If you’ve been reading our blogs and websites, you’ll often see reference to scoria. Scoria, also known as lava rock, has numerous properties which make it a great building material. The key breakthrough for earthbag building was Kelly Hart’s house made with bags of scoria. His house stays comfortable year round in a very cold climate. This blog post recaps some of the most practical applications of scoria-filled earthbags.
Scoria is perfect for superinsulated earthbag walls. It’s low cost, all natural, rot proof, fireproof, doesn’t attract pests, and is lightweight and easy to work with. Anyone can handle bags of lightweight fill material such as scoria by themselves. It’s almost like handling bags of popcorn.
Previous posts have explained how to build insulated earthbag houses with scoria. Insulating Earthbag Walls with Tube Sandbags describes how to use tube sandbags filled with scoria as an outer layer of insulation. Earthbag Building in Cold Climates explains how bags can be sewn to create two compartments – earth in one side and insulation in the other. In extremely cold or extremely hot climates I would fill the bags with 100% insulation (or all earth in a hot climate if insulation wasn’t available).
Earthbag foundations offer many advantages over reinforced concrete foundations and work well with many types of sustainable buildings. In particular, they are low-cost, fast and easy to build, require no cement (a major expense and cause of global warming), and require no forms or expensive equipment. Scoria-filled bags create a shallow, frost-protected foundation, and therefore eliminate the need for rigid foam insulation and extensive excavation. This one step alone could save you thousands over conventional foundations.
Earthbags are ideal for building greenhouses due to their resistance to moisture damage. When filled with insulation such as perlite or scoria, earthbag walls and foundations enable you to grow plants year-round.
Kelly Hart’s free Dome Building Guide shows step-by-step construction of how to build earthbag domes. His method of using scoria-filled earthbags is the easiest, fastest dome building method that’s been developed so far. Scoria is great for building domes since the aggregates tend to lock together and form stable walls that can withstand high compression loads. Tie courses together with twine for best results.
i’ve yet to try any form of earthbag building. (i’m dying to experiment). i’m curious though if it would help to mix the scoria with earth to create a more solid “stabilized” tube or bag? maybe 75% scoria to 25% earth? i worry that if the tube became exposed and deteriorated would the bag rupture allowing the scoria to crumble out and eventually led the dome to collapse.
i live in new hampshire so i’m concerned about wet and winter conditions for earthbag construction. also, do you know if scoria is readily available in the northeast? i haven’t shopped around yet. maybe it will be too expensive since there are no volcanos around here that i know of. ;)
You can mix clay or a thin slurry of lime or cement with scoria to create a more stable wall, but you’ll lose some insulation value and increase labor. Kelly Hart used 100% scoria with good results.
Long, straight walls need buttressing, post and beam, temporary braces, etc. for stability.
Contact landscape suppliers. Or better yet, do a Google search for the nearest volcano and buy it direct by the truck load.
And as always, test things out on a small tool shed or garage before tackling something complicated.
We have used 50% scoria and 50% earth in our bag mix for a dome and was amazed at how stable the walls were. I am very exited about using just scoria or perlite on the outside of an earth bag wall for insulation as a breathable alternative to foam that will pull double duty as a french drain.
I also just saw a you tube video on building a monolithic wall with hemp and lime and wondering if anyone had any comments about it.
Here is what they say:
Hemp, lime, and water create a light weight building material stronger than concrete and breathable. Lime turns the high cellulose levels in the hemp into a mineral, virtually petrifying it. It performs better than straw bale and won’t rot, mold, or be eaten by termites. It is applied like a cross between rammed earth and poured earth and doesn’t require hard compaction. Lite compaction leaves air pockets for insulation and the chemical action of the lime and hemp creates thermal mass.
Her is the link
Thanks,
Neil
Neil, thanks for the input. Glad to hear of your experience using scoria. Feel free to share any additional details you might have learned.
Hempcrete has real good potential. But due to the ridiculous laws in the US it may not catch on in the states. Wikipedia has a decent summary: http://en.wikipedia.org/wiki/Hempcrete They point out how hempcrete has 1/20th the strength of concrete and so a post and beam frame is required.
We’re currently investigating vetiver grass/lime as another alternative. Vetiver provides valuable erosion resistance in the rainy season, and can be cut in the dry season and used for thatch or possibly vetiver/lime in blocks or earthbags. Useful properties include its natural resistance to termites and moisture resistance.
I am very interested in using scoria in my earthbag cabin that I will be building this spring. Due to very cold winters in the mountains of Pennsylvania I think that 100% scoria filled bags are the way to go. However, I’m having a hard-time finding it here. Would expanded perlite be a viable alternative? Or is it too fragile for building walls? The structure will be a roundhouse with a basic shed roof.
Perlite will 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. Perlite’s R-value is 2.7 per inch. So 15″ would give you around R-40 walls. Even the tiniest heater would keep a small roundhouse warm. How big is the roundhouse?
