Natural building utilizes minimally processed, nontoxic materials and systems used appropriately for the climate, site and intended use. It incorporates primarily natural materials rather than high embodied energy, processed commercial materials, with a focus on getting these natural materials from as local a source as possible–ideally, from the building site itself.
“Cob” is an old English term meaning loaf or lump. Cob construction uses a mixture of clay, sand and straw, and is used mostly in a structural, or load-bearing, capacity as house and building walls. But it is an extremely versatile medium that can be used for garden walls, benches, bas relief, bread ovens, and more. Building with cob is like building a big clay pot, as it requires no formwork or preforming in to blocks. It is magnificently sculptural and lends itself well to curvilinear and freeform walls and artistic expression. The high thermal mass of cob helps reduce temperature swings by storing and releasing heat, and the clay can help control humidity – beneficial properties in Central Texas and elsewhere.
Houses made of bales of straw can be load bearing (carrying the load of the roof) or they can used to create the walls (infill) of a post and beam structure. Walls are generally plastered with lime on the exterior and earth on the interior, but other finish options are possible as well. The thick walls created by the 18″ thick bales (most Texas bales) not only provide phenomenal insulation (about R-43) but give a look of solidity and lovely thick-wall aesthetic at window and door openings. Appropriately designed and constructed straw bale buildings do well in nearly all climates, including hot and humid Central Texas.
Light clay straw walls are made by tossing loose straw with clay slip, then lightly packing the mixture into slip forms. It is a non-loadbearing material placed as infill between columns and intermediate supporting structure such as timber frame or steel. Light clay straw walls offer both insulative and thermal mass properties, with the balance depending upon the ratio of clay to straw. It can be used as perimeter walls or interior walls (even between 2×4 wood studs) and typically receives coatings of lime or earthen plaster.
Earthbag construction is a relatively quick process that uses woven polypropylene feed bags filled with clayey or sandy soil, sand or gravel to form foundations, walls, benches, and more. Beautiful curvey forms and domes can be created using earthbags.
Beautiful, thick, solid earthen walls can be created by ramming slightly moistened earth in sturdy forms. The resulting walls can be finished with a clear sealer to reveal the earthen stiations, or plastered with lime or earth plaster. Colored clays or artifacts such as geodes and fossils can be cast into rammed earth walls for added interest. Rammed earth walls are load-bearing and offer very high thermal mass but low insulating properties.
Slightly moistened earth is compressed under great pressure in to blocks that are immediately ready to put into the wall. Compressed earth blocks (CEB) can be used to form perimeter or interior load bearing walls or can be used for infill within another, structural system. These versatile blocks have been successfully used in both residential and commercial construction, and their familiar shape and method of laying can make acceptance in the conventional construction world more likely.
Here is a partial list of wall systems and properties compiled into this helpful format by Michael G. Smith.
