Building Earthen Homes Using the Original DIY Material
Categories: Building Methods
Adobe, cob, compressed earth blocks and clay-straw building methods are labor-intensive but tremendously rewarding.
Dirt is the original DIY material. In cultures all over the world, people have used earth to fashion everything from bowls to buildings. We know this because so many earthen homes are still around, including buildings hundreds and even thousands of years old. In recent decades, interest in earth construction has risen. What follows are some of the pros and cons of the different types of earthen building, including adobe, cob, compressed earth blocks and clay-slip straw, and some practical tips on things you may want to try as well as those you’ll want to avoid.
Earthen Home Basics
Some basic features and practices apply to most earthen building types. First, the old adage: Give your buildings a good pair of boots and a hat. That means lifting your earthen walls up away from water on a solid foundation and covering them with big overhangs.
I’m a fan of outdoor rooms, so I prefer large patio overhangs all around, except for the south side of the building if we need passive solar heat gain in the winter. To prevent water damage, keep earthen walls covered during construction. A sensible approach in wet climates is to build the roof first on a post-and-beam structure, and then infill with bricks, cob or clay-slip straw.
Second, dirt isn’t insulation. Light, fluffy and airtight assemblies prevent heat flow; massive, dense ones do not. Some people don’t get this. I think the confusion arises because of a unique feature of the most common earthen building climate: hot and arid. In such regions, exterior temperatures tend to fluctuate above and below desired interior temperatures (hot during the day, cold at night). Thick-mass walls can act as an effective form of dynamic insulation. In all other climates, in my view, earthen materials aren’t appropriate for exterior wall systems if your goal is to build the most energy-efficient building. They instead should be used inside the insulated envelope as interior walls, floors and plasters to add mass, soundproofing and beauty to the building.
Third, earthen building is not easy, simple or cheap. Dirt is heavy, and you need to move a lot of it around to build. That’s not easy. Anything heavy that has the potential of falling on your head needs to be taken seriously. As for cheap, a big plus of earthen building is definitely that much of the material can often be found on site and is ubiquitous and inexpensive. But earthen building is labor-intensive, so what you save in materials, you may pay back in hard work. Of course, much of it may be slave labor (i.e., you), but to make an apples-to-apples comparison with conventional approaches, you have to value that time. I could make a free building out of gold if the gold were salvaged and the labor were unpaid.
Finally, the best earthen building approach is one that has a history in your area, because that means local expertise is available, and problem solving and code approval will likely be easier. Being a novice without elders to guide you along can be a lonely, difficult enterprise. Building is hard, serious work — wonderfully rewarding if done right and potentially calamitous if done wrong.
Anatomy of Dirt: Mixes for Earthen Building
Adobe. Adobe construction consists of air-dried earthen blocks laid like bricks with a mud mortar. A mix of water, clay subsoil, sand (as needed) and straw is placed in forms, and the resulting blocks are stacked under a cover to air dry. A typical block size is 10 by 14 inches. Door and window frames are usually structural boxes (known as “bucks”). Because adobe can be mass produced and is modular and storable, it lends itself to the slower, phased approach typical of do-it-yourself construction. Because you’re making the blocks, you can adjust their dimensions to meet your needs.
Cob. Cob is similar to adobe except that the mix is placed by hand into loaf-sized clumps and allowed to dry in the wall before plastering. It’s most often mixed with hands and feet on a tarp, though mixing can also be mechanized with equipment, such as a tractor. Cob can be sculptural and can easily accommodate elements such as glass bottle “windows” (bottle blocks), benches, niches, coat racks, shelves and other features. As with adobe, windows and doors are framed with bucks. Cob is more expressive and flexible than adobe is, but perhaps also more labor-intensive. Unlike adobe, cob can’t be made in advance and stored, so it’s less adaptable to phased DIY construction. To get the best of both worlds, consider combining adobe and cob construction: Use adobe for the main walls and cob for sculptural elements and sections where you desire bottle windows or other “accessories.” Cob is also excellent for interior mass.
Compressed Earth Blocks (CEB). Soil can be compressed in a form to make blocks for use in walls and floors. This requires considerable force, so a CEB machine is necessary. These machines can be either manual or powered. I haven’t seen one for less than $1,500 — they can be much more expensive — but if you need a lot of blocks, a CEB machine can pay for itself quickly. The process involves working the dirt through a screen to remove rocks and break up the clay. Portland cement powder is often added to increase strength. The mix is set in the press, compressed, and then stacked to dry and cure. Block size and shape are determined by the form, and some machines allow for a variety of form options. The cured blocks are laid much like brick. I’ve used a mortar made up of the same mix as the block itself.