Perlite Roundhouses

A reader asked if perlite is a good choice for building earthbag roundhouses. Perlite should work if you make some adaptations. It has excellent R-value and so, in my opinion, is well worth a little extra effort. In fact, it’s a top choice because of its R-value of 2.7 per inch. So 15″ would give you around R-40 walls. That’s roughly double a superinsulated house. Even the tiniest heater would keep a small roundhouse warm if you have adequate roof and floor insulation. The general rule is to have at least as much insulation in the roof as you do in the walls. Otherwise heat will take the path of least resistance and leak through the roof.


Perlite is a generic term for naturally occurring siliceous rock that’s been heated and expanded. It can expand 4 to 20 times its original volume. The tiny air spaces provide excellent insulation. Perlite is a form of natural glass and is classified as chemically inert. More details at and Perlite Institute.

Suggested building techniques:
– Lay lengths of baling twine every 18″ or so on every third course. When the wall starts to get a little wobbly, tie ¾”-1″ bamboo poles on opposite sides of the wall and tie tightly together. You could also use saplings, rebar, 3/4″x3/4″ hardwood “rippers” from cabinet shops or sawmills, etc.
– Minimize windows and doors because this will weaken the design. If you want extra doors and windows, then use posts in these areas.
– Tubes are more prone to rolling, but maybe you can get them to work. Hopefully you can experiment with both bags and tubes and report your results so we can share with others.
– You could add a binder such as clay for added stability. Even if you lose a tiny bit of R-value, your house will still be extremely cozy.
– Use a post and beam design if the roundhouse is more than about 15′ interior diameter.
– Use 22″ wide bags for larger roundhouses when building with bags of perlite, rice hulls or vermiculite.
– Use a wood bond beam and lightweight roof such as metal roofing.
– Consider adding a sleeping loft. That would be my first choice on cold winter nights.
– Keep it small and simple and you’ll be fine.

If you build with perlite, please keep us posted of your project. Perlite-filled earthbag building is untested and even though I’m confident it will work, we need first-hand accounts to help demonstrate its practicality. An ideal testing scenario would be someone in Alaska or Canada who can build a small roundhouse.

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35 thoughts on “Perlite Roundhouses”

  1. hello sir/ma,
    i will love to ask if your reputable company offer training program so i can apply.
    thank you in advance,i look forward to hear from you soon.
    peter sunday

  2. I am planning on building some earthbag houses in southern mo. soon an would like to have some insulation as to suppliment the earth berming I plan to do. I’m planning on tubes and a few heavy timbers. Smallish but maybe up to around 600-800 sq ft.

  3. Thanks, Owen. Yup, I’m concerned with the weight of the bags. I’m a woman of a certain age and can no longer do the heavy lifting. I suspected that thermal mass is more efficient in my climate than insulation but maybe a combo of rice hulls (available locally) and earth would still be effective or maybe I just need to find smaller bags that I can lift over my head.

    My neighbor’s house is obscured by a bamboo grove…he probably won’t notice any gone missing (kidding)

  4. What would you use as a foundation? A rubble trench as you would for dirt? I’ll be building in the hot and humid Texas climate with a concrete slab for the floor. I’m considering a round pole barn of sorts and using the insulation bags as infill. Also considering tearing apart old sofas and mattresses that people give away and mixing in some borax for fireproofing.

    • That’s a good plan. But you don’t need maximum insulation in your climate. Massive earthbag walls should work fine. At most, maybe add 2″ of insulation on the exterior. Standard earthbags will be much more stable (although more labor intensive). Maybe what you’re really after is saving labor? Then you can do what you’re describing. In addition to the posts, add some wood or bamboo poles on each side of the wall as pinning. Put them on opposite sides of the wall and tie together tightly. This will stiff even loose bags into a sturdy wall, especially after plaster.

      I like scoria in foundation bags so the foundation is insulated. But again, you probably don’t need it.

  5. aloha
    have land in hawaii
    want to build eathbag structures for guests to stay in while learning alterative ways to live WITH the land in compfort and style.
    cinder is the main thing availble here, my land is 3500ft gets to 46 f at night and is windy
    do bags get filled dry, damp, wet??? what is the best ratio for mixture??? what is the best thing to seal/cover???
    is there local assistance for little old ladies like me to learn how to build my own???

    we have one permitted earth bag home on island.


