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Has anyone heard or experienced any down side to using pumice as a building material for the floor or walls?

GBA Editor | Posted in Green Products and Materials on

I am thinking of building with a 12 part pumice to 1 part cement mix as wall material with standard 3 layer stucco cement as wall covering . I heard New mexico pumice had possible radon emissions ?
I am wondering about moisture condensation inside of wall at the dew point leading to mold ?
I live in western oregon with high humidity ,the pumice would come from our abundant pumice fields.
pumice can’t rot can’t burn and has 1.5R per inch .
The only down side I see is there are no codes for pumice and would need to get an engineers report for earthquake stability using rebar.

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  1. Armando Cobo | | #1

    I’ve heard of a couple of builders in NM that build with PumiceCrete, for more info:
    1. Scott Machardy in Taos, NM, [email protected]
    2. LX&R Design, Abiquiu, NM, [email protected]

    My HERS rater in NM has worked two homes built with pumice; unfortunately the owner-builders have moved overseas. One thing he told me is to make sure your rater knows that the Rvalue depends on the mix design and density, and that you can find those values at the ASHRAE Fundamental

    There is a book called Building with Pumice, which you can buy or read it on this site:

    Near your neck of the woods there is The Rockwood residence, Hayden Island, OR ( and was done 20+ years a go with a wall design similar to ThermoWall system. Also you can check:

  2. Riversong | | #2

    Apparently, there are a great number of buildings, including high-rise, that have been built from pumice concrete. The compressive strength may be significantly reduced depending on the size and ratio of aggregate (as little as 400 psi with 50 pcf cured weight).

    While the insulative value will be far superior to the 0.08/inch of concrete, you won't get close to 1.5/inch. It will have an R-value of 0.25 to 0.50 per inch, depending on density.

  3. Sunlover in a Cloudy Clime | | #3

    I've been researching the heck out of this, the last few weeks, all over the internet (and the world; pumice is used for building almost everywhere). The bad news: pumice is not R2/inch, and probably rarely R1.5.

    The R-value of pumice is almost certainly going to vary, from volcano to volcano and eruption to eruption, and even with the stage of the eruption, with the composition of the parent rock, and of the entrapped gasses; also the winds will carry lighter, frothier, better-insulating pumice farther from the volcano. That said, the only scientific evaluation of pumice' R-value I've found is one done by Oak Ridge National Lab (ORNL), and to get the gist of it, you have to read the report on the report;

    Actually this paper is the results of two tests muddled together. In the first they tested pumice as received, at about 40% moisture; the R-value was only ~0.6 dF. ft^2 h / BTU. As they progressively oven dried the sample, its R-value rose to ~1.2, at a bone-dry density of ~25 PCF. In the second test, pumice tested at ~20% moisture was 0.53 dF. ft^2 h / BTU; dried to 7% moisture, that rose to R0.85. Somehow they got a surface-to-surface performance of R-1 / inch out of that. But, as they explain, the thermal mass factor (and lack of thermal bridging through framing) increases the apparent value of the insulation. A frame wall in Phoenix (the best of six climates tested) would need 97% more insulation R-value to perform as well; a frame wall in Minneapolis, the worst case, would still need a 38% higher apparent R-value to produce the same heating/cooling loads. I'm considering 18 inches of compacted pumice under a thin, solar slab floor; if I assign it an apparently mid-range R-value of 1, but bump that up by 138%, it might perform about like an R25 frame wall. Or it might not.

    A 65 cu yd semi-trailer full would cover 1170 sf 18 in. deep; at $9 / yd, cost $585; trucked from Chemult, Oregon to Portland, ~ $850, or ~ $1450 total (South Central Pumice LLC). By the unit, 2" XPS is ~$50/4x8 ft sheet 2" thick (Parr LBR); 1170/32 sf is 37 sheets, or $1850. Even at only R0.85/inch, 18 inches would be ~R15, half-again the required performance under a heated slab, (but still half what I'd like) for less money than XPS, and far less environmental impact--XPS is a nasty thing to do to a planet you like.

    What I'm not yet certain of is whether the moisture-Radon barrier (MRB) should go on top of the pumice, between it and the slab, (which can be permeable, or not) or under the pumice. And if the MRB goes on top, do I need a capillary break between pumice and ground?

    As for walls, ORNL says that adding Portland cement (in heavy pumice block, maybe 100 pcf) reduces the R-value by a factor of 10; lightweight block, 50-60 pcf, would still likely perform five times worse than loose pumice. The best I can do, even for pumice-crete, with all the voids in it, is around R0.5 per inch. There are cements other than Portland, most of which are "greener"; I'd like to know how pumice would work with (foamed?) magnesium oxy-chloride. But the most promising method I've seen so far is earthbag construction. You can use the new continuous polypropylene tubes, but any livestock operation will have feed sacks to give away. As ORNL only got a surface-to-surface value of R16 for 16-inch pumice-bag-walls, though, I might look for bags more like 20 inches wide (and something more than 3" deep, when filled). Plaster inside with a thick, sand-aggregate (stores more heat) or earthen plaster for thermal mass, and outside with a pumice-lime plaster (gets stronger with age). KEEP BOTH PERMEABLE and the wall should perform like cobb, drying from the inside out. USE BIG OVERHANGS to keep the rain off the walls. I've often wondered why Frank Lloyd Wright called them Prairie Style houses when they were so obviously designed for the Pacific Northwest.

    That's my two cents. Anyone?

    Sunlover in a Cloudy Clime

    1. AdamPNW | | #6

      I’ve been considering using loose pumice as insulation under an earthen floor. But how would one go about compacting the pumice without crushing it (and therefore decreasing the r-value)?

  4. GBA Editor
    Martin Holladay | | #4

    The vapor barrier (usually thick polyethylene) should be installed immediately under the slab. The concrete slab is your air barrier. For more information, see Polyethylene Under Concrete Slabs.

  5. erickoperek | | #5

    Insulating value of pumice is determined by density; lightweight pumice contains more air bubbles and so is a better insulator = has a higher R-value. Use the lightest weight pumice conveniently available. Crushing pumice destroys air bubbles thus decreasing insulation value. Use cavity wall construction: Outside wall -- Interior Filled with Pumice -- Inside Wall. Pumice insulation should be about 2 feet = 24 inches thick to give R-40 insulation value in cold climates = design house with thick walls because pumice is not a high R-value insulation. Use pumice because it is local and cheap.

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