How Earthbag Homes Work

Earthbag homes are inexpensive, with many builders able to use the soil that's on-site to build the home. Bags are usually low-priced in bulk, particularly misprinted bags that companies sell at reduced rates. But we'll talk more about materials in earthbag construction in the next section. While earthbag structures are labor-intensive, they are relatively easy to build with little experience and a crew of only a few people, cutting down on construction and contracting costs.

Earthbags could be used as infill for a conventionally framed house, but typically they're more monolithic structures, meaning that the entire structure is made from the same materials throughout. Other than the bags and the barbed wire that holds the bags together, earthbag building is a natural building method that eliminates the use of limited resources. For example, an earthbag dome building can eliminate 95 percent of lumber that is currently used to build a stick frame house [source: Hunter and Kiffmeyer].

Earthbag homes also appear to be structurally sound and safe. Architect Nader Khalili conducted structural integrity tests under the supervision of the International Conference of Building Officials. He tested earthbag domes under conditions simulating seismic, wind and snow loads, and the tests surpassed the 1991 Uniform Building Code's rigorous requirements by 200 percent [source: Hunter, Kiffmeyer]. Anecdotal accounts tell of the structures surviving fires, floods and hurricanes. Properly plastering the walls will keep out mold, insects and rodents.

Earthbag walls also exhibit high levels of thermal mass, which is the measure of a material's ability to absorb, store and transfer heat. Earthbag walls that are greater than 12 inches thick (0.3 m) exhibit the thermal flywheel effect. At the hottest time of the day, the walls will absorb the heat, but they don't release it into the structure until the walls start cooling at night. This creates about a 12-hour-delay between the time in which the walls soak in the heat and when they release it. For this reason, earthbag homes work best in locales where there is a significant difference between daytime and nighttime temperatures.

Once an earthbag home is complete, you would never guess that the walls are made of dirt. The walls are strong enough to hang cabinets and pictures, and plumbing and electricity are installed just as in a conventional home. Lofts can provide extra space. Earthbag builders Kaki Hunter and Donald Kiffmeyer speak about the warmth and "spiritual" benefits of living within earthen walls, but others have pointed out that for some, earthbag walls are too heavy and "organic"-looking [source: Kennedy]. We'll talk about other potential disadvantages of earthbag construction later.

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On the next page, we'll take a closer look at the main materials of earthbag homes.

As you might guess from the name, choosing the earth and the bags are important steps in earthbag construction. Essentially, any type of soil can be used, but knowing the makeup of soil can help you to achieve the best mix.

Soil is made up of clay, silt, sand and gravel. Silt is extremely fine-grained, and using too much in an earthbag structure will weaken it. Gravel, or jagged pieces of rock, is sometimes used in earthbags, mostly at a foundational level, but builders primarily use a mix of clay and sand. Clay serves as the glue to hold sand together, while the loose, gritty particles of sand form the bulk of an earth wall's stability. Coarse, jagged sands are best because there are lots of sides for the other grains to adhere to. Earthbag builders Kaki Hunter and Donald Kiffmeyer recommend a ratio of 70 percent sand to 30 percent clay.

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­N­o matter the mixture, topsoil should not be used. Grasses, twigs and the miscellaneous debris found in topsoil will eventually decompose, leaving cavities in the bag and undermining the structural integrity of the building. Once these are cleared from the soil, most clean soil can be used. It's also possible to import soil from a local gravel yard, where reject materials from gravel usually have an appropriate sand-to-clay ratio.

Playing around with moisture content will also affect the composition of the soil, with moistened material creating a more stable structure because it presses everything together. A measure of about 10 percent moisture will work well [source: Hunter and Kiffmeyer]. How can you tell the right ratio? Soil with about 10 percent moisture will form a ball in your hand, but it will shatter if dropped.

The bags are typically 50-pound (23-kg) or 100-pound (45-kg) sacks of polypropylene or burlap. Burlap isn't as durable as polypropylene, but it is a more natural material. Generally, the weaker the fill material, the stronger the bag should be. It's also possible to use long sandbag tubing, such as the superadobe tubes developed by Nader Khalili. The lack of seams in the wall could possibly lead to greater wall stability, but some builders find these harder to work with, as they are cumbersome and sometimes roll after they've been placed on the wall.

Several other materials are necessary to build an earthbag home:

Enough talk about earthbag homes. Let's build one.

