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Quebec oven insulation? - Forno Bravo Forum: The Wood-Fired Oven Community


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Quebec oven insulation?

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  • Quebec oven insulation?

    I recently completed construction of a Quebec oven using mainly Kiko Denzers recommendations for materials and the insulating layer, a mixture of clay, sand and wood chips about five inch thick. Plaster was clay, sand and straw probably no more than 1/2 inch thick. My problem is the oven continues to lose heat to the outside as indicated by the temperature outside of the oven, too hot to lay your hand on when fired. I am considering applying a layer of the insulating blanket that is offered in your store and recommended in construction of your ovens. Is the blanket material breathable, if so could I possibly apply it to the outside of the layers already set, possibly on some thick clay slip and than plaster over? The oven is fairly large, 36 X 48 inside baking surface and I had hoped to bake at least two loads of artisan bread per firing if not three.

    Thanks for any advise
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  • #2
    Re: Quebec oven insulation?

    You don't say how recently you finished the oven or how many times you've fired it. If it is new it will take many firings to dry out properly. You might just need to keep firing it and you will find its performance improves enormously.
    Kindled with zeal and fired with passion.


    • #3
      Re: Quebec oven insulation?

      Thank's David

      I have only put a couple of small drying fires in the oven since applying the thermal layer, after the first insulating layer I lite a couple of hotter fires and the thermal layer opened up down the middle so I applied a couple more inches of insulation material (clay slip saw dust and wood chip mixture) and than about 1/2 to 3/4 inch of clay plaster, fired it repaired the plaster crack in same area of where the original insulating layer opened up, than fired it for about 3 hours with a moderate fire (not what I would call roaring, possibly wood a little wet) spread out the coals for around 1/2 hour, pulled them out closed up to soak with heat, checked temperature with high heat oven guage on hearth temperature was 500 degrees, cleaned out the rest of the deck and loaded up with 16 one pound loaves sourdough. Thirty minutes later they were not browned and I didn't get any oven spring from the hearth. I must say I was pretty sure the oven wasn't ready really hot enough and possibly not ready for baking but wanted to give it a try anyway.

      At any rate I will continue to fire if for drying and give it another try, I will say that I do still smell some burning in the insulating layer, I supose this is the saw dust and chips buring out to allow for the porus layer of clay that is needed to properly insulate.

      Thanks for the advise and if you have any other tips on curing an earth oven would love to hear them.



      • #4
        Re: Quebec oven insulation?

        I'm not convinced that the clay/wood particle mixture will be a workable insulation layer. It might just be adding to thermal mass, even if it's completely dried out. On that subject, as with any clay oven, that will have to be kept absolutely bone dry to work at all, and I'm not sure a coat of stucco can do that for you.

        Another consideration is the insulation under the oven and floor. It's the same clay/wood mixture?

        Now, mixing clay and sawdust is how they make refractory insulation bricks, which are about as insulating as vermiculite concrete, but they are fired at high temperature, where the organic material is completely burnt out, and the brick is rendered waterproof.

        I think the best plan is to get your oven, as built, as dry as you can, and see how it behaves. If you can't get it to work, I'd get rid of the organic stuff and insulate it properly. Refractory insulation isn't cheap, and you don't want to do this more than once.
        My geodesic oven project: part 1, part 2


        • #5
          Re: Quebec oven insulation?

          Thanks for the advice, I'll keep firing to achieve a bone dry oven, I hope??

          The insulation directly under the hearth is a mixture of portland and vermiculite about 5 or 6 to one vermiculite to the portland about 5 inches thick, below that is urbanite and sand packed solid to the foundation.

          When you say insulating properly with refractory material again would the insulating blankets that are sold at the forno bravo store be acceptable for a clay oven as the oven does need to breath. My thought would be if it comes to this to remove all of the organic material down to the the thermal layer, 4 to 5 inch thick clay/sand mixture, and reinsulate with the woven ceramic blanket up to at least three inches. My question is about installing over a clay oven, would it be acceptable to apply a layer of fairly thick clay slip to the oven in sections as the blanket is applied and than plaster over with either clay or lime plaster to maintain the breathability of the oven. Or is there a better way to install the blanket material if it come to that or is there a more acceptable way to go about reinsulating.



          • #6
            Re: Quebec oven insulation?

            If you do decide to invest in a ceramic blanket to put over the dome, you may want to pull out the phone book to see if there is a commercial insulation vendor in your community. They will likely have the ceramic blanket. It is used to wrap high temp pipes. When I built my dome, I purchased the ceramic board from FB and ended up sourcing the blanket locally. The difference was that buying from FB is very convenient and sourcing locally helps avoid expensive freight charges.