|Technique||Materials needed||Structural properties||Thermal properties||Best applications||Advantages||Disadvantages|
|Straw bale||Straw bales, sticks or bamboo for pinning; baling twine||Load-bearing or infill; good earthquake resistance||Excellent insulation; moderate thermal mass (better with thick plaster)||Exterior walls in most climates; quick, temporary structures||Goes up relatively quickly. Fairly easy to permit. Increasing pool of experienced designers/builders.||Very susceptible to moisture damage; bales must be stored carefully; unfinished building must be protected from rain. Needs thick plaster for protection.|
|Cob||Clay soil, sand, straw||Load-bearing or infill; moderate earthquake resistance||Good thermal mass; low insulation||Exterior walls in moderate or sunny climates; interior walls; ovens and hearths; benches; garden walls; greenhouses; floors||Highly sculptural; enormous design flexibility. Combines well with other materials. Simple to learn.||Labor intensive; goes up slowly, especially in cool, moist conditions. Prone to water damage. Permitting may take persistence.|
|Adobe||Clay soil, sand, straw or other fiber||Load-bearing or infill; poor earthquake resistance||Good thermal mass; low insulation||Exterior walls in moderate or sunny climates; interior walls; ovens and hearths; domes and vaults in dry, non-earthquake regions.||Adobe blocks can be made in one place and transported. When blocks are made, wall goes up fast. Many pros in SW. Simple to learn.||Making and storing adobe blocks takes a lot of space and dry weather. Prone to damage from water or earthquakes.|
|Rammed earth||Clay soil with high content of sand; often stabilized with cement & reinforced with steel||Load-bearing; good earthquake resistance||Very good thermal mass; low insulation||Exterior walls in moderate or sunny climates; benches and garden walls.||Contractors, engineers and permits available in CA and elsewhere.||Very labor or machine intensive. Requires forms. Professionally built RE can be expensive and uses non-natural materials.|
|Earthbags or Super-adobe||Woven polyprop-ylene feed sacks filled with clay soil, sandy soil, sand, or gravel; may be stabilized with cement; barbed wire||Load-bearing; foundations for other wall systems; good earthquake resistance||Very good thermal mass; low insulation (unless filled with a light fill like pumice or scoria)||Foundations for cob, straw bale, etc.; exterior walls in moderate or sunny climates; benches, garden walls and retaining walls; domes and vaults in dry, non-earthquake regions.||Relatively quick earth building technique. Allows the use of a wide range of fill materials. Simple to learn.||Poly bags very susceptible to UV damage; must be protected from direct sun; long-term durability of unstabilized earthbags unknown.|
|Stone||Stones; may be dry-stacked or mortared with a mixture of sand, cement, lime and/or clay||Load-bearing; foundations; poor earthquake resistance unless reinforced with steel||Very good thermal mass; very low insulation||Foundations, basements, retaining walls; fireplaces and hearths; floors and patios; exterior walls in non-earthquake regions with mild climates.||Very durable, even in wet conditions and in contact with ground.||Very labor intensive.|
|Light clay or Slipstraw||Straw (or wood chips, hemp hurds, or other suitable material); clay slip||Infill||Insulation and thermal mass vary with mix; insulation can be high per thickness||Remodels; exterior and interior walls in many climates.||Walls can be any thickness. Combines well with standard stud framing or timber framing.||Requires forms, so walls are generally straight. Prone to water damage. Wood required for frame and forms.|
|Wattle and daub||Straight, flexible sticks (or bamboo); clay soil; chopped straw and/or manure||Infill||Poor insulation; low thermal mass (thick plaster increases mass)||Interior walls; unheated structures such as outhouses, sheds, etc.; exterior walls in hot tropics.||Walls can be very thin. Uncovered wattle is very decorative.||Requires lots of straight flexible sticks which can be difficult to find. Labor intensive. Prone to water damage.|
|Clay wattle||Clay soil; long straw; sticks||Infill||Poor insulation; low thermal mass unless wall is quite thick||Interior walls; unheated structures such as outhouses, sheds, etc.; exterior walls in hot tropics.||Walls can be very thin, curved and sculptural. Simple to learn.||New, little-known technique. Requires long straw. Prone to water damage.|
|Cordwoodmasonry||Wood cut into short lengths; mortar may include cement, lime, clay, sand, sawdust; lime/sawdust or perlite insulation||Infill or load-bearing (round structures only); poor earthquake resistance||Good insulation; moderate thermal mass||Exterior and interior walls.||Decorative. Easy to attach wooden framing and furniture.||Wood must be sound and very dry. Tendency for wood to expand and contract, cracking mortar and creating drafts.|
|Papercrete, Fibrous cement, Fidobe or Hybrid adobe||Recycled paper pulp; sand; cement or clay soil||Infill or load-bearing||Good insulation; thermal mass varies with mix||Exterior and interior walls; floors; plasters||Very versatile techniques. Walls easily modified.||New technique. Requires a specialized mixer. Questionable water resistance.|