Clay soil provides a versatile building material, used for thousands of years to create beautiful and durable structures.
Every region of the world has an indigenous method for building walls of clay. Clay materials can be formed into blocks (such as adobe) or built monolithically (such as rammed earth or cob).
Walls made with clay are really a mixture of two ingredients: clay & sand. Coarse sand provides strength, similar to aggregate in concrete. Clay is the sticky ingredient, providing glue to hold everything together. Many types of clay construction contain a third ingredient: straw (or other fiber). The long straw knits the wall together, providing an internal network, similar to reinforcing
bars in concrete.
The binding nature of clay is activated by adding water. Wet clay is sticky and malleable. Once dried, the clay holds its shape and remains strong.
Clay is ubiquitous in nearly every region of the world. Harvesting is as simple as digging, since clay is found in soil. Ideal soil makeup requires a minimum of 20% clay, with little to no organic humousy soil and less than 10% silt. The techniques used to build clay walls are extremely intuitive and are therefore quick and easy to learn. However, construction is slow and heavy, and often requires a drying period before finishing. Careful construction timing is crucial.
sculpted clay wall creates this winding staircase, with integral handrail | cob wall that absorbs low winter sun for free heat inside (stays cool in summer) |
Best Use
Use clay walls to capture and store heat for later dissipation.
Clay walls provide excellent thermal mass. Thermal mass materials are like a rechargeable battery for heat energy: mass absorbs heat when in excess and releases heat when the mass is warmer than its surroundings. This means mass materials even out fluctuations in temperatures, always creating an average. Well-positioned mass walls capture free solar heat in winter, when the sun peaks low in the sky. Or can be used to absorb heat from a fire and store it for dissipation after the fire goes out. Shaded clay walls provide free air conditioning, absorbing heat and humidity out of the air.
Cheat Sheet | CLAY WALLS |
CLAY WALLSMade with sticky clay soils, coarse sand for strength, and, usually, agricultural fibers (such as straw) to create strong, heavy, high-thermal mass walls. Clay walls are typically finished with vapor-permeable plasters, made from clay or lime. Techniques are quick to learn and intuitive. Almost no tools are needed, and most are simple and can be hand-made. Clay walls are NOT insulating | AT A GLANCER-0.125 per inch of thickness meets all fire safety codes meets hurricane wind-load codes inexpensive (almost free) material high labor to build can be monolithic or formed into bricks BEST USEuse to capture and store heat energy(such as solar heat or from a fire) LINK TO GLOSSARY |
The Ingredients
SAND
Coarse sand and small stones serve as aggregate in any clay wall, similar to aggregate in concrete. Sand provides compressive strength (for pushing forces). Sand also helps control shrinkage, and therefore reduces cracking. This is because clay absorbs water and swells when wet, then shrinks again when dry. Whereas sand does not change volume whether it is wet or dry. Usually the proportion of sand comprises between 70% and 75% of the clay recipe. Maximizing the amount of sand that still creates a dust-free clay surface results in the strongest possible wall.
STRAW
Long pieces of straw or other plant fibers help knit the wall together, similar to reinforcing bars in concrete. Fiber creates internal tensile strength (to resist pulling forces), which provides additional resistance to cracking as the clay dries and shrinks. Rammed earth is the only clay material that never contains fiber. The best fibers are pliable and have good surface texture for the clay to bond to. The fibers remain resistant to rot once the wall is dry, since biodegradation requires persistent moisture.
CLAY
Clay provides the key ingredient that glues the sand and straw together into a cohesive material. Wetting clay creates a sticky, malleable material. Once wet clay is shaped and allowed to dry, the particles remain stuck together to create a strong monolithic material. Harvesting clay involves digging, since it appears as a component of soils in nearly every region of the world.
building with cob is intuitive, tactile, hands-on sculpting | pressing wet clay, sand, & straw mix into adobe molds | rammed earth wall with layers of different colored clays |
What's So Magical About Clay?
Clay becomes uniquely sticky when wet, and then strong and cohesive when dry. The reason lies in the clay particles, that absorb water and become fat. These fat particles act like suction cups that stick well to one another. When wet, those clay particles also slide, which explains why wet clay malleable.
This ability...to wet the clay, create a sticky glue, shape as desired, and then dry into a strong material...is precisely what makes clay a magical ingredient in natural buildings. This property allows clay to be used for so many applications: pottery, bricks, walls, floors, plasters, and more!!
Benefits | Challenges |
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PERFORMANCE
RULES OF CLAY1. Clay is sticky when wet and strong when dry. The stickiness is what makes clay a superior binder for clay wall materials, plasters, paints, etc. 2. Clay materials are heavy, averaging 120 to 200 lbs. per cubic foot. So each linear foot of wall can weigh over 1,000 lbs. Clay walls require solid support, usually directly down to the ground. 3. Clay acts as thermal mass, NOT insulation. So if you will heat your space, keep clay materials as interior walls only.
