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


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Vermiculite insulation

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  • Vermiculite insulation

    Hello folks, I built my oven last year and mostly so far, so good.
    however I'm having issues with insulating and covering the dome.
    When I first built it I used a 3inch layer of vermiculite, which i covered with a layer of render (sand,cement,lime). When the oven was fired the render cracked and since then steam escapes through the cracks each time it is hot.
    I deduced from this that the vermiculite firstly hadn't dried fully initially then since then rain had got through the cracks/render worsening the problem.
    This past week I've removed the render, the vermiculite was very wet underneath. I've since lit the oven to dry the vermiculte and then applied another 3 inch layer of vermiculite to increase the insulation.
    My questions to everyone are
    1) How long do I leave the vermiculite to dry before rendering again?
    2) Should I fire the oven in the meantime to speed up the process?
    3) What can I do to make sure the render is waterproof? (I bought a bottle of pink liquid from the local builders merchant is meant to improve this, is this enough?)
    4) Is it inevitable that the render will crack/moisture needs to escape? Should I place vents in the render to allow it to escape?
    Overall I know I should probably just build a cover/enclosure for the oven because it rains a lot here (cornwall UK), but I'd really like to keep the dome shape.
    Any help greatly appreciated, many thanks, James

  • #2
    Re: Vermiculite insulation

    Hello James,

    I'm in the process of insulating my dome this week. Not that this would help your situation but I was curious...Why did you decide to not use the ceramic insulative blanket on the dome? I was planning on using 2-3 inches of blanket and then loose vermiculite. The loose vermiculite will be trapped between a stone enclosure and the ceramic blanket. The stone enclosure will be my exterior finish. I wasn't going to use any cement board as the enclosure just build a stone wall around the dome. My concern is that moisture will leak in through the mortar joints of the stone exterior. Not exactly sure what's going to happen...

    Any thoughts? Anyone?

    My Oven Progress: http://www.fornobravo.com/forum/f8/p...uild-7155.html

    If you fail to plan, you plan to fail!


    • #3
      Re: Vermiculite insulation

      You should have a vapor barrier inside the stone wall, a liquid membrane will be the easiest and best.

      For the OP, I d not have a good answer for you in regards to drying out the vermiculite. Maybe leave a hole at the top for the steam to exit, then cap it later. What I WOULD do is keep the oven tightly tarped until the final coat was done so it is not 1 step forward 2 steps back.


      • #4
        Re: Vermiculite insulation

        I'm not sure dome enclosures are really suitable for wet climates like the UK. You may want to think about putting some kind of proper roofing on your oven, not forgetting to flash around the chimney.

        Insulation, of any sort, really won't work at all if it wet, even damp.

        After all, in your climate you wouldn't live in a house with just some stucco spread on the roof, would you?
        My geodesic oven project: part 1, part 2


        • #5
          Re: Vermiculite insulation

          You should cure the oven or expel the moisture from the vermiculite layer before rendering the outside. The steam pressure building up inside is what causes the cracking IMO
          Kindled with zeal and fired with passion.


          • #6
            Re: Vermiculite insulation

            I live on Vancouver Island B.C, which is also wet. I also used a stucco (render) finish, and cover it with a tarp in the "rainy season". You can never fully completely waterproof your oven. You just have to manage it. You will have to "season" it every spring to drive the moisture out.

            I put a 1 inch vent in the top that communicates thru the stucco and into the insulation. I covered this with a decorative copper dish. I also have 3 - 1 inch vents at the bottom at the structural slab level around the perimeter to assist with air movement thru the insulation. I capped the holes with one of these minivents to keep insects out.:
            Attached Files
            Last edited by Neil2; 08-31-2010, 04:13 PM.