Last Updated: Oct 16, 2021
It has been said that a home is only as good as its foundation. Think about much we rely on our home’s foundation: it must hold up the weight of the rest of the house, resist wind and soil pressure, withstand frost, keep out water, pests, and soil, be its own conditioned living space, and serve as the actual footprint—and here we mean from a design standpoint, not the ecological footprint—of the home.
Most homes’ foundations are made from concrete—either cast-in-place, precast, or concrete masonry units (CMUs). Insulated concrete forms (ICFs) are also becoming more prevalent. What’s the problem with concrete? According to BuildingGreen.com, concrete “contains extraordinary embodied fossil fuel energy and a primary contributor to global warming. Concrete has many advantages as foundation material but should be used as sparingly as possible.” That’s what we mean by “footprint” in the headline: the ecological footprint of a home increases when concrete is used. So, what are some alternatives?
As part of our research into innovative products for homes, we found one exciting way of making the foundation for a building. It’s called an earthbag foundation. It’s made by filling long, skinny bags with dry soil or gravel, then tamping and stacking them. Next, two or more courses of bags are stacked with barbed wire in between to hold them together to make the desired height.
The bags are made of a polypropylene material in long tubes used for farm feed sacks. However, the polypropylene sacks need to be protected from sunlight, so they don’t degrade. This can be done by backfilling earth against them or extending the building’s siding down over them.
The gravel and earth are directly from your building site or a local quarry, so there are virtually no transportation costs—making this one of the most sustainable ways to build a solid foundation. Can just any earth be shoveled into the bags? According to Endeavour, an innovative learning, building, and living center, “a wide variety of material can be used in the bags, as long as it has an aggregate content capable of being tamped to a high degree of compaction.”
For this reason, the embodied energy and environmental footprint of an earthbag foundation are far lower than that of a concrete foundation.
The earthbag method has been used for structures all around the world, and not just for houses. A great example is the Trillium Lakelands Teachers’ Union Office in Lindsay, Ontario, Canada. This beautiful 2,400 square foot office building was built with many sustainable features, including photovoltaics, straw bale walls, and an earthbag foundation.
And, you can use the earth wherever you are located. They used road-base gravel and a small amount of a lime/metakaolin binder to provide a mixture that tamps well and stays coherent after curing, even if the bag is damaged or removed. It is also possible to use aggregate and clay in the bags.
1. Design limitations.
Not every building can be built with this type of foundation; this is the primary con. The earthbag foundation is primarily for buildings where the living space will be above grade, without a basement. Like any shallow foundation, it must be frost-protected with well-drained gravel and a skirt of insulation in the ground. Tall buildings would not work well either: as the wall gets taller, they cannot support the weight. As a result, most earthbag buildings are one story high.
Because earthbags are thick and heavy, they make thick walls. Interior walls made of earthbags, for example, would not be desirable, as they would take up too much space. According to Kelly Hart, author of Essential Earthbag Construction: The Complete Step-by-Step Guide (New Society Publishers),
"Earthbags make great domes, but structures should be no larger than about 20 feet (6 meters) in diameter, and they cannot be hemispherical; catenary arches are the best dome shape to build. Domes need to be circular at the base so that all of the forces around them are equally balanced; otherwise, there is the risk of deformation and failure. Earthbag domes are best limited to fairly arid climates, as it is difficult to assure that the final plaster will always be watertight in wetter climates. Vaults should be avoided; they are too unstable. Walls that have many openings for doors and windows are probably best framed with wood because there are limits to how many such openings can be placed in an earthbag wall."
2. Variable insulation.
Because the bags are filled with earth, and the soil is typically not an excellent insulator, earthbags by themselves are not the best for insulation. In addition, extra insulation might need to be added in cooler climates, adding to the project's complexity and cost.
3. Potential permitting issues.
Since earthbags are not commonplace, not every building authority will readily provide a permit. However, earthbag foundations are still an "alternative" form of building, so if you need a permit, check with the permitting official first before you get too far down the road.
4. Mechanical, Electrical, and Plumbing Fixtures.
Earthbags are not conducive to having water pipes, air ducts, or electrical wires run through them. So if you plan to have these things, it's best to plan additional materials for your mechanical, electrical, and plumbing systems.
5. Versatility.
Yes, this was listed as a pro due to the versatility in design, but it is a con: earthbag homes are not that versatile if you think about furnishings, finishings, and remodeling. The walls might not hold cabinetry and fixtures, for example, and may need additional materials for reinforcement. In addition, it's challenging to remodel earthbag homes: imagine cutting through earthbags to add a door or a window. So, it's best to plan and love what you end up with.
Earthbag foundations are a low-cost, easy-to-construct alternative foundation for homes that have a much smaller environmental footprint than any other option. But beware: it’s not for everyone.