Questions about the Earthbag/Papercrete Home
Questions answered by Kelly Hart

Q: I was wondering, why you choose earthbags over the other choices you have obviously researched.

A: At the time that I was designing my house, I also was exposed to a variety of interesting alternative approaches to building, and decided to experiment with some of them. I was intrigued with earthbags for several reasons: they can be filled with natural, earthen materials; they can be flexibly arranged into a wide range of shapes; if protected from the sun, the bags and their contents will not rot over time; they can be fashioned into domes, so that other tensile roof materials are not needed; they are inexpensive; they are simple to use without much experience.

Q: Could you briefly outline the main construction challenges of building your earthbag house?

A: The earthbag/papercrete home that I built was definitely experimental in nature. I had heard of the use of sandbags for building structures, particularly through the work of Nader Khalili in California, but all of his work was focused on using adobe soil, which solidifies into a solid block, so that ultimately the bag itself is not needed structurally. I wanted to fill the bags with an insulating material, because at over 8,000 ft elevation in the mountains of Colorado, the house would be comfortable. I chose scoria, a local light-weight volcanic rock as a filler material...but this had never been done before. With this loose material, the bag itself would become part of the structural fabric.

Another challenge was that my design called for a rather large (30' X 20') elliptical dome as part of the structure. This is larger than anything attempted before with earthbags, as far as I know. Would this be possible? I didn't know. At first I tried making this as a free-standing stack of bags...and it started to deform by the time I nearly had it completed. So I had to dismantle much of it and start over with a pole-frame internal support structure.

Another challenge was creating the arched span over the 6' doorways with earthbags...again something that had never been done before. Also I spanned the six-foot vaulted entry to the house with earthbags...not easily done.

Then there was the challenge of designing the two-story earthbag connecting space between the two primary domes with the entire earthbag portion both resting on the existing domes and on wood-framed supports on one side.

A whole other unknown was how papercrete would function as an external and internal plaster over the bags. This material is virtually untested in this application. Just making the mixer for the papercrete was a big job.

Of course another major challenge was that I accomplished virtually all of this by myself, without the benefit of a crew for assistance. It took me about three years to build this house.

Q: Are you happy with how your earthbag/papercrete house is working out?

A: I am quite satisfied with how our house is working. We have had no water problems at all. The papercrete has been holding up pretty well, but after about five years is showing signs of erosion in areas that are hard hit by storms. I would basically build the house the same way if I were to start over again today, although I might put a finish coat of harder stucco over the papercrete. Another change I would make is to make the larger dome circular instead of oblong, because the circular shape is much more stable and I wouldn't have to deal with the unequal forces that the oblong shape creates. I am especially pleased with the thermal efficiency of the house. It is comfortable both winter and summer. I'm sure this is because I filled the bags with the crushed volcanic rock that is available in our area.

Q: How do you heat your house?

A: The house is designed for passive solar heating, and that works pretty well when we have sun, but on extremely cold days, or during overcast weather, we have both a wood stove and a couple of small propane direct-vented heaters in the bedroom and in one of the offices. We also have solar hot water in the hot tub and preheating the domestic hot water, and this heat can also be used for supplementary heat.

Q: Is it practical for a man nudging 60 to attempt this project on his own? One advantage - unlimited time (I hope!)

A: Hey, I'm nudging 60 and I did it!

Q: I am interested in building an earthbag home. Are you aware of any builders who can construct a home in Florida? My husband and I saw your home on HGTV, but realize that it took you 3 years of working full time on your home. We would not be able to make that kind of time investment. If you have know of any builders to refer us to I would appreciate it.

A: It did take me that long to build my home, but I did 95% of it by myself, which most people wouldn't do. Earthbag building could proceed nearly as fast as other methods of building, if a crew is organized and experienced. Unfortunately finding such a crew would likely be difficult, because building this way is not very well known. The best you could expect is to find a local builder who is willing to help you and willing to learn how to build this way himself...or to train the crew yourself. It is not difficult to learn how to do, but does require some special knowledge.

