By: Tim Motis
Published: 2019-07-24


Patrick Trail, an ECHO Asia staff member, compiled a picture-based guide for constructing earthbag houses for seed storage in Asia. Earthbag houses have been used by multiple seed banks in Asia as an alternative to more costly conventional structures. At this writing, construction of an earthbag house at ECHO in Florida is also underway. Content below is drawn from insights gleaned at both ECHO locations.

Introduction

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Figure 2. Earthbag house at ECHO in Thailand. Source: ECHO Asia Staff

What is an earthbag house?

An earthbag house consists of a roof over walls made of grain bags filled with soil. The bags are stacked in layers along the perimeter of a foundation or pad, with space left for a door. Then the stacked bags are plastered over with mud (Figure 2).

What are some advantages of earthbag houses?

Earthbag structures are made from local, inexpensive materials. An ECHO earthbag house in Thailand was built at a cost of 750 USD (Trail et al. 2019). The simplicity of earthbag houses means that people with little or no construction experience can quickly learn the necessary skills. Earthbag houses are strong, but are also flexible enough to resist earthquakes (Geiger and Zemskova 2016). The walls are durable, non-toxic, and resistant to fire and insects.

Why are earthbag houses good for seed storage? 

In the warm tropics, temperature and humidity tend to be high and can fluctuate widely. Under such conditions, seeds deteriorate due to premature germination, rotting, insect pests, and rapid metabolism of food reserves. 

Seeds maintain viability best when they are kept consistently dry and cool. Vacuum sealing (to exclude humidity) or desiccants (to absorb moisture) can help keep seed moisture low. However, these technologies do not address temperature.

Earthbag houses stabilize storage temperature in comparison to outside air. An ECHO earthbag house in Thailand reduced average maximum air temperature from 44°C (outside) to 28.5°C (inside the earthbag house); minimum temperatures increased slightly from 10°C (outside) to 11.5°C (inside the earthbag house) (Trail et al. 2019). Underground storage is another way to moderate temperatures, but seeds in an aboveground structure are easier to access than buried containers or underground bunkers would be.

How does an earthbag house work? 

Earthen walls have high thermal mass, which means they absorb and transfer heat or cold. In warm climates, the exterior walls absorb heat during the day. With earth walls at least 30 cm thick, it takes about 12 hours for that energy to move to the interior of the structure (Hunter and Kiffmeyer 2004). At night, the walls stop absorbing heat as the outside temperature drops. The walls release heat at night, keeping the inside temperature warmer than that outside. Lightweight materials used for the roof (straw/thatch) and placed between the ceiling and roof (rice hulls or plastic bottles) have low thermal mass, so they do not absorb and transfer heat well. They are, however, good insulators. Insulating materials block transfer of heat to the interior of the building. The properties of the walls, roof and ceiling work together to moderate against extreme temperatures. 

What are the limitations of an earthbag house?

Humidity inside an earthbag house can be high if outdoor humidity is high. Disease-causing fungi proliferate at or above 65% humidity. If the humidity inside an earthbag house is high, keep the seeds in sealed containers. Earthbag houses work best where it cools off at night; they are less effective if day and night time temperatures do not differ much. In hot climates, an earthbag house will not keep temperatures as low as can be achieved with air-conditioning or refrigeration, but the improvement over ambient conditions will still be significant. 

Site Selection

Select a site based on project needs. For example, choose a place that is accessible to those managing the seed collection. If possible, build the earthbag house near trees or other structures that provide protection from the sun. Avoid low-lying areas that could easily flood. To reduce labor and hauling costs, build where suitable soil and other materials are easily accessed.

Materials and Design

Materials vary with design choices and with what is available locally. Below are a few guidelines to consider for the main structural components of an earthbag house:

Foundation

Pick-axes and shovels are needed for digging a trench around the perimeter; the trench will be filled to form a footer for the walls of the earthbag house. Note that the shape of the foundation determines the shape of the structure. A round design maximizes structural strength and requires the least amount of materials (Toevs 2019). Whatever shape you choose, Geiger and Zemskova (2016) recommend digging the trench 60 cm wide by 60 to 90 cm deep (down to the subsoil). Fill the trench with rubble/gravel, with the largest rocks at the bottom of the trench. 

