Plastering Earthbags
Questions answered by Kelly Hart
Earthen
Lime Plaster
Cement Stucco
Papercrete
Bag Burning
Waterproofing
Wire Mesh
UV Protection
Sloping Interiors
Q: Can you tell me if an earth plaster can be used on the exterior of an earthbag building in a wet climate, or would I have to use a stucco cover for the sake of waterproofing the walls?
A: An earthen plaster can be used on the exterior of an earthbag building, but it might be advisable to stabilize the plaster some with Portland cement or emulsified asphalt...or plan on repairing the plaster regularly. Unless the walls are exposed to direct rainfall, it should not be necessary to waterproof the walls.
Q: We have had a friend level a driveway and house pad for us with a backhoe and discovered a very sticky substance in the volcanic soil that seems to be clay-like. You can squish it together, bend it and it holds its form very well. We are planning to make a cob mix with it to put on the inside walls of the earthbag dome home and create an earthen floor. Do you think that would work, this clay-like substance mixed with straw and sand and water?
A: The soil that you describe does indeed sound like clay, which is one component of good cob, adobe or rammed earth. But you really only want about 15-30% clay, with the rest sand for a good mix. You can figure out the relative % of your natural soil by doing a jar test. Fill a clear glass jar about 1/3 full of the soil, then add water to nearly the top and shake the whole mixture vigorously. Then let it stand for a day to settle out. Eventually the solids will settle to the bottom and the water will be fairly clear on top, with perhaps any organic material floating on that. On the very bottom will be the heavier aggregate, then above that the courser sand and then finer sand, and finally on the very top will usually be a clearly marked line of clay. Then you can compare the amount of each layer to determine the composition of your soil.
Q: Is a mud mixture or mud/cement/sand an alternative? What mix?
A: I'm sure there are many formulations that would work. I would suggest some cement for stabilization, although I know of people using lime for the same reason. Pure mud plasters would require more maintenance.
Q: I really want a living roof. What are our options?
A: I know that with their Honey House, Kaki and Doni had that roof planted with grass for a year or two, and talked about this working for them. It is a bit steeper than I would imagine would work. Perhaps embedding some fish net or something in the soil would help keep it up there on the dome.
Q: What is used to finish walls and ceilings in a domed or vaulted structure?
A: Usually it is recommended that a mix of about one third clay with the rest sand is best for an earthen plaster, because the straight clay has a tendency to expand or shrink, depending on its moisture content and the sand tends to stabilize this. A finish coat of a lime plaster will produce a more durable finish. I have also used papercrete to plaster the interior of an earthbag dome.
Q: I LOVE the idea of earthbag housing but I come from a wet country (Southern New Zealand) and also don't like concrete. How do I make an exterior surface that will stand up to wet conditions without using concrete? Do you need chicken wire when you plaster your house or will the baling twine be enough? Would old fishing net do the job?
A: You could use an earthen plaster stabilized with emulsified asphalt or you could use a lime plaster...or both. For a dome that might receive lots of rain, you might want to first cover the bags with plastic sheeting before the plaster. In this case you would definitely need some chicken wire or possibly fish net to hold the plaster in place. Otherwise, baling twine might suffice.
Q: How do you stop the bottom of your plaster on the outside of your walls from getting soggy if you live in a wet climate? What do you put on the bottom of your walls?
It depends on what sort of plaster you are using how necessary this might be; you can use a cement-based stucco for the first foot or two, if needed.
Q: I paid a visit to CalEarth and took the tour. I live only 20 miles away from the center. When I showed them a photo of the progress on my dome, they were impressed. They asked what I was going to coat the structure with and when I told them I was going to make cob from earth, cement, straw and water, they said if I used straw I didn't need the cement or if I used cement, I didn't need straw. With my soil having little or no clay I'm wondering if they are right. What do you think?
A: Traditional cob is a mixture of sand, clay and chopped straw, but earthen plasters can take many forms. Most earthen plasters do not use straw, unless it is wanted for texture or appearance. For a dome, an unstabilized earthen plaster on the outside will certainly require regular maintenance, which you most likely do not want to commit to. That leaves the option of stabilizing the plaster with something, such as cement, lime, emulsified asphalt, or some other industrial product. Without much clay in your soil, straw alone will not hold it together. If it were me, I would go with Portland cement, since it is likely more durable over time.
This brings up a related question about how waterproof the building will be. Because domes do not have roofs with eaves, the wall/roof is completely exposed to the weather. In your arid climate, your soil-packed bags covered with an earthen plaster will probably be fine. In a more humid environment, this system might lead to leaks, so then it might be necessary to put a moisture barrier between the bags and the plaster. In your situation, it is probably better to leave the whole thing breathable.
Q: I am wondering what would be the best material and best way to cover the surface of the earth bags after we finish the construction of the dome. In Africa we used cement for covering the surface. However, I feel that is not very natural. It looks not very pleasant for a dome located at a beautiful and remote location. Would you kindly instruct me about the proportion of the content of the surface, to keep it waterproofed and nice looking?
