either a brick floor or one of your kits?


    Check out my pictures here:

    If at first you don't succeed... Skydiving isn't for you.


    • #3

      Hey Les,

      That's right. You attach it to the concrete hearth with refractory caulk (which we are stocking), and you are done. You can assemble either a Forno Bravo oven, or a Pompeii Oven on top of it.

      Pizza Ovens
      Outdoor Fireplaces


      • #4

        This is great. It's a lot like the fiberfrax board I am using under my floor bricks, and it's cheaper too. How much psi can it tolerate before squashing? With the weight of the oven directly on top of it, can it take the pressure without compression?

        This makes perlcrete a thing of the past. Bravo indeed!

        - Fio
        There is nothing quite so satisfying as drinking a cold beer, while tending a hot fire, in an oven that you built yourself, and making the best pizza that your friends have ever had.


        • #5

          1. So do we add this to the instructions?? Alf early on told me that, if I get his right, will dispense with the reinforced concrete and opt for a welded frame and mild steel tray and then cast the insulting layer on top of this. Now with this we can dispense with both the casting of the reinforced hearth and the vermiculte/cement.

          2. Is it still cheaper to do this when you throw in shipping costs say to Ohio?


          • #6
            Shipping won't be bad

            You only need 2-3 sheets, depending on your oven size, and it's light, so shipping around the country won't be a killer. Not bad at all.

            The compression strength is good, so your oven won't squash it. I will look it up and post it.
            Pizza Ovens
            Outdoor Fireplaces


            • #7
              Caveman not understand.

              Has anyone tried this yet in a practical pizza oven? Where is it made? What is it made of? How long will it last in weather? Do I need to seal it against moisture? Where can I get it for free?


              • #8
                Hey Caveman,

                Always the best questions. I made a spec sheeet, and posted it here:


                It's made in Europe and imported by a furnace supply company. We are now stocking it in California. The two main components are 45% Silica and 45% Calcium Oxide. We are doing the first installations under ovens now, so there is more to come on that. It will stand up to the sands of time, and has a good track record in more demanding industrial applications.

                Like any insulator, it doesn't want to get wet, and needs to be set inside your waterproof enclosure. I don't know if the insulation would break down, but it would take heat out of the oven to drive out the moisture. But then any water that made it to the floor insulation would probably have already done damage to your woven ceramic insulation around the dome.

                A funny aside. One of our competitors (that I like poking fun at) still recomends that you use 6" of SAND in the hearth -- between two layers of concrete. Talk about a sponge attracted to water. I have been to one party and talked with someone who attended a different party/different oven, where the hearth sand had absorbed water from winter rains, and the oven just would not heat up. It was stuck at 350F after hours of firing. We had take out pizza, and later heard that the oven took over a week of fires to dry out.

                Finally, I don't think this is something you can find in somebody's back yard. Oh well. I have heard that crushed glass and ashes from wood fires are good old-fahsioned insulators. If you live near a volcano, tufa works well.

                Pizza Ovens
                Outdoor Fireplaces


                • #9
                  Thermal Conductivity

                  I have been corresponding with a builder regarding strength of vermiculite cement mix, volumes of 5 gallon buckets and how to get a 6:1 mix. The supplier of the Vermiculite provided a nice handy chart that showed the material strength, density and the thermal conductivity (k). k, is the physical property of a material that indicates its ability to conduct heat. The smaller the k the better it will insulate. According to, a 6:1 ratio ranges from 0.69 to 0.73 and an average compressive strength of 155 psi, cork, fiberglass, mineral wool on the other hand can be as low as 0.04., however cork will squish like no tomorrow.

                  Thermal conductivity of this product is a bit better, plus the numbers show the conductivity at higher than standard day temperatures which is till good.

                  Are the units in the Compressive Strength correct? PSI maybe? The spec say 377 lbs/cu ft which doesn't make engineering sense in terms of units. If it was meant to be lbs/sq ft then that is equal to 2.6 psi - that means you can poke a hole in it with your index finger. A brick spreads the surface area out and may not crush it, more on the lines of 0.22 psi.

                  Note in the spec:

                  Modulus of Rupture: This measurement shows the maximum bending load that a beam can support. The number indicates the stress required to cause failure. The higher number means that a greater force is required to cause failure.

                  Compression Strength: Compressive strength is the capacity of a material to withstand axially directed pushing forces. In this case think of loading up the item against a surface where it can?t bend. Easy way to look at this is in terms of jack hammering. If the hammer isn?t hit onto the concrete walk way hard enough nothing will happen. Hit it really hard then the head of the hammer will crush/pulverize the concrete. Now if you loaded up the concrete to the same load that was applied by the hammer then a static load would also crush the concrete. See any California Highway for a correlation. Now if you had foam then of course it will compress thus the terminology, compressive strength.
                  Last edited by jengineer; 07-06-2006, 02:58 PM.


