Using Miscellaneous Materials to Fill Earthbags
Questions answered by Kelly Hart

Q: Concerning earthbags, can they be filled with 'woodchip'. I have plenty of woodchips from some pine trees that we had to remove. How would 'peat' work? Living in Ireland, we have plenty of peat and I understand that it has loose particles. This should help its ability to hold onto heat?

A: You certainly can fill earthbags with almost anything, including woodchips and peat. The efficacy of doing this depends on how you intend to use the bags. Neither woodchips nor peat would be able to withstand much pressure from weight, so you would expect some deformation over time, which could be disastrous in some circumstances. On the other hand, if the bags were not expected to hold much weight (such as when they are placed in a roof cavity), then they might be quite useful to insulate that area. Both woodchips and peat are subject to rot if they are moist, so that is another concern that should be addressed.

Q: I live in northern Vermont and am very interested in building a round, yurt like home with earthbags. With the need for insulation & some mass - in your opinion, how would lightclay (wood chips & clay) work as a supporting wall?

A: I suspect that this would hold up quite well, especially if you created a solid wooden or concrete bond beam at the top for the roof to sit on. Also you might let it all settle awhile before plastering (making sure to keep the sun off of the bags.

Q: "Vermiculite" as a viable product for insulation of your green home. Vermiculite or "Asbestos" as it is better known, is a well documented product for its cause of lung cancer and silicosis-like effect on tissues. I would hope that you would mention these particular warnings to anyone that might use the "Vermiculite" idea from your page.

A: Thank you for alerting me to this potential problem with vermiculite, which I was not aware of. In doing some on-line research into the question, I have discovered that pure vermiculite does not contain asbestos, but that there was a period of time when asbestos contaminated vermiculite mined in Libby, Montana did contain some asbestos. This contaminated vermiculite found its way into some residential insulation products sold up to about 1996 in the U.S. and Canada. If disturbed, this insulation can become a health hazard if the dust is breathed. That mine in Libby has been closed down for several years and is no longer producing this contaminated vermiculite. Current sources of vermiculite are carefully monitored for this potential problem, especially because of the alert that has been issued by the U.S. EPA. It is my understanding that unless you go digging in someone's attic to obtain the old contaminated vermiculite, there is little danger in using newly available vermiculite.

Q: I was wondering if you think sawdust in an earthbag as infill in a Larsen truss wall would work, and if so how well? We are building in Kansas.

A: A bit of research reveals that Larsen trusses are lightweight rigid frames that are installed outside existing load-bearing walls, as a method of creating a large cavity for super-insulating a house and providing structural support for the new sheathing on the wall. Sawdust has been traditionally used as insulation, often mixed with lime to discourage insects, in wall cavities in the old days. I see no reason why your idea wouldn't work, especially if all general precautions for safeguarding the wall from moisture are made. I am not sure what the R-value of sawdust is, but would expect it to be similar to cellulose insulation, and therefore pretty good.

Q: What about using sawdust as fill material?

A: Sawdust might work, but I have never heard of anybody using it. Possible problems are: rot if it ever gets wet long enough, compaction over time, insect infestation (termites?). I know that sawdust has been used historically for insulation, and sometimes it is mixed with lime to inhibit mold or insect infestation.

Q: I'm trying to find urbanite, but so far all I've been able to find is broken up asphalt from a parking lot. Is this an ok substitute for cement? Will it off-gas? It would be used in a stem wall for a straw bale building on top of a rubble trench. Both sides would be coated in a lime/perlite stucco for better insulation.

A: I would not use asphalt chunks, not only because they might off-gas, but because they are not really all that solid over time and can easily "melt" and therefore settle. You are much better off using concrete chunks or gravel.

Q: I do like the scoria approach, but I don't think we have much in the way of volcanic rock in my neck of the woods.  I'm going to check into perlite to see if that's available hereabouts and if so, if it would be an affordable alternative.  But if that doesn't work, I'll probably have to scrap the bagging idea and go with some other approach. 

A: (Owen) Perlite, scoria, vermiculite and rice hulls are all good options.  Rice hulls are still experimental and it would be helpful to get more info on how they've worked for others.  Although rice hulls are resistant to decay, they will rot if they get wet, so I would use them only in dry areas with a good roof (no domes).  Price each one and see what's the most affordable.  For scoria, check with landscape suppliers.  You can get perlite and vermiculite from gardening supply outlets.  Try to buy things direct by the truck load.