  6. I was wondering if anyone knew anything about the building codes(any restrictions) on earth-bag construction in Bonner county Idaho. Specifically the Priest River area. My husband and I are looking at land in that area and want to build an earth bag home there.

        • That PDF indicates that a single inch of loose fill perlite is R-3.12. Wow! If you can fill earthbags with it and they remain structurally and dimensionally stable, this could be big.

          • Yes, perlite and other insulation materials have a lot of potential. There are some hurdles to overcome, however. Perlite is quite pricey and using it to fill thick walls may break the budget. But there are usually workarounds. Maybe you live near a perlite factory and get ‘factory seconds’ that are somewhat imperfect. Also, walls of loose fill material shift quite easily. Again, there are steps you could take to increase stability. I’ve covered this in numerous blog posts: lash vertical pins (rebar, bamboo, saplings, etc.) on opposite sides of the wall and tie together tightly with baling twine. Here’s a good link called Earthbag Scoria Casita that explains numerous ways of increasing stability when working with loose fill materials:

            Note: Scoria is more stable and easier to work with than perlite. You’d likely want to use additional techniques to stabilize perlite walls. Ex: mix perlite with a little clay.

  7. Very interesting read! I’m a Perlite and Vermiculite manufacturer in North East Ohio. I’ve never seen anyone use Perlite in this fashion before and find it truly intriguing. If someone needs some help with locating Perlite and/or Vermiculite let me know. I’ll keep watching everyone’s progress.

    • I am looking at using earthbag construction in northern Idaho and actually found this forum in my search for perlite. It’s good to know that it is a viable possibility. I plan to use a number of different methods in my building, including ideas from earthbag and earthship designs.

      So yes I am in the process of rounding up information on bulk sources of perlite / vermiculite that is not to far of a drive from the northern Idaho region.

    • hi,
      thanks for your offer of assistance.
      i wonder if you could provide an estimate of the bulk price of perlite. i imagine the price would fluctuate but just a ballpark figure would help to determine if perlite is a feasible option.
      what is the name of your company? do you ship to new hampshire and if so how much would shipping costs be? do you know of any other perlite manufacturers that are closer to new hampshire?

      it looks like i’ll finally have a chance to experiment with an earthbag dome this spring/summer. i’m thinking of a 10′ dome with perlite and some sort of binder in a raschel? bags or tubes.
      i want to incorporate a rocket stove into the design of the dome and i’ll see how well the dome can keep a comfortable temperature next winter.

      thanks again,

      • We don’t sell perlite. This was just an idea for cold climates. Use Google to locate suppliers near you.

        Perlite will fall through raschel bags. Use regular poly bags or tubes.

      • hi Owen,
        i know you aren’t in the business of selling perlite. my comment was directed to Michael Dunlavey.

        i’d rather use raschel bags. the idea is to mix the perlite with clay or cement stabilized earth. i hope to do some experimental test bags in the next couple of weeks.

        • Mixing perlite with clay etc. would work, but you’d lose R-value. The optimum solution depends on your climate. For extremely cold climates, I would use 100% insulation in bags or tubes for maximum insulation or add a layer of insulated bags on the exterior.

      • Depending on what grade Perlite you want to use and the quantities I might be able to help you. If you are looking for the medium construction grade Perlite our wholesale price is $4.50 per 4cu.ft. poly bag F.O.B. In order for me to sell to you I would have to ensure you purchase a full 53′ truckload/ 1008 4cu.ft. bags and have a way to off load them/ tow motor. Shipping is very expensive, especially going East. If not I might be able to find a distributor near your area. Most are horticultural distributors. If you talk to them they might be willing to order the construction grade for you. You’d just have to work out the payment portion.

        Either way feel free to email me and I see what I can do to help. Mike at pvpind dot com.

  8. Owen,

    I was a bit surprised with your statement:
    “The general rule is to have at least as much insulation in the roof as you do in the walls. Otherwise heat will take the path of least resistance and leak through the roof.”