Options exist for the floor. If you choose a concrete floor, you will need to build a pad with cement, sand, and rebar. The perimeter of the pad—on which the walls are built— rests on the footer, with the remainder of the pad supported by packed earth or gravel. In this case, the foundation consists of a concrete pad and footer. Concrete readily absorbs moisture and dries out more slowly than soil. To prevent the pad from absorbing ground moisture, plastic is needed as a moisture barrier between the ground and cement.

In a dry climate, you might make the floor simply of compacted earth or gravel. In that case, the foundation would consist simply of the footer.

Walls

Walls are made mainly of soil with enough clay to stick together and harden (10% to 30% clay according to Stouter 2011). On page 14 of Earthbag Building in the Humid Tropics, Stouter (2011) explains how to test the soil for the right texture and moisture; for example, if a ball of soil shatters when dropped 1.5 m, the soil needs more clay and water. Too much clay in the mixture is also a problem. A mixture with more than 40% clay could result in unstable walls due to excessive shrinking and swelling (Toevs 2019).

Sift the soil as needed, to remove rocks and debris. At ECHO Florida, we used a mixture of 20% clay with 80% sand. At ECHO Asia in Thailand, we used 60% soil (estimated to have equal parts clay and fine sand) and 40% rice hulls. The rice hulls (and alternative lightweight materials such as volcanic rock) provide insulation, to reduce the amount of heat stored and radiated back inside the building. This is recommended for areas where temperatures stay high day and night. Rice hulls also resist rot and insect attack. Keep in mind that the mixture of soil and any lightweight material needs to be strong enough to bear the weight of the walls and roof. To test the strength of the mix, Stouter (2011) recommends filling an earthbag with moistened fill, tamping it, and then allowing it to dry for 1-2 weeks; a 25 cm span of earthbag should be able to support a 54 kg person.

Bags are another significant component of the walls. Usually, grain bags of various types and sizes are used. Geiger (2019) suggests bags that are roughly 46 cm X 76 cm. The bags must be strong enough to hold the weight and shape of the walls during construction, but their long-term strength is not critical, since the earth inside them will harden and the bags will be covered with plaster. Polypropylene bags are a good option, because they do not tear easily. 

Other materials are needed for the walls: barbed (four point) and non-barbed (16-gauge thickness) wire, wood for a door and doorframe, and plaster. Mud plaster can be made from the same soil that is used to fill the bags. Stouter (2011) discusses plaster options in more detail (see references section). Tools required for wall construction include wire cutters, a tamping tool (this can be made from cement if needed), a leveling device such as a bubble level, and buckets for filling bags with soil. 

Ceiling and roof

Joists and rafters are needed to support the ceiling and roof, respectively. These can be made with bamboo, wood, or metal. In many earthbag houses, ceiling joists rest on a cement beam that is built on the top layer of bags. The ceiling support structure of ECHO’s earthbag houses rests directly on the walls (Figure 3). We suggest plywood for the ceiling. For insulation on top of the ceiling, pile local materials that are unlikely to mold, such as used/empty plastic bottles, or rice hulls (used by ECHO in Thailand; can be bagged to keep them dry). A thatched roof is often made using straw (Figure 3).

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Figure 3. Structural support for the roof and ceiling of an ECHO earthbag house in Thailand (A) and Florida (B). In Thailand (A), a plywood ceiling rests on metal bars welded to rebar pylons (an example of one is indicated by the red arrow) hammered into the top of the earthen wall. In Florida (B), wooden joists rest on plywood (indicated by red arrows) inserted between two bags. Source: ECHO Asia staff (A) and Tim Motis (B).

