GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Perlite as insulation

eyremountllc | Posted in PassivHaus on

I am wondering about whether it may be okay to use perlite as insulation under a basement slab. I am planning a Passive House and under the current plan, I would use 4 inches EPS. According to passive house, the EPS would also be under the footings. I am a little concerned about the structural stability of EPS under the concrete footings. I am considering using perlite granules (R-2.7/in), maybe about 6 inches of it underneath the entire foundation. Are there structural issues to be concerned with?
Another application is adding perlite into the concrete mix that is to be poured into the ICF forms. In the original plan, I planned to add 3 inches of EPS on the outside of the ICFs, but this makes my walls about 15 inches thick. I am wondering if I can add enough perlite in the concrete to bring the R-value of the concrete to a level that I can skip the additional EPS and again without sacrificing strength.
Any comments or advice? Thanks in advance

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Riversong | | #1

    I doubt that expanded perlite would have the compressive strength to bear a foundation, and it's relatively uniform size and shape would not allow it to be compacted.

    Adding it to the concrete in the ICF cores would weaken the concrete and almost certainly not be allowed by the ICF manufacturer.

    XPS is the foam board generally installed under foundations. It has higher typical compressive strength and 25% more R-value per inch.

    If you want to improve the R-value of the foundation, you'd be better off with a standard 8" concrete wall with 4" XPS on the exterior. Alternatively, use the ThermoMass system, which places up to 4" XPS between two whythes of 4" concrete connected by 12" oc fiberglass ties with the tensile strength of ½" rebar. In either case, you'd have the advantage of the interior thermal mass which you would lose with the ICF wall, and with the midline foam there is no need for insect, UV or physical damage protection.

  2. GBA Editor
    Martin Holladay | | #2

    European researchers have experimented with "styrofoam concrete" for years. The idea, as you suggest, is to add EPS beads (or perhaps perlite or other insulating beads) to concrete.

    When I worked as a volunteer doing earthquake relief work in Armenia in 1990, our team was building medical clinics in mountain villages in the Spitak region.

    Among the sponsors of our effort was a German group that was convinced that styrofoam concrete was the wave of the future. We used wall panels made from styrofoam concrete -- the walls were shipped from Germany. These styrofoam concrete wall panels were finished on the exterior with synthetic stucco.

    When it came time to place the concrete floors, the German engineer advising our group told us to mix the styrofoam concrete on site. The Soviet-era Ready-Mix truck would show up. We'd tell the driver to park the truck, and we'd climb up to the back of the truck and pour huge bags of styrofoam beads into the truck, and then the driver would mix it all up. Although we did our best to perform volume calculations to be sure the ratio was right, we really had no idea how much concrete was in the truck, and neither did the driver.

    The resulting concrete was crumbly, not very strong, and incredibly difficult to finish. It was not trowel-friendly.

    After my experience in Armenia, my advice is unequivocal: stay away from stryofoam concrete.

  3. Riversong | | #3

    But Martin, that was a Soviet concrete mixer. No wonder it didn't work - that's why the Soviet Union collapsed (maybe the Berlin wall was made of styrofoam concrete?)

    Americans, on the other hand, can do anything we put our minds to. That's why we won the Cold War and have taken over the world ;-)

  4. GBA Editor
    Martin Holladay | | #4

    As far as I could tell, the Soviet Union manufactured decent concrete trucks. Their hand tools, however, were pathetic. Even carpenters' hammers looked like they were modeled on the hammer on the Soviet flag.

    I supervised a crew of Armenian workers for a housing construction project in Stepanavan. We ended up importing all necessary hand tools from the U.S.

    Although the Soviet Union had batch plants and concrete trucks, they didn't have a good understanding of the need to keep water ratios low. (Either that, or the problem was due to the fact that happy hour usually began at breakfast.) Lots of soupy concrete -- some of which I had to send back to the plant, rejected, in spite of the fact there was a big shortage of cement and concrete in those days. Undoubtedly some hapless customer got the load I rejected.

  5. mike eliason | | #5

    high-density EPS (e.g. insulfoam) has been used under footings on a few NW passivhaus projects, if i recall correctly.

    applicability of a product like glapor's foam glass insulation in lieu of perlite?

  6. Roger Lin | | #6

    Interesting about the glapor product! One of the reason I started thinking about using perlite was during last month's passive house training (phase 2), Millcell (similar to Glapor, I think) was mentioned as an insulation option for under the slab. I've been curious about the cost differences between EPS and Millcell. I have not had any luck finding millcell on google. It almost seems like all the websites have been removed. Anyways, Millcell is supposed to be recycled expanded foam glass, which perlite is expanded glass/volcanic rock. Perlite has been used underfloor but primarily under floating concrete floors. See,

    Perhaps I should find out from the perlite institute whether there were tests done for perlite application underneath a basement concrete floor.

    I think if some significant R-value can be gained from adding perlite without sacrificing strength, it may be smart to consider doing it. Perlite, I hear comes in different forms, it may not actually look like the styrofoam beads that we are used to. I recently heard that some manufacturers are grinding it into a powder form which makes it easier to apply. In China, I hear they mix the perlite powder into EIFS and coating buildings with it to enhance R-value.

    With that said, I think Martin has a good point about mixing concrete on site, you can't really know for sure whether the ratio is right.

  7. Kevin Dickson, MSME | | #7

    Here's the common XPS foam that is much stronger than the soil itself:

    Apparently its 100psi rating is based on 10% compression. Your structural engineer would decide how much load it should actually take, but the actual load is more like 10psi.

  8. Riversong | | #8


    The OP wasn't interested in XPS, but for those who are, the standard 25 psi XPS is more than adequate to support a 2-storey residential structure.

  9. alan | | #9

    perlite and cement mix hollow blocks are used in South Africa to build single story houses and it seems to meet their codes

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |