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.
40 thoughts on “Using Scoria for Earthbag Building”
Hello, we are looking at building our first earth back structure in northern Arizona where the temperatures go from 10° to 100°. My initial thought was to build an external roof via a pole barn architecture, this could allow the bags to stay dry and protect them from sun during the building process. We have access to easily source cinders (local scoria) in my intent was to perhaps fill the bags with 100% cinders material. Looking at a rubble trench foundation. This is our first build ever and it will be under 600 ft² likely 30×20 type dimension. Square structure but I know there are pros and cons. My concern is reading on some of the blogs that 100% of scoria bags may not be able to stabilize, is this accurate? Any advice for newbies?
I think that your roof idea is a good one. I built my earthbag house in Colorado (see http://earthbagbuilding.com/projects/hart.htm ) using mostly bags full of scoria, and it worked out just fine. The scoria has a sharp enough surface to pack together rather solidly and does not really compress much further.
Thank you so much for the reply, I will dig into your step-by-step project in more detail. Thanks for sending that link very helpful. I would prefer to do a rectangle, it seems like that could be a challenge over a traditional circle structure with scoria. Any thoughts on that after working with the scoria bags?
Rectilinear structures should do fine, as long as you observe the guidelines for reinforcement with periodic vertical pinning and the need for buttressing.
Ok, thank you. if I am using the bags as buttresses, should we lay them out in the corners or middle of 27′ wall? Likely both perhaps. Also what are your thoughts on keeping the wall from pushing inward if I only have the buttresses on the outside? I will not have roof pressure as it will be an externally supported roof structure. I know this is a long wall run which is why I’m nervous.
Buttresses can either be inside (walls can serve this function) or outside and in either case they need to be well connected with the wall, preferably with partial bags inserted into the wall as it is built. The corners do not require buttressing, unless you are building for high earthquake probability. Buttresses should occur approximately every 10 feet, so with a 27′ wall you would need two buttresses. Remember you will also have a very solid and well connected bond beam at the top of the wall that the roof can rest on and be attached to.
I truly appreciate you taking the time to help us all as we stumble through this process. I have purchased the roundhouse plans and can’t decide whether to use scoria for insulation or road-base for strength in the walls. I was planning to use the raschel mesh tubes.
I live near Austin Texas; would you recommend one or the other for my location? Also, if I use scoria, Can I use the raschel tubes and scoria for the first two foundation rows or should I use the double bag poly as is recommended in most of the literature? I will have the 6mil plastic to prevent moisture from the exterior soils
Yes to using double bags for scoria foundation.
For the walls you could use either one. Road base is the most solid, scoria is the fastest and easiest. You could even combine the two, for example: Road base up to chest high or so and then switch to scoria to speed construction.
This is why it’s always best to make a demonstration corner as I show in my videos, or build a tool shed, etc. so you can decide what’s best for you. Physical (human) strength is a big factor. Tamping earthbags is hard work.
Living where scoria is scarce, and expensive to bring in, I’d like to ask about anyone’s experience with Perlite, which is available here.
It will work if you follow the steps in this article: Scoria Casita https://naturalbuildingblog.siterubix.com/earthbag-scoria-casita/
The walls need extra reinforcing to be stable as explained in the article. You could also add a little stabilizer to improve strength and stability. Ex: mix in a thin slurry of cement — sort of like salad dressing, just enough to bind it together. Experiment first until you find what works best.
Would using scoria filled bags instead of gravel for the stemwall be structurally sound for high seismic areas? 4 rows of scoria (1 and half row under threshold, followed by earth filled soil? Since it is very lightweight compared to gravel and earth filled bags, would it compromise the integrity of the walls? i have a caliche clay/sand soil which compacts into a pretty solid/strong bag. i have access to scoria and it is way cheaper than gravel where i live. Thank you.
Scoria would act just like gravel to dissipate stresses. As long as the scoria bags are well connected to the earthbags then I see no problem. This includes using a tie down methods using 1/4″ poly rope or poly strapping and rebar pinning. Tubes are stronger and faster than bags. Add large buttresses as you see in the Nepal designs. Limit the length of straight walls. Use a reinforced concrete bond beam, lightweight roof, etc.
