Using Scoria for Earthbags
Questions answered by Kelly Hart

Q: I have just bought some land in Lexington, Oklahoma ... and I am planning all that I want to do and how I want to build... and I need to know where can I get scoria?

A: Scoria is a type of volcanic stone that is often available in regions where old volcanic activity has occurred. I don't know whether it is available in your area. It can often be purchased at landscape supply outlets, since it is commonly used for landscaping, but it may be quite expensive if purchased far away from the natural sources.

Q: My wife and I plan on building next spring and we are searching for a source for scoria or pumice in Michigan. Most, if not all, sources are limited to landscape suppliers and cannot supply a large volume. What is the going price per ton or cubic yard in the west?

A: The source for scoria where I live is Colorado Lava. Their number is 800 528 2765 and the going rate seems to be around $24.50 per ton, 24 ton max capacity for a semi, $2.50 per mile for delivery (2010) . The mine is just outside Antonito, Colorado, on the New Mexico side of the border with their main offices in Illinois. www.coloradolava.com

Q: We live on the Big Island of Hawaii and need to build a quick structure about 400-600 sq' We are thinking that Earthbag building is the way to go. We have cinder available to us as filler for the bags. Do you think it would be wise to also mix in cement?

A: I used a kind of cinder (scoria) to fill the bags for my dome home, and it worked great...wonderful insulation. This was 3/4" minus gravel, without much dust in it. Larger aggregate might not work as well. I did not use cement, and I don't think it is is necessary.

Q: I have begun to build a 30X20 studio, with a basement 11X12. I was not able to find the scoria that you use so am having to use blue gravel, 3/4" crushed and it is very heavy! I could sure use some help, my body is complaining loudly.

A: The crushed volcanic rock (scoria) is certainly easier to work with than ordinary gravel would be; a bag of it only weighs about 35 pounds. It is also better insulation because of all the trapped air in the rock. Regular gravel will probably give you some insulation, for the same reason, but probably not as much.

The scoria also packs into something that becomes fairly solid, which your gravel may or may not do. If it is crushed, and not smooth, then it will likely pack pretty well. This could become an issue if there is any tendency for the bags to move or roll once they are in place in the wall; some gravels have a tendency to do this.

Q: Won´t there be a problem if I place the heavy roof on the walls made with bags filled with balastre (the red volcanic rock)?  You know, because it´s fragile and it´s light.

A: I wouldn't worry about the "balastre" being able to hold up your roof. Once it is compacted a bit it is just a strong as solid adobe. If you want to be sure that it will be plenty solid, you might make a steel-reinforced concrete "bond beam" at the top that will help distribute the weight of your roof structure.

I'm not sure we are talking about the same thing. You mentioned that you used crushed volcanic rock in your Colorado site. I am talking about lava rocks abundant in my property. Can I crush my lava rocks with hand tools, or maybe a bulldozer to get some fine powder for the mix? Would the resultant powder be of any quality for the earthbags?

From what I can see in the pictures the lava rock you have there is similar to what I used. The main useful quality about what I used (which was red) is that it is very lightweight, full of air pockets, so that it acts as a natural insulating material. If it is crushed to a powder, then this quality is diminished considerably, so I made sure that I eliminated as much powder as I could before using it. I used a pre-crushed gravel that averaged about 3/4 inch, but somewhat larger or smaller is also possible. If you were able to use this, then you wouldn't necessarily need to use any cement at all...I didn't.

Q: My husband and I have just purchased land in the hills of Montana and will start a test dome shortly in preparation for building a home next year. I have been searching locally for scoria for the bag filling. All I can find is 3/8" or 1.5". I'm a bit worried by that - the 3/8" may be on the small side and so may not have adequate insulating properties, but the 1.5" may be too bulky and untampable (if that's a word!). Perhaps we could mix them.

A: I think that your suggestion to mix the two sizes would be the best, but try to order the stone with as little fines as possible (or even screen it out yourself) since the fines will also lessen the insulative value.

Q: Are scoria and/or perlite filled earthbags as sturdy and even bullet proof as are those filled with sand or dirt?

A: Scoria or perlite filled bags are not quite as solid as an adobe soil would be, but they are plenty solid for this sort of building and the insulation value makes them well worth the extra expense...especially in that climate. They may not be as bullet-proof however.

