Moisture Proofing Earthbag Buildings
Questions answered by Kelly Hart

Q: I am interested in building an earthbag home and would like to berm it more than you have because, well, this is Texas and we are very hot and humid in my location. How did you battle moisture from berming the home against the earthbags?

A: I used a double layer of 6 mil polyethylene plastic where ever I intended to berm the structure. As I was building up the courses of earthbags, I simply tucked the plastic between the courses a couple of bags higher than where I knew the soil would come to, and let it dangle downward until I could do the back filling.

Q: I'm wondering if the earthbag technique, or a form of it, has been tried in the Pacific Northwest, where wet winters are the norm.

A: I am not aware of any earthbag structures built in the Pacific Northwest, but there likely are some. While it is true that most earthbag buildings have been built in more arid locations, there are some in wetter climates, such as Thailand, Hawaii, Mongolia, Belize, Honduras, Haiti and the Philippines. I might suggest trying a more conventional vertical walled structure with a roof that has good eaves to shed water away from the walls, rather than a dome building that takes the full brunt of the storms on the earthbag wall. There have been some successful earthbag domes built in wet climates though.

Q: What is the best way to waterproof an earthbag home in a hot, humid climate (we get about 40" of rain here annually and relative humidity is rarely below 80%)?

A: The best way to finish an earthbag wall in a hot humid climate is, first of all, keep as much water off the wall as possible, with a good roof with an eave. Next, do NOT use a moisture barrier; leave the wall as breathable as you can. And use a natural earthen plaster, especially on the inside.  It has been proven that earthen blocks (adobe or CEB) handle hot humid conditions better than any other type of wall system. The clay in the earth will actually help absorb and release humidity while keeping the interior temperature much more stable and moderate.

Q: I read your Q & A page and became quite concerned about waterproofing and domes...I'm concerned when you say that my house is likely to leak?! This is not really desirable, should I redesign all the roofs? This would be a great shame, as I specifically chose the dome for its aesthetic qualities.

A: Just because domes may be hard to waterproof doesn't mean that you can't do it successfully; you just need to be careful about what you do. I don't think that you can really have a breathable dome that is also waterproof in your climate. You are going to have to have a waterproof plaster on the outside, and stucco is probably your best option for this. It should be fairly rich in Portland cement and have a mesh embedded to keep it from chipping away eventually. Over this stucco I would recommend a coat or two of latex or acrylic waterproof paint that is designed for roofs. This is because the stucco will probably have some cracks here and there and this will keep water from seeping in.

On the inside, I suggest using a breathable plaster, like a natural earthen plaster (clay and sand) or a lime plaster. Then if moisture does happen to get through your outside plaster, at least the bags can eventually dry out from the inside.

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: I built most of an earthbag dome in MA and I am trying to figure out an ingenious way to cover the bags with something permanent before winter really hits. I have considered concrete but I am worried about having a moisture problem inside after, and losing the breathing of the earthen walls. The dome's a bit underground but it has 3-4 ft of gravel under it, a peripheral drain, and another drain uphill from it. I started to cob plaster the outside but didn't finish and don't have time to now. I hope to be able to get eaves up on it to protect the exterior walls from precipitation in the next two weeks. I may not be here in the spring to work on it and i don't want all of my work to crumble.. some of the bags are losing integrity from being in the sun. I wrapped some Tyvek around it to shield it a bit. I may do stonework over the whole thing at some point. Any ideas for a strong and quick solution?

A: Well, you have hit on what I think is the Achilles heal of earthbag domes: the lack of good, natural options for a breathable, but waterproof covering. Most solutions for waterproofing will also render it not breathable. And nothing is particularly quick and easy....but you better do something soon or the bags will completely disintegrate.

My general solution for a waterproof plaster on a dome is a concrete-based plaster covered by a coat or two of roof paint. The stucco will likely develop some fine cracks, but the paint should take care of that. An elaborate solution that would leave the dome breathable is a completely separate roof system that covers the dome, allowing air circulation between the bags and the second roof.

Q: I have a question for you about earth bag building on a slope. We live on the small caribbean island of Grenada and are building on the site of my husband's grandmother's house. We have cleared the site of the rubble from the original house that was almost completely destroyed during Hurricane Ivan. Though the land is almost level where we propose to build, we have had to dig into the slope to open the area where the back of the house will be situated. This means that the floor level will be below ground in the back, but above ground in the front. We are concerned about drainage and are planning a rock & gravel foundation including a French drain, will this be sufficient? Our soil is clay base but drains quickly. Is it a problem to butt the back wall of the house up against the embankment? Would it be better/stronger to bring the back wall 2/3 feet away from the embankment? We are planning to construct several small earth bag retaining walls up the mountainside behind the house site, but would like to do that after the house is complete.

A: A rubble trench foundation with a French drain should keep most moisture out of the building. I think that you could berm up against the earthbags in that back wall safely, as long as the wall is sufficiently buttressed to withstand the pressure. I also suggest that you place a plastic curtain on the outside of the bags that will have the dirt up against, as insurance that moisture will not penetrate. This is basically what I did with the house that I built, and it worked out just fine.

Q: Is it possible to plaster the bags with just sand, water, and lime? Yes, I would do multiple layers. And then paint it for water and weather protection? In regards to the exterior surface of the building.

