Protecting Earthbags UV Exposure
Questions answered by Kelly Hart

Comment: I started building my house 2 months ago, and we did have to take a small break of 3 weeks for money problems. When we came back we found out all the bags have been destroyed by the sun! For only 2 months this is weird!  I share this with you so you can put this on your site for other people to see what the sun can do! We have to start all over again!

Reply: This is a perfect example of what the sun can do to the polypropylene bags in just a few short weeks! Obviously, to avoid this it is essential to keep the bags covered at all times, except when you are actually working on a wall section. And they should be plastered as soon as possible.

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: I am wondering could you use sod instead of concrete to protect earth bags from the UV rays? Just make an earth sheltered raw earth bag home with chicken wire or something to hold it in place, along with a dome shape and wooden "damns" to prevent the sod from falling off?

A: Yes, if you can assure that the sod will stay in place over time, then I'm sure this would protect the poly bag material from UV deodorization. Kaki and Doni tried this with their first dome, and eventually gave up because of difficulty keeping the sod intact on the dome.

Q: What are the pro's and cons of lime render vs cement plaster?

A: Lime render is much more benign environmentally since it reabsorbs the CO2 given off in its manufacture over time. It also is more breathable, a definite plus for most wall systems. Cement based stucco is more durable and seldom needs retouching.

Q: Painting bags to protect from the sun makes so much sense rather than having to deal with tarps and plastic and 8 men. Can we use a cheap latex paint? dark colour?

A: I would use the cheapest latex paint you can find, since it is only a temporary covering until the plaster goes on. The color doesn't matter. Sometimes you can find old or discarded paint that can be mixed together for this job.

Q: How much sun is too much? We put the bags up in October and covered the site with a tarp. At some point the weather exposed some of the bags, I would say the bags have seen as much as 4 months of a Vermont winter sun. We'll stucco the bags in the next 2 weeks.

A: You can tell if the bags have deteriorated too much by poking them with your finger. If you can tear the fabric at all this way, then it is usually too far gone to be very dependable. Obviously, it depends on how extensive the damage might be as to how serious it is. A spot or two of this softness won't affect things much, but a large area is not good. I have seen bags rot from the sun within a month of very intense Southwest sunlight. Even with a larger area of damage, you might be able to save the situation by wrapping them with chicken wire or other mesh reinforcement material before you apply the plaster. This is not a bad idea regardless of the state of the bags, since it will make your stucco more permanent.

Q: I live in Ghana, West Africa. About a month ago, a crew from the US came and built an earthbag roundhouse near our home. The building was left unplastered and uncovered from then until now. I went to the location today and saw the state of the structure. There is a large amount of deterioration in all of the south-facing bags. I contacted the team and offered my assistance if the structure can be saved; I would like to know what you deem the most effective way to save the house from further damage. I took a mason to see the site and start plastering day after tomorrow, but as the bags are filled with pure sand, it seems there will have to be more extensive work done to reinforce the damaged bags. What do you suggest?

A: It always makes me sad when I hear stories like this. We try to emphasize how important it is to keep the bags covered from the sunlight, but obviously some people don't heed the warning. If the fill was more stable than the sand, you might be able to save the walls with mesh and plaster alone, but with pure sand it is more problematic. There are adhesives that can be used to glue more good fabric to the damaged places, but if the entire south-facing wall was left for a month in the direct sun, then all of the fabric is probably compromised and fragile. It may be necessary to dismantle the wall and start over again!

I was thinking that if we found some heavyish material or paper and wrapped that wall and covered that with chicken wire, it might hold.

Well, what you propose might work, but it would be uncertain over time. Paper would not be strong enough to stay intact. If the builders are willing to live with the uncertainty, it might be worth it. If you were able to fashion some long wire staples out of heavy wire and use these to securely attach some wire mesh to the wall, then the plaster would be more likely to stay intact and protect the wall.

Q: I am building some terraced hills using earthbags in the backyard. I will be driving long nails on the foundation bags to fix them to the ground and then stack bags on top. I was wondering if there is a uv resistant bag I could use? Would burlap hold up? I know I could use poly bags but I'm not sure if the stucco render I do overtop will hold up to the weather in NJ. Any recommendations? I was even thinking of spraying plastidip on the poly bags.

A: They do make UV resistant bags, but these really just postpone the process of degradation; they don't completely arrest it. Burlap does resist UV much better than polypropylene, but they succumb to moisture readily and rot, so they are not generally a better option. The only permanent solution is a good plaster done right away, and ordinary cement stucco is probably the most permanent of all...even in NJ. Using a good stucco mesh will help keep it all intact.

Q: Mike and I started a scoria bag dome about 8 months ago. We got about 7' vertical height done on the 14' inner diameter building before we ran out of decent weather. We tarped very well, but now realize that some of the tarps were "foil" backed and some were not, and this may have contributed to the bag problem. We have noticed that some of the bags have veritably turned to dust under gentle pressure. We replaced 8 buttress bags whose corners had disintegrated, applied gorilla tape to any spots that seemed weak (there were quite a few - some bags are now all tape on the parts that show).

We know we need to stop everything now and plaster. My concern is that we are going to try and plaster over an unstable substrate. If we use too much pressure when applying the plaster, the bags may collapse. Do you have any suggestions?

A: I can empathize with your dilemma; it is a shame that the bags have disintegrated to that extent. They must have been exposed to a fair amount of UV at some point... I don't know of anything else that will do that. Obviously bags of scoria rely much more heavily on the strength of the poly bag to maintain their firmness.

In order to save your work you clearly need to create a firm skin on both sides of the wall. I would consider a stucco with some mesh embedded in it on both sides. Plastic mesh is said to last longer than metal as it won't rust over time. In order to fix it to the wall I suggest coming up with a way poke to wire or poly twine directly through from one side to the other. Then you can tie it on one side and pull it taught and connect it to the other side. Some sort of large needle-like device that can be pushed through with the wire attached to it. Strawbale builders often use such a long needle to custom form bales. I hope that you can salvage your work somehow.

Q: I would like to ask about painting and breathing. The rains have started, but for curing the fill material (mix of clay and sand) maybe we will have to wait till rainy season is finished. We painted all the courses with thin paint to protect bags from sun mainly. Our construction is slow, and maybe we will have to do this again, though now we have less sun than 1 month to go. But will bags dry under paint coating?

A: Painting the bags will certainly slow down the curing, which is why tarps are often better. I have never done this, so I don't know whether it will stop the fill from drying completely.

Q: I am building my house with roundwood timber framing. I was planning on using cordwood but really like earthbags. I live in the PNW of the US and have lots of wood but no clay on my land. I am not a big fan of working with cob anyway (although like the look and sustainability) so I was wondering how covering earthbags with a layer of vapor barrier house wrap then sawmilled timber siding would hold up over time?

A: The main function of plaster is to protect the bags from abrasion, weather and the sun. Covering the walls with wood would take care of all of these, so it seems like a good thing to. I'm not sure you would actually need the house wrap because the wall will naturally breath anyway.

Q: I am considering building a 13’x22’ two story addition onto my house and wanted to know if and how I could install a traditional vinyl siding to the earthbags instead of plastering them. I live in a cold wet area and do not want to have to keep up with maintenance of the plaster. Can I screw furring strips to the bags and then side over that way?

A: You would have to attach furring strips to the bags somehow, perhaps with heavy twine or wire wrapped around bags as the walls are built.


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