The Big Picture of Natural Building

My definition of natural building is this:

Use materials you have locally in abundance, that can be harvested without environmental harm, and build in a way that ensures health & energy efficiency in your climate.

Several variables determine which material is best suited for your application, including: thermal performance, budget, sweat equity goals, and skill of your work force. This means no single solution will suit every situation. And it means you will need to understand the properties of various building materials around you.

How do you know which materials work best for your application?

The answer is simple: IT DEPENDS! (of course!)To help sort out which materials are better suited for your application, it's important to understand some basic material properties. Then I'll explain how to use those attributes to your greatest advantage.

STRAW insulates. What this means is that a wall built of strawbales or other straw-based material slows down heat energy traveling from one side of the wall to the other. A good insulator acts like a down jacket that keeps your body heat inside the jacket instead of getting disbursed to the cold winter air. A well-insulated house will use less energy to heat in winter than a poorly insulated house, because the insulation keeps the heat inside. If you use an air conditioner in summer, insulation will keep the heat outside, so again you need less energy to keep cool.

CLAY materials act as thermal mass. Clay walls store heat energy because thermal mass is like storage battery for heat (or cool) energy. This means clay materials are good at absorbing heat energy from the sun or a fire and storing that heat. When the air temperature drops lower than the temperature of the clay material, the battery stored heat releases back into the air. In this way, clay absorbs a lot of heat energy and then releases the heat over time, long after the heat source is gone. Conversely, a shaded thermal mass with no heat input will stay cool in the summer and absorb heat energy out of the warmer air around it (thus having a net cooling effect, like free air conditioning).

How to best use these characteristics to your advantage?

The answer depends on your climate and what you are building. If you will use energy to change the inside temperature and keep it something different from the outside temperature, then you want a good insulator...ie, straw. If you live in a mild climate where the temperature swings are day-to-night instead of seasonal, then you want thermal mass to average out temperature swings...ie, clay. Thermal mass can also provide highly beneficial interior elements in conjunction with passive solar design, to capture heat from the southern sun in winter (when the sun is low) but remain shaded in summer (when the sun is high in the sky).

STRAW works best...

CLAY works best...

anywhere you are trying to keep the inside temperature different from the exterior temperature; the insulating strawbales will help keep the temperature exchange to a minimum, so the energy used to change the inside temperature will be minimized
added to clay materials for internal structure (armature)

as thermal mass built around a masonry heater or rocket stove (or near a wood burning stove), where the cob can absorb heat from the fire, and store the heat energy even after the fire is out
for trombe walls in passive solar design, with the cob thermal mass inside, where it is warmed by sun coming through South-facing glass
for any interior element when you are trying to keep the inside cool; this can be the same thermal mass used to keep warm in winter as long as there is no heat source warming it when you want to stay cool

How I use natural materials...

I use a combination of strawbale and clay walls. I design for a climate that requires several months of heat in the winter. Using insulating strawbales for exterior walls ensures only minimal energy is needed to heat. I then position clay elements to the interior: surrounding a wood-burning heat source or positioned so low winter sun warms it from the south. A well-positioned clay wall captures free heat from the sun in cold months. That same clay wall is shaded in summer and draws heat out of the space (like a cave). Clay walls inside a well-insulated envelope regulate the interior temperature in both winter & summer for free.

Comparison of Natural Wall Materials

quick-look comparison of various natural materials and methods

MATERIAL	DESCRIPTION	BEST USE	BENEFITS	CHALLENGES
STRAWBALE	Strawbale wall construction uses big fuzzy agricultural bricks of straw, stacked and secured. The surfaces are coated with breathable, tactile clay or lime plasters	Use to save energy for heating or cooling, or wherever you benefit from high insulation	•high insulation •simple installation •inexpensive •renewable •thick walls with deep window sills •healthy spaces	•careful detailing required •must use breathable finishes •requires interaction with permit officials •labor intensive
LIGHT CLAY	Light clay straw is made by coating straw fibers with liquid clay and tamping the mixture into a wall cavity. The clay glues the straw fibers together. to create a solid infill material.	Use where modest insulation or good sound absorption is a benefit	•excellent acoustic performance •simple installation •very inexpensive •prevents air leakage	•requires ample airflow during drying •requires cavity wall system (such as stud framing) •labor intensive •limited testing data
HEMPCRETE	Hempcrete is a combination of hemp hurds (from woody hemp stalks) glued together with sticky lime. The mixture is tamped into a wall cavity and then plastered on both sides.	Use as insulation to save on heating & cooling, or where walls experience occasional wetting.	•high insulation •can handle some dampness •sequesters CO2 •regulates humidity •prevents air leakage	•lime binder is caustic during installation •requires cavity wall system (such as stud framing) •labor intensive •currently costly in USA
CORDWOOD	Cordwood features logs, round or split, all cut to the same length. The logs are mortared together, with their length creating. the wall-width.	Use if you have wood in abundance and benefit from modest insulation	•modest insulation •no finish plastering required •uses short pieces of wood •logs create the wall surface aesthetic	•slow to build •logs must be cured (dried) to prevent shrinking in place •wood-intensive •pointing between logs can be tricky
COB	Cob is a mixture of clay soil, sand aggregate, and straw fiber. When wet, the material is malleable and can be easily sculpted. As. the cob dries, it becomes exceedingly strong.	Use as mass (heat battery) for passive solar design or in climates where cooling is always beneficial	•sculptable, curve-friendly walls •can be structural •simple, low-tech •very inexpensive •intuitive •regulates humidity •good thermal mass	•lengthy drying time •creates humidity during drying •cannot freeze when wet •non-insulating
ADOBE	Adobe is exactly the same as cob, but formed into over-sized bricks. The bricks are dried in the sun and then stacked with a clay-based mortar.	Use as mass (heat battery) for passive solar design or in climates where cooling is always beneficial	•can be pre-made •can be structural •simple, low-tech •can custom-size •regulates humidity •good thermal mass •no finish plaster needed	•need space to make & sun-dry bricks •protect from rain during brick production •niches require planning •non-insulating
RAMMED EARTH	Rammed Earth is a mixture of clay binder and sand aggregate that is tamped into formwork. The tamping transforms the mixture essentially into sedimentary rock.	Use as mass (heat battery) for passive solar design or in climates where cooling is always beneficial	•no finish plaster needed •can be structural •beautiful striated effect •excellent thermal mass •regulates humidity	•requires formwork •easiest if straight •niches require planning •non-insulating •critical to test materials to find strongest recipe
WATTLE & DAUB	Wattle is woven material, used to create whatever shape you like (walls, shelves, fencing). Daub is a wet mixture of clay soil and plant fibers (such as straw).	Use as non-structural partitions where insulation is not required	•nearly free interior wall system •thin walls •simple, low-tech •intuitive	•non-structural •usually requires finish plaster •low thermal mass •non-insulating