A: The best exterior plaster for earthbag domes in a damp climate is debatable. It sounds like where you will build in Hawaii is on the margin in terms of appropriate climate for such building. It is really hard to assure that domes will not leak over time in a damp climate. Lime plasters do OK when protected by eaves on a vertical wall, but I would not expect them to hold up as well over a dome. The advantage of lime is that it makes a breathable plaster, but you don't want this. Will paint adhere to it? Maybe, but if you ever have a leak and the lime plaster absorbs some moisture, chances are the bond between the paint and the lime will loosen, and then you are in trouble. I have used roofing paint over cement stuccoed domes before, and this has worked...but it must be well maintained.

Q: This is the dome of a friend in Colombia and is located in a rainy climate. The soil is stabilized with hydraulic lime, it has a double wall and wire. The foundation is 60 centimeters deep and is filled with cement and stone; above it is a concrete plate of 6 cm. The tubes of superadobe may have stayed moist longer than necessary due to rain during the construction process despite being covered with plastic. It has lime plaster on the outside that has not worked. It feels moist inside and you see many cracks inside, and blown, cracked plaster outside. Apart from a purely visual "diagnosis", we wonder how to help the owner learn how to waterproof the outside and inside.

A: I feel so sad when I see something like this. The dome house is really quite beautiful and it is such a shame that it has these major problems. We don't advise people to build earthbag domes in wet climates for this very reason. Short of building a separate roof over the structure, it is next to impossible to guarantee that moisture won't find its way through the plaster to the inside and cause the sorts of problems encountered with this dome.

The person who posted the images on Facebook is mostly focusing on failures due to fundamental stresses within the design, but I think the main problem is the lack of having a waterproof skin, which allows all of these other problems to occur.

My recommendation to save the building is to make a permanent separate roof over the entire structure, with eaves to keep the rain off the lower parts of the walls as well. There is a diagram of one such roof shown at the beginning of this project. First build the roof, then wait until the entire structure has a chance to dry out completely, then repair and replaster the walls where necessary. I hope they can save the building; so much work and love into building it obviously.

Q: We are building an earth bermed earthbag one room cabin. The back wall is pretty straight, and it's about 30 feet long. The side walls will be partially bermed in. We have dug and filled the foundation trench, and laid three layers of bags for the "stem wall". But we have not inserted any moisture barrier. I have read that you need a moisture barrier and I'm wondering, if we wait to complete the wall, and then drape it over the bags, and infill it, what happens to moisture that collects at the bottom of the wall where it's just "hanging" and not tucked in in some way? Should there be a barrier sort of tucked under the bags somewhat or anything? Or can you just do that after the whole wall is up?

A: I have done it both ways, placing the liner after the wall is built and then backfilling, and starting out with the liner in place, secured at the bottom. I like the second option better, because then you can backfill as the wall rises and, if the plastic is big enough, you can drape it over the wall when you are not working to protect the bags from sunlight.

Q: In your Q and A's, you answer questions about waterproofing the walls that are to be bermed with 6 mill plastic, but what do you recommend to control condensation and levels of humidity in the interior with hot, humid air hitting a cold wall of earth? Are there any earth bermed houses in hot, humid climates that don't need air conditioners, dehumidification, or mechanical ventilation?

A: Scientific studies have shown that earthen wall materials provide one of the best ways of mitigating humidity in a house. They can absorb and release a great deal of humidity without ill effect. So if there is a thick earthen plaster that is not sealed and can breathe this would be best. And if there are earthbags filled with earth behind the plaster, this would give another level of protection. This article gives some good information about building in hot, humid climates.

Q: Is it necessary to lay down a moisture/vapor barrier before we start the walls for a recessed building?

A: Here is one idea that I recently did: I made an underground pantry with earthbags for walls, but before I started to back fill the first few courses of bags, I laid a large sheet of black 6 mil polyethylene against the outside of the wall. The dirt held it in place very securely, and I could either lay the plastic back and away while I was working on the wall, or I could pull it over the top of the bags and down inside to protect the wall from exposure to UV when I wasn't working on it. This worked very well.

Q: I have one concern about building bermed or underground: mold. Will earthbag walls be able to breath if they are bermed in a humid climate? I know we would need a heavy plastic water barrier between the wall and the berm, but then will the wall be able to breath without making the inside more humid and unbearable?

A: I have never known of an earthbag structure to support mold. Earthen systems like earthbags are unequaled in their ability to safely absorb humidity without harm. I would leave the interior breathable with an earthen plaster over the bags and you would be fine. Also, if you do go this way, it is good to provide a bit of insulation between the bags and the berm so the ground doesn't suck heat from the house.

Q: We have pretty much decided earthbags won't work here. It is very humid (but cool) and has a very heavy rain season. Everything I read says earthbag isn't good for rain.

A: A rainy climate can be an issue with earthbag domes, but that is a special case. Otherwise, earthbags are well suited to a rainy, humid climate, as long as you have a good roof overhead. In fact earthen materials are better able to handle humidity that most other building materials, as been proven by tests. The problem with domes is that it is hard to assure that the domed roof remains sealed from rain over time, and this introduces the risk of moisture being trapped in the bags, softening the soil, and leading to possible collapse. So If you want to build earthbag domes in rainy climates, it is best to use cement-stabilized fill material, so at least the risk of collapse is minimized.


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