Q: I went to the Lava Rock facility to get a price list and to see the scoria.  Their 2009 price list showed one ton of the 3/8 inch or less material for $16 a ton.  I have a feeling you used something larger than that.  Am I correct?

A: Yes, we used a 3/4" minus aggregate and it worked fine. The 3/8" would also work, but might be slightly less effective as insulation. The problem with any of this is the "fines" or dust that often comes with it, which tends to clog up the air pores and add to the weight. If you can ask them to screen out these fines, then either of these sizes should work.

Q: The building people are concerned with what the material is made of. Since I have the same type of climate that you do in CO I want to go with the lava rock.

A: While lava rock worked well for me, since it provided excellent insulation, I suspect that the building officials will recommend the adobe-like soil.

Q: I am very interested in the use scoria as fill material. However, when I referenced this in Kaki Hunter's book, their take was that, while the scoria has excellent insulative qualities, it does not pack down well from a 3/4" size material, resulting in lumpy bags that do not fit well in an earthbag wall. Did you have any such problems in your building, and if so how did you get around it.

A: This is one point that I disagree with Kaki and Doni about in their book. I know that they suggest mixing the scoria with soil so that it will pack well. The problem with this is that it severely diminishes the insulative value of the scoria.

As you know I used straight scoria in most of the bags in the large domed house that I built over a decade ago. (see http://earthbagbuilding.com/projects/hart.htm) The scoria does not pack quite like adobe soil does, but it packs quite firmly none-the-less, certainly firmly enough to hold its shape in the dome configuration. All those sharp angles of the 3/4" minus aggregate interconnect in such a way that once they are tamped they tend to hold that shape, especially after being plastered on both sides. That house has not shifted or sagged in any way since the day I finished it, and I expect it to still be here a century from now.

Q: If I wanted to build a 1500 square foot barn/house using the lava rock, how much scoria would I need to buy?

A: Of course there are many variables, but given standard 50# earthbags and a single story structure, you might need about 1800 bags (minus doors and windows), which at about 35 lbs/bag of scoria would weigh about 28 tons.

Q: I see you talking quite often about scoria, or crushed volcanic rock as an alternative to rammed earthbags. When you say "crushed", how fine are we talking about? Is the scoria that you find in landscaping stores good enough or does it need to be crushed more finely?

A: The scoria that I have used is around 3/4" minus, but somewhat larger or smaller should also work. The less fines, or powdery dust, the better. Landscape stone comes in many sizes; I wouldn't used anything much greater than 1 1/2" as fill for earthbags.

Q: The land I will be building on is several acres of Hawaiian lava. Do you think if we used a bulldozer to crush some of the lava like we do to create entry/exit routes and level a pad for a small home, that we could use it as fill material?

A: I have often advocated that folks in Hawaii use their local cinder for earthbag building. You don't want it crushed too finely though; the dust will clog the pores and it won't be good for insulation. What you want is a cinder gravel that is up to maybe 1" or less.

Q: Would scoria-filled bags support both the concrete roof and the lateral forces of being buried 5'? It would not have a concrete foundation. I plan on using 18x30" woven poly bags.

A: My sense is, from my own experience, that earthbags filled with scoria will support a great deal of weight, especially if you create a solid reinforced concrete bond beam at the top that supports the roof structure.

Q: I am seriously considering using your method of making an earthbag building with scoria. What are the ramifications of using this method on a linear straight wall building with roofing?

A: You can certainly make a linear straight wall using scoria-filled earthbags. You just need to make sure that it is sufficiently supported with buttresses or other reinforcement, such as pinning with steel rods and a solid concrete bond beam at the top.

Q: Do you have any suggestions as where I could find bulk crushed volcanic rock? I have only seen amounts typically used for small projects.

A: Volcanic stone is primarily available in the western US, but you might check at landscape supply companies about availability in your area.

Q: I was wondering if anyone has done any work on evaluating the R-value of scoria bag walls yet? It is pretty obvious to anyone that 380mm thick scoria walls ought to be sufficient for the climate in Northern New Zealand, but the officals are having trouble making the numbers add up.

