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Hearth question - Forno Bravo Forum: The Wood-Fired Oven Community



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Hearth question

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  • Hearth question

    Hello all This is my first post on Here. I started my Pompei Oven Last week. And so far have built the base, and poured the structural slab, and the insulating slab. My question is this: How thick should the insulating slab be? Also, my slab is 69" x 84", I used 3 full bags of Perlite along with 2 large bags (66 or 88lbs) of Portland cement, and topped up with sand to achieve 3/4 yard on finished mix. This was done by a company here in the Niagara Region called "U-Cart Concrete", who delivered the mix to our Café to be poured on-site. Did I use enough Perlite? The insulating slab is 3" thick, will this be thick enough? If not, should I first lay down a first layer of firebrick, then lay my floor down on top of that? Any help would be greatly appreciated.
    I will post pics on next thread (soon as I shrink them down to under 100kb)
    Last edited by Frenchcanuck; 07-11-2007, 10:20 PM.

  • #2
    Re: Hearth question

    Firebrick doesn't insulate - It adds thermal mass. You say "Our cafe" meaning it's a comercial oven fired daily, so added thermal mass won't hurt you, but I think you're a bit short of insulation. This is the only chance you'll ever get to insulate under the oven, so I'd think of another couple of inches of perlite concrete, or even better, an inch of cal-sil board, which would have the added advantage of giving you a really flat area to lay your brick floor.
    My geodesic oven project: part 1, part 2


    • #3
      Re: Hearth question

      Thx for the reply dmun I went by the Oven plans supplied by FB and it says to use 2-3 four cu/ft bags of either perlite or vermiculite mixed in with the portland cement, I opted to use 3 bags and also some sand. Also FB plans say the top insulating layer should be 3.5" thick ( I ended up with 3" ) will it really make that big of a difference? Should the plans be modified to say : Insulating layer should be 5"/ 5.5" thick? for future builders that is.
      I am in Niagara, Ontario Canada, Home Depot is the place to buy most of the materials, I am not sure if they will carry the cal-sil board that you mentioned..... I will check today. Is that board hard or flexible? Forgive me I am new to this.... Cheers


      • #4
        Re: Hearth question

        The board is rigid and brittle. You won't find Cal-Sil or any high temp insulation at Home Depot. Hopefully some of the Canucks will chime in with local suppliers, but you may have to order. See this thread, I found a good spreadsheet that lists all types of insualtion, specs, and manufacturers.


        I agree with Dmun. If this is commercial you will need alot more insulation. I would think 2 inches of board underneath. In fact, if I were to do again, I would abandon the Vermiculite layer altogether and go with insulation board only.
        Last edited by wlively; 07-12-2007, 06:55 AM. Reason: typos
        Wade Lively


        • #5
          Re: Hearth question


          I'm about 75 kms north and east of Toronto and have a high mass barrel vault oven that I use for bread in my microbakery operation. I agree entirely with what's been said about the insulation procedures. You'll probably find retrofitting cal sil board underneath everything on the hearth will prevent heat bleed. I'd also go for another layer on top of your vermic layer. As Dmun says, it will give you a very flat surface on which to lay your brick. Too, you won't have another shot at this, so insulate as much as you can. Believe me, if you don't, holding enough heat for baking will be a problem, and your wood consumption will be alarmingly high. This goes for both bread and pizza.

          I've had real difficulty finding cal sil board in our area. You might try contacting companies that install woodstoves, fireplaces and such. However, the frustration can be taken out of the equation by simply ordering it from Forno Bravo or another on-line supplier. Unlike our cousins to the south, specialty items like this can be hard to find outside of industrial suppliers who aren't interested in selling small amounts. Being where you are, though, it might be easier to take a drive to Buffalo and buy it there.

          The real advantage of cal sil board, apart from it's superior heat stopping abilities, is that you only need two inches of it, while you'd need six of vermic/Portland to get anywhere close. That means you can add it and not run into too many problems adjusting the height of the hearth.

          I used a vermic/Portland layer on my oven four years ago. If I were to do it again, I'd use cal sil all the way. Thinner, simpler, cheaper in the long run, less labour intensive, etc., etc.

          By the way, welcome aboard, eh. Maybe check out my website at :: Mary G's Artisan Breads :: Traditional wood-fired brick-oven breads made to uncompromising gourmet standards. You're not all that far away, and if it would be helpful, swing by my place to have a look at my oven. Send me an email through the forum about that.

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


          • #6
            Re: Hearth question


            An insulating product that I think could be good is from www.roxul.com. Their product is a 'wool' insulation made from rock and slag and can withstand temperatures up to 2150F. It is also a Canadian product that seems to be available across the country in building supply stores such as Home Hardware etc. They have a variety of products from home insulation to boards and blankets. It might be worth a look.

            Ian Johnson
            Kaleden, BC


            • #7
              Re: Hearth question