BASICS
Although cob can be the hard and fibrous cylinder part of a corn plant that holds together the many kernels of tasty sweet corn, it can also mean “lump” or “loaf” – a dialectal English term that within the context of earth building are softball sized clumps of sand, clay, and straw used to hand sculpt the walls of structures and homes. This results in another term for cob which is sometimes called “monolithic adobe.”
To make cob, builders will first take clay (the binding material) and sand (the aggregate) and mix those ingredients with water and straw to form a thick, sticky, muddy, dough-like substance. Cob builders vary in their opinions as to what proportions will make a good cob mix, with testing being the overall determinant as to how well the onsite or acquired material properties will bind and hold up. According to cob experts, the mixture should include a lot of sand and enough water and clay to bind the sand. The amount of straw is determined by how much the builder can comfortably introduce into the mix. Experts say the advantage of using lots of straw is that it adds reinforcement and tensile strength for a cob wall. An example of a basic mixture is one-part wet clay (clay pudding), four parts sand, and one part straw. Although builders can purchase masonry sand from a local supply store and bagged clay from a distributor, the cheaper option is for builders to use nearby sub-soil or the sub-soil from the building site as many regions of the world will have suitable clay and sand to make cob.
When it comes time for the actual mixing and laying of the cob, builders can do the mixing part by using powered tools or by inviting or employing a group of people to walk and stomp on the cob ingredients with their bare feet. The barefoot group mixing method can also incorporate some additional fun by adding music and dancing to the mix. As cob builders and their helpers have all the materials wetted and mixed, they can break up the cob into softball-sized globs or the infamous lumps or loaves that are then melded and sculpted to form thick load-bearing monolithic walls that rarely require posts or beams to be a part of a structure.
HISTORY
Humans have used cob as a building material for thousands of years in many parts of the world. Although cob went through a period of close extinction as it competed with contemporary building methods, it has undergone a revival since the mid-1990s. According to the Cob Research Institute, cob-like walls appear to have originated in North Africa starting in the 11th century and by the 12th century, Europeans started using cob in various forms. For Great Britain, cob structures became the norm in many parts of the country by the 15th century and then gained widespread use by the 19th century. As the British settled into parts of North America, Australia, and New Zealand, they also brought the cob technique with them during the 1700s and 1800s. Outside of British and European influence, indigenous people throughout the world also developed cob techniques of their own. The Arabian Peninsula has firm evidence of cob history, with Yemen having cob structures that are over 700 years old and nine stories tall.
Throughout the world, the cob technique subsided by the late 1800s due to the mass-production of industrialized brick and other building. However, as many long-neglected cob homes needed repair by the 1980s, it prompted preservationists to consider and use traditional building techniques. The resurgence became evident in the United Kingdom as the country adopted new building regulations in 1985 that accepted historical materials. As restoration advocates further used cob, public attitudes started to change and as a result, traditional cob cottages became historically valued and viewed as picturesque. By the 1990s, England’s strong cob building revival came into full swing.
The 1990s was also the time period the Cob Cottage Company in Oregon started making hard, stable cob mixtures by incorporating amendments or using more sand or clay as needed. In previous centuries, cob builders used whatever soil was on hand and used little or no amendments, as there were no transportation methods to make amendments readily available. The Oregon method differs from traditional cob in that there is a high proportion of coarse sand and long, strong straw strands in order to prevent cracking. The components of the method also strengthen the earthen mass of a wall assembly. The method also has enabled cob to become a contemporary method for building.
In order for cob to be recognized as a legitimate construction method and for it to be understood among building professionals within the United States and Canada, the Cob Research Institute and its members achieved adoption of their code proposal by the International Code Council (ICC) in 2019. The organization’s proposal is now an appendix to the International Residential Code (IRC), a set of residential building standards that are often adopted or followed by local jurisdictions, building code officials, and professionals within the building industry. The Cob Research Institute views the code adoption as a major step in fulfilling the organization’s mission of removing legal obstacles for building with cob and the organization is continuing its mission by conducting more research and submitting proposal updates for additional ICC adoption.