A: I have also puzzled over this question of plastering earthbag domes. The most common approach does seem to be to use a cement stucco, but as you say this is not so natural. On the other hand, a simple earthen plaster on a dome would require regular maintenance for sure, because it would tend to wash away with the weather. Some degree of stabilization would be needed. Another option would be a lime plaster, which is fairly durable and more natural than cement...or possibly an earthen plaster at first which is then given a final coat of lime plaster. I think that a lime plaster over an earthen plaster would be a reasonable approach to explore. Both of these are naturally breathable, and the lime may shed water sufficiently to keep the interior dry. I haven't tried it on a dome, nor am I aware of anyone actually trying it. But this doesn't mean that it isn't a good idea. I think some experimentation is in order. Unlike Portland cement, lime reabsorbs the CO2 that is released in its manufacture while it cures, so it is a net zero polluter in this regard. Lime has been traditionally used in Europe for centuries as a plaster over cob and rammed earth buildings.
None of these guarantee that the domes won't leak in a heavy rain, because cracks might occur...even with the cement stucco. The only way to be certain of no leaks might be to place a plastic curtain over the dome before plastering, which then would require the use of some sort of netting or wire mesh to hold it in place.
On the dome house that I built in Colorado, I used papercrete for a plaster, and it worked well in that rather dry climate...but papercrete requires machinery to make, and also uses some cement, so it is not so natural either. And it would not be very good for wetter climates.
Q: We have a small winter home in Baja Mexico and want to construct a workshop/generator room using sand filled bags. I would like to plaster the finished structure (inside and out) and would also appreciate any advice as to what wire or ? would be best to use.
A: Using some form of stucco netting or chicken wire with the plaster will make it more durable over time.
Comment: I found a great solution to render the bags, using a mix of 55% earth, 30% coarse sand, and only 15% (white) cement, plus some red and yellow oxides. The plaster is alive and looks great. Much better than the crazy amount of cements we used to render our first structure! Also I have oiled the entire house with linseed oil and am very content with that too, it feels so much better than these toxic stains...
Comment: I have some interesting info about earthen plaster. I used sand, cattails, clay and water on my garden bed. What is interesting is how the rocks embedded in the mixture makes the plaster even stronger. I also experimented with a little bit of wheat paste. Each section of the garden bed is an experiment. The section that is doing the best in rain and freezing temperatures is the section with the imbedded rocks, which I sealed with flour paste, I also added the paste to the mix. After the rocks where imbedded, I waited about an 1 hr until the plaster hardened up a bit then put a layer of flour paste on the plaster around the rocks, it did an amazing job to seal it up.
Q: We have read and seen a lot on the internet and some CalEarth books and we started practicing on building with sandbags (read recycled cement bags). We started building a perimeter wall, but we keep having problems with plastering our bags. We don't seem to find the right mixture/balance of clay and sand. When we carry out the suspended jar test, we don't get any layers. It is a big blur. But the soil feels sticky, and pliable. We also use cactus juice in our plastering and limewash, and we finish of with a lime coat. After strong rain last week parts of the wall got washed away. Would you be able to have a look at some pictures taken at our site and give recommendations? We are planning to build a dome, but now we are not too sure about the durability after a rainstorm?
A: (Owen) It sounds like your soil is not suitable for plaster. Marginal soil can work in the bags, but earthen plaster is more critical. An earthen plaster subcoat ruined the domes in the Philippines. http://earthbagbuilding.wordpress.com/2009/07/23/a-sad-but-educational-footnote/
Just a guess, but maybe you have silt for soil. Talk to a local engineer who does construction. They'll be able to tell you. It's probably best to eliminate the earthen plaster and use only lime or cement plaster on the bags.
We warn people all the time that domes are a poor choice in rainy climates. Domes evolved in desert regions. They're very difficult to waterproof, will need regular maintenance, and all the bags need to be stabilized.
(Kelly): The use of mesh or netting (either metal or plastic) attached to the wall before plastering will help keep it all intact.
Q: I've got nine courses laid and two or three more courses to go on my garden wall, and am looking for plastering options for my poly bags. I'd like to put a Type S hydrated lime plaster on, but from what I've read I'm not sure that this would adhere directly to the bags. They suggest an earthen plaster undercoat, but I don't really want to do that as I'm not sure my clay is the right type. (It dries to rock-hard state in the summer heat and has lots of shale in it which would be difficult to winnow out.) Do you think I could slap it directly on the bags?
A: My experience with lime plaster is that it works best in thin applications, partly because it takes a long time to dry, but also because it does shrink quite a bit. Like clay, it needs to be mixed with sand for greater durability and less shrinkage. I used a lime plaster over papercrete on the interior of my earthbag house, and this worked well. In your situation I might suggest an earthen plaster first, and then a thinner coat of the lime. A bit of latex paint can be mixed with this to keep it from rubbing off when touched. I am sure that your clay will work fine if it is mixed with 2 or three parts of sand for every part of clay.
I was working with a lime putty that I had prepared in advance and then added white sand and a tad of white Portland cement to make it set up quicker. It can take quite awhile to harden, especially in a humid environment. In Mexico, I tried mixing straight hydrated lime with sand in ratios simillar to cement stucco, and applied this as the complete plaster on the inside of the little dome project. This was during the dry season, and it hardened up within a few days and seemed to be quite durable, without much cracking...I was impressed.
Q: We're planning on plastering the building with lime and lime-wash. I know that this must hold up pretty well in the tropics because all of the old Spanish forts in old San Juan had lime mortars and plasters. But I want to know how often will we have to apply a lime wash to keep the plaster fresh.
