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Pumice as Insulation - Forno Bravo Forum: The Wood-Fired Oven Community

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I'm Peter Reinhart! Ask Me Anything! Monday, February 15, 2016 7:00-8:00 pm EST

To kick off our AMA feature, we have invited author, chef and master bread maker and host of Pizza Quest, Peter Reinhart, to be our first host! Peter will be in the Forum on Monday, February 15th, from 7:00 - 8:00 pm EST. If you are unable to be online during the live session, you can post your questions in the sticky post. Peter will answer those questions during the live session on February 15th. You can view Peter's answers to your questions as well as what happened during the live session in the session thread.

Ask Me Anything New Forum Feature

You will notice a new forum at the top of the main page called, "Ask Me Anything". This forum will be used for live one hour "Ask Me Anything" (AMA) sessions hosted by people who are knowledgeable in different areas pertaining to wood fired ovens. How it works:
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Pumice as Insulation

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  • Pumice as Insulation

    Greetings from New Zealand, a country with more than its fair share of volcanoes. As a result we have a lot of pumice.
    Does anyone have any advice on using pumice as insulation? Can I use the same mix as for for perlite or vermiculite concrete (5 pumice:1 cement)

  • #2
    Re: Pumice as Insulation

    I am trying to find out the same thing. Good luck. I think it will be ok as long as the pummice is not to fine. Otherwise it would be like using sand.

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    • #3
      Re: Pumice as Insulation

      Originally posted by Jimney View Post
      I am trying to find out the same thing. Good luck. I think it will be ok as long as the pummice is not to fine. Otherwise it would be like using sand.
      Actually I think it 's the opposite. Fine vermiculite is easier to make a workable mix than coarse stuff. Suggest you crush it up to grains of max size of about 6mm and discard powder. That's the usual technique of using crushed insulating firebricks as aggregate. It's time consuming hard work but should work ok.
      Kindled with zeal and fired with passion.

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      • #4
        Re: Pumice as Insulation

        Crushed pumice is not an insulator. Pumice in macro form is an insulator because of the millions of tiny air bubbles. Crush it, and you just have glass dust.

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        • #5
          Re: Pumice as Insulation

          Originally posted by david s View Post
          Actually I think it 's the opposite. Fine vermiculite is easier to make a workable mix than coarse stuff. Suggest you crush it up to grains of max size of about 6mm and discard powder. That's the usual technique of using crushed insulating firebricks as aggregate. It's time consuming hard work but should work ok.
          If you can break it up to small grains as I suggested you will still have grains containing air, but discard the dust.
          Kindled with zeal and fired with passion.

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          • #6
            Re: Pumice as Insulation

            Just covered this in a recent thread. The data I can find says pumice has a thermal conductivity of 0.58 in the metric scale. Perlite is 0.031 and Vermiculite is 0.058. As you can see there is a full order of magnitude between the insulating ability of Perlite and Vermiculite and that of Pumice.

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            • #7
              Re: Pumice as Insulation

              Again, you have to go back to what your design criteria are. If you want to use it as an aggregate in a structural concrete member, then it is considered insulating (and lightweight). If you are using it as insulation, then it is not so good, and the smaller you pound it, the worse it is.

              In macro form, shaped into bricks, it is not that bad if it is cheap and available, and in aggregate (not fines) form it makes a decent insulation for lightweight structural concrete.

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              • #8
                Re: Pumice as Insulation

                Yes it is, for the structural slab, but do not confuse that with using it as an insulator. Using the criteria for structural concrete, it is considered an insulator, using the criteria of insulation, it is a poor choice in granular form that gets worse the finer the granules.

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                • #9
                  Re: Pumice as Insulation

                  Originally posted by shuboyje View Post
                  Just covered this in a recent thread. The data I can find says pumice has a thermal conductivity of 0.58 in the metric scale. Perlite is 0.031 and Vermiculite is 0.058. As you can see there is a full order of magnitude between the insulating ability of Perlite and Vermiculite and that of Pumice.
                  I couldn't find appropriate data on the thermal conductivity of pumice, but with such a high number you quoted, I'm sure if you take another look it will be referring to concrete with pumice added as an aggregate rather than pumice alone.

                  If pumice is freely locally available and folk have the energy and time to process it into a material suitable for insulation then I think that is the noble path to take.
                  Kindled with zeal and fired with passion.

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                  • #10
                    Re: Pumice as Insulation

                    Thanks guys. For some reason i was emailed only one of your responses, so I went ahead with using it as underfloor insulation to start with. This 100 mm thick base sits on a 90mm thick concrete slab
                    I used 7 mm pumice and mixed a fairly dry mix at 5:1 with cement. It had the consistency of fluffy popcorn (but smaller). I added beer bottles to the base and lightly packed the pumicecrete around them.
                    I didn't tamp the pumicecrete down as that would remove the air voids, which would lessen the insulation.
                    The concept looks good as it appears to have a lot of air in it.
                    I believe it can only be better than sawdust and clay mixes.
                    Incidentally, in New Zealand, hot water cylinders used to be slated with pumice. That's where I got the idea.

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