Clay Building Overview

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.

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.

sculpted clay wall creates this winding staircase, with integral handrail

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Good thermal mass properties (acting like a rechargeable battery for heat energy storage)

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Simple, easy-to-learn construction techniques

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Uses low-tech inexpensive tools

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Can be used structurally

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Inexpensive material that is ubiquitous around the world

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Natural and completely biodegradable materials, that create a healthy indoor space

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Absorbs excess humidity (acting like a passive dehumidifier)

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Aesthetics of a thick-walled building with large widow sills

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Can be used additively and subtractively

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Promotes healthy indoor air quality

Benefits

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cob wall that absorbs low winter sun for free heat inside (stays cool in summer)

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.

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!!

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Does not provide insulation, so should only be used where thermal mass is beneficial (often misunderstood with clay walls)

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Cannot be used for exterior walls in cold climates or where insulation is required for building codes

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Wet clay walls (such as cob) require substantial drying time, often months

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Cannot freeze during drying time

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Requires breathable finishes, usually natural plasters

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Extremely heavy materials (literally tons!)

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Extremely labor intensive, so can be expensive if hiring out construction labor and time-consuming if building yourself

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Requires testing to determine the proper recipe for your materials (there are many types of clay)

Challenges

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.

THERMAL MASS

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

ingredients.

RAMMED EARTH

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.

Performance

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

FIRE PROOF

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

proof.

MOISTURE

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.

RELATIVE COSTS

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.

BUILDING CODE

Clay walls meet current U.S. building

codes...provided they are not used

where insulating walls are required.

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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.