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Medium vs Fine grade Vermiculite? - Forno Bravo Forum: The Wood-Fired Oven Community

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Medium vs Fine grade Vermiculite?

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  • Medium vs Fine grade Vermiculite?

    Hi,

    I've been reading up on the insulation hearth and as far as i can tell no-one has asked this question, but apoligies if has laready been crossed.

    For making the insulation hearth, which grade of vermiculite is best, fine or medium?

    Here's the link to the shop im going to buy them from ScotPlantsDirect|Perlite and Vermiculite| Orders 49.99 and Over Free Delivery - Growing Needs - Perlite / Vermiculite

    Many thanks for any help

    Cheers Ross

  • #2
    Re: Medium vs Fine grade Vermiculite?

    I would go with the medium. During my build, I needed one more bag and it ended up being the fine stuff; think sawdust. (was in the same package, just a different store). I think when it is mixed with the Portland, it will not have as good as insulation as the coarser product.

    Les...
    Last edited by Les; 07-13-2010, 03:04 PM. Reason: spelling
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    • #3
      Re: Medium vs Fine grade Vermiculite?

      Nice one, cheers Les.

      Also, your oven looks a peach!

      Cheers Ross

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      • #4
        Re: Medium vs Fine grade Vermiculite?

        The larger particles will insulate better. You don't really want dense cement in there, it's just glue to hold the vermiculite in a solid, so the bigger the vermiculite chunks are, the tougher time heat will have diffusing through the insulation.

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        • #5
          Re: Medium vs Fine grade Vermiculite?

          I live in metro Detroit area and need to know where to find vermiculite? Thanks in advance.

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          • #6
            Re: Medium vs Fine grade Vermiculite?

            Hi Rolaed,

            I couldn't tell you anywhere in specific, but here in Scotland it was dead easy to get a hold of. It is used in gardens so most decent gardening stores sell it. It is also on amazon (in the UK).

            Cheers Ross

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            • #7
              Re: Medium vs Fine grade Vermiculite?

              Also, thanks for the advice Kebwi

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              • #8
                Re: Medium vs Fine grade Vermiculite?

                The density of vermiculite seems to be the same whether it is fine medium or coarse, so theoretically it should make no difference to the insulation value. I had to switch to using fine grade when I couldn't get the coarser stuff, but I found that it makes a more consistent end product (less prone to crumble away) and easier to apply, so now I use the fine stuff all the time. But try it yourself and make up your own mind.
                Kindled with zeal and fired with passion.

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                • #9
                  Re: Medium vs Fine grade Vermiculite?

                  Thanks David. I've ordered a 100 litre bag of medium grade to use for the insulating hearth. I'll maybe buy a bag of fine stuff for the igloo.

                  Cheers Ross

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                  • #10
                    Re: Medium vs Fine grade Vermiculite?

                    I finally found a local store that sells it(garden supplies place). I guess Vermiculite is used in gardening too.

                    Question to all:

                    On the hearth slab, I have heard two stories. First you put Vermiculite and then on top the cement. Or visa versa, first the cement and then the vermiculite. How deep should each layer be?

                    Same for the dome. Put the vermiculite on first and then the cement/stucco coat?

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                    • #11
                      Re: Medium vs Fine grade Vermiculite?

                      Im not one of the experts, but as far as i was aware you lay a 4 inch structural hearth (pure concrete) and then lay a 4 inch insulating hearth by mixing the vermiculite with the concrete on top of that.

                      Cheers Ross

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                      • #12
                        Re: Medium vs Fine grade Vermiculite?

                        I have heard two stories. First you put Vermiculite and then on top the cement.
                        This approach has been entirely discredited: This leaves entirely too much thermal mass inside the insulation envelope. Support slab first, then insulation, then brick hearth.

                        Four inches of vermiculite concrete, top and bottom, is the minimum. The stucco coat is for waterproofing/finish. You don't want to seal the dome right away, do your curing fires first to drive most of the water out of the insulation. Vermiculite concrete absorbs a LOT of water.
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                        • #13
                          Re: Medium vs Fine grade Vermiculite?

                          So one has to wait until the 4" cement slab cures before pouring in the 4" vermiculite?

                          So the 2x6s used to form the cement slab are removed and then placed on top of the cured slab and vermiculite poured into it.

                          I am just trying to document the steps.

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                          • #14
                            Re: Medium vs Fine grade Vermiculite?

                            with regards to a hearth slab which is say 4" or 100mm,half can be concrete and the top half can be a mix of vermiculite with portland cement,this is amble.then toped with fire bricks it will be 8" inches of thermal mass,cant get any better than that.

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                            • #15
                              Re: Medium vs Fine grade Vermiculite?

                              Thanks.

                              How about the cladding on the dome. Same thing. 2" vermiculite/portland cement and 2" concrete on top?

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