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Floor Insulation materials & Thermal conductivity chart

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  • Floor Insulation materials & Thermal conductivity chart

    Hi Y'all,

    I am researching underfloor insulation alternatives as perlite, vermiculite and cal sil boards are expensive as hell where i live. I have some questions and would love to see what you guys think

    I am considering a 600 mm x 900 mm x 50 mm ceramic fiber board which i can shell out for but one board will not entirely cover a firebrick floor of radius 17 inches. I would rather get ceramic blanket than a 2nd ceramic fiber board and fill the gaps in the floor with it

    Is it good design to make a frame out of 1.5 inch angle iron > place a 1 inch sheet of ceramic blanket on it > then place the 9 x 4.5 x 3 inch standard firebricks on it > and then glue and layer 3 inches of fiber blanket under the iron frame?

    I think the iron frame being trapped between ceramic sheets might heat up too much? Idea is to use the iron frame so the firebrick can rest on it but it does not crush the blanket



    OR skip the iron frame and just rest the brick directly on the ceramic blanket? i have seen some videos of people doing this in kiln builds, but those bricks are like 1.5 inches thick. The ones I plan to use are standard fire bricks 3 inches thick, i feel the weight will crush the blanket and diminish any insulative properties
    Click image for larger version

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    Another idea along the same direction is cut a 600 mm square out of the ceramic board place it in the center of the floor and then then fill the rest of the missing sides of the floor circle loosely packed ceramic blanket. In my CAD drawing i see on the edges it is just a brick or half a brick that will rest on the blanket so it is less likely to squish.

    With the remaining 600 mm x 300 mm I can cut out 50 mm cubes and surround the round brick floor with these equally spaced insulation cubes > finally rest the 4 dome sections on the insulation cubes and stuff the gaps under the dome with fiber blanket > each 1/4 dome section should be about 50 KG and the compressive strength of the ceramic board which i think is around 0.08 to 0.12 Mpa should handle it alright.


    I also have easy access to Pumice, Glass wool, Magnesia block and CLC block and am thinking of skipping ceramic fiber board and ceramic fiber blanket completely, i will save a lot of money


    My other alternative is to build a cartwheel type frame from angle iron and place a sheet metal cup like floor on top and fill it with clc block > topped with mag block > topped with pumice > I will do this if the insulation value will be enough to do pizza on day 1 and bread on day 2 and 3


    Click image for larger version

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    Some questions I can't find answers to > why use 10:1 perlite mix for the floor, would it not be better to just dry fill and loose fill the perlite and then top it with fireclay and fire board? What do you guys think about magnesia block, pumice and CLC block?


    I am also sharing a chart of thermal conductivity of various materials which i have culled from the internet, can someone Kindly review and correct the data, i am not sure of the accuracy of these numbers:
    Material Density kg/m3 Thermal conductivity W/mK
    Calcium Silicate Board 330-900 0.048
    ceramic-fiber-board 250/300/360 0.14
    ceramic-fiber-blanket 64/96/128 0.30/0.23/0.20
    insulating-firebrick 600 0.23
    Perlite 32 0.051
    Vermiculite Brick 485 0.167
    Vermiculite Expanded 300 0.069
    Vermiculite Expanded 220 0.071
    Vermiculite Insulating Powder 270 0.121
    Exfoliated Vermiculite NA 0.04
    Concrete foam Vermiculite NA 0.14
    Pumice Specific mass 700 0.23
    Glass wool NA 0.032
    Paper flakes NA 0.039
    Cellular Light Weight Concrete Block 800 0.53
    Magnesia Soldering Jewelers Block NA from 10 to 0.181







  • #2
    If you look up the thermal conductivity of steel, it is orders of magnitude higher than required for an underfloor insulation , (around 50 W/mK), so any steel under the floor should be avoided. Additionally as the underfloor is a bit of a moisture trap both heat and moisture accelerate corrosion. Ceramic fibre blanket does not have sufficient strength to support an oven let alone floor bricks and will compress reducing or even almost eliminating any insulation value. Both loose perlite and loose vermiculite can be purchased cheaply as they are used agriculturally. If having difficulty in sourcing it try hydroponics suppliers. The loose material can easily be solidified by mixing with cement and water. A 5:1 vermiculite or perlite to cement is recommended for underfloor insulation to attain the required strength. A 10:1 mix is not strong enough for the underfloor but is preferred for over the dome which carries a much lighter load and doubles the insulation value (over the 5:1 mix) but is still firm enough to act as a suitable substrate for an exterior render. The disadvantage of casting your own insulating slab is that it requires considerable water in the mix which requires elimination, preferably before building over it.

    Check the attachments for further explanation.

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    Attached Files
    Last edited by david s; 07-28-2021, 08:35 PM.
    Kindled with zeal and fired with passion.

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    • #3
      Hey dave nice chart on the vermicrete
      Adding the sand completely changed the vermcrete's thermal profile

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