THERMAL MASSAll clay walls act as thermal mass, which means the material performs like rechargeable batteries for heat energy. Clay absorbs excess heat when cooler than its surroundings, and releases heat when it is warmer. Adding a heat source to the clay (from the sun or a fire) charges the battery. The stored heat dissipates when the mass is warmer than the surroundings. Similarly, clay mass walls absorb excess heat from air in summer, passively cooling the surrounding air...similar to a cave. Clay walls do NOT act as insulation!! (They do not prevent the flow of heat from one side to the other.)
COB AND ADOBECob and adobe use the same recipe: 25% to 30% pure clay with 70% to 75% sand. In regions around the world that have indigenous clay construction methods, you also find soils that have exactly these ratios in them. The clay is the sticky binder and the sand supplies strong aggregate. Long straw is added for tensile (pulling) strength.
The difference between cob & adobe is that cob is sculpted in place, whereas adobe is formed into bricks (and dried in the sun). They otherwise contain the same ingredients.
RAMMED EARTHRammed earth walls provide the densest type of clay wall, and thus the highest thermal mass per thickness. Rammed earth walls do not contain fiber/straw. Instead, the mixture includes only sand (~70%) and clay (~30%). The mixture of clay and sand is compacted (rammed) into forms. The ramming turns the loose mixture into a dense, strong, sedimentary stone-like material. | FIRE PROOFClay materials are inherently non-flammable. You know how fire fighters dig a trench around forest fires to contain them? They are clearing out flammable brush and exposing non-flammable earth. Similarly, walls built of that same earth are also fire proof. MOISTUREOne of the coolest properties of clay is the ability to regulate humidity. Clay is hygroscopic, meaning it acts as a desiccant, absorbing humidity when surrounding air exceeds 50%. When humidity drops, the clay releases moisture back into the air. This constant absorption and release of humidity cannot be seen. The clay walls never feel wet or sticky. They just absorb and then release humidity. As a result, spaces with ample clay do not experience peaks in humidity. My favorite example of this is taking a shower. When you have ample clay inside your bathroom, you won't see fog on your mirror after taking a hot steamy shower. The clay absorbs the steam before it can condense on your mirror. The humidity then releases slowly the rest of the day. RELATIVE COSTSMaterial costs for clay walls are dirt cheap. (Sorry, couldn't resist.) Clay, sand, and straw cost significantly less than materials used in conventional construction. (Material costs make up about 1/2 of conventional construction costs, compared to only 1/4 of a natural home.) The main difference from conventional construction: clay walls require a lot of heavy labor to build. If you are building yourself, you save significant costs by using clay materials. However, labor costs can be significant, if you hire out the construction force (instead of building yourself). Techniques for mixing and building are extremely easy to learn. Clay is highly intuitive to work with. The feasibility of building high quality with little experience is positively within reach. BUILDING CODEClay walls meet current U.S. building codes...provided they are not used where insulating walls are required. |
How to test soil for clay content
Determining if your soil has clay is shockingly simple: just add some water. Really! Remember that clay particles become fat and sticky when they absorb water and swell. This means you can take a handful of clay, add water, knead it in your hands for a bit and then observe if the mixture becomes sticky. Wet sand will not become sticky. Wet organic gardening soil will not become sticky. Only clayey soils become sticky by adding water. It's that simple. So there you have it, a low-tech, 30 second test to determine if your soil contains sticky clay.
If you want to get more scientific and determine what else your soil contains (and in what proportions), another simple test separates the soil components into layers. This test is commonly called the "shake test" because it involves adding a soil sample to a straight-sided jar, adding water, and shaking. The agitation washes any sand aggregates clean of the smaller clay particles. The heavy sand then sinks to the bottom of the jar, while the clay remains suspended in the water for a longer period of time. Because the settling times are different for each component of the soil, they settle in layers...allowing you to measure the proportion of each in your soil.
Watch a demonstration of testing clay soil here:
Recommended Reads
Here are my favorite books about clay construction materials
(click the cover of the book to purchase)
Building with Cob: If you are looking for one book on cob wall construction, this is my favorite. Detailed. Step-by-step photos. And lots of construction tips. | The Rammed Earth House: Great book on rammed earth construction, including info on how to test materials, create a strong recipe, and build formwork. | The Hand Sculpted House: This book explores an intuitive connection with cob construction, including design considerations, construction tips, and many sketches. | Build your own Earth Oven: A great way to test out clay construction is to tackle a small project...like a clay oven! And this how-to book explains all you need to know. | Earthen Floors: Start-to-finish guide to preparing, installing, and finishing a beautiful floor made from clay...the finishing touch for your strawbale home. |
A wealth of good information 😎