Q: I was wondering if you have any experience with recycled products such as asphalt or concrete fines?  I found a local gravel yard with recycled materials and was thinking about using this instead of the typical earthbag material.  I have successful bonded recycled asphalt and recycled fines and have strengthened them by adding 15% to 30% clay.  My only worry is that it won't stand the test of time for whatever reason.  The test blocks seem strong, but most of them crack when I try to drive a screw into them.  Before building with earthbags must the samples be able take a driven screw without braking?  If so, does it matter the screw type.  In other words, must it be a masonry screw?

A: I don't have any experience with either of the materials that you mention, but I do know that "road base" has been a common fill material for earthbags with good results. The web pages about concrete fines suggests that it can be used as a road base, so it  might also work well. The recycled asphalt seems more questionable to me, partly because I know that asphalt tends to "melt" when it get warm, and so it might not be as stable over time, especially if there ever were a fire in the structure.

While some earthbag fill materials might be able to take screws without cracking, this is certainly not a requirement for building with earthbags. I built a house using crushed volcanic stone in the bags, and these would not hold screws at all. On the other hand, a good consolidated adobe fill should be able to accept long deck screw quite well. There are always other ways of mounting wall fixtures than using screws directly in the walls, such as placing wooden stakes at strategic places or even through bolting with threaded rods.

Q: Do you think one layer of scoria / perlite / pumice / vermiculite would be strong enough to support an earthbag (compressed earth only fill) structure? as compressed earth is much heavier (12' high).

A: I know that scoria would do fine under such compression, and I am pretty sure that both pumice and perlite would also; vermiculite or rice hulls might compress too much.

Q: I'm trying to build an earth bag dome here in Merida, Yucatan. The soil is pretty bare and mostly composed of limestone mother rock. There is an abundance of limestone dust that is not very expensive here and that I would like to use for bag filling, instead of the cement, lime, red earth, stone dust mix that my neighbours are using to build their earth dome house. My plan is for a 5 meter diameter dome. simple. Is their anything I could add to the limestone dust/gravel to stabilize it?

A: My guess is that if you just add enough water to dampen the limestone dust and then tamp it well in the bags that it will pack into a rather solid block. You might try this and see. If it doesn't "set up" after it dries, then you will need to add some cement or perhaps clay to it. I think you are going to have to experiment with a variety of mixes to see what works.

Q: What about using straw or rice hulls in a dome earthbag home that is reinforced with curved beams and wire net?

A: I would not advise using bags of straw or rice hulls for a dome structure. The possibility of the straw or hulls rotting from moisture is too great. If the dome is protected on the outside with an adequate moisture barrier, you still have the danger of condensation forming from within.

Q: I am assisting a group of friends to construct a 6' high, 700' long "Living" wall to insulate their eco-village from a busy road way. Earth Bags are the top contender for the project. My work with Robert has me wonder if the bags can be filled with light clay and wood chips or straw? The two reasons for considering light clay are 1) To reduce weight (labor) 700' feet is a lot of earth. 2) Teach everyone involved about earth bag and light clay building systems. It doesn't appear that anyone has done this. Based on my experience with Eco Nest I can't see why not?

A: (Owen Geiger) I think this can work if you add some reinforcing -- either zig zags, curves, buttresses, wider bags at bottom of wall and narrower bags at the top, benches, mesh, pinning, and so on. You don't need to do all of these things, just enough to ensure stability. Also, you consider putting a water resistant cap along the top of the wall such as clay roofing tile.

Q: Fellow Arkansans, Paul and Lisa Majors, filled their earthbags with 100% crushed lime and claim that when cured it's like concrete. What kind of fill, and in what proportions, would you recommend to support heavy loads?

A: Crushed lime seemed to work great for the Majors, and it would for you too if it is available at a good price. Any solid fill material like the lime or soil with a percentage of clay in it should be quite solid enough. But really practically any kind of fill can be used and still find a way to attach things to the wall.

Q: Is it possible to use a clay-slip straw mixture to fill polypropylene bags for an 8ft dome sauna? Of course it will shrink as it dries and is not recommended for weight bearing, but perhaps a small dome would be okay?

A: I can see several reasons why this ultimately would not work. Light clay-straw needs to dry out very quickly or the straw can mold, and being packed into the bags will deter this. Also, even if the material dried sufficiently, there is the danger, especially in a sauna, of it absorbing more humidity and molding or deforming. Seems too risky to me.

Q: I would like to know if you think its possible to mix the soil with some lighter material in order (like chunks of styrofoam, rice husks, etc) to make the lifting process easier.