From what I understand, the “casita” project is near Taos, and Kelly’s house is near Crestone, CO — relatively low-risk seismic areas.
I know there’s not much objective data, but with barbed wire, buttressing, pinning and strapping — and a light roof — would you judge a vertical-walled scoria project (with a rectangular layout) to be feasible in a highly seismic area? Specifically, I’m thinking of Hawaii’s big island — hence my wanting to use scoria.
Thanks for all of your work in this area.
It’s possible. I would use opposing rebar every 2′ apart tied together with scoria tubes between and covered with cement plaster.
We are putting a roof on the wall to protect it . Can we use gravel in the bags ? I keep seeing lots about that — but not sure that it suits my needs . I will have to make my way to a computer – having a time navigating threw the site on my phone .
Gravel bags are good, although you’ll need some way to stabilize the walls as they get higher. In addition to the previously mentioned wall stabilizing methods, you could add a bit of cement to the gravel.
Hi ! I am building a 6 ft hay bail privacy wall…varying lengths . Already have my rumble trench dug and filled and want to do earth bags as a base . Been reading of the scoria and thought that was the way to go . But after searching far and wide can’t seem to get any local. I’m in south Eastern Pa so it get snow in winter and humid in summer. Any suggestions on which way to go ?
Straw bales in a privacy wall will rot in PA. They need to be protected by a roof.
Our blog lists numerous lightweight fill materials such as pumice, expanded clay, scoria. You’ll have to track down something locally available.
Lightweight materials require special techniques so they’re stable — curved walls, added benches, posts, strapping, rebar pins, etc.
We are located in Minnesota thinking of doing the Torus design. Since we get extreme cold and humidty want to know whats best for the earth bags. We were thinking doing earthbag muxture and then another layer of the scoria on the exterior. Would that work or what would you recomend? Also with Torus design could it be single family, 5 bdrms, 4 baths, 1 family room and 1 living room. also want 2 bathrooms between 2 bedrooms on each side and make larger master bath and only 1 laundry and 1 mech room be a room for storage for freezer and things. Any help and input would be appreciated. Thanx
I suggest using straw bales on top of a gravel bag foundation. This would be the fastest, easiest method for you and provide excellent insulation. It’s best to do post and beam. Finish the roof before adding the bales so they never get wet.
Every plan can easily be modified. This is best done by ordering my CAD plans and working with someone local who will make sure everything is just the way you want it.
Hi, Im getting close to purchasing land in the San Luis valley area of Colorado for building my earthbag home. Ive read a lot of good things about using scoria for insulation in earthbags but I dont know where to buy it- I Googled “Scoria colorado” and a number of other search terms and got nowhere. In my wanderings on this site I read something about a mine near Antonito. At this stage in my plans I was planning on building a half circle- with ~8ft high walls- and earth berming the circular walls – leaving the flat side facing the south. Im wondering if scoria filled bags need more buttressing or support than dirt filled bags (especially with the earth berming) than dirt filled bags since they’re lighter?
The scoria mine is just over the border in NM south of Antonito. The phone number is posted somewhere on our site. Try our Resources page: http://www.earthbagbuilding.com/resources.htm
Scoria will shift if you don’t add clay, etc. to stabilize it. As Kelly Hart has proven, you don’t have to stabilize it if you use a few special techniques (external pinning, twine ties, buttressing, etc.). His dome guide has some tips: http://www.earthbagbuilding.com/articles/riceland.htm
Read about the earthbag scoria casita. He provides lots of tips that will help you a lot. https://naturalbuildingblog.siterubix.com/earthbag-scoria-casita/
Consider building something like this: http://www.instructables.com/id/How-to-Build-an-Insulated-Earthbag-House/
That would work great with scoria if you follow all the advice listed above. It would be a near zero energy home. Scoria is one of the great, underutilized, hidden gems of natural building for all the reasons we describe on this blog. (Search ‘scoria’ in the search engine on the right side of page.)
Stability: Read about Kelly’s dome home here: http://www.earthbagbuilding.com/projects/hart.htm You might want to watch his DVD (maybe available in a local library). He has a curved wall between two domes that has withstood backfill for years with no problems.
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.
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 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
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.
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.
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.
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?
– 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 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
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’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.