Earthbags can be used as infill for a more conventionally framed home, but for this section, we'll look at how to build an earthbag dome.

Rubble trench foundations, meaning a trench filled with rocks, gravel or broken concrete, are commonly used with earthbag homes. The first layer of bags can either be placed at ground level or slightly below ground level, in the trench.

Bags should be filled on-site, right before they are placed. Earthbags can be filled in several ways. For example, some builders have constructed bag stands that hold a big bag upright so that a person can shovel soil into it. As the walls get higher, bags could be lifted in place partially filled and then finished by handing up cans of dirt.

As bags are filled, they can be sewed closed with twine, but this isn't required. An alternative would be to place the open end of the bag, folded in with neat corners, against the sealed end of the adjoining bag. Tightly placing the bags keeps them closed and ensures the wall's structural strength. Bags should be placed so that the places where the bags on the previous row meet are covered by the bags on the subsequent row, just like brickwork.

After a row of bags has been laid, the tamper presses the bags down into place. This keeps the earth from shifting and keeps each layer level. This compression is also what forces the dirt inside an earthbag to become a solid, self-supporting form, so that if the bag were somehow removed, the dirt wall would still stand. Between each layer of bags, place one or two strands of barbed wire along the top of the row to hold the next row of bags in place. Even if the barbed wire pokes a hole in the bag, the tamping of the dirt ensures that the wall remains solid; large holes can be patched with duct tape.

To add windows and doors to the structure, use wooden forms to mark the place where the window or door will be, and then place earthbags around it. It may be necessary to create custom bags by using varying amounts of dirt. For example, fan bags are used to put arches over the windows or doors.

The bags are gradually stepped in to build the dome. A simple homemade compass or tripod guide can aid in positioning the bags of the dome. One such compass could consist of a pole placed in the center of the floor. At the top of the pole, an L-shaped piece of material can be attached with an adjustable clamp. Rotating the pole and adjusting the metal inward will help evenly place bags. As a general rule, earthbag builders Kaki Hunter and Donald Kiffmeyer recommend stepping in a maximum of one-fourth the width of the previous bag to maintain structural integrity. In other words, a builder working with 12-inch (0.3-m) bags would step the next layer in about 3 inches (7.6 cm), so that the other 9 inches (22.9 cm) are still resting on the previous layer.

When complete, the structure should be plastered as soon as possible to avoid prolonged exposure to the sun. Mud plasters are most commonly used on earthbag structures, but lime and cement plasters may also be used if you place a mesh over the bags to hold it in place. A roof can be finished with coverings such as shingles or tiles if desired.

Before you start piling earthbags pell-mell in your backyard, learn the challenges of earthbag construction next.

Most of the challenges of building with earthbags have to do with the hoops you have to jump through before construction starts. Earthbag construction, because it is not widely known or used, poses some difficulties in dealing with building officials, banks and insurers.

Despite the structural testing conducted on earthbag homes, there is no mention of them in building codes, except in the city of Hesperia and in San Bernardino County in California, thanks to architect Nader Khalili's work with local building officials.

Outside of Hesperia, it will take a lot more work on the part of the person who wants an earthbag home. Earthbag construction has been primarily concentrated in Colorado, New Mexico and California [source: Barnes, Kang, Cao]. Many officials outside these areas might be unfamiliar with earthbag construction, requiring the loan applicant to provide the research. Many banks just aren't ready to take a chance on alternative construction, and dome structures are rarely financed by banks. Working with a smaller, independent bank that is more familiar with the area might yield better results than working with a bigger bank.

Still, even if a bank is interested, there might be other difficulties. Because earthbag housing is still rare, it will be difficult in many areas to find a comparable home, which is often the key to selling an unusual building method to a bank. A bank will use appraised values of comparable homes to set a base value for the loan applicant's home. If there are comparable homes, but they haven't been on the market in the past six to nine months, then they can't be used as comparable values. If an appraiser can't determine the value of the home, then banks can't give the money to build or buy it.

In terms of actually constructing the house, there are some limits on design. To remain structurally sound, earthbag homes are relatively small. For a domed earthbag home, 20 feet (6 m) is generally the recommended maximum diameter of the building [source: Hunter, Kiffmeyer]. Bigger structures can be accomplished by building a series of interconnected domes, or by extending underground.

To learn more about earthbag homes, see the links on the next page.