A: I wouldn't advise you to mix in styrofoam or rice hulls because they will weaken the bonds and make the mix fragment easier. The way to deal with the weight is to fill the bags in place and not lift them, but gently lay them down into place.

Q: I am preparing to build a roundhouse earthbag shed in the Florida panhandle. When I called a local gravel and dirt hauling place and told him about my project, he suggested that crushed shells might be a good material. He said they used them for road base many years ago and they compact well and make brick like material. I plan to go get a sample of several types of soil from him to test them and will get a sample of crushed shells (mostly oyster). I'm pretty sure it is crushed small enough that it won't cut the bag because I've seen it used on driveways and it looks tiny. Other than following the various tests you've documented for soil, are there any other issues I might want to consider before using crushed oyster shells as my bag fill material?

A: I see no reason why this shouldn't work. In fact there was a project in the Bahamas several years ago where they did a similar thing using sand and coral.

Q: I live in central Texas and am wondering about using 'regular' concrete bags one can buy at Lowes or Home Deport. A 50lb bag of concrete is about $2.50 and based on the size of storage I would like to build would not be too expensive. What are your thoughts about the effectiveness of using this as the "earth" material? My plan was to lay and wet the bags so that they solidify.

A: Wetting bags of concrete to create retaining walls is an old method that works if the bag material will wick enough moisture into the concrete mix (some bags have a plastic liner that would prohibit this.) Of course you still have the remains of the bag to deal with or plaster over.

This approach will cost you quite a bit more than using standard earthbags (which can be purchased for about 25 cents apiece and are larger). Usually the soil can be accessed freely on site. And even if you need to add a bit of cement to stabilize it, the cost would be considerably less.

Q: We are a small recycling & waste management company based in Malawi, East Africa. We would like to start a project using used woven polypropylene bags. We are collecting these bags from various customers we have been considering using them for earthbags. One of the main problems we face in our recycling efforts is that we collect a lot of broken and whole glass bottles as a service to customers, but we cannot do anything with it. It is not economically viable to recycle the glass in Malawi, and the transport costs when exporting glass is astronomical. I was wondering whether it would be viable to use the broken glass as a filler for the earthbags?

A: I have not heard of anybody doing this, but that doesn't mean that it couldn't work. The glass itself is non-toxic and non-biodegradable so it might be a good candidate for use as fill material in earthbags. My primary concern would be that the broken glass could easily cut the fabric of the bags so that you would lose containment. Perhaps double bagging would help with this problem. You would still need to tamp the bags some to compact the glass so that it doesn't compress with the weight from above.

Whole glass bottles have frequently been used to build partial, or even complete, walls when they are mortared into place. It might be that carefully placed bottles inside bags could also be used for building walls, although this might require sorting the bottles into various sizes and be too much work. Whole glass jars actually provide some insulation because of the trapped air, and to a lesser degree the broken shards might do the same when contained in bags and plastered on both sides. I think this is an idea worth pursuing that will take some tests on your part to see how practical it is.

Q: Can you think of any issues with using fly ash for the inside bag material ? I am close to a power plant and wanted to ask them if I could get some fly ash for free to use.

A: I would be hesitant to use fly ash for this purpose. Here is a quote I found with a brief internet search:

"The Environmental Protection Agency is thinking about classifying the byproduct of coal power plants as toxic waste. Probably not something you want to hear if you live anywhere near a power plant. It's called fly ash. It's what's left when companies like Lakeland Electric, TECO and Florida Progress burn coal, capture the energy, and generate electricity. Here's what concerns scientists: it contains mercury, arsenic and lead, which can cause cancer, heart attacks and stroke." Fly ash is commonly used as an additive to concrete, and in that use it is probably much safer.

Q: I am building a dome in southeastern Arizona my soil is pretty sandy so I have been mixing it with a percent of portland cement and I a getting good results. My problem is that mixing is rather time consuming. I want to just be able to fill the bags which leads me to my question:

I know that you used crushed lava rock. If I am unable to get crushed lava rock (scoria) in my area, what other types of rock could I use? Is drain rock acceptable? Any type of locally available crushed rock? I seem to remember reading somewhere that you are of the opinion that you can fill the bags with "almost" anything.

A: Domes are a special case when it comes to appropriate fill material. Because the bags also end up being your roof, you need to be confident that they will never deform if, for some reason, the fill gets damp. For this reason we usually recommend stabilizing the soil with cement, as you have been doing. Drain rock may not pack into a solid block, another requirement for dome construction. The crushed volcanic stone does pack pretty well because of all the sharp edges.


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