A: I think that lime washes are often renewed every few years to keep them fresh looking and help protect the plaster.
Q: I've built a dome house using Mr. Khalili's basic plan. I've plastered that structure with the same basic mix, screened for a finer texture. Things went well. The plaster "alligatored" a bit where it dried too quickly. But that is not really a concern. The finish is somewhat softer than I would prefer. I am in a high humidity climate. Will painting the house with lime wash made up from hydrated lime powder cause the outside skin of the earthen to harden a little more? Will multiple coats improve the hardness? Currently applied earthen plaster ratios; 20% clay 65% clean, sharp, washed river sand, 15% portland cement
A: Lime plaster is often used to provide a somewhat weather resistant coating over traditional cob or earthen plastered buildings. These walls, however, are usually also protected by roof eaves and do need to be renewed periodically. Lime plaster is fairly soft and is not waterproof, which is often a good thing in that it preserves the breathability of the wall system.
Providing an adequate moisture barrier on an earthbag dome is something that needs careful attention. Even conventional cement-based stucco cannot be relied upon to provide long term assurance that leaks won't occur. I have used latex-based roof coatings over stucco in an attempt to waterproof a dome, but this is neither natural nor permanent.
The earthbag/superadobe community is still looking for the perfect solution to waterproofing domes. Please let us know if you find a good solution!
Q: I was wondering if a limestone plaster or stucco on the outside would work or be durable?
A: Yes you can use a limestone plaster on the exterior and this would especially be a good choice if you are making a vertical wall home as opposed to a dome. Lime may not provide the needed moisture protection if there is not a roof with an eave over the building.
Q: Could I cover a dirtbag house with traditional cement rendering or would it be too heavy - or too difficult for an amateur to apply?
A: I know of several bag domes that are covered with ordinary cement stucco.
Q: What mix would you use for your plaster that would be relatively inexpensive since we have such a large surface area to cover?
A: Stucco mixes often combine both cement and lime. Something on the order of 25 parts sand, 2 parts cement and 1 part lime should work...but you might see if this is strong enough, and also discuss it with the locals who will be doing the job.
Q: Could I simply spray stucco over my earthbags when finished laying them? Any idea where to get a stucco sprayer on a budget?
A: Yes you can use stucco as a plaster over earthbags, and this could be sprayed. Probably it would be best to use a stucco netting to help keep the plaster from cracking too much and keep it attached to the bags. Depending on the location of the bag surface, you might want to have a moisture barrier over the bags before the stucco netting it placed to help keep moisture from getting through any cracks that do appear. For instance this might be more important with a dome than with a protected wall. Stucco spraying equipment can probably be rented...or you might just want to hire an experienced crew with their own equipment.
Q: There is a lot of rice grown in Texas, so I am thinking that filling the bags with rice hulls would work best. However, I'm not sure what type of plastering material to use over the earthbags. Papercrete doesn't seem to be the right material, since it is usually quite humid here. Can you suggest the best material for me to use?
A: I think that the choice of plaster could be critical to the success of this project. Rice Hulls are bound to compress quite a bit with pressure from above, so a weak plaster might fail over time. For this reason, I would recommend an ordinary cement stucco that is well reinforced with mesh. If this were done both inside and out, then you would in effect be creating a large SIP (structural insulated panel), with the rice hull bags as the insulating filler and the stucco as the exterior membrane. It has been shown that with strawbale walls, the plaster is often the load-bearing part. You might construct a small wall in this fashion to convince yourself and the authorities that it can withstand the necessary pressure of a roof structure, etc.
Q: If I plaster with cement stucco, is chicken-wire mesh necessary? Is there a less expensive option?
A: You can plaster without the mesh, but the stucco will be more durable over time with it. I believe that there are several other stucco mesh options, but I don't know the costs.
A: (Owen Geiger): Poly (synthetic) fish net is less expensive than conventional wire plaster mesh. It's been used with earthbags with good success. If you look around you'll find some that are incredibly inexpensive. Must be made in China or somewhere similar with low cost production. And they're very strong. You can use twine to tie it to the bags.
Q: Just how thick is a cement plaster layer used on earthbag? And does it take a lot of cement to fill in the uneven face of the bags? What is the minimum thickness? For me to propose this material to the regular building people I need more specific info.
A: Earthbags do need to be covered with a plaster, or be protected somehow from the sun, but this does not have to necessary be with cement. Because of the cavities between bags there is some natural variation in the thickness of any plaster, but I would say a minimum of about an inch would be appropriate.
Q: What's the best way to finish the bags for a humid environment? We like the smooth plastered look.
A: Probably an ordinary cement stucco is the most durable, especially in your climate.
Q: At one point on your site you suggest using Portland cement mixed with lime. Would that be a good bet, and if so, where can I find instructions as to proportions? How about a mix of P. cement, lime, and some of my clay-ey dirt that I soften up with water?
A: Portland cement, lime and sand is a conventional mix for masonry mortar, mixed 2:1:9...and I am sure this would make a very durable and sticky plaster. I wouldn't add clay to this, since clay and Portland cement are somewhat at odds with each other.
Q: I have an acre of land in the Seattle area. It is rural forest and my private road is an old dirt logging road. I have excavated part of a hill next to the road and it is about 10' high. I want to put a raised bed/retaining wall there. Serving the purpose of reinforcing the dirt wall and providing seating next to the road and a planter bed. I wonder if you have any ideas for plaster for a retaining wall that gets rained on frequently and on the earth side obviously has earth packed against it?