Q: I have been considering taking a class at CalEarth on superadobe, but the price is quite hefty for someone my age (24). I have been checking out your site and thought I'd ask if you think it necessary to take this class or if it is also quite possible to build a sturdy earthbag home through book and internet research and experimentation?

A: I built my earthbag home without the benefit of one of the CalEarth workshops (partly for the same reason that you mention), but I am pretty comfortable with experimenting with various construction technologies and have a lot of experience in doing this.

Q: I am interested in combining homes of super adobe, bags, earth and papercrete. What kind of costs for about 800 sq ft?

A: Earthbag building can be one of the least expensive ways to go, since the materials themselves are fairly inexpensive. It is all of the other things that most structures need that add up to more money, so the walls are just part of the cost. I built my earthbag/papercrete house for $16/sf, about ten years ago...but I did almost all of the work myself, and I am very good at scrounging for supplies...and it took me three years to do so.

Q: How has the papercrete plaster on your earthbag house stood the test of time?

A: Overall, the structure is as sound as the day I finished it. I'm sure there were also a few cracks in the PC plaster that developed over time, but I never worried much about them because of the nature of the wall system employed.

As you know, the papercrete was used a plaster over the earthbags, both inside and out. After about a decade of exposure to the elements, the exterior PC plaster was showing signs of erosion in certain places, most notably at the base of the embedded glass where moisture would accumulate and on the more horizontal aspects, such as over the door eyebrows and the top of the vaulted entry way.

The folks who bought the house were concerned about this, and so was I since I expect that house to last for centuries. We discussed how to protect the plaster generally, and at my suggestion they went to considerable expense to have a professional stucco crew do a stucco job over the entire house, complete with more wire mesh embedded in it.

Papercrete makes an excellent substrate for almost any further plaster work, since it is so amazingly dimensionally stable under most any condition. The lime plaster I troweled over the interior PC has held up very well, and I expect the exterior stucco to do the same.

This does mean that the structure is not quite a breathable as it was with just the PC plaster, but I think it is sufficiently breathable, especially since it can still breath quite well inside.

I was always amazed at how well the combination of earthbags filled with scoria and plastered with PC performed as a skin for the house. At least in this rather arid environment there was never a drop of moisture that came through that package. I think that the PC acted like a sponge and just held the moisture at that level, allowing it to soon evaporate rather quickly. It was never damp long enough to develop any tendency to support mold.

I would never recommend that folks use PC as a solid material for a roof, either on a dome like mine, or on more conventional roofs. All of the major problems I know about (and there have been several of them in the Crestone area) where mold has taken hold, have been either in situations where the PC formed the roof or was in contact with the earth near the foundation. In several cases attempts were made to seal the roof with tar or Elastomeric paint or some such impermeable barrier, and these always failed eventually. Moisture would find a way to enter through small cracks and then get trapped into the absorbent PC. The moisture barrier would then effectively prevent the PC from drying out and in the warm interior environment mold is the natural result. One person saved his house by creating an elaborate tent-like structure over the entire house, with an air space between the two so the PC could dry.

I do know of a few instances of the use of PC as a plaster over strawbales, and this has also worked pretty well. The PC is naturally more absorbent than the straw, so it tends to wick any moisture away from the straw and actually help with the molding potential...at least that is my take on it.

Q: If I were to use scoria or another insulating material (have been looking into ways to use perlite, for example), would this material be piled against the exposed bag? How do you keep the material in this envelope while backfilling? It seems there might be some challenge to keeping the material at a consistent thickness to the height of where the plastic sheeting comes out of the wall while you are backfilling. Any advice on this process would be illuminating.

A: As I recall, when I did this I did the backfilling in stages. At the point when the bag wall was up at about the height where the plastic apron would be tucked between the bags, I laid the plastic out over the berm material so that the space between the plastic and the wall could be filled with scoria. Then I added more berm soil and scoria (maybe a foot) and tamped it in place. I continued this process until the entire area was filled and I could lay the plastic over the bag wall and continue going up.


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