Construction Steps 

 
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Figure 4. Foundation of earthbag house at ECHO in Asia. Source: ECHO Asia staff

1) Build the foundation and place the doorframe 

A solid, level foundation (Figure 4) supports the entire structure and helps maintain a uniform wall height. As mentioned in the materials and design section, this involves a gravel-filled trench for a footer, with walls resting on the footer or on a concrete pad. 

At ECHO in Florida and Thailand, the foundation consisted of a footer and concrete pad. This approach is useful in locations that receive a lot of moisture during the monsoon rains. For ECHO’s Florida earthbag structure, we laid plastic between the ground and cement pad/floor, and also between the surface of the floor and the first layer of earth bags. The second moisture barrier was added as an extra precaution to make sure that any moisture absorbed by the concrete pad cannot wick up into the bottom layer of earth bags. With the foundation in place, construct the doorframe and place it on the foundation. 

2) Fill bags with soil

Fill the bags about two-thirds full of soil. By not filling the bags completely full, they will be more workable when placed on the foundation. The empty space also means bag material will be left at the top end; this can be folded over or stitched shut with wire, to keep soil from spilling out. 

NOTE: If you are building an earthbag house without a cement pad as part of the foundation, consider placing several layers of double-bagged gravel above the footer. This will protect the bottom of the structure against water erosion that could otherwise destroy earthen walls. Earth-filled bags can then be stacked on top of the gravel-filled bags. 

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Figure 5. Placing an earth-filled bag on earthbag house wall. Source: ECHO Asia staff

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Figure 6. Pole for measuring distance of each bag from the center of a circular earthbag house. Source: Cody Kiefer

3) Stack earth-filled bags on the foundation 

Place bags around the perimeter on top of the foundation, one layer at a time (Figure 5). To maintain a true circle as the walls are built, tie a string or rope to a center pole (Figure 6), cut or mark it to indicate the desired distance from the center to the inside edge of the wall, and place each bag so that the inside edge is always the same distance from the center. For accurate measurements, keep the string level when measuring. For each layer of bags, first put all the bags in place and then tamp them so they are flat and level (Figure 7); the bags will lock into place as they push against each other (Toevs, 2019).

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Figure 7. Tamping bags to form walls of an earthbag house. Source: Cody Kiefer

We placed barbed wire between each layer of bags (Figure 8). The wire provides an interlocking matrix that holds the bags together, both between layers and within layers. We used bricks to hold two strands of barbed wire in place while we stacked the bags for each layer (Figure 8A). At ECHO in Florida, we tied pieces of 16-gauge rebar tie wire to each strand of barbed wire (Figure 8B), at intervals of 35-45 cm. The ends of these non-barbed wires, extending 6-8 cm on either side of the wall, were used to fasten chicken wire/stucco mesh to the interior and exterior walls (see step 3). To keep the bags from snagging on the wire as we were positioning them, we placed a flat, smooth piece of sheet metal over the barbed wire (Figure 8C). Once the bag was in place, we slid the metal out, allowing the bag to be pierced and thereby caught hold of by the barbs.

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Figure 8. Barbed wire (A) and rebar tie wire tied to barbed wire (B) for interlocking earthbag wall components. Photo C shows a piece of sheet metal used to prevent bags from catching on the barbed wire during placement. Source: Cody Kiefer

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Figure 9. Wooden anchor (A), between every third layer of bags (B), to which the doorframe was attached. Source: Cody Kiefer (A) and Tim Motis (B)

The doorframe tends to be held in place by the weight of the bags pressing against the sides. In ECHO’s Florida structure, we placed pieces of wood (Figure 9A) adjacent to the doorframe on each side, between every third layer of bags (Figure 9B); these served as points of attachment for added stability.

Add layers of bags until the desired height is reached. The ECHO earthbag house in Florida is about 2 m tall, which allows most people to be in the structure without having to stoop to keep their head from touching the ceiling. Geiger and Zemskova (2016) state that the ratio of wall height to wall thickness should not be more than 8. The Florida earthbag house, which has not been plastered as of this writing, has a wall thickness of 30.5 cm. With a 200 cm wall height, our height to thickness ratio before plastering is 6.6 (200 divided by 30.5).

