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Insulation under hearth

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  • Insulation under hearth

    Hello everyone . I am just starting the construction of a 36" diameter wood oven. Where i am in canada i cant seem to find or purchase ceramic board insulation for under the hearth. I can obtain Roxul 80 which is a dense( not as dense as ceramic) board at 1 1/2" thick. My thought is to use a single layer? ( or double layer) and then put a 1/4" concrete backer board on top in order to ensure the weight dispersion of the dome will never compress the insulation. It would also provide a very flat surface for the firebricks. Ceramic fiberboard has a density of 16lb/ft3 and roxul 80 has a density of 8lb/ft3.
    The other question i have is the type of brick for the oven vent and arch. Do you need to use fire brick or can you use regular brick for this area as it is not exposed to the higher temps?

    Thanks so much everyone i am super stoked to get this project underway.

    Cheers, N

  • #2
    Recommended "compressive strength" for floor insulation should be around 70 PSI at 5% compression. The Roxul 80 product only has about 3 PSI at 10% compression. It is way too soft for floor insulation. Look at refractory suppliers, you will not find CaSi board at a big box store. If this fails, you can go with a perlite or vermuculite/portland mix at a 5 to 1 ratio just twice the thickness equivalent to CaSi. Don't head the Roxul route, the ovens are really heavy and need a solid base. The vent area gets exposed to fairly high temps and subject to flame impingment, fire bricks recommended.
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    • #3
      Thank you so much for the quick response. Will the perlite/vermiculite at twice the thickness( 4" then) give same insulation value?

      Thanks again.



      • #4
        Here's table on pcrete K values and strengths. I will leave it up to you to determine, as I mention in the previous post it takes roughly twice the thickness to be equal to CaSi. Have you downloaded and reviewed the Free e-plans from Forno Bravo, it is a good baseline and although dated has a lot of pertinent information. Also most answer to questions can be found on the blog via the search command.

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        • #5
          Thank you for that. makes sense to go with the perlite as the price for the ceramic board is around $500

          Cheers, n