A: If it were me, I would probably opt for a cement-based stucco plaster over the exposed bags, mainly because the wall will be completely exposed to the weather and this will require the least maintenance over time. Stabilized earthen plasters or lime plasters are other options. Use of a stucco mesh will further stabilize the plaster. Retaining walls of this sort do need to deal with the possible build-up of water behind them, so a French drain around the base and/or periodic weep holes in the wall might be a good idea.
Q: What is the best plastering material for moisture and wind?
A: The most durable plaster under severe weather conditions would also be cement-based stucco with a mesh material embedded in it. If the cement ratio is high enough, it would be virtually waterproof.
Q: I live in rural north Portugal which can have torrential rain-storms. Would you recommend any special outside render-covering of the earthbag structure for this particular situation?
A: The ideal render for waterproofing an earthbag dome is still a matter of speculation. The conventional approach seems to be the use of cement-stabilized stucco, which can be enhanced with latex. If this is inadequate then it can be painted with various exterior roofing paints. Structures that have conventional roofs with eaves are much more forgiving, and stabilized earthen plasters or lime renders will work fine.
Q: One thing that startled me was the story of the domes that were built in Philippines for the health clinic. They used a layer of mud plastering prior to plastering over it with the cement plaster, and it obviously failed. My understanding was that the failure occurred because the cement does not adhere well to that type of earth plaster. The plaster must have cracked off in a number of places, and then the exposed earthen plaster (which probably had seeds in it) came to life (plants grew out of it).
A: Yes, the outer layer of fill and plaster did fail on that Clinic, probably for several reasons. Cement-based stucco is very hard to keep from cracking some, and all it takes is a few little cracks to let moisture in. Then once it has penetrated it cannot easily escape, so moisture accumulates to germinates seeds and cause other problems. For this same reason it is generally not advisable to stucco over adobe walls. Lime plasters are much more breathable than cement-based ones, so this problem is less likely to occur. But in a rainy climate lime plasters may not provide enough moisture protection on domes. This is the perennial problem with earthbag domes: How to seal them from moisture? CalEarth is still trying to come up with a solution to this. Last I heard they were going to try Elastomeric paint on the domes they plan to build in Haiti. In Mexico, I basically did the same thing, using a commercial latex-based roof sealer to paint the cement stuccoed dome that I built with students at a school recently.
Q: I read about using paper, water, and cement as a plaster. I don't know how it will withstand our termites so I will have to give it a try.
A: As you may know, we used "papercrete" to plaster both sides of our domes, and it has worked out very well. Papercrete is basically made with water, paper and Portland cement, but the paper is turned back into a pulp in the process of mixing so that nothing is readable any more. A more durable final coat can be made by adding quite a bit of sand to the mix. This material does seem to resist termites and other insects.
Q: I plastered my earthbag building with papercrete. Then I left it for one year without roofing it. I am from Dar-es-Salaam in East Africa. My building is under the shade of big trees. After an annual rainfall period, I found out that the papercrete turned black in color. I have some knowledge in mushroom substrate science. When the paper in in the composition stays wet for long time it becomes a good medium for fungus to grow, which decomposes the paper. As a result, the papercrete deteriorates and becomes unsuitable. Please advice me on this problem.
A: I am very sorry to hear about your problems with the papercrete. I do not advise people to use papercrete in damp or humid conditions, because of this very problem. I had a little mold form inside my earthbag/papercrete dome, which I then plastered with a thick layer of lime plaster, which took care of the problem. Another possibility would be to kill the mold with bleach or something, and then replaster your walls with a cement stucco.
Q: Will papercrete work on a 45 degree slope? My current design is a scoria-filled earthbag pithouse with a 45 degree roof. It's not a matter of getting it to bond like on a ceiling, but rather if it will hold up to the weather and keep water out. This design is only for dry climates.
A: I would have to say after my experience with the domes in Crestone, that I would not expect PC to hold up for a very long time. By the time we sold it (after 7 years) the PC was eroding in places where there was more severe elemental abrasion, such as at the bottom of the windows or on the top of the entry vault. The folks who bought it have noticed this and asked my opinion about what to do to protect it, and I have suggested that they actually do a final stucco coating to make it all much more permanent.
Q: In the mix of paper with cement what was is the rough percentage of cement for papercrete?
A: Basically the mix that I use mostly is: 50/50 paper and cement by dry weight, and the amount of water doesn't really matter...just enough to make the slurry. This is for "pure" papercrete, without sand. For a more durable product that doesn't shrink as much and is less likely to burn, add some sand to the mix, and reduce the amount of paper proportionally by wet volume. It is best to experiment and come up with a mix that works for what you are doing.
Q: For stuccoing walls, it seems to me that paper and cement mix does not get wasted as much as a mix of fine earth and cement. Although not a major cost, we have to throw a couple of layers of mix in our Hawaiian dome after some of the mix just does not hold on the wall. Seems like a crafts and artist job, the better with your hands the less waste you get.
A: Yes, it is a tricky business getting plaster to stick, especially to the inside of domes. I finally came up with a technique of throwing just a thin layer of papercrete on the bags at first, and then adding more after it set up. Pure stucco is much heavier, and so even more difficult to make stick.