4) Construct the roof with an insulated ceiling 

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Figure 10. Bamboo (A) and metal (B) as options for rafters. Note rice hulls placed on top of plywood ceiling for insulation. Source: ECHO Asia staff

Figure 10 shows bamboo (10A) and metal (10B) rafters. Bamboo is likely more available than metal framing or lumber, but the latter two options are more permanent. In Florida’s subtropical climate, untreated bamboo lasts for about two years, whereas bamboo treated to resist decomposition and insect attack lasts as long as conventional lumber (20-30 years) when under the protection of a roof (Toevs, 2019; see Bielema 2017 for information on treating bamboo). Make the rafters long enough for an overhang that will shield most of the exterior wall surface from the sun and rain. Secure the ceiling—using nails, screws, or wire— onto joists made with wooden poles, cut lumber (Figure 3B), bamboo (Figure 10A) or metal (Figure 10B). Place insulation material on top of the ceiling (Figure 10B), and secure thatch to the rafters (Figure 3A).

5) Plaster the walls

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Figure 11. Mixing plaster to cover the walls of an earthbag house. Source: ECHO Asia staff

Once the walls and roof are built, cover the walls with plaster. At ECHO Asia, we made plaster by adding water to the same soil/rice hull mixture that we used to fill the bags (Figure 11). In Florida, since our soil for plaster only had 20% clay, we applied that plaster to chicken wire/stucco mesh attached to the walls, so that it would stick better; this step would be unnecessary if the clay content in the soil used for plaster is closer to 30-35% (Toevs, 2019). 

Conclusion

Consider an earthbag structure for stabilizing seed storage temperature in areas where air conditioning and refrigeration are not feasible. ECHO staff in Asia have found earthbag buildings to be especially helpful for community seed banks in need of low-cost, sustainable storage space. Temperature control with earthbag storage could be combined with vacuum sealing (Bicksler 2015) to also reduce humidity, thereby extending the life of seeds. See the References and Further Reading sections below for much more information and detail on earthbag construction.

References:

Bielema, C. 2017. Bamboo for Construction. ECHO Technical Note #92.

Bicksler, A. 2015. Bicycle Pump Vacuum Sealer for Seed Storage. ECHO Development Notes 126:1-2.

Geiger, O. 2019. Step by Step Earthbag Construction. EarthbagBuilding.com. Website accessed 16 July 2019.

Hunter, K. and D. Kiffmeyer. 2004. Earthbag Building: The Tools, Tricks, and Techniques. New Society Publishers.

Geiger, O. and K. Zemskova. 2016. Earthbag Technology – Simple, Safe and Sustainable. Nepal Engineers’ Association Technical Journal XLIII-EC30 (1):78-90.

Stouter, P. 2011. Earthbag Building in the Humid Tropics: Simple Structures 2nd edition. SCRIBD.

Toevs, E. 2019. Personal Communication.

Trail, P., Y. Danmalidoi, S.M. Pler, A. Bicksler, and B. Thansrithong. 2019. Low-Cost Natural Building Options for Storing Seed in Tropical Southeast Asia. ECHO Asia Notes 38:6-8.

Further reading

More on the cost of earthbag houses:

Haft, R., H. Husain, A. Johnson, and J. Price. 2010. Green Building in Haiti

Actual costs vary with size and design choices. Haft et al. (2010) reported a cost of $2,168.95 USD for an earthbag house in Port-au-Prince, Haiti. Appendix B of the book Earthbag Building (Hunter and Kiffmeyer 2004) provides guidance in determining cost of labor and materials. A seed storage structure does not need windows, which reduces cost in comparison to a home. 

General information:

Hart, K. 2018. Essential Earthbag Construction: The Complete Step-by-Step Guide (Sustainable Building Essentials Series). New Society Publishers.