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Thickness of Vermiculite/Concrete Insulation Base

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  • Thickness of Vermiculite/Concrete Insulation Base


    I am having trouble figuring out how thick to lay my vermicrete slab on top of my concrete support slab. I've seen anywhere from 2'' to 8'' thick of a 5:1 mixture. I would like to keep the thickness and weight down to a minimum, if possible.

    I am building a dome, most likely out of firebrick, and I think it is most cost effective for me to go with the vermic/concrete mixture as opposed to Ceramic Fiber board.


  • #2
    Min 4" 5 to 1 p or vcrete is abt the same as 2" of CaSi or AlSI board.
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    • #3
      The big advantage of a 5:1 vermicrete slab is in cost. The downside is you have to make it thicker than cal sil. for the same performance. It also contains lots of water that needs drying before building over it.
      Kindled with zeal and fired with passion.


      • #4
        Sorry for questioning, but how come is the recommended ratio specified to 5:1 (~200 psi) while the FB board is ratad to 75 psi as compressive strength? Has anyone tried a leaner (~8:1) mix which collapsued?

        discussion from the past:


        • #5
          Try making an 8:1 vermicrete and I think you’ll agree that it’s not strong enough. Maybe the finer texture of the FB board, which I’m not familiar with, is adequate. Some builders have reported issues with it compressing if it gets wet. Cal sil board can get wet but it’s strength doesn’t alter. Probably the best solution is foamglass which doesn’t absorb water.
          Last edited by david s; 05-06-2020, 12:59 PM.
          Kindled with zeal and fired with passion.


          • #6
            Okej, thanks. As I understand, it is quite trickey to value compressive test results of insulating materials as they are so fragile. What the hand feels does not agree with the measured value and according to your description it seems reasonable.

            By the way, I got 7 m3 EPS-concrete poured in the basement a year ago. It is 450 kg/m3, 0.08 W/mK & 2 MPa (300 PSI) and I agree its pretty brittle to the touch.

            When I calculate the pressure from the dome (32" ID, 300 kg) , I get a silly low pressure of about 0.012 MPa (1.7 PSI). By just reading the numbers, the safety margin is high, hence the question.
            Last edited by Petter; 05-06-2020, 10:33 PM. Reason: Wrong pressure. 0.012 MPa is correct.