    As you know, hot air tends to rise rather than seeking a “path of least resistance.” Otherwise, heat is always seeking equilibrium unless stopped by insulation or delayed by thermal mass.

    Actually, a more common rule of thumb is that with a rectangular house that is at least fairly well sealed, approximately 65% of heat gain or loss is through the roof. Thus, in either heating or cooling climates, the roof should have *more* insulation than the walls, if working with minimums. In Southern areas, an additional radiant barrier in the attic is also a very good idea.

    R40 walls would be about what a typical three-string straw bale wall would have, I believe. In either the far North or in the deep South, personally I’d shoot for about R60 in the roof.

    Another place to be careful, of course, is with any wall or roof penetrations. Air leaks are a huge factor in lowering effective R values. A perlite bag wall would itself be excellent, but careful detailing around any windows and doors would be an extremely good idea in any climate in which the perlite itself would be good.

    I fully agree that inadequate foundation insulation is very common in many areas. However, before I moved to Ukraine I spent most of my life in Texas, where deep ground temperatures are often within the comfort zone; thus, an insulation “umbrella” extending out from the house foundation but under ground to keep below-foundation temperatures closer to deep ground temperatures would often obviate the need for much insulation below the foundation itself. However, a highly insulated wall such as the perlite bags would be extremely effective in reducing the outside heat from gaining entry to the house during hot weather–which is often found about seven months a year there.

    • I’m not going to debate semantics here about heat/hot air or leaks/rising. These are quick blog posts to point people in the right direction.

      If you have superinsulated R-40 walls, you’ll be okay with R-40 ceilings. Sure, more is better. I’m talking about minimal insulation.

    • David,

      Are you living in Ukraine now?
      It would be interesting to get acquainted if possible.
      Interesting to meet other person interested in earthbag building here.

      alexey.tigarev AT

  9. Hi Owen,

    I know, I know… I’m the guru of Corten Steel boxes… but;

    Years ago, on the site of a failed hydroponics project in “upper Idaho”, we scavenged the perlite and mixed it with local soil/clay, to build a “bagged bunker”. barbed wire in between… fairly conventional earthbag build.

    Okay, it was “oblongish” instead of “round”, we manhandled a few beams into it to create an upper loft for sleeping, and we put a wood stove smack dab in the middle. (Anyone know where “smack dab” comes from? I use it all the time, but I’m not sure how it got there…)

    Anyway, low tech roofing. We used a “conventional roof stuffed with (bleah!) fiberglass batts” and then canted it to allow snow run off and some clerestory windows. We strapped the whole shooting match together over an insulated concrete slab…

    (Constructed from rigid insulation boards we also scavenged.)

    Using advanced math (and a smart aleck from Idaho State U), we calculated our final results at about r25ish in the walls. The Roof was built from tripled 2x8s (screwed and glued – 16″ O.C.) and the thickest ply we could steal… um… borrow… um… scavenge. ;)

    They got lots of snow.

    Waterproof membrane (roll-on) and then more scavenged stuff… Metal roofing went over the top.

    As I recall, we built the whole thing for under a grand. Hauling in the concrete and hand-mixing it with river water was the hardest part.

    The Perlite worked as insulation in the walls, and the little bunker did it’s job. I’m told that it’s still being used as a “Hiker’s Camp” to this day, up “near” the Canadian Border.

    I know some Forestry guys who have used it in the last few years. I’ll see if I can find you some photos.


    aka Ronin

    • Fascinating story. Thanks for sharing. And yes, please send photos of you can get them. Any extra details like ratio of soil/perlite, type of soil, amount of compression (if any), size of bags, etc. would be helpful.

  10. Thanks for the post. Not having found any suppilers of scoria in new hampshire with a quick google search I was trying to get some pricing for perlite. I found a company in ma. But they havent gotten back to me.

    Any thoughts on the price of perlite?
    My main concern is affordable housing so if perlite is more expensive than the alternatives maybe I’ll just end up in a log cabin.

    Maybe mixing 50 or 75% earth in the bags will keep the cost down. I’m planning on a dome not a roundhouse

  11. If you’re planning to build an earthbag structure with perlite, please email me at strawhouses [at] so we can discuss some building details. For instance, I would use vertical strapping that goes completely around the wall and over the bond beam.


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