Q: The Mongolian Dada burned the plastic off the bags to stucco. Is that what you advise?
A: I have never burned the bags away, especially because I filled my bags with a loose volcanic gravel, so this would not have worked at all. Some people seem to think that removing the poly material makes the plaster stick better. I have found that it sticks pretty well without. I also tied poly twine around the bags to give the plaster something positive to hang onto.
Q: You mentioned burning off the bag after assembly. Would there be a need for a wire mesh covering after the burning of bags and before plastering?
A:
I don't recommend burning away the bags since this is not essential, and only works if the fill material is very solidly consolidated. Wire (or synthetic stucco) mesh will likely make the plaster more durable over time, but is not essential either.
Q: We want to built an earthbag dome in the South of Chile, where there are heavy rains almost throughout the whole year. How can we make sure, that the dome is waterproof?
A: I wish that I could give a simple answer to this question, but there is no tried and true method of insuring that an earthbag dome will remain waterproof. The common thing to do is to use a cement-based plaster and hope to the best. I recently did this and discovered that it still leaked during a heavy rainy period. So then I resorted to painting the exterior with a latex roof paint that was especially formulated to seal concrete roofs.
Another approach might be to line the bags with plastic sheeting before it is plastered, but a technique for doing this has not been thoroughly worked out. It would likely require using a stucco mesh over the plastic to keep the plaster intact.
Q: You don't recommend water proofing coat on a normal construction, it seems like water proofing would be beneficial.
A: The trouble with water proofing is that is inhibits the breathability of the wall system, which is generally beneficial for both occupants and the wall. Under some circumstances a moisture barrier is necessary, such as when the area will be bermed with earth.
Q: I am planning to build a super adobe dome house, in a place where it rains a lot, using the pp tubing for the walls, Nader Khalili's style. What can I use to water proof the dome?
A: This is a good question, and one that should take a lot of consideration. You certainly don't want the adobe to get very wet, even when it is encased in the tubing. The tubing should be plastered with something on the outside to protect it from the sunlight. I would think that the most sure approach might be to drape a layer or two of plastic sheeting over the dome before you apply the plaster, using some wire mesh over the plastic to help reinforce the plaster and hold it in place.
Q: You said your house is made of earthbag and papercrete, and in the section on papercrete it is said that papercrete will act as a sponge unless coated with a protective layer. I'm wondering what sort of material would work as a protective layer to shield the papercrete plaster from rain?
A: With my house I left the papercrete unsealed so that the whole house breathes, and in our arid SW climate this has worked well. The papercrete simply soaks up moisture when it precipitates, and then releases it back to the atmosphere. With the earthbags as the central main component of the wall, water never makes its way into the house. In a wetter climate, it might be necessary to seal the papercrete, and this can be done with a mix of silicone caulk and mineral spirits, or a similar commercial product.
Q: I was thinking of using a painted on waterproofing membrane, like roof tar, or something of the sorts, before backfilling a berm. Any suggestions?
A: I would not expect this to provide adequate protection against the eventual penetration of moisture into the wall. What I did with great success was to line the exterior portions of the earthbags with two layers of at least 6 mil polyethylene sheeting over all areas to be backfilled (and above grade by maybe a foot). This can be folded right into the appropriate course of bags while building.
Q: I am interested in building an earthbag home down here in Louisiana. I am curious to know if you have any tips on how to gutter around the dome in order to harvest rainwater. I am considering building a pole frame and using rice hulls (which are abundant here) to fill the bags.
A: In order to harvest rainwater from an earthbag dome you will need to use a plaster that will easily shed the water and not contaminate it. I would imagine that a cement-rich stucco would shed the water. Obviously some molded gutter arrangement would need to be built in to the exterior plaster design. With cement stucco, you may need to have an impervious layer of plastic beneath it to keep water from seeping into the dome from cracks that will likely develop. This has been a problem with similar domes that I am familiar with.
Q: I have a question about sealing the papercrete on the outside from heavy rain. Thorsen Foundation Sealer followed by an Elastomeric paint was suggested, what do you think?
A: People have successfully sealed papercrete with sealers or paint, but there is the risk of actually sealing in the moisture so that it can't escape if it does find a way in. I generally prefer leaving it breathable for this reason. I never had any moisture come through my papercrete/earthbag domes.
Q: With the application of concrete with the plaster that is applied to the walls and outer surface of the building, is there any breathability? This is a quality that is normally so wonderful with an adobe structure.
A: The earthbags themselves are breathable, as is the fill material, so it depends on what they are plastered with how breathable the wall will be. Earthen plasters and lime plasters are quite breathable; cement stucco is not as breathable, but can be formulated to be somewhat breathable; papercrete is breathable. The question of breathability becomes more of an issue with domes, where the walls/roof must also shed water...
Q: I saw some designs where a roof was placed over the earth bag walls. What is the advantage/ disadvantage of this design over using the earth bags to form a domed roof?
A: There is certainly no structural need for a separate roof. I suspect that the choice was made for either aesthetic or moisture proofing reasons. How do you keep the moisture out of an earthbag dome? With my domes, I used scoria to fill the bags and plastered them with papercrete, which in the fairly arid southwest US was fine. In a damper climate, like Malaysia, further measures would have to be taken, such as a moisture-proof plaster or the use of a plastic liner over the dome before it gets plastered. With either of these choices, it can be tricky to assure that no moisture enters the structure.