Suggested techniques:
– Lay lengths of baling twine every 18″ or so on every other course. When the wall starts to get a little wobbly, tie 1″ bamboo poles on opposite sides of the wall and tie tightly together with the twine. 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 probably more prone to rolling, but maybe you can get them to work. Hopefully you can experiment with both bags and tubes.
– 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.
– Remember to insulate the roof as least as much as the walls or the heat will just shoot through the roof.
– Use a post and beam design if the roundhouse is more than about 15′ interior diameter.
– Use 22″ wide bags for larger roundhouses. Wider bags will be more stable.
– Use a lightweight roof such as metal roofing if you don’t use post and beam.
– Keep it small and simple and you’ll be fine.
The most recent issue of The Last Straw Journal (#60) has an article about using perlite under floors. This indicates that perlite holds up under compression. (But the limit in earthbag walls is untested, so use a lightweight roof or post and beam.)
Please, please keep us posted. Perlite is untested and even though I’m sure it will work, we need first-hand accounts to help demonstrate its practicality and win people over.
We are planning a rather large roundhouse: 30 ft diameter. I suppose I will need to go with post and beam if I’m using the perlite. I was hoping to avoid wood framing. I found perlite here in PA for a decent price (about $100 for 55 cubic feet bulk bags) but it is still way more than I was hoping to pay considering I need around 1000 cu ft of it. Thanks for the advice, I be sure to update if I go with the perlite.
Yes, you’ll need post and beam. But that doesn’t necessarily mean it has to be a lot more expensive. If you don’t live in an area with building codes, you can use locally harvested wood poles. You can gather them from national forests with a firewood permit or buy from tree farms who sell them dirt cheap to pulp mills.
$1,800 is reasonable when you factor in how quickly you’ll recoup your investment (maybe in one year). It’s common to pay that much or more per heating season for home heating. Maybe you’ll get a discount if you buy a big truck load. And while you’re at it, check on prices for vermiculite, which has just slightly lower R-value.
Would you recommend using scoria or pumice in bags, I am building in a humid, tropical climate. And at what ratio to earth? Thank you.
Both will work. It mostly depends on what you have locally available. What’s the least expensive?
Scoria has more air spaces, at least the little I’ve seen. Maybe it varies from place to place. Trapped air in all those air spaces will create a more insulated wall. But this is not a big issue in the tropics, so use whatever is most affordable.
I recommend clean 1/2″ aggregates. Big chunks will be difficult to work with and may tear the bags.
I found a supplier for cinders. The company offers 3/8 -1/8 inch or 3/4 – 3/8 inch. The company also has what they call a dirty screen cinder which has a lot of fines (3/8 down to sand). It might not work in mesh bags. Which size would be best? The 3/4, the 3/8 or the dirty mix in regular bags?
Kelly Hart used 3/8″. I think that size is easiest to work with and fills the bags evenly. You want clean material for maximum insulation, no fines.
I have been researching some more bag and tube choices. I have found a nice mesh tube from Volm that looks like it would be perfect and economical. The only problem seems to be the mesh size. It looks to be about 1/2 in x 5/8in. I am wondering if the larger scoria (3/4-3/8) might make this mesh workable. The mesh tubing is very strong and is priced right.
I remember reading that Kelly had some problems using poly tubing with the scoria. I am wondering if the problems of building with tubing and scoria could be overcome by using a mesh tube. It would seem the scoria and wire would make a very strong bond between the courses without the poly interfering.
My planning now involves possibly using this mesh, the scoria, barb wire for a 20 ft dome.
Sounds like a good plan. Be sure to order one sample and test to see if the lava rock will go through, if the material is strong enough, etc. Be sure to get 18″ wide bags/tubes measured flat when empty.
I would use poly baling twine not wire to tie courses together. There should be less slippage, but the poly will help tie everything together. Ranch supply stores and feed stores typically carry twine. Kelly had trouble with tubes rolling when filled with scoria, so there’s something to watch out for.
Scoria is a TOP choice. It’s sort of a “sleeper” because it wasn’t adequately covered in Doni and Kaki’s book. You’re going to save a huge amount of time and effort. But be sure to keep an eye on things as you’re building. Add a brace if the wall looks a little unsteady. It will be super strong once finished (especially with a loft with ends of logs/joists secured in the walls), but may need temporary bracing. Even wood framed walls have temporary braces while being built.
Also note how Kelly added buttresses inside doors where the dome is weakest: http://www.earthbagbuilding.com/articles/riceland.htm
Seems like doing double wide courses would be better with scoria. More stable and quieter. Could also build in more alcoves, and plumbing would be so much easier.
That’s an option, but also a lot of extra work and materials. It might be practical if the system is mechanized: feed scoria through a hopper to fill continuous tubes, etc.
Regular scoria houses are extremely quiet. You’d have 18″ wide walls with 15″ of insulation!
I have a place that is built on stilts because of the angle of the terrain. The floor is cold in winter and I’m wondering if putting lava rock under the building would help keep it warmer.
Yes. But if the house is high above the ground, then use floor insulation with higher R-value.