MATERIAL PROPERTIES
Insulation Characteristics:
Similar to adobe, cob walls are a relatively high-mass wall system, with a low thermal resistance (R-value) rating. The International Residential Code (IRC) appendix for cob construction assigns an R-value of 0.22 per inch of cob thickness for cob walls with a density of 110 pounds per cubic foot. Although cob has a low R-value, depending on location, experts say thick cob walls, such as three-foot-thick cob walls, could potentially achieve favorable insulating values. The thermal mass of cob can also absorb heat and depending on climate conditions, the material could help stabilize indoor temperatures year-round. This feature is favorable for hotter regions, such as the southwestern part of the United States, where there are vast temperature swings between day and nighttime hours.
For colder climates, the code proposal by the Cob Research Institute suggests adding insulation to the face of cob walls if the insulation assembly complies with other code requirements for finishes that integrate support, vapor permeance, and weight limits. Although some jurisdictions may reduce thermal requirements for cob by treating the material like concrete block, brick, or rammed earth, there are some jurisdictions, even in warmer regions, that will require a minimum insulation value for an exterior mass wall. This requirement then leads to a situation of needing to build with thicker cob walls, but what gets tricky is that code enforcement officials may not allow thick cob walls in regions where there are structural and safety concerns because of earthquakes. According to John Fordice of the Cob Research Institute, the CobBauge Project at the University of Plymouth has been devising ways to insulate cob walls by wrapping them with lighter mixtures of clay and natural fibers which include hemp and straw.
Fire Resistance:
Because humans have been using earth to build ovens and fireplaces for centuries, proponents of cob consider the material fire resistant along with having the favorable feature of using non-toxic material. Although cob builders claim fire resistance for cob, the current International Residential Code says cob walls are not fire-resistance rated. The Cob Research Institute is trying to change that and has been collaborating with other organizations to develop a fire rating of cob within the realm of ASTM E119 laboratory testing. The Cob Research Institute says it is seeking the code update so builders can construct cob as a firewall between dwelling units or within property lines of densely built areas. According to the organization’s website, a collaborator of the organization had two fire tests done in December 2021 for a tri-density cob wall and a mono-density wall, with each withstanding ASTM E119 fire exposure for 120 minutes. The organization’s collaborator also requested comprehensive load testing of the mono-density assembly to assess the structural integrity of the structure after exposure, and it performed well. Because of the testing, the Cob Research Institute introduced a list of certain specifications for a cob wall to have a two-hour fire resistance rating in its proposal for an IRC update. Among the specifications are a minimum bottom wall thickness of 12 inches, a minimum top wall thickness of 10 inches, and a minimum compressive strength of 85 psi for a section.
Durability and Strength:
According to the Cob Research Institute, the laboratory compressive strength of a cob mix depends on many factors or the ranges of each component material, including the water content. Using different test methods and a focus of certain components, the lowest average psi was 41, which included long straw and 231 psi when building with smaller walls. For flexural and rupture test results, the highest psi value was 142 (modulus, conventional mix). The current code for cob construction does dictate that all cob walls shall have a minimum compressive strength of 60 psi and braced wall panels shall have a minimum of 85 psi.
Before building a cob structure, cob experts recommend builders make test samples by molding several cobs together and letting them dry for a week. The dried samples can then be dropped, scratched, and nailed to determine if the ingredients are binding and working properly. The International Residential Code also requires builders to demonstrate the compressive strength of a cob mix (used for structural and non-structural walls) to a building official by testing five samples by an approved laboratory or with approved on-site testing.
Because cob in its raw form is susceptible to deterioration from harsh weather elements or long periods of rain, experts recommend that cob structures have a good foundation and a roof with a decent overhang. The structure should also have protective plaster that allows the walls to breathe while defending the structure from rain and windy elements. For earthquake resistance, the Cob Research Institute has reviewed studies showing that both unreinforced and reinforced cob walls (based on design by Oregon Cob Cottage Company) are suitable in locations with severe earthquakes, but the International Residential Code does currently require a structural design by a licensed civil engineer for cob buildings constructed in many seismic regions of the United States.
Indoor Air Quality:
Cob walls are hygroscopic, meaning they can stabilize indoor humidity levels by taking in a lot of vapors and releasing the moisture vapor when conditions allow. This in turn helps with indoor air quality by preventing mold and keeping the indoor air cleaner.
Sound Transmission:
Cob walls will generally have a high sound insulation ability due to their mass and thickness, similar to adobe.
