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Laying fire bricks on insulation

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  • Laying fire bricks on insulation

    Hi, I am building pizza oven and need some advice. I have ceramic fibre board for installation but am unsure if I can lay fire bricks directly on top. I've heard you need to put a layer of castable refractory on top of the ceramic fibre board ( not sure if it should be insulating refractory or not), also heard just put sand on ceramic board and lay directly on top. Any advice on this please? David

  • #2
    You don't need to put anything between the bricks and the CF board for performance, but you might need to use something to serve as a leveling compound. My boards went down flat so I just put my bricks directly on the board. I actually found I had quite a bit of variation in the thickness of my bricks, at least enough that on a level surface some bricks were proud relative to others. Just needed to sort enough of similar thickness to do the floor.
    If you do need to level the floor the FB plans suggest a mixture of sand, fire clay and water to make a paste and spread with a notched trowel. This seemed like an additional hassle to me so I was glad to get by without. Whatever you use you don't want to bond your floor bricks to your insulation. They aren't going to go anywhere once you get them surrounded with the first row of bricks and if you ever need to remove one you'll be glad they aren't glued down.
    My build thread


    • #3
      Thanks for the response JRpizza, I wasnt too sure if the CF was enough insulation as I was reading you need 100mm of insulation but the CF board only comes in 25 or 50mm thickness. Am I right to have the CF board about 25mm overlapping the fire bricks, then the insulation blanket sits on top of the CF, or does it need to be sized the same as the outside brick dome and the insulation blanket goes over the outside if it?


      • #4
        I'd suggest reading up on insulation thickness in different posts on the forum to see what others do. There is a point of diminishing returns with too much insulation, but more is better than less. I ended up going with 5" as I got a good deal on 2.5" board and did two layers. I probably would have done 4" if it was more cost effective but glad I did 5". If you have 100mm under your floor the oven won't care if it is 4 layers @25 or two @50. You don't want any bricks you heat to touch the hearth, so insulation goes both under floor and dome bricks. I build mine with the dome bricks right out to the edge as my dome OD was 48" and my board arrangement put me at 48" without much waste.
        My build thread


        • #5
          Ok cheers for the info. I'll go with 4" thick. I'll have a read up on whether to use vermiculite or perlite for dome.


          • #6
            Perlite is a slightly better insulator than vermiculite, but once you add cement to it there’s no measurable difference in its insulating capacity. Even a quite lean 10:1 mix contains as much mass (thermal conductivity) from the cement as from the perlite or vermiculite as it’s 10x heavier.
            Perlite contains dust which can be an annoying irritant to inhale, whilst vermiculite doesn’t have this property. I use a 50/50 mix of each. Get which ever is the best price. For me they’re the same.
            Last edited by david s; 03-14-2021, 06:51 PM.
            Kindled with zeal and fired with passion.


            • #7
              JRPizza and DaveS,

              Is there a concern long-term with ceramic fiber board deteriorating over time and releasing ceramic fibers into the cooking dome? Is there a way to add a separation layer of some sort between the ceramic fiber board and firebrick floor? Seems like constant expansion and contraction of the firebrick floor during heating and cooling would abrade the ceramic fiberboard surface over time.

              It may not be a concern but I'm not completely sure of the durability of these ceramic fiber board materials.

              Thank you,



              • #8
                Rick, I just laid my bricks on top of the my Insblok board and have not heard of anybody else doing anything to mitigate the possibility of CF board fibers migrating up through the small spaces between bricks and contaminating the cooking area. I was very careful to make sure that no blanket fibers could get into my oven and used a Nomex type gasket material for my heat break so I would not have glass fibers dropping down on the cooking floor. I also have CF board inside my door but that is welded up tight.

                PS, you posted a visitor's message to yourself asking about builds in cold climates that is kind of hidden from general view and probably won't generate any answers. Best bet would be to do a detailed forum search and if you don't have any luck making a post.
                The forum search engine leaves something to be desired so try copy/paste the link below - it generated about 351 hits.
                frost fornobravo
                My build thread


                • #9
                  I use ceramic fibre under the floor, I find it hard to imagine how loose fibres could migrate up into your food but it is a sensible question as CF products should be given full respect!
                  Floor bricks do indeed expand and contract but it is not like large gaps appear as the amount each brick expands is very slight and only noticeable when many bricks expand as a mass.
                  In any case what happens is the gaps between the bricks are filled with ash so if any fibres were to attempt a rise to the surface they would not only have a tiny space to migrate through but a space filled with ash.
                  Ceramic fibre board will form a hard, non dusting, surface when it is expose to high heat although I doubt if enough heat would penetrante the bricks to make that happen, I dont actually know to be honest.
                  If this is a majour concern to you, then a simple method would be to seal the fibreboard surface with a waterglass product or ceramic board sealer.

                  Finally there are several CFBs available that are said to be completely safe and body soluble , or in other words wont sit in your lungs for 20 years but dissolve and pass through your system.
                  Last edited by fox; 09-27-2021, 12:28 AM.