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Perlite Size For Insulation 1-3mm or 3-6mm - Forno Bravo Forum: The Wood-Fired Oven Community

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  • Perlite Size For Insulation 1-3mm or 3-6mm

    The store I'm going to buy my perlite from (for insulation) has two sizes available 1-3mm and 3-6mm. They both are the same price and come in 100L bags for a reasonable price.
    I wanted to know what people thought would be better given the 5:1 (perliteortland cement) under insulation and 7:1 over dome ratio.

    Some factors I'm considering are.
    - insulation factor
    - strength
    - drainage

    I am thinking the larger 3-6mm size will have larger voids which may be a good thing (more insulation?) or a bad based on if the portland cement is all going to drain to the bottom through the voids making it weaker. Any input is welcome. I'm currently leaning towards the larger size so long as the voids won't result in problems (like loss of strength), as I think there will be less powder to deal with.Thanks!

  • #2
    Iíve mixed tons of the stuff and have often pondered the same questions. Here are some things Iíve learned. The smaller the grain size the more water is required. That therefore means more water to eliminate. Ideally with any concrete mix a variety grain aggregate size is desirable. The leaner the mix, the more difficult it is to apply, but also the weaker it will be and more insulative it will be. The correct amount of water is vital. Too little and the mix will not hold together and will make hand application difficult. Too much water and the cement will wash off the grains resulting in an inconsistent mix. Water addition will also vary if the stuff already contains some water from being rained on or if the weather has been really humid. A 100 litre bag should weigh around 10 kg. For every 10 litres of perlite or vermiculite you need to add around 3 litres of water. Best to mix in a barrow with a shovel, mixing dry ingredients first the slowly adding and mixing the water. Using a mixer results in the stuff sticking to to sides and mixer blades. With a barrow and shovel you can see and feel the mix much better. The correct amount of water is when it just starts to pool a little in the bottom of the barrow. For a lean mix you can make it far more workable by adding a handful of powdered clay for every litre of cement added. Getting back to grain size Iíve also found a mix of 50/50 perlite vermiculite makes a more workable mix than either of them alone(not 100% sure why), perhaps getting a variety of grain size in the mix.Perlite contains fine dust that is an irritant the breathe in so wear a dust mask and use the hold your breath technique. Vermiculite doesnít seem to have the same characteristics. If I were in your position I'd be choosing the coarser perlite.The attached table is also really useful. Click image for larger version  Name:	image_83170.jpg Views:	1 Size:	332.7 KB ID:	411046
    Last edited by david s; 02-16-2019, 12:51 PM.
    Kindled with zeal and fired with passion.

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    • #3
      Hey David! Thanks so much for all the tips, I went for 3 bags of 3-6 mm and 1 bag of 1-3mm (just to help fill the voids a little). I'll try to get some clay to aid in the mixing process too.
      I'll try and dry it out for afew weeks before hand too, so at least I can be sure it'll be easier to get consistent hydration. Thanks again!

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      • #4
        Originally posted by peter64 View Post
        Hey David! Thanks so much for all the tips, I went for 3 bags of 3-6 mm and 1 bag of 1-3mm (just to help fill the voids a little). I'll try to get some clay to aid in the mixing process too.
        I'll try and dry it out for afew weeks before hand too, so at least I can be sure it'll be easier to get consistent hydration. Thanks again!
        Consistent hydration is not a problem because of all the free water contained in the mix. Eliminating the free water is the problem especially if you build straight over it. Attached is an experiment i did drying a 5:1 insulating slab that you may find useful.
        Vermicrete insulating slab copy.doc.zip

        Kindled with zeal and fired with passion.

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        • #5
          Thanks again David this is also very useful infomation. I'll leave I'll try to leave the base at least 2 weeks before I start laying the firebricks.

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