Building Earthen Homes Using the Original DIY Material
Categories: Building Methods
Clay-Slip Straw. Clay-slip straw (also known as slip straw, light straw clay andLeichtlehmbau) 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.
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).
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.
Want to build your own green home? Read Clarke Snell's advice in Your Green Dream Home: First Things to Consider before you hire a contractor.