GALLERY
Additional Notes
Cob involves using readily available and inexpensive materials that do not create a lot of pollution in producing them, including the potential of using earthen materials from the building site or local materials. Because of cob’s inability to restrict heat flow, it may take extra energy to maintain a comfortable temperature inside the home. For builders choosing to work with cob in colder climates, cob will require extra consideration and planning to ensure a more energy efficient structure.
Cob lends itself well to freedom of design and personal and playful expression. Builders can craft and sculpt curved walls, niches, arches, vaults, benches, and other unique features that display personalized beauty and character. Cob structures can also provide a feeling of coziness and security with their thick walls.
FAQ
The material cost for cob walls can be lower compared to conventional homes, as there will be fewer or no costs for wall materials such as cement, lumber, bricks, and metal. Cob wall ingredients can also be inexpensive, especially if site soils are used. If subsoil is not available at the building site or from a nearby location, there will be an additional cost to acquire the clay and sand from sellers, including the straw needed for the project, but those materials are likely cheaper than conventional wall materials. Labor may be the biggest cost factor in building a cob exterior, but it can become less expensive with skill sharing workshops and having work parties with friends, family, and neighbors. Don’t forget that cob walls are only one element of a home, which can include conventional or unconventional floors, roofs, and mechanical systems, etc.
The “cob-ability” of nearby soil is often determined first by the noticeable traits the sub-soil has or its composition of water-absorbent and non-water-absorbent minerals or rocks. The non-water-absorbent portion of sub-soil can vary with how much gravel, sand, and silt it has or if the soil has one dominant form of an aggregate. Sand that is ¼ inch in size or smaller is the most optimal for cob. For the water-absorbent portion of sub-soil, if it is apparent there is a lot of clay in the subsoil, an owner-builder will likely need to add sand to create a suitable mix. Testing the soil with a jar or shake test and doing it several times will ultimately help determine if the sub-soil is suitable as is or if its current composition will require more sand or clay. A shake test involves taking crushed sub-soil from the site and filling a jar one-third full of the soil and then filling the rest of it with water and then shaking it vigorously and examining how the soil particles settle by layer. If the jar content shows mostly clay or sand, the cob builder will then be able to add one ingredient or the other to make a good cob mix.
Adobe and cob are both earth-based products primarily made of sand, clay, straw, and water, but the difference is that builders set and dry adobe into blocks prior to construction, whereas cob is molded and sculpted into walls while it is still wet. One advantage cob has over adobe is if a project is in a damp climate and or is in a region with limited dry periods, builders can still lay cob within those environments.
It is important to note concrete is also a mud mix, but it differs from cob in that its aggregates are gravel and sand and its binder is Portland cement. Portland cement is made from lime, which is often made from limestone, and other minerals that are baked in a kiln. Modern versions of cob and concrete include additives to increase tensile strength. For cob, the additive is strands of straw, and for concrete, it is steel rebar.
The response “it depends” is often part of the answer for how fast a cob structure can be built. According to the Cob Cottage Company, a prominent collective that has promoted cob building in the United States for many years, the group builds a foot in wall height per day during dry weather. The group also says the construction pace depends on weather and the size of the crew. The perspective by Cob Cottage Company is that cob building should be done at a peaceful, meditative, and artistic pace rather than racing to build fast. Another advantage of cob is that once a wall is done, there is not a need for sheet rock, taping, spackling, and painting, but cob walls will still need to be finished with a plaster.
Additional Reading
Reference Material:
The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage by Ianto Evans, Michael G. Smith, and Linda Smiley, Chelsea Green Publishing, 2002.
The International Code Council – Appendix AU
International Residential Code (IRC) Appendix: AU-Cob Construction (Monolithic Adobe)
Organizations:
Cob Cottage Company – Coquille, OR
http://www.cobcottage.com/
Cob Research Institute – Berkeley, CA
https://www.cobcode.org/
University of Plymouth CobBauge Project – Plymouth, UK
https://www.plymouth.ac.uk/research/cornerstone-heritage/cobbauge-project
Earthen Construction Initiative – San Antonio, TX
https://www.earthenci.org/
Endeavour Sustainable Building School – Peterborough, OT
https://endeavourcentre.org/
Cob Workshops – (crowdsourced list of cob building workshops)
https://cobworkshops.org/
