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 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.
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!!
Does not provide insulation, so should only be used where thermal mass is beneficial (often misunderstood with clay walls)
Cannot be used for exterior walls in cold climates or where insulation is required for building codes
Wet clay walls (such as cob) require substantial drying time, often months
Cannot freeze during drying time
Requires breathable finishes, usually natural plasters
Extremely heavy materials (literally tons!)
Extremely labor intensive, so can be expensive if hiring out construction labor and time-consuming if building yourself
Requires testing to determine the proper recipe for your materials (there are many types of clay)
sculpted clay bench, shaped for ergonomics
RULES OF CLAY
1. 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.
All 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 & ADOBE
Cob 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
Rammed 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.
clay wall separating entry & kitchen
sculpted clay sink with oil sealer to waterproof
cob mass wall warmed by a masonry heater
sculpted cob niche & embedded glass block
Clay 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
One 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.
Material 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.
Clay walls meet current U.S. building
codes...provided they are not used
where insulating walls are required.
Want to learn more? Here are some articles to get you started.
AT A GLANCE R-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 USE use to capture and store heat energy (such as solar heat or from a fire) LINK TO GLOSSARY Read the glossary description here.
CLAY WALLS Made 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.
rammed earth wall with layers of different colored clays
pressing wet clay, sand, & straw mix into adobe molds
building with cob is intuitive, tactile, hands-on sculpting
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.
Here are my favorite books about clay construction (click the cover to purchase)
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.