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Community and Q&A

Perlite compressive strength?

Jerry Liebler | Posted in Energy Efficiency and Durability on

I’m considering the use of bagged or loose perlite to insulate below a basement slab. Can it also be used under footings? I can’t seem to find it’s load bearing capacity. Has anyone used a “penetrometer” on either loose or bagged perlite concrete block fill ( the grade recommended for below slabs)? Penetrometers are used to judge the weight bearing capacity of soils. Though the Perlite institute advocates use under slabs it would be comforting to know it’s load bearing capacity. when tested as a “soil”. Actual test data would answer the questions about where it can be used.

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  1. GBA Editor
    Martin Holladay | | #1

    Every few months, it seems, you post a new question about using perlite to insulate a slab or a concrete footing.

    The first time was in October 2012: Lightweight insulating (perlite) concrete footing?

    The second time was in March 2013: Can insulating concrete be used for footings?

    Now is the third time. You write, "I can't seem to find the load bearing capacity of perlite." The answer was partially addressed in the thread you started in March 2013. In that thread, Ron Keagle wrote, "I made some phone calls to the Perlite Institute and a few distributors with an inquiry about the possible use of Perlite concrete either as the footing, or as an independent layer beneath the concrete footing. All contacts told me that they have never heard of that being done and their immediate reaction was that Perlite concrete would not have the necessary compressive strength."

    Of course, this time around, perhaps a new reader will provide additional information.

  2. GBA Editor
    Martin Holladay | | #2

    Maybe your persistence will bear some useful fruit. I decided to do some research. You are probably better off using perlite concrete, not loose perlite or bagged perlite.
    "Each Perlite particle resembles a glassy froth of bubbles. ... This structure imparts lightness, natural insulating properties and limited compressive strength."
    "Perlite also has a high compressive strength, although not quite as high [as Calcium Silicate insulation], requiring 74 psi to reach 5% compression. Mineral wool with a nominal 8 pcf density reaches 5% compression with only 0.42 psi."
    "For most uses, to maintain a proper balance between insulation value and compressive strength [when mixing perlite concrete], a 1:6 (one part Portland cement by volume to 6 parts perlite by volume) mix is chosen with a density between 384 and 480 kg/m3. This provides ... a compressive strength of 125 to 200 psi (986 to 1378 Pa)."
    "Perlite conforming to ASTM C549, and provided in easy-to-install, lightweight bags (either plastic or paper) may be used as insulation below concrete floors."

    The image below is a table showing the compressive strength of a variety of perlite concrete mixes. (Source:

  3. Jerry Liebler | | #3

    You are absolutely correct, my initial questions are UN-answered What I've recently discovered is that the load bearing capacity of ordinary dirt is also unknown and is, however, routinely MEASURED by the use of a simple instrument, called a "penetrometer" What I'm now hoping is that someone here has used a 'penetrometer' on perlite, after all it's just a particular kind of dirt. Treckhaus in Oregon used perlite, in bags as part of the insulation under a slab and as you found the perlite institute advocates something similar but what is the safe loading for such a slab? Evidently the Treckhaus example shows that perlite can support at least 100 pounds per square foot (50 PSF weight of a 4" slab + live load). Footings are designed to spread the load on them to an area sufficient to support the building on the available soil. If the soil can only support 2000 pounds per sq foot the footings will need to be larger (and add their own weight as well) than if the soil can support 8000 pounds per sq ft.

  4. Alan B | | #4

    Just use foam, its proven.
    When your looking at something thats unknown long term your taking a huge risk, if you lose your going to be out a lot of money and it will take a lot of work to repair. Do you know if Perlite has a fatigue limit? For long term use you would need to know.
    So stick with the safe, effective and tested solution.

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