Q: How do you attach waterproof sheet to your roof without making holes in it?
A: Sometimes you can drape the sheeting over parts, or embed it between courses of bags to hold it. Otherwise, it might be necessary to use staples and caulk places where they tear the plastic.
Q: If one was able to bury the earthbags how would one be able to seal the structure to prevent water damage, frost and other issues that the bags may have to deal with by being under ground?
A: I used two layers of 6 mil poly sheeting to waterproof my earth-covered pantry. See the lower photos at http://greenhomebuilding.com/storeyourfood.htm.
Q: Would it be better to put loose rubble around the outside of the bags so that no soil would be in contact with them?
A: Once the bags are moisture-proofed with plastic, there is no problem with backfilling with soil.
Q: Although my boyfriend and I don't yet have the piece of land on which to build our dream earthbag dome, we have every intention of doing so once the opportunity arises. Although the climate in the South of France gives us lovely long hot summers, we also get very cold and wet winters, so good waterproofing will be essential. I do not want to go for the roof option however, because for me the lack of roof is the whole attraction to earth dome houses. Something that has occurred to me is that I could kill two birds with one stone by covering the dome in a layer of thinnish stones with water resistant cement? In France it is common practice to build brieze block houses and then cover them in slabs of thin stone so that the house looks like it was built entirely of stone, so such stone 'tiles' are easily available. This way I will hopefully not only make it more rainproof but also make the dome house look like the traditional Southern French stone 'borie' (pictured above), tiny dome shelters that the shepherds used in the olden days. I've attached a few photos so you can see. The French authorities are notoriously bureaucratic and repressive when it comes to errant individuals not conforming to the 'slavery-to-big-industry' ethic that sums up the French establishment approach to all things alternative. I feel that if i were to do a small earth dome covered in a thinnish layer of non porous stone, it would look so much like a borie that they might turn a blind eye to my lack of conformism to the brieze block religion that reigns out here. My question would be, would this be enough to really waterproof the structure? Like you I have my concerns about using a plastic sheeting on top of the bags for the possible problem of breathability of the structure....perhaps covering in a thin layer of stones might work?
A: You have hit on one of the big quandaries with earthbag domes: how to make them waterproof and breathable at the same time. I did this on a house that I built by filling the bags with crushed volcanic stone, and then plastering the dome with papercrete, but this may not be appropriate in all situations and climates. Covering the dome before plastering with plastic will keep it dry inside, but also stop the breathability, as you suggest. The stone borie that you refer to are wonderful examples of vernacular and functional architecture. They remind me of the Italian Trulli buildings made with limestone (see http://en.wikipedia.org/wiki/Trullo ), which are waterproof. Perhaps a similar technique could be used to cover an earthbag dome to make it waterproof, as well as breathable. You would need a good supply of flat stone to do this effectively. Your idea of using small stones that are cemented together seems a bit risky, in that it is very difficult to assure that leaks won't penetrate the assemblage over time. I think that some experimentation is in order.
A: (Owen Geiger) One issue is the added labor. You're basically building two houses. Earthbag building is labor intensive and, as I'm sure you realize, stacking thousands of stones is no small chore. It will make for a lovely exterior finish, but just be aware of what you're getting into. I would advise building small and possibly hiring someone with this type of stone building experience. Also realize there will be extra foundation work if you use stone.
Ventilation: Add cross ventilation -- windows on both sides. Add fans that vent to the exterior for anything that produces smoke or humidity -- cooking stove, bathroom, etc.
I would use easily repairable earthen plaster on the interior. In the event of a leak, it would be much easier to deal with than lime or cement plaster. Earth plaster is low cost, doesn't mold and it excels at stabilizing humidity levels in the home. Also, the acoustics would be excellent.
Q: I was wondering whether breathability with the plastic covering has to be a problem if the dome at least has an air vent so that the air can circulate from the outside to the inside, even if it's just through this one space? And yes, the local French bories are indeed related to the Trulli and, from what I gather, waterproof too, as long as they're not tumbling down that is...although the name 'borie' means 'bad hut' in old provencal, so the locals didn't seem to think much of them...still, these structures are obviously strong as they have often remained standing hundreds of years.
A: Well, I do think that while breathability of the skin of a building is desirable, it is not necessary for the health of the building or the occupants, as long as any issues related to possible condensation on the inside are addressed and, as you suggest, there is adequate ventilation of the space. I placed exhaust air vents at the peak of my domes, and provided ample inlet air vents near the base, and the venturi effect of upward air was very noticeable.
Q: If an EB home was built with a dome shape, could shingles still be used if the owner wished?
A: The trick with putting shingles on an EB dome is attaching them. You would need to have a solid material (probably wood) to nail them to, and this is hard to do with the dome shape, although not impossible. If you went to the trouble to create a roof structure to nail the shingles to, you might as well not bother with the earthbag dome, in a way, and just use the structure for the roof and insulate it in a traditional way.
Q:
What is the best plastering for the outside walls, the area suffers from heavy winter snows and light rains in the summer.
A:
If the walls will be protected by a roof, then a lightly cement-stabilized earthen plaster or a lime plaster should suffice; if you are building a dome, then you might want a more waterproof, heavy cement stucco.
