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.
Clay-Slip Straw. Clay-slip straw (also known as slip straw, light straw clay and Leichtlehmbau) is straw covered in a clay slurry that’s made by soaking clay in water until it becomes suspended. It’s mixed by hand or with machinery such as a mortar mixer. Clay-slip straw is strong enough to hold itself up, but not strong enough to carry significant loads such as a roof, so it must be installed in a frame attached to a skeletal structure. This more complex frame can be an additional cost. The clay-slip straw mix is placed in forms and packed in place. After the mixture is sufficiently dry to support the next layer, the forms are moved up the wall and another layer of material is installed. The wall has to dry before being plastered, and because the wall has considerable water content, this drying can take a number of weeks.
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Clay-slip straw is lighter and easier to place than other mixes. It also has the highest resistance to heat flow (R-value) per inch. These values vary considerably based on density of the mix and several other factors, so determining the actual performance of your mix is difficult. In one study, R-values ranged between R-1 and R-2 per inch for mixes using the same materials in different proportions. Compare this with about R-3.5 per inch for cellulose insulation. Thicker walls mean more insulation, and this makes clay-slip straw perhaps the only DIY earth home method I’d use for an exterior wall system in a cold climate.
Earth Plasters. All of the earthen wall materials described thus far are typically finished with plaster. Earthen walls need to be “hygroscopic,” a fancy word that means something has the ability to take in and release water vapor in response to humidity changes. Earthen walls also need to be covered with permeable plaster. Portland cement-based plasters are not recommended, because they are less water-permeable. Typically, earth and lime plasters are the right choices. Earth plasters are a mix of clay, sand and water. Chopped straw is often added, especially in base coats. Other materials from wheat paste to glue to urine (yes, urine) can be added to adjust the mix’s properties, such as its water resilience.
Earthen Floors. Earthen floors come in a number of styles, such as puddled, tamped and laid. They all use clay, with just enough water content to allow the binding clay to find its way around the other materials. After it’s in place, the mix is generally compacted (tamped). Sometimes layers of the mix are applied and tamped. Another approach is to lay compressed earth block as you would thick tile. After it’s dry, the floor is usually finished to reduce dust, increase surface strength and limit water permeability (see “Better Natural Finishes for Earthen Materials,” below).
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Sample First, Then Build
Earthen building materials start as a combination of binder (clay) and aggregate (sand). Straw is often added to help the materials hold together. When mixed in water, clay particles slip between sand particles. As the mix dries, the clay particles contract, locking the sand in place to create a structural material that can hold its own weight as well as support other things, such as a roof. The ideal ratio of clay varies, but one guideline is 75 percent sand to 25 percent clay.
The best approach is to make sample batches of mix, form them into balls, loaves, little walls — whatever — and test their strength. None of these tests can tell you for sure whether your dirt will hold up a roof, so if you’ll be using your earth walls as load-bearing walls, hire an engineer to be safe.
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Better Natural Finishes for Earthen Materials
Finishing earthen plasters and floors may be a central reason for their slow acceptance in mainstream building. Earth plasters dry slowly, which makes them difficult to incorporate into conventional construction schedules. After they dry, they can dust or flake. Convincing someone that dirt can make a durable floor can be tricky.
Deciding when to use a “breathable” finish and when to use a waterproof finish is an important piece of the puzzle for earthen wall and floor systems. Soil contains fungi spores that can colonize if exposed to moisture. If mold gets into earthen walls or floors, it can potentially lead to poor indoor air quality, a problem that can be hard to fix.
For some areas, breathable finishes are the best option. In other areas, the moisture must be stopped from entering in the first place or it will cause mold and possibly structural degradation.
One option for DIY earthen floor finish is linseed oil, but linseed oil alone requires too many coats, dries out, feeds mold, and doesn’t have the surface density or hardness required for floors. At Earthpaint, we combined two natural finishes typically used for wood floors. These products use pine and cashew resins to create more durable surfaces without providing a food source for mold.
Earthen walls function best if they are water repellent but permeable to water vapor. If the walls are coated with a mold-proof primer and are water repellent yet breathable, moisture and mold won’t be able to cause trouble in the walls. Eliminating dust and hardening the surface while preserving lighter color tones of the plaster is vital.
It took several tries to find the right finish. Oil-based products darkened the walls too much. Potassium silicate sealer hardened up nicely but didn’t repel water. Water-based sealers had too much sheen and were not breathable.
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We settled on a beeswax and water glass (sodium silicate) emulsion that is breathable, light in color, and has a pleasant, natural sheen. It repels water, hardens surfaces and eliminates dust.
— Tom Rioux, founder of Earthpaint, which sells its deck stain, nontoxic paint, floor finish and mold abatement products factory-direct.
Clarke Snell works at the Laboratory for Innovative Housing at the University of North Carolina, Charlotte, where he is also a graduate student in architecture. We highly recommend his book, Building Green: A Complete How-to Guide to Alternative Building Methods.
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