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Underfloor Insulation - looking at the options

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  • Underfloor Insulation - looking at the options

    I'm part ways into a lockdown oven build. The stand is taking shape using recycled clay building bricks and the bottom concrete layer is on its way shortly so I'm looking at underfloor insulation next and picked up a thread that mentioned "Thermalite" blocks. Worth a look, I thought as they are cheap. Laid on their flats, a square metre would cost about 20 and probably cheaper. They are bulky old things so your insulation layer would be 100 mm thick. Has anyone tried them in an oven build? Would they be stable at oven temperatures or is cracking likely?

    The thermal conductivity ("U value") of a 100mm Thermalite Turbo block is 0.11 W/mK and the U value of the Victas Ceramic Fibre Board is 0.08 at 400C (Lower is better insulation). From another forum, I have found that in the "CIBSE" guide that vermiculite concrete has a U value of 0.17 (not sure for what thickness). I understand that you need a minimum of 100 mm thickness for Vermiculite concrete? Please, note that I am no thermal conductivity expert!

    So the order of good insulation appears to be ceramic FB > Thermalite block > Vermiculite concrete. But cost-wise the order is ceramic FB > Vermiculite concrete > Thermalite block.

    A Thermatlite Turbo measuring 100 x 215 x 440 will cost around 1.70. I'm contacting the manufacturer to see what they think.

    Experience, thoughts, corrections to my data? ...all would be received gratefully!

  • #2
    The numbers seem reasonable. Vermiculite concrete is common, so the cement binder in the aircrete should manage the cooking floor temperature.

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    • #3
      I contacted Forterra, who manufacture Thermalite blocks and they said that they did not recommend laying blocks on their flats as they had not been checked for loading in this orientation. I don't think this is an issue in a wood-fired oven situation! However, more significantly they said that that did not recommend that the blocks were used next to fire as they believed that they would split.

      I guess this is the end of the road for Thermalite and similar blocks?

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      • #4
        How did you get on in the end. I was thinking of using the same thing as I have loads left over from a build.
        This thread shows them being used https://community.fornobravo.com/for...rth-insulation
        My 32" oven, grill & smoker build https://community.fornobravo.com/for...oven-and-grill

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