Q: We need to do a bermed or underground structure. How do you use the earthbags underneath the soil for walls? Do you plaster or waterproof the outside of the filled bags with some type of waterproof material? We want it to remain as natural a landscape as possible after construction. The area is high dessert in the midwest. Rainfall is about 13 inches per year normally.
A: I think that your project would be a good one for earthbags. The earthbag house that I built in Colorado was partially bermed, and I used 2 layers of 6-mil plastic polyethylene between the bag wall and the berm, and this worked out well. You can see pictures of this above.
Q: We have come to the conclusion that a Bermed Earthbag home is the final step for us. We are concerned about moisture and earthbag building a bermed home. I see by your drawings that you put 6 ml poly between the soil walls and the earthbags? Does this mean that one needs to leave the bags exposed for a while to cure them before plastering them? In the environment out there the sun could really damage poly bags before applying the plaster if we wait too long.
A: You are right that it is important to keep sun exposure of the bags to a minimum, so if your fill material needs to cure or dry out some before the bags are lined with plastic and backfilled you might try to allow some airflow to the bag wall using a temporary tarp for a period of time. I used crushed volcanic stone to fill the bags of the house I built in Colorado, so there was no need to wait for curing. Also, bags will continue to breathe and cure even if one side is covered as long as the inside is left unplastered for awhile, or a breathable plaster is used.
Q: I am building in Washington state, where it is cold an wet, and I read somewhere in your questions and answers that one can waterproof by putting heavy duty pond liner and then plaster over it, but how do you go about making plaster attach to a pond liner?
A: If you were to plaster over a pond liner or other such moisture barrier, you would need to cover the liner with wire mesh or stucco netting first to make sure that the plaster stays in place.
Q:
My husband wants to build the small dome as seen in Mother Earth as a studio for me...this makes me very happy! We are however a little confused by the different "plans" to build the dome. We would like for it to look like the one in Mother Earth...Was this one plastered? or is the earth sides and roof enough to keep it water proofed? I would imagine there would also have to be some sort of vent for air as well.
A (Owen): This dome was not plastered. In the article I explain how to cover it with 6 mil poly in layers. I live in a rainy climate and so far there have been no leaks. Another issue is the living roof. It's beautiful, for sure, but it's a lot of work! It's like tending a garden. Rain washes soil and nutrients downhill, and so you need to regularly add compost to the top and water it to prevent drying out. Plus, you need to weed, trim, add new plants, etc. regularly. I love my dome and so this extra work is acceptable to me, but many would not want to do this. It would be much faster in the long run to plaster the dome.
Q: Does anyone have any ideas for creating a good skin for a dome without the use of cement?
A: Waterproofing a dome is sort of the achilles heal of the system I think. I don't really know of a foolproof method. The domes I built in Colorado were completely breathable, with papercrete as the plaster and the bags filled with crushed scoria. This worked because the papercrete held the moisture from entering and then it simply evaporated when the sun came out...but this was in the arid SW US, and I wouldn't recommend it in damper climates.
I have always assumed that cement-rich stucco would do the job, much it does with ferrocement cisterns. But I just got back from working on our little dome project in Mexico, and now I have my doubts about even this. We used sand/cement ratio of about 7:1, applied two coats, and used an latex additive on the first coat. After un unusual spell of nearly three days of continual rain, there was quite a lot of water that had enter through the plaster. I think that this dome will eventually have to be painted with a roof sealer to waterproof it.
Domes that were subsequently covered with soil have been successfully waterproofed by layering plastic sheeting over them before the soil, but this does require quite a bit of maintenance over time to keep from eroding, etc.
Another potential option is to apply a waterproof coating directly on the bags before being plastered. This was done in New Zealand recently at a workshop (see this page), but this has not weathered the test of time, so the jury is still out about this.
Q: Why is it important in your opinion that an earthbag structure be breathable? I plan to use Grancrete on the exterior of my earthbag structure, and this is waterproof. I might use it on the inside as well.
A: I am in favor of breathable walls in general, just because they tend to be healthier for both the walls and the inhabitants, but this is not always possible. Earthbag domes will usually need to be sealed to keep out water, for instance. I see no reason why you couldn't use the magnesium cement to cover the earthbags, once you work out the best way to do this.
C: The proper way to waterproof domes has been a recurring question on theCalEarth forum over the last several years. The only one that seems quite good is the asphalt paint though that is ecologically quite unfriendly and it stops the walls from breathing. Most of the domes we built were primarily for the sake of the workshops. I think at the moment only one is utilized, this one was finished with the asphalt paint. Also the ones in Pakistan were finished like that, I don't know how they are holding up, I doubt they are actually used as they were also just for demonstration.
Q: How can a slick plastic bag that gets brittle from exposure to the elements promote adequate adhesion to any surface material unless it's solidly mechanically anchored?
A: Because of all the deep crevasses inherent in earthbag walls, I think that plaster has more to hold onto than just the plastic. Also, wire netting and other wire will help a lot in keeping the integrity of the wall system.
Q: How to attach the netting/lathe to the bags? I've read that strawbalers use some kind of "pin" to attach netting to their bales, but I'm thinking you would want something rough or with some bends in it to keep from easily pulling out. Any thoughts or other ideas for 'pinning' the poultry netting to the bags?
A: I have used large wire staples of the sort that strawbalers use, and they work pretty well if you hammer them in at angles so they don't pull out easily.
Q: Could you describe these 'staples' for me? (length, size of wire, width of the bend, etc.) so I have a better idea what you're talking about?
A: I manufactured the staples myself, using rather heavy duty wire that you can barely bend with your hands. The U shape has ends that are about 6 to 8 inches long and have about 1 to 2 inches between them, and you can usually pound these in with a hammer or large stone.
How do you attach the chicken wire?
I made large wire "staples" out of heavy gauge wire bent in a U shape and pounded them into the bags to hold the wire in critical places. Or, if you are not lining the bags with plastic before plastering, you can periodically lay pieces of twine across the bags when you lay them and use these later to tie up the mesh.
Wouldn't pounding metal staples through your plastic waterproofing defeat the purpose?
If you do it carefully, they are nearly self sealing, and if not you can seal them with a bit of silicon caulk. Also, you use them sparingly.
Q: I've been looking at hay bale net wrap for a while now as an alternative to chicken wire. The problem is that it comes in 48 and 60 inch widths, which is hard to work with. I found some pallet wrap that comes in 20 inch widths, and even has a handheld roller. I know that many people just stucco right to the sandbags, too. Please share your input on this.
A: I have done both, using chicken wire mesh and none at all, and they have both worked. I did place poly twine around three courses of bags and tied it periodically, so that there was some degree of positive grip...and I'm sure this helped in the areas without the mesh. I used the chicken wire on the outside where I knew it would see more weather. And I was using papercrete plaster, so I kept the mesh to nothing less that the 2" wire, since anything smaller tended to gum up too soon. Owen has used old fish net and says that it also works well. I expect that ultimately the mesh will increase the longevity of the plaster, but this is just a guess.
Q: I read somewhere that earthen plasters don't need the chicken wire/lathe cover (because the clay sticks to the bags just fine) but lime plasters still need the support of the wire. Is that correct? Does that include the "cradles" mentioned in the K&H book, like around corners or under arches, or are those not really necessary either?
A: I am not sure about this distinction, but in general I think that a mesh of some sort is usually a good idea because it is positive insurance that the plaster will remain over time. I like the cradle idea under arches because this is a very difficult area to get a mesh to stay put, and the cradle is easy to do during construction.
Q: You mentioned burning off the bag after assembly. Would there be a need for a wire mesh covering after the burning of bags and before plastering?
A: I don't recommend burning away the bags since this is not essential, and only works if the fill material is very solidly consolidated. Wire (or synthetic stucco) mesh will likely make the plaster more durable over time, but is not essential either.
Q: I leave the finish rough and before applying the second coat, I wet the wall down before applying the cement mixture. Do I need to use the mesh in between each coat? I planned on doing 4 coats.
A: It would be unusual to use any more that one layer of plaster mesh, and this is usually embedded in the first coat. Beyond that, as long as the subsequent layers are left fairly rough so that the next coats will have something to "key" into, this is all that is necessary.
Q: Would latex paint work to block uv rays, to paint the inside and the outside as you go stacking bags?
A: This might work, at least for awhile, but I wouldn't rely on it without testing the theory over several months to see how well it works. Somehow keeping a tarp over the project during construction seems easier to me.
Q: I would prefer to plaster my earthbag house in one session after completion of construction. This means the structure would be exposed to sunlight for up to 6 months or more. Can you recommend an interim plaster or coating that is very easy apply to just protect the bags from deterioration in this time, and that would also provide a good base for the final plaster proper?
A: With my house, I did two applications of papercrete as a plaster, with the first one going on as I progressed, to some extent, and the second being applied once I had finished all of the bag work. The second coat also got a unifying layer of chicken wire mesh embedded in it. This has worked well, but after about 10 years, some of the papercrete is beginning to wear away, and think a final coat of ordinary cement stucco would be in order to preserve it for a longer period.
It can be rather cumbersome to make papercrete, since you need the right equipment, whereas ordinary stucco is much more commonly available. A similar two-stage approach could be used with this as well. Earthen plasters are another possibility; you might want to stabilize the second coat with a bit of Portland cement or emulsified asphalt to make it more durable.
No matter how you organize your plastering, I would still recommend using tarps to keep the bags covered until they can be covered with a plaster, except for the area where you are working. It is surprising how quickly the UV will destroy the polypropylene, even if it has an inhibitor.
Q: I would like to ask you a question about my next step with the earthbag retaining walls I have built. Winter is closing in on us, and my builders and I are wondering if we can berm the earth behind these walls without any concrete or stucco on them?
A: As far as the earthbags are concerned, the most important thing is to get them protected from the sun. There is no problem with berming the soil right up against the bags, unless you need a moisture barrier there.
Q: Do you have any suggestions as to what can be used to plaster the interior? As it is working against gravity, I am stumped as to what to use and how to keep it from falling off.
A: Adobe soil has been used, that is soil that is mostly sand but with 15-30% clay to bind it together. Regular cement stucco has also been used. We had the same situation with our domes, and I used polypropylene baling twine wrapped around the earthbags as I built the structure (circling 3 bags at a time) to give the interior plaster something positive to grab onto. Also I used a very light mix of pure papercrete (no sand added) on the initial coat. This I threw up on the wall by hand and let it splat and dry in place with a very rough texture. It is amazing that the stuff would actually adhere to horizontal surfaces and dry in place. Once it has dried, more can be added to fill in all of the voids and make as smooth a wall as desired.