source: Tim Reith
Cob is an old English word for "lump". It refers to lumps of earthen material that are sculpted together to form walls or other features. Cob is a mixture of clay or clay-rich soil, sand, fiber (generally straw), and water. This is a truly ancient building material, one that can be found (or variations thereof) across the globe and spanning back thousands of years. The benefits of cob are many, and like many of the other materials in the natural builder's toolbox, can be integrated into a structure to take advantage of its strength and beauty in a non-toxic, low-impact, inexpensive, user-friendly way.
Roots in Earth
Best known to us in the west is cob's long tradition in the UK, where thousands of cob buildings — many still standing and inhabited — have been constructed over the last few centuries. True to their vernacular, these structures have very clean lines, simple designs, and evoke the imagery of thick-walled, white-washed British country cottages. Cob — and, with the exception of adobe, earthen construction in general — is something of a newcomer in the US. With its development in the United States has come a change in character and feel, and many new cob structures exhibit cob's inherent creative potential, sporting curved walls, niches and inlays, moldings and sculpted reliefs. It is cob's ability to be sculpted that sets it apart from so many other forms of earthen construction: no forms, molds, or other rectilinear confinements are required for cob's functionality. Apart from obeying basic rules of physics and cob's own working properties, there are virtually endless design possibilities with the medium, from the flat, plumb wall to the sculpture of a dolphin leaping over the tile-studded arch. Cob is also by no means limited to walls — built-in benches, chimneys, even ovens have been traditionally and are still frequently built of cob or cob-variant mixes. Again, when it comes to designing and building with cob, the sky's the limit.
[back to top]
Being primarily made of earth, cob is as inexpensive as the soil from which it is made; if this happens to be material pulled out of the site during excavation, it could well be that your investment in a cob wall is little more than a load of sand and some bales of straw. Quite simply, cob is made by mixing clay-rich soil &mdash or if not available, pure clay — with sand, adding in loose straw, and hydrating with water to get the desired consistency. Cob can be thought of very much along the same lines as plaster when analyzing the nature and function of its ingredients. There are three basic categories of purpose: binder, structure, and fiber. The binder is the clay; it holds the mix together. The structure is the sand; it provides the body of the mix, giving the cob its strength, stability, and durability. The fiber is generally straw; lengths of straw run interstitially throughout the cob wall, providing tensile strength and helping to hold the body of the wall together to resist forces at play against it. See the plasters section for more on the trinity of binder/structure/fiber.
Mixing cob can be very low tech and labor-intensive, or can be done with the aid of machinery. Often, especially in smaller owner-built or community-built structures, cob is mixed by foot. Truly; one adds the materials sequentially and strategically onto tarps, and stomps in such a way as to quickly and efficiently mix the material together. The mix is then transported to the workplace in the tarp. Smaller batches can also be made by hand, although this is a much more exhausting process than by using one's feet and body weight. If one finds oneself in the position of needing to save on labor or time, machines can be used to mix the cob; bobcats or similar small front-end loaders can lift, dump, smash, and mix larger volumes of material quickly, as well as transport sand, soil, and mixed cob easily around the site. For even larger projects, tractors or large equipment can be used to do the job.
Cob is built one cob at a time, and sculpted into form. Cobs are knitted together side to side and between courses to create a seamless, monolithic structure; care
is taken to maintain proper form, plumb and level, and the wall can easily be added onto or carved back as need be throughout the process. Cob needs to be of a certain
moistness to be able to effectively mix and shape. However, this moistness — and the absence of formwork, except for creating arches or other void spaces —
limits the progress; cob needs to "set up" &mdash that is, dry and stiffen a bit — between courses to avoid slumping and distortion, and ensure structural
integrity. This rate depends on the nature of the mix and the weather; often, it is paced nicely with the rate of mixing and labor available for construction. It also
gives ample time to allow the walls to develop in a thoughtful and organized manner, and provide many opportunities for creative expression as the walls are under
[back to top]
The "Mass" in Massive
As can be judged by its weight, cob is a very massive material, meaning it has a lot of weight relative to its volume — high density. This mass serves a number of positive functions. For one, cob is very structural in compression when built in good proportion (i.e. not too tall and skinny); this allows for cob walls to fully support roof loads without the additional need for a frame. For another, the mass — in the form of clay — helps to mitigate moisture issues. Clay has a fantastic ability to hold large amounts of water without weakening, deforming, or otherwise failing. Therefore, whether in liquid or vapor form, sourced from rain or condensation, the high volumes of clay serve to act as a 'moisture battery' to assuage damage that might otherwise be done by elevated levels of moisture. Please note this is a feature specific in this case to clay; one would not enjoy similar benefits from a concrete wall — in fact, quite the opposite, as while concrete is generally porous enough to wick moisture in contact, it lacks the capacity to store moisture. From a thermal standpoint, and again universal to all mass regardless of composition, mass can serve as a great benefit in stabilizing temperature swings. Not unlike clay's ability to store large amounts of water, mass in general has a great ability to store heat. Take the example of a sun-drenched rock, upon which a piece of black steel is placed. The metal will become incredibly hot while in the sun; shortly after sunset, the steel will loose its heat. The boulder upon which it sits, embodying many times more mass, will not heat up to the same temperature, as the steel; rather, it will feel pleasantly warm. And this warmth will be sustained long after the steel has returned to ambient temperature. This same heat-retention characteristic serves well in a structure, as high-mass homes can help buffer temperature swings, particularly diurnal swings, or those that occur between daylight and nighttime. When well-designed, high internal mass in a structure will store excess heat generated by daytime sunshine and reduce overheating, and radiate it back into the home at night, offsetting nighttime cooling.
[back to top]
So how does this mass serve us in this cold climate? It should be noted that mass can work both ways; if a large amount of mass is left to get very cold, it will take a tremendous amount of heat energy to bring it up to a comfortable temperature (think of a clammy, stone-walled castle). The trick is to keep the mass at that comfortable temperature in the first place. In a cold climate, mass that is very well insulated is much less likely to cool down to an uncomfortable temperature &mdash common sense. This is particularly an issue in structures that are heated incidentally, such as camps, as opposed to continuously-heated structures, such as a residence. If a cob home is kept heated to 65 deg. Far. beginning in the fall, it will take a relatively small amount of energy to keep the mass at this temperature; this is the same energy efficiency benefit enjoyed by radiant floor concrete slabs. However, if that cob is left to cool to 50 deg. F., it will take much more heat to bring the mass up to temperature to gain its benefit. Determining heat-loss calculations depends on many factors, and this high-mass, low-insulation scenario may not dramatically effect the performance of the house, depending on the size and design; the moral of the story is to use your mass correctly. Ideally, in this climate, it should be insulated. That said, there are people living comfortably in cob houses in Vermont, and in such cases, keep the home fires burning. And being fireproof, cob can be gracefully integrated to form the oven, the chimney, and the wall.
As mentioned above, cob's natural ability to mitigate excess moisture is well received in this wet environment. Inherently durable — a well-protected wall
will last many hundreds of years, as observed in Europe — cob can withstand the ravages of water well. That said, the better protection offered, the longer the
life and less need for maintenance. Plaster is the ubiquitous finish for cob structures, and appropriate design
strategies to avoid water infiltration in the first place can make the difference between a temporary shelter and a legacy building. In all, by historic precedence and
in modern context alike, cob has earned a place of stature in cold and wet climates, in the northeastern US and beyond.
[back to top]