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.
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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
cob wall that absorbs low winter sun for free heat inside (stays cool in summer)
Cheat Sheet
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.
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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.
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building with cob is intuitive, tactile, hands-on sculpting
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The Ingredients
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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.
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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.
rammed earth wall with layers of different colored clays
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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!!
pressing wet clay, sand, & straw mix into adobe molds
Benefits
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• Good thermal mass properties (acting like a rechargeable battery for heat energy storage)
• Simple, easy-to-learn construction techniques
• Uses low-tech inexpensive tools
• Can be used structurally
• Inexpensive material that is ubiquitous around the world
• Natural and completely biodegradable materials, that create a healthy indoor space
• Absorbs excess humidity (acting like a passive dehumidifier)
• Aesthetics of a thick-walled building with large widow sills
• Can be used additively and subtractively
• Promotes healthy indoor air quality
Challenges
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• 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)
Performance
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.
sculpted clay bench, shaped for ergonomics
clay wall separating entry & kitchen
sculpted clay sink with oil sealer to waterproof
cob mass wall warmed by a masonry heater
FIRE RESISTANCE
Loose straw is highly flammable. Which
leads people to believe that strawbale walls
must also be flammable. However, quite the
opposite is true.
Fire needs 3 things: fuel, oxygen, and a
spark. The difference between loose straw
and a tight strawbale, is that there is not
enough oxygen inside a strawbale to
support a flame. In addition, strawbale
walls are finished with thick plasters...the
same materials used for fire-proofing.
Testing supports the fire-resistance.
Strawbale walls pass a two-hour
rating in a small-scale fire test (ASTM-
119), outperforming most standard types of
construction. Flame spread characteristics
and smoke development (ASTM E-84) also
exceed code requirements, with a flame
spread index of 10 and a smoke
development of 350.
MOISTURE
By far, the biggest concern with strawbale
walls...as with any construction materials...is
moisture. Any material that stays
persistently damp can support mold growth.
When biodegradable materials remain damp
they begin to decompose. This is true for
wood as well as straw.
Moisture infiltrates a wall in one of
two ways: as liquid water (such as from
a leaky pipe or roof) or as vapor/humidity.
Preventing liquid water migration requires
careful detailing (at wall penetrations and
horizontal surfaces). Humidity only becomes
a concern if it condenses inside the wall.
Which means eliminating cold spots inside
the wall (where humidity condenses into a
liquid) and using only vapor-permeable
finishes that allow humidity to migrate freely
through a wall.
Bales should be dry before
installation (15% or less moisture) and the
tops of all walls must be protected from rain
during construction.
INSTALLING ELECTRICAL
Installing wiring is actually more
simple with strawbale walls than with
conventional studs. This is because there is
no need to drill studs and snake the wires
through. Wires are simply surface
mounted to the bales using 4" landscape
staples or tucked between the bales. Plaster
is a minimum of 1-1/4" thick, which meets
the code requirement for wire location
behind the wall surface.
Junction boxes are attached to structural
elements or are screwed to wooden wedges
driven tightly into the monolithic bales.
sculpted cob niche & embedded glass block
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 building (click the text link below each book)
