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: 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 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: Do you have any other 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.

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 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: 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: 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.

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: 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: 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: 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: 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 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: 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: 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: 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: I just started building a water catchment tank this week end. Do you have any suggestion on coating the inside with something so no sharp edges will cut the liner of the catchment? I was thinking to just do cement stucco on the whole thing.

A: A cement stucco would certainly do the job, but it would ideally have some wire mesh to keep it there while it cures and to hold it all together. The process of assembling such a mesh could puncture the liner as well. There is also the question of protecting the liner from the UV. One approach would be to use two layers of plastic to assure that you have some protection if the first one is cut or degrades. On my dam and pond I covered the bottom and sloping sides with drain rock and dry stacked larger stones up the steeper sides, then cemented in flat stones around the rim...and this worked well. You could do something similar with bricks.

Q: We have two eartrhbag buttresses that are partially exposed (jutting out beyond roof extension) getting consistently rained on about 6 months of the year. The plaster is crumbling slightly at about 2' up from the ground. Doesn't seem to be a structural issue- more aesthetic- but we worry about it over time. Can we do a larger ratio mix of concrete to waterproof perhaps the bottom half of the buttress? Would that leave enough of the wall open to breathability?

This area of the house is obviously more vulnerable to this kind of erosion; the bottom couple of feet on any wall gets more beat-up due to back-splash from heavy rains. It may well be that a higher ratio of cement to stabilize the plaster would help with the maintenance issue. I doubt that this would adversely affect the overall breathability of the house.

 For the top layer of plaster-paint I think we did a 10% cement 90% cal  (limestone) mix with about 10 to 1 water to latex plus powdered pigment originally. Also, how thick should the plaster coat be ideally in this situation to protect the bags? And then how thick should the limestone paint be on top of that?

I would suggest at least an inch of plaster over the bags for protection. I believe that cob buildings in Europe often have an exterior lime coating to protect the cob, and reduce the frequency of maintenance. This would be a thicker coating than the limewash that you are applying. I applied a lime putty to the interior of my earthbag house (you can see some of the interior pictures towards the bottom of http://www.earthbagbuilding.com/projects/hart.htm ). This final plaster was about 1/4 inch thick and was made with lime putty, white sand and a bit of white Portland cement (to make it set up faster). Lime plaster has the advantage of being breathable (and also reabsorbing the CO2 that is released in its manufacture).

We are considering ways we might extend the roof out further also...

This would obviously help with your erosion problem.

We also have to re-plaster the interior and exterior walls about once a year or two, as we get cracks in the earth plaster and then the limestone paint starts chipping... Is that normal ? Is this an indicator of too much expansion then contraction in the walls due to too much moisture?

Earthen plasters traditionally need periodic maintenance. It might be that the humidity variations are contributing to the expansion and contraction of your plaster. It is possible that the original plaster has too much clay content, which would lead to this problem. It is hard to say what the best solution is at this point, other than the periodic repairs that you are making.

Q: I was wondering if you had any ideas about how to recover the outside papercrete on our dome to keep it breathable and get it to stick? 

A: I think that probably the best bet would be to apply a coat of cement-based stucco as an finish coat. I think that if no moisture barrier is placed over the papercrete this will breath well enough. To make it stay there and not fragment over time, it will probably be necessary to embed some stucco mesh (either metal or synthetic) in it. This whole job could be done by the Macias stucco crew in a few days probably. I like this solution because it would probably solve the problem for a very long time, and keep the place looking nice. You could choose a suitable color coat.

Another possibility would be to put a final coat of lime plaster over the papercrete; this would definitely be breathable, but would likely require more maintenance over time.

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: 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: 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: 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: 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.

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: 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: 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: 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: 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.