The earthbag stemwall goes in at the off-grid straw bale cabin site

To be really honest here, foundations are the least exciting part of any natural building project for me.  Most of the fun stuff like timber framing, building with straw bales, applying clay plaster, or building with cob represents the “glory” of the natural building world. Foundations don’t have the same sex appeal, for lack of a better phrase. However, a good foundation is one of the most important parts of a home.

The small off-grid home we’re helping friends build here in the Appalachian foothills is getting a bunch of work done to it recently. The biggest task of the moment is building the earthbag stemwall, which we need to finish before our upcoming Straw Bale Workshop in July. We opted for an earthbag stemwall due to the availability of appropriate materials, the relative ease of construction, and the unique siting of the house. All things considered, I think going the earthbag route has been a good choice. Here are some of our considerations for this particular choice.

Choosing to Build an Earthbag Stemwall

A foundation can take on many different forms. Stone or “urbanite” are good masonry choices, and in really rough terrain or wet climates, building up on piers is a potential option. Concrete represents a kind of “last resort” for me personally, as it comes loaded with high embodied energy and cannot be easily repaired or re-purposed if and when that time ever comes. Honestly, the options for building a home with ground contact is a difficult task to achieve with all natural materials. Earthbags fill an interesting void in the array of choices.

Earthbags get filled with a mix of readily available clay subsoil and gravel

Earthbags are nothing more than woven plastic sacks (or polypropylene, to be more specific) filled with a combination of clay subsoil and aggregate. Recipes for fill can vary according to need. The earthbags themselves are kept dry by being built upon a rubble trench, and extensive drainage work around the home. A large porch overhang protects fully half the home from direct weather as well.

Because the home site is inaccessible by anything other than foot or four wheel drive truck, all the material needs to be reasonably transported in small loads. The actual bags themselves take up barely any volume, of course, and clay subsoil is readily available around the site thanks to all of the grading that happened earlier last year. And we are able to get gravel up by the pickup load without too much trouble.

Some other benefits of choosing earthbags include that they are pretty forgiving, unlike stone, which requires a lot of time and skill to collect and stack. There are other challenges with using stone, too — if they’re dry-stacked, you need some reasonable way to keep drafts and rodents out. If they’re mortared, the stones are far less repairable. Concrete was not an option for this cabin (due to access), even if we wanted to use it. Also important to note is the octagonal shape of this house — earthbags easily accommodate an irregular house plan such as this.

So that’s where earthbags came in. They are low cost, require less time to complete, and frankly they’re easier to work with than other comparable choices. Outside inputs are either low (the amount of total plastic is not significant) or readily available and abundant (gravel for fill from a local quarry, and clay from the site itself.) Note too that we’re building straw bale walls on top of the earthbags (with a toe-up between the two). These two methods are a good match as well.

All things considered, earthbags offer some unique benefits in this particular natural building design and location.

Next time… I’ll describe the actual guts of our stemwall design, and include a cutaway drawing of how we designed the stemwall to accommodate the straw bale walls, and include insulation and protection from UV and moisture damage.