                  • #10
                    Vermiculite Concrete

                    I have to say that I'm impressed after reading the attachment. I'm having trouble following this through to a conclusion. I'd like to know if this says that the compressive strength of this and perlite concrete really is sufficient to hold up a firebrick dome and floor. Also did this conclude that the vermiculite and perlite versions of concrete will provide the appropriate amount of insulation for this application. And of course, does the new insulation technique do as well.

                    I'm already poured in the hearth area and my floor is down, so I probably wouldn't make any changes unless someone says "Stop already!"

                    Thanks for the great research.
                    "The road of excess leads to the Palace of Wisdom."


                    • #11
                      Hi Mike,

                      Vermiculite has great compression strength and will easily hold up your oven, arch, etc. As someone once noted, vermiculite concrete is used under swimming pools, that are a lot heavier than your oven!

                      On the Super Isol compression numbers, I will have to check. It has lots of little air holes, but it has good compression strength -- I just tried to push my thumb into it, and cannot make an impression. I had to push hard with the cap of a pen to make an indentation.

                      Pizza Ovens
                      Outdoor Fireplaces


                      • #12
                        Mike - definitely go for it. I am just nit picking and trying to keep my brain thinking. I probably should have made my post a private post to James. Doing a Google search I think that the units are wrong (ft^3 instead of in^2). that is also supported by quick experiment that James did.

                        If you search the threads you will find that the compressive strength of vermiculite has been discussed and approved.

                        Or you could read my ramblings below:

                        POOL COMPRESSION CALCULATION (Simplified)
                        1 gallon of water equals 8.33 lbs.
                        A 5 gallon pail has a diameter of 11.125" and a height of 13.25"
                        5 gallon pail/bucket contains .75 cubic ft.

                        take a column of water in a pool that has the dimensions of the 5 gallon bucket and a pool depth of 12 feet. What is the compressive equivalent pressure?

                        the pool is 12 feet high or 144 inches and the bucket is 13.25 inches : 144/13.25 = 10.9 buckets call it 11

                        there are 5 gallons/bucket and you have 11 buckets so you have a total of 55 gallons

                        there are 8.33 pounds/gallon of water and you have 55 gallons : 8.33 X 55 = 458 lbs round it up to 460 lbs.

                        that weight is on a diameter of 11.25 inches or on a surface area of 99.4 in^2 (Area = pi d^2 / 4)

                        so you have 458 lbs pushing down on 99 in^2 or 458/99 = 4.6 psi

                        vermiculite & cement = vermiculite concrete, at 6:1 mix has an average compressive strength of 155 psi

                        POMPEII COMRESSION STRENGTH
                        I am going to make some gross assumptions (lean toward conservatism) here.

                        Assume the weight of the oven is evenly spread about the circumference of your first ring.
                        Assume that each brick weighs 10 lbs (conservatism since they really are closer to 8).
                        Assume that the dome takes 200 bricks (a 42 inch dome is 180 bricks)
                        Assume you decide to use a fa?ade of bricks, say 350 and instead of bearing the weight on their own circle you manage to make the dome bear the weight.
                        Assume the width of the weight bearing brick is 3 inches and the inner diameter of the oven is 42 inches thus the outer diameter is 45 inches (that will give you the area that bears the load)
                        Assume the opening for the door is 15%

                        Area of a 45 inch circle is 1590 in2
                        Area of a 42 inch circle is 1385 in2
                        Subtract the inner circle from the outer to get the area of a Closed Oven is 205 in2.

                        Remove 15% for the door and your bearing area is 174 in2

                        that area is loaded up with 550 bricks at 10 pounds each or 5500 lbs

                        How much pressure is it made to bear? 5500/174 ~ 32 psi way below the compressive strength of 155psi

                        Last edited by jengineer; 07-07-2006, 10:50 AM.


                        • #13
                          Now I get it


                          Thanks for expanding the logic on this. I haven't had any doubts about the perlite or vermiculite concrete strength because of the information that's been presented so far. Now that you've done the math it is reassuring but more than that it's fun to see the math problem worked out. I just couldn't see where you were going at first.

                          "The road of excess leads to the Palace of Wisdom."


                          • #14
                            glue for insulating board

                            If I were to glue the new whizzy insulating board to the vermiculite layer under the hearth what type of glue would you suggest Would silicon work? Is it worth the trouble re heat retention in the hearth ?


                            • #15


                              I'd go for Bulldog Premium construction adhesive. It gets stronger with heat. Not much of a chance the board would come down. Alternately, try to find Bond Lock. We use it for stone, and it's even stronger.

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