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



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Hearth insulation

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  • Hearth insulation

    Ok, I was following Alan Scott and put a 2" layer of vermiculite / portland on the bottom of the hearth, then I ended up with a 5 1/2 layer of structural concrete. No I am thinking I should lay in an additional layer of vermiculite on top of the concrete? Any Thoughts?

    36" Pompeii

    Oh, I wanted a 42" oven and screwed up the hearth pour. Looking at the attached photos, can I get away with pouring vermiculite / concrete extending the hearth all the way down to the block?


    Last edited by rnrick; 09-19-2009, 01:43 PM. Reason: additions info

  • #2
    Re: Hearth insulation

    Alan Scott did a lot to popularize wood ovens in North America but I can't, for the life of me, understand putting insulation under the structural suspended slab and isolating the slab from the walls. The thermal mass is enormous and in the wrong place.

    Insulation should go between the structural suspended slab and the hearth bricks. If you don't have vertical room to inset this layer, knock it down and start over.

    You can probably extend the suspended slab (and bury the exposed rebar). Fill in the gap if you want since the structural slab is no longer going to be part of your thermal mass. Form and pour a 4 inch vermiculete layer on the slab and underneath the footprint of the proposed hearth and soldier bricks.

    How does this affect your hearth height ? My hearth is 44 inches high, which is perfect for 6'0" and my 5'4" wife. In my opinion many hearths are being built a bit low.
    Last edited by Neil2; 09-19-2009, 03:04 PM.


    • #3
      Re: Hearth insulation


      I have all the vertical height I want. 2 or 3 inches isnt a problem...

      So whatta think about pouring an additional layer of insulation?

      Knocking it down aint gonna happen, 1300 lbs of cement with 5/8 bar.




      • #4
        Re: Hearth insulation

        "So whatta think about pouring an additional layer of insulation?"

        This will work just fine. (My posting above must have crossed yours during an edit).


        • #5
          Re: Hearth insulation

          5/8 rebar? Wow. I guess it's that suspended slab.

          You're looking good. A little more insulation will make you much more happy, and your oven much more practical.
          My geodesic oven project: part 1, part 2


          • #6
            Re: Hearth insulation

            You absolutely must insulate on top of the slab. You could get away with pouring vermiculite concrete all the way to the edge. It has the compressive strength to support the oven. Will you pour it right over the gap of the suspended hearth?

            You can definitely get a larger oven on that base. You could just pour more structural concrete to extend the hearth (and pretty much fill in the suspended slab), and then add 2" of fornobravo insulating board. Otherwise you need to add 4" of vermiculite concrete. I do agree with neil that the hearths are generally low for the most comfortable use, but they look more symmetrical if they are not too high...

            I think it is lucky that you figured it out now! I started out thinking about the Alan Scott version too, but I am very happy with the mass in the pompeii that I built!

            Also, before you pour vermi-crete to the edge of the stand, think about how you are going to attach your walls, it is a little crumbly for drilling a bolt into...

            My Oven Thread:


            • #7
              Re: Hearth insulation

              Drake and others,

              Thanks you for your input. I was thinking of either the vermiculite all the way out to the edge, filling the voids (didnt know if it would support bigger oven)
              or bringing the structural concrete up the 4 inches or so, then pouring the vermiculite (3.5 inches) over the whole thing.

              Which way would you go. Using the structural concrete will make my walls much easier.....

              Any concerns on the transition between the concrete and cracking?



              • #8
                Re: Hearth insulation

                "Any concerns on the transition between the concrete and cracking?"

                If you are going to expand the structural slab, try to get some rebar in around the perimeter. Any internal cracking at the the join is not going to be a problem.

                Also agree with Drake, now is the time to consider how to anchor your walls.


                • #9
                  Re: Hearth insulation

                  Ok, got the structural cement poured at 1 am..... Neighbors love me.

                  Photos coming of the insulation and wall fix... Time to mix some vermiculite.

                  Oh, scored the vermiculite for $13.00 a bag from a wholesaler! Its the small wins that make it fun sometimes.

                  Thanks again for the input! Makes my own crazy ideas valid.