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Perlite or Vermiculite insulation ? - Forno Bravo Forum: The Wood-Fired Oven Community


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Perlite or Vermiculite insulation ?

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  • Perlite or Vermiculite insulation ?

    I am currently waiting for the oven cladding to dry. Its about 5" thick in most places, a little thicker round the top. I am not sure if I should now add Vermiculite insulation, mixed in 6:1 with Portland cement, or loose Perlite.
    I have both available, they cost exactly the same per bag, and I could use loose or a cement mixture, since I already build a outside wall, and now have a gap of about 4-5" between outside wall and oven cladding.
    Any pros or cons of either. What insulates better ?
    Thanks for any suggestions.


  • #2
    Re: Perlite or Vermiculite insulation ?

    I was talking to the owner of a refractory business today, and I mentioned the use of vermiculite or perlite. He was quite emphatic that perlite wouldn't perform as well as the vermiculite, but I didn't think to ask him why.



    • #3
      Re: Perlite or Vermiculite insulation ?

      try this page:

      SCHUNDLER COMPANY--Perlite Block and Cavity Fill--Superior to EPS Inserts and Beads


      Tests peformed by Dynatherm Engineering reveal that perlite is 6% more eflective as a core fill insulation than vermiculite. Although the physical characteristics of the two materials are similar, the closed cell structure and lower thermal conductivity of perlite account for perlite's superiority.
      it's probably a bit biased

      but if you'd like to buy the actual Journal of the Test, it can be found here:
      Last edited by Mitchamus; 11-09-2009, 03:04 PM.
      My 2nd Build:
      Is here


      • #4
        Re: Perlite or Vermiculite insulation ?

        I used 6 perlite (coarse grained) + 6 vermiculite + 1 portland.

        I found that the perlite made the vermiculite mix more "workable".

        A 12:1 mix has more insulation value than a 6:1 mix and is more than strong enough for the layer over the dome. (I used the 6:1 ratio for the insulation layer between the suspended slab and the hearth bricks.)

        If you have containment, you can also add it loose of course. If using loose material, I would use straight vermiculite - the particles are larger and less likely to "leak" out of any small joints.
        Last edited by Neil2; 11-09-2009, 03:24 PM.


        • #5
          Re: Perlite or Vermiculite insulation ?

          Originally posted by Neil2 View Post
          A 12:1 mix has more insulation value than a 6:1 mix and is more that strong enough for the layer over the dome.
          If you going for an enclosure around your dome - loose fill is the way to go.
          My 2nd Build:
          Is here


          • #6
            Re: Perlite or Vermiculite insulation ?

            Practical experience has shown that the two are interchangeable. I just chose what was affordable (perlite), as I put a lot on top of my oven, as well as behind surrounding the firebox of my fireplace.
            My geodesic oven project: part 1, part 2


            • #7
              Re: Perlite or Vermiculite insulation ?

              Thanks guys, I guess I`ll try to loose fill it with Vermiculite then. I will let it dry for a total of 3 weeks, and then try a big fire and see how long I have to heat it to feel heat on the outside of the cladding, so I know how long it takes to fully heat the oven.
              I already had a fire just for the firebrick and arches, before I poured the cladding. I saw some smoke creeping through some tiny cracks, and closed those with clay before I added the cladding layer. So that first part is all dry already.