A: My estimate of R-40 for 15" walls of bagged scoria was based on empirical experience. The earthbag domed house that I built at over 8000 feet in Colorado performed very well for passive solar heating, requiring very little additional heat. This was comparable to other local strawbale structures. I don't know of any formal thermal studies to refer you to, unfortunately. I did notice that perlite, which seems similar to me, is listed at having somewhere between R-2.5 and R-4 per inch, which would put my estimate of R-2.6/inch for scoria at the lower end of this scale.

Q: I am planning on building a (20' circle interior) roundhouse using timber frame (logged off property) and was wondering about using the scoria (double bagged) walls infill. Roof would also use harvested logs as well as same for a half loft. Should I use scoria for the entire wall infill and same above for loft wall? If so how stable would the wall actually be if it is scoria? Just wondering about the light weight of the scoria. I figured I would not be able to use the scoria without timber frame without the logs weighing the walls down.

A: There is no reason why you couldn't do as you propose, using bags of scoria as infill in a post and beam structure. But the log supports are not a necessity, as the bags of scoria can support a lot of weight. When I built the earthbag dome home with bags of scoria, I created a large loft by resting the log vigas directly on the bags, pinning them into place with rebar pins, without any problem.

Q: I am wondering however if the Scoria has to "breathe" like soil does? Does it hold any moisture at all (given that it is rock)? Ideally my husband would like to put 1/2 logs on the outside to give the look of a log cabin but keeping the insulation value of the scoria. Is this possible or would putting wood siding trap moisture?

A: Scoria will absorb a bit of moisture if it is immersed in water, but otherwise doesn't tend to hold water, and even if it did it would not decompose. So I wouldn't worry about this, and adding log siding should not be a problem. The domed house that I built with bags of scoria was eventually covered with a cement stucco on the outside and has shown no negative results from this. The inside was breathable with a papercrete/lime plaster finish. The domed house that I built with bags of scoria was eventually covered with a cement stucco on the outside and has shown no negative results from this. The inside was breathable with a papercrete/lime plaster finish.

Q: If we use scoria will their be sufficient strength in the walls to hold up the second story? And do we need to put a bond beam around the top of the bags between floors?

A: Bags of scoria can support a second story. A bond beam between the floors would be a very good idea.

Q: I need 40 tons of scoria for my project. I can get it in 3/4”-1” screened and/or 3/8” screened. Would you recommend I get a 50/50 combination of both sizes or just the 3/4”-1”?

A: A 50/50 mix sounds good to me.

Q: Have you ever used scoria as bag fill for the long tube bags, or only in the smaller bags?

A: I have mostly used scoria in smaller bags, but I did try it in some long tubes once in a dome situation where the course was leaning inward, and I noticed that there was a tendency for the tube to want to roll off the wall because the scoria doesn't pack as solidly as soil.

So then would you not recommend using scoria in long tube bags for domes? I’m making a 12’ done with two 10’ apses.

With domes that small I'm sure that there would be no tendency to roll off... the circle is just too tight.

Q: Do you use water and cement in a mix with scoria?

A: I didn't use any water or cement when I built my domed scoria/earthbag house. It does pack to some extent and generally wants to stay where it is tamped.

Q: I’m worried about using scoria in long bags over windows. It seems like I could make permanent forms that I don’t have to pull out, and that would solve the problem of the bag slumping. Does that seem like a good solution to you?

A: Over windows I would advise cementing the scoria in long bags so they are stabilized, in a similar way to using pumicecrete.

Q: I was able to find a pumice supplier, and  may I ask what range of sizes would be suitable for earthbag fill?

A: I found that about 3/4" and minus was about right, but you don't really want the fines, as they lessen the insulation value, so have those screened out if possible.

Q: I am building a yurt on a raised wooden platform and wondered whether using scoria-filled bags lined with marine plywood within an octagonal timber frame as a yurt floor would be an effective substitute for SIPs? Alternatively, I'm sure I could use straw.

A: I think that you could certainly use bags of scoria as a skirting around a wooden platform deck for your yurt.  This would provide nearly as good insulation as strawbales would, with the advantage that the scoria is not vulnerable to rot. This skirting could be protected with marine plywood I suppose, or it could be plastered with a durable plaster.

Q: Did you enjoy building with scoria? Did it work and last as well as you'd hoped?

A: Yes, building with scoria was a good choice. It is light and easy to carry and place the bags, even up tall ladders. And the insulation value was excellent. After two decades the house has not settled or cracked. I would do it again.


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