Clay Wall 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.

Photo left: sculpted clay wall creates this winding staircase, with integral handrail

Photo right: 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

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.

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.

Photo left: building with cob is intuitive, tactile, hands-on sculpting

Photo middle: rammed earth wall with layers of different colored clays

Photo right: pressing wet clay, sand, & straw mix into adobe molds

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!!

Benefits

• High insulating qualities(R-2 to 2.7 per inch, minimum)

• High sound absorption coefficient

• Simple, easy-to-learn construction techniques

• Structure can be completely standard

• Inexpensive material that is often available locally

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

• Renewable agricultural waste material, needing only a single growth season

• Requires very little energy to produce

• Diverts farming waste material

• Aesthetics of a thick-walled building with large widow sills

Challenges

• Requires careful detailing to prevent liquid water infiltration

• Requires breathable finishes, usually natural plasters, which may necessitate research or hiring a consultant

• Necessitates educating yourself, the builder, and (often) building permit officials

• Requires more interaction with permit officials in regions that do not currently have building codes for strawbale construction in place

• Hired labor can be expensive, especially for plaster finishes

Photo left: sculpted clay bench, shaped for ergonomics

Next: clay wall separating entry & kitchen

Next: sculpted clay sink with oil sealer to waterproof

Next: cob mass wall warmed by a masonry heater

Photo right: sculpted cob niche & embedded glass block

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 AND 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.

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.

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.

Build naturally...Live sustainably